Esoteric programming language coded with images

Piet is an esoteric programming language where the programs are encoded as images and resemble abstract paintings. Spot-on Mondrians (pictures) are the hook, but a wide range of pixelated styles are possible; the logic of the program can be exposed in the image.

Prime Number Generator

Sylvain Tintillier provides a method of generating prime numbers using Piet. Figuring out how it works is easy, he says, "Just look at the bitmap!"