gl_Position is the clip-space position of a particular vertex. gl_FragCoord.xyz contains the window-space position of the fragment. These are ...
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GLSL-Passingcoordinatestofragmentshader?
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Ihavethefollowingshaders
VertexShader
#version330
layout(location=0)invec3inPosition;
layout(location=1)invec4inColour;
layout(location=2)invec2inTex;
layout(location=3)invec3inNorm;
uniformmat4transformationMatrix;
uniformmat4projectionMatrix;
outvec4ShadowCoord;
voidmain()
{
gl_Position=projectionMatrix*transformationMatrix*vec4(inPosition,1.0);
ShadowCoord=gl_Position;
}
FragmentShader
#version330
invec4ShadowCoord;
outvec4FragColor;
uniformsampler2DheightmapTexture;
voidmain()
{
FragColor=vec4(vec3(clamp(ShadowCoord.x/500,0.0,1.0),clamp(gl_FragCoord.y/500,0.0,1.0),0.0),1.0);
}
WhatIwouldexpectthistodoistodrawthescene,withgreeninthetopleft,yellowinthetopright,redinthebottomrightandblackinthebottomleft.IndeedthisiswhathappenswhenIreplaceShadowCoord.xwithgl_FragCoord.xinmyfragmentshader.Whythen,doIonlygetaverticalgradientofgreen?EvidentlysomethingscrewyishappeningtomyShadowCoord.Anyoneknowwhatitis?
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askedSep3,2013at10:35
rspencerrspencer
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"Anyoneknowwhatitis?"No.BecausethecontentsofShadowCoorddependentirelyonthecontentsoftwomatricesthatwehavenoideawhattheyare.Wedon'tevenknowifyou'reusingaperspectiveororthographicproject.
– NicolBolas
Sep3,2013at10:49
Ok,butthepointisthatifIreplaceShadowCoordwithgl_FragCoordinthefragshaderthenitworks.Isgl_FragCoordnotderivedfromgl_Position?IfsowhydoesShadowCoordnotundergothesameinterpolation?FYI,itsaperspectiveprojectionofacameratransformation.
– rspencer
Sep3,2013at10:51
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Yes,gl_FragCoordis"derivedfromgl_Position".Thatdoesnotmeanthatitisequaltoit.
gl_Positionistheclip-spacepositionofaparticularvertex.gl_FragCoord.xyzcontainsthewindow-spacepositionofthefragment.Thesearenotthesamespaces.Togetfromonetotheother,thegraphicshardwareperformsanumberoftransformations.
Transformationsthatdonothappentoothervalues.That'swhythefragmentshaderdoesn'tcallitgl_Position;becauseit'snotthatvalueanymore.It'sanewvaluecomputedfromtheoriginalbasedonvariousstate(theviewport&depthrange)thattheuserhasset.
User-definedvertexparametersareinterpolatedastheyare.Nothing"screwy"ishappeningwithyouruser-definedinterpolatedparameter.Thesystemisdoingexactlywhatitshould:interpolatingthevalueyoupassedacrossthetriangle'ssurface.
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answeredSep3,2013at10:59
NicolBolasNicolBolas
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Rightthatmakessense.Howwouldonetellthesystemtodothoseprocessesonadifferentvariablethen?Say,eventually,ShadowCoordisgivenbysomeothertransformationmatrix,andIwanttothengetthecoordinatesofits'fragment'.WouldIhavetomanuallyimplementwhatisdescribedinthelink?
– rspencer
Sep3,2013at11:05
1
"Howwouldonetellthesystemtodothoseprocessesonadifferentvariablethen?"Youdon't;you'llhavetocomputeityourself.It'sjustanothermatrixmultiply;youcanjustfolditintothebackendofthematrixyou'realreadyusing.
– NicolBolas
Sep3,2013at11:14
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