基本的光照計(jì)算公式

1、基本的光照計(jì)算公式TheBasicLightingModelOpenGLandDirect3Dprovidealmostidenticalfixed-functionlightingmodels.Inourexample,wewilluseasimplifiedversionthatwewillrefertoastheBasicmodel.TheBasicmodel,liketheOpenGLandDirect3Dmodels,modifiesandextendstheclassicPhongmodel.IntheBasicmodel,anobjectssurfacecoloristhesum

2、ofemissive,ambient,diffuse,andspecularlightingcontributions.Eachcontributiondependsonthecombinationofthesurfacesmaterialproperties(suchasshininessandmaterialcolor)andthelightsourcesproperties(suchaslightcolorandposition).Werepresenteachcontributionasafloat3vectorthatcontainsthered,green,andbluecolor

3、components.Thishigh-levelequationdescribestheBasicmodelmathematically:surfaceColor=emissive+ambient+diffuse+specularTheEmissiveTermTheemissivetermrepresentslightemittedorgivenoffbyasurface.Thiscontributionisindependentofalllightsources.TheemissivetermisanRGBvaluethatindicatesthecoloroftheemittedligh

4、t.Ifyouweretoviewanemissivematerialinacompletelydarkroom,itwouldappeartobethiscolor.Theemissivetermcansimulateglowing.Figure5-2illustratestheemissivetermconceptually,andFigure5-3showsarenderingofapurelyemissiveobject.Therenderingisunderstandablyboring,becausetheemissivecoloristhesameallovertheobject

5、.Unlikeintherealworld,anobjectsemissiveglowdoesnotactuallyilluminateothernearbyobjectsinthescene.Anemissiveobjectisnotitselfalightsourceitdoesnotilluminateotherobjectsorcastshadows.Anotherwaytothinkoftheemissivetermisthatitisacoloraddedaftercomputingalltheotherlightingterms.Moreadvaneedglobalillumin

6、ationmodelswouldsimulatehowtheemittedlightaffectstherestofthescene,butthesemodelsarebeyondthescopeofthisbook.ObjectSurfaceFigure5-2TheEmissiveTermFigure5-3RenderingtheEmissiveTermHereisthemathematicalformulationweusefortheemissiveterm:emissive=Kwhere:kisthemateria1semissivecolor.eTheAmbientTermTheam

7、bienttermaccountsforlightthathasbouncedaroundsomuchinthescenethatitseemstocomefromeverywhere.Ambientlightdoesnotappeartocomefromanyparticulardirection;rather,itappearstocomefromalldirections.Becauseofthis,theambientlightingtermdoesnotdependonthelightsourceposition.Figure5-4illustratesthisconcept,and

8、Figure5-5showsarenderingofanobjectthatreceivesonlyambientlight.Theambienttermdependsonamaterialsambientreflectanee,aswellasthecoloroftheambientlightthatisincidentonthematerial.Liketheemissiveterm,theambienttermonitsownisjustaconstantcolor.Unliketheemissivecolor,however,theambienttermisaffectedbytheg

9、lobalambientlighting.IncomingAmbientLightReflectedAmbientLightObjectSurfaceObjectSurfaceFigure5-4TheAmbientTermFigure5-5RenderingtheAmbientTermHereisthemathematicalformulationweusefortheambientterm:ambient=KgxglobalAmbientwhere:kisthematerialsambientreflectaneeandaglobalAmbientisthecoloroftheincomin

10、gambientlight.TheDiffuseTermThediffusetermaccountsfordirectedlightreflectedoffasurfaceequallyinalldirections.Ingeneral,diffusesurfacesareroughonamicroscopicscale,withsmallnooksandcranniesthatreflectlightinmanydirections.Whenincomingraysoflighthitthesenooksandcrannies,thelightbouncesoffinalldirection

11、s,asshowninFigure5-6.DiffuseObjectIVTagniedFigure5-6DiffuseLightScatteringTheamountoflightreflectedisproportionaltotheangleofincideneeofthelightstrikingthesurface.Surfaceswithadullfinish,suchasadustychalkboard,aresaidtobediffuse.Thediffusecontributionatanyparticularpointonasurfaceisthesame,regardles

12、sofwheretheviewpointis.Figure5-7illustratesthediffuseterm,andFigure5-8showsarenderingofadiffuseobject.IncomingLightReflectedDiffuseLihtObjectSurfaceFigure5-8RenderingtheDiffuseTermObjectSurfaceHereisthemathematicalformulationweuseforthediffuseterm(illustratedinFigure5-9):diffuse=KdxlightColorxmax(NL

13、,0)where:Kdisthematerialsdiffusecolor,lightColoristhecoloroftheincomingdiffuselight,Nisthenormalizedsurfacenormal,Listhenormalizedvectortowardthelightsource,andPisthepointbeingshaded.Figure5-9CalculatingDiffuseLightingThevectordotproduct(orinnerproduct)ofthenormalizedvectorSVandLisameasureoftheangle

14、betweenthetwovectors.Thesmallertheanglebetweenthevectors,thegreaterthedot-productvaluewillbe,andthemoreincidentlightthesurfacewillreceive.Surfacesthatfaceawayfromthelightwillproducenegativedot-productvalues,sothemax(NL,0)intheequationensuresthatthesesurfacesshownodiffuselighting.TheSpecularTermThesp

15、eculartermrepresentslightscatteredfromasurfacepredominantlyaroundthemirrordirection.Thespeculartermismostprominentonverysmoothandshinysurfaces,suchaspolishedmetals.Figure5-10illustratestheconceptofspecularreflection,andFigure5-11showsarenderingofacompletelyspecularobject.ReflectedSpecuEarLightObject

16、SurfaceObjectSurfaceFigure5-10TheSpecularTermFigure5-11RenderingtheSpecularTermUnliketheemissive,ambient,anddiffuselightingterms,thespecularcontributiondependsonthelocationoftheviewer.Iftheviewerisnotatalocationthatreceivesthereflectedrays,theviewerwillnotseeaspecularhighlightonthesurface.Thespecula

17、rtermisaffectednotonlybythespecularcolorpropertiesofthelightsourceandmaterial,butalsobyhowshinythesurfaceis.Shiniermaterialshavesmaller,tighterhighlights,whereaslessshinymaterialshavehighlightsthataremorespreadout.Figure5-12showssomeexamplesofshininess,withtheshininessexponentincreasingfromlefttorig

18、ht.Shinins=5Shlninas,s=20SliiniriM=50Shinira-100Figure5-12ExamplesofDifferentShininessExponentsHereisthemathematicalformulationweuseforthespecularterm(illustratedinFigure5-13):specular=K$xlightColorxfacingx(max(NH,0)shininesswhere:Ksisthematerialsspecularcolor,lightColoristhecoloroftheincomingspecularlight,Nisthenormalizedsurfacenormal,Visthenormalizedvectortowardtheview