Question

I have a large set of vertices and currently use glColorPointer to specify their color. The problem is, that glColorPointer only accepts a size of 3 or 4 as its first parameter but the value of R, G and B for each vertex is identical.

Of course, I could use glVertexAttribPointer to specify each color value as an attribute of size one and duplicate it in a shader, but I’m looking for a way to do this in the fixed function pipeline.
Calling glColor1* is unfortunately out of the question given the amount of vertices (Yes, I tried it).

Any creative solution that squeezes the value into something else is also OK.

Answer 1

I think without shaders this won’t be possible since, well, glColorPointer only accepts a size of 3 or 4, like you already found out (there should also be no glColor1, only glColor3 and glColor4).

You might trick glColorPointer to use your tightly packed array by specifying a size of 3 but a stride of 1 (ubyte) or 4 (float). But this will give you a color of (Ri, Ri+1, Ri+2) for vertex i and there is no way to adjust it to (Ri, Ri, Ri), since the color matrix is not applied to per-vertex colors but only to pixel images.

So without shaders you don’t have much creativity left. What you could do is use a 1D texture of size 256, which contains all the grey colors from (0, 0, 0) to (255, 255, 255) in order. Then you can just use your per-vertex color as 1D texture coordinate into that texture. But I’m not sure if that would really buy you anything in either space or time.

The easy and straightforward way is to use vertex attributes and a shader to implement the unpacking of the attribute into the fragment color. This takes just a few lines of shader code and should be the preferred solution. The real difficulty here is the artificial restriction, not the problem space itself. Such cases should be handled by lifting the restriction.