Writing to depth buffer from opengl compute shader

Question

Generally on modern desktop OpenGL hardware what is the best way to fill a depth buffer from a compute shader and then use that depth buffer for graphics pipeline rendering with triangles etc?

Specifically I am wondering about concerns regards HiZ. Also I wonder if it's better to do compute shader modifications to the depth buffer before or after the graphics rendering?

If the compute shader is run after the graphics rendering I assume the depth buffer will typically be decompressed behind the scenes. But I worry done the other way around the depth buffer may be in a decompressed/non-optimal state for the graphics pipeline?

Answer 1

As far as i know, you cannot bind textures with any of the depth formats as images, and thus cannot write to depth format textures in compute shaders. See glBindImageTexture documentation, it lists the formats that your texture format must be compatible to. Depth formats are not among them and the specification says the depth formats are not compatible to the normal formats.

Texture copying functions have the same compatibility restrictions, so you can't even e.g. write to a normal texture in the compute shader and then copy to a depth texture. glCopyImageSubData does not explicitly have that restriction but i haven't tried it and it's not part of the core profile anymore.

What might work is writing to a normal texture, then rendering a fullscreen triangle and setting gl_FragDepth to values read from the texture, but that's an additional fullscreen pass.

I don't quite understand your second question - if your compute shader stuff modifies the depth buffer, the result will most likely be different depending on whether you do it before or after regular rendering because different parts will be visible or occluded.

But maybe that question is moot since it seems you cannot manually write into depth buffers at all - which might also answer your third question - by not writing into depth buffers you cannot mess with the compression of it :)

Please note that i'm no expert in this, i had a similar problem and looked at the docs/spec myself, so this all might be wrong :) Please let me know if you manage to write to depth buffers with compute shaders!