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:
Change clip control to 0/1
Use a floating point depth buffer.
Clear the depth buffer to 0
glBindFramebuffer(GL_FRAMEBUFFER, fbo); glClearDepth(0.0f); glClear(GL_DEPTH_BUFFER_BIT);
Change depth test to grater
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.