This is a strange flood-fill algorithm I stumbled upon a while back and I have no idea why it does its thing like it does.
What it does is sort of pretty though. You can hit SPACE to start with different variations of the theme -- that mostly but change the order in which pixels are added to the as-of-yet-unseen list. Hitting S will reset the step variable, for interesting results; D decrements it, I increments, H halves and M doubles (multiplies by two). C toggles coloring the pixels by their step value.
The source is unobfuscated, so please take a look, and if you know what's going on, let me know. Twitter @akx, email (the same handle) at iki dot fi.
If something seems broken, try Chrome. Ps. mouseover this box to hide it.