I was having trouble with cone tracing and decided to first try a basic GI algorithm based on a pattern of raycasts. Here is the result:

You can see this is pretty noisy, even with 25 raycasts per voxel. Cone tracing uses an average sample, which eliminates the noise problem, but it does introduce more inaccuracy into the lighting.

Next I wanted to try a more complex scene and get an estimate of performance. You may recognize the voxelized scene below as the "Sponza" scene frequently used in radiosity testing:

Direct lighting takes 368 milliseconds to calculate, with voxel size of 0.25 meters. If I cut the voxel grid down to a 64x64x64 grid then lighting takes just 75 milliseconds.

These speeds are good enough for soft GI that gradually adjusts as lighting changes, but I am not sure if this will be sufficient for our purposes. I'd like to do real-time screen-independent reflections.

I thought about it, and I thought about it some more, and then when I was done with that I kept thinking about it. Here's the design I came up with:

The final output is a 3D texture containing light data for all six possible directions.  (So a 256x256x256 grid of voxels would actually be 1536x256x256 RGB, equal to 288 megabytes.) The lit voxel array would also be six times as big. When a pixel is rendered, three texture lookups are performed on the 3D texture and multiplied by the normal of the pixel. If the voxel is empty, there is no GI information for that volume, so maybe a higher mipmap level could be used (if mipmaps are generated in the last step). The important thing is we only store the full-resolution voxel grid once.

The downsampled voxel grid use an alpha channel for coverage. For example, a pixel with 0.75 alpha would have six out of eight solid child voxels.

I do think voxelization is best performed on the CPU due to flexibility and the ability to cache static objects.

Direct lighting, in this case, would be calculated from shadowmaps. So I have to implement the clustered forward renderer before going forward with this.