I guess I don’t understand your issue with multiprocessing. Are you talking about using a -n option with the first (feeder) rtrace process, the second one, or both? Either or both should work, though I don’t know how big the gains will be unless one of them is doing a lot of interreflection calculations. The pay-off for multiprocessing tends to diminish if it only takes a tiny amount of time for each primary ray. You spend most of the time on i/o and book-keeping at that point.