Draft. May be completely wrong. Please edit and improve!
The Biomess are pseudoranomly generated via a combination of "Wang tiles" and "Pixel scenes".
Wang Tiles
These are slotted together, as can mostly easily be understood by looking at Horscht's awesome web-based wang tiler
Examples of the wang tiles from the Coal Pits (full, and zoomed section) can be seen below.
Pixel scenes
Pixel scenes are added to the rendered biome.
Material colors
The colors in the wang tiles represent materials, as assigned by data/materials.xml and documented in the image at right.