This project is a cellular automata that simulates falling sand. The falling is deterministic: each sand particle will fall to the same place each time the program is run. Some iterations eventually stop, some continue indefinitely.

The fun is that the project is implemented on a fragment shader. There is no array of grains of sand. Each pixel decides whether a grain of sand from a neighboring pixel will end up in it or not. This logic required a lot of banging my head against the wall, because the slightest mistake caused sand to disappear into nowhere or appear out of nothing and clutter up the entire space.

0…9 — set speed
space — pause
s — save picture
drag over — add more sand
shift+drag — erase