Is there a way to find out if a light is enabled in GLSL that doesn't involve passing attributes or creating a ton of different shaders?
What about using NVidia's C for Graphics (Cg)? Can I do it with Cg?
I am now convinced that you can't do it. But now I ask: why not?
If GLSL is anything like HLSL (DirectX's equivalent), which I suspect it is, I think you are stuck with passing attributes.
:-(
unforgiven3 is right - GLSL doesn't provide access to the fixed pipe enable states. You have to pass it in as a uniform.
You could set the light color to (0,0,0,0) and either branch on that, or just let it loop over the now non-contributing lights.
According to my understanding of "Enable or Not to Enable" part of GLSL Common Mistakes this is a philosophical reason: shaders are written to override fixed pipeline, therefore they shouldn't rely on fixed pipeline states. You gotta use a different shader for each capability you need.
Also I guess this might be a design choice based on the fact that branches are expensive on the GPU and someone must have thought that you shouldn't be doing things like
if (gl_LightSource[0].enabled)
{ ... }
in the first place.