@David

Only tracing every fifth pixel is a good optimisation -- but you can vastly improve the picture quality if you follow by another step: If the four pixels in a square all hit the same object, interpolate the texture and intensity, but if they don't, follow all the "missing" rays for the intervening pixels. I did that with a raytracer I made in '86 (except I followed every fourth pixel instead of every fifth), and I found that (with relatively simple scenes), I got immense speed-ups and only a little degradation in picture quality. In an animated sequence, I doubt you would notice the difference.