Download pdf - CS380: Introduction to Computer Graphics Texture Mapping … · 2018. 5. 3. · 18/05/03 1 Min H. Kim (KAIST) Foundations of 3D Computer Graphics, S. Gortler, MIT Press, 2012 CS380:

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MinH.Kim(KAIST) Foundationsof3DComputerGraphics,S.Gortler,MITPress,2012

CS380:IntroductiontoComputerGraphicsTextureMapping

Chapter15

MinH.KimKAISTSchoolofComputing


SUMMARYMaterials

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LightblobfromPVCplastic•  Recall:Givenanyvector(notnecessarilyofunitnorm)andaunitnormalvector,wecancomputethebouncevector(mirrorreflection)ofas

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w

n

w

B(w) = 2( w ⋅ n)n − w


Diffusereflectancemodel•  “Diffuse”materials,likeroughwood,appearequallybrightwhenobservedfromalldirection–  Thus,whencalculatingthecolorofapointonadiffusesurface,wedonotneedtousethevectoratall.

4©SzymonRusinkiewicz(Princeton)

v

v

fr = const.=ρπ

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Blinn-Phongreflectancemodel•  SimpleBRDFdescribingspecularreflection,modeledinOpenGL

©SzymonRusinkiewicz(Princeton)

n is normalh is halfway between I and v

fr =ρπ+ ks (n ⋅h)

α


TEXTUREMAPPINGChapter15

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Texturemapping•  Wehavealreadyseenandusedtexturemapping

•  Inbasictexturing,wesimply‘glue’partofanimageontoatrianglebyspecifyingtexturecoordinatesatthethreevertices.

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Texturemapping•  BunchofOpenGLcodestoloadatextureandsetvariousparameters(lin/const,mipmap,wrappingrules).

•  Auniformvariableisusedtopointtothedesiredtextureunit.

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Texturemapping•  Varyingvariablesareusedtostoretexturecoordinates.

•  Inthissimplestincarnation,wejustfetchr,g,bvaluesfromthetextureandsendthemdirectlytotheframebuffer.

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Texturemapping•  Alternatively,thetexturedatacouldbeinterpretedas,say,thediffusematerialcolorofthesurfacepoint,whichwouldthenbefollowedbythediffusematerialcomputationdescribedearlier.

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Texturemapping(main.cpp)•  initGLState()

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…glActiveTexture(GL_TEXTURE0);glGenTextures(1,&h_texture);glBindTexture(GL_TEXTURE_2D,h_texture);glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_WRAP_S,GL_CLAMP);glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_WRAP_T,GL_CLAMP);glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR);glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR);inttwidth,theight;packed_pixel_t*pixdata=ppmread("reachup.ppm",&twidth,&theight);assert(pixdata);glTexImage2D(GL_TEXTURE_2D,0,GL_SRGB,twidth,theight,0,GL_RGB,GL_UNSIGNED_BYTE,pixdata);free(pixdata);...


Texturemapping•  initShaders()•  display()

•  Texturelocation(0,0)àlowerleft,(1,1)àupperright

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h_texUnit0=safe_glGetUniformLocation(h_program,"texUnit0");h_aTexCoord=safe_glGetAttribLocation(h_program,"aTexCoord");

safe_glUniform1i(h_texUnit0,0);

GlfloatsqTex[12]={0,0,1,1,1,0,0,0,0,1,1,1};

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Texturemapping

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#version130uniformfloatuVertexScale;uniformmat4uProjMatrix;uniformmat4uModelViewMatrix;invec2aVertex;invec2aTexCoord;invec3aColor;outvec3vColor;outvec2vTexCoord;voidmain(){gl_Position=vec4(uProjMatrix*uModelViewMatrix*aVertex);vColor=aColor;vTexCoord=aTexCoord;}

•  Vertexshader


Texturemapping

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#version130uniformsampler2DuTexUnit0;//texturedatainvec2vTexCoord;//texturecoordinatesoutvec4fragColor;voidmain(){vec4texColor0=texture2D(uTexUnit0,vTexCoord);fragColor=texColor0;}

•  Fragmentshader

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Normalmapping•  Thedatafromatexturecanalsobeinterpretedinmoreinterestingways.

•  Innormalmapping,ther,g,bvaluesfromatextureareinterpretedasthethreecoordinatesofthenormalatthepoint.

•  Thisnormaldatacanthenbeusedaspartofsomematerialsimulation,

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Normalmapping

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Originalmesh4Mtriangles

Simplifiedmesh500triangles

Simplifiedmeshandnormalmapping500triangles

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Normalmapping•  Normaldatahasthreecoordinatevaluesofunitvectors,eachintherange[-1…1],whileRGBtexturesstorethreevalues,eachintherange[0…1](0…255)

•  Soneedsomeconversions:– R=normal_x/2.0+0.5;– normal_x=2*R-1;

•  Sometimeneedtodealwithcoordinatesystemconversions.

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Environmentcubemaps•  Texturescanalsobeusedtomodeltheenvironmentinthedistancearoundtheobjectbeingrendered.

•  Inthiscase,wetypicallyusesixsquaretexturesrepresentingthefacesofalargecubesurroundingthescene.

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+y

-y

-z +z+x-x

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CapturingEnvironmentMaps•  Photographingalightprobeproducesanenvironmentmaprepresentingincidentradiancefromalldirections.

19 B(w) = 2( w ⋅ n)n − w


CapturingEnvironmentMaps

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•  Howtoremovethecamerafromtheenvironmentmap?

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Spherevs.Cube

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SphericalEnvironmentMap&CubeMap

•  Galileo’sTomb

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http://www.pauldebevec.com/Probes/

Lightprovecaptured

Equirectangular(EQ)image

Six-facecubemap

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Environmentcubemaps•  Eachtexturepixelrepresentsthecolorasseenalongonedirectionintheenvironment.

•  Thisiscalledacubemap.GLSLprovidesacube-texturedatatype,samplerCubespecificallyforthispurpose.

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Environmentcubemaps•  Duringtheshadingofapoint,wecantreatthematerialatthatpointasaperfectmirrorandfetchtheenvironmentdatafromtheappropriateincomingdirection.

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Environmentmapshader•  Wecalculateinthepreviouslecture.

•  Thisbouncedvectorwillpointpointstowardstheenvironmentdirection,whichwouldbeobservedinamirroredsurface.

•  Bylookingupthecubemap,usingthisdirection,wegivethesurfacetheappearanceofamirror.

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B(v)

B(w) = 2( w ⋅ n)n − w


Environmentmapshader•  Fragmentshader

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#version130uniformsampler2DuTexUnit0;invec3nNormal;invec4vPosition;outvec4fragColor;vec3reflect(vec3w,vec3n){returnn*(dot(w,n)*2.0)-w;//bouncevector}voidmain(){vec3normal=normalize(vNormal);vec3reflected=reflect(normalize(vec3(-vPosition)),normal);vec4texColor0=textureCube(uTexUnit0,reflected);fragColor=vec4(texColor0.r,texColor0.g,texColor0.b,1.0);;}

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Environmentmapshader•  -vPositionrepresentstheviewvector•  textureCubeisaspecialGLSLfunctionthattakesadirectionvectorandreturnsthecolorstoredatthisdirectioninthecubetexturemap.

•  Hereweassumeeye-coordinates,butframechangesmaybeneeded.

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v


Environmentmapshader•  Thiscanbeusedforrefraction.

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Projectortexturemapping(cameramapping)

•  Therearetimeswhenwewishtoglueourtextureontoourtrianglesusingaprojectormodel,insteadoftheaffinegluingmodel.

•  Forexample,wemaywishtosimulateaslideprojectorilluminatingsometrianglesinspace.

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Projectortexturemapping

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RECAP:projection

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xnwn

ynwn

−wn

⎡

⎣

⎢⎢⎢⎢⎢

⎤

⎦

⎥⎥⎥⎥⎥

=

xcyc−wc

⎡

⎣

⎢⎢⎢⎢⎢

⎤

⎦

⎥⎥⎥⎥⎥

=

1 0 0 00 1 0 0− − − −0 0 −1 0

⎡

⎣

⎢⎢⎢⎢

⎤

⎦

⎥⎥⎥⎥

xeyeze1

⎡

⎣

⎢⎢⎢⎢⎢

⎤

⎦

⎥⎥⎥⎥⎥

[0,0,0,1]t

p

•  Canonicalsquarespace:

xn = − xeze, yn = − ye

ze

−ze

ye

wn


Projectortexturemapping•  Theslideprojector(≈camera)ismodeledusing4by4,modelviewandprojectionmatrices,

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Ms andPs . xtwt

ytwt

−wt

⎡

⎣

⎢⎢⎢⎢⎢

⎤

⎦

⎥⎥⎥⎥⎥

= PsMs

xoyozo1

⎡

⎣

⎢⎢⎢⎢⎢

⎤

⎦

⎥⎥⎥⎥⎥

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Projectortexturemapping•  Withthetexturecoordinatesdefinedas

•  Tocolorapointonatrianglewithobjectcoordinates,wefetchthetexturedatastoredatlocation

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xt =

xtwt

wt

andyt =ytwt

wt

[xo , yo ,zo ,1]t

[xt , yt ]t


Projectortexturemapping•  Thethreequantitiesareallaffinefunctionsof.Thusthesequantitieswillbeproperlyinterpolatedoveratrianglewhenimplementedasvaryingvariables.

•  Inthefragmentshader,weneedtodividebytoobtaintheactualtexturecoordinates.

•  Whendoingprojectortexturemapping,wedonotneedtopassanytexturecoordinatesoftrianglesasattributevariablestoourvertexshader.

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xtwt , ytwt and wt

(xo, yo, zo )

wt

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Projectortexturemapping•  Wesimplyusetheobjectcoordinatesalreadyavailabletous.

•  Wedoneedtopassintheobjectcoordinatesviathenecessaryprojectormatrices(affine/projectionmatrices),usinguniformvariables.

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Projectortexturemapping•  Projectorvertexshader

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#version130uniformmat4uModelViewMatrix;uniformmat4uProjMatrix;uniformmat4uSProjMatrix;uniformmat4uSModelViewMatrix;invec4aVertex;outvec4aTexCoord;voidmain(){vTexCoord=uSProjMatrix*uSModelViewMatrix*aVertex;gl_Position=uProjMatrix*uModelViewMatrix*aVertex;}

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Projectortexturemapping•  Projectorfragmentshader

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#version130uniformsampler2DvTexUnit0;invec4aTexCoord;outvec4fragColor;voidmain(){vec2tex2;tex2.x=vTexCoord.x/vTexCoord.w;tex2.y=vTexCoord.y/vTexCoord.w;vec4texColor0=texture2D(vTexUnit0,tex2);fragCoor=texColor0;}

xtwt

ytwt

−wt

⎡

⎣

⎢⎢⎢⎢⎢

⎤

⎦

⎥⎥⎥⎥⎥

=PsMs

xoyozo1

⎡

⎣

⎢⎢⎢⎢⎢

⎤

⎦

⎥⎥⎥⎥⎥


Projectortexturemapping•  Conveniently,OpenGLevengivesusaspecialcalltexture2DProj(vTexUnit0,pTexCoord),thatactuallydoesthedivideforus.

•  Inconveniently,whendesigningourslideprojectormatrixuSProjMatrix,wehavetodealwiththefactthatthecanonicaltextureimagedomaininOpenGListheunitsquare,whoselowerleftandupperrightcornershavecoordinatesusedforthedisplaywindow.

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[0,0]t and[1,1]t

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Multipassrendering•  Moreinterestingrenderingeffectscanbeobtainedusingmultiplerenderingpassesoverthegeometryinthescene.

•  Inthisapproach,theresultsofallbutthefinalpassarestoredofflineandnotdrawntothescreen.

•  Todothis,thedataisrenderedintosomethingcalled,aFrameBufferObject,orFBO.

•  Afterrendering,theFBOdataisthenloadedasatexture,andthuscanbeusedasinputdatainthenextrenderingpass.

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Multipassrendering

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1stpassrendering(cubemap) 2ndpassrendering(mirrorball)

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Multipassrendering

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1stpassrendering(cubemap) 2ndpassrendering(mirrorball)


Shadowmapping•  Theideaistofirstcreateandstoreaz-bufferedimagefromthepointofviewofthelight,andthencomparewhatweseeinourviewtowhatthelightsawititsview.

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Shadowmapping•  Ifapointobservedbytheeyeisnotobservedbythelight,thentheremustbesomeoccludingobjectinbetween,andweshoulddrawthatpointasifitwereinshadow.

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1stpassrendering


Shadowmapping•  Inafirstpass,werenderintoanFBOthesceneasobservedfromsomecamerawhoseorigincoincideswiththepositionofthepointlightsource.Letusmodelthiscameratransformas:

•  forappropriatematrices,.

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xtwt

ytwt

ztwt

wt

⎡

⎣

⎢⎢⎢⎢⎢⎢

⎤

⎦

⎥⎥⎥⎥⎥⎥

=PsMs

xoyozo1

⎡

⎣

⎢⎢⎢⎢⎢

⎤

⎦

⎥⎥⎥⎥⎥

Ps and Ms

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Shadowmapping•  Duringthisfirstpass,werenderthescenetoanFBOusingasthemodelviewmatrixandastheprojectionmatrix.

•  IntheFBO(oflightview),westore,notthecolorofthepoint,butratheritsvalue.

•  Duetoz-buffering,thedatastoredatapixelintheFBOrepresentsthevalueofthegeometryclosesttothelightalongtherelevantlineofsight.ThisFBOisthentransferredtoatexture.

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Ms

Ps

zt

zt


Shadowmapping•  Duringthesecondrenderingpass,werenderourdesiredimagefromtheeye’spointofview,

•  butforeachpixel(fragmentshader),weneedtocomparethedistancesof(vPosition-to-light)andthedepthmapfromlight

•  tocheckifthepointweareobservingwasalsoobservedbythelight,orifitwasblockedbysomethingcloserinthelight’sview.

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•  Todothis,weusethesamecomputationthatwasdonewithprojectortexturemapping

•  Doingso,inthefragmentshader,wecanobtainthevaryingvariablesassociatedwiththepoint.

•  Wethencomparethisvalue(projector)withthevalue(light)storedatinthetextureoflight.

Shadowmapping

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xt , yt andzt[xo , yo ,zo ,1]

t

zt

[xt , yt ]t

zt

2ndpassrendering


Shadowmapping•  Ifthesevaluesagreethenwearelookingatapointthatwasalsoseenbythelight;suchapointisnotinshadowandshouldbeshadedaccordinglyinfragmentshader.

•  Conversely,ifthesevaluesdisagree,thenthepointwearelookingatwasoccludedinthelight’simage,isinshadowandshouldbeshadedassuch.

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