I am looking for a way to store the functions in an array, either by using function pointers, or by storing the actual equations (e.g. ...
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glslfunctionpointer(orequivalent)
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Iamattemptingtocalloneofmanyfunctionsbasedonthevalueofavariable.Thevariableissetduringruntime,socodeontheCPUwillnotwork.Usinganif/switchstatementwillbeslowduetothelargenumberofpossibilities.[probablyincorrect]
Iamlookingforawaytostorethefunctionsinanarray,eitherbyusingfunctionpointers,orbystoringtheactualequations(e.g.texcoord.x*2)inanarray.
Examplepsuedo-code:
invec2texcoord;
outvec4color;
intnumber;//Assignedanumberbetween0and300duringruntime
floatfunc1(void){
returntexcoord.x+texcoord.y;
}
floatfunc2(void){
returntexcoord.x*2;
}
...
floatfunc299(void){
returntexcoord.y-7;
}
voidmain(void){
number=;
floatoutput=func();//wereaCPU-generatedvalue,andthatseemsunlikelyinyourcase.
Theonlygeneralsolutionyouhaveisaswitchstatement.Andquitefrankly,there'snothingwrongwiththat.
Imean,you'realreadygoingtoabsolutelymurderyourperformancebydoingthiskindofconditionanyway.Nomatterhowitgetsimplemented,ifdifferentinstancesinthesamewavefrontareexecutingseparatecode,you'rekindascrewedperformance-wise.
Nottomention,aswitchstatementdoesnotnecessarilygetslowerbasedonhowmanyconditionsthereare.Howitgetsimplementeddependsentirelyonthehardware.Ifjumptablesareavailable,itcanbedonereasonablyefficiently(ignoringtheaboveperformanceissue,ofcourse).
Also,thereisColonelThirtyTwo'sidea,whereyouencodeeachfunction'soperationasadot-productwithaconstantvector.Obviously,thislimitswhatyourvariousfunctionscanactuallyexecute.Butifitworksinyourcase,itworks.
1Ifyouwanttocontestthis,considerthatSPIR-VprovidesananalogtoeveryfeatureofGLSL,nomatterhowredundant,silly,orunnecessary...exceptshadersubroutines.
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answeredDec26,2015at0:30
NicolBolasNicolBolas
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@jjxtra:Itwouldjustgetclosedasaduplicateofthis.
– NicolBolas
May9,2018at17:06
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TherearenofunctionpointersinGLSL,butitispossibletodefineastructurewithalistoffunctionnames:
structfunctions_list{
intsin;
intcos;
inttan;
intfract;
};
constfunctions_listfunctions=functions_list(1,2,3,4);
Usingthislistoffunctionnames,itispossibletosimulateacallback:
floatcallback(intfunc,floatarg){
if(func==functions.sin)
returnsin(arg);
elseif(func==functions.cos)
returncos(arg);
elseif(func==functions.tan)
returntan(arg);
elseif(func==functions.fract)
returnfract(arg);
else
return0.0;
}
These"functionpointers"canbeusedtosimulatehigher-orderfunctionsinGLSL.
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editedJan25,2021at18:46
NicolBolas
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answeredJul2,2019at19:18
AndersonGreenAndersonGreen
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