Normalmap
From Blood Wiki
Normal maps are images that store per-pixel surfaces normals directly in the RGB values of an image. They can dramatically increase object detalisation without adding more polygons. Therefore, normalmaps have a bunch fo limits and quirks.
Recommended articles on normalmaps:
In Darkplaces Engine, normalmaps are OpenGL-oriented (X stores horizontal, Y vertical) and can additionally store alpha-component which represents height that will be use for parallex mapping.