Hardware Shadow Mapping

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1、HardwareShadowMappingCassEverittAshuRegeCemCebenoyancass@nvidia.comarege@nvidia.comcem@nvidia.comWithShadowsWithoutShadowsIntroductionShadowsmake3Dcomputergraphicslookbetter.Withoutthem,scenesoftenfeelunnaturalandflat,andtherelativedepthsofobjectsinthescenecanbevery

2、unclear.Thetroublewithrenderinghighqualityshadowsisthattheyrequireavisibilitytestforeachlightsourceateachrasterizedfragment.Forraytracers,addinganextravisibilitytestistrivial,butforrasterizers,itisnot.Fortunately,thereareanumberofcommoncaseswherethelightvisibilityte

3、stcanbeefficientlyperformedbyarasterizer.Thetwomostcommontechniquesforhardwareacceleratedcomplexshadowingarestenciledshadowvolumesandshadowmapping.Thisdocumentwillfocusonusingshadowmappingtoimplementshadowingforspotlights.Shadowmappingisanimage-basedshadowingtechniq

4、uedevelopedbyLanceWilliams[8]in1978.Itisparticularlyamenabletohardwareimplementationbecauseitmakesuseofexistinghardwarefunctionality–texturinganddepthbuffering.Theonlyextraburdenitplacesonhardwareistheneedtoperformahigh-precisionscalarcomparisonforeachtexelfetchedfr

5、omtheshadowmaptexture.Shadowmapsarealsoattractivetoapplicationprogrammersbecausetheyareveryeasytouse,andunlikestenciledshadowvolumes,theyrequirenoadditionalgeometryprocessing.Hardwareacceleratedshadowmapping[5]isavailabletodayonGeForce3GPUs.ItisexposedinOpenGL[4]thr

6、oughtheSGIX_shadowandSGIX_depth_textureextensions[6],andinDirect3D8throughaspecialtextureformat.Figure1.ThesediagramsweretakenfromMarkKilgard’sshadowmappingpresentationatGDC2001.Theyillustratetheshadowingcomparisonthatoccursinshadowmapping.HowItWorksThecleverinsight

7、ofshadowmappingisthatthedepthbuffergeneratedbyrenderingthescenefromthelight’spointofviewisapre-computedlightvisibilitytestoverthelight’sviewvolume.Thelight’sdepthbuffer(a.k.a.theshadowmap)partitionstheviewvolumeofthelightintotworegions:theshadowedregionandtheunshado

8、wedregion.Thevisibilitytestisoftheformpz£shadow_map(px,py)wherepisapointinthelight’simagespace.Shadowmappingreallyhappensinthetextureunit,