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Thetransi-tionzonebetweenscleraandcorneais calledthelimbusandhasafuzzyappearance.Theeyecontainsthreebasiclayers:thefibrous(corneo-scleral)coat,theuveaoruvealtract(choroid,ciliarybodyandiris)andtheneurallayer(retina).Thesecoatssurroundthecontentsoftheeye,whicharethelens,theaqueoushumorandthevitreousbody[Forresteretal.2001].Thecornea,withanindex ofrefractionof1.376,is responsibleformostoftherefractive poweroftheeye.Afterthecornea,lightpassesthroughtheaqueoushu-mor(index ofrefraction1.336),thelens(index ofrefractionrangesfrom1.386intheouterlayersto1.406atthecenter)andthevit-reousbody(index ofrefraction1.337)beforeit reachestheretina[Hecht1987]DrawingbyKarenLefohn.Usedwithpermission.2TheHumanEye2.1OverviewThehumaneyeis anorgandesignedforphoto-reception;i.e.con-vertinglightenergytonerve actionpotentials.Theseactionpo-tentialsaresubsequentlyrelayedtotheopticnerve andthebrain,wherefurtherprocessingoccurs,resultinginconsciousvision.Allstructuresintheeyearesubservienttothisphysiologicalprocess[Forresteret al.2001].Thelargeststructureoftheeyeisthesclera(Figure1),whichis approximatelysphericalwitha radiusofaround11.5mm.Thecorneasitssomewhatinfrontofthisstructureandhasa radiusof7.8mm.Thescleratakesaround56thofthecircumferenceoftheeye,whilethecorneais responsiblefortheother16th.

Whilerecentadvancesinmodelingandrenderinghave generallyfacilitatedtheproductionofrealisticimagery, certainspecializedmodelingandrenderingtasksremaindifficult.Onesuchchallengeis presentedbythehumaneye,whichexhibitsintricatedetailintheiristhatis complex enoughtoserve asanalternative tofingerprintsforpersonalidentification[Daugman1993].Oneofthebiggestchallengesinrealisticrenderingis knowingwhatfeaturesofa modelareimportantforrealism,andwhatcanbeignored.Thisissueis particularlyvexinginirissynthesisbecausetheanatomicalliteratureisincomplete,andthesalientaspectsofirisappearancehave notbeenstudied.However, wehave a par-ticularlyfortunatesituationinirissynthesis:artificialeyemakers(ocularists)have developeda procedureforphysicalirissynthesisthatresultsin eyeswithalltheimportantappearancecharacteristicsofrealeyes.Theirprocedurehasbeenrefinedoverdecades,andthemostcompletevalidationonecouldaskforis providedbytheperformanceoftheirproductsintherealworld.Inthispaper, wepresentanapproachthatenablesusersotherthantrainedoculariststocreatea realisticlookinghumaneye,pay-ingparticularattentionto theiris.We draw fromdomainknowledgeprovidedbyoculariststoprovidea toolkitthatallowsthecompo-sitionofa humanirisbylayeringsemi-transparenttextures.Thesetextureslookdecidedlypaintedandunrealistic.Thecompositedre-sult,however, providesa senseofdepthtotheirisandtakesonalevel ofrealismthatwebelieve hasnotbeenachievedbefore.Previousworkonrenderingeyeshasconcentratedpredomi-nantlyonproducinggeometryforfacialanimation[Parke andWa-ters1996]orformedicalapplications[Sagaretal.1994].Someworkhasfocusedonaccuratelymodelingthecornea[Halsteadet al.1996;Barsky etal.1999].Incontrast,thegoalofthisworkis toallow theeasycreationofrealisticlookingirisesforboththeocularprostheticsandentertainmentindustries.InSection2 weintroducetheterminologyfordescribingtheanatomyofhumaneyesandirises,bothintermsofmedicalappli-cationsandfromthepointofviewofocularists.Ourcontributiontothefieldofgraphicsincludesthetranslationofthewayocularistsworktoa simple,semi-automatedmethodofmodelingandrender-inghumanirises.Thismethodis presentedinSection3,whileourresultsarediscussedinSection4.ConclusionsaredrawninSec-tion5.Figure1:Thehumaneye. DrawingbyKarenLefohn.Usedwithpermission.2TheHumanEye2.1OverviewThehumaneyeis anorgandesignedforphoto-reception;i.e.con-vertinglightenergytonerve actionpotentials.Theseactionpo-tentialsaresubsequentlyrelayedtotheopticnerve andthebrain,wherefurtherprocessingoccurs,resultinginconsciousvision.Allstructuresintheeyearesubservienttothisphysiologicalprocess[Forresteret al.2001].Thelargeststructureoftheeyeisthesclera(Figure1),whichis approximatelysphericalwitha radiusofaround11.5mm.Thecorneasitssomewhatinfrontofthisstructureandhasa radiusof7.8mm.Thescleratakesaround56thofthecircumferenceoftheeye,whilethecorneais responsiblefortheother16th. Thetransi-tionzonebetweenscleraandcorneais calledthelimbusandhasafuzzyappearance.Theeyecontainsthreebasiclayers:thefibrous(corneo-scleral)coat,theuveaoruvealtract(choroid,ciliarybodyandiris)andtheneurallayer(retina).Thesecoatssurroundthecontentsoftheeye,whicharethelens,theaqueoushumorandthevitreousbody[Forresteretal.2001].Thecornea,withanindex ofrefractionof1.376,is responsibleformostoftherefractive poweroftheeye.Afterthecornea,lightpassesthroughtheaqueoushu-mor(index ofrefraction1.336),thelens(index ofrefractionrangesfrom1.386intheouterlayersto1.406atthecenter)andthevit-reousbody(index ofrefraction1.337)beforeit reachestheretina[Hecht1987]