I have more than one light bounce working now, and it looks a lot nicer than single-bounce GI. The ambient light here is pure black. All light is coming off from the direct light, and bouncing off surfaces.
It will take some time to get the details worked out, and more bounces will require more memory. I'm actually kind of shocked how good looking it is. This is just a single 128x128x128 volume texture at 0.25 meters per voxel. Light leaks seem to be not a problem, even at that low resolution, even with thin walls.
You can see that absolutely no direct light is hitting the red wall on the left, and the ambient light is pure black. Yet light is still bouncing off the floor onto the wall, and then lighting the dragon very faintly with red light. (The exposure here is higher than it should be so it looks a little washed out):
There's a lot here for me to take in. Real-time global illumination is such a rabbit hole that keeps unfolding new layers of complexity and possibilities. It seems like every few days my mind is totally blown and I look at the system differently, and the new way makes so much sense. In the last three days I have said to myself "I'll just try one more thing..." and then that thing opens up a lot of new possibilities. April is nearly over, and I have been working on just this feature since December!
I've got a bunch of stuff to work out, but it looks like this will run really really well in VR.