Question

This fragment shader code compiles in Windows, but throws an error in OS X (Mountain Lion, using a 3.2 core context).

#version 150 core
in int vinstance_id;
uniform uint object_id[16];
out uint id_map;

void main() {
    id_map = object_id[vinstance_id];
}

The error is:

ERROR: 0:2: int can't be an in in the fragment shader

I don’t think that’s right: In the GLSL 1.50 spec, section 4.3.4 says about fragment shaders:

Fragment inputs can only be signed and unsigned integers and integer vectors, float …

Is this a driver bug?

Answer 1

No, you didn’t read the spec thoroughly.

Fragment shader inputs that are signed or unsigned integers or integer vectors must be qualified
with the interpolation qualifier flat.

(Roughly in the middle of page 32 of linked document, if anyone wants to verify)