GLSL 4.0: Using Subroutines to Select Shader Functionality

The value of this variable can be set from the OpenGL side, ... avoiding a conditional statement or a shader swap can be very valuable. Home»GLSL4.0:UsingSubroutinestoSelectShaderFunctionality GLSL4.0:UsingSubroutinestoSelectShaderFunctionality BacktoPosts GLSL4.0:UsingSubroutinestoSelectShaderFunctionality AhrenStevens-Taylor2022-05-27T09:58:52+00:00 OpenGL4.0ShadingLanguageCookbook Over60highlyfocused,practicalrecipestomaximizeyourOpenGLShadinglanguageuse      InmanywaysitissimilartofunctionpointersinC.Auniformvariableservesasthepointerandisusedtoinvokethefunction.ThevalueofthisvariablecanbesetfromtheOpenGLside,therebybindingittooneofafewpossibledefinitions.Thesubroutine’sfunctiondefinitionsneednothavethesamename,butmusthavethesamenumberandtypeofparametersandthesamereturntype. Asingleshadercouldbewrittentoprovideseveralshadingalgorithmsintendedforuseondifferentobjectswithinthescene.Whenrenderingthescene,ratherthanswappingshaderprograms(orusingaconditionalstatement),wecansimplychangethesubroutine’suniformvariabletochoosetheappropriateshadingalgorithmaseachobjectisrendered. Sinceperformanceiscrucialinshaderprograms,avoidingaconditionalstatementorashaderswapcanbeveryvaluable.Withsubroutines,wecanimplementthefunctionalityofaconditionalstatementorshaderswapwithoutthecomputationaloverhead. Inthefollowingimage,weseeanexampleofarenderingthatwascreatedusingsubroutines.TheteapotontheleftisrenderedwiththefullADSshadingmodel,andtheteapotontherightisrenderedwithdiffuseshadingonly.Asubroutineisusedtoswitchbetweenshaderfunctionality. Gettingready Aswithpreviousrecipes,providethevertexpositionatattributelocation0andthevertexnormalatattributelocation1.UniformvariablesforalloftheADScoefficientsshouldbesetfromtheOpenGLside,aswellasthelightpositionandthestandardmatrices. We’llassumethat,intheOpenGLapplication,thevariableprogramHandlecontainsthehandletotheshaderprogramobject. Howtodoit… Tocreateashaderprogramthatusesasubroutinetoswitchbetweenpure-diffuseandADSshading,usethefollowingcode: Usethefollowingcodeforthevertexshader: #version400 subroutinevec3shadeModelType(vec4position,vec3normal); subroutineuniformshadeModelTypeshadeModel; layout(location=0)invec3VertexPosition; layout(location=1)invec3VertexNormal; outvec3LightIntensity; structLightInfo{ vec4Position;//Lightpositionineyecoords. vec3La;//Ambientlightintensity vec3Ld;//Diffuselightintensity vec3Ls;//Specularlightintensity }; uniformLightInfoLight; structMaterialInfo{ vec3Ka;//Ambientreflectivity vec3Kd;//Diffusereflectivity vec3Ks;//Specularreflectivity floatShininess;//Specularshininessfactor }; uniformMaterialInfoMaterial; uniformmat4ModelViewMatrix; uniformmat3NormalMatrix; uniformmat4ProjectionMatrix; uniformmat4MVP; voidgetEyeSpace(outvec3norm,outvec4position) { norm=normalize(NormalMatrix*VertexNormal); position=ModelViewMatrix*vec4(VertexPosition,1.0); } subroutine(shadeModelType) vec3phongModel(vec4position,vec3norm) { //TheADSshadingcalculationsgohere(see:“Using //functionsinshaders,”and“Implementing //per-vertexambient,diffuseandspecular(ADS)shading”) … } subroutine(shadeModelType) vec3diffuseOnly(vec4position,vec3norm) { vec3s=normalize(vec3(Light.Position–position)); return Light.Ld*Material.Kd*max(dot(s,norm),0.0); } voidmain() { vec3eyeNorm; vec4eyePosition; //Getthepositionandnormalineyespace getEyeSpace(eyeNorm,eyePosition); //Evaluatetheshadingequation.Thiswillcalloneof //thefunctions:diffuseOnlyorphongModel. LightIntensity=shadeModel(eyePosition,eyeNorm); gl_Position=MVP*vec4(VertexPosition,1.0); } Usethefollowingcodeforthefragmentshader: #version400 invec3LightIntensity; layout(location=0)outvec4FragColor; voidmain(){ FragColor=vec4(LightIntensity,1.0); } IntheOpenGLapplication,compileandlinktheaboveshadersintoashaderprogram,andinstalltheprogramintotheOpenGLpipeline. WithintherenderfunctionoftheOpenGLapplication,usethefollowingcode: GLuintadsIndex= glGetSubroutineIndex(programHandle, GL_VERTEX_SHADER,”phongModel”); GLuintdiffuseIndex= glGetSubroutineIndex(programHandle, GL_VERTEX_SHADER,“diffuseOnly”); glUniformSubroutinesuiv(GL_VERTEX_SHADER,1,&adsIndex); …//Rendertheleftteapot glUniformSubroutinesuiv(GL_VERTEX_SHADER,1,&diffuseIndex); …//Rendertherightteapot Howitworks… Inthisexample,thesubroutineisdefinedwithinthevertexshader.Thefirststepinvolvesdeclaringthesubroutinetype. subroutinevec3shadeModelType(vec4position, vec3normal); ThisdefinesanewsubroutinetypewiththenameshadeModelType.Thesyntaxisverysimilartoafunctionprototype,inthatitdefinesaname,aparameterlist,andareturntype.Aswithfunctionprototypes,theparameternamesareoptional. Aftercreatingthenewsubroutinetype,wedeclareauniformvariableofthattypenamedshadeModel. subroutineuniformshadeModelTypeshadeModel; ThisvariableservesasourfunctionpointerandwillbeassignedtooneofthetwopossiblefunctionsintheOpenGLapplication. Wedeclaretwofunctionstobepartofthesubroutinebyprefixingtheirdefinitionwiththesubroutinequalifier: subroutine(shadeModelType) Thisindicatesthatthefunctionmatchesthesubroutinetype,andthereforeitsheadermustmatchtheoneinthesubroutinetypedefinition.WeusethisprefixforthedefinitionofthefunctionsphongModelanddiffuseOnly.ThediffuseOnlyfunctioncomputesthediffuseshadingequation,andthephongModelfunctioncomputesthecompleteADSshadingequation. WecalloneofthetwosubroutinefunctionsbyutilizingthesubroutineuniformshadeModelwithinthemainfunction. LightIntensity=shadeModel(eyePosition,eyeNorm); Again,thiscallwillbeboundtooneofthetwofunctionsdependingonthevalueofthesubroutineuniformshadeModel,whichwewillsetwithintheOpenGLapplication. WithintherenderfunctionoftheOpenGLapplication,weassignavaluetothesubroutineuniformwiththefollowingsteps.First,wequeryfortheindexofeachsubroutinefunctionusingglGetSubroutineIndex.Thefirstargumentistheprogramhandle.Thesecondistheshaderstage.Inthiscase,thesubroutineisdefinedwithinthevertexshader,soweuseGL_VERTEX_SHADERhere.Thethirdargumentisthenameofthesubroutine.WequeryforeachfunctionindividuallyandstoretheindexesinthevariablesadsIndexanddiffuseIndex. Toselecttheappropriatesubroutinefunction,weneedtosetthevalueofthesubroutineuniformshadeModel.Todoso,wecallglUniformSubroutinesuiv.Thisfunctionisdesignedforsettingmultiplesubroutineuniformsatonce.Inourcase,ofcourse,wearesettingonlyasingleuniform.Thefirstargumentistheshaderstage(GL_VERTEX_SHADER),thesecondisthenumberofuniformsbeingset,andthethirdisapointertoanarrayofsubroutinefunctionindexes.Sincewearesettingasingleuniform,wesimplyprovidetheaddressoftheGLuintvariablecontainingtheindex,ratherthanatruearrayofvalues.Ofcourse,wewoulduseanarrayifmultipleuniformswerebeingset.Ingeneral,thearrayofvaluesprovidedasthethirdargumentisassignedtosubroutineuniformvariablesinthefollowingway.Theithelementofthearrayisassignedtothesubroutineuniformvariablewithindexi.Sincewehaveprovidedonlyasinglevalue,wearesettingthesubroutineuniformatindexzero. Youmaybewondering,“Howdoweknowthatoursubroutineuniformislocatedatindexzero?Wedidn’tqueryfortheindexbeforecallingglUniformSubroutinesuiv!”ThereasonthatthiscodeworksisthatwearerelyingonthefactthatOpenGLwillalwaysnumbertheindexesofthesubroutinesconsecutivelystartingatzero.Ifwehadmultiplesubroutineuniforms,wecould(andshould)queryfortheirindexesusingglGetSubroutineUniformLocation,andthenorderourarrayappropriately. Finally,weselectthephongModelfunctionbysettingtheuniformtoadsIndexandthenrendertheleftteapot.WethenselectthediffuseOnlyfunctionbysettingtheuniformtodiffuseIndexandrendertherightteapot. There’smore… Asubroutinefunctiondefinedinashadercanmatchmultiplesubroutinetypes.Inthatcase,thesubroutinequalifiercancontainacomma-separatedlistofsubroutinetypes.Forexample,ifasubroutinematchedthetypestype1andtype2,wecouldusethefollowingqualifier: subroutine(type1,type2) Thiswouldallowustousesubroutineuniformsofdifferingtypestorefertothesamesubroutinefunction. Summary Withsubroutineswecanimplementthefunctionalityofaconditionalstatementorshaderswapwithoutthecomputationaloverhead. Furtherresourcesonthissubject: TipsandTricksforGettingStartedwithOpenGLandGLSL4.0[Article] OpenGL4.0:UsingUniformBlocksandUniformBufferObjects[Article] OpenGL4.0:BuildingaC++ShaderProgramClass[Article] TheBasicsofGLSL4.0Shaders[Article] GLSL4.0:DiscardingFragmentstoCreateaPerforatedLook[Article] AhrenStevens-Taylor GameDevelopmentNews,Tutorials 0Comments Like:0 Sharethispost Facebook Twitter LinkedIn Email Reddit LeaveaReplyCancelreplyYouremailaddresswillnotbepublished.Requiredfieldsaremarked*Comment*Name* Email* Website Savemyname,email,andwebsiteinthisbrowserforthenexttimeIcomment. 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