GLSL Tutorial – Uniform Variables - Lighthouse3d.com

GLSL Tutorial – Uniform Variables ... Uniform variables act as constants, at least for the duration of a draw call. The application feeds these variables to the ... About Tutorials VerySimple*Libs CGStuff Books Tutorials»GLSLTutorial-Core»GLSLTutorial–UniformVariables  Addcomments   Prev:AttributeVariables Next:UniformBlocks   Uniformvariablesactasconstants,atleastforthedurationofadrawcall.Theapplicationfeedsthesevariablestothegraphicspipelineandtheyareaccessibleinallstagesofthepipeline,i.e.anyshadercanaccesseveryuniformvariable,aslongasitdeclaresthevariable.Thesevariablesarereadonly,asfarasshadersareconcerned. Uniformscanbegroupedinblocks,ordefinedindividually.Inherewewillstarttolookattheindividualdefinitions,andthenextsectionwilllookatuniformblocks. Insideashader,auniformisdefinedusingtheuniformkeyword.Forinstancetodefineavec4namedmyVarwecouldwriteinashader uniformvec4myVar; Uniformscanalsobeinitializedinsidetheshader,forinstancetoinitializeavec4wecouldproceedasfollows: uniformvec4myVar={0.5,0.2,0.7,1.0}; In-shader-initializationisgreatsinceitrelievesusfromhavingtosetuniforms.Wecansetadefaultvalueintheshaderinitialization,andonlyhavetosetavaluefromtheapplicationifwerequireadifferentvaluefortheuniformvariable. Insidetheapplication,tosetauniformvariable,wemustfirstgetitslocation,whichwecanretrievewiththefollowingfunction: [stextbox] GLintglGetUniformLocation(GLuintprogram,constchar*name); Params: program:thehandletothelinkedprogram name:thenameoftheuniformvariable Return:thelocationofthevariable,or-1ifthenamedoesnotcorrespondtoanactiveuniformvariable. [/stextbox] Anactiveuniformvariableisavariablethatitisactuallyusedinsidetheshader,notjustdeclared.Thecompilerisfreetothrowawayvariablesthatarenotusedinthecode.Therefore,evenifauniformisdeclaredintheshader,aslongasitisnotused,itsreportedlocationcanbe-1. Thereturnvalue,assumingthattheprogramislinkedandthatthevariableiseffectivelybeingusedinthecode,isthelocationofthevariable,whichlatercanbeusedtosetitsvalue(s).Inordertosetthevalues,OpenGLoffersalargefamilyoffunction,tocoverforalldatatypes,andseveralwaysofsettingthevalues.Forinstance,considerthevariablemyVarasdefinedabove.Tosetthevec4,andassumingpasanhandletoalinkedprogram,wecouldwrite: GLintmyLoc=glGetUniformLocation(p,"myVar"); glProgramUniform4f(p,myLoc,1.0f,2.0f,2.5f,2.7f); or,wecouldwrite floatmyFloats[4]={1.0f,2.0f,2.5f,2.7f}; GLintmyLoc=glGetUniformLocation(p,"myVar"); glProgramUniform4fv(p,myLoc,1,myFloats); Thesignatureofthesetwofunctionsisasfollows: [stextbox] voidglProgramUniform4f(GLuintprogram,GLintlocation,GLfloatf1,…,GLfloatf4); voidglProgramUniform4fv(GLuintprogram,GLintlocation,GLsizeicount,constGLfloat*values); Params: program:thehandletothelinkedprogram location:theuniform’slocationinprogramp f1…f4,values:thevaluestosettheuniform count:thenumberofitemstoset.Forbasicdatatypesthiswillalwaysbeone.Whenconsideringarrays,thenthisvaluerepresentsthenumberofelementsinthearraytobeset. [/stextbox] TherearesimilarfunctionsforallbasicdatatypesinGLSLandforthepredefinedmatrices. Arrays Considernowthatinourshaderwehaveanarraytypeofvariable,declaredasfollows: uniformvec4color[2]; Fromtheapplicationpointofview,wecanlookatthevariableasanarrayandsetallthecomponentsatonce.Thisistrueformostbasictypes,notstructs,seebelow. floatmyColors[8]={1.0f,0.0f,0.0f,0.0f,0.0f,1.0f,0.0f,0.0f}; GLintmyLoc=glGetUniformLocation(p,"color"); glProgramUniform4fv(p,myLoc,2,myColors); NoticethatwesetthethirdparameterofglProgramUniform4fvto2,sincewearesettingtwoelementsofthearray,i.e.twovec4.TheapplicationarraymyColorshaseightfloatsasrequired. Anotherapproachwouldbetoseteachelementindividually.Forinstance,assumethatweonlywanttosetthesecondelementoftheGLSLarray,i.e.color[1].Thenwecanwrite: GLfloataColor[4]={0.0f,1.0f,1.0f,0.0f}; myLoc=glGetUniformLocation(p,"color[1]"); glProgramUniform4fv(p,myLoc,1,aColor); ThereisaglProgramUniformfunctionforeachbasicdatatypeandtheycomeupintwoflavoursasshownforthevec4case: [stextbox] voidglProgramUniform4f(GLuintprogram,GLintlocation,GLfloatv1,…,GLfloatv4); voidglProgramUniform4fv(GLuintprogram,GLintlocation,GLsizeicount,GLfloat*v); Params: program:thelinkedprogramhandle location:thelocationofthevariable v1..v4:thevaluesforeachcomponentofthevec4 count:thenumberofvec4toset v:apointertowherethefloatdatamaybefound [/stextbox] Formatrices,thereisaspecialversionofthisfunction,seebelowtheexampleforamat4: [stextbox] voidglProgramUniformMatrix4fv(GLuintprogram,GLintlocation,GLsizeicount,GLbooleantranspose,GLfloat*v); Params: program:thelinkedprogramhandle location:thelocationofthevariable count:thenumberofvec4toset transpose:iftruethematrixwillbetransposedpriortoloadingtotheuniform v:apointertowherethefloatdatamaybefound [/stextbox] Structs InGLSLwecandefinestructs,inawaysimilartoC.Forinstance,thefollowingsnippetofcodedeclaresastructwithtwovec4s,anddefinesauniformvariableofthattype. structColors{ vec4Color1; vec4Color2; }; uniformColorsmyColors; Tousethestruct’sfieldsinsideashaderweusethesamenotationasinC.Forinstance,thefollowingmainfunctionassumesthedeclarationsabove: voidmain() { vec4aColor=myColors.Color1+myColors.Color2; ... } TosetthesevariablesintheapplicationwealsousethesamenotationasinC.Wecan’tsetthestructasawhole,asitcan’tgetalocationforit.Wemustsetitfieldbyfield.Thefollowingexampleshowshow: floatmyFloats[8]={0.0,1.0,0.0,0.0,1.0,0.0,0.0,0.0}; GLintmyLoc=glGetUniformLocation(p,"myColors.Color1"); glProgramUniform4fv(p,myLoc,1,myFloats); myLoc=glGetUniformLocation(p,"myColors.Color2"); glProgramUniform4fv(p,myLoc,1,&(myFloats[4])); Workingwitharraysofstructsissimple.Considerthefollowinguniformdeclaration: uniformColorsmyColors[2]; Intheshaderwecanwritethefollowing: voidmain() { vec4aColor=myColors[0].Color1+myColors[1].Color2; ... } Insideourapplicationweproceedinthesamemanner: GLintmyLoc; floatmyFloats[8]={0.0,1.0,0.0,0.0,1.0,0.0,0.0,0.0}; floatmyOtherFloats[8[={1.0,1.0,0.0,0.0,0.0,1.0,1.0,0.0}; myLoc=glGetUniformLocation(p,"myColors[0].Color1"); glProgramUniform4fv(p,myLoc,1,myFloats); myLoc=glGetUniformLocation(p,"myColors[0].Color2"); glProgramUniform4fv(p,myLoc,1,&(myFloats[4])); myLoc=glGetUniformLocation(p,"myColors[1].Color1"); glProgramUniform4fv(p,myLoc,1,myOtherFloats); myLoc=glGetUniformLocation(p,"myColors[1].Color2"); glProgramUniform4fv(p,myLoc,1,&(myOtherFloats[4])); Likethis:LikeLoading...  Prev:AttributeVariables Next:UniformBlocks     4Responsesto“GLSLTutorial–UniformVariables” Larrysays: 29/05/2014at4:34PM Nicearticle–thanks Suggestedtypofix: “…itreliefsusfromhavingtosetuniforms.” shouldbe “…itrelievesusfromhavingtosetuniforms.” Reply ARFsays: 03/06/2014at12:16PM Fixed,Thanks! Reply MingJinsays: 20/06/2013at2:58PM voidglGetProgramUniform4f(GLuintprogram,GLintlocation,GLfloatv1,…,GLfloatv4); Ithinkthefunctionshouldinsteadbe“glProgramUniform4f”? Reply ARFsays: 23/06/2013at2:18PM Fixed.Thanks. Reply LeaveaReply Cancelreply ThissiteusesAkismettoreducespam.Learnhowyourcommentdataisprocessed. 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