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 poten

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.

Answer 3

In addition to the other answers, you could maybe employ the gl_PrimitiveID variable, that's an input to the fragment shader (don't know since which version) and is incremented implicitly for each triangle. You could then use this to lookup the color (either from a 40k buffer texture of colors or color indices into a 15 color color map, or just some direct computation from the primitive id). But don't ask me about the performance of this approach.