Procedural textures are images created using noise patterns and other visual algorithms.
However these patterns can look mechanical and repetitive. Several layers of painted and automatic patterns are usually combined to reduce repetition.
Procedural textures for games are often made with node-based texture-editing tools. For example: Substance Designer, Filter Forge, DDO, World Machine.
- Procedural Content Generation for Real-Time 3D Applications Part 1: Oil Rush explains how the Unigine team used Filter Forge for texture synthesis.