Painting a Mountain Landscape with Mathematics in a Shader

HN commenters point to it as the best demonstration of procedural graphics from a master, accessible to beginners yet deep enough for advanced viewers.

Inigo Quilez live-paints a mountain landscape using only math in a shader, building up from a basic polynomial noise surface through fractal terrain, lighting and atmosphere. The code runs in real time on Shadertoy so viewers can edit it as they follow along.