I remember how slow the process was, it could take several hours/days to generate an image full of reflections, but in the end the results were usually stunning...
Link: http://www.povray.org/
Before it was renamed "povray", it was called "dkbtrace". I printed the entire dkbtrace source and studied it, to see how a real Ray Tracer worked; and through it, I learned how efficient vtbls work (it was 1990 C, and though C++ was already starting to become visible and popular, it was still "that new language that may or may not become popular" - dkbtrace implemented all the OO inside).
Thanks, D.K.B.