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Endurion

OpenGL Shader management with lights

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Hi,

I'm rather new with shaders (I've just managed to get my first shader to compile and work in a game). I however have a neat abstracted library under my belt that for now works with the good old fixed function pipeline.

The library abstracts the renderers in DLLs and thus allows me to exchange DX8, DX9 or OpenGL renderer. The display states are set indirectly, I provide a method SetShader with enum for example render flat (texture modulates with vertex color), render with flat with alpha test, etc. This seems to fit quite nicely with real shaders.

However a nuisance comes with lights. For fixed function I have my 8 lights which I simply enable/disable. Due to abstracting I want to have it working without D3DX. I chose HLSL with precompiled shaders. But how should I approach lights?

As it looks I'd have to compile every shader in at least 8 variantes for every light. That feels cumbersome. Is that really the way to go?

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Very interesting question indeed, I'd also be interested in knowing more.
In the past, I had some success by setting up internal system-specific conventions ("the engine_lights array contains the lights to evaluate, it counts engine_active_lights entries). Unfortunately, I rapidly went mad at managing the different light used (directional, omni, spot, shadowcasting y/n, projective y/n).
I have then tried various approaches but so far I failed in finding a satisfying generic solution. My best bet is that one should start from final product's requirements and very likely deal with the problem at different levels.
Deferred shading sounds very interesting to this regard but I am still not sold on it.

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Quote:
Original post by Endurion
As it looks I'd have to compile every shader in at least 8 variantes for every light. That feels cumbersome. Is that really the way to go?
On the last game I worked on, the main shader file was compiled into ~200 different versions (depending on which options were required).
This is sometimes called the "shader permutation problem", and there's no simple answer.

Other approaches include dynamic branching in the shader, or simply always computing 8 lights in the shader (and setting black-coloured lights when you want to disable them).

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Why have you got a boner over D3DX to compile them?

Our engine uses an effect enum for the core material and then makes its own combos of lighting, bump-mapping, shadows etc from a set of flags. To do this the combos are stored in an array indexed by the flags and if that array is empty the handling code goes and constructs a shader (by using strcat to physically write one) for the job.

I have to say that we've tried a number of times to find a way to get us nice effects without needing users to write millions of shaders whilst also having the engine work on something more limited like the Wii/PSP/etc. And this is the first design that ticked all the boxes and felt just right.

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