2006 opex miyazaki_volumetric display_3d scanning

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Volumetric display system based on three-dimensional scanning of inclined optical image

Daisuke Miyazaki, Kensuke Shiba, Koji Sotsuka, and Kenji Matsushita

Department of Physical Electronics and Informatics, Graduate School of Engineering, Osaka City Universi ty, 3-3-138, Sugimoto, Sumiyoshi-ku, Osaka 558-8585 , Japan

[email protected]

ABSTRACT: A volumetric display system based on three-dimensional(3D) scanning of an inclined image is reported. An optical image of a two-dimensional (2D) display, which is a vector-scan display monitor placedobliquely in an optical imaging system, is moved laterally by a galvanomet-ric mirror scanner. Inclined cross-sectional images of a 3D object are dis-played on the 2D display in accordance with the position of the image planeto form a 3D image. Three-dimensional images formed by this display sys-tem satisfy all the criteria for stereoscopic vision because they are real im-ages formed in a 3D space. Experimental results of volumetric imagingfrom computed-tomography images and 3D animated images are presented.

2006 Optical Society of AmericaOCIS codes: (120.2040) Displays; (100.6890) Three-dimensional image processing

References and links

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3. M. Emoto, T. Niida, and F. Okano, Repeated vergence adaptation causes the decline of visual functions inwatching stereoscopic television, J. Disp. Tech. 1, 328-340 (2005).

4. B. G. Blundell and A. J. Schwarz, Volumetric Three-Dimensional Display Systems, (John Wiley & Sons,New York, 2000).

5. I. N. Kompanetsa and S. A. Gonchukov, Volumetric displays,in Current Research on Image Processingfor 3D Information Displays, V. V. Petrov, ed., Proc. SPIE 5821, 125-136 (2005).

6. A. C. Traub, "Stereoscopic display using varifocal mirror oscillations," Appl. Opt. 6, 1085-1087 (1967).7. S. Suyama, M. Date, and H. Takada, "Three-dimensional display system with dual-frequency liquid-crystalvarifocal lens," Jpn. J. Appl. Phys. 39, 480-484 (2000).

8. P. Soltan, M. Lasher, W. Dahlke, N. Acantilado, and M. McDonald, "Laser-projected 3-D volumetric dis-plays," in Cockpit Displays IV: Flat Panel Displays for Defense Applications, D. G. Hopper, ed., Proc. SPIE3057, 496-506 (1997).

9. C.-C. Tsao and J. Chen, "Volumetric display by moving screen projection with a fast-switching spatial lightmodulator," in Projection Displays IV, M. H. Wu, ed., Proc. SPIE 3296, 191-197 (1998).

10. G. E. Favalora, J. Napoli, D. M. Hall, R. K. Dorval, M. G. Giovinco, M. J. Richmond, and W. S. Chun, 100Million-voxel volumetric display, in Cockpit Displays IX: Displays for Defense Applications, D. G. Hop-per, ed., Proc. SPIE 4712, 300-312 (2002).

11. D. Miyazaki and K. Matsushita, "Volume-scanning three-dimensional display that uses an inclined imageplane," Appl. Opt. 40, 3354-3358 (2001).

12. D. Miyazaki, M. Lasher, and Y. Fainman, "Fluorescent volumetric display excited by a single infraredbeam," Appl. Opt. 44, 5281-5285 (2005).

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A. Sullivan, DepthCube solid-state 3D volumetric display, in Stereoscopic Displays and Virtual RealitySystems XI, A. J. Woods, J. O. Merritt, S. A. Benton, M. T. Bolas, eds., Proc. SPIE 5291, 279-284 (2004).15. E. Downing, L. Hesselink, J. Ralston, and R. Macfarlane, A three-color, solid-state, three-dimensional dis-

play, Science 273, 1185-1189 (1996).

#73293 - $15.00 USD Received 25 July 2006; revised 23 October 2006; accepted 27 October 2006

(C) 2006 OSA 25 December 2006 / Vol. 14, No. 26 / OPTICS EXPRESS 12760


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1. Introduction

Information processing technology for three-dimensional (3D) images is undergoing rapidprogress and is being widely adopted in various fields. The development of displays that pro-duce natural-looking stereoscopic images is expected to make such 3-D image systems moreuseful. The majority of commercially available stereoscopic displays are based on binocular

parallax. These displays have some problems, including visual confusion and fatigue causedby inconsistencies in the 3D visual information, such as between focus and binocular conver-gence [1-3]. Volumetric displays can provide 3D images, which satisfy all the criteria ofstereoscopic vision without the need for special glasses because the 3D real images that theyproduce are made up of light spots arranged in 3D space by optical scanning [4-5]. In volu-metric display systems, a light spot or a two-dimensional (2D) image is rapidly scanned in 3Dspace. A realistic full 3D image can then be observed as a result of persistence of vision.Researchers have proposed various 3D scanning methods using, for example, rotation of aprojection screen [8-10], and movement of an image plane with a varifocal lens [6,7].

We have previously described the basic principles of a volumetric 3D display system inwhich an inclined 2D image is moved in lateral directions perpendicular to an optical axisusing a mirror scanner [11, 12]. In those reports, we presented experimental results of a pre-liminary volumetric display system using an 8 8 array of light emitting diodes to confirmthe principles of our method [11]. In this paper, we report experimental results of a volumet-

ric display system, which produces higher resolution 3D images using a vector-scan cathode-ray-tube (CRT) monitor.

In Section 2, we recap the principles of the volumetric display system using an inclinedimage plane, and we discuss some advantages and problems of this method. In Section 3, wedescribe the volumetric display system that we constructed. In Section 4, we describe ex-perimental results of stereoscopic 3D images formed by the volumetric display system, andwe give some conclusions in Section 5.

2. Principle of volumetric display using inclined image plane

Figure 1 is a schematic diagram of our proposed volumetric display system. A 2D displaydevice is placed obliquely in an optical imaging system, in which a rotating mirror (mirrorscanner) is inserted. In this figure, the 2D display device is placed perpendicularly to the xzplane and is inclined with respect to the yz plane in the object space of the imaging system.An inclined planar image is formed in the image space of the optical system. When the mirror

is rotated about a shaft parallel to the y-axis, the inclined planar image in the image space,whose coordinate system isx',y',z', is moved laterally in the x' direction. A locus of the mov-ing image can be observed as a series of moving afterimages as a result of the high-speed rota-tion of the mirror. Inclined cross-sectional images of a 3D object are displayed on the 2Ddisplay device in accordance with the position of the image plane. A 3D real image is thusformed as a stack of 2D cross-sectional images.




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Fig. 1. Schematic diagram of the volumetric display system using an inclined 2D display de-vice and a mirror scanner.

In a non-telecentric imaging system, the magnification of an image varies with the depth.Therefore, cross-sectional patterns that are displayed on the 2D display device must be de-

formed in advance by taking into consideration this change in magnification to form a non-deformed 3D image. In general, the distance moved by an image in response to rotation of themirror varies according to the distance from the lens. This causes the angle of the inclinedplanar image to change with the angle of the mirror. Therefore, it is necessary to prepare sec-tional images at angles corresponding to the changing angle of the image plane. A 4-fopticalsystem composed of two lenses and a mirror scanner placed at the common focal plane ofboth lenses is telecentric in both image and object spaces. In this optical system, neither themagnification nor the distance that the image moves depends on the depth.

Fig. 2. The relationship of the 3D image region and the movement of the inclined image plane.

The angle of the 2D display device affects the scanning efficiency in terms of scanningdistance and number of scanning planes required. Let the widths of the display region of a 3Dimage be, a, b, and c in the x', y', and z' directions, respectively. We assume that the angle ofthe inclined image plane, is constant for each shifted image position of the mirror scanner.Thus, the image must be moved by the mirror scanner through a distance,

d= a +

c

tan , (1)

in order to scan the entire 3D image region, as shown in Fig. 2. Thus, the distance, d, be-comes shorter at a larger angle of the inclined image plane. If the frame rate of the 3D imagesisf, and the pixel interval in the x' direction of the 3D image is g, the refresh rate of the 2Ddisplay device, r, is required to be




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r=f

ga + c

tan . (2)

Accordingly, a larger angle of the inclined image plane moderates the refresh rate requirementof the display device. In addition, the size of the 2D image, which is parallel to the xz plane,needs to be c/sin . Thus, the required area for the 2D display device is smaller for larger an-gles of the inclined image plane. In conclusion, a larger angle of the image plane is therefore

more desirable. In the experimental system we constructed, the angle of the inclined imageplane was 45 degrees because it was easy to generate cross-sectional images from voxel dataat this angle and to match the pixel pitch for every point of the 3D image.

3. Experimental volumetric display system using a vector-scan CRT monitor

A schematic diagram and a photograph of the constructed system are shown in Figs. 3 and 4,respectively. Concave mirrors were used as optical imaging elements because it is easier toproduce concave mirrors with larger diameter and larger numerical aperture compared withrefractive lenses. The diameter of the concave mirror was 152 mm, and the focal length was152 mm. Two concave mirrors were arranged adjacent to each other in the vertical direction.A vector-scanning CRT monitor was used to display cross-sectional images of the object to beviewed. The CRT monitor was inclined upward by 45 degrees relative to the optical axis. Agalvanometric mirror scanner of dimensions 100 mm in length and 50 mm in width was used.The long sides of the mirror were aligned parallel to the horizontal direction. The galvano-

metric mirror was controlled by a triangular wave signal synchronized to the CRT monitorsignal produced in each frame. A real optical image of the 2D image displayed on the CRTmonitor was formed by the two concave mirrors and the galvanometric mirror. The distancefrom the monitor to the concave mirror and the distance from the galvanometric mirror to thetwo concave mirrors were adjusted to be equal to the focal length of the concave mirror inorder to make the optical system telecentric. The optical image was moved in the verticaldirection by rotating the mirror on a horizontal shaft.

Fig. 3. Schematic diagram of the volumetric display system.




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Fig. 4 Photograph of the volumetric display system.

To produce flicker-free 3D images, every 3D scanning period must be completed withinthe afterimage (persistence) threshold of the human eye, namely, a few tens of milliseconds[13]. It is necessary to switch the cross-sectional images displayed on the CRT monitor at afrequency equal to the product of the 3D frame rate and the number of cross-sectional images.

Since it is difficult to achieve such a high frame rate using a raster-scan display device, thevector-scan CRT monitor was used in this experiment. The vector-scan CRT monitor candraw an image consisting of a small number of lines quickly. The display area was 82 mmhigh and 102 mm wide. The brightness controller generated a brightness signal for the CRTmonitor, to turn the monitor off in the return path of the reciprocating motion of the galvano-metric mirror to avoid displaying an image twice.

Display data stored in a computer was converted into an analog signal and sent to the vec-tor-scan CRT monitor via a 24-bit video interface. The red, green, and blue (RGB) color sig-nals of the video interface were used for the x-coordinate, the y-coordinate, and the bright-ness, respectively. The allocation of the signals can be exchanged, because the formats of theRGB signals is equivalent. A 60-Hz vertical synchronizing signal of the video interface wasused as a trigger to generate a signal for controlling the galvanometric mirror. Each cross-sectional image could be drawn in a period corresponding to the 38-kHz frequency of the vec-tor-scan CRT monitor. The resolution of each cross-sectional image was 256 256 becausethe red and green signals from the video interface, specifying the x- and y-coordinates, were8-bit signals. The memory capacity of the video interface restricted the number of samplingpoints of each cross-sectional image and the number of cross-sectional, which were 1280 and1040, respectively.




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(a)

(b)

Fig. 5. Square wave shapes displayed on the vector-scan CRT monitor with frequencies of (a)2.2 MHz and (b) 5.8 MHz.

Fig. 6. Frequency response of vector-scan CRT monitor.

We drew square wave patterns on the CRT display to measure its frequency response.Figure 5 shows the images displayed the CRT monitor when square waves of frequencies 2.2MHz and 5.8 MHz were input. The broken lines show the ideal square waves responses. Thewaveforms were distorted due to the limited time response of the CRT monitor. Figure 6shows the degradation of the amplitude of the square wave pattern versus frequency. Themaximum modulation frequency of the signal input to the CRT monitor from the VGA inter-face was about 35 MHz, which was too high for drawing the required patterns. To reduce thetemporal frequency of the signal, we inserted dark points for each point, corresponding to thetransition time of the CRT, to prevent unwanted bright lines appearing during the transitionperiod. We observed the quality of several 3-D images, which contained the various numbers

of the dark points. We decided to use nine dark points, because it was the minimum numberfor keeping the quality of the 3-D image.




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4. Experimental results of stereoscopic image formation by the volumetric display sys-tem

Here, we show experimental results of stereoscopic 3D images formed by the volumetric dis-play system. We used 3D computer tomography (CT) images as the object data. A stack ofcross-sectional CT image slices was processed using 3D graphics software to extract regionsof interest, to interpolate the slices, and to smooth the 3D images. In principle, a 3D image

formed by the volumetric display was a phantom image; that is to say, a hidden (occluded)part of the object could be seen through a front part of the object even though the slices over-lapped each other. To solve this problem, we removed hidden surfaces of the object using 3-D graphics software. Only the front-most points on contours of the object were retained ineach cross-sectional image. We determined the coordinates and the degrees of brightness ofthe points on the contours of the cross-sectional images inclined at 45 degrees to display thethose images on the vector-scan CRT monitor.

Figures 7(a) and 7(b) show photographs of a stereoscopic 3D image observed from thefront and from the left at an oblique angle, respectively. The 3D images changed smoothlydepending on the observer position. We confirmed that the 3D images had natural depth per-ception. The maximum viewing angles in the horizontal and vertical directions were about 30degrees and 15 degrees, respectively. The image region was a cube of 6 cm on each side.These image size and viewing angle can be predicted from the observation region restrictedby the numerical aperture of the optical system described in reference [11]. If the sum of the

width of a 3-D image li and the effective aperture of a mirror scanner lm is smaller than thediameter of a lensD (li + lm D (li < D), lo is expressed by

lo =s(D li)

f li. (4)

The maximum viewing angle can be estimated by 2 tan-1

(lo/2s). The estimation of the ex-

perimental system is in agreement with the observed results.

Figure 8 shows a movie of a spinning 3D image. A 3D movie of the spinning object wascreated from the CT images using the 3D graphics software. Each frame of the 3-D moviewas coded for the volumetric display in advance and was transferred from the computer to theCRT monitor continually to display a continually moving 3-D image.

There were undesired vertical trails from bright points in the 3D images formed by the ex-perimental system. They were caused by persistence of the phosphors of the CRT monitor.In order to form clearer images, it will be necessary to use phosphors with low persistence, oranother type of high-speed display device.




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(a)

(b)

Fig. 7. Three-dimensional images formed with the volumetric display. The 3D images were

viewed (a) from the front and (b) from the left at an oblique angle.

Fig. 8. (100KB) Movie of a volumetric image of a spinning human skull.

5. Discussions

One of the keys in developing a practical volumetric display is an effective 3-D scanningmethod. Table 1 shows a comparative table of the proposed method and other major volumet-ric display methods, which based on a rotating screen [10], a stack of switchable liquid crystalscattering shutters [14], two-beam activation of upconversion fluorescence [15], and a movingimage plane with a varifocal lens [6], respectively. Some advantages of our method, such asthe uniformity of voxel attribute are caused by translational motion of an optical real imagewithout changing the focal length of an imaging system. Moreover, optical image formationin the air without a project screen is advantageous for constructing an interactive system to

access the 3D image directly using 3D positioning devices. The restriction of viewing angleby the numerical aperture of the optical system is a disadvantageous point due to observationof an optical real image. Large lenses with a large numerical aperture and a large scanningmirror are required in order to increase the observation area. However, a large numerical ap-erture causes larger optical aberrations, and it is not straightforward to construct a large mirrorscanner. Especially, aberrations of curvature and distortion of the image field affected the


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distortion of the 3-D images in the experiments. These issues and tradeoffs must be consid-ered when designing and constructing the system.

The proposed volumetric display method is suitable for 3D imaging applications, whichrequire accurate grasp of the 3-D structure of volumetric data, such as 3D medical imaging byx-ray computerized tomography or magnetic resonance, because of the uniform voxel attrib-ute. The experimental system is, however, not suitable for displaying 3D images containingmany valued voxels because of the restriction of the memory capacity and the time response

of the CRT display unit. Moreover, luminous persistence of the CRT disturbed fine imaging.These disadvantages were caused by the performance of the display unit used in the experi-mental system. The quality of a 3-D image can be improved by using a 2D display device,which has high time-response and low persistence, such as a DMD used in other volumetricdisplay systems [10, 14]. Monochromatic image is another disadvantage of the prototype sys-tem. Multicolored imaging can be achieved by providing three display units for three primarycolors in the similar way as other volumetric displays [10, 14].

Table 1 Comparison of the proposed method with conventional volumetric displays.

SystemLimitation of Size of 3-D image space

Attributes of voxel Viewing angle

Rotating screen[10]

Size of Rotating screenrestricted by the drivingpower

Enlargementof voxelsalong the rotationdirection as the distancefrom the rotation axis.

Invisible voxels on thescreen parallel to theviewing direction.

360

Stack of switchableliquid crystal scat-tering shutters [14]

Limitation of the imagedepth by the depth offocus of the projectionoptical system

Larger separation ofvoxels in depth directionthan other direction

Limitation of the number

of voxels bytransparency of the panel

< 180

Two beam activa-tion of upconver-sion fluorescence[15]

Size of fluorescentmedium

Beam alignmentproblem for small voxel

Gohst voxel problem

360

Moving imageplane with a varifo-cal lens [6]

Size of varifocal mirror Change of voxel sizedepending on depth dueto the change ofmagnification

Depending on numericalaperture of the opticalsystem

Proposed methodSize of optical imagingcomponents

Uniformity of size,density, and brightness

Depending on numericalaperture of the opticalsystem




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Table 1 Comparison of the proposed method with conventional volumetric displays. (Continued)

System Color reproduction Optical aberrationsInteraction with the 3Dimage space

Rotating screen Achieved Small Impossible

Stack of switchableliquid crystal scat-tering shutters

Achieved Small Impossible

Two beam activa-tion of upconver-sion fluorescence

Difficult of individualactivation of eachwavelengths

Small Impossible

Moving imageplane with a varifo-cal lens

Possible byimprovement of adisplay unit

Certain aberrations dueto large numericalaperture and varifocallens

Possible

Proposed method

Possible by

improvement of adisplay unit

Certain aberrations due

to large numericalaperture

Possible

6. Conclusions

We have demonstrated a volumetric display system based on 3D scanning using an inclinedimage plane. Three-dimensional images consisting of 256 256 256 pixels can be formedusing a vector-scan CRT monitor for displaying 2D cross-sectional images and a galvanomet-ric mirror scanner for scanning a 3D volume. It was possible to observe stereoscopic 3D im-ages in which all stereoscopic visual factors were satisfied. Improvement of the CRT displayunit, especially suppression of the luminous persistence and the frequency response, isrequired to achieve fine image formation.

Acknowledgment

This research was partially supported by the Ministry of Education, Science, Sports and Cul-ture, Grant-in-Aid for Scientific Research (C), 17560034, 2005, 2006.



2006 opex miyazaki_volumetric display_3d scanning

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