Andreas M KunzkunzimesmavtethzchSwiss Federal Institute ofTechnology ZurichCenter of Product DevelopmentTannenstrasse 3 (CLA E 143)8092 Zurich Switzerland

Christian P SpagnocspagnoimesmavtethzchSwiss Federal Institute ofTechnology ZurichCenter of Product DevelopmentTannenstrasse 3 (CLA E 18)8092 Zurich Switzerland

Presence Vol 13 No 2 April 2004 178ndash192

copy 2004 by the Massachusetts Institute of Technology

Construction of a Three-SidedImmersive TelecollaborationSystem

Abstract

In this article the setup and working principle of a new telecollaboration systemldquoblue-crdquo is described This system is an attempt to meet the rising expectationsfrom industry of an IT-supported telecollaboration system One basic requirement isthat a three-dimensional representation of objects be possible together with three-dimensional representations of the remote users Since gesture and mimicry repre-sent an important information channel during a discussion a realistic 3D video rep-resentation is used instead of simple animated avatars

A simultaneous projection and image acquisition of the user in a telecollaborationsystem is necessary to allow simultaneous work of all team members Thus in theintroduced system problems had to be overcome such as providing simultaneouslyillumination for the image acquisition by the cameras and darkness for a bright pro-jection to be seen by the user A new approach was taken to integrate the cam-eras into the system by placing them behind active projection walls which can beswitched from transparent to opaque electrically Unlike other systems the camerasare therefore not visible to the user who thus behaves more naturally In additionsince the cameras are placed outside of the projection room there is more spaceto move inside the immersive environment

The article describes the technology and functionality of the system as well as thegathered experiences

1 Introduction

Advancing globalization and increasing demand for flexibility to quicklyreact to changing customer needs are influencing the way in which business isdone Short response and development time are key factors for success Infor-mation and communication are the primary competitive tools in the 21st cen-tury Increasing product complexity involves a growing number of people andcompanies which are often spread all over the world As a result the need fortravel and communication is increasing The Internet and e-mail technologieshave opened new avenues for communication Video conferencing has alsobeen available for over a decade now But even the combination of all thesetechnologies cannot always replace a face-to-face meeting Thus the goal is torealize a new generation of telecollaboration systems where the userrsquos texturesand silhouettes are captured in real time and then represented as a 3D modelin the other installation

178 PRESENCE VOLUME 13 NUMBER 2

First steps have already been taken to use VR for dis-tributed collaboration Computing tools support infor-mation exchange and simple communication fairly wellHowever collaboration on complex issues is not wellsupported Successful models of computer-supportedcollaborative work (CSCW) are still rare (Elspass et al1999) So far in virtual meetings human beings areinadequately represented and disembodied throughtext voice or 2D video projections The representationof people is therefore not natural As a result the levelof presence of the remote person is low and the separa-tion between remote collaborator and task can be quitefrustrating Ideally the remote person should be em-bodied by a full-size photorealistic 3D representationSuch a representation is ideally visualized with SpatiallyImmersive Displays

Spatially Immersive Displays (SID) have become in-creasingly popular over the past decade They enable theuser to work in virtual spaces while being physically sur-rounded with a panorama of imagery (Lantz 1996)The most common SID used today is the CAVE fromthe University of Illinois Chicago or one of its variants(Cruz-Neira Sandin amp DeFanti 1993) This environ-ment typically comprises a cubic room with up to sixback-projection units Distributed versions of SIDs al-low users to interact with remote collaborators in tele-collaboration applications These systems usually inte-grate 2D video-based human avatars possibly enhancedwith prereconstructed geometric models into the vir-tual environments (Gross Kunz Meier amp Staadt2000) The Cabin of the University of Tokyo is a five-sided SID consisting of three wall projections a floorprojection and a ceiling projection (Ogi Yamada ampHirose 1999) In 1997 the Cabin was extended to anetworked environment by connecting it to other SIDsto allow telecollaboration The user is captured withinfrared cameras placed on tripods inside the SID Infra-red cameras have been chosen because of the low lightconditions inside the SID A black-and-white represen-tation of the user is then transferred to the remote in-stallations Immersive projection systems demand highresolution The resolution of a display wall can be in-creased by using multiple projectors For systems withlarge numbers of projectors automatic projection cali-

bration is mandatory In 1998 a display wall was built atPrinceton University using eight LCD projectors Eachprojector is connected to a node of a computer clusterand an automatic projection calibration is integratedinto the system (Wallace Chen amp Li 2003) In No-vember 2000 the system was scaled up with 24 DigitalLight Processing (DLP) projectors (Chen et al 2002)

While stereo projection has already reached a highlevel of quality the acquisition of the user with simulta-neous projection is still very challenging This often re-sults in compromises between acquisition and projec-tion Infrared cameras are often used due to the poorlight conditions inside SIDs Sometimes the cameras areplaced in front of the projection which therefore de-creases the immersion quality and the size of the work-ing area In some cases only one camera is used allow-ing only a 2D representation of the user

2 Contribution

In the presented work the above mentioned issueshave been addressed and new solutions have been devel-oped to build the ldquoblue-crdquo an innovative prototype of ahighly immersive projection and video-acquisition vir-tual environment for collaborative work The new sys-tem makes it possible to record live video streams ofusers and to simultaneously project virtual reality scenesIn a virtual meeting the participants are embodied by arealistic high-quality representation fully 3D renderedand supporting motion and speech in real time (Elspasset al 1999) In order to increase the immersion a sixchannel audio system was also installed The user aspart of the real world is therefore synthesized by imageand sound and integrated in the virtual world Thus theSID evolves from a virtual reality system to an aug-mented virtuality system

Special solutions had to be found to allow simulta-neous projection and picture acquisition The camerasare placed behind active projection screens which canbe switched to a transparent state to perform pictureacquisition This solution allows integrating the camerasin a SID without interfering with the projected imagesThe controversy between darkness for a sharp projection

Kunz and Spagno 179

and brightness for picture acquisition has also been ad-dressed An active LED illumination in combinationwith modified shutter glasses has been implementedto allow an acquisition with color cameras while notdisturbing the user The goal of the so-called blue-cinstallation is to build collaborative immersive virtualenvironments which integrate real humans as three-dimensional objects Two installations are intercon-nected to allow bidirectional collaboration and in-teraction among people sharing virtual spaces Thepeople are captured in real time full color and three-dimensionally Each installation consists of a projectionsystem and acquisition hardware that operate in the vi-sual wavelength spectrum (see Figure 1)

Essentially two systems must be combined into oneinstallation an immersive visualization system and anacquisition system The acquisition has to take place inthe visual wavelength spectrum and in full color Multi-ple cameras are used which enable a 3D reconstructionof the acquired person In order to proof the concept ofnetworked collaboration two setups were realized lo-cated at different sites of the campus

3 System Overview

As a central element a special projection screen isused that can be switched from opaque to transparent

and back The idea is to use time multiplexing to com-bine the projection and acquisition During the opaquestate of the projection screen the projection is runningand an image is projected onto the screen During thetransparent state the cameras can look through the pro-jection screen and take images of the user inside theinstallation The switchable projection screen has to besynchronized with the acquisition system If the screenswitches quickly enough between the transparent andopaque states no flickering will be observable and thecameras will not be visible to the user Phase-dispersedliquid crystal (PDLC) glass panels are used as a projec-tion screen The panels are based on liquid crystal (LC)technology and can be switched electrically to transpar-ent and opaque Although these glass panels are com-mercially available they were not primarily designed forprojection but rather as an architectural element Thusmeasurements were performed to find out whether theycould be used as shuttered projection screens The max-imum available width of the panels is 950 mm For aprojection room of 285 m 285 m three of thesepanels are installed per side

With the use of actively shuttered projection screensthe projection and acquisition have to take place at dif-ferent points in time This time multiplexing betweenprojection and acquisition has two or three differentphases depending on the projection For a normalmonoscopic projection two time phases are necessaryone for projection and one for acquisition In combina-tion with an active stereo projection three phases arerequired one for the left-eye projection one for theright-eye projection and one for picture acquisitionThis sequence is repeated with a frequency that is highenough to avoid flickering that can be noticed by theuser

An important question was how to illuminate the userwithout disturbing him or her and without altering theprojection quality and the degree of immersion Theillumination has to be in the visible light spectrum sincecolor cameras are used The solution is to make use ofthe three phases and to activate a flash illumination onlyduring picture acquisition As protection from the lightthe user has to wear shutter glasses which are synchro-nized with the illumination and which are modified by

Figure 1 Principle of the blue-c

180 PRESENCE VOLUME 13 NUMBER 2

including a third phase when both lenses for the leftand right eye are darkened These shutter glasses areneeded anyway for the active stereoscopic projectionFigure 2 illustrates such a projection sequence which isfollowed by an acquisition sequence

During the third time phase when both glasses aredarkened an active illumination is triggered The illumi-nation is preferably done with white light in order toachieve a true-color texture recognition It is possible touse a strobe light or white light-emitting diodes(LEDs) Figure 3 shows how this active illumination canbe kept out of the userrsquos eyes by integrating a thirdphase into the shutter glasses (Kunz amp Spagno 2001aKunz amp Spagno 2001b)

In the left-hand pair of images in Figure 3 the illumi-nation is shown as seen without the third phase in theright-hand pair of images the illumination is shownafter the third phase within the shutter glasses is acti-vated In each pair the second image shows an objectthat is only illuminated by ambient light

With sufficiently bright illumination it is possible toacquire good-quality images The cameras for recording

the texture of the front of the user have to be placed infront of user without being in his or her field of view(Fuchs et al 1994) Therefore the best position for thecameras would be behind the projection screen wherethey cannot be seen by the user (Figure 4) The activeprojection screens allow the cameras to be arrangedaround the projection room The layout of the blue-c isa three-sided projection room with a quadratic groundplan This quadratic layout has been used for the firsttime in the cave (Cruz-Neira Sandin amp DeFanti1993) No edge-blending is required since the differentprojections are on different screens

The working principle of the blue-c relies on an exactsynchronization among all components involved Cam-eras flash illumination shutter glasses projector shut-ters and the active projection screen need a hard syn-chronization to allow stereoscopic projection togetherwith image acquisition

The triggering sequence of all components can beseen in Figure 5 The camera triggering frequency is setto acquire images only every seventh cycle of the basictriggering frequency of 625 Hz This corresponds to89 images per second This value was chosen to allowenough time for the acquisition system to process theincoming images Although the flash illumination andthe triggering of the PDLCs could be reduced to89 Hz we kept the higher triggering frequency in or-der to avoid flickering that would be disturbing to theuser since the human eye is very time-sensitive at lowerfrequencies

Figure 2 The three time phases (time intervals)

Figure 3 Active illuminations without and with a third phase

Figure 4 blue-c layout

Kunz and Spagno 181

4 The Components in Detail

41 Mechanical Construction

Since a magnetic tracking system was chosen thebasis of the construction is a wooden platform with atotal height of 540 mm (see Figure 6) The trackingemitter is built into the middle of the wooden platformand thus is not visible to the user The two lower frontloudspeakers and the subwoofer are also built into theplatform The floor of the platform can be easily openedto access all the built-in components The upper struc-ture is built of fiber-reinforced composites (glass andcarbon) These are stiff enough to keep the LC projec-tion panels safely in place and at the same time stillsmall and attractive from a design point of view Cablechannels are built into the beams providing sufficientspace for the cables of all components placed on theupper part of the structure including the cables for theactive projection screens Together with the woodenplatform there is enough space for all the cables andconnectors for the system The chosen materials woodand fiber-reinforced composites assure an accuratefunctioning of the tracking system The cables and theloudspeakers which are at a distance of 15 m from the

emitter of the tracking system do not have any impor-tant influence on its accuracy The results from prelimi-nary tests show that the electrically switched glass panesalso do not interfere with the tracking system

42 Hardware Construction

Figure 7 gives an overview of all components ofthe blue-c and its connection scheme A total of 6 pro-jectors and four monitors are connected to an SGIOnyx The 16 cameras are connected via sixteenFireWire cables to the computer cluster The active pro-jection panels the 6 ferro-electric shutters in front ofthe projectors the active illumination the cameras andthe IR-emitters are synchronized by custom-made syn-chronization electronics The individual componentswill be discussed in the following sections

421 Active Projection Screen Phase dis-persed liquid crystal (PDLC) glass panels offer the re-quired features for a switchable projection screen Theycan be controlled electrically and can be switched in afew milliseconds or less between a transparent and anopaque state Different glass manufacturers have switch-

Figure 5 Triggering timings

182 PRESENCE VOLUME 13 NUMBER 2

able glass panels available with an integrated PDLClayer To change the optical property of a PDLC glasspanel an electrical field has to be applied perpendicularto the PDLC film A voltage in the range of 50 V to100 V is necessary to generate this electrical field ThePDLC glass panels consist of two glass layers coatedwith a transparent electrode on the inside surface and aPDLC film in between Each of the two conductors has

to be contacted by an electrode On the glass panelsused the two copper electrodes are located on the twoshorter edges so as to be invisible when the glass panelsare installed The current flowing through the transpar-ent electrode has to be limited to avoid its overheatingand therefore damaging the PDLC glass panel Themanufacturer recommends not exceeding 300 mA for a2240 mm 950 mm panel with the electrodes placedon the short edges as installed in the blue-c To opti-mize the synchronization electronics the electricalproperties of the panels have to be analyzed

In order to analyze the electrical properties of thePDLC panels the current response to an applied volt-age was measured As with other liquid crystal devicesthe PDLCrsquos should not be exposed to any DC voltagewhich could cause ionic buildup damaging the liquidcrystal structure Therefore only AC measurementswere performed There is also an upper frequency limitdue to current limitations For a voltage of 80 V themaximum frequency is around 100 Hz (Figure 8)

To calculate the electrical behavior of the PDLC glasspanels an equivalent circuit of three components asproposed by Drzaic (1995) is used (Figure 9)

Based on this circuit the complex impedance is

Z R1 R2

1 CR2)2j CR22

1(CR22

This equation has a real and an imaginary part and thusonly two of three variables could be calculated How-ever since more measurements at different frequencies

Figure 6 The mechanical construction of the blue-c

Figure 7 Component overview

Kunz and Spagno 183

were performed an approximation was calculated mini-mizing the mean square error (MSE) between the calcu-lated and the measured voltages The approximated val-ues are

R1 1745 R2 2494 C 11F

The electronic control of the PDLC panes can be ad-justed to eliminate any DC components in the drivingvoltage In addition an external DC decoupling circuithas been added to guarantee that no DC voltage dam-ages the glass panels even if a fault in the driving circuitoccurs This DC decoupling consists of a capacitor CD

and a resistor RD (Figure 10) The values are 60 F forthe capacitor and 10 k for the resistor

The glass panes show a very stable performance overtime We do not observe any significant change either inthe electrical behavior or the stability of the LC filmThe system worked for many days without any prob-

lems although the panes are driven at their upper limitin frequency and allowable current

422 Projectors and Active Shutters Theactive LC projection screens do not support passive ste-reo projection due to their depolarization propertiesTherefore an active projection system was chosen forthe blue-c In the year 2000 when the design and theconstruction of the system were started active stereoDLP projectors were just emerging and still much tooexpensive for our budget The state-of-the-art CRTprojectors were also still quite expensive compared tothe light output Furthermore we were looking for asystem that could be turned black during the cameraacquisition period that is 625 times per second Thedecision was taken to use two three-chip LCD projec-tors (Sanyo XF12 3500 ANSI lumen) per side and ex-ternal LC shutters in front of the projection lens Withthese shutters it is possible to generate an active stereoprojection by alternating the light output of the twoprojectors and blocking it during the picture acquisitionperiod

The two projectors on each side are mounted in arack for a passive stereo projection installation (Figure11) The LC shutters are custom-made ferro-electricliquid crystal shutters with an aperture size of140 mm 140 mm The shutters are mounted in frontof the 121 projection lenses This lens allows projec-tion directly onto the projection wall without any addi-tional mirrors Using no mirrors in the projection pathfacilitates the alignment of the two LCD projectors foractive stereo projection The shutters do not have to besynchronized with the Onyx and projectors thus leav-ing the shutter times completely flexible This principlewould not work with a single-chip DLP projector sincethe sequential generation of the three colors by a ldquocolor

Figure 8 Voltage and current measured on a panel of

2240 mm 950 mm at 100 Hz

Figure 9 Equivalent circuit diagram of the PDLC glass panel

Figure 10 DC decoupling

184 PRESENCE VOLUME 13 NUMBER 2

wheelrdquo would require a complicated synchronization ofthe shutters with the projectors However today 3-chipDLP projectors could be used instead of the two LCprojectors The shutters in front of the projectors haveto be switched on and off at a frequency of approxi-mately 60 Hz

Twisted Nematic (TN) liquid-crystal shutters as usedin shutter glasses can offer a contrast ratio of 10001 orbetter (Lipton 2001) They switch in about 25 msfrom black to transparent and in less than 1 ms fromtransparent to black (Lipton 1991 Woods amp Tan2002 Bougrioua et al 2002) Ferro-electric (FE)liquid-crystal shutters offer about the same contrastratio as TN shutters but much faster switching timesThe FE shutters used in front of the projectors are140 mm 140 mm and offer a contrast ratio of ap-proximately 7501 The switching time from 10 to90 transparency and from 90 to 10 transparency isbelow 100 s (see Figure 12)

The upper measurement in Figure 12 shows the volt-age measured on the shutter The mean value of thisvoltage is zero as required The lower measurementshows the optical response of the shutter measured by aphototransistor and a light source The response is veryfast and guarantees an excellent separation between thetwo stereo channels

A TN shutter switches to black if a voltage is appliedand to transparent if the voltage is removed An FE

shutter in contrast switches to black if a voltage of onepolarity is applied and to transparent if a voltage of theopposite polarity is applied It is important that theshutter not be subject to any DC voltage as it will bedamaged The DC decoupling is achieved by connect-ing a capacitor CD of 1 F in series and a resistor RD of1 k in parallel to the shutter (see also Figure 10)

Mounting the shutters directly in front of the projec-torrsquos lens did not cause temperature problems as ex-pected Even after several hours of projection the tem-perature was beneath the limit tolerated by thetemperature-sensitive devices

423 Active Illumination The requirements onthe blue-c illumination are very demanding For theprojection both the inside and the outside of the pro-jection room have to be dark On the other handbright illumination is important for good-quality imageacquisition The illumination has a big impact on thequality of the extracted silhouettes and acquired tex-tures A person inside the blue-c should be illuminateduniformly from all sides and no shadow should be pro-jected on the floor The illumination has to be directedaway from the PDLC glass panels to avoid contrast-lossand reflections

Figure 11 Stereo projection rack with LC shutters

Figure 12 Ferro-electric LC shutter (driving voltage optical

response)

Kunz and Spagno 185

An active illumination is used to satisfy the needs ofboth projection and image acquisition The active lightis off during projection and on for the image acquisi-tion Therefore the light has to provide illumination foronly a few milliseconds and must be able to turn on andoff with a frequency between 60 and 70 Hz First testswere done with a strobe light A high voltage dischargein a xenon flash lamp produces a bright light pulse of avery short duration Frequencies in the range of 60 Hzare no problem for this type of lamp Despite the fastswitching this type of light has some important disad-vantages First light is emitted for only a few millisec-onds The light intensity integrated over time is there-fore only mediocre In addition the high voltagedischarge in the very short time period causes heavyelectromagnetic pulses which can interfere with sur-rounding electronic components The discharges alsocause a disturbing acoustic noise Finally the strobelight is focused in one spot This is a handicap whentrying to distribute the light uniformly in the projectionroom All these disadvantages led us to look for otherillumination solutions

The requirement of short switching times restricts thenumber of suitable light sources considerably Light-emitting diodes (LEDs) have very short switching timesand are thus suitable for our application Today whiteLEDs are commonly available and the light efficiency isabout twice that of an incandescent bulb (Zorpette2002) LEDs can be easily grouped in clusters to givemore light output The clusters are flat can be of anyshape and the light output can be distributed homoge-neously over the cluster if desired A total of 9984white 5 mm LEDs have been integrated into the blue-cThe opening angle (50 intensity) is 50 degrees and thelight output is approximately 2200 mcd per LEDThese LEDs were the brightest available on the marketin 2002 The LEDs are configured into 32 clusterseach containing 312 LEDs The clusters are reassem-bled in groups of two four or five and mounted inlong aluminum profiles

The LED clusters have to be placed carefully in theblue-c to allow a good quality image acquisition Thecameras are mostly placed outside the blue-c and facemore or less horizontally to the user through the PDLC

glass panels Under this aspect a diffused frontal illumi-nation of the user would be the most suitable But afrontal light would also shine directly into the oppositecameras and furthermore would put too much light onthe glass panels making them appear gray In additionit would be difficult to find a place to mount the clus-ters without interfering with the projection and thecameras

The solution is to distribute the LED clusters alongthe upper and lower edges of the projection room TheLEDs can be adjusted in such a way that they com-pletely illuminate the user minimizing at the same timethe amount of light being directed at the glass panelsand at the opposite cameras The additional mask infront of the LEDs helps to further diminish the amountof scattered light reaching the glass panels Figure 13shows a person between two opposite projection wallsas is the case in the blue-c for the two sidewalls Thelight is coming from all four edges and most parts of theperson are illuminated by more than one light sourceFigure 14 shows one illumination cluster Five of themare integrated in the aluminum profile and placed at theedge between the floor and the left glass panels

Realizing the illumination with the diodes it turnedout that the specifications of the LED were not reachedby every single element This resulted in failures andthus in nonactive LED lines in the cluster After the cur-

Figure 13 Placement of the LED clusters

186 PRESENCE VOLUME 13 NUMBER 2

rent through the entire cluster was reduced this prob-lem disappeared

424 Cameras Sixteen FireWire cameras areplaced around the blue-c on a special aluminum struc-ture The structure is based on standard aluminum ele-ments and offers the maximum flexibility in the place-ment of cameras and other components Each camera isconnected via FireWire to one node of the PC clusterThe signal is amplified every 5 m with a repeater Up tothree repeaters are used for each camera

Dragonflytrade cameras are used since they offer goodsensitivity and a high picture acquisition frequency of upto 15 Hz However cameras with an even higher sensi-tivity would be useful in order to reduce the intensity ofthe flash illumination On the other hand the pictureacquisition frequency is high enough since the bottle-neck is in the processing of the acquired images Initialtests showed that a repetition frequency of 10 Hz seemsto be sufficient for collaborative work

425 Synchronization Electronics The syn-chronization electronics of the blue-c were built modu-larly to offer flexibility in adding and changing compo-nents The main module is the frequency-controlmodule which is equipped with a reprogrammable PIC16F877 microcontroller This module generates thetriggering sequence to drive the other modules of the

synchronization unit Three PDLC glass panel drivermodules two shutter driver modules and one trigger-interface module are integrated into the synchronizationunit together with the synchronization control moduleThe LED driver and IR-emitter driver are placed out-side of the synchronization unit and are triggered by thetrigger interface module

Figure 15 gives an overview of the synchronizationelectronics All modules of the synchronization unit areinterconnected with a data bus In addition a powerbus is integrated into the unit to supply the differentmodules with the required voltages The LED driverand IR-emitter driver are located outside of the syn-chronization unit together with their own power sup-plies to reduce electromagnetic interference caused bythe currents

Using a microcontroller for the synchronization of allcomponents proved to be crucial Since every compo-nent including identical parts behaves slightly differ-ently (eg the glass panes) it is easy to create individu-alized triggering signals using the microcontroller Thismakes up for the increased effort of installing and pro-gramming the processor

426 Frequency Control Module The fre-quency control module is based on a PIC 16F877 mi-crocontroller This microcontroller can be repro-grammed in-circuit that is without unplugging thechip from the board Twenty-two inputndashoutput ports ofthe microcontroller are mapped to the data bus They

Figure 14 An LED cluster of the blue-c

Figure 15 Synchronization electronics

Kunz and Spagno 187

can be set individually as outputs or inputs Each ofthese ports is connected to a LED to show its state Themicrocontroller is clocked with a frequency of 20 MHzto offer high timing accuracy

427 PDLC Glass-Panel Driver Module Todrive the nine PDLC glass panels a total of three glass-panel driver modules are used (see Figure 16) In eachcase three glass panels in parallel are connected to thetwo outputs of a module The two outputs of a modulecan be switched separately to either 8 V or 85 V If bothoutputs are switched to the same value no voltage isapplied to the glass panels If the outputs are switchedto different values a voltage of 77 V or 77 V is ap-plied to the glass panels in series with the decouplingcapacitor Each of the two outputs is switched directlyby the corresponding input coming from the data busA current-limit control is superimposed on the voltageswitching

Figure 16 shows a simplified circuit consisting of aPDLC glass-panel driver module connected to threeglass panels with their DC decoupling circuits Each ofthe two incoming TTL signals switches an optocoupleroutput to a preset voltage of 8 V or 85 V The two out-puts of the optocouplers are connected to two APEXPA46 operational amplifiers The APEX PA64 has anintegrated adjustable current-limit control and can runat voltages up to 150 V and currents up to 5 A Theoperational amplifiers are used as followers thus theinput voltage is mapped directly to the output as long asthe current-limit is not exceeded The PDLC glass-panel driver modules are connected to a 95-V power

supply However the outputs of the module have beenset to switch only between 8 V and 85 V This givesenough margin to the output voltage from the powersupply and ground to avoid saturating the operationalamplifier The current-limit of the operational amplifieris set to 900 mA which corresponds to a current-limitof 300 mA per panel

428 Shutter Driver Module The shutterdriver module also has two inputs and two outputs Itconsists of two independent circuits with an opto-coupler and a half-bridge consisting of two MOSFETsand a half-bridge driver (Figure 17) The optocoupler isused to protect the other modules of the synchroniza-tion electronics in case a fault occurs in this module orin a connected component The output of the opto-coupler is connected to a half-bridge driver (IR2111)that drives two MOSFETs The bridge driver ensuresthat the two MOSFETs in the half-bridge are neverturned on at the same time The two half-bridge out-puts switch the voltage rail-to-rail without any current-limit In order to drive the ferro-electric shutters themodule is operated with a voltage of 12 V Three shut-ters are connected in parallel between the two half-bridges of a module The two half-bridges are alwaysswitched to opposite states The bridge voltage is there-fore either 12 V or 12 V as required by the shuttersfor switching

429 Trigger Interface Module The trigger-interface module is a multifunctional module It allows achoice between up to 8 lines out of the 22 data lines onthe bus Each line can be connected to a 5 V or 12 V

Figure 16 PDLC glass-panel driver moduleFigure 17 Shutter driver module

188 PRESENCE VOLUME 13 NUMBER 2

driver and configured as input or output For the blue-call channels used were configured as 5-V outputs Onechannel is used to trigger the FireWire cameras A sec-ond one is used to trigger the LED illumination Twomore channels are used to synchronize the shutterglasses and are connected to the IR-emitter driver

4210 LED Driver A total of 9984 LEDs areinstalled in different clusters in the blue-c The LEDclusters have to be driven with a voltage of 375 V and atotal current of up to 25 A The power supply for theLED cluster is installed in the floor of the blue-c andconnected directly to the LED clusters This reduces thewiring distances of the high-current connections andthus the probability of electro-magnetic interferenceEach cluster has an integrated MOSFET to switch theLEDs on and off The triggering signal comes directlyfrom the trigger-interface module

4211 Shutter Glasses The LED illuminationis triggered 625 times per second The user does notsee any flickering because this frequency is above thetemporal resolution of the human eye However thebright light inside the blue-c diminishes the contrast ofthe projected image considerably To protect the userfrom the active light an apposite third phase has beenintegrated into the stereo glasses to keep the light outof the userrsquos eyes The projection of the images for theleft and the right eye are followed by a third phase inwhich both LC shutters of the shutter glasses are dark-ened The LED illumination is on only during thisphase This helps to fade out most of the light from theillumination

In the blue-c we use the NuVision 60GX shutterglasses from MacNaughton The shutter glasses and theIR-emitter have a built-in microcontroller We havereprogrammed it to include the third phase Like mostshutter glasses the NuVision use twisted nematic LCshutters The transparent-to-black transition timeof these shutters is quite fast while the black-to-transparent switching time takes considerably longerFigure 18 shows the switching time of the NuVisionshutter glasses The upper measurement shows thetransparency of a shutter where a high value corre-

sponds to a high transparency The lower measurementshows the voltage applied to the IR-emitter As can beseen from the measurement there is no significant delaybetween sending a signal to the emitter and the begin-ning of the switching response However the switchingtime from 10 to 90 transparency takes approximately33 ms while the switching from 90 to 10 transpar-ency takes about 05 ms These values correspond to thevalues measured by Woods and Tan (Woods amp Tan2002) with other shutter glasses For practical use thereis a work-around involving some blanking time betweenthe projections for the left and right eye

5 Overall Setup

Two complete collaboration setups were de-signed as shown in Figure 19 In the upper image inFigure 19 the first prototype is shown essentially a lab-oratory setup One lesson learned from this is that inmany cases only the front screen is used for collabora-tion This allows for the creation of a more design-oriented setup in a public room (as shown in the lowerpart of Figure 19) using only a single screen Since thecamera positions are known from the first prototypethere was no longer any need for a flexible installation

Figure 18 Optical response of the shutter glasses

Kunz and Spagno 189

The goal of the second setup (bottom image in Fig-ure 19) was thus not to reproduce the first installationbut rather to create an asymmetric installation for thecollaboration With the asymmetric setup applicationswere first created transferring 3D sound and 3D imagesof the people from one installation to the other Figure20 shows a simple collaboration between two individu-als engaged in a dialogue Each person is in an installa-tion at a different location on the campus

Since the first system is an experimental setup a longinitialization time was required before the collaborationcould start Thus the system is not used for hands-oncollaboration yet Further work will shorten the initial-ization phase and will allow easier access to the system

6 Conclusions

The active projection screen is driven at its physi-cal limits The transition time from opaque to transpar-ent (10 to 90) takes approximately 2 ms while the

transition from transparent to opaque (90 to 10)takes approximately 6 ms The slow transition fromtransparent to opaque is dictated by the internal relax-ation forces of the LC film and cannot be acceleratedelectrically This non-ideal behavior can be minimizedby optimizing the time pattern of the different compo-nents For the user just the hotspot of the projectorsbecomes a little more evident but it remains at an ac-ceptable level The active projection screens offer a verybright and homogenous image

The junctions between the three projection-screenpanels are visible but not disturbing The LC film of theactive projection panels is installed between two glasspanels The surface of these glass panels is treated withan antireflective coating Nevertheless some reflectionsfrom the other projection screens are evident The re-flection can be compared to the reflection on a CRTtelevision in a medium-bright illuminated room Thereflection is noticeable when the user concentrates on itand disappears as soon as he or she concentrates on theapplication Since the projectors are darkened duringthe transparent state of the projection screens the cam-era acquisition is not disturbed by any reflection Thecameras are positioned in such a way as to see the userrsquosentire body even if the user gets very close to the pro-jection screens

Figure 19 Two different collaboration setups based on the blue-c

principle

Figure 20 Collaboration between two individuals

190 PRESENCE VOLUME 13 NUMBER 2

The channel separation of the stereoscopic projectionis very good and the total performance of the projec-tion is convincing The brightness is higher than tradi-tional CRT stereo projectors at a lower price per stereoprojection rack The six-channel audio system proved tobe very powerful The user can precisely localize a simu-lated noise or sound source The subwoofer located inthe wooden platform is also important for good audioperformance In addition it can transmit vibrations tothe user at lower frequencies and this acoustic stimulusplays an important role in high immersion

The mechanical structure was shown to meet the re-quired demands Its stability and rigidity give a strongsense of confidence in the blue-c The user is never un-easy about touching the projection screen or walkingaround as in some other VR installations The flexibilityin the placement of new components and cables as wellas the quick access to all the components proved to bea very valuable feature of the blue-c construction

7 Further Work

The two setups showed that collaboration over anetwork using 3D representations is technically possibleHowever there are more problems to overcome In itstechnical aspects the system should be downsized in costand weight with the goal being a mobile installation Herenew stereo projection systems would also be tested that areeasier to install and adjust Also new projection materialssuch as active Perspex will be evaluated As a first step inthis direction we will build a demonstrator that would besuitable for presentations on conferences and exhibitionsThe demonstrator could be used to show the core tech-nology of the blue-c itself but also as another input stationfor a multipoint collaboration (Disz Papka Pellegrino ampStevens 1995 Lehner amp DeFanti 1997 Fuchs et al2002 Bailett Calahan amp McCluskey 1998)

Further work on the blue-c will also deal with imple-menting the technology in existing processes for in-stance the product development process or the educa-tion process Thus applications for the blue-c will begenerated that offer benefits to the user when using the

3D capability in collaborative work Further informationcan be found at httpblue-cethzch

Acknowledgments

This project was funded as an internal research project no0-23803-00 by the Swiss Federal Institute of TechnologyZurich We would like to thank all members of the blue-cproject

References

Bailett A Calahan J amp McCluskey S (1998) Coordina-tion at different stages of the product development processRampD Management 28 237ndash247

Bougrioua F Oepts W Vleeschouwer H D AlexanderE Neyts K amp Pauwels H (2002) Determination of theswitching response in reflective twisted nematic liquid crys-tal displays The Japan Society of Applied Physics 41 5676ndash5681

Chen H Wallace G Gupta A Li K Funkhouser T ampCook P (2002) Experiences with scalability of displaywalls Proceedings of IPTVR 2002

Cruz-Neira C Sandin D J amp DeFanti T A (1993)Surrounding-screen projection-based virtual reality Thedesign and implementation of the CAVE Proceedings ofSIGGRAPH 1993 135ndash142

Disz T Papka M Pellegrino M amp Stevens R (1995)Sharing visualization experiences among remote virtual en-vironments Proceedings of the International Workshop onHigh Performance Computing for Computer Graphics andVisualization 135ndash142

Drzaic P S (1995) Liquid Crystal Dispersion World Scien-tific Singapore

Elspass W VanGool L Gross M H Kunz A Meier MSchmitt G Staadt O et al (1999) The BlueCave ETHZurich internal research proposal

Fuchs H Bishop G Arthur K McMillan L Bajcsy RLee S et al (1994) Virtual space teleconferencing using asea of cameras (Tech Rep No TR94-033) Chapel Hill

Kunz and Spagno 191

First steps have already been taken to use VR for dis-tributed collaboration Computing tools support infor-mation exchange and simple communication fairly wellHowever collaboration on complex issues is not wellsupported Successful models of computer-supportedcollaborative work (CSCW) are still rare (Elspass et al1999) So far in virtual meetings human beings areinadequately represented and disembodied throughtext voice or 2D video projections The representationof people is therefore not natural As a result the levelof presence of the remote person is low and the separa-tion between remote collaborator and task can be quitefrustrating Ideally the remote person should be em-bodied by a full-size photorealistic 3D representationSuch a representation is ideally visualized with SpatiallyImmersive Displays

Spatially Immersive Displays (SID) have become in-creasingly popular over the past decade They enable theuser to work in virtual spaces while being physically sur-rounded with a panorama of imagery (Lantz 1996)The most common SID used today is the CAVE fromthe University of Illinois Chicago or one of its variants(Cruz-Neira Sandin amp DeFanti 1993) This environ-ment typically comprises a cubic room with up to sixback-projection units Distributed versions of SIDs al-low users to interact with remote collaborators in tele-collaboration applications These systems usually inte-grate 2D video-based human avatars possibly enhancedwith prereconstructed geometric models into the vir-tual environments (Gross Kunz Meier amp Staadt2000) The Cabin of the University of Tokyo is a five-sided SID consisting of three wall projections a floorprojection and a ceiling projection (Ogi Yamada ampHirose 1999) In 1997 the Cabin was extended to anetworked environment by connecting it to other SIDsto allow telecollaboration The user is captured withinfrared cameras placed on tripods inside the SID Infra-red cameras have been chosen because of the low lightconditions inside the SID A black-and-white represen-tation of the user is then transferred to the remote in-stallations Immersive projection systems demand highresolution The resolution of a display wall can be in-creased by using multiple projectors For systems withlarge numbers of projectors automatic projection cali-

bration is mandatory In 1998 a display wall was built atPrinceton University using eight LCD projectors Eachprojector is connected to a node of a computer clusterand an automatic projection calibration is integratedinto the system (Wallace Chen amp Li 2003) In No-vember 2000 the system was scaled up with 24 DigitalLight Processing (DLP) projectors (Chen et al 2002)

While stereo projection has already reached a highlevel of quality the acquisition of the user with simulta-neous projection is still very challenging This often re-sults in compromises between acquisition and projec-tion Infrared cameras are often used due to the poorlight conditions inside SIDs Sometimes the cameras areplaced in front of the projection which therefore de-creases the immersion quality and the size of the work-ing area In some cases only one camera is used allow-ing only a 2D representation of the user

2 Contribution

In the presented work the above mentioned issueshave been addressed and new solutions have been devel-oped to build the ldquoblue-crdquo an innovative prototype of ahighly immersive projection and video-acquisition vir-tual environment for collaborative work The new sys-tem makes it possible to record live video streams ofusers and to simultaneously project virtual reality scenesIn a virtual meeting the participants are embodied by arealistic high-quality representation fully 3D renderedand supporting motion and speech in real time (Elspasset al 1999) In order to increase the immersion a sixchannel audio system was also installed The user aspart of the real world is therefore synthesized by imageand sound and integrated in the virtual world Thus theSID evolves from a virtual reality system to an aug-mented virtuality system

Special solutions had to be found to allow simulta-neous projection and picture acquisition The camerasare placed behind active projection screens which canbe switched to a transparent state to perform pictureacquisition This solution allows integrating the camerasin a SID without interfering with the projected imagesThe controversy between darkness for a sharp projection

Kunz and Spagno 179

and brightness for picture acquisition has also been ad-dressed An active LED illumination in combinationwith modified shutter glasses has been implementedto allow an acquisition with color cameras while notdisturbing the user The goal of the so-called blue-cinstallation is to build collaborative immersive virtualenvironments which integrate real humans as three-dimensional objects Two installations are intercon-nected to allow bidirectional collaboration and in-teraction among people sharing virtual spaces Thepeople are captured in real time full color and three-dimensionally Each installation consists of a projectionsystem and acquisition hardware that operate in the vi-sual wavelength spectrum (see Figure 1)

Essentially two systems must be combined into oneinstallation an immersive visualization system and anacquisition system The acquisition has to take place inthe visual wavelength spectrum and in full color Multi-ple cameras are used which enable a 3D reconstructionof the acquired person In order to proof the concept ofnetworked collaboration two setups were realized lo-cated at different sites of the campus

3 System Overview

As a central element a special projection screen isused that can be switched from opaque to transparent

and back The idea is to use time multiplexing to com-bine the projection and acquisition During the opaquestate of the projection screen the projection is runningand an image is projected onto the screen During thetransparent state the cameras can look through the pro-jection screen and take images of the user inside theinstallation The switchable projection screen has to besynchronized with the acquisition system If the screenswitches quickly enough between the transparent andopaque states no flickering will be observable and thecameras will not be visible to the user Phase-dispersedliquid crystal (PDLC) glass panels are used as a projec-tion screen The panels are based on liquid crystal (LC)technology and can be switched electrically to transpar-ent and opaque Although these glass panels are com-mercially available they were not primarily designed forprojection but rather as an architectural element Thusmeasurements were performed to find out whether theycould be used as shuttered projection screens The max-imum available width of the panels is 950 mm For aprojection room of 285 m 285 m three of thesepanels are installed per side

With the use of actively shuttered projection screensthe projection and acquisition have to take place at dif-ferent points in time This time multiplexing betweenprojection and acquisition has two or three differentphases depending on the projection For a normalmonoscopic projection two time phases are necessaryone for projection and one for acquisition In combina-tion with an active stereo projection three phases arerequired one for the left-eye projection one for theright-eye projection and one for picture acquisitionThis sequence is repeated with a frequency that is highenough to avoid flickering that can be noticed by theuser

An important question was how to illuminate the userwithout disturbing him or her and without altering theprojection quality and the degree of immersion Theillumination has to be in the visible light spectrum sincecolor cameras are used The solution is to make use ofthe three phases and to activate a flash illumination onlyduring picture acquisition As protection from the lightthe user has to wear shutter glasses which are synchro-nized with the illumination and which are modified by

Figure 1 Principle of the blue-c

180 PRESENCE VOLUME 13 NUMBER 2

including a third phase when both lenses for the leftand right eye are darkened These shutter glasses areneeded anyway for the active stereoscopic projectionFigure 2 illustrates such a projection sequence which isfollowed by an acquisition sequence

During the third time phase when both glasses aredarkened an active illumination is triggered The illumi-nation is preferably done with white light in order toachieve a true-color texture recognition It is possible touse a strobe light or white light-emitting diodes(LEDs) Figure 3 shows how this active illumination canbe kept out of the userrsquos eyes by integrating a thirdphase into the shutter glasses (Kunz amp Spagno 2001aKunz amp Spagno 2001b)

In the left-hand pair of images in Figure 3 the illumi-nation is shown as seen without the third phase in theright-hand pair of images the illumination is shownafter the third phase within the shutter glasses is acti-vated In each pair the second image shows an objectthat is only illuminated by ambient light

With sufficiently bright illumination it is possible toacquire good-quality images The cameras for recording

the texture of the front of the user have to be placed infront of user without being in his or her field of view(Fuchs et al 1994) Therefore the best position for thecameras would be behind the projection screen wherethey cannot be seen by the user (Figure 4) The activeprojection screens allow the cameras to be arrangedaround the projection room The layout of the blue-c isa three-sided projection room with a quadratic groundplan This quadratic layout has been used for the firsttime in the cave (Cruz-Neira Sandin amp DeFanti1993) No edge-blending is required since the differentprojections are on different screens

The working principle of the blue-c relies on an exactsynchronization among all components involved Cam-eras flash illumination shutter glasses projector shut-ters and the active projection screen need a hard syn-chronization to allow stereoscopic projection togetherwith image acquisition

The triggering sequence of all components can beseen in Figure 5 The camera triggering frequency is setto acquire images only every seventh cycle of the basictriggering frequency of 625 Hz This corresponds to89 images per second This value was chosen to allowenough time for the acquisition system to process theincoming images Although the flash illumination andthe triggering of the PDLCs could be reduced to89 Hz we kept the higher triggering frequency in or-der to avoid flickering that would be disturbing to theuser since the human eye is very time-sensitive at lowerfrequencies

Figure 2 The three time phases (time intervals)

Figure 3 Active illuminations without and with a third phase

Figure 4 blue-c layout

Kunz and Spagno 181

4 The Components in Detail

41 Mechanical Construction

Since a magnetic tracking system was chosen thebasis of the construction is a wooden platform with atotal height of 540 mm (see Figure 6) The trackingemitter is built into the middle of the wooden platformand thus is not visible to the user The two lower frontloudspeakers and the subwoofer are also built into theplatform The floor of the platform can be easily openedto access all the built-in components The upper struc-ture is built of fiber-reinforced composites (glass andcarbon) These are stiff enough to keep the LC projec-tion panels safely in place and at the same time stillsmall and attractive from a design point of view Cablechannels are built into the beams providing sufficientspace for the cables of all components placed on theupper part of the structure including the cables for theactive projection screens Together with the woodenplatform there is enough space for all the cables andconnectors for the system The chosen materials woodand fiber-reinforced composites assure an accuratefunctioning of the tracking system The cables and theloudspeakers which are at a distance of 15 m from the

emitter of the tracking system do not have any impor-tant influence on its accuracy The results from prelimi-nary tests show that the electrically switched glass panesalso do not interfere with the tracking system

42 Hardware Construction

Figure 7 gives an overview of all components ofthe blue-c and its connection scheme A total of 6 pro-jectors and four monitors are connected to an SGIOnyx The 16 cameras are connected via sixteenFireWire cables to the computer cluster The active pro-jection panels the 6 ferro-electric shutters in front ofthe projectors the active illumination the cameras andthe IR-emitters are synchronized by custom-made syn-chronization electronics The individual componentswill be discussed in the following sections

421 Active Projection Screen Phase dis-persed liquid crystal (PDLC) glass panels offer the re-quired features for a switchable projection screen Theycan be controlled electrically and can be switched in afew milliseconds or less between a transparent and anopaque state Different glass manufacturers have switch-

Figure 5 Triggering timings

182 PRESENCE VOLUME 13 NUMBER 2

able glass panels available with an integrated PDLClayer To change the optical property of a PDLC glasspanel an electrical field has to be applied perpendicularto the PDLC film A voltage in the range of 50 V to100 V is necessary to generate this electrical field ThePDLC glass panels consist of two glass layers coatedwith a transparent electrode on the inside surface and aPDLC film in between Each of the two conductors has

to be contacted by an electrode On the glass panelsused the two copper electrodes are located on the twoshorter edges so as to be invisible when the glass panelsare installed The current flowing through the transpar-ent electrode has to be limited to avoid its overheatingand therefore damaging the PDLC glass panel Themanufacturer recommends not exceeding 300 mA for a2240 mm 950 mm panel with the electrodes placedon the short edges as installed in the blue-c To opti-mize the synchronization electronics the electricalproperties of the panels have to be analyzed

In order to analyze the electrical properties of thePDLC panels the current response to an applied volt-age was measured As with other liquid crystal devicesthe PDLCrsquos should not be exposed to any DC voltagewhich could cause ionic buildup damaging the liquidcrystal structure Therefore only AC measurementswere performed There is also an upper frequency limitdue to current limitations For a voltage of 80 V themaximum frequency is around 100 Hz (Figure 8)

To calculate the electrical behavior of the PDLC glasspanels an equivalent circuit of three components asproposed by Drzaic (1995) is used (Figure 9)

Based on this circuit the complex impedance is

Z R1 R2

1 CR2)2j CR22

1(CR22

This equation has a real and an imaginary part and thusonly two of three variables could be calculated How-ever since more measurements at different frequencies

Figure 6 The mechanical construction of the blue-c

Figure 7 Component overview

Kunz and Spagno 183

were performed an approximation was calculated mini-mizing the mean square error (MSE) between the calcu-lated and the measured voltages The approximated val-ues are

R1 1745 R2 2494 C 11F

The electronic control of the PDLC panes can be ad-justed to eliminate any DC components in the drivingvoltage In addition an external DC decoupling circuithas been added to guarantee that no DC voltage dam-ages the glass panels even if a fault in the driving circuitoccurs This DC decoupling consists of a capacitor CD

and a resistor RD (Figure 10) The values are 60 F forthe capacitor and 10 k for the resistor

The glass panes show a very stable performance overtime We do not observe any significant change either inthe electrical behavior or the stability of the LC filmThe system worked for many days without any prob-

lems although the panes are driven at their upper limitin frequency and allowable current

422 Projectors and Active Shutters Theactive LC projection screens do not support passive ste-reo projection due to their depolarization propertiesTherefore an active projection system was chosen forthe blue-c In the year 2000 when the design and theconstruction of the system were started active stereoDLP projectors were just emerging and still much tooexpensive for our budget The state-of-the-art CRTprojectors were also still quite expensive compared tothe light output Furthermore we were looking for asystem that could be turned black during the cameraacquisition period that is 625 times per second Thedecision was taken to use two three-chip LCD projec-tors (Sanyo XF12 3500 ANSI lumen) per side and ex-ternal LC shutters in front of the projection lens Withthese shutters it is possible to generate an active stereoprojection by alternating the light output of the twoprojectors and blocking it during the picture acquisitionperiod

The two projectors on each side are mounted in arack for a passive stereo projection installation (Figure11) The LC shutters are custom-made ferro-electricliquid crystal shutters with an aperture size of140 mm 140 mm The shutters are mounted in frontof the 121 projection lenses This lens allows projec-tion directly onto the projection wall without any addi-tional mirrors Using no mirrors in the projection pathfacilitates the alignment of the two LCD projectors foractive stereo projection The shutters do not have to besynchronized with the Onyx and projectors thus leav-ing the shutter times completely flexible This principlewould not work with a single-chip DLP projector sincethe sequential generation of the three colors by a ldquocolor

Figure 8 Voltage and current measured on a panel of

2240 mm 950 mm at 100 Hz

Figure 9 Equivalent circuit diagram of the PDLC glass panel

Figure 10 DC decoupling

184 PRESENCE VOLUME 13 NUMBER 2

wheelrdquo would require a complicated synchronization ofthe shutters with the projectors However today 3-chipDLP projectors could be used instead of the two LCprojectors The shutters in front of the projectors haveto be switched on and off at a frequency of approxi-mately 60 Hz

Twisted Nematic (TN) liquid-crystal shutters as usedin shutter glasses can offer a contrast ratio of 10001 orbetter (Lipton 2001) They switch in about 25 msfrom black to transparent and in less than 1 ms fromtransparent to black (Lipton 1991 Woods amp Tan2002 Bougrioua et al 2002) Ferro-electric (FE)liquid-crystal shutters offer about the same contrastratio as TN shutters but much faster switching timesThe FE shutters used in front of the projectors are140 mm 140 mm and offer a contrast ratio of ap-proximately 7501 The switching time from 10 to90 transparency and from 90 to 10 transparency isbelow 100 s (see Figure 12)

The upper measurement in Figure 12 shows the volt-age measured on the shutter The mean value of thisvoltage is zero as required The lower measurementshows the optical response of the shutter measured by aphototransistor and a light source The response is veryfast and guarantees an excellent separation between thetwo stereo channels

A TN shutter switches to black if a voltage is appliedand to transparent if the voltage is removed An FE

shutter in contrast switches to black if a voltage of onepolarity is applied and to transparent if a voltage of theopposite polarity is applied It is important that theshutter not be subject to any DC voltage as it will bedamaged The DC decoupling is achieved by connect-ing a capacitor CD of 1 F in series and a resistor RD of1 k in parallel to the shutter (see also Figure 10)

Mounting the shutters directly in front of the projec-torrsquos lens did not cause temperature problems as ex-pected Even after several hours of projection the tem-perature was beneath the limit tolerated by thetemperature-sensitive devices

423 Active Illumination The requirements onthe blue-c illumination are very demanding For theprojection both the inside and the outside of the pro-jection room have to be dark On the other handbright illumination is important for good-quality imageacquisition The illumination has a big impact on thequality of the extracted silhouettes and acquired tex-tures A person inside the blue-c should be illuminateduniformly from all sides and no shadow should be pro-jected on the floor The illumination has to be directedaway from the PDLC glass panels to avoid contrast-lossand reflections

Figure 11 Stereo projection rack with LC shutters

Figure 12 Ferro-electric LC shutter (driving voltage optical

response)

Kunz and Spagno 185

An active illumination is used to satisfy the needs ofboth projection and image acquisition The active lightis off during projection and on for the image acquisi-tion Therefore the light has to provide illumination foronly a few milliseconds and must be able to turn on andoff with a frequency between 60 and 70 Hz First testswere done with a strobe light A high voltage dischargein a xenon flash lamp produces a bright light pulse of avery short duration Frequencies in the range of 60 Hzare no problem for this type of lamp Despite the fastswitching this type of light has some important disad-vantages First light is emitted for only a few millisec-onds The light intensity integrated over time is there-fore only mediocre In addition the high voltagedischarge in the very short time period causes heavyelectromagnetic pulses which can interfere with sur-rounding electronic components The discharges alsocause a disturbing acoustic noise Finally the strobelight is focused in one spot This is a handicap whentrying to distribute the light uniformly in the projectionroom All these disadvantages led us to look for otherillumination solutions

The requirement of short switching times restricts thenumber of suitable light sources considerably Light-emitting diodes (LEDs) have very short switching timesand are thus suitable for our application Today whiteLEDs are commonly available and the light efficiency isabout twice that of an incandescent bulb (Zorpette2002) LEDs can be easily grouped in clusters to givemore light output The clusters are flat can be of anyshape and the light output can be distributed homoge-neously over the cluster if desired A total of 9984white 5 mm LEDs have been integrated into the blue-cThe opening angle (50 intensity) is 50 degrees and thelight output is approximately 2200 mcd per LEDThese LEDs were the brightest available on the marketin 2002 The LEDs are configured into 32 clusterseach containing 312 LEDs The clusters are reassem-bled in groups of two four or five and mounted inlong aluminum profiles

The LED clusters have to be placed carefully in theblue-c to allow a good quality image acquisition Thecameras are mostly placed outside the blue-c and facemore or less horizontally to the user through the PDLC

glass panels Under this aspect a diffused frontal illumi-nation of the user would be the most suitable But afrontal light would also shine directly into the oppositecameras and furthermore would put too much light onthe glass panels making them appear gray In additionit would be difficult to find a place to mount the clus-ters without interfering with the projection and thecameras

The solution is to distribute the LED clusters alongthe upper and lower edges of the projection room TheLEDs can be adjusted in such a way that they com-pletely illuminate the user minimizing at the same timethe amount of light being directed at the glass panelsand at the opposite cameras The additional mask infront of the LEDs helps to further diminish the amountof scattered light reaching the glass panels Figure 13shows a person between two opposite projection wallsas is the case in the blue-c for the two sidewalls Thelight is coming from all four edges and most parts of theperson are illuminated by more than one light sourceFigure 14 shows one illumination cluster Five of themare integrated in the aluminum profile and placed at theedge between the floor and the left glass panels

Realizing the illumination with the diodes it turnedout that the specifications of the LED were not reachedby every single element This resulted in failures andthus in nonactive LED lines in the cluster After the cur-

Figure 13 Placement of the LED clusters

186 PRESENCE VOLUME 13 NUMBER 2

rent through the entire cluster was reduced this prob-lem disappeared

424 Cameras Sixteen FireWire cameras areplaced around the blue-c on a special aluminum struc-ture The structure is based on standard aluminum ele-ments and offers the maximum flexibility in the place-ment of cameras and other components Each camera isconnected via FireWire to one node of the PC clusterThe signal is amplified every 5 m with a repeater Up tothree repeaters are used for each camera

Dragonflytrade cameras are used since they offer goodsensitivity and a high picture acquisition frequency of upto 15 Hz However cameras with an even higher sensi-tivity would be useful in order to reduce the intensity ofthe flash illumination On the other hand the pictureacquisition frequency is high enough since the bottle-neck is in the processing of the acquired images Initialtests showed that a repetition frequency of 10 Hz seemsto be sufficient for collaborative work

425 Synchronization Electronics The syn-chronization electronics of the blue-c were built modu-larly to offer flexibility in adding and changing compo-nents The main module is the frequency-controlmodule which is equipped with a reprogrammable PIC16F877 microcontroller This module generates thetriggering sequence to drive the other modules of the

synchronization unit Three PDLC glass panel drivermodules two shutter driver modules and one trigger-interface module are integrated into the synchronizationunit together with the synchronization control moduleThe LED driver and IR-emitter driver are placed out-side of the synchronization unit and are triggered by thetrigger interface module

Figure 15 gives an overview of the synchronizationelectronics All modules of the synchronization unit areinterconnected with a data bus In addition a powerbus is integrated into the unit to supply the differentmodules with the required voltages The LED driverand IR-emitter driver are located outside of the syn-chronization unit together with their own power sup-plies to reduce electromagnetic interference caused bythe currents

Using a microcontroller for the synchronization of allcomponents proved to be crucial Since every compo-nent including identical parts behaves slightly differ-ently (eg the glass panes) it is easy to create individu-alized triggering signals using the microcontroller Thismakes up for the increased effort of installing and pro-gramming the processor

426 Frequency Control Module The fre-quency control module is based on a PIC 16F877 mi-crocontroller This microcontroller can be repro-grammed in-circuit that is without unplugging thechip from the board Twenty-two inputndashoutput ports ofthe microcontroller are mapped to the data bus They

Figure 14 An LED cluster of the blue-c

Figure 15 Synchronization electronics

Kunz and Spagno 187

can be set individually as outputs or inputs Each ofthese ports is connected to a LED to show its state Themicrocontroller is clocked with a frequency of 20 MHzto offer high timing accuracy

427 PDLC Glass-Panel Driver Module Todrive the nine PDLC glass panels a total of three glass-panel driver modules are used (see Figure 16) In eachcase three glass panels in parallel are connected to thetwo outputs of a module The two outputs of a modulecan be switched separately to either 8 V or 85 V If bothoutputs are switched to the same value no voltage isapplied to the glass panels If the outputs are switchedto different values a voltage of 77 V or 77 V is ap-plied to the glass panels in series with the decouplingcapacitor Each of the two outputs is switched directlyby the corresponding input coming from the data busA current-limit control is superimposed on the voltageswitching

Figure 16 shows a simplified circuit consisting of aPDLC glass-panel driver module connected to threeglass panels with their DC decoupling circuits Each ofthe two incoming TTL signals switches an optocoupleroutput to a preset voltage of 8 V or 85 V The two out-puts of the optocouplers are connected to two APEXPA46 operational amplifiers The APEX PA64 has anintegrated adjustable current-limit control and can runat voltages up to 150 V and currents up to 5 A Theoperational amplifiers are used as followers thus theinput voltage is mapped directly to the output as long asthe current-limit is not exceeded The PDLC glass-panel driver modules are connected to a 95-V power

supply However the outputs of the module have beenset to switch only between 8 V and 85 V This givesenough margin to the output voltage from the powersupply and ground to avoid saturating the operationalamplifier The current-limit of the operational amplifieris set to 900 mA which corresponds to a current-limitof 300 mA per panel

428 Shutter Driver Module The shutterdriver module also has two inputs and two outputs Itconsists of two independent circuits with an opto-coupler and a half-bridge consisting of two MOSFETsand a half-bridge driver (Figure 17) The optocoupler isused to protect the other modules of the synchroniza-tion electronics in case a fault occurs in this module orin a connected component The output of the opto-coupler is connected to a half-bridge driver (IR2111)that drives two MOSFETs The bridge driver ensuresthat the two MOSFETs in the half-bridge are neverturned on at the same time The two half-bridge out-puts switch the voltage rail-to-rail without any current-limit In order to drive the ferro-electric shutters themodule is operated with a voltage of 12 V Three shut-ters are connected in parallel between the two half-bridges of a module The two half-bridges are alwaysswitched to opposite states The bridge voltage is there-fore either 12 V or 12 V as required by the shuttersfor switching

429 Trigger Interface Module The trigger-interface module is a multifunctional module It allows achoice between up to 8 lines out of the 22 data lines onthe bus Each line can be connected to a 5 V or 12 V

Figure 16 PDLC glass-panel driver moduleFigure 17 Shutter driver module

188 PRESENCE VOLUME 13 NUMBER 2

driver and configured as input or output For the blue-call channels used were configured as 5-V outputs Onechannel is used to trigger the FireWire cameras A sec-ond one is used to trigger the LED illumination Twomore channels are used to synchronize the shutterglasses and are connected to the IR-emitter driver

4210 LED Driver A total of 9984 LEDs areinstalled in different clusters in the blue-c The LEDclusters have to be driven with a voltage of 375 V and atotal current of up to 25 A The power supply for theLED cluster is installed in the floor of the blue-c andconnected directly to the LED clusters This reduces thewiring distances of the high-current connections andthus the probability of electro-magnetic interferenceEach cluster has an integrated MOSFET to switch theLEDs on and off The triggering signal comes directlyfrom the trigger-interface module

4211 Shutter Glasses The LED illuminationis triggered 625 times per second The user does notsee any flickering because this frequency is above thetemporal resolution of the human eye However thebright light inside the blue-c diminishes the contrast ofthe projected image considerably To protect the userfrom the active light an apposite third phase has beenintegrated into the stereo glasses to keep the light outof the userrsquos eyes The projection of the images for theleft and the right eye are followed by a third phase inwhich both LC shutters of the shutter glasses are dark-ened The LED illumination is on only during thisphase This helps to fade out most of the light from theillumination

In the blue-c we use the NuVision 60GX shutterglasses from MacNaughton The shutter glasses and theIR-emitter have a built-in microcontroller We havereprogrammed it to include the third phase Like mostshutter glasses the NuVision use twisted nematic LCshutters The transparent-to-black transition timeof these shutters is quite fast while the black-to-transparent switching time takes considerably longerFigure 18 shows the switching time of the NuVisionshutter glasses The upper measurement shows thetransparency of a shutter where a high value corre-

sponds to a high transparency The lower measurementshows the voltage applied to the IR-emitter As can beseen from the measurement there is no significant delaybetween sending a signal to the emitter and the begin-ning of the switching response However the switchingtime from 10 to 90 transparency takes approximately33 ms while the switching from 90 to 10 transpar-ency takes about 05 ms These values correspond to thevalues measured by Woods and Tan (Woods amp Tan2002) with other shutter glasses For practical use thereis a work-around involving some blanking time betweenthe projections for the left and right eye

5 Overall Setup

Two complete collaboration setups were de-signed as shown in Figure 19 In the upper image inFigure 19 the first prototype is shown essentially a lab-oratory setup One lesson learned from this is that inmany cases only the front screen is used for collabora-tion This allows for the creation of a more design-oriented setup in a public room (as shown in the lowerpart of Figure 19) using only a single screen Since thecamera positions are known from the first prototypethere was no longer any need for a flexible installation

Figure 18 Optical response of the shutter glasses

Kunz and Spagno 189

The goal of the second setup (bottom image in Fig-ure 19) was thus not to reproduce the first installationbut rather to create an asymmetric installation for thecollaboration With the asymmetric setup applicationswere first created transferring 3D sound and 3D imagesof the people from one installation to the other Figure20 shows a simple collaboration between two individu-als engaged in a dialogue Each person is in an installa-tion at a different location on the campus

Since the first system is an experimental setup a longinitialization time was required before the collaborationcould start Thus the system is not used for hands-oncollaboration yet Further work will shorten the initial-ization phase and will allow easier access to the system

6 Conclusions

The active projection screen is driven at its physi-cal limits The transition time from opaque to transpar-ent (10 to 90) takes approximately 2 ms while the

transition from transparent to opaque (90 to 10)takes approximately 6 ms The slow transition fromtransparent to opaque is dictated by the internal relax-ation forces of the LC film and cannot be acceleratedelectrically This non-ideal behavior can be minimizedby optimizing the time pattern of the different compo-nents For the user just the hotspot of the projectorsbecomes a little more evident but it remains at an ac-ceptable level The active projection screens offer a verybright and homogenous image

The junctions between the three projection-screenpanels are visible but not disturbing The LC film of theactive projection panels is installed between two glasspanels The surface of these glass panels is treated withan antireflective coating Nevertheless some reflectionsfrom the other projection screens are evident The re-flection can be compared to the reflection on a CRTtelevision in a medium-bright illuminated room Thereflection is noticeable when the user concentrates on itand disappears as soon as he or she concentrates on theapplication Since the projectors are darkened duringthe transparent state of the projection screens the cam-era acquisition is not disturbed by any reflection Thecameras are positioned in such a way as to see the userrsquosentire body even if the user gets very close to the pro-jection screens

Figure 19 Two different collaboration setups based on the blue-c

principle

Figure 20 Collaboration between two individuals

190 PRESENCE VOLUME 13 NUMBER 2

The channel separation of the stereoscopic projectionis very good and the total performance of the projec-tion is convincing The brightness is higher than tradi-tional CRT stereo projectors at a lower price per stereoprojection rack The six-channel audio system proved tobe very powerful The user can precisely localize a simu-lated noise or sound source The subwoofer located inthe wooden platform is also important for good audioperformance In addition it can transmit vibrations tothe user at lower frequencies and this acoustic stimulusplays an important role in high immersion

The mechanical structure was shown to meet the re-quired demands Its stability and rigidity give a strongsense of confidence in the blue-c The user is never un-easy about touching the projection screen or walkingaround as in some other VR installations The flexibilityin the placement of new components and cables as wellas the quick access to all the components proved to bea very valuable feature of the blue-c construction

7 Further Work

The two setups showed that collaboration over anetwork using 3D representations is technically possibleHowever there are more problems to overcome In itstechnical aspects the system should be downsized in costand weight with the goal being a mobile installation Herenew stereo projection systems would also be tested that areeasier to install and adjust Also new projection materialssuch as active Perspex will be evaluated As a first step inthis direction we will build a demonstrator that would besuitable for presentations on conferences and exhibitionsThe demonstrator could be used to show the core tech-nology of the blue-c itself but also as another input stationfor a multipoint collaboration (Disz Papka Pellegrino ampStevens 1995 Lehner amp DeFanti 1997 Fuchs et al2002 Bailett Calahan amp McCluskey 1998)

Further work on the blue-c will also deal with imple-menting the technology in existing processes for in-stance the product development process or the educa-tion process Thus applications for the blue-c will begenerated that offer benefits to the user when using the

3D capability in collaborative work Further informationcan be found at httpblue-cethzch

Acknowledgments

This project was funded as an internal research project no0-23803-00 by the Swiss Federal Institute of TechnologyZurich We would like to thank all members of the blue-cproject

References

Bailett A Calahan J amp McCluskey S (1998) Coordina-tion at different stages of the product development processRampD Management 28 237ndash247

Bougrioua F Oepts W Vleeschouwer H D AlexanderE Neyts K amp Pauwels H (2002) Determination of theswitching response in reflective twisted nematic liquid crys-tal displays The Japan Society of Applied Physics 41 5676ndash5681

Chen H Wallace G Gupta A Li K Funkhouser T ampCook P (2002) Experiences with scalability of displaywalls Proceedings of IPTVR 2002

Cruz-Neira C Sandin D J amp DeFanti T A (1993)Surrounding-screen projection-based virtual reality Thedesign and implementation of the CAVE Proceedings ofSIGGRAPH 1993 135ndash142

Disz T Papka M Pellegrino M amp Stevens R (1995)Sharing visualization experiences among remote virtual en-vironments Proceedings of the International Workshop onHigh Performance Computing for Computer Graphics andVisualization 135ndash142

Drzaic P S (1995) Liquid Crystal Dispersion World Scien-tific Singapore

Elspass W VanGool L Gross M H Kunz A Meier MSchmitt G Staadt O et al (1999) The BlueCave ETHZurich internal research proposal

Fuchs H Bishop G Arthur K McMillan L Bajcsy RLee S et al (1994) Virtual space teleconferencing using asea of cameras (Tech Rep No TR94-033) Chapel Hill

Kunz and Spagno 191

and brightness for picture acquisition has also been ad-dressed An active LED illumination in combinationwith modified shutter glasses has been implementedto allow an acquisition with color cameras while notdisturbing the user The goal of the so-called blue-cinstallation is to build collaborative immersive virtualenvironments which integrate real humans as three-dimensional objects Two installations are intercon-nected to allow bidirectional collaboration and in-teraction among people sharing virtual spaces Thepeople are captured in real time full color and three-dimensionally Each installation consists of a projectionsystem and acquisition hardware that operate in the vi-sual wavelength spectrum (see Figure 1)

Essentially two systems must be combined into oneinstallation an immersive visualization system and anacquisition system The acquisition has to take place inthe visual wavelength spectrum and in full color Multi-ple cameras are used which enable a 3D reconstructionof the acquired person In order to proof the concept ofnetworked collaboration two setups were realized lo-cated at different sites of the campus

3 System Overview

As a central element a special projection screen isused that can be switched from opaque to transparent

and back The idea is to use time multiplexing to com-bine the projection and acquisition During the opaquestate of the projection screen the projection is runningand an image is projected onto the screen During thetransparent state the cameras can look through the pro-jection screen and take images of the user inside theinstallation The switchable projection screen has to besynchronized with the acquisition system If the screenswitches quickly enough between the transparent andopaque states no flickering will be observable and thecameras will not be visible to the user Phase-dispersedliquid crystal (PDLC) glass panels are used as a projec-tion screen The panels are based on liquid crystal (LC)technology and can be switched electrically to transpar-ent and opaque Although these glass panels are com-mercially available they were not primarily designed forprojection but rather as an architectural element Thusmeasurements were performed to find out whether theycould be used as shuttered projection screens The max-imum available width of the panels is 950 mm For aprojection room of 285 m 285 m three of thesepanels are installed per side

With the use of actively shuttered projection screensthe projection and acquisition have to take place at dif-ferent points in time This time multiplexing betweenprojection and acquisition has two or three differentphases depending on the projection For a normalmonoscopic projection two time phases are necessaryone for projection and one for acquisition In combina-tion with an active stereo projection three phases arerequired one for the left-eye projection one for theright-eye projection and one for picture acquisitionThis sequence is repeated with a frequency that is highenough to avoid flickering that can be noticed by theuser

An important question was how to illuminate the userwithout disturbing him or her and without altering theprojection quality and the degree of immersion Theillumination has to be in the visible light spectrum sincecolor cameras are used The solution is to make use ofthe three phases and to activate a flash illumination onlyduring picture acquisition As protection from the lightthe user has to wear shutter glasses which are synchro-nized with the illumination and which are modified by

Figure 1 Principle of the blue-c

180 PRESENCE VOLUME 13 NUMBER 2

including a third phase when both lenses for the leftand right eye are darkened These shutter glasses areneeded anyway for the active stereoscopic projectionFigure 2 illustrates such a projection sequence which isfollowed by an acquisition sequence

During the third time phase when both glasses aredarkened an active illumination is triggered The illumi-nation is preferably done with white light in order toachieve a true-color texture recognition It is possible touse a strobe light or white light-emitting diodes(LEDs) Figure 3 shows how this active illumination canbe kept out of the userrsquos eyes by integrating a thirdphase into the shutter glasses (Kunz amp Spagno 2001aKunz amp Spagno 2001b)

In the left-hand pair of images in Figure 3 the illumi-nation is shown as seen without the third phase in theright-hand pair of images the illumination is shownafter the third phase within the shutter glasses is acti-vated In each pair the second image shows an objectthat is only illuminated by ambient light

With sufficiently bright illumination it is possible toacquire good-quality images The cameras for recording

the texture of the front of the user have to be placed infront of user without being in his or her field of view(Fuchs et al 1994) Therefore the best position for thecameras would be behind the projection screen wherethey cannot be seen by the user (Figure 4) The activeprojection screens allow the cameras to be arrangedaround the projection room The layout of the blue-c isa three-sided projection room with a quadratic groundplan This quadratic layout has been used for the firsttime in the cave (Cruz-Neira Sandin amp DeFanti1993) No edge-blending is required since the differentprojections are on different screens

The working principle of the blue-c relies on an exactsynchronization among all components involved Cam-eras flash illumination shutter glasses projector shut-ters and the active projection screen need a hard syn-chronization to allow stereoscopic projection togetherwith image acquisition

The triggering sequence of all components can beseen in Figure 5 The camera triggering frequency is setto acquire images only every seventh cycle of the basictriggering frequency of 625 Hz This corresponds to89 images per second This value was chosen to allowenough time for the acquisition system to process theincoming images Although the flash illumination andthe triggering of the PDLCs could be reduced to89 Hz we kept the higher triggering frequency in or-der to avoid flickering that would be disturbing to theuser since the human eye is very time-sensitive at lowerfrequencies

Figure 2 The three time phases (time intervals)

Figure 3 Active illuminations without and with a third phase

Figure 4 blue-c layout

Kunz and Spagno 181

4 The Components in Detail

41 Mechanical Construction

Since a magnetic tracking system was chosen thebasis of the construction is a wooden platform with atotal height of 540 mm (see Figure 6) The trackingemitter is built into the middle of the wooden platformand thus is not visible to the user The two lower frontloudspeakers and the subwoofer are also built into theplatform The floor of the platform can be easily openedto access all the built-in components The upper struc-ture is built of fiber-reinforced composites (glass andcarbon) These are stiff enough to keep the LC projec-tion panels safely in place and at the same time stillsmall and attractive from a design point of view Cablechannels are built into the beams providing sufficientspace for the cables of all components placed on theupper part of the structure including the cables for theactive projection screens Together with the woodenplatform there is enough space for all the cables andconnectors for the system The chosen materials woodand fiber-reinforced composites assure an accuratefunctioning of the tracking system The cables and theloudspeakers which are at a distance of 15 m from the

emitter of the tracking system do not have any impor-tant influence on its accuracy The results from prelimi-nary tests show that the electrically switched glass panesalso do not interfere with the tracking system

42 Hardware Construction

Figure 7 gives an overview of all components ofthe blue-c and its connection scheme A total of 6 pro-jectors and four monitors are connected to an SGIOnyx The 16 cameras are connected via sixteenFireWire cables to the computer cluster The active pro-jection panels the 6 ferro-electric shutters in front ofthe projectors the active illumination the cameras andthe IR-emitters are synchronized by custom-made syn-chronization electronics The individual componentswill be discussed in the following sections

421 Active Projection Screen Phase dis-persed liquid crystal (PDLC) glass panels offer the re-quired features for a switchable projection screen Theycan be controlled electrically and can be switched in afew milliseconds or less between a transparent and anopaque state Different glass manufacturers have switch-

Figure 5 Triggering timings

182 PRESENCE VOLUME 13 NUMBER 2

able glass panels available with an integrated PDLClayer To change the optical property of a PDLC glasspanel an electrical field has to be applied perpendicularto the PDLC film A voltage in the range of 50 V to100 V is necessary to generate this electrical field ThePDLC glass panels consist of two glass layers coatedwith a transparent electrode on the inside surface and aPDLC film in between Each of the two conductors has

to be contacted by an electrode On the glass panelsused the two copper electrodes are located on the twoshorter edges so as to be invisible when the glass panelsare installed The current flowing through the transpar-ent electrode has to be limited to avoid its overheatingand therefore damaging the PDLC glass panel Themanufacturer recommends not exceeding 300 mA for a2240 mm 950 mm panel with the electrodes placedon the short edges as installed in the blue-c To opti-mize the synchronization electronics the electricalproperties of the panels have to be analyzed

In order to analyze the electrical properties of thePDLC panels the current response to an applied volt-age was measured As with other liquid crystal devicesthe PDLCrsquos should not be exposed to any DC voltagewhich could cause ionic buildup damaging the liquidcrystal structure Therefore only AC measurementswere performed There is also an upper frequency limitdue to current limitations For a voltage of 80 V themaximum frequency is around 100 Hz (Figure 8)

To calculate the electrical behavior of the PDLC glasspanels an equivalent circuit of three components asproposed by Drzaic (1995) is used (Figure 9)

Based on this circuit the complex impedance is

Z R1 R2

1 CR2)2j CR22

1(CR22

This equation has a real and an imaginary part and thusonly two of three variables could be calculated How-ever since more measurements at different frequencies

Figure 6 The mechanical construction of the blue-c

Figure 7 Component overview

Kunz and Spagno 183

were performed an approximation was calculated mini-mizing the mean square error (MSE) between the calcu-lated and the measured voltages The approximated val-ues are

R1 1745 R2 2494 C 11F

The electronic control of the PDLC panes can be ad-justed to eliminate any DC components in the drivingvoltage In addition an external DC decoupling circuithas been added to guarantee that no DC voltage dam-ages the glass panels even if a fault in the driving circuitoccurs This DC decoupling consists of a capacitor CD

and a resistor RD (Figure 10) The values are 60 F forthe capacitor and 10 k for the resistor

The glass panes show a very stable performance overtime We do not observe any significant change either inthe electrical behavior or the stability of the LC filmThe system worked for many days without any prob-

lems although the panes are driven at their upper limitin frequency and allowable current

422 Projectors and Active Shutters Theactive LC projection screens do not support passive ste-reo projection due to their depolarization propertiesTherefore an active projection system was chosen forthe blue-c In the year 2000 when the design and theconstruction of the system were started active stereoDLP projectors were just emerging and still much tooexpensive for our budget The state-of-the-art CRTprojectors were also still quite expensive compared tothe light output Furthermore we were looking for asystem that could be turned black during the cameraacquisition period that is 625 times per second Thedecision was taken to use two three-chip LCD projec-tors (Sanyo XF12 3500 ANSI lumen) per side and ex-ternal LC shutters in front of the projection lens Withthese shutters it is possible to generate an active stereoprojection by alternating the light output of the twoprojectors and blocking it during the picture acquisitionperiod

The two projectors on each side are mounted in arack for a passive stereo projection installation (Figure11) The LC shutters are custom-made ferro-electricliquid crystal shutters with an aperture size of140 mm 140 mm The shutters are mounted in frontof the 121 projection lenses This lens allows projec-tion directly onto the projection wall without any addi-tional mirrors Using no mirrors in the projection pathfacilitates the alignment of the two LCD projectors foractive stereo projection The shutters do not have to besynchronized with the Onyx and projectors thus leav-ing the shutter times completely flexible This principlewould not work with a single-chip DLP projector sincethe sequential generation of the three colors by a ldquocolor

Figure 8 Voltage and current measured on a panel of

2240 mm 950 mm at 100 Hz

Figure 9 Equivalent circuit diagram of the PDLC glass panel

Figure 10 DC decoupling

184 PRESENCE VOLUME 13 NUMBER 2

wheelrdquo would require a complicated synchronization ofthe shutters with the projectors However today 3-chipDLP projectors could be used instead of the two LCprojectors The shutters in front of the projectors haveto be switched on and off at a frequency of approxi-mately 60 Hz

Twisted Nematic (TN) liquid-crystal shutters as usedin shutter glasses can offer a contrast ratio of 10001 orbetter (Lipton 2001) They switch in about 25 msfrom black to transparent and in less than 1 ms fromtransparent to black (Lipton 1991 Woods amp Tan2002 Bougrioua et al 2002) Ferro-electric (FE)liquid-crystal shutters offer about the same contrastratio as TN shutters but much faster switching timesThe FE shutters used in front of the projectors are140 mm 140 mm and offer a contrast ratio of ap-proximately 7501 The switching time from 10 to90 transparency and from 90 to 10 transparency isbelow 100 s (see Figure 12)

The upper measurement in Figure 12 shows the volt-age measured on the shutter The mean value of thisvoltage is zero as required The lower measurementshows the optical response of the shutter measured by aphototransistor and a light source The response is veryfast and guarantees an excellent separation between thetwo stereo channels

A TN shutter switches to black if a voltage is appliedand to transparent if the voltage is removed An FE

shutter in contrast switches to black if a voltage of onepolarity is applied and to transparent if a voltage of theopposite polarity is applied It is important that theshutter not be subject to any DC voltage as it will bedamaged The DC decoupling is achieved by connect-ing a capacitor CD of 1 F in series and a resistor RD of1 k in parallel to the shutter (see also Figure 10)

Mounting the shutters directly in front of the projec-torrsquos lens did not cause temperature problems as ex-pected Even after several hours of projection the tem-perature was beneath the limit tolerated by thetemperature-sensitive devices

423 Active Illumination The requirements onthe blue-c illumination are very demanding For theprojection both the inside and the outside of the pro-jection room have to be dark On the other handbright illumination is important for good-quality imageacquisition The illumination has a big impact on thequality of the extracted silhouettes and acquired tex-tures A person inside the blue-c should be illuminateduniformly from all sides and no shadow should be pro-jected on the floor The illumination has to be directedaway from the PDLC glass panels to avoid contrast-lossand reflections

Figure 11 Stereo projection rack with LC shutters

Figure 12 Ferro-electric LC shutter (driving voltage optical

response)

Kunz and Spagno 185

An active illumination is used to satisfy the needs ofboth projection and image acquisition The active lightis off during projection and on for the image acquisi-tion Therefore the light has to provide illumination foronly a few milliseconds and must be able to turn on andoff with a frequency between 60 and 70 Hz First testswere done with a strobe light A high voltage dischargein a xenon flash lamp produces a bright light pulse of avery short duration Frequencies in the range of 60 Hzare no problem for this type of lamp Despite the fastswitching this type of light has some important disad-vantages First light is emitted for only a few millisec-onds The light intensity integrated over time is there-fore only mediocre In addition the high voltagedischarge in the very short time period causes heavyelectromagnetic pulses which can interfere with sur-rounding electronic components The discharges alsocause a disturbing acoustic noise Finally the strobelight is focused in one spot This is a handicap whentrying to distribute the light uniformly in the projectionroom All these disadvantages led us to look for otherillumination solutions

The requirement of short switching times restricts thenumber of suitable light sources considerably Light-emitting diodes (LEDs) have very short switching timesand are thus suitable for our application Today whiteLEDs are commonly available and the light efficiency isabout twice that of an incandescent bulb (Zorpette2002) LEDs can be easily grouped in clusters to givemore light output The clusters are flat can be of anyshape and the light output can be distributed homoge-neously over the cluster if desired A total of 9984white 5 mm LEDs have been integrated into the blue-cThe opening angle (50 intensity) is 50 degrees and thelight output is approximately 2200 mcd per LEDThese LEDs were the brightest available on the marketin 2002 The LEDs are configured into 32 clusterseach containing 312 LEDs The clusters are reassem-bled in groups of two four or five and mounted inlong aluminum profiles

The LED clusters have to be placed carefully in theblue-c to allow a good quality image acquisition Thecameras are mostly placed outside the blue-c and facemore or less horizontally to the user through the PDLC

glass panels Under this aspect a diffused frontal illumi-nation of the user would be the most suitable But afrontal light would also shine directly into the oppositecameras and furthermore would put too much light onthe glass panels making them appear gray In additionit would be difficult to find a place to mount the clus-ters without interfering with the projection and thecameras

The solution is to distribute the LED clusters alongthe upper and lower edges of the projection room TheLEDs can be adjusted in such a way that they com-pletely illuminate the user minimizing at the same timethe amount of light being directed at the glass panelsand at the opposite cameras The additional mask infront of the LEDs helps to further diminish the amountof scattered light reaching the glass panels Figure 13shows a person between two opposite projection wallsas is the case in the blue-c for the two sidewalls Thelight is coming from all four edges and most parts of theperson are illuminated by more than one light sourceFigure 14 shows one illumination cluster Five of themare integrated in the aluminum profile and placed at theedge between the floor and the left glass panels

Realizing the illumination with the diodes it turnedout that the specifications of the LED were not reachedby every single element This resulted in failures andthus in nonactive LED lines in the cluster After the cur-

Figure 13 Placement of the LED clusters

186 PRESENCE VOLUME 13 NUMBER 2

rent through the entire cluster was reduced this prob-lem disappeared

424 Cameras Sixteen FireWire cameras areplaced around the blue-c on a special aluminum struc-ture The structure is based on standard aluminum ele-ments and offers the maximum flexibility in the place-ment of cameras and other components Each camera isconnected via FireWire to one node of the PC clusterThe signal is amplified every 5 m with a repeater Up tothree repeaters are used for each camera

Dragonflytrade cameras are used since they offer goodsensitivity and a high picture acquisition frequency of upto 15 Hz However cameras with an even higher sensi-tivity would be useful in order to reduce the intensity ofthe flash illumination On the other hand the pictureacquisition frequency is high enough since the bottle-neck is in the processing of the acquired images Initialtests showed that a repetition frequency of 10 Hz seemsto be sufficient for collaborative work

425 Synchronization Electronics The syn-chronization electronics of the blue-c were built modu-larly to offer flexibility in adding and changing compo-nents The main module is the frequency-controlmodule which is equipped with a reprogrammable PIC16F877 microcontroller This module generates thetriggering sequence to drive the other modules of the

synchronization unit Three PDLC glass panel drivermodules two shutter driver modules and one trigger-interface module are integrated into the synchronizationunit together with the synchronization control moduleThe LED driver and IR-emitter driver are placed out-side of the synchronization unit and are triggered by thetrigger interface module

Figure 15 gives an overview of the synchronizationelectronics All modules of the synchronization unit areinterconnected with a data bus In addition a powerbus is integrated into the unit to supply the differentmodules with the required voltages The LED driverand IR-emitter driver are located outside of the syn-chronization unit together with their own power sup-plies to reduce electromagnetic interference caused bythe currents

Using a microcontroller for the synchronization of allcomponents proved to be crucial Since every compo-nent including identical parts behaves slightly differ-ently (eg the glass panes) it is easy to create individu-alized triggering signals using the microcontroller Thismakes up for the increased effort of installing and pro-gramming the processor

426 Frequency Control Module The fre-quency control module is based on a PIC 16F877 mi-crocontroller This microcontroller can be repro-grammed in-circuit that is without unplugging thechip from the board Twenty-two inputndashoutput ports ofthe microcontroller are mapped to the data bus They

Figure 14 An LED cluster of the blue-c

Figure 15 Synchronization electronics

Kunz and Spagno 187

can be set individually as outputs or inputs Each ofthese ports is connected to a LED to show its state Themicrocontroller is clocked with a frequency of 20 MHzto offer high timing accuracy

427 PDLC Glass-Panel Driver Module Todrive the nine PDLC glass panels a total of three glass-panel driver modules are used (see Figure 16) In eachcase three glass panels in parallel are connected to thetwo outputs of a module The two outputs of a modulecan be switched separately to either 8 V or 85 V If bothoutputs are switched to the same value no voltage isapplied to the glass panels If the outputs are switchedto different values a voltage of 77 V or 77 V is ap-plied to the glass panels in series with the decouplingcapacitor Each of the two outputs is switched directlyby the corresponding input coming from the data busA current-limit control is superimposed on the voltageswitching

Figure 16 shows a simplified circuit consisting of aPDLC glass-panel driver module connected to threeglass panels with their DC decoupling circuits Each ofthe two incoming TTL signals switches an optocoupleroutput to a preset voltage of 8 V or 85 V The two out-puts of the optocouplers are connected to two APEXPA46 operational amplifiers The APEX PA64 has anintegrated adjustable current-limit control and can runat voltages up to 150 V and currents up to 5 A Theoperational amplifiers are used as followers thus theinput voltage is mapped directly to the output as long asthe current-limit is not exceeded The PDLC glass-panel driver modules are connected to a 95-V power

supply However the outputs of the module have beenset to switch only between 8 V and 85 V This givesenough margin to the output voltage from the powersupply and ground to avoid saturating the operationalamplifier The current-limit of the operational amplifieris set to 900 mA which corresponds to a current-limitof 300 mA per panel

428 Shutter Driver Module The shutterdriver module also has two inputs and two outputs Itconsists of two independent circuits with an opto-coupler and a half-bridge consisting of two MOSFETsand a half-bridge driver (Figure 17) The optocoupler isused to protect the other modules of the synchroniza-tion electronics in case a fault occurs in this module orin a connected component The output of the opto-coupler is connected to a half-bridge driver (IR2111)that drives two MOSFETs The bridge driver ensuresthat the two MOSFETs in the half-bridge are neverturned on at the same time The two half-bridge out-puts switch the voltage rail-to-rail without any current-limit In order to drive the ferro-electric shutters themodule is operated with a voltage of 12 V Three shut-ters are connected in parallel between the two half-bridges of a module The two half-bridges are alwaysswitched to opposite states The bridge voltage is there-fore either 12 V or 12 V as required by the shuttersfor switching

429 Trigger Interface Module The trigger-interface module is a multifunctional module It allows achoice between up to 8 lines out of the 22 data lines onthe bus Each line can be connected to a 5 V or 12 V

Figure 16 PDLC glass-panel driver moduleFigure 17 Shutter driver module

188 PRESENCE VOLUME 13 NUMBER 2

driver and configured as input or output For the blue-call channels used were configured as 5-V outputs Onechannel is used to trigger the FireWire cameras A sec-ond one is used to trigger the LED illumination Twomore channels are used to synchronize the shutterglasses and are connected to the IR-emitter driver

4210 LED Driver A total of 9984 LEDs areinstalled in different clusters in the blue-c The LEDclusters have to be driven with a voltage of 375 V and atotal current of up to 25 A The power supply for theLED cluster is installed in the floor of the blue-c andconnected directly to the LED clusters This reduces thewiring distances of the high-current connections andthus the probability of electro-magnetic interferenceEach cluster has an integrated MOSFET to switch theLEDs on and off The triggering signal comes directlyfrom the trigger-interface module

4211 Shutter Glasses The LED illuminationis triggered 625 times per second The user does notsee any flickering because this frequency is above thetemporal resolution of the human eye However thebright light inside the blue-c diminishes the contrast ofthe projected image considerably To protect the userfrom the active light an apposite third phase has beenintegrated into the stereo glasses to keep the light outof the userrsquos eyes The projection of the images for theleft and the right eye are followed by a third phase inwhich both LC shutters of the shutter glasses are dark-ened The LED illumination is on only during thisphase This helps to fade out most of the light from theillumination

In the blue-c we use the NuVision 60GX shutterglasses from MacNaughton The shutter glasses and theIR-emitter have a built-in microcontroller We havereprogrammed it to include the third phase Like mostshutter glasses the NuVision use twisted nematic LCshutters The transparent-to-black transition timeof these shutters is quite fast while the black-to-transparent switching time takes considerably longerFigure 18 shows the switching time of the NuVisionshutter glasses The upper measurement shows thetransparency of a shutter where a high value corre-

sponds to a high transparency The lower measurementshows the voltage applied to the IR-emitter As can beseen from the measurement there is no significant delaybetween sending a signal to the emitter and the begin-ning of the switching response However the switchingtime from 10 to 90 transparency takes approximately33 ms while the switching from 90 to 10 transpar-ency takes about 05 ms These values correspond to thevalues measured by Woods and Tan (Woods amp Tan2002) with other shutter glasses For practical use thereis a work-around involving some blanking time betweenthe projections for the left and right eye

5 Overall Setup

Two complete collaboration setups were de-signed as shown in Figure 19 In the upper image inFigure 19 the first prototype is shown essentially a lab-oratory setup One lesson learned from this is that inmany cases only the front screen is used for collabora-tion This allows for the creation of a more design-oriented setup in a public room (as shown in the lowerpart of Figure 19) using only a single screen Since thecamera positions are known from the first prototypethere was no longer any need for a flexible installation

Figure 18 Optical response of the shutter glasses

Kunz and Spagno 189

The goal of the second setup (bottom image in Fig-ure 19) was thus not to reproduce the first installationbut rather to create an asymmetric installation for thecollaboration With the asymmetric setup applicationswere first created transferring 3D sound and 3D imagesof the people from one installation to the other Figure20 shows a simple collaboration between two individu-als engaged in a dialogue Each person is in an installa-tion at a different location on the campus

Since the first system is an experimental setup a longinitialization time was required before the collaborationcould start Thus the system is not used for hands-oncollaboration yet Further work will shorten the initial-ization phase and will allow easier access to the system

6 Conclusions

The active projection screen is driven at its physi-cal limits The transition time from opaque to transpar-ent (10 to 90) takes approximately 2 ms while the

transition from transparent to opaque (90 to 10)takes approximately 6 ms The slow transition fromtransparent to opaque is dictated by the internal relax-ation forces of the LC film and cannot be acceleratedelectrically This non-ideal behavior can be minimizedby optimizing the time pattern of the different compo-nents For the user just the hotspot of the projectorsbecomes a little more evident but it remains at an ac-ceptable level The active projection screens offer a verybright and homogenous image

The junctions between the three projection-screenpanels are visible but not disturbing The LC film of theactive projection panels is installed between two glasspanels The surface of these glass panels is treated withan antireflective coating Nevertheless some reflectionsfrom the other projection screens are evident The re-flection can be compared to the reflection on a CRTtelevision in a medium-bright illuminated room Thereflection is noticeable when the user concentrates on itand disappears as soon as he or she concentrates on theapplication Since the projectors are darkened duringthe transparent state of the projection screens the cam-era acquisition is not disturbed by any reflection Thecameras are positioned in such a way as to see the userrsquosentire body even if the user gets very close to the pro-jection screens

Figure 19 Two different collaboration setups based on the blue-c

principle

Figure 20 Collaboration between two individuals

190 PRESENCE VOLUME 13 NUMBER 2

The channel separation of the stereoscopic projectionis very good and the total performance of the projec-tion is convincing The brightness is higher than tradi-tional CRT stereo projectors at a lower price per stereoprojection rack The six-channel audio system proved tobe very powerful The user can precisely localize a simu-lated noise or sound source The subwoofer located inthe wooden platform is also important for good audioperformance In addition it can transmit vibrations tothe user at lower frequencies and this acoustic stimulusplays an important role in high immersion

The mechanical structure was shown to meet the re-quired demands Its stability and rigidity give a strongsense of confidence in the blue-c The user is never un-easy about touching the projection screen or walkingaround as in some other VR installations The flexibilityin the placement of new components and cables as wellas the quick access to all the components proved to bea very valuable feature of the blue-c construction

7 Further Work

The two setups showed that collaboration over anetwork using 3D representations is technically possibleHowever there are more problems to overcome In itstechnical aspects the system should be downsized in costand weight with the goal being a mobile installation Herenew stereo projection systems would also be tested that areeasier to install and adjust Also new projection materialssuch as active Perspex will be evaluated As a first step inthis direction we will build a demonstrator that would besuitable for presentations on conferences and exhibitionsThe demonstrator could be used to show the core tech-nology of the blue-c itself but also as another input stationfor a multipoint collaboration (Disz Papka Pellegrino ampStevens 1995 Lehner amp DeFanti 1997 Fuchs et al2002 Bailett Calahan amp McCluskey 1998)

Further work on the blue-c will also deal with imple-menting the technology in existing processes for in-stance the product development process or the educa-tion process Thus applications for the blue-c will begenerated that offer benefits to the user when using the

3D capability in collaborative work Further informationcan be found at httpblue-cethzch

Acknowledgments

This project was funded as an internal research project no0-23803-00 by the Swiss Federal Institute of TechnologyZurich We would like to thank all members of the blue-cproject

References

Bailett A Calahan J amp McCluskey S (1998) Coordina-tion at different stages of the product development processRampD Management 28 237ndash247

Bougrioua F Oepts W Vleeschouwer H D AlexanderE Neyts K amp Pauwels H (2002) Determination of theswitching response in reflective twisted nematic liquid crys-tal displays The Japan Society of Applied Physics 41 5676ndash5681

Chen H Wallace G Gupta A Li K Funkhouser T ampCook P (2002) Experiences with scalability of displaywalls Proceedings of IPTVR 2002

Cruz-Neira C Sandin D J amp DeFanti T A (1993)Surrounding-screen projection-based virtual reality Thedesign and implementation of the CAVE Proceedings ofSIGGRAPH 1993 135ndash142

Disz T Papka M Pellegrino M amp Stevens R (1995)Sharing visualization experiences among remote virtual en-vironments Proceedings of the International Workshop onHigh Performance Computing for Computer Graphics andVisualization 135ndash142

Drzaic P S (1995) Liquid Crystal Dispersion World Scien-tific Singapore

Elspass W VanGool L Gross M H Kunz A Meier MSchmitt G Staadt O et al (1999) The BlueCave ETHZurich internal research proposal

Fuchs H Bishop G Arthur K McMillan L Bajcsy RLee S et al (1994) Virtual space teleconferencing using asea of cameras (Tech Rep No TR94-033) Chapel Hill

Kunz and Spagno 191

including a third phase when both lenses for the leftand right eye are darkened These shutter glasses areneeded anyway for the active stereoscopic projectionFigure 2 illustrates such a projection sequence which isfollowed by an acquisition sequence

During the third time phase when both glasses aredarkened an active illumination is triggered The illumi-nation is preferably done with white light in order toachieve a true-color texture recognition It is possible touse a strobe light or white light-emitting diodes(LEDs) Figure 3 shows how this active illumination canbe kept out of the userrsquos eyes by integrating a thirdphase into the shutter glasses (Kunz amp Spagno 2001aKunz amp Spagno 2001b)

In the left-hand pair of images in Figure 3 the illumi-nation is shown as seen without the third phase in theright-hand pair of images the illumination is shownafter the third phase within the shutter glasses is acti-vated In each pair the second image shows an objectthat is only illuminated by ambient light

With sufficiently bright illumination it is possible toacquire good-quality images The cameras for recording

the texture of the front of the user have to be placed infront of user without being in his or her field of view(Fuchs et al 1994) Therefore the best position for thecameras would be behind the projection screen wherethey cannot be seen by the user (Figure 4) The activeprojection screens allow the cameras to be arrangedaround the projection room The layout of the blue-c isa three-sided projection room with a quadratic groundplan This quadratic layout has been used for the firsttime in the cave (Cruz-Neira Sandin amp DeFanti1993) No edge-blending is required since the differentprojections are on different screens

The working principle of the blue-c relies on an exactsynchronization among all components involved Cam-eras flash illumination shutter glasses projector shut-ters and the active projection screen need a hard syn-chronization to allow stereoscopic projection togetherwith image acquisition

The triggering sequence of all components can beseen in Figure 5 The camera triggering frequency is setto acquire images only every seventh cycle of the basictriggering frequency of 625 Hz This corresponds to89 images per second This value was chosen to allowenough time for the acquisition system to process theincoming images Although the flash illumination andthe triggering of the PDLCs could be reduced to89 Hz we kept the higher triggering frequency in or-der to avoid flickering that would be disturbing to theuser since the human eye is very time-sensitive at lowerfrequencies

Figure 2 The three time phases (time intervals)

Figure 3 Active illuminations without and with a third phase

Figure 4 blue-c layout

Kunz and Spagno 181

4 The Components in Detail

41 Mechanical Construction

Since a magnetic tracking system was chosen thebasis of the construction is a wooden platform with atotal height of 540 mm (see Figure 6) The trackingemitter is built into the middle of the wooden platformand thus is not visible to the user The two lower frontloudspeakers and the subwoofer are also built into theplatform The floor of the platform can be easily openedto access all the built-in components The upper struc-ture is built of fiber-reinforced composites (glass andcarbon) These are stiff enough to keep the LC projec-tion panels safely in place and at the same time stillsmall and attractive from a design point of view Cablechannels are built into the beams providing sufficientspace for the cables of all components placed on theupper part of the structure including the cables for theactive projection screens Together with the woodenplatform there is enough space for all the cables andconnectors for the system The chosen materials woodand fiber-reinforced composites assure an accuratefunctioning of the tracking system The cables and theloudspeakers which are at a distance of 15 m from the

emitter of the tracking system do not have any impor-tant influence on its accuracy The results from prelimi-nary tests show that the electrically switched glass panesalso do not interfere with the tracking system

42 Hardware Construction

Figure 7 gives an overview of all components ofthe blue-c and its connection scheme A total of 6 pro-jectors and four monitors are connected to an SGIOnyx The 16 cameras are connected via sixteenFireWire cables to the computer cluster The active pro-jection panels the 6 ferro-electric shutters in front ofthe projectors the active illumination the cameras andthe IR-emitters are synchronized by custom-made syn-chronization electronics The individual componentswill be discussed in the following sections

421 Active Projection Screen Phase dis-persed liquid crystal (PDLC) glass panels offer the re-quired features for a switchable projection screen Theycan be controlled electrically and can be switched in afew milliseconds or less between a transparent and anopaque state Different glass manufacturers have switch-

Figure 5 Triggering timings

182 PRESENCE VOLUME 13 NUMBER 2

able glass panels available with an integrated PDLClayer To change the optical property of a PDLC glasspanel an electrical field has to be applied perpendicularto the PDLC film A voltage in the range of 50 V to100 V is necessary to generate this electrical field ThePDLC glass panels consist of two glass layers coatedwith a transparent electrode on the inside surface and aPDLC film in between Each of the two conductors has

to be contacted by an electrode On the glass panelsused the two copper electrodes are located on the twoshorter edges so as to be invisible when the glass panelsare installed The current flowing through the transpar-ent electrode has to be limited to avoid its overheatingand therefore damaging the PDLC glass panel Themanufacturer recommends not exceeding 300 mA for a2240 mm 950 mm panel with the electrodes placedon the short edges as installed in the blue-c To opti-mize the synchronization electronics the electricalproperties of the panels have to be analyzed

In order to analyze the electrical properties of thePDLC panels the current response to an applied volt-age was measured As with other liquid crystal devicesthe PDLCrsquos should not be exposed to any DC voltagewhich could cause ionic buildup damaging the liquidcrystal structure Therefore only AC measurementswere performed There is also an upper frequency limitdue to current limitations For a voltage of 80 V themaximum frequency is around 100 Hz (Figure 8)

To calculate the electrical behavior of the PDLC glasspanels an equivalent circuit of three components asproposed by Drzaic (1995) is used (Figure 9)

Based on this circuit the complex impedance is

Z R1 R2

1 CR2)2j CR22

1(CR22

This equation has a real and an imaginary part and thusonly two of three variables could be calculated How-ever since more measurements at different frequencies

Figure 6 The mechanical construction of the blue-c

Figure 7 Component overview

Kunz and Spagno 183

were performed an approximation was calculated mini-mizing the mean square error (MSE) between the calcu-lated and the measured voltages The approximated val-ues are

R1 1745 R2 2494 C 11F

The electronic control of the PDLC panes can be ad-justed to eliminate any DC components in the drivingvoltage In addition an external DC decoupling circuithas been added to guarantee that no DC voltage dam-ages the glass panels even if a fault in the driving circuitoccurs This DC decoupling consists of a capacitor CD

and a resistor RD (Figure 10) The values are 60 F forthe capacitor and 10 k for the resistor

The glass panes show a very stable performance overtime We do not observe any significant change either inthe electrical behavior or the stability of the LC filmThe system worked for many days without any prob-

lems although the panes are driven at their upper limitin frequency and allowable current

422 Projectors and Active Shutters Theactive LC projection screens do not support passive ste-reo projection due to their depolarization propertiesTherefore an active projection system was chosen forthe blue-c In the year 2000 when the design and theconstruction of the system were started active stereoDLP projectors were just emerging and still much tooexpensive for our budget The state-of-the-art CRTprojectors were also still quite expensive compared tothe light output Furthermore we were looking for asystem that could be turned black during the cameraacquisition period that is 625 times per second Thedecision was taken to use two three-chip LCD projec-tors (Sanyo XF12 3500 ANSI lumen) per side and ex-ternal LC shutters in front of the projection lens Withthese shutters it is possible to generate an active stereoprojection by alternating the light output of the twoprojectors and blocking it during the picture acquisitionperiod

The two projectors on each side are mounted in arack for a passive stereo projection installation (Figure11) The LC shutters are custom-made ferro-electricliquid crystal shutters with an aperture size of140 mm 140 mm The shutters are mounted in frontof the 121 projection lenses This lens allows projec-tion directly onto the projection wall without any addi-tional mirrors Using no mirrors in the projection pathfacilitates the alignment of the two LCD projectors foractive stereo projection The shutters do not have to besynchronized with the Onyx and projectors thus leav-ing the shutter times completely flexible This principlewould not work with a single-chip DLP projector sincethe sequential generation of the three colors by a ldquocolor

Figure 8 Voltage and current measured on a panel of

2240 mm 950 mm at 100 Hz

Figure 9 Equivalent circuit diagram of the PDLC glass panel

Figure 10 DC decoupling

184 PRESENCE VOLUME 13 NUMBER 2

wheelrdquo would require a complicated synchronization ofthe shutters with the projectors However today 3-chipDLP projectors could be used instead of the two LCprojectors The shutters in front of the projectors haveto be switched on and off at a frequency of approxi-mately 60 Hz

Twisted Nematic (TN) liquid-crystal shutters as usedin shutter glasses can offer a contrast ratio of 10001 orbetter (Lipton 2001) They switch in about 25 msfrom black to transparent and in less than 1 ms fromtransparent to black (Lipton 1991 Woods amp Tan2002 Bougrioua et al 2002) Ferro-electric (FE)liquid-crystal shutters offer about the same contrastratio as TN shutters but much faster switching timesThe FE shutters used in front of the projectors are140 mm 140 mm and offer a contrast ratio of ap-proximately 7501 The switching time from 10 to90 transparency and from 90 to 10 transparency isbelow 100 s (see Figure 12)

The upper measurement in Figure 12 shows the volt-age measured on the shutter The mean value of thisvoltage is zero as required The lower measurementshows the optical response of the shutter measured by aphototransistor and a light source The response is veryfast and guarantees an excellent separation between thetwo stereo channels

A TN shutter switches to black if a voltage is appliedand to transparent if the voltage is removed An FE

shutter in contrast switches to black if a voltage of onepolarity is applied and to transparent if a voltage of theopposite polarity is applied It is important that theshutter not be subject to any DC voltage as it will bedamaged The DC decoupling is achieved by connect-ing a capacitor CD of 1 F in series and a resistor RD of1 k in parallel to the shutter (see also Figure 10)

Mounting the shutters directly in front of the projec-torrsquos lens did not cause temperature problems as ex-pected Even after several hours of projection the tem-perature was beneath the limit tolerated by thetemperature-sensitive devices

423 Active Illumination The requirements onthe blue-c illumination are very demanding For theprojection both the inside and the outside of the pro-jection room have to be dark On the other handbright illumination is important for good-quality imageacquisition The illumination has a big impact on thequality of the extracted silhouettes and acquired tex-tures A person inside the blue-c should be illuminateduniformly from all sides and no shadow should be pro-jected on the floor The illumination has to be directedaway from the PDLC glass panels to avoid contrast-lossand reflections

Figure 11 Stereo projection rack with LC shutters

Figure 12 Ferro-electric LC shutter (driving voltage optical

response)

Kunz and Spagno 185

An active illumination is used to satisfy the needs ofboth projection and image acquisition The active lightis off during projection and on for the image acquisi-tion Therefore the light has to provide illumination foronly a few milliseconds and must be able to turn on andoff with a frequency between 60 and 70 Hz First testswere done with a strobe light A high voltage dischargein a xenon flash lamp produces a bright light pulse of avery short duration Frequencies in the range of 60 Hzare no problem for this type of lamp Despite the fastswitching this type of light has some important disad-vantages First light is emitted for only a few millisec-onds The light intensity integrated over time is there-fore only mediocre In addition the high voltagedischarge in the very short time period causes heavyelectromagnetic pulses which can interfere with sur-rounding electronic components The discharges alsocause a disturbing acoustic noise Finally the strobelight is focused in one spot This is a handicap whentrying to distribute the light uniformly in the projectionroom All these disadvantages led us to look for otherillumination solutions

The requirement of short switching times restricts thenumber of suitable light sources considerably Light-emitting diodes (LEDs) have very short switching timesand are thus suitable for our application Today whiteLEDs are commonly available and the light efficiency isabout twice that of an incandescent bulb (Zorpette2002) LEDs can be easily grouped in clusters to givemore light output The clusters are flat can be of anyshape and the light output can be distributed homoge-neously over the cluster if desired A total of 9984white 5 mm LEDs have been integrated into the blue-cThe opening angle (50 intensity) is 50 degrees and thelight output is approximately 2200 mcd per LEDThese LEDs were the brightest available on the marketin 2002 The LEDs are configured into 32 clusterseach containing 312 LEDs The clusters are reassem-bled in groups of two four or five and mounted inlong aluminum profiles

The LED clusters have to be placed carefully in theblue-c to allow a good quality image acquisition Thecameras are mostly placed outside the blue-c and facemore or less horizontally to the user through the PDLC

glass panels Under this aspect a diffused frontal illumi-nation of the user would be the most suitable But afrontal light would also shine directly into the oppositecameras and furthermore would put too much light onthe glass panels making them appear gray In additionit would be difficult to find a place to mount the clus-ters without interfering with the projection and thecameras

The solution is to distribute the LED clusters alongthe upper and lower edges of the projection room TheLEDs can be adjusted in such a way that they com-pletely illuminate the user minimizing at the same timethe amount of light being directed at the glass panelsand at the opposite cameras The additional mask infront of the LEDs helps to further diminish the amountof scattered light reaching the glass panels Figure 13shows a person between two opposite projection wallsas is the case in the blue-c for the two sidewalls Thelight is coming from all four edges and most parts of theperson are illuminated by more than one light sourceFigure 14 shows one illumination cluster Five of themare integrated in the aluminum profile and placed at theedge between the floor and the left glass panels

Realizing the illumination with the diodes it turnedout that the specifications of the LED were not reachedby every single element This resulted in failures andthus in nonactive LED lines in the cluster After the cur-

Figure 13 Placement of the LED clusters

186 PRESENCE VOLUME 13 NUMBER 2

rent through the entire cluster was reduced this prob-lem disappeared

424 Cameras Sixteen FireWire cameras areplaced around the blue-c on a special aluminum struc-ture The structure is based on standard aluminum ele-ments and offers the maximum flexibility in the place-ment of cameras and other components Each camera isconnected via FireWire to one node of the PC clusterThe signal is amplified every 5 m with a repeater Up tothree repeaters are used for each camera

Dragonflytrade cameras are used since they offer goodsensitivity and a high picture acquisition frequency of upto 15 Hz However cameras with an even higher sensi-tivity would be useful in order to reduce the intensity ofthe flash illumination On the other hand the pictureacquisition frequency is high enough since the bottle-neck is in the processing of the acquired images Initialtests showed that a repetition frequency of 10 Hz seemsto be sufficient for collaborative work

425 Synchronization Electronics The syn-chronization electronics of the blue-c were built modu-larly to offer flexibility in adding and changing compo-nents The main module is the frequency-controlmodule which is equipped with a reprogrammable PIC16F877 microcontroller This module generates thetriggering sequence to drive the other modules of the

synchronization unit Three PDLC glass panel drivermodules two shutter driver modules and one trigger-interface module are integrated into the synchronizationunit together with the synchronization control moduleThe LED driver and IR-emitter driver are placed out-side of the synchronization unit and are triggered by thetrigger interface module

Figure 15 gives an overview of the synchronizationelectronics All modules of the synchronization unit areinterconnected with a data bus In addition a powerbus is integrated into the unit to supply the differentmodules with the required voltages The LED driverand IR-emitter driver are located outside of the syn-chronization unit together with their own power sup-plies to reduce electromagnetic interference caused bythe currents

Using a microcontroller for the synchronization of allcomponents proved to be crucial Since every compo-nent including identical parts behaves slightly differ-ently (eg the glass panes) it is easy to create individu-alized triggering signals using the microcontroller Thismakes up for the increased effort of installing and pro-gramming the processor

426 Frequency Control Module The fre-quency control module is based on a PIC 16F877 mi-crocontroller This microcontroller can be repro-grammed in-circuit that is without unplugging thechip from the board Twenty-two inputndashoutput ports ofthe microcontroller are mapped to the data bus They

Figure 14 An LED cluster of the blue-c

Figure 15 Synchronization electronics

Kunz and Spagno 187

can be set individually as outputs or inputs Each ofthese ports is connected to a LED to show its state Themicrocontroller is clocked with a frequency of 20 MHzto offer high timing accuracy

427 PDLC Glass-Panel Driver Module Todrive the nine PDLC glass panels a total of three glass-panel driver modules are used (see Figure 16) In eachcase three glass panels in parallel are connected to thetwo outputs of a module The two outputs of a modulecan be switched separately to either 8 V or 85 V If bothoutputs are switched to the same value no voltage isapplied to the glass panels If the outputs are switchedto different values a voltage of 77 V or 77 V is ap-plied to the glass panels in series with the decouplingcapacitor Each of the two outputs is switched directlyby the corresponding input coming from the data busA current-limit control is superimposed on the voltageswitching

Figure 16 shows a simplified circuit consisting of aPDLC glass-panel driver module connected to threeglass panels with their DC decoupling circuits Each ofthe two incoming TTL signals switches an optocoupleroutput to a preset voltage of 8 V or 85 V The two out-puts of the optocouplers are connected to two APEXPA46 operational amplifiers The APEX PA64 has anintegrated adjustable current-limit control and can runat voltages up to 150 V and currents up to 5 A Theoperational amplifiers are used as followers thus theinput voltage is mapped directly to the output as long asthe current-limit is not exceeded The PDLC glass-panel driver modules are connected to a 95-V power

supply However the outputs of the module have beenset to switch only between 8 V and 85 V This givesenough margin to the output voltage from the powersupply and ground to avoid saturating the operationalamplifier The current-limit of the operational amplifieris set to 900 mA which corresponds to a current-limitof 300 mA per panel

428 Shutter Driver Module The shutterdriver module also has two inputs and two outputs Itconsists of two independent circuits with an opto-coupler and a half-bridge consisting of two MOSFETsand a half-bridge driver (Figure 17) The optocoupler isused to protect the other modules of the synchroniza-tion electronics in case a fault occurs in this module orin a connected component The output of the opto-coupler is connected to a half-bridge driver (IR2111)that drives two MOSFETs The bridge driver ensuresthat the two MOSFETs in the half-bridge are neverturned on at the same time The two half-bridge out-puts switch the voltage rail-to-rail without any current-limit In order to drive the ferro-electric shutters themodule is operated with a voltage of 12 V Three shut-ters are connected in parallel between the two half-bridges of a module The two half-bridges are alwaysswitched to opposite states The bridge voltage is there-fore either 12 V or 12 V as required by the shuttersfor switching

429 Trigger Interface Module The trigger-interface module is a multifunctional module It allows achoice between up to 8 lines out of the 22 data lines onthe bus Each line can be connected to a 5 V or 12 V

Figure 16 PDLC glass-panel driver moduleFigure 17 Shutter driver module

188 PRESENCE VOLUME 13 NUMBER 2

driver and configured as input or output For the blue-call channels used were configured as 5-V outputs Onechannel is used to trigger the FireWire cameras A sec-ond one is used to trigger the LED illumination Twomore channels are used to synchronize the shutterglasses and are connected to the IR-emitter driver

4210 LED Driver A total of 9984 LEDs areinstalled in different clusters in the blue-c The LEDclusters have to be driven with a voltage of 375 V and atotal current of up to 25 A The power supply for theLED cluster is installed in the floor of the blue-c andconnected directly to the LED clusters This reduces thewiring distances of the high-current connections andthus the probability of electro-magnetic interferenceEach cluster has an integrated MOSFET to switch theLEDs on and off The triggering signal comes directlyfrom the trigger-interface module

4211 Shutter Glasses The LED illuminationis triggered 625 times per second The user does notsee any flickering because this frequency is above thetemporal resolution of the human eye However thebright light inside the blue-c diminishes the contrast ofthe projected image considerably To protect the userfrom the active light an apposite third phase has beenintegrated into the stereo glasses to keep the light outof the userrsquos eyes The projection of the images for theleft and the right eye are followed by a third phase inwhich both LC shutters of the shutter glasses are dark-ened The LED illumination is on only during thisphase This helps to fade out most of the light from theillumination

In the blue-c we use the NuVision 60GX shutterglasses from MacNaughton The shutter glasses and theIR-emitter have a built-in microcontroller We havereprogrammed it to include the third phase Like mostshutter glasses the NuVision use twisted nematic LCshutters The transparent-to-black transition timeof these shutters is quite fast while the black-to-transparent switching time takes considerably longerFigure 18 shows the switching time of the NuVisionshutter glasses The upper measurement shows thetransparency of a shutter where a high value corre-

sponds to a high transparency The lower measurementshows the voltage applied to the IR-emitter As can beseen from the measurement there is no significant delaybetween sending a signal to the emitter and the begin-ning of the switching response However the switchingtime from 10 to 90 transparency takes approximately33 ms while the switching from 90 to 10 transpar-ency takes about 05 ms These values correspond to thevalues measured by Woods and Tan (Woods amp Tan2002) with other shutter glasses For practical use thereis a work-around involving some blanking time betweenthe projections for the left and right eye

5 Overall Setup

Two complete collaboration setups were de-signed as shown in Figure 19 In the upper image inFigure 19 the first prototype is shown essentially a lab-oratory setup One lesson learned from this is that inmany cases only the front screen is used for collabora-tion This allows for the creation of a more design-oriented setup in a public room (as shown in the lowerpart of Figure 19) using only a single screen Since thecamera positions are known from the first prototypethere was no longer any need for a flexible installation

Figure 18 Optical response of the shutter glasses

Kunz and Spagno 189

The goal of the second setup (bottom image in Fig-ure 19) was thus not to reproduce the first installationbut rather to create an asymmetric installation for thecollaboration With the asymmetric setup applicationswere first created transferring 3D sound and 3D imagesof the people from one installation to the other Figure20 shows a simple collaboration between two individu-als engaged in a dialogue Each person is in an installa-tion at a different location on the campus

Since the first system is an experimental setup a longinitialization time was required before the collaborationcould start Thus the system is not used for hands-oncollaboration yet Further work will shorten the initial-ization phase and will allow easier access to the system

6 Conclusions

The active projection screen is driven at its physi-cal limits The transition time from opaque to transpar-ent (10 to 90) takes approximately 2 ms while the

transition from transparent to opaque (90 to 10)takes approximately 6 ms The slow transition fromtransparent to opaque is dictated by the internal relax-ation forces of the LC film and cannot be acceleratedelectrically This non-ideal behavior can be minimizedby optimizing the time pattern of the different compo-nents For the user just the hotspot of the projectorsbecomes a little more evident but it remains at an ac-ceptable level The active projection screens offer a verybright and homogenous image

The junctions between the three projection-screenpanels are visible but not disturbing The LC film of theactive projection panels is installed between two glasspanels The surface of these glass panels is treated withan antireflective coating Nevertheless some reflectionsfrom the other projection screens are evident The re-flection can be compared to the reflection on a CRTtelevision in a medium-bright illuminated room Thereflection is noticeable when the user concentrates on itand disappears as soon as he or she concentrates on theapplication Since the projectors are darkened duringthe transparent state of the projection screens the cam-era acquisition is not disturbed by any reflection Thecameras are positioned in such a way as to see the userrsquosentire body even if the user gets very close to the pro-jection screens

Figure 19 Two different collaboration setups based on the blue-c

principle

Figure 20 Collaboration between two individuals

190 PRESENCE VOLUME 13 NUMBER 2

The channel separation of the stereoscopic projectionis very good and the total performance of the projec-tion is convincing The brightness is higher than tradi-tional CRT stereo projectors at a lower price per stereoprojection rack The six-channel audio system proved tobe very powerful The user can precisely localize a simu-lated noise or sound source The subwoofer located inthe wooden platform is also important for good audioperformance In addition it can transmit vibrations tothe user at lower frequencies and this acoustic stimulusplays an important role in high immersion

The mechanical structure was shown to meet the re-quired demands Its stability and rigidity give a strongsense of confidence in the blue-c The user is never un-easy about touching the projection screen or walkingaround as in some other VR installations The flexibilityin the placement of new components and cables as wellas the quick access to all the components proved to bea very valuable feature of the blue-c construction

7 Further Work

The two setups showed that collaboration over anetwork using 3D representations is technically possibleHowever there are more problems to overcome In itstechnical aspects the system should be downsized in costand weight with the goal being a mobile installation Herenew stereo projection systems would also be tested that areeasier to install and adjust Also new projection materialssuch as active Perspex will be evaluated As a first step inthis direction we will build a demonstrator that would besuitable for presentations on conferences and exhibitionsThe demonstrator could be used to show the core tech-nology of the blue-c itself but also as another input stationfor a multipoint collaboration (Disz Papka Pellegrino ampStevens 1995 Lehner amp DeFanti 1997 Fuchs et al2002 Bailett Calahan amp McCluskey 1998)

Further work on the blue-c will also deal with imple-menting the technology in existing processes for in-stance the product development process or the educa-tion process Thus applications for the blue-c will begenerated that offer benefits to the user when using the

3D capability in collaborative work Further informationcan be found at httpblue-cethzch

Acknowledgments

This project was funded as an internal research project no0-23803-00 by the Swiss Federal Institute of TechnologyZurich We would like to thank all members of the blue-cproject

References

Bailett A Calahan J amp McCluskey S (1998) Coordina-tion at different stages of the product development processRampD Management 28 237ndash247

Bougrioua F Oepts W Vleeschouwer H D AlexanderE Neyts K amp Pauwels H (2002) Determination of theswitching response in reflective twisted nematic liquid crys-tal displays The Japan Society of Applied Physics 41 5676ndash5681

Chen H Wallace G Gupta A Li K Funkhouser T ampCook P (2002) Experiences with scalability of displaywalls Proceedings of IPTVR 2002

Cruz-Neira C Sandin D J amp DeFanti T A (1993)Surrounding-screen projection-based virtual reality Thedesign and implementation of the CAVE Proceedings ofSIGGRAPH 1993 135ndash142

Disz T Papka M Pellegrino M amp Stevens R (1995)Sharing visualization experiences among remote virtual en-vironments Proceedings of the International Workshop onHigh Performance Computing for Computer Graphics andVisualization 135ndash142

Drzaic P S (1995) Liquid Crystal Dispersion World Scien-tific Singapore

Elspass W VanGool L Gross M H Kunz A Meier MSchmitt G Staadt O et al (1999) The BlueCave ETHZurich internal research proposal

Fuchs H Bishop G Arthur K McMillan L Bajcsy RLee S et al (1994) Virtual space teleconferencing using asea of cameras (Tech Rep No TR94-033) Chapel Hill

Kunz and Spagno 191

4 The Components in Detail

41 Mechanical Construction

Since a magnetic tracking system was chosen thebasis of the construction is a wooden platform with atotal height of 540 mm (see Figure 6) The trackingemitter is built into the middle of the wooden platformand thus is not visible to the user The two lower frontloudspeakers and the subwoofer are also built into theplatform The floor of the platform can be easily openedto access all the built-in components The upper struc-ture is built of fiber-reinforced composites (glass andcarbon) These are stiff enough to keep the LC projec-tion panels safely in place and at the same time stillsmall and attractive from a design point of view Cablechannels are built into the beams providing sufficientspace for the cables of all components placed on theupper part of the structure including the cables for theactive projection screens Together with the woodenplatform there is enough space for all the cables andconnectors for the system The chosen materials woodand fiber-reinforced composites assure an accuratefunctioning of the tracking system The cables and theloudspeakers which are at a distance of 15 m from the

emitter of the tracking system do not have any impor-tant influence on its accuracy The results from prelimi-nary tests show that the electrically switched glass panesalso do not interfere with the tracking system

42 Hardware Construction

Figure 7 gives an overview of all components ofthe blue-c and its connection scheme A total of 6 pro-jectors and four monitors are connected to an SGIOnyx The 16 cameras are connected via sixteenFireWire cables to the computer cluster The active pro-jection panels the 6 ferro-electric shutters in front ofthe projectors the active illumination the cameras andthe IR-emitters are synchronized by custom-made syn-chronization electronics The individual componentswill be discussed in the following sections

421 Active Projection Screen Phase dis-persed liquid crystal (PDLC) glass panels offer the re-quired features for a switchable projection screen Theycan be controlled electrically and can be switched in afew milliseconds or less between a transparent and anopaque state Different glass manufacturers have switch-

Figure 5 Triggering timings

182 PRESENCE VOLUME 13 NUMBER 2

able glass panels available with an integrated PDLClayer To change the optical property of a PDLC glasspanel an electrical field has to be applied perpendicularto the PDLC film A voltage in the range of 50 V to100 V is necessary to generate this electrical field ThePDLC glass panels consist of two glass layers coatedwith a transparent electrode on the inside surface and aPDLC film in between Each of the two conductors has

to be contacted by an electrode On the glass panelsused the two copper electrodes are located on the twoshorter edges so as to be invisible when the glass panelsare installed The current flowing through the transpar-ent electrode has to be limited to avoid its overheatingand therefore damaging the PDLC glass panel Themanufacturer recommends not exceeding 300 mA for a2240 mm 950 mm panel with the electrodes placedon the short edges as installed in the blue-c To opti-mize the synchronization electronics the electricalproperties of the panels have to be analyzed

In order to analyze the electrical properties of thePDLC panels the current response to an applied volt-age was measured As with other liquid crystal devicesthe PDLCrsquos should not be exposed to any DC voltagewhich could cause ionic buildup damaging the liquidcrystal structure Therefore only AC measurementswere performed There is also an upper frequency limitdue to current limitations For a voltage of 80 V themaximum frequency is around 100 Hz (Figure 8)

To calculate the electrical behavior of the PDLC glasspanels an equivalent circuit of three components asproposed by Drzaic (1995) is used (Figure 9)

Based on this circuit the complex impedance is

Z R1 R2

1 CR2)2j CR22

1(CR22

This equation has a real and an imaginary part and thusonly two of three variables could be calculated How-ever since more measurements at different frequencies

Figure 6 The mechanical construction of the blue-c

Figure 7 Component overview

Kunz and Spagno 183

were performed an approximation was calculated mini-mizing the mean square error (MSE) between the calcu-lated and the measured voltages The approximated val-ues are

R1 1745 R2 2494 C 11F

The electronic control of the PDLC panes can be ad-justed to eliminate any DC components in the drivingvoltage In addition an external DC decoupling circuithas been added to guarantee that no DC voltage dam-ages the glass panels even if a fault in the driving circuitoccurs This DC decoupling consists of a capacitor CD

and a resistor RD (Figure 10) The values are 60 F forthe capacitor and 10 k for the resistor

The glass panes show a very stable performance overtime We do not observe any significant change either inthe electrical behavior or the stability of the LC filmThe system worked for many days without any prob-

lems although the panes are driven at their upper limitin frequency and allowable current

422 Projectors and Active Shutters Theactive LC projection screens do not support passive ste-reo projection due to their depolarization propertiesTherefore an active projection system was chosen forthe blue-c In the year 2000 when the design and theconstruction of the system were started active stereoDLP projectors were just emerging and still much tooexpensive for our budget The state-of-the-art CRTprojectors were also still quite expensive compared tothe light output Furthermore we were looking for asystem that could be turned black during the cameraacquisition period that is 625 times per second Thedecision was taken to use two three-chip LCD projec-tors (Sanyo XF12 3500 ANSI lumen) per side and ex-ternal LC shutters in front of the projection lens Withthese shutters it is possible to generate an active stereoprojection by alternating the light output of the twoprojectors and blocking it during the picture acquisitionperiod

The two projectors on each side are mounted in arack for a passive stereo projection installation (Figure11) The LC shutters are custom-made ferro-electricliquid crystal shutters with an aperture size of140 mm 140 mm The shutters are mounted in frontof the 121 projection lenses This lens allows projec-tion directly onto the projection wall without any addi-tional mirrors Using no mirrors in the projection pathfacilitates the alignment of the two LCD projectors foractive stereo projection The shutters do not have to besynchronized with the Onyx and projectors thus leav-ing the shutter times completely flexible This principlewould not work with a single-chip DLP projector sincethe sequential generation of the three colors by a ldquocolor

Figure 8 Voltage and current measured on a panel of

2240 mm 950 mm at 100 Hz

Figure 9 Equivalent circuit diagram of the PDLC glass panel

Figure 10 DC decoupling

184 PRESENCE VOLUME 13 NUMBER 2

wheelrdquo would require a complicated synchronization ofthe shutters with the projectors However today 3-chipDLP projectors could be used instead of the two LCprojectors The shutters in front of the projectors haveto be switched on and off at a frequency of approxi-mately 60 Hz

Twisted Nematic (TN) liquid-crystal shutters as usedin shutter glasses can offer a contrast ratio of 10001 orbetter (Lipton 2001) They switch in about 25 msfrom black to transparent and in less than 1 ms fromtransparent to black (Lipton 1991 Woods amp Tan2002 Bougrioua et al 2002) Ferro-electric (FE)liquid-crystal shutters offer about the same contrastratio as TN shutters but much faster switching timesThe FE shutters used in front of the projectors are140 mm 140 mm and offer a contrast ratio of ap-proximately 7501 The switching time from 10 to90 transparency and from 90 to 10 transparency isbelow 100 s (see Figure 12)

The upper measurement in Figure 12 shows the volt-age measured on the shutter The mean value of thisvoltage is zero as required The lower measurementshows the optical response of the shutter measured by aphototransistor and a light source The response is veryfast and guarantees an excellent separation between thetwo stereo channels

A TN shutter switches to black if a voltage is appliedand to transparent if the voltage is removed An FE

shutter in contrast switches to black if a voltage of onepolarity is applied and to transparent if a voltage of theopposite polarity is applied It is important that theshutter not be subject to any DC voltage as it will bedamaged The DC decoupling is achieved by connect-ing a capacitor CD of 1 F in series and a resistor RD of1 k in parallel to the shutter (see also Figure 10)

Mounting the shutters directly in front of the projec-torrsquos lens did not cause temperature problems as ex-pected Even after several hours of projection the tem-perature was beneath the limit tolerated by thetemperature-sensitive devices

423 Active Illumination The requirements onthe blue-c illumination are very demanding For theprojection both the inside and the outside of the pro-jection room have to be dark On the other handbright illumination is important for good-quality imageacquisition The illumination has a big impact on thequality of the extracted silhouettes and acquired tex-tures A person inside the blue-c should be illuminateduniformly from all sides and no shadow should be pro-jected on the floor The illumination has to be directedaway from the PDLC glass panels to avoid contrast-lossand reflections

Figure 11 Stereo projection rack with LC shutters

Figure 12 Ferro-electric LC shutter (driving voltage optical

response)

Kunz and Spagno 185

An active illumination is used to satisfy the needs ofboth projection and image acquisition The active lightis off during projection and on for the image acquisi-tion Therefore the light has to provide illumination foronly a few milliseconds and must be able to turn on andoff with a frequency between 60 and 70 Hz First testswere done with a strobe light A high voltage dischargein a xenon flash lamp produces a bright light pulse of avery short duration Frequencies in the range of 60 Hzare no problem for this type of lamp Despite the fastswitching this type of light has some important disad-vantages First light is emitted for only a few millisec-onds The light intensity integrated over time is there-fore only mediocre In addition the high voltagedischarge in the very short time period causes heavyelectromagnetic pulses which can interfere with sur-rounding electronic components The discharges alsocause a disturbing acoustic noise Finally the strobelight is focused in one spot This is a handicap whentrying to distribute the light uniformly in the projectionroom All these disadvantages led us to look for otherillumination solutions

The requirement of short switching times restricts thenumber of suitable light sources considerably Light-emitting diodes (LEDs) have very short switching timesand are thus suitable for our application Today whiteLEDs are commonly available and the light efficiency isabout twice that of an incandescent bulb (Zorpette2002) LEDs can be easily grouped in clusters to givemore light output The clusters are flat can be of anyshape and the light output can be distributed homoge-neously over the cluster if desired A total of 9984white 5 mm LEDs have been integrated into the blue-cThe opening angle (50 intensity) is 50 degrees and thelight output is approximately 2200 mcd per LEDThese LEDs were the brightest available on the marketin 2002 The LEDs are configured into 32 clusterseach containing 312 LEDs The clusters are reassem-bled in groups of two four or five and mounted inlong aluminum profiles

The LED clusters have to be placed carefully in theblue-c to allow a good quality image acquisition Thecameras are mostly placed outside the blue-c and facemore or less horizontally to the user through the PDLC

glass panels Under this aspect a diffused frontal illumi-nation of the user would be the most suitable But afrontal light would also shine directly into the oppositecameras and furthermore would put too much light onthe glass panels making them appear gray In additionit would be difficult to find a place to mount the clus-ters without interfering with the projection and thecameras

The solution is to distribute the LED clusters alongthe upper and lower edges of the projection room TheLEDs can be adjusted in such a way that they com-pletely illuminate the user minimizing at the same timethe amount of light being directed at the glass panelsand at the opposite cameras The additional mask infront of the LEDs helps to further diminish the amountof scattered light reaching the glass panels Figure 13shows a person between two opposite projection wallsas is the case in the blue-c for the two sidewalls Thelight is coming from all four edges and most parts of theperson are illuminated by more than one light sourceFigure 14 shows one illumination cluster Five of themare integrated in the aluminum profile and placed at theedge between the floor and the left glass panels

Realizing the illumination with the diodes it turnedout that the specifications of the LED were not reachedby every single element This resulted in failures andthus in nonactive LED lines in the cluster After the cur-

Figure 13 Placement of the LED clusters

186 PRESENCE VOLUME 13 NUMBER 2

rent through the entire cluster was reduced this prob-lem disappeared

424 Cameras Sixteen FireWire cameras areplaced around the blue-c on a special aluminum struc-ture The structure is based on standard aluminum ele-ments and offers the maximum flexibility in the place-ment of cameras and other components Each camera isconnected via FireWire to one node of the PC clusterThe signal is amplified every 5 m with a repeater Up tothree repeaters are used for each camera

Dragonflytrade cameras are used since they offer goodsensitivity and a high picture acquisition frequency of upto 15 Hz However cameras with an even higher sensi-tivity would be useful in order to reduce the intensity ofthe flash illumination On the other hand the pictureacquisition frequency is high enough since the bottle-neck is in the processing of the acquired images Initialtests showed that a repetition frequency of 10 Hz seemsto be sufficient for collaborative work

425 Synchronization Electronics The syn-chronization electronics of the blue-c were built modu-larly to offer flexibility in adding and changing compo-nents The main module is the frequency-controlmodule which is equipped with a reprogrammable PIC16F877 microcontroller This module generates thetriggering sequence to drive the other modules of the

synchronization unit Three PDLC glass panel drivermodules two shutter driver modules and one trigger-interface module are integrated into the synchronizationunit together with the synchronization control moduleThe LED driver and IR-emitter driver are placed out-side of the synchronization unit and are triggered by thetrigger interface module

Figure 15 gives an overview of the synchronizationelectronics All modules of the synchronization unit areinterconnected with a data bus In addition a powerbus is integrated into the unit to supply the differentmodules with the required voltages The LED driverand IR-emitter driver are located outside of the syn-chronization unit together with their own power sup-plies to reduce electromagnetic interference caused bythe currents

Using a microcontroller for the synchronization of allcomponents proved to be crucial Since every compo-nent including identical parts behaves slightly differ-ently (eg the glass panes) it is easy to create individu-alized triggering signals using the microcontroller Thismakes up for the increased effort of installing and pro-gramming the processor

426 Frequency Control Module The fre-quency control module is based on a PIC 16F877 mi-crocontroller This microcontroller can be repro-grammed in-circuit that is without unplugging thechip from the board Twenty-two inputndashoutput ports ofthe microcontroller are mapped to the data bus They

Figure 14 An LED cluster of the blue-c

Figure 15 Synchronization electronics

Kunz and Spagno 187

can be set individually as outputs or inputs Each ofthese ports is connected to a LED to show its state Themicrocontroller is clocked with a frequency of 20 MHzto offer high timing accuracy

427 PDLC Glass-Panel Driver Module Todrive the nine PDLC glass panels a total of three glass-panel driver modules are used (see Figure 16) In eachcase three glass panels in parallel are connected to thetwo outputs of a module The two outputs of a modulecan be switched separately to either 8 V or 85 V If bothoutputs are switched to the same value no voltage isapplied to the glass panels If the outputs are switchedto different values a voltage of 77 V or 77 V is ap-plied to the glass panels in series with the decouplingcapacitor Each of the two outputs is switched directlyby the corresponding input coming from the data busA current-limit control is superimposed on the voltageswitching

Figure 16 shows a simplified circuit consisting of aPDLC glass-panel driver module connected to threeglass panels with their DC decoupling circuits Each ofthe two incoming TTL signals switches an optocoupleroutput to a preset voltage of 8 V or 85 V The two out-puts of the optocouplers are connected to two APEXPA46 operational amplifiers The APEX PA64 has anintegrated adjustable current-limit control and can runat voltages up to 150 V and currents up to 5 A Theoperational amplifiers are used as followers thus theinput voltage is mapped directly to the output as long asthe current-limit is not exceeded The PDLC glass-panel driver modules are connected to a 95-V power

supply However the outputs of the module have beenset to switch only between 8 V and 85 V This givesenough margin to the output voltage from the powersupply and ground to avoid saturating the operationalamplifier The current-limit of the operational amplifieris set to 900 mA which corresponds to a current-limitof 300 mA per panel

428 Shutter Driver Module The shutterdriver module also has two inputs and two outputs Itconsists of two independent circuits with an opto-coupler and a half-bridge consisting of two MOSFETsand a half-bridge driver (Figure 17) The optocoupler isused to protect the other modules of the synchroniza-tion electronics in case a fault occurs in this module orin a connected component The output of the opto-coupler is connected to a half-bridge driver (IR2111)that drives two MOSFETs The bridge driver ensuresthat the two MOSFETs in the half-bridge are neverturned on at the same time The two half-bridge out-puts switch the voltage rail-to-rail without any current-limit In order to drive the ferro-electric shutters themodule is operated with a voltage of 12 V Three shut-ters are connected in parallel between the two half-bridges of a module The two half-bridges are alwaysswitched to opposite states The bridge voltage is there-fore either 12 V or 12 V as required by the shuttersfor switching

429 Trigger Interface Module The trigger-interface module is a multifunctional module It allows achoice between up to 8 lines out of the 22 data lines onthe bus Each line can be connected to a 5 V or 12 V

Figure 16 PDLC glass-panel driver moduleFigure 17 Shutter driver module

188 PRESENCE VOLUME 13 NUMBER 2

driver and configured as input or output For the blue-call channels used were configured as 5-V outputs Onechannel is used to trigger the FireWire cameras A sec-ond one is used to trigger the LED illumination Twomore channels are used to synchronize the shutterglasses and are connected to the IR-emitter driver

4210 LED Driver A total of 9984 LEDs areinstalled in different clusters in the blue-c The LEDclusters have to be driven with a voltage of 375 V and atotal current of up to 25 A The power supply for theLED cluster is installed in the floor of the blue-c andconnected directly to the LED clusters This reduces thewiring distances of the high-current connections andthus the probability of electro-magnetic interferenceEach cluster has an integrated MOSFET to switch theLEDs on and off The triggering signal comes directlyfrom the trigger-interface module

4211 Shutter Glasses The LED illuminationis triggered 625 times per second The user does notsee any flickering because this frequency is above thetemporal resolution of the human eye However thebright light inside the blue-c diminishes the contrast ofthe projected image considerably To protect the userfrom the active light an apposite third phase has beenintegrated into the stereo glasses to keep the light outof the userrsquos eyes The projection of the images for theleft and the right eye are followed by a third phase inwhich both LC shutters of the shutter glasses are dark-ened The LED illumination is on only during thisphase This helps to fade out most of the light from theillumination

In the blue-c we use the NuVision 60GX shutterglasses from MacNaughton The shutter glasses and theIR-emitter have a built-in microcontroller We havereprogrammed it to include the third phase Like mostshutter glasses the NuVision use twisted nematic LCshutters The transparent-to-black transition timeof these shutters is quite fast while the black-to-transparent switching time takes considerably longerFigure 18 shows the switching time of the NuVisionshutter glasses The upper measurement shows thetransparency of a shutter where a high value corre-

sponds to a high transparency The lower measurementshows the voltage applied to the IR-emitter As can beseen from the measurement there is no significant delaybetween sending a signal to the emitter and the begin-ning of the switching response However the switchingtime from 10 to 90 transparency takes approximately33 ms while the switching from 90 to 10 transpar-ency takes about 05 ms These values correspond to thevalues measured by Woods and Tan (Woods amp Tan2002) with other shutter glasses For practical use thereis a work-around involving some blanking time betweenthe projections for the left and right eye

5 Overall Setup

Two complete collaboration setups were de-signed as shown in Figure 19 In the upper image inFigure 19 the first prototype is shown essentially a lab-oratory setup One lesson learned from this is that inmany cases only the front screen is used for collabora-tion This allows for the creation of a more design-oriented setup in a public room (as shown in the lowerpart of Figure 19) using only a single screen Since thecamera positions are known from the first prototypethere was no longer any need for a flexible installation

Figure 18 Optical response of the shutter glasses

Kunz and Spagno 189

The goal of the second setup (bottom image in Fig-ure 19) was thus not to reproduce the first installationbut rather to create an asymmetric installation for thecollaboration With the asymmetric setup applicationswere first created transferring 3D sound and 3D imagesof the people from one installation to the other Figure20 shows a simple collaboration between two individu-als engaged in a dialogue Each person is in an installa-tion at a different location on the campus

Since the first system is an experimental setup a longinitialization time was required before the collaborationcould start Thus the system is not used for hands-oncollaboration yet Further work will shorten the initial-ization phase and will allow easier access to the system

6 Conclusions

The active projection screen is driven at its physi-cal limits The transition time from opaque to transpar-ent (10 to 90) takes approximately 2 ms while the

transition from transparent to opaque (90 to 10)takes approximately 6 ms The slow transition fromtransparent to opaque is dictated by the internal relax-ation forces of the LC film and cannot be acceleratedelectrically This non-ideal behavior can be minimizedby optimizing the time pattern of the different compo-nents For the user just the hotspot of the projectorsbecomes a little more evident but it remains at an ac-ceptable level The active projection screens offer a verybright and homogenous image

The junctions between the three projection-screenpanels are visible but not disturbing The LC film of theactive projection panels is installed between two glasspanels The surface of these glass panels is treated withan antireflective coating Nevertheless some reflectionsfrom the other projection screens are evident The re-flection can be compared to the reflection on a CRTtelevision in a medium-bright illuminated room Thereflection is noticeable when the user concentrates on itand disappears as soon as he or she concentrates on theapplication Since the projectors are darkened duringthe transparent state of the projection screens the cam-era acquisition is not disturbed by any reflection Thecameras are positioned in such a way as to see the userrsquosentire body even if the user gets very close to the pro-jection screens

Figure 19 Two different collaboration setups based on the blue-c

principle

Figure 20 Collaboration between two individuals

190 PRESENCE VOLUME 13 NUMBER 2

The channel separation of the stereoscopic projectionis very good and the total performance of the projec-tion is convincing The brightness is higher than tradi-tional CRT stereo projectors at a lower price per stereoprojection rack The six-channel audio system proved tobe very powerful The user can precisely localize a simu-lated noise or sound source The subwoofer located inthe wooden platform is also important for good audioperformance In addition it can transmit vibrations tothe user at lower frequencies and this acoustic stimulusplays an important role in high immersion

The mechanical structure was shown to meet the re-quired demands Its stability and rigidity give a strongsense of confidence in the blue-c The user is never un-easy about touching the projection screen or walkingaround as in some other VR installations The flexibilityin the placement of new components and cables as wellas the quick access to all the components proved to bea very valuable feature of the blue-c construction

7 Further Work

The two setups showed that collaboration over anetwork using 3D representations is technically possibleHowever there are more problems to overcome In itstechnical aspects the system should be downsized in costand weight with the goal being a mobile installation Herenew stereo projection systems would also be tested that areeasier to install and adjust Also new projection materialssuch as active Perspex will be evaluated As a first step inthis direction we will build a demonstrator that would besuitable for presentations on conferences and exhibitionsThe demonstrator could be used to show the core tech-nology of the blue-c itself but also as another input stationfor a multipoint collaboration (Disz Papka Pellegrino ampStevens 1995 Lehner amp DeFanti 1997 Fuchs et al2002 Bailett Calahan amp McCluskey 1998)

Further work on the blue-c will also deal with imple-menting the technology in existing processes for in-stance the product development process or the educa-tion process Thus applications for the blue-c will begenerated that offer benefits to the user when using the

3D capability in collaborative work Further informationcan be found at httpblue-cethzch

Acknowledgments

This project was funded as an internal research project no0-23803-00 by the Swiss Federal Institute of TechnologyZurich We would like to thank all members of the blue-cproject

References

Bailett A Calahan J amp McCluskey S (1998) Coordina-tion at different stages of the product development processRampD Management 28 237ndash247

Bougrioua F Oepts W Vleeschouwer H D AlexanderE Neyts K amp Pauwels H (2002) Determination of theswitching response in reflective twisted nematic liquid crys-tal displays The Japan Society of Applied Physics 41 5676ndash5681

Chen H Wallace G Gupta A Li K Funkhouser T ampCook P (2002) Experiences with scalability of displaywalls Proceedings of IPTVR 2002

Cruz-Neira C Sandin D J amp DeFanti T A (1993)Surrounding-screen projection-based virtual reality Thedesign and implementation of the CAVE Proceedings ofSIGGRAPH 1993 135ndash142

Disz T Papka M Pellegrino M amp Stevens R (1995)Sharing visualization experiences among remote virtual en-vironments Proceedings of the International Workshop onHigh Performance Computing for Computer Graphics andVisualization 135ndash142

Drzaic P S (1995) Liquid Crystal Dispersion World Scien-tific Singapore

Elspass W VanGool L Gross M H Kunz A Meier MSchmitt G Staadt O et al (1999) The BlueCave ETHZurich internal research proposal

Fuchs H Bishop G Arthur K McMillan L Bajcsy RLee S et al (1994) Virtual space teleconferencing using asea of cameras (Tech Rep No TR94-033) Chapel Hill

Kunz and Spagno 191

able glass panels available with an integrated PDLClayer To change the optical property of a PDLC glasspanel an electrical field has to be applied perpendicularto the PDLC film A voltage in the range of 50 V to100 V is necessary to generate this electrical field ThePDLC glass panels consist of two glass layers coatedwith a transparent electrode on the inside surface and aPDLC film in between Each of the two conductors has

to be contacted by an electrode On the glass panelsused the two copper electrodes are located on the twoshorter edges so as to be invisible when the glass panelsare installed The current flowing through the transpar-ent electrode has to be limited to avoid its overheatingand therefore damaging the PDLC glass panel Themanufacturer recommends not exceeding 300 mA for a2240 mm 950 mm panel with the electrodes placedon the short edges as installed in the blue-c To opti-mize the synchronization electronics the electricalproperties of the panels have to be analyzed

In order to analyze the electrical properties of thePDLC panels the current response to an applied volt-age was measured As with other liquid crystal devicesthe PDLCrsquos should not be exposed to any DC voltagewhich could cause ionic buildup damaging the liquidcrystal structure Therefore only AC measurementswere performed There is also an upper frequency limitdue to current limitations For a voltage of 80 V themaximum frequency is around 100 Hz (Figure 8)

To calculate the electrical behavior of the PDLC glasspanels an equivalent circuit of three components asproposed by Drzaic (1995) is used (Figure 9)

Based on this circuit the complex impedance is

Z R1 R2

1 CR2)2j CR22

1(CR22

This equation has a real and an imaginary part and thusonly two of three variables could be calculated How-ever since more measurements at different frequencies

Figure 6 The mechanical construction of the blue-c

Figure 7 Component overview

Kunz and Spagno 183

were performed an approximation was calculated mini-mizing the mean square error (MSE) between the calcu-lated and the measured voltages The approximated val-ues are

R1 1745 R2 2494 C 11F

The electronic control of the PDLC panes can be ad-justed to eliminate any DC components in the drivingvoltage In addition an external DC decoupling circuithas been added to guarantee that no DC voltage dam-ages the glass panels even if a fault in the driving circuitoccurs This DC decoupling consists of a capacitor CD

and a resistor RD (Figure 10) The values are 60 F forthe capacitor and 10 k for the resistor

The glass panes show a very stable performance overtime We do not observe any significant change either inthe electrical behavior or the stability of the LC filmThe system worked for many days without any prob-

lems although the panes are driven at their upper limitin frequency and allowable current

422 Projectors and Active Shutters Theactive LC projection screens do not support passive ste-reo projection due to their depolarization propertiesTherefore an active projection system was chosen forthe blue-c In the year 2000 when the design and theconstruction of the system were started active stereoDLP projectors were just emerging and still much tooexpensive for our budget The state-of-the-art CRTprojectors were also still quite expensive compared tothe light output Furthermore we were looking for asystem that could be turned black during the cameraacquisition period that is 625 times per second Thedecision was taken to use two three-chip LCD projec-tors (Sanyo XF12 3500 ANSI lumen) per side and ex-ternal LC shutters in front of the projection lens Withthese shutters it is possible to generate an active stereoprojection by alternating the light output of the twoprojectors and blocking it during the picture acquisitionperiod

The two projectors on each side are mounted in arack for a passive stereo projection installation (Figure11) The LC shutters are custom-made ferro-electricliquid crystal shutters with an aperture size of140 mm 140 mm The shutters are mounted in frontof the 121 projection lenses This lens allows projec-tion directly onto the projection wall without any addi-tional mirrors Using no mirrors in the projection pathfacilitates the alignment of the two LCD projectors foractive stereo projection The shutters do not have to besynchronized with the Onyx and projectors thus leav-ing the shutter times completely flexible This principlewould not work with a single-chip DLP projector sincethe sequential generation of the three colors by a ldquocolor

Figure 8 Voltage and current measured on a panel of

2240 mm 950 mm at 100 Hz

Figure 9 Equivalent circuit diagram of the PDLC glass panel

Figure 10 DC decoupling

184 PRESENCE VOLUME 13 NUMBER 2

wheelrdquo would require a complicated synchronization ofthe shutters with the projectors However today 3-chipDLP projectors could be used instead of the two LCprojectors The shutters in front of the projectors haveto be switched on and off at a frequency of approxi-mately 60 Hz

Twisted Nematic (TN) liquid-crystal shutters as usedin shutter glasses can offer a contrast ratio of 10001 orbetter (Lipton 2001) They switch in about 25 msfrom black to transparent and in less than 1 ms fromtransparent to black (Lipton 1991 Woods amp Tan2002 Bougrioua et al 2002) Ferro-electric (FE)liquid-crystal shutters offer about the same contrastratio as TN shutters but much faster switching timesThe FE shutters used in front of the projectors are140 mm 140 mm and offer a contrast ratio of ap-proximately 7501 The switching time from 10 to90 transparency and from 90 to 10 transparency isbelow 100 s (see Figure 12)

The upper measurement in Figure 12 shows the volt-age measured on the shutter The mean value of thisvoltage is zero as required The lower measurementshows the optical response of the shutter measured by aphototransistor and a light source The response is veryfast and guarantees an excellent separation between thetwo stereo channels

A TN shutter switches to black if a voltage is appliedand to transparent if the voltage is removed An FE

shutter in contrast switches to black if a voltage of onepolarity is applied and to transparent if a voltage of theopposite polarity is applied It is important that theshutter not be subject to any DC voltage as it will bedamaged The DC decoupling is achieved by connect-ing a capacitor CD of 1 F in series and a resistor RD of1 k in parallel to the shutter (see also Figure 10)

Mounting the shutters directly in front of the projec-torrsquos lens did not cause temperature problems as ex-pected Even after several hours of projection the tem-perature was beneath the limit tolerated by thetemperature-sensitive devices

423 Active Illumination The requirements onthe blue-c illumination are very demanding For theprojection both the inside and the outside of the pro-jection room have to be dark On the other handbright illumination is important for good-quality imageacquisition The illumination has a big impact on thequality of the extracted silhouettes and acquired tex-tures A person inside the blue-c should be illuminateduniformly from all sides and no shadow should be pro-jected on the floor The illumination has to be directedaway from the PDLC glass panels to avoid contrast-lossand reflections

Figure 11 Stereo projection rack with LC shutters

Figure 12 Ferro-electric LC shutter (driving voltage optical

response)

Kunz and Spagno 185

An active illumination is used to satisfy the needs ofboth projection and image acquisition The active lightis off during projection and on for the image acquisi-tion Therefore the light has to provide illumination foronly a few milliseconds and must be able to turn on andoff with a frequency between 60 and 70 Hz First testswere done with a strobe light A high voltage dischargein a xenon flash lamp produces a bright light pulse of avery short duration Frequencies in the range of 60 Hzare no problem for this type of lamp Despite the fastswitching this type of light has some important disad-vantages First light is emitted for only a few millisec-onds The light intensity integrated over time is there-fore only mediocre In addition the high voltagedischarge in the very short time period causes heavyelectromagnetic pulses which can interfere with sur-rounding electronic components The discharges alsocause a disturbing acoustic noise Finally the strobelight is focused in one spot This is a handicap whentrying to distribute the light uniformly in the projectionroom All these disadvantages led us to look for otherillumination solutions

The requirement of short switching times restricts thenumber of suitable light sources considerably Light-emitting diodes (LEDs) have very short switching timesand are thus suitable for our application Today whiteLEDs are commonly available and the light efficiency isabout twice that of an incandescent bulb (Zorpette2002) LEDs can be easily grouped in clusters to givemore light output The clusters are flat can be of anyshape and the light output can be distributed homoge-neously over the cluster if desired A total of 9984white 5 mm LEDs have been integrated into the blue-cThe opening angle (50 intensity) is 50 degrees and thelight output is approximately 2200 mcd per LEDThese LEDs were the brightest available on the marketin 2002 The LEDs are configured into 32 clusterseach containing 312 LEDs The clusters are reassem-bled in groups of two four or five and mounted inlong aluminum profiles

The LED clusters have to be placed carefully in theblue-c to allow a good quality image acquisition Thecameras are mostly placed outside the blue-c and facemore or less horizontally to the user through the PDLC

glass panels Under this aspect a diffused frontal illumi-nation of the user would be the most suitable But afrontal light would also shine directly into the oppositecameras and furthermore would put too much light onthe glass panels making them appear gray In additionit would be difficult to find a place to mount the clus-ters without interfering with the projection and thecameras

The solution is to distribute the LED clusters alongthe upper and lower edges of the projection room TheLEDs can be adjusted in such a way that they com-pletely illuminate the user minimizing at the same timethe amount of light being directed at the glass panelsand at the opposite cameras The additional mask infront of the LEDs helps to further diminish the amountof scattered light reaching the glass panels Figure 13shows a person between two opposite projection wallsas is the case in the blue-c for the two sidewalls Thelight is coming from all four edges and most parts of theperson are illuminated by more than one light sourceFigure 14 shows one illumination cluster Five of themare integrated in the aluminum profile and placed at theedge between the floor and the left glass panels

Realizing the illumination with the diodes it turnedout that the specifications of the LED were not reachedby every single element This resulted in failures andthus in nonactive LED lines in the cluster After the cur-

Figure 13 Placement of the LED clusters

186 PRESENCE VOLUME 13 NUMBER 2

rent through the entire cluster was reduced this prob-lem disappeared

424 Cameras Sixteen FireWire cameras areplaced around the blue-c on a special aluminum struc-ture The structure is based on standard aluminum ele-ments and offers the maximum flexibility in the place-ment of cameras and other components Each camera isconnected via FireWire to one node of the PC clusterThe signal is amplified every 5 m with a repeater Up tothree repeaters are used for each camera

Dragonflytrade cameras are used since they offer goodsensitivity and a high picture acquisition frequency of upto 15 Hz However cameras with an even higher sensi-tivity would be useful in order to reduce the intensity ofthe flash illumination On the other hand the pictureacquisition frequency is high enough since the bottle-neck is in the processing of the acquired images Initialtests showed that a repetition frequency of 10 Hz seemsto be sufficient for collaborative work

425 Synchronization Electronics The syn-chronization electronics of the blue-c were built modu-larly to offer flexibility in adding and changing compo-nents The main module is the frequency-controlmodule which is equipped with a reprogrammable PIC16F877 microcontroller This module generates thetriggering sequence to drive the other modules of the

synchronization unit Three PDLC glass panel drivermodules two shutter driver modules and one trigger-interface module are integrated into the synchronizationunit together with the synchronization control moduleThe LED driver and IR-emitter driver are placed out-side of the synchronization unit and are triggered by thetrigger interface module

Figure 15 gives an overview of the synchronizationelectronics All modules of the synchronization unit areinterconnected with a data bus In addition a powerbus is integrated into the unit to supply the differentmodules with the required voltages The LED driverand IR-emitter driver are located outside of the syn-chronization unit together with their own power sup-plies to reduce electromagnetic interference caused bythe currents

Using a microcontroller for the synchronization of allcomponents proved to be crucial Since every compo-nent including identical parts behaves slightly differ-ently (eg the glass panes) it is easy to create individu-alized triggering signals using the microcontroller Thismakes up for the increased effort of installing and pro-gramming the processor

426 Frequency Control Module The fre-quency control module is based on a PIC 16F877 mi-crocontroller This microcontroller can be repro-grammed in-circuit that is without unplugging thechip from the board Twenty-two inputndashoutput ports ofthe microcontroller are mapped to the data bus They

Figure 14 An LED cluster of the blue-c

Figure 15 Synchronization electronics

Kunz and Spagno 187

can be set individually as outputs or inputs Each ofthese ports is connected to a LED to show its state Themicrocontroller is clocked with a frequency of 20 MHzto offer high timing accuracy

427 PDLC Glass-Panel Driver Module Todrive the nine PDLC glass panels a total of three glass-panel driver modules are used (see Figure 16) In eachcase three glass panels in parallel are connected to thetwo outputs of a module The two outputs of a modulecan be switched separately to either 8 V or 85 V If bothoutputs are switched to the same value no voltage isapplied to the glass panels If the outputs are switchedto different values a voltage of 77 V or 77 V is ap-plied to the glass panels in series with the decouplingcapacitor Each of the two outputs is switched directlyby the corresponding input coming from the data busA current-limit control is superimposed on the voltageswitching

Figure 16 shows a simplified circuit consisting of aPDLC glass-panel driver module connected to threeglass panels with their DC decoupling circuits Each ofthe two incoming TTL signals switches an optocoupleroutput to a preset voltage of 8 V or 85 V The two out-puts of the optocouplers are connected to two APEXPA46 operational amplifiers The APEX PA64 has anintegrated adjustable current-limit control and can runat voltages up to 150 V and currents up to 5 A Theoperational amplifiers are used as followers thus theinput voltage is mapped directly to the output as long asthe current-limit is not exceeded The PDLC glass-panel driver modules are connected to a 95-V power

supply However the outputs of the module have beenset to switch only between 8 V and 85 V This givesenough margin to the output voltage from the powersupply and ground to avoid saturating the operationalamplifier The current-limit of the operational amplifieris set to 900 mA which corresponds to a current-limitof 300 mA per panel

428 Shutter Driver Module The shutterdriver module also has two inputs and two outputs Itconsists of two independent circuits with an opto-coupler and a half-bridge consisting of two MOSFETsand a half-bridge driver (Figure 17) The optocoupler isused to protect the other modules of the synchroniza-tion electronics in case a fault occurs in this module orin a connected component The output of the opto-coupler is connected to a half-bridge driver (IR2111)that drives two MOSFETs The bridge driver ensuresthat the two MOSFETs in the half-bridge are neverturned on at the same time The two half-bridge out-puts switch the voltage rail-to-rail without any current-limit In order to drive the ferro-electric shutters themodule is operated with a voltage of 12 V Three shut-ters are connected in parallel between the two half-bridges of a module The two half-bridges are alwaysswitched to opposite states The bridge voltage is there-fore either 12 V or 12 V as required by the shuttersfor switching

429 Trigger Interface Module The trigger-interface module is a multifunctional module It allows achoice between up to 8 lines out of the 22 data lines onthe bus Each line can be connected to a 5 V or 12 V

Figure 16 PDLC glass-panel driver moduleFigure 17 Shutter driver module

188 PRESENCE VOLUME 13 NUMBER 2

driver and configured as input or output For the blue-call channels used were configured as 5-V outputs Onechannel is used to trigger the FireWire cameras A sec-ond one is used to trigger the LED illumination Twomore channels are used to synchronize the shutterglasses and are connected to the IR-emitter driver

4210 LED Driver A total of 9984 LEDs areinstalled in different clusters in the blue-c The LEDclusters have to be driven with a voltage of 375 V and atotal current of up to 25 A The power supply for theLED cluster is installed in the floor of the blue-c andconnected directly to the LED clusters This reduces thewiring distances of the high-current connections andthus the probability of electro-magnetic interferenceEach cluster has an integrated MOSFET to switch theLEDs on and off The triggering signal comes directlyfrom the trigger-interface module

4211 Shutter Glasses The LED illuminationis triggered 625 times per second The user does notsee any flickering because this frequency is above thetemporal resolution of the human eye However thebright light inside the blue-c diminishes the contrast ofthe projected image considerably To protect the userfrom the active light an apposite third phase has beenintegrated into the stereo glasses to keep the light outof the userrsquos eyes The projection of the images for theleft and the right eye are followed by a third phase inwhich both LC shutters of the shutter glasses are dark-ened The LED illumination is on only during thisphase This helps to fade out most of the light from theillumination

In the blue-c we use the NuVision 60GX shutterglasses from MacNaughton The shutter glasses and theIR-emitter have a built-in microcontroller We havereprogrammed it to include the third phase Like mostshutter glasses the NuVision use twisted nematic LCshutters The transparent-to-black transition timeof these shutters is quite fast while the black-to-transparent switching time takes considerably longerFigure 18 shows the switching time of the NuVisionshutter glasses The upper measurement shows thetransparency of a shutter where a high value corre-

sponds to a high transparency The lower measurementshows the voltage applied to the IR-emitter As can beseen from the measurement there is no significant delaybetween sending a signal to the emitter and the begin-ning of the switching response However the switchingtime from 10 to 90 transparency takes approximately33 ms while the switching from 90 to 10 transpar-ency takes about 05 ms These values correspond to thevalues measured by Woods and Tan (Woods amp Tan2002) with other shutter glasses For practical use thereis a work-around involving some blanking time betweenthe projections for the left and right eye

5 Overall Setup

Two complete collaboration setups were de-signed as shown in Figure 19 In the upper image inFigure 19 the first prototype is shown essentially a lab-oratory setup One lesson learned from this is that inmany cases only the front screen is used for collabora-tion This allows for the creation of a more design-oriented setup in a public room (as shown in the lowerpart of Figure 19) using only a single screen Since thecamera positions are known from the first prototypethere was no longer any need for a flexible installation

Figure 18 Optical response of the shutter glasses

Kunz and Spagno 189

The goal of the second setup (bottom image in Fig-ure 19) was thus not to reproduce the first installationbut rather to create an asymmetric installation for thecollaboration With the asymmetric setup applicationswere first created transferring 3D sound and 3D imagesof the people from one installation to the other Figure20 shows a simple collaboration between two individu-als engaged in a dialogue Each person is in an installa-tion at a different location on the campus

Since the first system is an experimental setup a longinitialization time was required before the collaborationcould start Thus the system is not used for hands-oncollaboration yet Further work will shorten the initial-ization phase and will allow easier access to the system

6 Conclusions

The active projection screen is driven at its physi-cal limits The transition time from opaque to transpar-ent (10 to 90) takes approximately 2 ms while the

transition from transparent to opaque (90 to 10)takes approximately 6 ms The slow transition fromtransparent to opaque is dictated by the internal relax-ation forces of the LC film and cannot be acceleratedelectrically This non-ideal behavior can be minimizedby optimizing the time pattern of the different compo-nents For the user just the hotspot of the projectorsbecomes a little more evident but it remains at an ac-ceptable level The active projection screens offer a verybright and homogenous image

The junctions between the three projection-screenpanels are visible but not disturbing The LC film of theactive projection panels is installed between two glasspanels The surface of these glass panels is treated withan antireflective coating Nevertheless some reflectionsfrom the other projection screens are evident The re-flection can be compared to the reflection on a CRTtelevision in a medium-bright illuminated room Thereflection is noticeable when the user concentrates on itand disappears as soon as he or she concentrates on theapplication Since the projectors are darkened duringthe transparent state of the projection screens the cam-era acquisition is not disturbed by any reflection Thecameras are positioned in such a way as to see the userrsquosentire body even if the user gets very close to the pro-jection screens

Figure 19 Two different collaboration setups based on the blue-c

principle

Figure 20 Collaboration between two individuals

190 PRESENCE VOLUME 13 NUMBER 2

The channel separation of the stereoscopic projectionis very good and the total performance of the projec-tion is convincing The brightness is higher than tradi-tional CRT stereo projectors at a lower price per stereoprojection rack The six-channel audio system proved tobe very powerful The user can precisely localize a simu-lated noise or sound source The subwoofer located inthe wooden platform is also important for good audioperformance In addition it can transmit vibrations tothe user at lower frequencies and this acoustic stimulusplays an important role in high immersion

The mechanical structure was shown to meet the re-quired demands Its stability and rigidity give a strongsense of confidence in the blue-c The user is never un-easy about touching the projection screen or walkingaround as in some other VR installations The flexibilityin the placement of new components and cables as wellas the quick access to all the components proved to bea very valuable feature of the blue-c construction

7 Further Work

The two setups showed that collaboration over anetwork using 3D representations is technically possibleHowever there are more problems to overcome In itstechnical aspects the system should be downsized in costand weight with the goal being a mobile installation Herenew stereo projection systems would also be tested that areeasier to install and adjust Also new projection materialssuch as active Perspex will be evaluated As a first step inthis direction we will build a demonstrator that would besuitable for presentations on conferences and exhibitionsThe demonstrator could be used to show the core tech-nology of the blue-c itself but also as another input stationfor a multipoint collaboration (Disz Papka Pellegrino ampStevens 1995 Lehner amp DeFanti 1997 Fuchs et al2002 Bailett Calahan amp McCluskey 1998)

Further work on the blue-c will also deal with imple-menting the technology in existing processes for in-stance the product development process or the educa-tion process Thus applications for the blue-c will begenerated that offer benefits to the user when using the

3D capability in collaborative work Further informationcan be found at httpblue-cethzch

Acknowledgments

This project was funded as an internal research project no0-23803-00 by the Swiss Federal Institute of TechnologyZurich We would like to thank all members of the blue-cproject

References

Bailett A Calahan J amp McCluskey S (1998) Coordina-tion at different stages of the product development processRampD Management 28 237ndash247

Bougrioua F Oepts W Vleeschouwer H D AlexanderE Neyts K amp Pauwels H (2002) Determination of theswitching response in reflective twisted nematic liquid crys-tal displays The Japan Society of Applied Physics 41 5676ndash5681

Chen H Wallace G Gupta A Li K Funkhouser T ampCook P (2002) Experiences with scalability of displaywalls Proceedings of IPTVR 2002

Cruz-Neira C Sandin D J amp DeFanti T A (1993)Surrounding-screen projection-based virtual reality Thedesign and implementation of the CAVE Proceedings ofSIGGRAPH 1993 135ndash142

Disz T Papka M Pellegrino M amp Stevens R (1995)Sharing visualization experiences among remote virtual en-vironments Proceedings of the International Workshop onHigh Performance Computing for Computer Graphics andVisualization 135ndash142

Drzaic P S (1995) Liquid Crystal Dispersion World Scien-tific Singapore

Elspass W VanGool L Gross M H Kunz A Meier MSchmitt G Staadt O et al (1999) The BlueCave ETHZurich internal research proposal

Fuchs H Bishop G Arthur K McMillan L Bajcsy RLee S et al (1994) Virtual space teleconferencing using asea of cameras (Tech Rep No TR94-033) Chapel Hill

Kunz and Spagno 191

were performed an approximation was calculated mini-mizing the mean square error (MSE) between the calcu-lated and the measured voltages The approximated val-ues are

R1 1745 R2 2494 C 11F

The electronic control of the PDLC panes can be ad-justed to eliminate any DC components in the drivingvoltage In addition an external DC decoupling circuithas been added to guarantee that no DC voltage dam-ages the glass panels even if a fault in the driving circuitoccurs This DC decoupling consists of a capacitor CD

and a resistor RD (Figure 10) The values are 60 F forthe capacitor and 10 k for the resistor

The glass panes show a very stable performance overtime We do not observe any significant change either inthe electrical behavior or the stability of the LC filmThe system worked for many days without any prob-

lems although the panes are driven at their upper limitin frequency and allowable current

422 Projectors and Active Shutters Theactive LC projection screens do not support passive ste-reo projection due to their depolarization propertiesTherefore an active projection system was chosen forthe blue-c In the year 2000 when the design and theconstruction of the system were started active stereoDLP projectors were just emerging and still much tooexpensive for our budget The state-of-the-art CRTprojectors were also still quite expensive compared tothe light output Furthermore we were looking for asystem that could be turned black during the cameraacquisition period that is 625 times per second Thedecision was taken to use two three-chip LCD projec-tors (Sanyo XF12 3500 ANSI lumen) per side and ex-ternal LC shutters in front of the projection lens Withthese shutters it is possible to generate an active stereoprojection by alternating the light output of the twoprojectors and blocking it during the picture acquisitionperiod

The two projectors on each side are mounted in arack for a passive stereo projection installation (Figure11) The LC shutters are custom-made ferro-electricliquid crystal shutters with an aperture size of140 mm 140 mm The shutters are mounted in frontof the 121 projection lenses This lens allows projec-tion directly onto the projection wall without any addi-tional mirrors Using no mirrors in the projection pathfacilitates the alignment of the two LCD projectors foractive stereo projection The shutters do not have to besynchronized with the Onyx and projectors thus leav-ing the shutter times completely flexible This principlewould not work with a single-chip DLP projector sincethe sequential generation of the three colors by a ldquocolor

Figure 8 Voltage and current measured on a panel of

2240 mm 950 mm at 100 Hz

Figure 9 Equivalent circuit diagram of the PDLC glass panel

Figure 10 DC decoupling

184 PRESENCE VOLUME 13 NUMBER 2

wheelrdquo would require a complicated synchronization ofthe shutters with the projectors However today 3-chipDLP projectors could be used instead of the two LCprojectors The shutters in front of the projectors haveto be switched on and off at a frequency of approxi-mately 60 Hz

Twisted Nematic (TN) liquid-crystal shutters as usedin shutter glasses can offer a contrast ratio of 10001 orbetter (Lipton 2001) They switch in about 25 msfrom black to transparent and in less than 1 ms fromtransparent to black (Lipton 1991 Woods amp Tan2002 Bougrioua et al 2002) Ferro-electric (FE)liquid-crystal shutters offer about the same contrastratio as TN shutters but much faster switching timesThe FE shutters used in front of the projectors are140 mm 140 mm and offer a contrast ratio of ap-proximately 7501 The switching time from 10 to90 transparency and from 90 to 10 transparency isbelow 100 s (see Figure 12)

The upper measurement in Figure 12 shows the volt-age measured on the shutter The mean value of thisvoltage is zero as required The lower measurementshows the optical response of the shutter measured by aphototransistor and a light source The response is veryfast and guarantees an excellent separation between thetwo stereo channels

A TN shutter switches to black if a voltage is appliedand to transparent if the voltage is removed An FE

shutter in contrast switches to black if a voltage of onepolarity is applied and to transparent if a voltage of theopposite polarity is applied It is important that theshutter not be subject to any DC voltage as it will bedamaged The DC decoupling is achieved by connect-ing a capacitor CD of 1 F in series and a resistor RD of1 k in parallel to the shutter (see also Figure 10)

Mounting the shutters directly in front of the projec-torrsquos lens did not cause temperature problems as ex-pected Even after several hours of projection the tem-perature was beneath the limit tolerated by thetemperature-sensitive devices

423 Active Illumination The requirements onthe blue-c illumination are very demanding For theprojection both the inside and the outside of the pro-jection room have to be dark On the other handbright illumination is important for good-quality imageacquisition The illumination has a big impact on thequality of the extracted silhouettes and acquired tex-tures A person inside the blue-c should be illuminateduniformly from all sides and no shadow should be pro-jected on the floor The illumination has to be directedaway from the PDLC glass panels to avoid contrast-lossand reflections

Figure 11 Stereo projection rack with LC shutters

Figure 12 Ferro-electric LC shutter (driving voltage optical

response)

Kunz and Spagno 185

An active illumination is used to satisfy the needs ofboth projection and image acquisition The active lightis off during projection and on for the image acquisi-tion Therefore the light has to provide illumination foronly a few milliseconds and must be able to turn on andoff with a frequency between 60 and 70 Hz First testswere done with a strobe light A high voltage dischargein a xenon flash lamp produces a bright light pulse of avery short duration Frequencies in the range of 60 Hzare no problem for this type of lamp Despite the fastswitching this type of light has some important disad-vantages First light is emitted for only a few millisec-onds The light intensity integrated over time is there-fore only mediocre In addition the high voltagedischarge in the very short time period causes heavyelectromagnetic pulses which can interfere with sur-rounding electronic components The discharges alsocause a disturbing acoustic noise Finally the strobelight is focused in one spot This is a handicap whentrying to distribute the light uniformly in the projectionroom All these disadvantages led us to look for otherillumination solutions

The requirement of short switching times restricts thenumber of suitable light sources considerably Light-emitting diodes (LEDs) have very short switching timesand are thus suitable for our application Today whiteLEDs are commonly available and the light efficiency isabout twice that of an incandescent bulb (Zorpette2002) LEDs can be easily grouped in clusters to givemore light output The clusters are flat can be of anyshape and the light output can be distributed homoge-neously over the cluster if desired A total of 9984white 5 mm LEDs have been integrated into the blue-cThe opening angle (50 intensity) is 50 degrees and thelight output is approximately 2200 mcd per LEDThese LEDs were the brightest available on the marketin 2002 The LEDs are configured into 32 clusterseach containing 312 LEDs The clusters are reassem-bled in groups of two four or five and mounted inlong aluminum profiles

The LED clusters have to be placed carefully in theblue-c to allow a good quality image acquisition Thecameras are mostly placed outside the blue-c and facemore or less horizontally to the user through the PDLC

glass panels Under this aspect a diffused frontal illumi-nation of the user would be the most suitable But afrontal light would also shine directly into the oppositecameras and furthermore would put too much light onthe glass panels making them appear gray In additionit would be difficult to find a place to mount the clus-ters without interfering with the projection and thecameras

The solution is to distribute the LED clusters alongthe upper and lower edges of the projection room TheLEDs can be adjusted in such a way that they com-pletely illuminate the user minimizing at the same timethe amount of light being directed at the glass panelsand at the opposite cameras The additional mask infront of the LEDs helps to further diminish the amountof scattered light reaching the glass panels Figure 13shows a person between two opposite projection wallsas is the case in the blue-c for the two sidewalls Thelight is coming from all four edges and most parts of theperson are illuminated by more than one light sourceFigure 14 shows one illumination cluster Five of themare integrated in the aluminum profile and placed at theedge between the floor and the left glass panels

Realizing the illumination with the diodes it turnedout that the specifications of the LED were not reachedby every single element This resulted in failures andthus in nonactive LED lines in the cluster After the cur-

Figure 13 Placement of the LED clusters

186 PRESENCE VOLUME 13 NUMBER 2

rent through the entire cluster was reduced this prob-lem disappeared

424 Cameras Sixteen FireWire cameras areplaced around the blue-c on a special aluminum struc-ture The structure is based on standard aluminum ele-ments and offers the maximum flexibility in the place-ment of cameras and other components Each camera isconnected via FireWire to one node of the PC clusterThe signal is amplified every 5 m with a repeater Up tothree repeaters are used for each camera

Dragonflytrade cameras are used since they offer goodsensitivity and a high picture acquisition frequency of upto 15 Hz However cameras with an even higher sensi-tivity would be useful in order to reduce the intensity ofthe flash illumination On the other hand the pictureacquisition frequency is high enough since the bottle-neck is in the processing of the acquired images Initialtests showed that a repetition frequency of 10 Hz seemsto be sufficient for collaborative work

425 Synchronization Electronics The syn-chronization electronics of the blue-c were built modu-larly to offer flexibility in adding and changing compo-nents The main module is the frequency-controlmodule which is equipped with a reprogrammable PIC16F877 microcontroller This module generates thetriggering sequence to drive the other modules of the

synchronization unit Three PDLC glass panel drivermodules two shutter driver modules and one trigger-interface module are integrated into the synchronizationunit together with the synchronization control moduleThe LED driver and IR-emitter driver are placed out-side of the synchronization unit and are triggered by thetrigger interface module

Figure 15 gives an overview of the synchronizationelectronics All modules of the synchronization unit areinterconnected with a data bus In addition a powerbus is integrated into the unit to supply the differentmodules with the required voltages The LED driverand IR-emitter driver are located outside of the syn-chronization unit together with their own power sup-plies to reduce electromagnetic interference caused bythe currents

Using a microcontroller for the synchronization of allcomponents proved to be crucial Since every compo-nent including identical parts behaves slightly differ-ently (eg the glass panes) it is easy to create individu-alized triggering signals using the microcontroller Thismakes up for the increased effort of installing and pro-gramming the processor

426 Frequency Control Module The fre-quency control module is based on a PIC 16F877 mi-crocontroller This microcontroller can be repro-grammed in-circuit that is without unplugging thechip from the board Twenty-two inputndashoutput ports ofthe microcontroller are mapped to the data bus They

Figure 14 An LED cluster of the blue-c

Figure 15 Synchronization electronics

Kunz and Spagno 187

can be set individually as outputs or inputs Each ofthese ports is connected to a LED to show its state Themicrocontroller is clocked with a frequency of 20 MHzto offer high timing accuracy

427 PDLC Glass-Panel Driver Module Todrive the nine PDLC glass panels a total of three glass-panel driver modules are used (see Figure 16) In eachcase three glass panels in parallel are connected to thetwo outputs of a module The two outputs of a modulecan be switched separately to either 8 V or 85 V If bothoutputs are switched to the same value no voltage isapplied to the glass panels If the outputs are switchedto different values a voltage of 77 V or 77 V is ap-plied to the glass panels in series with the decouplingcapacitor Each of the two outputs is switched directlyby the corresponding input coming from the data busA current-limit control is superimposed on the voltageswitching

Figure 16 shows a simplified circuit consisting of aPDLC glass-panel driver module connected to threeglass panels with their DC decoupling circuits Each ofthe two incoming TTL signals switches an optocoupleroutput to a preset voltage of 8 V or 85 V The two out-puts of the optocouplers are connected to two APEXPA46 operational amplifiers The APEX PA64 has anintegrated adjustable current-limit control and can runat voltages up to 150 V and currents up to 5 A Theoperational amplifiers are used as followers thus theinput voltage is mapped directly to the output as long asthe current-limit is not exceeded The PDLC glass-panel driver modules are connected to a 95-V power

supply However the outputs of the module have beenset to switch only between 8 V and 85 V This givesenough margin to the output voltage from the powersupply and ground to avoid saturating the operationalamplifier The current-limit of the operational amplifieris set to 900 mA which corresponds to a current-limitof 300 mA per panel

428 Shutter Driver Module The shutterdriver module also has two inputs and two outputs Itconsists of two independent circuits with an opto-coupler and a half-bridge consisting of two MOSFETsand a half-bridge driver (Figure 17) The optocoupler isused to protect the other modules of the synchroniza-tion electronics in case a fault occurs in this module orin a connected component The output of the opto-coupler is connected to a half-bridge driver (IR2111)that drives two MOSFETs The bridge driver ensuresthat the two MOSFETs in the half-bridge are neverturned on at the same time The two half-bridge out-puts switch the voltage rail-to-rail without any current-limit In order to drive the ferro-electric shutters themodule is operated with a voltage of 12 V Three shut-ters are connected in parallel between the two half-bridges of a module The two half-bridges are alwaysswitched to opposite states The bridge voltage is there-fore either 12 V or 12 V as required by the shuttersfor switching

429 Trigger Interface Module The trigger-interface module is a multifunctional module It allows achoice between up to 8 lines out of the 22 data lines onthe bus Each line can be connected to a 5 V or 12 V

Figure 16 PDLC glass-panel driver moduleFigure 17 Shutter driver module

188 PRESENCE VOLUME 13 NUMBER 2

driver and configured as input or output For the blue-call channels used were configured as 5-V outputs Onechannel is used to trigger the FireWire cameras A sec-ond one is used to trigger the LED illumination Twomore channels are used to synchronize the shutterglasses and are connected to the IR-emitter driver

4210 LED Driver A total of 9984 LEDs areinstalled in different clusters in the blue-c The LEDclusters have to be driven with a voltage of 375 V and atotal current of up to 25 A The power supply for theLED cluster is installed in the floor of the blue-c andconnected directly to the LED clusters This reduces thewiring distances of the high-current connections andthus the probability of electro-magnetic interferenceEach cluster has an integrated MOSFET to switch theLEDs on and off The triggering signal comes directlyfrom the trigger-interface module

4211 Shutter Glasses The LED illuminationis triggered 625 times per second The user does notsee any flickering because this frequency is above thetemporal resolution of the human eye However thebright light inside the blue-c diminishes the contrast ofthe projected image considerably To protect the userfrom the active light an apposite third phase has beenintegrated into the stereo glasses to keep the light outof the userrsquos eyes The projection of the images for theleft and the right eye are followed by a third phase inwhich both LC shutters of the shutter glasses are dark-ened The LED illumination is on only during thisphase This helps to fade out most of the light from theillumination

In the blue-c we use the NuVision 60GX shutterglasses from MacNaughton The shutter glasses and theIR-emitter have a built-in microcontroller We havereprogrammed it to include the third phase Like mostshutter glasses the NuVision use twisted nematic LCshutters The transparent-to-black transition timeof these shutters is quite fast while the black-to-transparent switching time takes considerably longerFigure 18 shows the switching time of the NuVisionshutter glasses The upper measurement shows thetransparency of a shutter where a high value corre-

sponds to a high transparency The lower measurementshows the voltage applied to the IR-emitter As can beseen from the measurement there is no significant delaybetween sending a signal to the emitter and the begin-ning of the switching response However the switchingtime from 10 to 90 transparency takes approximately33 ms while the switching from 90 to 10 transpar-ency takes about 05 ms These values correspond to thevalues measured by Woods and Tan (Woods amp Tan2002) with other shutter glasses For practical use thereis a work-around involving some blanking time betweenthe projections for the left and right eye

5 Overall Setup

Two complete collaboration setups were de-signed as shown in Figure 19 In the upper image inFigure 19 the first prototype is shown essentially a lab-oratory setup One lesson learned from this is that inmany cases only the front screen is used for collabora-tion This allows for the creation of a more design-oriented setup in a public room (as shown in the lowerpart of Figure 19) using only a single screen Since thecamera positions are known from the first prototypethere was no longer any need for a flexible installation

Figure 18 Optical response of the shutter glasses

Kunz and Spagno 189

The goal of the second setup (bottom image in Fig-ure 19) was thus not to reproduce the first installationbut rather to create an asymmetric installation for thecollaboration With the asymmetric setup applicationswere first created transferring 3D sound and 3D imagesof the people from one installation to the other Figure20 shows a simple collaboration between two individu-als engaged in a dialogue Each person is in an installa-tion at a different location on the campus

Since the first system is an experimental setup a longinitialization time was required before the collaborationcould start Thus the system is not used for hands-oncollaboration yet Further work will shorten the initial-ization phase and will allow easier access to the system

6 Conclusions

The active projection screen is driven at its physi-cal limits The transition time from opaque to transpar-ent (10 to 90) takes approximately 2 ms while the

transition from transparent to opaque (90 to 10)takes approximately 6 ms The slow transition fromtransparent to opaque is dictated by the internal relax-ation forces of the LC film and cannot be acceleratedelectrically This non-ideal behavior can be minimizedby optimizing the time pattern of the different compo-nents For the user just the hotspot of the projectorsbecomes a little more evident but it remains at an ac-ceptable level The active projection screens offer a verybright and homogenous image

The junctions between the three projection-screenpanels are visible but not disturbing The LC film of theactive projection panels is installed between two glasspanels The surface of these glass panels is treated withan antireflective coating Nevertheless some reflectionsfrom the other projection screens are evident The re-flection can be compared to the reflection on a CRTtelevision in a medium-bright illuminated room Thereflection is noticeable when the user concentrates on itand disappears as soon as he or she concentrates on theapplication Since the projectors are darkened duringthe transparent state of the projection screens the cam-era acquisition is not disturbed by any reflection Thecameras are positioned in such a way as to see the userrsquosentire body even if the user gets very close to the pro-jection screens

Figure 19 Two different collaboration setups based on the blue-c

principle

Figure 20 Collaboration between two individuals

190 PRESENCE VOLUME 13 NUMBER 2

The channel separation of the stereoscopic projectionis very good and the total performance of the projec-tion is convincing The brightness is higher than tradi-tional CRT stereo projectors at a lower price per stereoprojection rack The six-channel audio system proved tobe very powerful The user can precisely localize a simu-lated noise or sound source The subwoofer located inthe wooden platform is also important for good audioperformance In addition it can transmit vibrations tothe user at lower frequencies and this acoustic stimulusplays an important role in high immersion

The mechanical structure was shown to meet the re-quired demands Its stability and rigidity give a strongsense of confidence in the blue-c The user is never un-easy about touching the projection screen or walkingaround as in some other VR installations The flexibilityin the placement of new components and cables as wellas the quick access to all the components proved to bea very valuable feature of the blue-c construction

7 Further Work

The two setups showed that collaboration over anetwork using 3D representations is technically possibleHowever there are more problems to overcome In itstechnical aspects the system should be downsized in costand weight with the goal being a mobile installation Herenew stereo projection systems would also be tested that areeasier to install and adjust Also new projection materialssuch as active Perspex will be evaluated As a first step inthis direction we will build a demonstrator that would besuitable for presentations on conferences and exhibitionsThe demonstrator could be used to show the core tech-nology of the blue-c itself but also as another input stationfor a multipoint collaboration (Disz Papka Pellegrino ampStevens 1995 Lehner amp DeFanti 1997 Fuchs et al2002 Bailett Calahan amp McCluskey 1998)

Further work on the blue-c will also deal with imple-menting the technology in existing processes for in-stance the product development process or the educa-tion process Thus applications for the blue-c will begenerated that offer benefits to the user when using the

3D capability in collaborative work Further informationcan be found at httpblue-cethzch

Acknowledgments

This project was funded as an internal research project no0-23803-00 by the Swiss Federal Institute of TechnologyZurich We would like to thank all members of the blue-cproject

References

Bailett A Calahan J amp McCluskey S (1998) Coordina-tion at different stages of the product development processRampD Management 28 237ndash247

Bougrioua F Oepts W Vleeschouwer H D AlexanderE Neyts K amp Pauwels H (2002) Determination of theswitching response in reflective twisted nematic liquid crys-tal displays The Japan Society of Applied Physics 41 5676ndash5681

Chen H Wallace G Gupta A Li K Funkhouser T ampCook P (2002) Experiences with scalability of displaywalls Proceedings of IPTVR 2002

Cruz-Neira C Sandin D J amp DeFanti T A (1993)Surrounding-screen projection-based virtual reality Thedesign and implementation of the CAVE Proceedings ofSIGGRAPH 1993 135ndash142

Disz T Papka M Pellegrino M amp Stevens R (1995)Sharing visualization experiences among remote virtual en-vironments Proceedings of the International Workshop onHigh Performance Computing for Computer Graphics andVisualization 135ndash142

Drzaic P S (1995) Liquid Crystal Dispersion World Scien-tific Singapore

Elspass W VanGool L Gross M H Kunz A Meier MSchmitt G Staadt O et al (1999) The BlueCave ETHZurich internal research proposal

Fuchs H Bishop G Arthur K McMillan L Bajcsy RLee S et al (1994) Virtual space teleconferencing using asea of cameras (Tech Rep No TR94-033) Chapel Hill

Kunz and Spagno 191

wheelrdquo would require a complicated synchronization ofthe shutters with the projectors However today 3-chipDLP projectors could be used instead of the two LCprojectors The shutters in front of the projectors haveto be switched on and off at a frequency of approxi-mately 60 Hz

Twisted Nematic (TN) liquid-crystal shutters as usedin shutter glasses can offer a contrast ratio of 10001 orbetter (Lipton 2001) They switch in about 25 msfrom black to transparent and in less than 1 ms fromtransparent to black (Lipton 1991 Woods amp Tan2002 Bougrioua et al 2002) Ferro-electric (FE)liquid-crystal shutters offer about the same contrastratio as TN shutters but much faster switching timesThe FE shutters used in front of the projectors are140 mm 140 mm and offer a contrast ratio of ap-proximately 7501 The switching time from 10 to90 transparency and from 90 to 10 transparency isbelow 100 s (see Figure 12)

The upper measurement in Figure 12 shows the volt-age measured on the shutter The mean value of thisvoltage is zero as required The lower measurementshows the optical response of the shutter measured by aphototransistor and a light source The response is veryfast and guarantees an excellent separation between thetwo stereo channels

A TN shutter switches to black if a voltage is appliedand to transparent if the voltage is removed An FE

shutter in contrast switches to black if a voltage of onepolarity is applied and to transparent if a voltage of theopposite polarity is applied It is important that theshutter not be subject to any DC voltage as it will bedamaged The DC decoupling is achieved by connect-ing a capacitor CD of 1 F in series and a resistor RD of1 k in parallel to the shutter (see also Figure 10)

Mounting the shutters directly in front of the projec-torrsquos lens did not cause temperature problems as ex-pected Even after several hours of projection the tem-perature was beneath the limit tolerated by thetemperature-sensitive devices

423 Active Illumination The requirements onthe blue-c illumination are very demanding For theprojection both the inside and the outside of the pro-jection room have to be dark On the other handbright illumination is important for good-quality imageacquisition The illumination has a big impact on thequality of the extracted silhouettes and acquired tex-tures A person inside the blue-c should be illuminateduniformly from all sides and no shadow should be pro-jected on the floor The illumination has to be directedaway from the PDLC glass panels to avoid contrast-lossand reflections

Figure 11 Stereo projection rack with LC shutters

Figure 12 Ferro-electric LC shutter (driving voltage optical

response)

Kunz and Spagno 185

An active illumination is used to satisfy the needs ofboth projection and image acquisition The active lightis off during projection and on for the image acquisi-tion Therefore the light has to provide illumination foronly a few milliseconds and must be able to turn on andoff with a frequency between 60 and 70 Hz First testswere done with a strobe light A high voltage dischargein a xenon flash lamp produces a bright light pulse of avery short duration Frequencies in the range of 60 Hzare no problem for this type of lamp Despite the fastswitching this type of light has some important disad-vantages First light is emitted for only a few millisec-onds The light intensity integrated over time is there-fore only mediocre In addition the high voltagedischarge in the very short time period causes heavyelectromagnetic pulses which can interfere with sur-rounding electronic components The discharges alsocause a disturbing acoustic noise Finally the strobelight is focused in one spot This is a handicap whentrying to distribute the light uniformly in the projectionroom All these disadvantages led us to look for otherillumination solutions

The requirement of short switching times restricts thenumber of suitable light sources considerably Light-emitting diodes (LEDs) have very short switching timesand are thus suitable for our application Today whiteLEDs are commonly available and the light efficiency isabout twice that of an incandescent bulb (Zorpette2002) LEDs can be easily grouped in clusters to givemore light output The clusters are flat can be of anyshape and the light output can be distributed homoge-neously over the cluster if desired A total of 9984white 5 mm LEDs have been integrated into the blue-cThe opening angle (50 intensity) is 50 degrees and thelight output is approximately 2200 mcd per LEDThese LEDs were the brightest available on the marketin 2002 The LEDs are configured into 32 clusterseach containing 312 LEDs The clusters are reassem-bled in groups of two four or five and mounted inlong aluminum profiles

The LED clusters have to be placed carefully in theblue-c to allow a good quality image acquisition Thecameras are mostly placed outside the blue-c and facemore or less horizontally to the user through the PDLC

glass panels Under this aspect a diffused frontal illumi-nation of the user would be the most suitable But afrontal light would also shine directly into the oppositecameras and furthermore would put too much light onthe glass panels making them appear gray In additionit would be difficult to find a place to mount the clus-ters without interfering with the projection and thecameras

The solution is to distribute the LED clusters alongthe upper and lower edges of the projection room TheLEDs can be adjusted in such a way that they com-pletely illuminate the user minimizing at the same timethe amount of light being directed at the glass panelsand at the opposite cameras The additional mask infront of the LEDs helps to further diminish the amountof scattered light reaching the glass panels Figure 13shows a person between two opposite projection wallsas is the case in the blue-c for the two sidewalls Thelight is coming from all four edges and most parts of theperson are illuminated by more than one light sourceFigure 14 shows one illumination cluster Five of themare integrated in the aluminum profile and placed at theedge between the floor and the left glass panels

Realizing the illumination with the diodes it turnedout that the specifications of the LED were not reachedby every single element This resulted in failures andthus in nonactive LED lines in the cluster After the cur-

Figure 13 Placement of the LED clusters

186 PRESENCE VOLUME 13 NUMBER 2

rent through the entire cluster was reduced this prob-lem disappeared

424 Cameras Sixteen FireWire cameras areplaced around the blue-c on a special aluminum struc-ture The structure is based on standard aluminum ele-ments and offers the maximum flexibility in the place-ment of cameras and other components Each camera isconnected via FireWire to one node of the PC clusterThe signal is amplified every 5 m with a repeater Up tothree repeaters are used for each camera

Dragonflytrade cameras are used since they offer goodsensitivity and a high picture acquisition frequency of upto 15 Hz However cameras with an even higher sensi-tivity would be useful in order to reduce the intensity ofthe flash illumination On the other hand the pictureacquisition frequency is high enough since the bottle-neck is in the processing of the acquired images Initialtests showed that a repetition frequency of 10 Hz seemsto be sufficient for collaborative work

425 Synchronization Electronics The syn-chronization electronics of the blue-c were built modu-larly to offer flexibility in adding and changing compo-nents The main module is the frequency-controlmodule which is equipped with a reprogrammable PIC16F877 microcontroller This module generates thetriggering sequence to drive the other modules of the

synchronization unit Three PDLC glass panel drivermodules two shutter driver modules and one trigger-interface module are integrated into the synchronizationunit together with the synchronization control moduleThe LED driver and IR-emitter driver are placed out-side of the synchronization unit and are triggered by thetrigger interface module

Figure 15 gives an overview of the synchronizationelectronics All modules of the synchronization unit areinterconnected with a data bus In addition a powerbus is integrated into the unit to supply the differentmodules with the required voltages The LED driverand IR-emitter driver are located outside of the syn-chronization unit together with their own power sup-plies to reduce electromagnetic interference caused bythe currents

Using a microcontroller for the synchronization of allcomponents proved to be crucial Since every compo-nent including identical parts behaves slightly differ-ently (eg the glass panes) it is easy to create individu-alized triggering signals using the microcontroller Thismakes up for the increased effort of installing and pro-gramming the processor

426 Frequency Control Module The fre-quency control module is based on a PIC 16F877 mi-crocontroller This microcontroller can be repro-grammed in-circuit that is without unplugging thechip from the board Twenty-two inputndashoutput ports ofthe microcontroller are mapped to the data bus They

Figure 14 An LED cluster of the blue-c

Figure 15 Synchronization electronics

Kunz and Spagno 187

can be set individually as outputs or inputs Each ofthese ports is connected to a LED to show its state Themicrocontroller is clocked with a frequency of 20 MHzto offer high timing accuracy

427 PDLC Glass-Panel Driver Module Todrive the nine PDLC glass panels a total of three glass-panel driver modules are used (see Figure 16) In eachcase three glass panels in parallel are connected to thetwo outputs of a module The two outputs of a modulecan be switched separately to either 8 V or 85 V If bothoutputs are switched to the same value no voltage isapplied to the glass panels If the outputs are switchedto different values a voltage of 77 V or 77 V is ap-plied to the glass panels in series with the decouplingcapacitor Each of the two outputs is switched directlyby the corresponding input coming from the data busA current-limit control is superimposed on the voltageswitching

Figure 16 shows a simplified circuit consisting of aPDLC glass-panel driver module connected to threeglass panels with their DC decoupling circuits Each ofthe two incoming TTL signals switches an optocoupleroutput to a preset voltage of 8 V or 85 V The two out-puts of the optocouplers are connected to two APEXPA46 operational amplifiers The APEX PA64 has anintegrated adjustable current-limit control and can runat voltages up to 150 V and currents up to 5 A Theoperational amplifiers are used as followers thus theinput voltage is mapped directly to the output as long asthe current-limit is not exceeded The PDLC glass-panel driver modules are connected to a 95-V power

supply However the outputs of the module have beenset to switch only between 8 V and 85 V This givesenough margin to the output voltage from the powersupply and ground to avoid saturating the operationalamplifier The current-limit of the operational amplifieris set to 900 mA which corresponds to a current-limitof 300 mA per panel

428 Shutter Driver Module The shutterdriver module also has two inputs and two outputs Itconsists of two independent circuits with an opto-coupler and a half-bridge consisting of two MOSFETsand a half-bridge driver (Figure 17) The optocoupler isused to protect the other modules of the synchroniza-tion electronics in case a fault occurs in this module orin a connected component The output of the opto-coupler is connected to a half-bridge driver (IR2111)that drives two MOSFETs The bridge driver ensuresthat the two MOSFETs in the half-bridge are neverturned on at the same time The two half-bridge out-puts switch the voltage rail-to-rail without any current-limit In order to drive the ferro-electric shutters themodule is operated with a voltage of 12 V Three shut-ters are connected in parallel between the two half-bridges of a module The two half-bridges are alwaysswitched to opposite states The bridge voltage is there-fore either 12 V or 12 V as required by the shuttersfor switching

429 Trigger Interface Module The trigger-interface module is a multifunctional module It allows achoice between up to 8 lines out of the 22 data lines onthe bus Each line can be connected to a 5 V or 12 V

Figure 16 PDLC glass-panel driver moduleFigure 17 Shutter driver module

188 PRESENCE VOLUME 13 NUMBER 2

driver and configured as input or output For the blue-call channels used were configured as 5-V outputs Onechannel is used to trigger the FireWire cameras A sec-ond one is used to trigger the LED illumination Twomore channels are used to synchronize the shutterglasses and are connected to the IR-emitter driver

4210 LED Driver A total of 9984 LEDs areinstalled in different clusters in the blue-c The LEDclusters have to be driven with a voltage of 375 V and atotal current of up to 25 A The power supply for theLED cluster is installed in the floor of the blue-c andconnected directly to the LED clusters This reduces thewiring distances of the high-current connections andthus the probability of electro-magnetic interferenceEach cluster has an integrated MOSFET to switch theLEDs on and off The triggering signal comes directlyfrom the trigger-interface module

4211 Shutter Glasses The LED illuminationis triggered 625 times per second The user does notsee any flickering because this frequency is above thetemporal resolution of the human eye However thebright light inside the blue-c diminishes the contrast ofthe projected image considerably To protect the userfrom the active light an apposite third phase has beenintegrated into the stereo glasses to keep the light outof the userrsquos eyes The projection of the images for theleft and the right eye are followed by a third phase inwhich both LC shutters of the shutter glasses are dark-ened The LED illumination is on only during thisphase This helps to fade out most of the light from theillumination

In the blue-c we use the NuVision 60GX shutterglasses from MacNaughton The shutter glasses and theIR-emitter have a built-in microcontroller We havereprogrammed it to include the third phase Like mostshutter glasses the NuVision use twisted nematic LCshutters The transparent-to-black transition timeof these shutters is quite fast while the black-to-transparent switching time takes considerably longerFigure 18 shows the switching time of the NuVisionshutter glasses The upper measurement shows thetransparency of a shutter where a high value corre-

sponds to a high transparency The lower measurementshows the voltage applied to the IR-emitter As can beseen from the measurement there is no significant delaybetween sending a signal to the emitter and the begin-ning of the switching response However the switchingtime from 10 to 90 transparency takes approximately33 ms while the switching from 90 to 10 transpar-ency takes about 05 ms These values correspond to thevalues measured by Woods and Tan (Woods amp Tan2002) with other shutter glasses For practical use thereis a work-around involving some blanking time betweenthe projections for the left and right eye

5 Overall Setup

Two complete collaboration setups were de-signed as shown in Figure 19 In the upper image inFigure 19 the first prototype is shown essentially a lab-oratory setup One lesson learned from this is that inmany cases only the front screen is used for collabora-tion This allows for the creation of a more design-oriented setup in a public room (as shown in the lowerpart of Figure 19) using only a single screen Since thecamera positions are known from the first prototypethere was no longer any need for a flexible installation

Figure 18 Optical response of the shutter glasses

Kunz and Spagno 189

The goal of the second setup (bottom image in Fig-ure 19) was thus not to reproduce the first installationbut rather to create an asymmetric installation for thecollaboration With the asymmetric setup applicationswere first created transferring 3D sound and 3D imagesof the people from one installation to the other Figure20 shows a simple collaboration between two individu-als engaged in a dialogue Each person is in an installa-tion at a different location on the campus

Since the first system is an experimental setup a longinitialization time was required before the collaborationcould start Thus the system is not used for hands-oncollaboration yet Further work will shorten the initial-ization phase and will allow easier access to the system

6 Conclusions

The active projection screen is driven at its physi-cal limits The transition time from opaque to transpar-ent (10 to 90) takes approximately 2 ms while the

transition from transparent to opaque (90 to 10)takes approximately 6 ms The slow transition fromtransparent to opaque is dictated by the internal relax-ation forces of the LC film and cannot be acceleratedelectrically This non-ideal behavior can be minimizedby optimizing the time pattern of the different compo-nents For the user just the hotspot of the projectorsbecomes a little more evident but it remains at an ac-ceptable level The active projection screens offer a verybright and homogenous image

The junctions between the three projection-screenpanels are visible but not disturbing The LC film of theactive projection panels is installed between two glasspanels The surface of these glass panels is treated withan antireflective coating Nevertheless some reflectionsfrom the other projection screens are evident The re-flection can be compared to the reflection on a CRTtelevision in a medium-bright illuminated room Thereflection is noticeable when the user concentrates on itand disappears as soon as he or she concentrates on theapplication Since the projectors are darkened duringthe transparent state of the projection screens the cam-era acquisition is not disturbed by any reflection Thecameras are positioned in such a way as to see the userrsquosentire body even if the user gets very close to the pro-jection screens

Figure 19 Two different collaboration setups based on the blue-c

principle

Figure 20 Collaboration between two individuals

190 PRESENCE VOLUME 13 NUMBER 2

The channel separation of the stereoscopic projectionis very good and the total performance of the projec-tion is convincing The brightness is higher than tradi-tional CRT stereo projectors at a lower price per stereoprojection rack The six-channel audio system proved tobe very powerful The user can precisely localize a simu-lated noise or sound source The subwoofer located inthe wooden platform is also important for good audioperformance In addition it can transmit vibrations tothe user at lower frequencies and this acoustic stimulusplays an important role in high immersion

The mechanical structure was shown to meet the re-quired demands Its stability and rigidity give a strongsense of confidence in the blue-c The user is never un-easy about touching the projection screen or walkingaround as in some other VR installations The flexibilityin the placement of new components and cables as wellas the quick access to all the components proved to bea very valuable feature of the blue-c construction

7 Further Work

The two setups showed that collaboration over anetwork using 3D representations is technically possibleHowever there are more problems to overcome In itstechnical aspects the system should be downsized in costand weight with the goal being a mobile installation Herenew stereo projection systems would also be tested that areeasier to install and adjust Also new projection materialssuch as active Perspex will be evaluated As a first step inthis direction we will build a demonstrator that would besuitable for presentations on conferences and exhibitionsThe demonstrator could be used to show the core tech-nology of the blue-c itself but also as another input stationfor a multipoint collaboration (Disz Papka Pellegrino ampStevens 1995 Lehner amp DeFanti 1997 Fuchs et al2002 Bailett Calahan amp McCluskey 1998)

Further work on the blue-c will also deal with imple-menting the technology in existing processes for in-stance the product development process or the educa-tion process Thus applications for the blue-c will begenerated that offer benefits to the user when using the

3D capability in collaborative work Further informationcan be found at httpblue-cethzch

Acknowledgments

This project was funded as an internal research project no0-23803-00 by the Swiss Federal Institute of TechnologyZurich We would like to thank all members of the blue-cproject

References

Bailett A Calahan J amp McCluskey S (1998) Coordina-tion at different stages of the product development processRampD Management 28 237ndash247

Bougrioua F Oepts W Vleeschouwer H D AlexanderE Neyts K amp Pauwels H (2002) Determination of theswitching response in reflective twisted nematic liquid crys-tal displays The Japan Society of Applied Physics 41 5676ndash5681

Chen H Wallace G Gupta A Li K Funkhouser T ampCook P (2002) Experiences with scalability of displaywalls Proceedings of IPTVR 2002

Cruz-Neira C Sandin D J amp DeFanti T A (1993)Surrounding-screen projection-based virtual reality Thedesign and implementation of the CAVE Proceedings ofSIGGRAPH 1993 135ndash142

Disz T Papka M Pellegrino M amp Stevens R (1995)Sharing visualization experiences among remote virtual en-vironments Proceedings of the International Workshop onHigh Performance Computing for Computer Graphics andVisualization 135ndash142

Drzaic P S (1995) Liquid Crystal Dispersion World Scien-tific Singapore

Elspass W VanGool L Gross M H Kunz A Meier MSchmitt G Staadt O et al (1999) The BlueCave ETHZurich internal research proposal

Fuchs H Bishop G Arthur K McMillan L Bajcsy RLee S et al (1994) Virtual space teleconferencing using asea of cameras (Tech Rep No TR94-033) Chapel Hill

Kunz and Spagno 191

An active illumination is used to satisfy the needs ofboth projection and image acquisition The active lightis off during projection and on for the image acquisi-tion Therefore the light has to provide illumination foronly a few milliseconds and must be able to turn on andoff with a frequency between 60 and 70 Hz First testswere done with a strobe light A high voltage dischargein a xenon flash lamp produces a bright light pulse of avery short duration Frequencies in the range of 60 Hzare no problem for this type of lamp Despite the fastswitching this type of light has some important disad-vantages First light is emitted for only a few millisec-onds The light intensity integrated over time is there-fore only mediocre In addition the high voltagedischarge in the very short time period causes heavyelectromagnetic pulses which can interfere with sur-rounding electronic components The discharges alsocause a disturbing acoustic noise Finally the strobelight is focused in one spot This is a handicap whentrying to distribute the light uniformly in the projectionroom All these disadvantages led us to look for otherillumination solutions

The requirement of short switching times restricts thenumber of suitable light sources considerably Light-emitting diodes (LEDs) have very short switching timesand are thus suitable for our application Today whiteLEDs are commonly available and the light efficiency isabout twice that of an incandescent bulb (Zorpette2002) LEDs can be easily grouped in clusters to givemore light output The clusters are flat can be of anyshape and the light output can be distributed homoge-neously over the cluster if desired A total of 9984white 5 mm LEDs have been integrated into the blue-cThe opening angle (50 intensity) is 50 degrees and thelight output is approximately 2200 mcd per LEDThese LEDs were the brightest available on the marketin 2002 The LEDs are configured into 32 clusterseach containing 312 LEDs The clusters are reassem-bled in groups of two four or five and mounted inlong aluminum profiles

The LED clusters have to be placed carefully in theblue-c to allow a good quality image acquisition Thecameras are mostly placed outside the blue-c and facemore or less horizontally to the user through the PDLC

glass panels Under this aspect a diffused frontal illumi-nation of the user would be the most suitable But afrontal light would also shine directly into the oppositecameras and furthermore would put too much light onthe glass panels making them appear gray In additionit would be difficult to find a place to mount the clus-ters without interfering with the projection and thecameras

The solution is to distribute the LED clusters alongthe upper and lower edges of the projection room TheLEDs can be adjusted in such a way that they com-pletely illuminate the user minimizing at the same timethe amount of light being directed at the glass panelsand at the opposite cameras The additional mask infront of the LEDs helps to further diminish the amountof scattered light reaching the glass panels Figure 13shows a person between two opposite projection wallsas is the case in the blue-c for the two sidewalls Thelight is coming from all four edges and most parts of theperson are illuminated by more than one light sourceFigure 14 shows one illumination cluster Five of themare integrated in the aluminum profile and placed at theedge between the floor and the left glass panels

Realizing the illumination with the diodes it turnedout that the specifications of the LED were not reachedby every single element This resulted in failures andthus in nonactive LED lines in the cluster After the cur-

Figure 13 Placement of the LED clusters

186 PRESENCE VOLUME 13 NUMBER 2

rent through the entire cluster was reduced this prob-lem disappeared

424 Cameras Sixteen FireWire cameras areplaced around the blue-c on a special aluminum struc-ture The structure is based on standard aluminum ele-ments and offers the maximum flexibility in the place-ment of cameras and other components Each camera isconnected via FireWire to one node of the PC clusterThe signal is amplified every 5 m with a repeater Up tothree repeaters are used for each camera

Dragonflytrade cameras are used since they offer goodsensitivity and a high picture acquisition frequency of upto 15 Hz However cameras with an even higher sensi-tivity would be useful in order to reduce the intensity ofthe flash illumination On the other hand the pictureacquisition frequency is high enough since the bottle-neck is in the processing of the acquired images Initialtests showed that a repetition frequency of 10 Hz seemsto be sufficient for collaborative work

425 Synchronization Electronics The syn-chronization electronics of the blue-c were built modu-larly to offer flexibility in adding and changing compo-nents The main module is the frequency-controlmodule which is equipped with a reprogrammable PIC16F877 microcontroller This module generates thetriggering sequence to drive the other modules of the

synchronization unit Three PDLC glass panel drivermodules two shutter driver modules and one trigger-interface module are integrated into the synchronizationunit together with the synchronization control moduleThe LED driver and IR-emitter driver are placed out-side of the synchronization unit and are triggered by thetrigger interface module

Figure 15 gives an overview of the synchronizationelectronics All modules of the synchronization unit areinterconnected with a data bus In addition a powerbus is integrated into the unit to supply the differentmodules with the required voltages The LED driverand IR-emitter driver are located outside of the syn-chronization unit together with their own power sup-plies to reduce electromagnetic interference caused bythe currents

Using a microcontroller for the synchronization of allcomponents proved to be crucial Since every compo-nent including identical parts behaves slightly differ-ently (eg the glass panes) it is easy to create individu-alized triggering signals using the microcontroller Thismakes up for the increased effort of installing and pro-gramming the processor

426 Frequency Control Module The fre-quency control module is based on a PIC 16F877 mi-crocontroller This microcontroller can be repro-grammed in-circuit that is without unplugging thechip from the board Twenty-two inputndashoutput ports ofthe microcontroller are mapped to the data bus They

Figure 14 An LED cluster of the blue-c

Figure 15 Synchronization electronics

Kunz and Spagno 187

can be set individually as outputs or inputs Each ofthese ports is connected to a LED to show its state Themicrocontroller is clocked with a frequency of 20 MHzto offer high timing accuracy

427 PDLC Glass-Panel Driver Module Todrive the nine PDLC glass panels a total of three glass-panel driver modules are used (see Figure 16) In eachcase three glass panels in parallel are connected to thetwo outputs of a module The two outputs of a modulecan be switched separately to either 8 V or 85 V If bothoutputs are switched to the same value no voltage isapplied to the glass panels If the outputs are switchedto different values a voltage of 77 V or 77 V is ap-plied to the glass panels in series with the decouplingcapacitor Each of the two outputs is switched directlyby the corresponding input coming from the data busA current-limit control is superimposed on the voltageswitching

Figure 16 shows a simplified circuit consisting of aPDLC glass-panel driver module connected to threeglass panels with their DC decoupling circuits Each ofthe two incoming TTL signals switches an optocoupleroutput to a preset voltage of 8 V or 85 V The two out-puts of the optocouplers are connected to two APEXPA46 operational amplifiers The APEX PA64 has anintegrated adjustable current-limit control and can runat voltages up to 150 V and currents up to 5 A Theoperational amplifiers are used as followers thus theinput voltage is mapped directly to the output as long asthe current-limit is not exceeded The PDLC glass-panel driver modules are connected to a 95-V power

supply However the outputs of the module have beenset to switch only between 8 V and 85 V This givesenough margin to the output voltage from the powersupply and ground to avoid saturating the operationalamplifier The current-limit of the operational amplifieris set to 900 mA which corresponds to a current-limitof 300 mA per panel

428 Shutter Driver Module The shutterdriver module also has two inputs and two outputs Itconsists of two independent circuits with an opto-coupler and a half-bridge consisting of two MOSFETsand a half-bridge driver (Figure 17) The optocoupler isused to protect the other modules of the synchroniza-tion electronics in case a fault occurs in this module orin a connected component The output of the opto-coupler is connected to a half-bridge driver (IR2111)that drives two MOSFETs The bridge driver ensuresthat the two MOSFETs in the half-bridge are neverturned on at the same time The two half-bridge out-puts switch the voltage rail-to-rail without any current-limit In order to drive the ferro-electric shutters themodule is operated with a voltage of 12 V Three shut-ters are connected in parallel between the two half-bridges of a module The two half-bridges are alwaysswitched to opposite states The bridge voltage is there-fore either 12 V or 12 V as required by the shuttersfor switching

429 Trigger Interface Module The trigger-interface module is a multifunctional module It allows achoice between up to 8 lines out of the 22 data lines onthe bus Each line can be connected to a 5 V or 12 V

Figure 16 PDLC glass-panel driver moduleFigure 17 Shutter driver module

188 PRESENCE VOLUME 13 NUMBER 2

driver and configured as input or output For the blue-call channels used were configured as 5-V outputs Onechannel is used to trigger the FireWire cameras A sec-ond one is used to trigger the LED illumination Twomore channels are used to synchronize the shutterglasses and are connected to the IR-emitter driver

4210 LED Driver A total of 9984 LEDs areinstalled in different clusters in the blue-c The LEDclusters have to be driven with a voltage of 375 V and atotal current of up to 25 A The power supply for theLED cluster is installed in the floor of the blue-c andconnected directly to the LED clusters This reduces thewiring distances of the high-current connections andthus the probability of electro-magnetic interferenceEach cluster has an integrated MOSFET to switch theLEDs on and off The triggering signal comes directlyfrom the trigger-interface module

4211 Shutter Glasses The LED illuminationis triggered 625 times per second The user does notsee any flickering because this frequency is above thetemporal resolution of the human eye However thebright light inside the blue-c diminishes the contrast ofthe projected image considerably To protect the userfrom the active light an apposite third phase has beenintegrated into the stereo glasses to keep the light outof the userrsquos eyes The projection of the images for theleft and the right eye are followed by a third phase inwhich both LC shutters of the shutter glasses are dark-ened The LED illumination is on only during thisphase This helps to fade out most of the light from theillumination

In the blue-c we use the NuVision 60GX shutterglasses from MacNaughton The shutter glasses and theIR-emitter have a built-in microcontroller We havereprogrammed it to include the third phase Like mostshutter glasses the NuVision use twisted nematic LCshutters The transparent-to-black transition timeof these shutters is quite fast while the black-to-transparent switching time takes considerably longerFigure 18 shows the switching time of the NuVisionshutter glasses The upper measurement shows thetransparency of a shutter where a high value corre-

sponds to a high transparency The lower measurementshows the voltage applied to the IR-emitter As can beseen from the measurement there is no significant delaybetween sending a signal to the emitter and the begin-ning of the switching response However the switchingtime from 10 to 90 transparency takes approximately33 ms while the switching from 90 to 10 transpar-ency takes about 05 ms These values correspond to thevalues measured by Woods and Tan (Woods amp Tan2002) with other shutter glasses For practical use thereis a work-around involving some blanking time betweenthe projections for the left and right eye

5 Overall Setup

Two complete collaboration setups were de-signed as shown in Figure 19 In the upper image inFigure 19 the first prototype is shown essentially a lab-oratory setup One lesson learned from this is that inmany cases only the front screen is used for collabora-tion This allows for the creation of a more design-oriented setup in a public room (as shown in the lowerpart of Figure 19) using only a single screen Since thecamera positions are known from the first prototypethere was no longer any need for a flexible installation

Figure 18 Optical response of the shutter glasses

Kunz and Spagno 189

The goal of the second setup (bottom image in Fig-ure 19) was thus not to reproduce the first installationbut rather to create an asymmetric installation for thecollaboration With the asymmetric setup applicationswere first created transferring 3D sound and 3D imagesof the people from one installation to the other Figure20 shows a simple collaboration between two individu-als engaged in a dialogue Each person is in an installa-tion at a different location on the campus

Since the first system is an experimental setup a longinitialization time was required before the collaborationcould start Thus the system is not used for hands-oncollaboration yet Further work will shorten the initial-ization phase and will allow easier access to the system

6 Conclusions

The active projection screen is driven at its physi-cal limits The transition time from opaque to transpar-ent (10 to 90) takes approximately 2 ms while the

transition from transparent to opaque (90 to 10)takes approximately 6 ms The slow transition fromtransparent to opaque is dictated by the internal relax-ation forces of the LC film and cannot be acceleratedelectrically This non-ideal behavior can be minimizedby optimizing the time pattern of the different compo-nents For the user just the hotspot of the projectorsbecomes a little more evident but it remains at an ac-ceptable level The active projection screens offer a verybright and homogenous image

The junctions between the three projection-screenpanels are visible but not disturbing The LC film of theactive projection panels is installed between two glasspanels The surface of these glass panels is treated withan antireflective coating Nevertheless some reflectionsfrom the other projection screens are evident The re-flection can be compared to the reflection on a CRTtelevision in a medium-bright illuminated room Thereflection is noticeable when the user concentrates on itand disappears as soon as he or she concentrates on theapplication Since the projectors are darkened duringthe transparent state of the projection screens the cam-era acquisition is not disturbed by any reflection Thecameras are positioned in such a way as to see the userrsquosentire body even if the user gets very close to the pro-jection screens

Figure 19 Two different collaboration setups based on the blue-c

principle

Figure 20 Collaboration between two individuals

190 PRESENCE VOLUME 13 NUMBER 2

The channel separation of the stereoscopic projectionis very good and the total performance of the projec-tion is convincing The brightness is higher than tradi-tional CRT stereo projectors at a lower price per stereoprojection rack The six-channel audio system proved tobe very powerful The user can precisely localize a simu-lated noise or sound source The subwoofer located inthe wooden platform is also important for good audioperformance In addition it can transmit vibrations tothe user at lower frequencies and this acoustic stimulusplays an important role in high immersion

The mechanical structure was shown to meet the re-quired demands Its stability and rigidity give a strongsense of confidence in the blue-c The user is never un-easy about touching the projection screen or walkingaround as in some other VR installations The flexibilityin the placement of new components and cables as wellas the quick access to all the components proved to bea very valuable feature of the blue-c construction

7 Further Work

The two setups showed that collaboration over anetwork using 3D representations is technically possibleHowever there are more problems to overcome In itstechnical aspects the system should be downsized in costand weight with the goal being a mobile installation Herenew stereo projection systems would also be tested that areeasier to install and adjust Also new projection materialssuch as active Perspex will be evaluated As a first step inthis direction we will build a demonstrator that would besuitable for presentations on conferences and exhibitionsThe demonstrator could be used to show the core tech-nology of the blue-c itself but also as another input stationfor a multipoint collaboration (Disz Papka Pellegrino ampStevens 1995 Lehner amp DeFanti 1997 Fuchs et al2002 Bailett Calahan amp McCluskey 1998)

Further work on the blue-c will also deal with imple-menting the technology in existing processes for in-stance the product development process or the educa-tion process Thus applications for the blue-c will begenerated that offer benefits to the user when using the

3D capability in collaborative work Further informationcan be found at httpblue-cethzch

Acknowledgments

This project was funded as an internal research project no0-23803-00 by the Swiss Federal Institute of TechnologyZurich We would like to thank all members of the blue-cproject

References

Bailett A Calahan J amp McCluskey S (1998) Coordina-tion at different stages of the product development processRampD Management 28 237ndash247

Bougrioua F Oepts W Vleeschouwer H D AlexanderE Neyts K amp Pauwels H (2002) Determination of theswitching response in reflective twisted nematic liquid crys-tal displays The Japan Society of Applied Physics 41 5676ndash5681

Chen H Wallace G Gupta A Li K Funkhouser T ampCook P (2002) Experiences with scalability of displaywalls Proceedings of IPTVR 2002

Cruz-Neira C Sandin D J amp DeFanti T A (1993)Surrounding-screen projection-based virtual reality Thedesign and implementation of the CAVE Proceedings ofSIGGRAPH 1993 135ndash142

Disz T Papka M Pellegrino M amp Stevens R (1995)Sharing visualization experiences among remote virtual en-vironments Proceedings of the International Workshop onHigh Performance Computing for Computer Graphics andVisualization 135ndash142

Drzaic P S (1995) Liquid Crystal Dispersion World Scien-tific Singapore

Elspass W VanGool L Gross M H Kunz A Meier MSchmitt G Staadt O et al (1999) The BlueCave ETHZurich internal research proposal

Fuchs H Bishop G Arthur K McMillan L Bajcsy RLee S et al (1994) Virtual space teleconferencing using asea of cameras (Tech Rep No TR94-033) Chapel Hill

Kunz and Spagno 191

rent through the entire cluster was reduced this prob-lem disappeared

424 Cameras Sixteen FireWire cameras areplaced around the blue-c on a special aluminum struc-ture The structure is based on standard aluminum ele-ments and offers the maximum flexibility in the place-ment of cameras and other components Each camera isconnected via FireWire to one node of the PC clusterThe signal is amplified every 5 m with a repeater Up tothree repeaters are used for each camera

Dragonflytrade cameras are used since they offer goodsensitivity and a high picture acquisition frequency of upto 15 Hz However cameras with an even higher sensi-tivity would be useful in order to reduce the intensity ofthe flash illumination On the other hand the pictureacquisition frequency is high enough since the bottle-neck is in the processing of the acquired images Initialtests showed that a repetition frequency of 10 Hz seemsto be sufficient for collaborative work

425 Synchronization Electronics The syn-chronization electronics of the blue-c were built modu-larly to offer flexibility in adding and changing compo-nents The main module is the frequency-controlmodule which is equipped with a reprogrammable PIC16F877 microcontroller This module generates thetriggering sequence to drive the other modules of the

synchronization unit Three PDLC glass panel drivermodules two shutter driver modules and one trigger-interface module are integrated into the synchronizationunit together with the synchronization control moduleThe LED driver and IR-emitter driver are placed out-side of the synchronization unit and are triggered by thetrigger interface module

Figure 15 gives an overview of the synchronizationelectronics All modules of the synchronization unit areinterconnected with a data bus In addition a powerbus is integrated into the unit to supply the differentmodules with the required voltages The LED driverand IR-emitter driver are located outside of the syn-chronization unit together with their own power sup-plies to reduce electromagnetic interference caused bythe currents

Using a microcontroller for the synchronization of allcomponents proved to be crucial Since every compo-nent including identical parts behaves slightly differ-ently (eg the glass panes) it is easy to create individu-alized triggering signals using the microcontroller Thismakes up for the increased effort of installing and pro-gramming the processor

426 Frequency Control Module The fre-quency control module is based on a PIC 16F877 mi-crocontroller This microcontroller can be repro-grammed in-circuit that is without unplugging thechip from the board Twenty-two inputndashoutput ports ofthe microcontroller are mapped to the data bus They

Figure 14 An LED cluster of the blue-c

Figure 15 Synchronization electronics

Kunz and Spagno 187

can be set individually as outputs or inputs Each ofthese ports is connected to a LED to show its state Themicrocontroller is clocked with a frequency of 20 MHzto offer high timing accuracy

427 PDLC Glass-Panel Driver Module Todrive the nine PDLC glass panels a total of three glass-panel driver modules are used (see Figure 16) In eachcase three glass panels in parallel are connected to thetwo outputs of a module The two outputs of a modulecan be switched separately to either 8 V or 85 V If bothoutputs are switched to the same value no voltage isapplied to the glass panels If the outputs are switchedto different values a voltage of 77 V or 77 V is ap-plied to the glass panels in series with the decouplingcapacitor Each of the two outputs is switched directlyby the corresponding input coming from the data busA current-limit control is superimposed on the voltageswitching

Figure 16 shows a simplified circuit consisting of aPDLC glass-panel driver module connected to threeglass panels with their DC decoupling circuits Each ofthe two incoming TTL signals switches an optocoupleroutput to a preset voltage of 8 V or 85 V The two out-puts of the optocouplers are connected to two APEXPA46 operational amplifiers The APEX PA64 has anintegrated adjustable current-limit control and can runat voltages up to 150 V and currents up to 5 A Theoperational amplifiers are used as followers thus theinput voltage is mapped directly to the output as long asthe current-limit is not exceeded The PDLC glass-panel driver modules are connected to a 95-V power

supply However the outputs of the module have beenset to switch only between 8 V and 85 V This givesenough margin to the output voltage from the powersupply and ground to avoid saturating the operationalamplifier The current-limit of the operational amplifieris set to 900 mA which corresponds to a current-limitof 300 mA per panel

428 Shutter Driver Module The shutterdriver module also has two inputs and two outputs Itconsists of two independent circuits with an opto-coupler and a half-bridge consisting of two MOSFETsand a half-bridge driver (Figure 17) The optocoupler isused to protect the other modules of the synchroniza-tion electronics in case a fault occurs in this module orin a connected component The output of the opto-coupler is connected to a half-bridge driver (IR2111)that drives two MOSFETs The bridge driver ensuresthat the two MOSFETs in the half-bridge are neverturned on at the same time The two half-bridge out-puts switch the voltage rail-to-rail without any current-limit In order to drive the ferro-electric shutters themodule is operated with a voltage of 12 V Three shut-ters are connected in parallel between the two half-bridges of a module The two half-bridges are alwaysswitched to opposite states The bridge voltage is there-fore either 12 V or 12 V as required by the shuttersfor switching

429 Trigger Interface Module The trigger-interface module is a multifunctional module It allows achoice between up to 8 lines out of the 22 data lines onthe bus Each line can be connected to a 5 V or 12 V

Figure 16 PDLC glass-panel driver moduleFigure 17 Shutter driver module

188 PRESENCE VOLUME 13 NUMBER 2

driver and configured as input or output For the blue-call channels used were configured as 5-V outputs Onechannel is used to trigger the FireWire cameras A sec-ond one is used to trigger the LED illumination Twomore channels are used to synchronize the shutterglasses and are connected to the IR-emitter driver

4210 LED Driver A total of 9984 LEDs areinstalled in different clusters in the blue-c The LEDclusters have to be driven with a voltage of 375 V and atotal current of up to 25 A The power supply for theLED cluster is installed in the floor of the blue-c andconnected directly to the LED clusters This reduces thewiring distances of the high-current connections andthus the probability of electro-magnetic interferenceEach cluster has an integrated MOSFET to switch theLEDs on and off The triggering signal comes directlyfrom the trigger-interface module

4211 Shutter Glasses The LED illuminationis triggered 625 times per second The user does notsee any flickering because this frequency is above thetemporal resolution of the human eye However thebright light inside the blue-c diminishes the contrast ofthe projected image considerably To protect the userfrom the active light an apposite third phase has beenintegrated into the stereo glasses to keep the light outof the userrsquos eyes The projection of the images for theleft and the right eye are followed by a third phase inwhich both LC shutters of the shutter glasses are dark-ened The LED illumination is on only during thisphase This helps to fade out most of the light from theillumination

In the blue-c we use the NuVision 60GX shutterglasses from MacNaughton The shutter glasses and theIR-emitter have a built-in microcontroller We havereprogrammed it to include the third phase Like mostshutter glasses the NuVision use twisted nematic LCshutters The transparent-to-black transition timeof these shutters is quite fast while the black-to-transparent switching time takes considerably longerFigure 18 shows the switching time of the NuVisionshutter glasses The upper measurement shows thetransparency of a shutter where a high value corre-

sponds to a high transparency The lower measurementshows the voltage applied to the IR-emitter As can beseen from the measurement there is no significant delaybetween sending a signal to the emitter and the begin-ning of the switching response However the switchingtime from 10 to 90 transparency takes approximately33 ms while the switching from 90 to 10 transpar-ency takes about 05 ms These values correspond to thevalues measured by Woods and Tan (Woods amp Tan2002) with other shutter glasses For practical use thereis a work-around involving some blanking time betweenthe projections for the left and right eye

5 Overall Setup

Two complete collaboration setups were de-signed as shown in Figure 19 In the upper image inFigure 19 the first prototype is shown essentially a lab-oratory setup One lesson learned from this is that inmany cases only the front screen is used for collabora-tion This allows for the creation of a more design-oriented setup in a public room (as shown in the lowerpart of Figure 19) using only a single screen Since thecamera positions are known from the first prototypethere was no longer any need for a flexible installation

Figure 18 Optical response of the shutter glasses

Kunz and Spagno 189

The goal of the second setup (bottom image in Fig-ure 19) was thus not to reproduce the first installationbut rather to create an asymmetric installation for thecollaboration With the asymmetric setup applicationswere first created transferring 3D sound and 3D imagesof the people from one installation to the other Figure20 shows a simple collaboration between two individu-als engaged in a dialogue Each person is in an installa-tion at a different location on the campus

Since the first system is an experimental setup a longinitialization time was required before the collaborationcould start Thus the system is not used for hands-oncollaboration yet Further work will shorten the initial-ization phase and will allow easier access to the system

6 Conclusions

The active projection screen is driven at its physi-cal limits The transition time from opaque to transpar-ent (10 to 90) takes approximately 2 ms while the

transition from transparent to opaque (90 to 10)takes approximately 6 ms The slow transition fromtransparent to opaque is dictated by the internal relax-ation forces of the LC film and cannot be acceleratedelectrically This non-ideal behavior can be minimizedby optimizing the time pattern of the different compo-nents For the user just the hotspot of the projectorsbecomes a little more evident but it remains at an ac-ceptable level The active projection screens offer a verybright and homogenous image

The junctions between the three projection-screenpanels are visible but not disturbing The LC film of theactive projection panels is installed between two glasspanels The surface of these glass panels is treated withan antireflective coating Nevertheless some reflectionsfrom the other projection screens are evident The re-flection can be compared to the reflection on a CRTtelevision in a medium-bright illuminated room Thereflection is noticeable when the user concentrates on itand disappears as soon as he or she concentrates on theapplication Since the projectors are darkened duringthe transparent state of the projection screens the cam-era acquisition is not disturbed by any reflection Thecameras are positioned in such a way as to see the userrsquosentire body even if the user gets very close to the pro-jection screens

Figure 19 Two different collaboration setups based on the blue-c

principle

Figure 20 Collaboration between two individuals

190 PRESENCE VOLUME 13 NUMBER 2

The channel separation of the stereoscopic projectionis very good and the total performance of the projec-tion is convincing The brightness is higher than tradi-tional CRT stereo projectors at a lower price per stereoprojection rack The six-channel audio system proved tobe very powerful The user can precisely localize a simu-lated noise or sound source The subwoofer located inthe wooden platform is also important for good audioperformance In addition it can transmit vibrations tothe user at lower frequencies and this acoustic stimulusplays an important role in high immersion

The mechanical structure was shown to meet the re-quired demands Its stability and rigidity give a strongsense of confidence in the blue-c The user is never un-easy about touching the projection screen or walkingaround as in some other VR installations The flexibilityin the placement of new components and cables as wellas the quick access to all the components proved to bea very valuable feature of the blue-c construction

7 Further Work

The two setups showed that collaboration over anetwork using 3D representations is technically possibleHowever there are more problems to overcome In itstechnical aspects the system should be downsized in costand weight with the goal being a mobile installation Herenew stereo projection systems would also be tested that areeasier to install and adjust Also new projection materialssuch as active Perspex will be evaluated As a first step inthis direction we will build a demonstrator that would besuitable for presentations on conferences and exhibitionsThe demonstrator could be used to show the core tech-nology of the blue-c itself but also as another input stationfor a multipoint collaboration (Disz Papka Pellegrino ampStevens 1995 Lehner amp DeFanti 1997 Fuchs et al2002 Bailett Calahan amp McCluskey 1998)

Further work on the blue-c will also deal with imple-menting the technology in existing processes for in-stance the product development process or the educa-tion process Thus applications for the blue-c will begenerated that offer benefits to the user when using the

3D capability in collaborative work Further informationcan be found at httpblue-cethzch

Acknowledgments

This project was funded as an internal research project no0-23803-00 by the Swiss Federal Institute of TechnologyZurich We would like to thank all members of the blue-cproject

References

Bailett A Calahan J amp McCluskey S (1998) Coordina-tion at different stages of the product development processRampD Management 28 237ndash247

Bougrioua F Oepts W Vleeschouwer H D AlexanderE Neyts K amp Pauwels H (2002) Determination of theswitching response in reflective twisted nematic liquid crys-tal displays The Japan Society of Applied Physics 41 5676ndash5681

Chen H Wallace G Gupta A Li K Funkhouser T ampCook P (2002) Experiences with scalability of displaywalls Proceedings of IPTVR 2002

Cruz-Neira C Sandin D J amp DeFanti T A (1993)Surrounding-screen projection-based virtual reality Thedesign and implementation of the CAVE Proceedings ofSIGGRAPH 1993 135ndash142

Disz T Papka M Pellegrino M amp Stevens R (1995)Sharing visualization experiences among remote virtual en-vironments Proceedings of the International Workshop onHigh Performance Computing for Computer Graphics andVisualization 135ndash142

Drzaic P S (1995) Liquid Crystal Dispersion World Scien-tific Singapore

Elspass W VanGool L Gross M H Kunz A Meier MSchmitt G Staadt O et al (1999) The BlueCave ETHZurich internal research proposal

Fuchs H Bishop G Arthur K McMillan L Bajcsy RLee S et al (1994) Virtual space teleconferencing using asea of cameras (Tech Rep No TR94-033) Chapel Hill

Kunz and Spagno 191

can be set individually as outputs or inputs Each ofthese ports is connected to a LED to show its state Themicrocontroller is clocked with a frequency of 20 MHzto offer high timing accuracy

427 PDLC Glass-Panel Driver Module Todrive the nine PDLC glass panels a total of three glass-panel driver modules are used (see Figure 16) In eachcase three glass panels in parallel are connected to thetwo outputs of a module The two outputs of a modulecan be switched separately to either 8 V or 85 V If bothoutputs are switched to the same value no voltage isapplied to the glass panels If the outputs are switchedto different values a voltage of 77 V or 77 V is ap-plied to the glass panels in series with the decouplingcapacitor Each of the two outputs is switched directlyby the corresponding input coming from the data busA current-limit control is superimposed on the voltageswitching

Figure 16 shows a simplified circuit consisting of aPDLC glass-panel driver module connected to threeglass panels with their DC decoupling circuits Each ofthe two incoming TTL signals switches an optocoupleroutput to a preset voltage of 8 V or 85 V The two out-puts of the optocouplers are connected to two APEXPA46 operational amplifiers The APEX PA64 has anintegrated adjustable current-limit control and can runat voltages up to 150 V and currents up to 5 A Theoperational amplifiers are used as followers thus theinput voltage is mapped directly to the output as long asthe current-limit is not exceeded The PDLC glass-panel driver modules are connected to a 95-V power

supply However the outputs of the module have beenset to switch only between 8 V and 85 V This givesenough margin to the output voltage from the powersupply and ground to avoid saturating the operationalamplifier The current-limit of the operational amplifieris set to 900 mA which corresponds to a current-limitof 300 mA per panel

428 Shutter Driver Module The shutterdriver module also has two inputs and two outputs Itconsists of two independent circuits with an opto-coupler and a half-bridge consisting of two MOSFETsand a half-bridge driver (Figure 17) The optocoupler isused to protect the other modules of the synchroniza-tion electronics in case a fault occurs in this module orin a connected component The output of the opto-coupler is connected to a half-bridge driver (IR2111)that drives two MOSFETs The bridge driver ensuresthat the two MOSFETs in the half-bridge are neverturned on at the same time The two half-bridge out-puts switch the voltage rail-to-rail without any current-limit In order to drive the ferro-electric shutters themodule is operated with a voltage of 12 V Three shut-ters are connected in parallel between the two half-bridges of a module The two half-bridges are alwaysswitched to opposite states The bridge voltage is there-fore either 12 V or 12 V as required by the shuttersfor switching

429 Trigger Interface Module The trigger-interface module is a multifunctional module It allows achoice between up to 8 lines out of the 22 data lines onthe bus Each line can be connected to a 5 V or 12 V

Figure 16 PDLC glass-panel driver moduleFigure 17 Shutter driver module

188 PRESENCE VOLUME 13 NUMBER 2

driver and configured as input or output For the blue-call channels used were configured as 5-V outputs Onechannel is used to trigger the FireWire cameras A sec-ond one is used to trigger the LED illumination Twomore channels are used to synchronize the shutterglasses and are connected to the IR-emitter driver

4210 LED Driver A total of 9984 LEDs areinstalled in different clusters in the blue-c The LEDclusters have to be driven with a voltage of 375 V and atotal current of up to 25 A The power supply for theLED cluster is installed in the floor of the blue-c andconnected directly to the LED clusters This reduces thewiring distances of the high-current connections andthus the probability of electro-magnetic interferenceEach cluster has an integrated MOSFET to switch theLEDs on and off The triggering signal comes directlyfrom the trigger-interface module

4211 Shutter Glasses The LED illuminationis triggered 625 times per second The user does notsee any flickering because this frequency is above thetemporal resolution of the human eye However thebright light inside the blue-c diminishes the contrast ofthe projected image considerably To protect the userfrom the active light an apposite third phase has beenintegrated into the stereo glasses to keep the light outof the userrsquos eyes The projection of the images for theleft and the right eye are followed by a third phase inwhich both LC shutters of the shutter glasses are dark-ened The LED illumination is on only during thisphase This helps to fade out most of the light from theillumination

In the blue-c we use the NuVision 60GX shutterglasses from MacNaughton The shutter glasses and theIR-emitter have a built-in microcontroller We havereprogrammed it to include the third phase Like mostshutter glasses the NuVision use twisted nematic LCshutters The transparent-to-black transition timeof these shutters is quite fast while the black-to-transparent switching time takes considerably longerFigure 18 shows the switching time of the NuVisionshutter glasses The upper measurement shows thetransparency of a shutter where a high value corre-

sponds to a high transparency The lower measurementshows the voltage applied to the IR-emitter As can beseen from the measurement there is no significant delaybetween sending a signal to the emitter and the begin-ning of the switching response However the switchingtime from 10 to 90 transparency takes approximately33 ms while the switching from 90 to 10 transpar-ency takes about 05 ms These values correspond to thevalues measured by Woods and Tan (Woods amp Tan2002) with other shutter glasses For practical use thereis a work-around involving some blanking time betweenthe projections for the left and right eye

5 Overall Setup

Two complete collaboration setups were de-signed as shown in Figure 19 In the upper image inFigure 19 the first prototype is shown essentially a lab-oratory setup One lesson learned from this is that inmany cases only the front screen is used for collabora-tion This allows for the creation of a more design-oriented setup in a public room (as shown in the lowerpart of Figure 19) using only a single screen Since thecamera positions are known from the first prototypethere was no longer any need for a flexible installation

Figure 18 Optical response of the shutter glasses

Kunz and Spagno 189

The goal of the second setup (bottom image in Fig-ure 19) was thus not to reproduce the first installationbut rather to create an asymmetric installation for thecollaboration With the asymmetric setup applicationswere first created transferring 3D sound and 3D imagesof the people from one installation to the other Figure20 shows a simple collaboration between two individu-als engaged in a dialogue Each person is in an installa-tion at a different location on the campus

Since the first system is an experimental setup a longinitialization time was required before the collaborationcould start Thus the system is not used for hands-oncollaboration yet Further work will shorten the initial-ization phase and will allow easier access to the system

6 Conclusions

The active projection screen is driven at its physi-cal limits The transition time from opaque to transpar-ent (10 to 90) takes approximately 2 ms while the

transition from transparent to opaque (90 to 10)takes approximately 6 ms The slow transition fromtransparent to opaque is dictated by the internal relax-ation forces of the LC film and cannot be acceleratedelectrically This non-ideal behavior can be minimizedby optimizing the time pattern of the different compo-nents For the user just the hotspot of the projectorsbecomes a little more evident but it remains at an ac-ceptable level The active projection screens offer a verybright and homogenous image

The junctions between the three projection-screenpanels are visible but not disturbing The LC film of theactive projection panels is installed between two glasspanels The surface of these glass panels is treated withan antireflective coating Nevertheless some reflectionsfrom the other projection screens are evident The re-flection can be compared to the reflection on a CRTtelevision in a medium-bright illuminated room Thereflection is noticeable when the user concentrates on itand disappears as soon as he or she concentrates on theapplication Since the projectors are darkened duringthe transparent state of the projection screens the cam-era acquisition is not disturbed by any reflection Thecameras are positioned in such a way as to see the userrsquosentire body even if the user gets very close to the pro-jection screens

Figure 19 Two different collaboration setups based on the blue-c

principle

Figure 20 Collaboration between two individuals

190 PRESENCE VOLUME 13 NUMBER 2

The channel separation of the stereoscopic projectionis very good and the total performance of the projec-tion is convincing The brightness is higher than tradi-tional CRT stereo projectors at a lower price per stereoprojection rack The six-channel audio system proved tobe very powerful The user can precisely localize a simu-lated noise or sound source The subwoofer located inthe wooden platform is also important for good audioperformance In addition it can transmit vibrations tothe user at lower frequencies and this acoustic stimulusplays an important role in high immersion

The mechanical structure was shown to meet the re-quired demands Its stability and rigidity give a strongsense of confidence in the blue-c The user is never un-easy about touching the projection screen or walkingaround as in some other VR installations The flexibilityin the placement of new components and cables as wellas the quick access to all the components proved to bea very valuable feature of the blue-c construction

7 Further Work

The two setups showed that collaboration over anetwork using 3D representations is technically possibleHowever there are more problems to overcome In itstechnical aspects the system should be downsized in costand weight with the goal being a mobile installation Herenew stereo projection systems would also be tested that areeasier to install and adjust Also new projection materialssuch as active Perspex will be evaluated As a first step inthis direction we will build a demonstrator that would besuitable for presentations on conferences and exhibitionsThe demonstrator could be used to show the core tech-nology of the blue-c itself but also as another input stationfor a multipoint collaboration (Disz Papka Pellegrino ampStevens 1995 Lehner amp DeFanti 1997 Fuchs et al2002 Bailett Calahan amp McCluskey 1998)

Further work on the blue-c will also deal with imple-menting the technology in existing processes for in-stance the product development process or the educa-tion process Thus applications for the blue-c will begenerated that offer benefits to the user when using the

3D capability in collaborative work Further informationcan be found at httpblue-cethzch

Acknowledgments

This project was funded as an internal research project no0-23803-00 by the Swiss Federal Institute of TechnologyZurich We would like to thank all members of the blue-cproject

References

Bailett A Calahan J amp McCluskey S (1998) Coordina-tion at different stages of the product development processRampD Management 28 237ndash247

Bougrioua F Oepts W Vleeschouwer H D AlexanderE Neyts K amp Pauwels H (2002) Determination of theswitching response in reflective twisted nematic liquid crys-tal displays The Japan Society of Applied Physics 41 5676ndash5681

Chen H Wallace G Gupta A Li K Funkhouser T ampCook P (2002) Experiences with scalability of displaywalls Proceedings of IPTVR 2002

Cruz-Neira C Sandin D J amp DeFanti T A (1993)Surrounding-screen projection-based virtual reality Thedesign and implementation of the CAVE Proceedings ofSIGGRAPH 1993 135ndash142

Disz T Papka M Pellegrino M amp Stevens R (1995)Sharing visualization experiences among remote virtual en-vironments Proceedings of the International Workshop onHigh Performance Computing for Computer Graphics andVisualization 135ndash142

Drzaic P S (1995) Liquid Crystal Dispersion World Scien-tific Singapore

Elspass W VanGool L Gross M H Kunz A Meier MSchmitt G Staadt O et al (1999) The BlueCave ETHZurich internal research proposal

Fuchs H Bishop G Arthur K McMillan L Bajcsy RLee S et al (1994) Virtual space teleconferencing using asea of cameras (Tech Rep No TR94-033) Chapel Hill

Kunz and Spagno 191

driver and configured as input or output For the blue-call channels used were configured as 5-V outputs Onechannel is used to trigger the FireWire cameras A sec-ond one is used to trigger the LED illumination Twomore channels are used to synchronize the shutterglasses and are connected to the IR-emitter driver

4210 LED Driver A total of 9984 LEDs areinstalled in different clusters in the blue-c The LEDclusters have to be driven with a voltage of 375 V and atotal current of up to 25 A The power supply for theLED cluster is installed in the floor of the blue-c andconnected directly to the LED clusters This reduces thewiring distances of the high-current connections andthus the probability of electro-magnetic interferenceEach cluster has an integrated MOSFET to switch theLEDs on and off The triggering signal comes directlyfrom the trigger-interface module

4211 Shutter Glasses The LED illuminationis triggered 625 times per second The user does notsee any flickering because this frequency is above thetemporal resolution of the human eye However thebright light inside the blue-c diminishes the contrast ofthe projected image considerably To protect the userfrom the active light an apposite third phase has beenintegrated into the stereo glasses to keep the light outof the userrsquos eyes The projection of the images for theleft and the right eye are followed by a third phase inwhich both LC shutters of the shutter glasses are dark-ened The LED illumination is on only during thisphase This helps to fade out most of the light from theillumination

In the blue-c we use the NuVision 60GX shutterglasses from MacNaughton The shutter glasses and theIR-emitter have a built-in microcontroller We havereprogrammed it to include the third phase Like mostshutter glasses the NuVision use twisted nematic LCshutters The transparent-to-black transition timeof these shutters is quite fast while the black-to-transparent switching time takes considerably longerFigure 18 shows the switching time of the NuVisionshutter glasses The upper measurement shows thetransparency of a shutter where a high value corre-

sponds to a high transparency The lower measurementshows the voltage applied to the IR-emitter As can beseen from the measurement there is no significant delaybetween sending a signal to the emitter and the begin-ning of the switching response However the switchingtime from 10 to 90 transparency takes approximately33 ms while the switching from 90 to 10 transpar-ency takes about 05 ms These values correspond to thevalues measured by Woods and Tan (Woods amp Tan2002) with other shutter glasses For practical use thereis a work-around involving some blanking time betweenthe projections for the left and right eye

5 Overall Setup

Two complete collaboration setups were de-signed as shown in Figure 19 In the upper image inFigure 19 the first prototype is shown essentially a lab-oratory setup One lesson learned from this is that inmany cases only the front screen is used for collabora-tion This allows for the creation of a more design-oriented setup in a public room (as shown in the lowerpart of Figure 19) using only a single screen Since thecamera positions are known from the first prototypethere was no longer any need for a flexible installation

Figure 18 Optical response of the shutter glasses

Kunz and Spagno 189

The goal of the second setup (bottom image in Fig-ure 19) was thus not to reproduce the first installationbut rather to create an asymmetric installation for thecollaboration With the asymmetric setup applicationswere first created transferring 3D sound and 3D imagesof the people from one installation to the other Figure20 shows a simple collaboration between two individu-als engaged in a dialogue Each person is in an installa-tion at a different location on the campus

Since the first system is an experimental setup a longinitialization time was required before the collaborationcould start Thus the system is not used for hands-oncollaboration yet Further work will shorten the initial-ization phase and will allow easier access to the system

6 Conclusions

The active projection screen is driven at its physi-cal limits The transition time from opaque to transpar-ent (10 to 90) takes approximately 2 ms while the

transition from transparent to opaque (90 to 10)takes approximately 6 ms The slow transition fromtransparent to opaque is dictated by the internal relax-ation forces of the LC film and cannot be acceleratedelectrically This non-ideal behavior can be minimizedby optimizing the time pattern of the different compo-nents For the user just the hotspot of the projectorsbecomes a little more evident but it remains at an ac-ceptable level The active projection screens offer a verybright and homogenous image

The junctions between the three projection-screenpanels are visible but not disturbing The LC film of theactive projection panels is installed between two glasspanels The surface of these glass panels is treated withan antireflective coating Nevertheless some reflectionsfrom the other projection screens are evident The re-flection can be compared to the reflection on a CRTtelevision in a medium-bright illuminated room Thereflection is noticeable when the user concentrates on itand disappears as soon as he or she concentrates on theapplication Since the projectors are darkened duringthe transparent state of the projection screens the cam-era acquisition is not disturbed by any reflection Thecameras are positioned in such a way as to see the userrsquosentire body even if the user gets very close to the pro-jection screens

Figure 19 Two different collaboration setups based on the blue-c

principle

Figure 20 Collaboration between two individuals

190 PRESENCE VOLUME 13 NUMBER 2

The channel separation of the stereoscopic projectionis very good and the total performance of the projec-tion is convincing The brightness is higher than tradi-tional CRT stereo projectors at a lower price per stereoprojection rack The six-channel audio system proved tobe very powerful The user can precisely localize a simu-lated noise or sound source The subwoofer located inthe wooden platform is also important for good audioperformance In addition it can transmit vibrations tothe user at lower frequencies and this acoustic stimulusplays an important role in high immersion

The mechanical structure was shown to meet the re-quired demands Its stability and rigidity give a strongsense of confidence in the blue-c The user is never un-easy about touching the projection screen or walkingaround as in some other VR installations The flexibilityin the placement of new components and cables as wellas the quick access to all the components proved to bea very valuable feature of the blue-c construction

7 Further Work

The two setups showed that collaboration over anetwork using 3D representations is technically possibleHowever there are more problems to overcome In itstechnical aspects the system should be downsized in costand weight with the goal being a mobile installation Herenew stereo projection systems would also be tested that areeasier to install and adjust Also new projection materialssuch as active Perspex will be evaluated As a first step inthis direction we will build a demonstrator that would besuitable for presentations on conferences and exhibitionsThe demonstrator could be used to show the core tech-nology of the blue-c itself but also as another input stationfor a multipoint collaboration (Disz Papka Pellegrino ampStevens 1995 Lehner amp DeFanti 1997 Fuchs et al2002 Bailett Calahan amp McCluskey 1998)

Further work on the blue-c will also deal with imple-menting the technology in existing processes for in-stance the product development process or the educa-tion process Thus applications for the blue-c will begenerated that offer benefits to the user when using the

3D capability in collaborative work Further informationcan be found at httpblue-cethzch

Acknowledgments

This project was funded as an internal research project no0-23803-00 by the Swiss Federal Institute of TechnologyZurich We would like to thank all members of the blue-cproject

References

Bailett A Calahan J amp McCluskey S (1998) Coordina-tion at different stages of the product development processRampD Management 28 237ndash247

Bougrioua F Oepts W Vleeschouwer H D AlexanderE Neyts K amp Pauwels H (2002) Determination of theswitching response in reflective twisted nematic liquid crys-tal displays The Japan Society of Applied Physics 41 5676ndash5681

Chen H Wallace G Gupta A Li K Funkhouser T ampCook P (2002) Experiences with scalability of displaywalls Proceedings of IPTVR 2002

Cruz-Neira C Sandin D J amp DeFanti T A (1993)Surrounding-screen projection-based virtual reality Thedesign and implementation of the CAVE Proceedings ofSIGGRAPH 1993 135ndash142

Disz T Papka M Pellegrino M amp Stevens R (1995)Sharing visualization experiences among remote virtual en-vironments Proceedings of the International Workshop onHigh Performance Computing for Computer Graphics andVisualization 135ndash142

Drzaic P S (1995) Liquid Crystal Dispersion World Scien-tific Singapore

Elspass W VanGool L Gross M H Kunz A Meier MSchmitt G Staadt O et al (1999) The BlueCave ETHZurich internal research proposal

Fuchs H Bishop G Arthur K McMillan L Bajcsy RLee S et al (1994) Virtual space teleconferencing using asea of cameras (Tech Rep No TR94-033) Chapel Hill

Kunz and Spagno 191

The goal of the second setup (bottom image in Fig-ure 19) was thus not to reproduce the first installationbut rather to create an asymmetric installation for thecollaboration With the asymmetric setup applicationswere first created transferring 3D sound and 3D imagesof the people from one installation to the other Figure20 shows a simple collaboration between two individu-als engaged in a dialogue Each person is in an installa-tion at a different location on the campus

Since the first system is an experimental setup a longinitialization time was required before the collaborationcould start Thus the system is not used for hands-oncollaboration yet Further work will shorten the initial-ization phase and will allow easier access to the system

6 Conclusions

The active projection screen is driven at its physi-cal limits The transition time from opaque to transpar-ent (10 to 90) takes approximately 2 ms while the

transition from transparent to opaque (90 to 10)takes approximately 6 ms The slow transition fromtransparent to opaque is dictated by the internal relax-ation forces of the LC film and cannot be acceleratedelectrically This non-ideal behavior can be minimizedby optimizing the time pattern of the different compo-nents For the user just the hotspot of the projectorsbecomes a little more evident but it remains at an ac-ceptable level The active projection screens offer a verybright and homogenous image

The junctions between the three projection-screenpanels are visible but not disturbing The LC film of theactive projection panels is installed between two glasspanels The surface of these glass panels is treated withan antireflective coating Nevertheless some reflectionsfrom the other projection screens are evident The re-flection can be compared to the reflection on a CRTtelevision in a medium-bright illuminated room Thereflection is noticeable when the user concentrates on itand disappears as soon as he or she concentrates on theapplication Since the projectors are darkened duringthe transparent state of the projection screens the cam-era acquisition is not disturbed by any reflection Thecameras are positioned in such a way as to see the userrsquosentire body even if the user gets very close to the pro-jection screens

Figure 19 Two different collaboration setups based on the blue-c

principle

Figure 20 Collaboration between two individuals

190 PRESENCE VOLUME 13 NUMBER 2

The channel separation of the stereoscopic projectionis very good and the total performance of the projec-tion is convincing The brightness is higher than tradi-tional CRT stereo projectors at a lower price per stereoprojection rack The six-channel audio system proved tobe very powerful The user can precisely localize a simu-lated noise or sound source The subwoofer located inthe wooden platform is also important for good audioperformance In addition it can transmit vibrations tothe user at lower frequencies and this acoustic stimulusplays an important role in high immersion

The mechanical structure was shown to meet the re-quired demands Its stability and rigidity give a strongsense of confidence in the blue-c The user is never un-easy about touching the projection screen or walkingaround as in some other VR installations The flexibilityin the placement of new components and cables as wellas the quick access to all the components proved to bea very valuable feature of the blue-c construction

7 Further Work

The two setups showed that collaboration over anetwork using 3D representations is technically possibleHowever there are more problems to overcome In itstechnical aspects the system should be downsized in costand weight with the goal being a mobile installation Herenew stereo projection systems would also be tested that areeasier to install and adjust Also new projection materialssuch as active Perspex will be evaluated As a first step inthis direction we will build a demonstrator that would besuitable for presentations on conferences and exhibitionsThe demonstrator could be used to show the core tech-nology of the blue-c itself but also as another input stationfor a multipoint collaboration (Disz Papka Pellegrino ampStevens 1995 Lehner amp DeFanti 1997 Fuchs et al2002 Bailett Calahan amp McCluskey 1998)

Further work on the blue-c will also deal with imple-menting the technology in existing processes for in-stance the product development process or the educa-tion process Thus applications for the blue-c will begenerated that offer benefits to the user when using the

3D capability in collaborative work Further informationcan be found at httpblue-cethzch

Acknowledgments

This project was funded as an internal research project no0-23803-00 by the Swiss Federal Institute of TechnologyZurich We would like to thank all members of the blue-cproject

References

Bailett A Calahan J amp McCluskey S (1998) Coordina-tion at different stages of the product development processRampD Management 28 237ndash247

Bougrioua F Oepts W Vleeschouwer H D AlexanderE Neyts K amp Pauwels H (2002) Determination of theswitching response in reflective twisted nematic liquid crys-tal displays The Japan Society of Applied Physics 41 5676ndash5681

Chen H Wallace G Gupta A Li K Funkhouser T ampCook P (2002) Experiences with scalability of displaywalls Proceedings of IPTVR 2002

Cruz-Neira C Sandin D J amp DeFanti T A (1993)Surrounding-screen projection-based virtual reality Thedesign and implementation of the CAVE Proceedings ofSIGGRAPH 1993 135ndash142

Disz T Papka M Pellegrino M amp Stevens R (1995)Sharing visualization experiences among remote virtual en-vironments Proceedings of the International Workshop onHigh Performance Computing for Computer Graphics andVisualization 135ndash142

Drzaic P S (1995) Liquid Crystal Dispersion World Scien-tific Singapore

Elspass W VanGool L Gross M H Kunz A Meier MSchmitt G Staadt O et al (1999) The BlueCave ETHZurich internal research proposal

Fuchs H Bishop G Arthur K McMillan L Bajcsy RLee S et al (1994) Virtual space teleconferencing using asea of cameras (Tech Rep No TR94-033) Chapel Hill

Kunz and Spagno 191

The channel separation of the stereoscopic projectionis very good and the total performance of the projec-tion is convincing The brightness is higher than tradi-tional CRT stereo projectors at a lower price per stereoprojection rack The six-channel audio system proved tobe very powerful The user can precisely localize a simu-lated noise or sound source The subwoofer located inthe wooden platform is also important for good audioperformance In addition it can transmit vibrations tothe user at lower frequencies and this acoustic stimulusplays an important role in high immersion

The mechanical structure was shown to meet the re-quired demands Its stability and rigidity give a strongsense of confidence in the blue-c The user is never un-easy about touching the projection screen or walkingaround as in some other VR installations The flexibilityin the placement of new components and cables as wellas the quick access to all the components proved to bea very valuable feature of the blue-c construction

7 Further Work

The two setups showed that collaboration over anetwork using 3D representations is technically possibleHowever there are more problems to overcome In itstechnical aspects the system should be downsized in costand weight with the goal being a mobile installation Herenew stereo projection systems would also be tested that areeasier to install and adjust Also new projection materialssuch as active Perspex will be evaluated As a first step inthis direction we will build a demonstrator that would besuitable for presentations on conferences and exhibitionsThe demonstrator could be used to show the core tech-nology of the blue-c itself but also as another input stationfor a multipoint collaboration (Disz Papka Pellegrino ampStevens 1995 Lehner amp DeFanti 1997 Fuchs et al2002 Bailett Calahan amp McCluskey 1998)

Further work on the blue-c will also deal with imple-menting the technology in existing processes for in-stance the product development process or the educa-tion process Thus applications for the blue-c will begenerated that offer benefits to the user when using the

3D capability in collaborative work Further informationcan be found at httpblue-cethzch

Acknowledgments

This project was funded as an internal research project no0-23803-00 by the Swiss Federal Institute of TechnologyZurich We would like to thank all members of the blue-cproject

References

Bailett A Calahan J amp McCluskey S (1998) Coordina-tion at different stages of the product development processRampD Management 28 237ndash247

Bougrioua F Oepts W Vleeschouwer H D AlexanderE Neyts K amp Pauwels H (2002) Determination of theswitching response in reflective twisted nematic liquid crys-tal displays The Japan Society of Applied Physics 41 5676ndash5681

Chen H Wallace G Gupta A Li K Funkhouser T ampCook P (2002) Experiences with scalability of displaywalls Proceedings of IPTVR 2002

Cruz-Neira C Sandin D J amp DeFanti T A (1993)Surrounding-screen projection-based virtual reality Thedesign and implementation of the CAVE Proceedings ofSIGGRAPH 1993 135ndash142

Disz T Papka M Pellegrino M amp Stevens R (1995)Sharing visualization experiences among remote virtual en-vironments Proceedings of the International Workshop onHigh Performance Computing for Computer Graphics andVisualization 135ndash142

Drzaic P S (1995) Liquid Crystal Dispersion World Scien-tific Singapore

Elspass W VanGool L Gross M H Kunz A Meier MSchmitt G Staadt O et al (1999) The BlueCave ETHZurich internal research proposal

Fuchs H Bishop G Arthur K McMillan L Bajcsy RLee S et al (1994) Virtual space teleconferencing using asea of cameras (Tech Rep No TR94-033) Chapel Hill

Kunz and Spagno 191

NC The University of North Carolina at Chapel Hill De-partment of Computer Science

Fuchs D K Lindemann U Jokele B Anderl R KleinerS Fadel G et al (2002) Sensitization to the distributeddevelopment process Proceedings of the TMCE 2002 801ndash812

Gross M Kunz A Meier M amp Staadt O (2000) Theblue-c Integrating real humans into a networked immersiveenvironment Proceedings of ACM Collaborative VirtualEnvironments 201ndash202

Kunz A amp Spagno C (2001a) Modified shutter glasses forprojection and picture acquisition in virtual environmentsProceedings of IEEE Virtual Reality 2001 Conference 281ndash282

Kunz A amp Spagno C (2001b) Novel shutter glass controlfor simultaneous projection and picture acquisition Proceed-ings of Immersive Projection Technology and Virtual Envi-ronments 257ndash266

Lantz E (1996) The future of virtual reality Head mounteddisplays versus spatially immersive displays Proceedings of the23rd Annual SIGGRAPH 485ndash486

Lehner V D amp DeFanti T A (1997) Distributed virtualreality Supporting remote collaboration in vehicle designComputer Graphics amp Applications 17(2) 13ndash17

Lipton L (1991) Selection devices for field-sequential ste-reoscopic displays A brief history Proceedings of StereoscopicDisplays and Applications II 274ndash282

Lipton L (2001) The stereoscopic cinema From film to dig-ital projection Proceedings of SMPTE 586ndash593

Ogi T Yamada T amp Hirose M (1999) Integrating livevideo for immersive environments Multi Media 6(3) July-September 14ndash22

Wallace G Chen H amp Li K (2003) Color gamut match-ing for tiled display walls Proceedings of IPTEGVE 2003293ndash302

Woods A J amp Tan S S L (2002) Characterizing sourcesof ghosting in time-sequential stereoscopic video displaysProceedings of Stereoscopic Displays and Virtual Reality Sys-tems IX 66ndash77

Zorpette G (2002) Let there be light IEEE Spectrum 3970ndash74

192 PRESENCE VOLUME 13 NUMBER 2