An Overview of the OpenGL Shading Language - InformIT

However, unlike ANSI C, main() does not return an integer value; ... GLSL switch statements also support “fall-through” cases—case ... SUMMERREAD-A-THON Save35%onbooks&eBookswithcodeREADATHON.Shopnow. Home > Articles ShaderFundamentals Oct24,2016 📄Contents ␡ ShadersandOpenGL OpenGL'sProgrammablePipeline AnOverviewoftheOpenGLShadingLanguage InterfaceBlocks CompilingShaders ShaderSubroutines SeparateShaderObjects SPIR-V ⎙Print +ShareThis Thischapterisfromthebook OpenGLProgrammingGuide:TheOfficialGuidetoLearningOpenGL,Version4.5withSPIR-V,9thEdition LearnMore Buy OpenGLProgrammingGuide:TheOfficialGuidetoLearningOpenGL,Version4.5withSPIR-V,9thEdition LearnMore Buy AnOverviewoftheOpenGLShadingLanguage ThissectionprovidesanoverviewoftheshadinglanguageusedwithinOpenGL.GLSLsharesmanytraitswithC++andJava,andisusedforauthoringshadersforallthestagessupportedinOpenGL,althoughcertainfeaturesareavailableonlyforparticulartypesofshaders.WewillfirstdescribeGLSL’srequirements,types,andotherlanguageconstructsthataresharedbetweenthevariousshaderstages,andthendiscussthefeaturesuniquetoeachtypeofshader. CreatingShaderswithGLSL Here,wedescribehowtocreateacompleteshader. TheStartingPoint Ashaderprogram,justlikeaCprogram,startsexecutioninmain().EveryGLSLshaderprogrambeginslifeasfollows: #version330core void main() {     //Yourcodegoeshere } The//constructisacommentandterminatesattheendofthecurrentline,justlikeinC.Additionally,C-type,multi-linecomments—the/*and*/type—arealsosupported.However,unlikeANSIC,main()doesnotreturnanintegervalue;itisdeclaredvoid.Also,aswithCanditsderivativelanguages,statementsareterminatedwithasemicolon.WhilethisisaperfectlylegalGLSLprogramthatcompilesandevenruns,itsfunctionalityleavessomethingtobedesired.Toaddalittlemoreexcitementtoourshaders,we’llcontinuebydescribingvariablesandtheiroperation. DeclaringVariables GLSLisatypedlanguage;everyvariablemustbedeclaredandhaveanassociatedtype.VariablenamesconformtothesamerulesasthoseforC:Youcanuseletters,numbers,andtheunderscorecharacter(_)tocomposevariablenames.However,adigitcannotbethefirstcharacterinavariablename.Similarly,variablenamescannotcontainconsecutiveunderscores;thosenamesarereservedinGLSL. Table2.1showsthebasictypesavailableinGLSL. Table2.1BasicDataTypesinGLSL Type Description float IEEE32-bitfloating-pointvalue double IEEE64-bitfloating-pointvalue int signedtwo´s-complement32-bitintegervalue uint unsigned32-bitintegervalue bool Booleanvalue Thesetypes(andlater,aggregatetypescomposedofthese)arealltransparent.Thatis,theirinternalformisexposedandtheshadercodegetstoassumewhattheylooklikeinternally. Anadditionalsetoftypes,theopaquetypes,donothavetheirinternalformexposed.Theseincludesamplertypes,imagetypes,andatomiccountertypes.Theydeclarevariablesusedasopaquehandlesforaccessingtexturemaps,images,andatomiccounters,asdescribedinChapter4,“Color,Pixels,andFragments.” ThevarioustypesofsamplersandtheirusesarediscussedinChapter6,“TexturesandFramebuffers.” VariableScoping Whileallvariablesmustbedeclared,theymaybedeclaredanytimebeforetheiruse(justasinC++).ThescopingrulesofGLSL,whichcloselyparallelthoseofC++,areasfollows: Variablesdeclaredoutsideofanyfunctiondefinitionhaveglobalscopeandarevisibletoallsubsequentfunctionswithintheshaderprogram. Variablesdeclaredwithinasetofcurlybraces(e.g.,functiondefinition,blockfollowingaloopor“if”statement,etc.)existwithinthescopeofthosebracesonly. Loopiterationvariables,suchasiintheloop for(inti=0;i<10;++i){      //loopbody } arescopedonlyforthebodyoftheloop. VariableInitialization Variablesmayalsobeinitializedwhendeclared.Forexample: int     i,numParticles=1500; float   force,g=-9.8; bool    falling=true; double  pi=3.1415926535897932384626LF; Integerliteralconstantsmaybeexpressedasoctal,decimal,orhexadecimalvalues.Anoptionalminussignbeforeanumericvaluenegatestheconstant,andatrailing‘u’or‘U’denotesanunsignedintegervalue. Floating-pointliteralsmustincludeadecimalpoint,unlessdescribedinscientificformat,asin3E-7.(However,therearemanysituationswhereanintegerliteralwillbeimplicitlyconvertedtoafloating-pointvalue.)Additionally,theymayoptionallyincludean‘f’or‘F’suffixasinConafloatliteral.Youmustincludeasuffixof‘lF’or‘LF’tomakealiteralhavetheprecisionofadouble. BooleanvaluesareeithertrueorfalseandcanbeinitializedtoeitherofthosevaluesorastheresultofanoperationthatresolvestoaBooleanexpression. Constructors Asmentioned,GLSLismoretypesafethanC++,havingfewerimplicitconversionbetweenvalues.Forexample, intf=false; willresultinacompilationerrorduetoassigningaBooleanvaluetoanintegervariable.TypeswillbeimplicitlyconvertedasshowninTable2.2. Table2.2ImplicitConversionsinGLSL TypeNeeded CanBeImplicitlyConvertedFrom uint int float int,uint double int,uint,float Thesetypeconversionsworkforscalars,vectors,andmatricesofthesetypes.Conversionswillneverchangewhethersomethingisavectororamatrix,orhowmanycomponentsithas.Conversionsalsodon’tapplytoarraysorstructures. Anyotherconversionofvaluesrequiresexplicitconversionusingaconversionconstructor.Aconstructor,asinotherlanguageslikeC++,isafunctionwiththesamenameasatype,whichreturnsavalueofthattype.Forexample, floatf=10.0; int   ten=int(f); usesanintconversionconstructortodotheconversion.Likewise,theothertypesalsohaveconversionconstructors:float,double,uint,bool,andvectorsandmatricesofthesetypes.Eachacceptsmultipleothertypestoexplicitlyconvertfrom.ThesefunctionsalsoillustrateanotherfeatureofGLSL:functionoverloading,wherebyeachfunctiontakesvariousinputtypes,butallusethesamebasefunctionname.Wewilldiscussmoreonfunctionsinabit. AggregateTypes GLSL’sbasictypescanbecombinedtobettermatchcoreOpenGL’sdatavaluesandtoeasecomputationaloperations. First,GLSLsupportsvectorsoftwo,three,orfourcomponentsforeachofthebasictypesofbool,int,uint,float,anddouble.Also,matricesoffloatanddoubleareavailable.Table2.3liststhevalidvectorandmatrixtypes. Table2.3GLSLVectorandMatrixTypes BaseType 2DVec 3DVec 4DVec MatrixTypes float vec2 vec3 vec4 mat2mat3mat4mat2x2mat2x3mat2x4mat3x2mat3x3mat3x4mat4x2mat4x3mat4x4 double dvec2 dvec3 dvec4 dmat2dmat3dmat4dmat2x2dmat2x3dmat2x4dmat3x2dmat3x3dmat3x4dmat4x2dmat4x3dmat4x4 int ivec2 ivec3 ivec4 ‐ uint uvec2 uvec3 uvec4 ‐ bool bvec2 bvec3 bvec4 ‐ Matrixtypesthatlistbothdimensions,suchasmat4x3,usethefirstvaluetospecifythenumberofcolumns,thesecondthenumberofrows. Variablesdeclaredwiththesetypescanbeinitializedsimilartotheirscalarcounterparts: vec3velocity=vec3(0.0,2.0,3.0); Convertingbetweentypesisequallyaccessible: ivec3steps=ivec3(velocity); Vectorconstructorscanalsobeusedtotruncateorlengthenavector.Ifalongervectorispassedintotheconstructorofasmallervector,thevectoristruncatedtotheappropriatelength. vec4color; vec3RGB=vec3(color);//nowRGBonlyhasthreeelements Scalarvaluescanbepromotedtovectors,butthat’stheonlywayavectorconstructortakesfewercomponentsthanitssizeindicates: vec3white=vec3(1.0);  //white=(1.0,1.0,1.0) vec4translucent=vec4(white,0.5); Matricesareconstructedinthesamemannerandcanbeinitializedtoeitheradiagonalmatrixorafullypopulatedmatrix.Inthecaseofdiagonalmatrices,asinglevalueispassedintotheconstructor,andthediagonalelementsofthematrixaresettothatvalue,withallothersbeingsettozero,asin Matricescanalsobecreatedbyspecifyingthevalueofeveryelementinthematrixintheconstructor.Valuescanbespecifiedbycombinationsofscalarsandvectorsaslongasenoughvaluesareprovidedandeachcolumnisspecifiedinthesamemanner.Additionally,matricesarespecifiedincolumn-majororder,meaningthevaluesareusedtopopulatecolumnsbeforerows(whichistheoppositeofhowCinitializestwo-dimensionalarrays). Forexample,wecouldinitializea3×3matrixinanyofthefollowingways: mat3M=mat3(1.0,2.0,3.0,               4.0,5.0,6.0,               7.0,8.0,9.0); vec3column1=vec3(1.0,2.0,3.0); vec3column2=vec3(4.0,5.0,6.0); vec3column3=vec3(7.0,8.0,9.0); mat3M=mat3(column1,column2,column3); oreven vec2column1=vec2(1.0,2.0); vec2column2=vec2(4.0,5.0); vec2column3=vec2(7.0,8.0); mat3M=mat3(column1,3.0, column2,6.0, column3,9.0); allyieldingthesamematrix, AccessingElementsinVectorsandMatrices Theindividualelementsofvectorsandmatricescanbeaccessedandassigned.Vectorssupporttwotypesofelementaccess:anamed-componentmethodandanarray-likemethod.Matricesuseatwo-dimensionalarray-likemethod. Componentsofavectorcanbeaccessedbyname,asin floatred=color.r; floatv_y=velocity.y; orbyusingazero-basedindexscheme.Thefollowingyieldidenticalresultstothepreviouslisting: floatred=color[0]; floatv_y=velocity[1]; Infact,asshowninTable2.4,therearethreesetsofcomponentnames,allofwhichdothesamething.Themultiplesetsareusefulforclarifyingtheoperationsthatyou’redoing. Table2.4VectorComponentAccessors ComponentAccessors Description (x,y,z,w) Componentsassociatedwithpositions (r,g,b,a) Componentsassociatedwithcolors (s,t,p,q) Componentsassociatedwithtexturecoordinates Acommonuseforcomponent-wiseaccesstovectorsisforswizzlingcomponents,asyoumightdowithcolors,perhapsforcolorspaceconversion.Forexample,youcoulddothefollowingtospecifyaluminancevaluebasedontheredcomponentofaninputcolor: vec3luminance=color.rrr; Likewise,ifyouneededtomovecomponentsaroundinavector,youmightdo color=color.abgr;//reversethecomponentsofacolor Theonlyrestrictionisthatonlyonesetofcomponentscanbeusedwithavariableinonestatement.Thatis,youcan’tdo vec4color=otherColor.rgz;//Error:'z'isfromadifferentgroup Also,acompile-timeerrorwillberaisedifyouattempttoaccessanelementthat’soutsidetherangeofthetype.Forexample, vec2pos; floatzPos=pos.z;//Error:no'z'componentin2Dvectors Matrixelementscanbeaccessedusingthearraynotation.Eitherasinglescalarvalueoranarrayofelementscanbeaccessedfromamatrix: mat4m=mat4(2.0); vec4zVec=m[2];       //getcolumn2ofthematrix floatyScale=m[1][1];//orm[1].yworksaswell Structures Youcanalsologicallygroupcollectionsofdifferenttypesintoastructure.Structuresareconvenientforpassinggroupsofassociateddataintofunctions.Whenastructureisdefined,itautomaticallycreatesanewtypeandimplicitlydefinesaconstructorfunctionthattakesthetypesoftheelementsofthestructureasparameters. structParticle{     floatlifetime;     vec3position;     vec3velocity; }; Particlep=Particle(10.0,pos,vel);//pos,velarevec3s Likewise,toreferenceelementsofastructure,usethefamiliar“dot”notationasyouwouldinC. Arrays GLSLalsosupportsarraysofanytype,includingstructures.AswithC,arraysareindexedusingbrackets([]).Therangeofelementsinanarrayofsizenis0...n–1.UnlikeinC,however,neithernegativearrayindicesnorpositiveindicesoutofrangearepermitted.AsofGLSL4.3,arrayscanbemadeoutofarrays,providingawaytohandlemultidimensionaldata.However,GLSL4.2andearlierversionsdonotallowarraysofarraystobecreated(thatis,youcannotcreateamultidimensionalarray). Arrayscanbedeclaredsizedorunsized.Youmightuseanunsizedarrayasaforwarddeclarationofanarrayvariableandlaterredeclareittotheappropriatesize.Arraydeclarationsusethebracketnotation,asin float     coeff[3];//anarrayof3floats float[3]  coeff;//samething int       indices[];//unsized.Redeclarelaterwithasize Arraysarefirst-classtypesinGLSL,meaningtheyhaveconstructorsandcanbeusedasfunctionparametersandreturntypes.Tostaticallyinitializeanarrayofvalues,youwoulduseaconstructorinthefollowingmanner: floatcoeff[3]=float[3](2.38,3.14,42.0); Thedimensionvalueontheconstructorisoptional. Additionally,similartoJava,GLSLarrayshaveanimplicitmethodforreportingtheirnumberofelements:thelength()method.Ifyouwouldliketooperateonallthevaluesinanarray,hereisanexampleusingthelength()method: for(inti=0;i> integer Bit-wiseoperations 7 <><=>= arithmetic Relationaloperations 8 ==!= any Equalityoperations 9 & integer Bit-wiseand 10 ∧ integer Bit-wiseexclusiveor 11 | integer Bit-wiseinclusiveor 12 && bool Logicalandoperation 13 ∧∧ bool Logicalexclusive-oroperation 14 || bool Logicaloroperation 15 a?b:c bool?any:any Ternaryselectionoperation(inline“if„operation;if(a)then(b)else(c)) 16 =+=-=*=/=%=<<=>>=&=∧=|= anyarithmeticarithmeticintegerinteger AssignmentArithmeticassignment 17 ,(comma) any Sequenceofoperations OverloadedOperators MostoperatorsinGLSLareoverloaded,meaningthattheyoperateonavariedsetoftypes.Specifically,arithmeticoperations(includingpre-andpost-incrementand-decrement)forvectorsandmatricesarewelldefinedinGLSL.Forexample,tomultiplyavectorandamatrix(recallingthattheorderofoperandsisimportant;matrixmultiplicationisnoncommutative,forallyoumathheads),usethefollowingoperation: vec3v; mat3m; vec3result=v*m; Thenormalrestrictionsapply,thatthedimensionalityofthematrixandthevectormustmatch.Additionally,scalarmultiplicationwithavectorormatrixwillproducetheexpectedresult.Onenotableexceptionisthatthemultiplicationoftwovectorswillresultincomponent-wisemultiplicationofcomponents;however,multiplyingtwomatriceswillresultinnormalmatrixmultiplication. vec2a,b,c; mat2m,u,v; c=a*b;//      c=(a.x*b.x,a.y*b.y) m=u*v;//      m=(u00*v00+u01*v10   u00*v01+u01*v11            //           u01*v00+u11*v10   u10*v01+u11*v11) Additionalcommonvectoroperations(e.g.,dotandcrossproducts)aresupportedbyfunctioncalls,aswellasvariousper-componentoperationsonvectorsandmatrices. ControlFlow GLSL’slogicalcontrolstructuresarethepopularif-elseandswitchstatements.AswiththeClanguage,theelseclauseisoptional,andmultiplestatementsrequireablock. if(truth){     //trueclause } else{     //falseclause } SimilartothesituationinC,switchstatementsareavailable(startingwithGLSL1.30)intheirfamiliarform: switch(int_value){     casen:       //statements       break;     casem:       //statements       break;     default:       //statements       break; } GLSLswitchstatementsalsosupport“fall-through”cases—casestatementsthatdonotendwithbreakstatements.Eachcasedoesrequiresomestatementtoexecutebeforetheendoftheswitch(beforetheclosingbrace).Also,unlikeinC++,nostatementsareallowedbeforethefirstcase.Ifnocasematchestheswitchandadefaultlabelispresent,thenitisexecuted. LoopingConstructs GLSLsupportsthefamiliarCformoffor,while,anddo...whileloops. Theforlooppermitsthedeclarationoftheloopiterationvariableintheinitializationclauseoftheforloop.Thescopeofiterationvariablesdeclaredinthismannerisonlyforthelifetimeoftheloop. for(inti=0;i<10;++i){     ... } while(n<10){     ... } do{     ... }while(n<10); Control-FlowStatements AdditionalcontrolstatementsbeyondconditionalsandloopsareavailableinGLSL.Table2.7describesavailablecontrol-flowstatements. Table2.7GLSLControl-FlowStatements Statement Description break Terminatesexecutionoftheblockofaloopandcontinuesexecutionafterthescopeofthatblock. continue Terminatesthecurrentiterationoftheenclosingblockofaloop,resumingexecutionwiththenextiterationoftheloop. return[result] Returnsfromthecurrentfunction,optionallyprovidingavaluetobereturnedfromthefunction(assumingreturnvaluematchesthereturntypeoftheenclosingfunction). discard Discardsthecurrentfragmentandceasesshaderexecution.Discardstatementsarevalidonlyinfragmentshaderprograms. Thediscardstatementisavailableonlyinfragmentprograms.Theexecutionofthefragmentshadermaybeterminatedattheexecutionofthediscardstatement,butthisisimplementation-dependent. Functions Functionspermityoutoreplaceoccurrencesofcommoncodewithafunctioncall.This,ofcourse,allowsforsmallercodeandfewerchancesforerrors.GLSLdefinesanumberofbuilt-infunctions,whicharelistedinAppendixC,aswellassupportforuser-definedfunctions.User-definedfunctionscanbedefinedinasingleshaderobjectandreusedinmultipleshaderprograms. Declarations FunctiondeclarationsyntaxisverysimilartoC,withtheexceptionoftheaccessmodifiersonvariables: returnTypefunctionName([accessModifier]type1variable1,                         [accessModifier]type2variable2,                           ...) {     //functionbody     returnreturnValue;//unlessreturnTypeisvoid } Functionnamescanbeanycombinationofletters,numbers,andtheunderscorecharacter,withtheexceptionthatitcanneitherbeginwithadigitnorwithgl_norcontainconsecutiveunderscores. Returntypescanbeanybuilt-inGLSLtypeoruser-definedstructureorarraytype.Arraysasreturnvaluesmustexplicitlyspecifytheirsize.Ifafunctiondoesn’treturnavalue,itsreturntypeisvoid. Parameterstofunctionscanalsobeofanytype,includingarrays(whichmustspecifytheirsize). Functionsmustbeeitherdeclaredwithaprototypeordefinedwithabodybeforetheyarecalled.JustasinC++,thecompilermusthaveseenthefunction’sdeclarationbeforeitsuse,oranerrorwillberaised.Ifafunctionisusedinashaderobjectotherthantheonewhereit’sdefined,aprototypemustbedeclared.Aprototypeismerelythefunction’ssignaturewithoutitsaccompanyingbody.Here’sasimpleexample: floatHornerEvalPolynomial(floatcoeff[10],floatx); ParameterQualifiers WhilefunctionsinGLSLareabletomodifyandreturnvaluesaftertheirexecution,there’snoconceptofapointerorreference,asinCorC++.Rather,parametersoffunctionshaveassociatedparameterqualifiersindicatingwhetherthevalueshouldbecopiedinto,oroutof,afunctionafterexecution.Table2.8describestheavailableparameterqualifiersinGLSL. Table2.8GLSLFunctionParameterAccessModifiers AccessModifier Description in Valuecopiedintoafunction(defaultifnotspecified) constin Read-onlyvaluecopiedintoafunction out Valuecopiedoutofafunction(undefineduponentranceintothefunction) inout Valuecopiedintoandoutofafunction Theinkeywordisoptional.Ifavariabledoesnotincludeanaccessmodifier,aninmodifierisimplicitlyaddedtotheparameter’sdeclaration.However,ifthevariable’svalueneedstobecopiedoutofafunction,itmusteitherbetaggedwithanout(forcopyout-onlyvariables)oraninout(foravariablebothcopiedinandcopiedout)modifier.Writingtoavariablenottaggedwithoneofthesemodifierswillgenerateacompile-timeerror. Additionally,toverifyatcompiletimethatafunctiondoesn’tmodifyaninput-onlyvariable,addingaconstinmodifierwillcausethecompilertocheckthatthevariableisnotwrittentointhefunction.Ifyoudon’tdothisanddowritetoaninput-onlyvariable,thewriteonlymodifiesthelocalcopyinthefunction. ComputationalInvariance GLSLdoesnotguaranteethattwoidenticalcomputationsindifferentshaderswillresultinexactlythesamevalue.ThesituationisnodifferentthanforcomputationalapplicationsexecutingontheCPU,wherethechoiceofoptimizationsmayresultintinydifferencesinresults.Thesetinyerrorsmaybeanissueformultipassalgorithmsthatexpectpositionstobecomputedexactlythesameforeachshaderpass.GLSLhastwomethodsforenforcingthistypeofinvariancebetweenshaders,usingtheinvariantorprecisekeywords. Bothofthesemethodswillcausecomputationsdonebythegraphicsdevicetocreatereproducibility(invariance)inresultsofthesameexpression.However,theydonothelpreproducethesameresultsbetweenthehostandthegraphicsdevice.Compile-timeconstantexpressionsarecomputedonthecompiler’shost,andthereisnoguaranteethatthehostcomputesinexactlythesamewayasthegraphicsdevice.Forexample: uniformfloatten;         //applicationsetsthisto10.0 constfloatf=sin(10.0);//computedoncompilerhost floatg=sin(ten);        //computedongraphicsdevice voidmain() {     if(f==g)            //fandgmightbenotequal         ; } Inthisexample,itwouldnotmatterifinvariantorprecisewasusedonanyofthevariablesinvolved,astheyaffectonlytwocomputationsdoneonthegraphicsdevice. TheinvariantQualifier Theinvariantqualifiermaybeappliedtoanyshaderoutputvariable.Itwillguaranteethatiftwoshaderinvocationseachsettheoutputvariablewiththesameexpressionandthesamevaluesforthevariablesinthatexpression,bothwillcomputethesamevalue. Theoutputvariabledeclaredasinvariantmaybeabuilt-invariableorauser-definedone.Forexample: invariantgl_Position; invariantcentroidoutvec3Color; Asyoumayrecall,outputvariablesareusedtopassdatafromonestagetothenext.Theinvariantkeywordmaybeappliedatanytimebeforeuseofthevariableintheshaderandmaybeusedtomodifybuilt-invariables.Thisisdonebydeclaringthevariableonlywithinvariant,aswasshownearlierforgl_Position. Fordebugging,itmaybeusefultoimposeinvarianceonallvaryingvariablesinshader.Thiscanbeaccomplishedbyusingthevertexshaderpreprocessorpragma. #pragmaSTDGLinvariant(all) Globalinvarianceinthismannerisusefulfordebugging;however,itmaylikelyhaveanimpactontheshader’sperformance.GuaranteeinginvarianceusuallydisablesoptimizationsthatmayhavebeenperformedbytheGLSLcompiler. ThepreciseQualifier Theprecisequalifiermaybeappliedtoanycomputedvariableorfunctionreturnvalue.Despiteitsname,itspurposeisnottoincreaseprecision,buttoincreasereproducibilityofacomputation.Itismostlyusedintessellationshaderstoavoidformingcracksinyourgeometry.TessellationshadingingeneralisdescribedinChapter9,“TessellationShaders,”andthereisadditionaldiscussioninthatchapteraboutausecaseforprecisequalification. Generally,youusepreciseinsteadofinvariantwhenyouneedtogetthesameresultfromanexpression,evenifvaluesfeedingtheexpressionarepermutedinawaythatshouldnotmathematicallyaffecttheresult.Forexample,thefollowingexpressionshouldgetthesameresultifthevaluesforaandbareexchanged.Itshouldalsogetthesameresultifthevaluesforcanddandexchanged,orifbothaandcareexchangedandbanddareexchanged,andsoon. Location=a*b+c*d; Theprecisequalifiermaybeappliedtoabuilt-invariable,uservariable,orfunctionreturnvalue. precisegl_Position; preciseoutvec3Location; precisevec3subdivide(vec3P1,vec3P2){...} Theprecisekeywordmaybeappliedatanytimebeforeuseofthevariableintheshaderandmaybeusedtomodifypreviouslydeclaredvariables. Onepracticalimpactinacompilerofusingpreciseisanexpressionliketheoneabovecannotbeevaluatedusingtwodifferentmethodsofmultiplicationforthetwomultiplyoperations—forexample,amultiplyinstructionforthefirstmultiplyandafusedmultiply-and-addinstructionforthesecondmultiply.Thisisbecausethesetwoinstructionswillgetslightlydifferentresultsforthesamevalues.Becausethatwasdisallowedbyprecise,thecompilerispreventedfromdoingthis.Becauseuseoffusedmultiply-and-addinstructionsisimportanttoperformance,itwouldbeunfortunatetocompletelydisallowthem.Sothereisabuilt-infunctioninGLSL,fma(),thatyoucanusetoexplicitlysaythisisokay. preciseoutfloatresult;   ... floatf=c*d; floatresult=fma(a,b,f); Ofcourse,youdothatonlyifyouweren’tgoingtohavethevaluesofaandcpermuted,asyouwouldbedefeatingthepurposeofusingprecise. ShaderPreprocessor ThefirststepincompilationofaGLSLshaderisparsingbythepreprocessor.SimilartotheCpreprocessor,thereareanumberofdirectivesforcreatingconditionalcompilationblocksanddefiningvalues.However,unlikeintheCpreprocessor,thereisnofileinclusion(#include). PreprocessorDirectives Table2.9liststhepreprocessordirectivesacceptedbytheGLSLpreprocessorandtheirfunctions. Table2.9GLSLPreprocessorDirectives PreprocessorDirective Description #define Controlthedefinitionofconstantsand #undef macrossimilartotheCpreprocessor. #if Conditionalcodemanagementsimilar #ifdef totheCpreprocessor,includingthedefinedoperator. #ifndef #else Conditionalexpressionsevaluateinteger #elif expressionsanddefinedvalues #endif (asspecifiedby#define)only. #errortext Causethecompilertoinserttext(uptothefirstnewlinecharacter)intotheshaderinformationlog. #pragmaoptions Controlcompilerspecificoptions. #extensionoptions SpecifycompileroperationwithrespecttospecifiedGLSLextensions. #versionnumber MandateaspecificversionofGLSLversionsupport. #lineoptions Controldiagnosticlinenumbering. MacroDefinition TheGLSLpreprocessorallowsmacrodefinitioninmuchthesamemannerastheCpreprocessor,withtheexceptionofthestringsubstitutionandconcatenationfacilities.Macrosmightdefineasinglevalue,asin #defineNUM_ELEMENTS10 orwithparameterslike #defineLPos(n)gl_LightSource[(n)].position Additionally,thereareseveralpredefinedmacrosforaidingindiagnosticmessages(thatyoumightissuewiththe#errordirective,forexample),asshowninTable2.10. Table2.10GLSLPreprocessorPredefinedMacros MacroName Description _LINE_ Linenumberdefinedbyonemorethanthenumberofnewlinecharactersprocessedandmodifiedbythe#linedirective _FILE_ Sourcestringnumbercurrentlybeingprocessed _VERSION_ IntegerrepresentationoftheOpenGLShadingLanguageversion Likewise,macros(excludingthosedefinedbyGLSL)maybeundefinedbyusingthe#undefdirective.Forexample, #undefLPos PreprocessorConditionals IdenticaltotheprocessingbytheCpreprocessor,theGLSLpreprocessorprovidesconditionalcodeinclusionbasedonmacrodefinitionandintegerconstantevaluation. Macrodefinitionmaybedeterminedintwoways.Usethe#ifdefdirective: #ifdefNUM_ELEMENTS   ... #endif Orusethedefinedoperatorwiththe#ifor#elifdirectives: #ifdefined(NUM_ELEMENTS)&&NUM_ELEMENTS>3   ... #elifNUM_ELEMENTS<7   ... #endif CompilerControl The#pragmadirectiveprovidesthecompileradditionalinformationregardinghowyouwouldlikeyourshaderscompiled. OptimizationCompilerOption Theoptimizeoptioninstructsthecompilertoenableordisableoptimizationoftheshaderfromthepointwherethedirectiveresidesforwardintheshadersource.Youcanenableordisableoptimizationbyissuingeither #pragmaoptimize(on) or #pragmaoptimize(off) respectively.Theseoptionsmaybeissuedonlyoutsideofafunctiondefinition.Bydefault,optimizationisenabledforallshaders. DebugCompilerOption Thedebugoptionenablesordisablesadditionaldiagnosticoutputoftheshader.Youcanenableordisabledebuggingbyissuingeither #pragmadebug(on) or #pragmadebug(off) respectively.Aswiththeoptimizeoption,theseoptionsmaybeissuedonlyoutsideofafunctiondefinition,andbydefault,debuggingisdisabledforallshaders. GlobalShader-CompilationOption Onefinal#pragmadirectivethatisavailableisSTDGL.Thisoptioniscurrentlyusedtoenableinvarianceintheoutputofvaryingvalues. ExtensionProcessinginShaders GLSL,likeOpenGLitself,maybeenhancedbyextensions.AsvendorsmayincludeextensionsspecifictotheirOpenGLimplementation,it’susefultohavesomecontrolovershadercompilationinlightofpossibleextensionsthatashadermayuse. TheGLSLpreprocessorusesthe#extensiondirectivetoprovideinstructionstotheshadercompilerregardinghowextensionavailabilityshouldbehandledduringcompilation.Foranyorallextensions,youcanspecifyhowyouwouldlikethecompilertoproceedwithcompilation: #extensionextension_name: forasingleextension,or #extensionall: whichaffectsthebehaviorofallextensions. TheoptionsavailableareshowninTable2.11 Table2.11GLSLExtensionDirectiveModifiers Directive Description require Flaganerroriftheextensionisnotsupportedoriftheall-extensionspecificationisused. enable Giveawarningiftheparticularextensionsspecifiedarenotsupported,orflaganerroriftheall-extensionspecificationisused. warn Giveawarningiftheparticularextensionsspecifiedarenotsupported,orgiveawarningifanyextensionuseisdetectedduringcompilation. disable Disablesupportfortheparticularextensionslisted(thatis,havethecompileractasiftheextensionisnotsupportedevenifitis)orallextensionsifallispresent,issuingwarningsanderrorsasiftheextensionwerenotpresent. +ShareThis 🔖SaveToYourAccount InformITPromotionalMailings&SpecialOffers ⚠ IwouldliketoreceiveexclusiveoffersandhearaboutproductsfromInformITanditsfamilyofbrands.Icanunsubscribeatanytime. 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Surveys Pearsonmayofferopportunitiestoprovidefeedbackorparticipateinsurveys,includingsurveysevaluatingPearsonproducts,servicesorsites.Participationisvoluntary.Pearsoncollectsinformationrequestedinthesurveyquestionsandusestheinformationtoevaluate,support,maintainandimproveproducts,servicesorsites,developnewproductsandservices,conducteducationalresearchandforotherpurposesspecifiedinthesurvey.ContestsandDrawings Occasionally,wemaysponsoracontestordrawing.Participationisoptional.Pearsoncollectsname,contactinformationandotherinformationspecifiedontheentryformforthecontestordrawingtoconductthecontestordrawing.Pearsonmaycollectadditionalpersonalinformationfromthewinnersofacontestordrawinginordertoawardtheprizeandfortaxreportingpurposes,asrequiredbylaw. Newsletters Ifyouhaveelectedtoreceiveemailnewslettersorpromotionalmailingsandspecialoffersbutwanttounsubscribe,[email protected]. ServiceAnnouncements Onrareoccasionsitisnecessarytosendoutastrictlyservicerelatedannouncement.Forinstance,ifourserviceistemporarilysuspendedformaintenancewemightsendusersanemail.Generally,usersmaynotopt-outofthesecommunications,thoughtheycandeactivatetheiraccountinformation.However,thesecommunicationsarenotpromotionalinnature. CustomerService Wecommunicatewithusersonaregularbasistoproviderequestedservicesandinregardtoissuesrelatingtotheiraccountwereplyviaemailorphoneinaccordancewiththeusers'wisheswhenausersubmitstheirinformationthroughourContactUsform.OtherCollectionandUseofInformation ApplicationandSystemLogs Pearsonautomaticallycollectslogdatatohelpensurethedelivery,availabilityandsecurityofthissite.Logdatamayincludetechnicalinformationabouthowauserorvisitorconnectedtothissite,suchasbrowsertype,typeofcomputer/device,operatingsystem,internetserviceproviderandIPaddress.Weusethisinformationforsupportpurposesandtomonitorthehealthofthesite,identifyproblems,improveservice,detectunauthorizedaccessandfraudulentactivity,preventandrespondtosecurityincidentsandappropriatelyscalecomputingresources. WebAnalytics Pearsonmayusethirdpartywebtrendanalyticalservices,includingGoogleAnalytics,tocollectvisitorinformation,suchasIPaddresses,browsertypes,referringpages,pagesvisitedandtimespentonaparticularsite.Whiletheseanalyticalservicescollectandreportinformationonananonymousbasis,theymayusecookiestogatherwebtrendinformation.TheinformationgatheredmayenablePearson(butnotthethirdpartywebtrendservices)tolinkinformationwithapplicationandsystemlogdata.Pearsonusesthisinformationforsystemadministrationandtoidentifyproblems,improveservice,detectunauthorizedaccessandfraudulentactivity,preventandrespondtosecurityincidents,appropriatelyscalecomputingresourcesandotherwisesupportanddeliverthissiteanditsservices. CookiesandRelatedTechnologies Thissiteusescookiesandsimilartechnologiestopersonalizecontent,measuretrafficpatterns,controlsecurity,trackuseandaccessofinformationonthissite,andprovideinterest-basedmessagesandadvertising.Userscanmanageandblocktheuseofcookiesthroughtheirbrowser.Disablingorblockingcertaincookiesmaylimitthefunctionalityofthissite. DoNotTrack ThissitecurrentlydoesnotrespondtoDoNotTracksignals. Security Pearsonusesappropriatephysical,administrativeandtechnicalsecuritymeasurestoprotectpersonalinformationfromunauthorizedaccess,useanddisclosure. Children Thissiteisnotdirectedtochildrenundertheageof13.Marketing Pearsonmaysendordirectmarketingcommunicationstousers,providedthat PearsonwillnotusepersonalinformationcollectedorprocessedasaK-12schoolserviceproviderforthepurposeofdirectedortargetedadvertising. SuchmarketingisconsistentwithapplicablelawandPearson'slegalobligations. Pearsonwillnotknowinglydirectorsendmarketingcommunicationstoanindividualwhohasexpressedapreferencenottoreceivemarketing. Whererequiredbyapplicablelaw,expressorimpliedconsenttomarketingexistsandhasnotbeenwithdrawn. PearsonmayprovidepersonalinformationtoathirdpartyserviceprovideronarestrictedbasistoprovidemarketingsolelyonbehalfofPearsonoranaffiliateorcustomerforwhomPearsonisaserviceprovider.Marketingpreferencesmaybechangedatanytime. Correcting/UpdatingPersonalInformation Ifauser'spersonallyidentifiableinformationchanges(suchasyourpostaladdressoremailaddress),weprovideawaytocorrectorupdatethatuser'spersonaldataprovidedtous.ThiscanbedoneontheAccountpage.Ifausernolongerdesiresourserviceanddesirestodeletehisorheraccount,pleasecontactusatcustomer-service@informit.comandwewillprocessthedeletionofauser'saccount. Choice/Opt-out UserscanalwaysmakeaninformedchoiceastowhethertheyshouldproceedwithcertainservicesofferedbyInformIT.Ifyouchoosetoremoveyourselffromourmailinglist(s)simplyvisitthefollowingpageanduncheckanycommunicationyounolongerwanttoreceive:www.informit.com/u.aspx.SaleofPersonalInformation Pearsondoesnotrentorsellpersonalinformationinexchangeforanypaymentofmoney. WhilePearsondoesnotsellpersonalinformation,asdefinedinNevadalaw,NevadaresidentsmayemailarequestfornosaleoftheirpersonalinformationtoNevadaDesignatedRequest@pearson.com. SupplementalPrivacyStatementforCaliforniaResidents CaliforniaresidentsshouldreadourSupplementalprivacystatementforCaliforniaresidentsinconjunctionwiththisPrivacyNotice.TheSupplementalprivacystatementforCaliforniaresidentsexplainsPearson'scommitmenttocomplywithCalifornialawandappliestopersonalinformationofCaliforniaresidentscollectedinconnectionwiththissiteandtheServices. SharingandDisclosure Pearsonmaydisclosepersonalinformation,asfollows: Asrequiredbylaw. Withtheconsentoftheindividual(ortheirparent,iftheindividualisaminor) Inresponsetoasubpoena,courtorderorlegalprocess,totheextentpermittedorrequiredbylaw Toprotectthesecurityandsafetyofindividuals,data,assetsandsystems,consistentwithapplicablelaw Inconnectionthesale,jointventureorothertransferofsomeorallofitscompanyorassets,subjecttotheprovisionsofthisPrivacyNotice Toinvestigateoraddressactualorsuspectedfraudorotherillegalactivities Toexerciseitslegalrights,includingenforcementoftheTermsofUseforthissiteoranothercontract ToaffiliatedPearsoncompaniesandothercompaniesandorganizationswhoperformworkforPearsonandareobligatedtoprotecttheprivacyofpersonalinformationconsistentwiththisPrivacyNotice Toaschool,organization,companyorgovernmentagency,wherePearsoncollectsorprocessesthepersonalinformationinaschoolsettingoronbehalfofsuchorganization,companyorgovernmentagency. Links Thiswebsitecontainslinkstoothersites.Pleasebeawarethatwearenotresponsiblefortheprivacypracticesofsuchothersites.WeencourageouruserstobeawarewhentheyleaveoursiteandtoreadtheprivacystatementsofeachandeverywebsitethatcollectsPersonalInformation.Thisprivacystatementappliessolelytoinformationcollectedbythiswebsite. RequestsandContact PleasecontactusaboutthisPrivacyNoticeorifyouhaveanyrequestsorquestionsrelatingtotheprivacyofyourpersonalinformation. 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