Why use Uber Shaders? Why not one giant shader with some ifs.

ill · 2012-04-30T07:31:23

I'm not sure how it works in the world of shaders but in CUDA, when you have branches, but all threads take the same branch, as I understand there is no slow down. The slowdown occurs from divergence as some of the processors have to sit idle if they need to take one branch and let the others take the other branch. So say I was doing a renderer in CUDA. I could say, enable light 0 and make it a point light. Disable light 1. Disable light 2. Then I'd have the entire program loaded and just run something like this: for(int light = 0; light < maxLights; light++) { if(gl_light[light].enabled) { switch(gl_light[light].type) { case LIGHT_SPOT: //do spot light stuff break; case LIGHT_POINT: //do point light stuff break; case LIGHT_DIRECTIONAL: //do diractional light stuff break; } } } There are branches, but all processors take the same path in the branches and don't cause divergence. In the world of Shaders I've seen people recommending something like uber shaders to handle different permutations of rendering states to avoid branches since they are supposed to be slower. But is it really slower in shaders when all of them take the same path in the code? You end up having to compile many different shaders and changing which one is loaded based on what you are rendering, which causes some slowdown. Is there a reason to not just have these much bigger shaders with some if statements?

Graphics and GPU Programming Programming

Started by ill April 26, 2012 07:41 AM

20 comments, last by Hodgman 11 years, 11 months ago

MJP

20,295

April 29, 2012 07:24 PM

When Cg and HLSL were first created they were pretty much identical. Since D3D10 however they have diverged quite a bit.

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Hodgman

52,717

April 30, 2012 07:31 AM

So Cg is similar to HLSL?

HLSL and Cg are almost exactly the same language -- both Microsoft and nVidia cooperated to create a "high level shading language" together. When they were done, MS published their version under the name "DirectX HLSL" and nVidia published theirs under the name "nVidia Cg", but they were both almost the same language (with different supporting tools/APIs/extensions).

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Why use Uber Shaders? Why not one giant shader with some ifs.

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Why use Uber Shaders? Why not one giant shader with some ifs.

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