I have this crazy idea.
What if you made a real-time 3D render that for each frame, progressed some number of light bouncing. In effect this is a ray tracer but it does the ray tracing in steps. First step is direct lightning, second step is 1 degree of indirect lightning and third is 2 degrees of indirect lightning.
For each step, you would accumulate the information in a new frame buffer. Obviously there are limitations to this because you are only considering what's on screen but is the idea sound? Can it be done? What does it look like?
I'm eager to try it out because it appears simple but I'm wondering if something like this doesn't already exist.
Basically a ray tracer implemented with real-time 3D rasterization that converges over time (with sub steps, more than 1 pass per frame) this convergence can be adjusted.