FastHolographiclikeStereogramsDisplayusingShellRenderingandaHolographicScreen

CandidoF.X.deMendonaa,AlexandreX.FalcoaCesarA.Vanninib,JosJ.Lunazzic

aInstituteofComputing,StateUniversityofCampinas,Campinas,SP,BrazilbMathandNaturalSciencesCenter,SacredHeartCollege,Bauru,SP,Brazil

cInstituteofOptics,StateUniversityofCampinas,Campinas,SP,Brazil

SendcorrespondencetoC.F.X.M.,Email:xavier@dcc.unicamp.br,Telephone:+55(19)7885875,FAX+55(19)7885847

Background:Displaysystemsbasedoncomputergraphicstechniquesusuallycreate2.5Dimagedisplayona2Dscreen.Toobtain3Dimagedisplay,asystemhastoexploitsomedepthcues,suchashorizontalandbinocularparallaxes,thatcanonlyberepresentedina3Dspace.Thistypeofsystemisdividedintwomajorparadigms:Stereography(thatmakesuseofstereoparallax)andHolography(thatmakesuseofalldepthcues).Stereographyconsistsofgivingtoeacheyeadifferentviewofthescene.Adrawbackinstereographyistheuseofauxiliarydevicesorviewingtricks(e.g,polarizedglasses,virtualrealityhelmet,anddivergentviewing).Insomecases,suchasinVirtualReality,aconsiderableamountofcomputationalresourcesisalsorequiredondetectingandredrawingscenesintheauxiliarydevicesforeverynewlocationoftheobserver.Holographyconsistsofaprocesstoregisterthreedimensionalinformationofoneobject.Traditionalholographymakesuseofhardcopyhologramstoregisterthe3Dinformationofobjectsasstereograms.Thisinformationisretrievedfordisplaybyilluminatingthehologramwithareferencelightbeam.Recently,holographiclikestereogramscanbevirtuallycreatedandstoredbyacomputerandsubsequentlydisplayedonaholographicscreen(i.e.atransparentfilmwithgratingandfocusingopticalproperties,see[1]).Themainadvantageofthissystemovertheothersisthatitrequireslesscomputationalefforttogenerateanddisplay3Dimages.Thistechniqueiscalledholoprojection.

MotivationandResearchAims:Ourmotivationforthisworkstemsatthenumerousapplicationsthatcanbedevelopedbyusingholoprojectiontoprovidereal3Dvisualization.

Inholoprojection,ouraimsofresearchare:

3Dimagedisplayinanaturalway(i.e.withouttheuseofauxiliarydevicesorviewingtricks,suchaspolarizedglasses,virtualrealityhelmet,detectionofobserver'slocation,divergentviewing,etc)3Dimagedisplaywithstereoandhorizontalparallaxessuchasseeninholograms.HorizontalparallaxshouldbeobservedwithoutextracomputationFast3Dvisualizationinmedicalimagingapplications.

Whatisitallabout?WedescribetheincrementalimprovementoftheresearchoverholoprojectionthathasresultedintwopreviousversionsofHoloprojectorsandanewversionthatisintroducedinthiswork.Wepurpose:

A3DdisplaysystemtovisualizeholographiclikestereogramsonaholographicscreenAfastholoprojectiontechniquebasedonshellrenderingtodiscretizehorizontalparallaxbycodingseveralviewsofa3Dobjectformingaholographiclikestereogram.

Wealsodemonstratethenewsystemusingmedicalimagedata.

FirstHoloprojector:Thefirstholoprojector[2]consistsofanoptomechanicalsystemwithawhitelightsource,threemirrorsconnectedtostepmotors(Dx,DyandDz),areflectivediffractiongrating,aprojectivelensandaholographicscreen(seeFigure1).ThemirrorsDxandDyplacethebeamoflightindifferentxandylocationsontothediffractiongrating.Duetothephysicsofthediffractiongrating,thewhitebeamoflightisdividedintotwobeamsoflight.Thefirstbeamoflight,whichcarriesonmostofthebrightness,isdisregarded.Thesecondbeamoflightisaspectralrangeoflight.Thisrangeoflightiswiderasfartherthegratingisfromtheprojectivelens,sincetheoriginoftherangeoflightistheplacewherethewhitebeamoflighthitsthediffractiongrating.Therangeoflightcrossestheprojectivelens,andduetotheconvergencepropertyofthelens,themirrorDzpositionsthecrossingpointtowardstheholographicscreen.Thentheobserverlookingattheholographicscreencanperceivethedepthofthevoxelformedbythecrossingpoint(seeFigure2,whereRE=righteyeandLE=lefteye).Thus,thesetofvoxelsformedbyallcombinationofmovementsofthemirrorsDxandDyformaplane(oralmostaplane)ofvoxels.ThemirrorDzwillplacethisplaneofvoxelstowardstheholographicscreenindifferentpositionsformingadisplayingvolume.Theobserverseesamovingpointoflightthatdrawsobjectsverypreciselyas,forexample,theedgesofacubeinreal3Dvolume.

FirstHoloprojector:Schema

FIGURE1

DepthEncodingProcess:

FIGURE2

SecondHoloprojector:InthesecondHoloprojector[3],themirrorsDxandDyareeliminatedfromtheset,andaLCDprojector(seeFigure3)replacesthelightsource.Asthewholeplanecanbedisplayedatatime,thescenemustbeslicedinanarrayofconsecutiveplanestransversallypositionedthroughtheholographicscreenbythemirrorDz.Thereflectivediffractiongratingisreplacedbyatransparentdiffractiongratingandtherestofthesystemremainsthesame.Theimagesarethenprojectedratherthanbeingmappedbythemovementofthemirrors.Thesceneshownbythissystemisgeneratedbyaraytracingprogrammodifiedtoallowtheslicingprocess.Theslicingprocessdepictsavirtualversionofhowabakerslicesaloafofbread.Figure4displaysastereopairofcuttingplanesoftheslicingprocessofascene.Eachsliceisdisplayedasatwodimensionaltexturedobjectthesetoftwodimensionalslicesbuildsavolumedisplay.

SecondHoloprojector:Schema

FIGURE3

SlicingProcess:

FIGURE4

ThirdHoloprojector:InthethirdHoloprojector,weeliminatethediffractiongrating,theobjectivelensandtheoscillatingmirrorDz.ItconsistssimplyofaLCDprojectorandaholographicscreen(seeFigure5).Todisplaya3Dobjectinthissystem,wefirstcreateanorderedsequenceofnviewsoftheobjectwithdiscretehorizontalparallaxandselectanorderedsequenceofnhuesinthespectrumofvisiblepurecolors.Toeachviewweassignadifferenthuefollowingtheorderofthesequences.Thuswehavecodednviewsformingaframeofholographiclikestereogram.Ananimationcanbecreatedasasequenceofframeslikethis.Byprojectingtheholographiclikestereogramsontotheholographicscreen,weobtainnviewsoftheobjectperframeandtheobservercansee3Dimagesoftheobjectperframewithdiscretehorizontalparallax.AstheLCDprojectorisbasedonRGBcolors,wehaveusedjustthreepurecolorsRed,GreenandBlueatthemoment.

Holoprojector3.0:Schema

FIGURE5

3DVisualization:MedicalImagingApplicationsThemainpointinthisworkistodemonstratethatthethirdversionoftheHoloprojector(Figure5)canbeusedinmedicalimagingapplicationstoprovidefast3Dimagedisplayofhumaninternalstructures.Inthiscase,therequirementforafastrenderingtechniqueiscrucial,sinceeachholographicframecorrespondstothree(inthecaseofthreepurecolors)renderedviewsofthestructurewithdiscretehorizontalparallax.Shellrendering,proposedbyUdupaandOdhner[4],canbeplacedasoneofthefastestrenderingtechniquesdevelopedsofar.Inthiswork,weadoptedaparticularimplementationoftheshelldatastructureforsurfacerendering.Inmedicalimaging,a3Dobjectmayberepresentedbyasetofvoxelsinthevicinityofitsboundary.Ashellconsistsofthissetofvoxelstogetherwithanumberofattributesassociatedwiththevoxelsinthisset.Theshellisstoredinaspecialdatastructurethatallowsrandomaccesstothevoxelsandtheirattributes.Shellrenderingisaveryfastvoxelsprojectiontechniquethatcreates2.5Dviewsoftheobjectfromtheshelldatastructure.Thus,foranygivenlocationoftheobserver,wecodethreeviewsoftheobject(i.e.Red,GreenandBlue)withdiscretehorizontalparallaxandusethethirdHoloprojectortodisplaythis3Dimageonourholographicscreen.

ExperimentsandResults:Ourprototypedisplaysystem(thirdHoloprojector)consistsofaNTSCLCD60Hzprojectorandaholographicscreen.Thisholographicscreencanshowanynumberofviewswithinabout6degreesofhorizontalparallax.WehavechosenanentireCTskull,wheretheboundaryisrepresentedby1,500,000voxels,andaSUNSparc464MBRAMforourexperiments.Sincetypicalprojectorssupportthreecolors,wehavecodedthreeviews(withresolution320x320pixels)oftheskullwithhorizontalparallaxtoformaholographiclikestereogram(seeFigure6).Thistasktakesabout1secondusingshellrendering.TheholographiclikestereogramissubsequentlydisplayedfreeofflickingontotheholographicscreenbytheNTSCprojector.Therefore,thewholesystemallowsreal3Dinteractivevisualizationwithoutauxiliarydevicesorviewingtricks.

ViewingEncodingProcess:

FIGURE6

ConcludingRemarks:Wehavepresentedanewtechniquebasedonshellrendering,whichcangenerateholographiclikestereogramsfromtypicalmedicalimagedatainrealtime.Wehavealsopresentedanew3Ddisplaysystemtovisualizeholographiclikestereogramsonaholographicscreen,withoutviewingtricksorauxiliarydevices.Today,oursystemhassomelimitations:6degreesofhorizontalparallax,thedistanceoftheprojection,therangedistanceoftheobserverand3codedviews.Thefirstthreelimitationscanberelaxedonthebuildingprocessofthescreenandthenumberofcodedviews(discretehorizontalparallax)canbeincreasedto64byusingagrayLCD2KHzprojector,availabletoday,anda60Hzarrangementof64huefiltersasshowninFigure7.Withthisarrangement,wecandisplayonourholographicscreen64viewsoftheobjectperframewithnoflicking.

Holoprojector4.0:Schema

FIGURE7

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