Integrating Dynamic Deformations into Interactive Volume(3)

Anyproxygeometrycanbeusedtogeneratethefrag-mentsthatsamplethevolume.Thisenablessamplingalongarbitrarylinesandplanes.Toensurethatthesesamplescanbereadfromtheframebuffer,itisimportantthatthelineorplaneisviewaligned.Ourstrategyforsamplinganarbitrarylineorplaneistorotatetheviewsothatitisparalleltotheimageplane,rendertheelement,andthenreadtheresultsfromtheframebuffer.

Wenotethatotherapproachesforinteractivevolumede-formationsthatrelyondeformingthemeshoftheproxyge-ometry(§6.1)wouldrequiremorecomplexapproachestorealizingthealternatesamplingstrategies.

Wediscussmethodsthatexploitthefreedominproxyge-ometrytoperformray-castingandarbitraryslicing,andtodisplaymultipledeformedvolumessimultaneously.4.1.PickingandProbing

Onesimplesamplingtechniqueinvolvestheproxygeome-tryofaline.Theuseofalineasproxygeometryallowsustodoalinearprobe,whichissimilartothatofaray

cast.

cTheEurographicsAssociation2006.Figure5:ArenderingofahumanheadcapturedviaCT

imaging(512x512x106).Thisrenderingdisplaysaclippingplanewithasamplingplanerenderedbothontheclippingplaneandinadetachedviewportontheimageplane.Weobtainlinearprobesbyrotatingobjectspacetoalignthedesiredlineparalleltotheimageplane.Wethenrasterizethisline,directingfragmentshaderoutputandhencevolumesamplingoutputtothebackframebuffer,givingusahigh-resolutionsamplingofthedeformedspace.Wecanthencopythislinetomainmemoryforusebyuser-interfaces.Ourmethodsimpli essuchlinearprobessinceweonlyneedtodrawasinglelinetoobtaintheraycastinformationthroughthedeformedvolume.Withoutencapsulatingthede-formationinsideofthefragmentshader,thislinewouldneedtobedividedintoseverallinestoperformapiece-wiselinearapproximationofthedeformation.

Theabilitytocomputeaprobecreatesanumberofuserinteractionpossibilities.Forexample,auser-interfacecalledraypickingmayneedtodetermineatwhatdepththe rstdeformed“surface”occursunderthemouse.Anumberofdifferentprobescouldbeusedtodeterminethisinforma-tion.Onesuchprobewouldscanthegeneratedlineforthe rstnon-zeroalphavalue.Anothersuchprobemightscanthelineforthe rstgradientwith“large”magnitude.Athirdprobecouldsumalphavaluesuntiltheyexceedone,imply-ingin niteabsorptionofanythingfartherback.4.2.Slicing

Asecondproxygeometrythatisusefulforuserinterfacesisthatofaplane.Thoughplanesareusedasproxygeometryinthefullvolumerendering,theyhaveanother,commonlysoughtafteruse.Slicingplanesareidealforremovingdepthcomplexitywhenexploringvolumes.

Ourimplementationcansamplearbitraryplanesbyview-aligningthemandrenderingtothebackbuffer.Thiscanei-therbedisplayedinaseparateviewport,orappliedasatex-turedpolygoninobjectspace.BothdisplaysareillustratedinFigure5.

Non-linear geometric deformation (or warping) is a useful tool for working with volumes. Unfortunately, the computational expense of performing the resampling needed to implement volume deformation has precluded its use in interactive applications. In this

Figure6:AcruderegistrationofamousetorsocapturedviaMRI(256x256x192),showninblue/grey,toasecondmousecapturedviaCT(256x256x385),showninblack/red.Bothdatasetsandtheirassociatedwarptexturesarepassedtothefragmentshaders,wheretheyareshadedandmixed.

Toillustratetheusefulnessoftheseslicingplanes,con-sideravolumeobtainedthroughCTimagingasshowninFigure5.Aslicingplanealignedalongthedeformedobjectspaceaxiscanallowascientisttocomparethedeformedsliceagainstastandardatlasofthehead.Additionally,arbi-trarilyorientedslicescanbeexaminedandcompared.4.3.MultipleVolumeDisplay

Athirdanduniquebene tofarbitraryproxygeometriesisforthedisplayofmultipledeformedvolumes.Tocorrectlysamplespace,aproxygeometrysamplecannotoverlapotherproxygeometrysamples.Thisimpliesthatinordertocor-rectlyrendertwovolumesinthesamespace,theirsamplesmustbetakenandcombinedfromthesameproxygeometry.Therefore,techniquesthatadaptivelytessellatetheproxyge-ometriesdependentonthedeformationwouldhaveto ndanadaptivetessellationthatsatis esthedeformationsofeachvolume.

Ourmethodallowsustorendermultiplevolumesusingstandardproxygeometries,passingbothtexturevolumestothefragmentshaders.Thefragmentshadersthenhavetheadditionalfreedomtocomputetheiremissionsandabsorp-tionsbasedondifferentcombinationstrategies:sum,differ-ence,emittedcolormixing,etc.AscanbeseeninFigure6,theserenderingscanbevisuallycomplexanddif culttoin-terpret.Makinguseofsimultaneousvolumedisplayisanareaforfuturework.

esofDynamicVolumeDeformation

Therenderingandsamplingtechniquesdescribedinsections3and4providethebuildingblocksneededforanumberofapplications.Wewilloutlinesomefeaturesthatwehaveimplementedthatshowtheversatilityofourapproach.5.1.InteractiveControlofVolumeDeformationsWehaveimplementeddeformationsthatareinteractivelycontrolledbyadjustingtheirparameters.Theprimaryuse

ofthisislandmarkdeformationwhereasetofuserspeci- edpointsarecontrolled.Thismaybeusedforperforminginteractiveregistration[FRR96]

Landmarkdeformationfunctionsuseasetofpointpairsasparameters,whereonepointineachpair,thesource,isinundeformedobjectspace,andthesecondpoint,thetarget,isindeformedobjectspace.Thegoaloftheselandmarkbaseddeformationfunctionsisto ndamappingthateitherexactlyorapproximatelymapsbetweenallsourcesandtheircor-respondingtargets.TwoexamplesofdeformationsthatcanbeusedaslandmarkdeformationfunctionsareThinPlateSplinesandHierarchicalB-Splines.

Toaddmeaningfulpointpairs,weneedtobeabletoeas-ilyspecifysigni cantsurfacefeaturepoints.Theraypicker(§4.1)ingaraypicker,wecanallowtheusertoholddownthemouseandmoveapointalongthesurfaceofthevolume,evenifithasbeendeformed.Thisallowstheusertospecifyacoordinatein2D,andallowsthesystemtoinferthedepthalongtheraythattheuserwantstoselectandwith nerresolutionthanisactuallyusedtorenderthevolume.

Onceplaced,controlpointscaneitherbemanipulatedbyusingtheraypickertodragthepointsalongaspeci edsur-face,ordraggedinviewalignedplanes.Sinceavastmajorityoftheworkfordisplayingthedeformedvolumeisof oadedtotheGPU,theCPUcanbeutilizedmoreforsolvingthede …… 此处隐藏：6134字，全部文档内容请下载后查看。喜欢就下载吧 ……