GL_DEPTH_CLAMP has no effect (reverse z)

Question

In an effort to improve cascaded shadow maps, I have looked into using GL_DEPTH_CLAMP and moving the near and far plane just around the actual view frustum instead of the global bounding box. But the use of GL_DEPTH_CLAMP appears to have no effect and the near plane clips the geometry.

I use reverse z as per Reversed-Z in OpenGL:

The tl/dr version is:

1. Change clip control to 0/1

   glClipControl(GL_LOWER_LEFT, GL_ZERO_TO_ONE);
   

2. Use a floating point depth buffer.

3. Clear the depth buffer to 0

   glBindFramebuffer(GL_FRAMEBUFFER, fbo);
   
   glClearDepth(0.0f);
   glClear(GL_DEPTH_BUFFER_BIT);
   

4. Change depth test to grater

   glDepthFunc(GL_GREATER);
   

5. Change the projection matrix to match

The reverse z works really fine, but apparently it messes with GL_DEPTH_CLAMP. If I switch off reverse z GL_DEPTH_CLAMP appears to work as designed; but many other bits in the code get messed up. (It's just not switchable anymore.)

If you look at the documentation of GL_DEPTH_CLAMP it states the following:

If enabled, the -wc ≤ zc ≤ wc plane equation is ignored by view volume clipping (effectively, there is no near or far plane clipping).

My guess is that GL_DEPTH_CLAMP's implementation is just not compatible with reverse z. But I feel like I am missing something. Any idea on how to get GL_DEPTH_CLAMP to working with reverse z?

See the following image of the depth buffers:

As you can see in the the first cascade, the near plane clips the geometry, it should be all white.

Answer 1

It is compatible, but instead of clipping it now clamps (hence the name). If the depth of your primitives is outside the [0,1] range it ends up behaving as if you have no depth-test at all. The only way to mitigate that is to either scale down the depth range so it fits in [0,1] or to use GL_NV_depth_buffer_float to disable clipping and clamping completely.