As reasonably-well documented, I like ray-tracing as a way of writing programs that generate pretty pictures. I wrote a simple ray-tracer aaaages ago, and only recently found the source. This got me tempted to write some extensions that I never got around to before.
So, I've finally implemented depth-of-field blurriness. It was quite fun. I got to do the standard checkerboard pattern, and some shiny spheres with colours from a colour wheel and stuff like that, so it's kept me quite amused. Ah, the winter break.