5
$\begingroup$

I'm using hardware tessellation to render a terrain mesh with variable LOD.

I have this snippet on my tessellation control shader:

int edge1 = gl_InvocationID;
int edge2 = (gl_InvocationID + 1) % 3;
tcs_out[gl_InvocationID].color = vec4(computeLODColor(gl_TessLevelOuter[edge1], gl_TessLevelOuter[edge2]), 1.0);

This should compute a color for the control point based on the tessellation level of the two edges that intersect on the point (using quad patches).

It's all working fine except for this: gl_TessLevelOuter[edge1] and gl_TessLevelOuter[edge1].

This indexing always return 0 (I've tried using it directly as a color). To test the problem, I tried this: gl_TessLevelOuter[gl_InvocationID]. It also has the same behavior.

If I do something like this:

if(gl_InvocationID == 0) {
    tcs_out[gl_InvocationID].color = vec4(computeLODColor(gl_TessLevelOuter[0], gl_TessLevelOuter[1]), 1.0);
}
else if(gl_InvocationID == 1) {
    tcs_out[gl_InvocationID].color = vec4(computeLODColor(gl_TessLevelOuter[1], gl_TessLevelOuter[2]), 1.0);
}
else if(gl_InvocationID == 2) {
    tcs_out[gl_InvocationID].color = vec4(computeLODColor(gl_TessLevelOuter[2], gl_TessLevelOuter[3]), 1.0);
}
else if(gl_InvocationID == 3) {
    tcs_out[gl_InvocationID].color = vec4(computeLODColor(gl_TessLevelOuter[3], gl_TessLevelOuter[0]), 1.0);
}

then it all works as expected.

I've checked the OpenGL specification and couldn't find anything about undefined behavior when indexing gl_TessLevelOuter by a non-constant value.

So the question is, am I misunderstanding something, doing something against the specification or is it a GL implementation bug?

I'm using an AMD Radeon HD 6700 with updated drivers, and there are no warnings compiling the shaders. (I've never seen a single shader compiler warning though, so not sure if that's another problem)

$\endgroup$
2
  • $\begingroup$ dynamic indexing can cause issues in some drivers $\endgroup$ Nov 17, 2015 at 10:21
  • $\begingroup$ Well, I guess that's the case, can't find a better explanation. $\endgroup$
    – MadEqua
    Nov 23, 2015 at 15:18

0

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Browse other questions tagged or ask your own question.