计算机图形学computer graphics课件18

TextureMappingSoftwareCollege,ShandongUniversityInstructor:ZhouYuanfengE-mail:yuanfeng.zhou@23ObjectivesIntroduceMappingMethodsTextureMappingEnvironmentMappingBumpMappingConsiderbasicstrategiesForwardvsbackwardmappingPointsamplingvsareaaveraging4TheLimitsofGeometricModelingAlthoughgraphicscardscanrenderover10millionpolygonspersecond,thatnumberisinsufficientformanyphenomenaCloudsGrassTerrainSkin,hairWaterandfire5ModelinganOrangeConsidertheproblemofmodelinganorange(thefruit)Startwithanorange-coloredsphereToosimpleReplacespherewithamorecomplexshapeDoesnotcapturesurfacecharacteristics(smalldimples)Takestoomanypolygonstomodelallthedimples6ModelinganOrange(2)Takeapictureofarealorange,scanit,and“paste”ontosimplegeometricmodelThisprocessisknownastexturemappingStillmightnotbesufficientbecauseresultingsurfacewillbesmoothNeedtochangelocalshapeBumpmapping+7ThreeTypesofMappingTextureMappingUsesimagestofillinsideofpolygonsEnvironment(reflectionmapping)UsesapictureoftheenvironmentfortexturemapsAllowssimulationofhighlyspecularsurfacesBumpmappingEmulatesalteringnormalvectorsduringtherenderingprocess8TextureMappinggeometricmodeltexturemapped9EnvironmentMapping10BumpMapping+=originalsurfacebumpmapmodifiedsurface11Wheredoesmappingtakeplace?MappingtechniquesareimplementedattheendoftherenderingpipelineVeryefficientbecausefewpolygonsmakeitpasttheclipper12Isitsimple?Althoughtheideaissimple---mapanimagetoasurface---thereare3or4coordinatesystemsinvolved2Dimage3Dsurface13CoordinateSystemsParametriccoordinatesMaybeusedtomodelcurvesandsurfacesTexturecoordinatesUsedtoidentifypointsintheimagetobemappedObjectorWorldCoordinatesConceptually,wherethemappingtakesplaceWindowCoordinatesWherethefinalimageisreallyproduced14TextureMappingparametriccoordinatestexturecoordinatesworldcoordinateswindowcoordinates15MappingFunctionsBasicproblemishowtofindthemapsConsidermappingfromtexturecoordinatestoapointasurfaceAppeartoneedthreefunctionsx=x(s,t)y=y(s,t)z=z(s,t)Butwereallywanttogotheotherwayst(x,y,z)16BackwardMappingWereallywanttogobackwardsGivenapixel,wewanttoknowtowhichpointonanobjectitcorrespondsGivenapointonanobject,wewanttoknowtowhichpointinthetextureitcorrespondsNeedamapoftheforms=s(x,y,z)t=t(x,y,z)Suchfunctionsaredifficulttofindingeneral

Parameterization1718Two-partmappingOnesolutiontothemappingproblemistofirstmapthetexturetoasimpleintermediatesurfaceExample:maptocylinder19CylindricalMappingparametriccylinderx=rcos(2piu)y=rsin(2piu)z=v/hmapsrectangleinu,vspacetocylinderofradiusrandheighthinworldcoordinatess=ut=vmapsfromtexturespaceCylindricalMapping20Mapping:

zz(a)texture(b)cylindricalxyrh(c)mappingxy(d)ba(c)11CylindricalMapping21Mapuandvtopointsa，b，c，d:(1)

:(2):(3):(4):Solving:Fromtexturetocylindricalparameters:Fromcylindricalparameterstotexture：22SphericalMappingWecanuseaparametricspherex=rcos2piuy=rsin2pucos2pivz=rsin2pusin2pivinasimilarmannertothecylinderbuthavetodecidewheretoputthedistortionSpheresareusedinenvironmentalmaps23BoxMappingEasytousewithsimpleorthographicprojectionAlsousedinenvironmentmapsPolycubemapping24ConformalmappingInmathematics,aconformalmapisafunctionwhichpreservesangles.2526SecondMappingMapfromintermediateobjecttoactualobjectNormalsfromintermediatetoactualNormalsfromactualtointermediateVectorsfromcenterofintermediate

intermediateactual27AliasingPointsamplingofthetexturecanleadtoaliasingerrorspointsamplesinu,v(orx,y,z)spacepointsamplesintexturespacemissbluestripes28AreaAveragingAbetterbutsloweroptionistouseareaaveragingNotethatpreimageofpixeliscurvedpixelpreimageCatmulltexturemappingSurfacesubdivision29PixeledSScreenSurfaceContinuousmapping30

(a)SphericalMapping(b)CylindricalMappingMip-map(1983,LanceWilliams)31Mip-mapActuallymip-mapisalookuptablefortexture32OpenGLTextureMapping34BasicStrategyThreestepstoapplyingatexturespecifythetexturereadorgenerateimageassigntotextureenabletexturingassigntexturecoordinatestoverticesPropermappingfunctionislefttoapplicationspecifytextureparameterswrapping,filtering35TextureMappingstxyzimagegeometrydisplay36TextureExampleThetexture(below)isa256x256imagethathasbeenmappedtoarectangularpolygonwhichisviewedinperspective37TextureMappingandtheOpenGLPipelinegeometrypipelineverticespixelpipelineimagerasterizerImagesandgeometryflowthroughseparatepipelinesthatjoinattherasterizer“complex”texturesdonotaffectgeometriccomplexity38Defineatextureimagefromanarrayof

texels(textureelements)inCPUmemory

Glubytemy_texels[512][512];DefineasanyotherpixelmapScannedimageGeneratebyapplicationcodeEnabletexturemappingglEnable(GL_TEXTURE_2D)OpenGLsupports1-4dimensionaltexturemapsSpecifyingaTextureImage1Dtexturemapping391Dtexturemappingunsignedcharcolorbar[1024];glTexImage1D(GL_TEXTURE_1D,0,GL_RGBA,256,0,GL_BGRA_EXT,GL_UNSIGNED_BYTE,colorbar);glTexParameteri(GL_TEXTURE_1D,GL_TEXTURE_WRAP_S,GL_CLAMP);Texturecoord:[0,1]4041DefineImageasaTextureglTexImage2D(target,level,components,

w,h,border,format,type,texels);

target:typeoftexture,e.g.GL_TEXTURE_2D level:usedformipmapping(discussedlater) components:elementspertexel w,h:widthandheightoftexelsinpixels border:usedforsmoothing(discussedlater) formatandtype:describetexels texels:pointertotexelarrayglTexImage2D(GL_TEXTURE_2D,0,3,512,512,0,GL_RGB,GL_UNSIGNED_BYTE,my_texels);42ConvertingATextureImageOpenGLrequirestexturedimensionstobepowersof2Ifdimensionsofimagearenotpowersof2gluScaleImage(format,w_in,h_in,

type_in,*data_in,w_out,h_out,

type_out,*data_out);data_in

issourceimagedata_out

isfordestinationimageImageinterpolatedandfilteredduringscaling43BasedonparametrictexturecoordinatesglTexCoord*()specifiedateachvertexst1,10,10,01,0(s,t)=(0.2,0.8)(0.4,0.2)(0.8,0.4)ABCabcTextureSpaceObjectSpaceMappingaTexture44TypicalCodeglBegin(GL_POLYGON); glColor3f(r0,g0,b0);//ifnoshadingused glNormal3f(u0,v0,w0);//ifshadingused glTexCoord2f(s0,t0); glVertex3f(x0,y0,z0); glColor3f(r1,g1,b1); glNormal3f(u1,v1,w1); glTexCoord2f(s1,t1); glVertex3f(x1,y1,z1); . .glEnd();Notethatwecanusevertexarraystoincreaseefficiency45InterpolationOpenGLusesinterpolationtofindpropertexelsfromspecifiedtexturecoordinatesCanbedistortionsgoodselectionoftexcoordinatespoorselectionoftexcoordinatestexturestretchedovertrapezoidshowingeffectsofbilinearinterpolation46TextureParametersOpenGLhasavarietyofparametersthatdeterminehowtextureisappliedWrappingparametersdeterminewhathappensifsandtareoutsidethe(0,1)rangeFiltermodesallowustouseareaaveraginginsteadofpointsamplesMipmappingallowsustousetexturesatmultipleresolutionsEnvironmentparametersdeterminehowtexturemappinginteractswithshading47WrappingModeClamping:ifs,t>1use1,ifs,t<0use0Repeating:uses,tmodulo1glTexParameteri(GL_TEXTURE_2D,

GL_TEXTURE_WRAP_S,GL_CLAMP)glTexParameteri(GL_TEXTURE_2D,

GL_TEXTURE_WRAP_T,GL_REPEAT)texturestGL_CLAMPwrappingGL_REPEATwrapping48MagnificationandMinificationTexturePolygonMagnificationMinificationPolygonTextureMorethanonetexelcancoverapixel(minification)ormorethanonepixelcancoveratexel(magnification)Canusepointsampling(nearesttexel)orlinearfiltering(2x2filter)toobtaintexturevalues49FilterModesModesdeterminedbyglTexParameteri(target,type,mode)

glTexParameteri(GL_TEXTURE_2D,GL_TEXURE_MAG_FILTER,GL_NEAREST);glTexParameteri(GL_TEXTURE_2D,GL_TEXURE_MIN_FILTER,GL_LINEAR);Notethatlinearfilteringrequiresaborderofanextratexelforfilteringatedges(border=1)50MipmappedTexturesMipmappingallowsforprefilteredtexturemapsofdecreasingresolutionsLessensinterpolationerrorsforsmallertexturedobjectsDeclaremipmaplevelduringtexturedefinitionglTexImage2D(GL_TEXTURE_*D,level,

…)GLUmipmapbuilderroutineswillbuildallthetexturesfromagivenimagegluBuild*DMipmaps(…)51Example

pointsamplingmipmappedpointsamplingmipmappedlinearfilteringlinearfiltering52TextureFunctionsControlshowtextureisappliedglTexEnv{fi}[v](

GL_TEXTURE_ENV,mode,param)

GL_TEXTURE_ENV_MODE

modesGL_MODULATE:modulateswithcomputedshade GL_BLEND:blendswithanenvironmentalcolor GL_REPLACE:useonlytexturecolorGL(GL_TEXTURE_ENV,GL_TEXTURE_ENV_MODE,GL_MODULATE);Setblendcolorwith

GL_TEXTURE_ENV_COLORDemo53PerspectiveCorrectionHintTexturecoordinateandcolorinterpolationeitherlinearlyinscreenspaceorusingdepth/perspectivevalues(slower)Noticeableforpolygons“onedge”glHint(

GL_PERSPECTIVE_CORRECTION_HINT,hint

)wherehintisoneofGL_DONT_CAREGL_NICESTGL_FASTEST54GeneratingTextureCoordinatesOpenGLcangeneratetexturecoordinatesautomaticallyglTexGen{ifd}[v]()specifyaplanegeneratetexturecoordinatesbasedupondistancefromtheplanegenerationmodesGL_OBJECT_LINEARGL_EYE_LINEAR

GL_SPHERE_MAP(usedforenvironmentalmaps)LinearmappingTextureis“attached”toobject:GLfloatparams={A,B,C,D};glTexGenfv(GL_S,GL_OBJECT_PLANE,params);glTexGeni(