register combiner problem

[gabnicu]

I need to "add" 4 textures (on a GeForce 3), choosing the maximum of them. Textures are RGBA,
Adding the 4 textures is simple, using register combiners or the old glTexEnv, but choosing the maximum among the 4 textures, for each pixel is not simple. I've read the documents for texture combiners, and couldn't find an appropriate function.
Any help would be most appreciated.

[Tom Nuydens]

The only conditional operation you have is "mux", and the condition is always "if (spare0.a <= 0.5)".

You can only access B and A individually, not R and G, so you have no way to put R and G in spare0.a, and hence no way to perform a mux on them. Assuming you want to do this for R, G, B and possibly A separately, I think there's no way it can be done.

It _should_ be possible if you only cared about luminance and not about RGB, though. For two textures T1 and T2, you could try to put 0.5 + T1 - T2 in spare0.a, then use mux to select either T1 or T2.

-- Tom