Drawing lots of textures with OpenGL can be slow, especially if we do it in several passes, or by rebinding textures a lot. That is why we need to pack textures into texture pages, and use several texture units with multitexturing to draw several texture at once. This presents some problems, as Freeciv uses plenty of layers of information, and graphics card today do not have very many texture units. Basically, the number of passes we need to do (redrawing everything) is texture pages / texture units. So the fewer texture pages we need, the better. We can assume that everyone today have 4 texture units on their graphics cards. But Freeciv has many more layers, and we want to finish drawing the terrain with one pass. We do this by rendering many of the possible combinations beforehand and st