Yeah, Piet's a curveball! It's a pretty cool (esoteric, totally useless) language however.
Piet's an esoteric, and GLSL is OpenGL's Shader Language. GLSL looks kind of C-like, while Piet looks like Mondrian paintings (for many programs, anyhow).
Yeah, I got that problem because I knew who Piet Mondrian was, so I took an educated guess.
If I hadn't read an article on esolangs last week, I would have missed Piet.