Has anyone experienced problems when using dFdx/dFdy derivatives in glsl with 16x multisample FBOs in OpenGL? I am using latest drivers on GTX670.
In my case I create the color attachment of the FBO with:
glTexImage2DMultisample(GL_TEXTURE_2D_MULTISAMPLE, 16, GL_RGBA, width, height, GL_FALSE);
When rendering on such an FBO, it appears that dFdx/dFdy return 0, whereas if I reduce the samples parameter to 8 (everything else remaining the same), the derivatives start working (so I am quite sure that the problem is not due to the shader code itself, at least not in a trivial way).
In a related note, does anyone know exactly what type of AA different values of the samples parameter correspond to? I suspect that the actual type of AA (CSAA, MSAA, etc.) when samples=16 may have something to do with my problem.
Any info/pointers would be greatly appreciated.