I'm working on a slightly older ATI Mobility Radeon 5650 graphics card, with Catalyst drivers version 22.214.171.124. I'm targeting OGL 3.3 (and glsl 330).
I have a test application that creates a Vertex Array Object with a simple set of 2 attribute arrays, one containing vertices of 3 floats each, and the other containing color values of 3 unsigned bytes each. I'm using glVertexAttribPointer and telling OGL to normalized my unsigned byte colors into floats for the shader.
Now into my array, I pack 6 lines representing the world axis, each using a different color. Then I pack a series of lines representing a 2D grid across the world XZ plane, all of one color.
I render in two steps using glDrawElements(). I bind my VAO, then the world axis is rendered at one line width. Next a second glDrawElements is called for the grid lines after changing line thickness again. I just offset my start offset and index counts to shift where I'm drawing from in the VAO for each call.
Now here is the issue:
On some (and only some) ATI cards, when I draw the grid lines, the last 6 lines seem to get random colors. Sometimes they are black/white, sometimes they are the colors of my world axis lines. It is significant that it is just the last 6 lines, as that is how far into the VAO the grid set (and its colors) are offset past the 6 lines of the world axis. The lines of the grid itself are correct, so I know this isnt as simple as just specifiying the wrong start offset/index count in glDrawElements.
Now, if I ignore trying to change line widths and just render the entire VAO in one pass, everything appears correctly.
Also, if I change to using GL_FLOAT for my source color component instead of GL_UNSIGNED_BYTE (and set normalize to GL_FALSE when calling glVertexAttribPointer), but change nothing else, it works fine using the two draw calls.
Lastly, the GL_UNSIGNED_BYTE based implementation works fine under all NVidia and Intel graphics adapters we tested. It even worked fine on a few desktop ATI cards.
For once, I'm starting to strongly suspect a driver bug instead of this being my fault... =P