Approach for writing a GLSL fragment shader with a solid color per triangle/face

Question

I have vertex and triangle data which contains a color for each triangle (face), not for each vertex. i.e. A single vertex is shared by multiple faces, each face potentially a different color.

How should I approach this problem in GLSL to obtain a solid color assignment for each face being rendered? Calculating and assigning a "vertex color" buffer by averaging the colors of a vertex's neighboring polys is easy enough, but this of course produces a blurry result where the colors are interpolated in the fragment shader.

What I really need shouldn't be interpolated color values at all, I'll have about 40k triangles shaded with approx 15 possible solid colors once this is working as intended.

Answer 1

While you maybe could do this in high end GLSL, the right way to do solid shading is to make unique vertices for every triangle. This is a trivial loop. For every vertex, count how many triangles share it. That's how often you have to replicate it. Make sure your loop to do this is O(n). Then just set each vertex color or normal to that of the triangle. Again one straight loop. Do not bother to optimize for shared colors, it is not worth it.

Edit much later, because this is a popular answer:

To do flat per face shading you can interpolate the vertex position in world or view space. Then in the fragment shader compute ddx(dFdx) and ddy(dFdy) of this variable. Take the cross product of those two vectors and normalize it - you got a flat normal! No mesh changes or per vertex data needed at all.

Answer 2

OpenGL does not have "per-face" attributes. See:

How can I specify per-face colors when using indexed vertex arrays in OpenGL 3.x?

Here are a few possible options I see:

1. Ditch the index arrays and use separate vertices for each face like starmole suggested
2. Create an index array for each color used. Use materials instead of vertex colors and change the material after drawing the triangles from the index array for each color.
3. If the geometry allows it, you can make sure the last vertex specified by the index array has the correct vertex color for the face, and then use GL_FLAT shading, or have the fragment shader only use at the last vertex color.