This replaces the compute shader pass for volume material properties
voxelization by a fragment shader that is run only once per pixel.
The fragment shader then execute the nodetree in a loop for each
individual froxel.
The motivations are:
- faster evaluation of homogenous materials: can evaluate nodetree
once and fast write the properties for all froxel in a loop.
This matches cycles homogenous material optimization (except that
it only considers the first hit).
- no invocations for empty froxels: not restricted to box dispach.
- support for more than one material: invocations are per pixel.
- cleaner implementation (no compute shader specific paths).
Implementation wise, this is done by adding a stencil texture when
rendering volumetric objects. It is populated during the occupancy
phase but it is not directly used (the stencil test is enabled but
since we use `imageAtomic` to set the occupancy bits, the fragment
shader is forced to be run). The early depth-test is then turned
on for the material properties pass, allowing only one fragment to
be invoked.
This fragment runs the nodetree at the desired frequency: once per
direction (homogenous), or once per froxel (heterogenous).
Note that I tried to use the frontmost fragment using a depth equal
test but it was failing for some reason on Apple silicon producing
flickering artifacts. We might reconsider this frontmost fragment
approach later since the result is now face order dependant when
an object has multiple materials.
Pull Request: https://projects.blender.org/blender/blender/pulls/119439
4a7e98be40