Ummm, this should be elsewhere... I'll send it to Isometric and Tiles, perhaps they can explain the theory behind parallax.

I think this should perhaps be in the Game Programming forum or somewhere similar. It might get more attention there. Thx, Kazgoroth [grin]

Anyway, if you mean how to actually make textures for parallax mapping in a paint package, it's relatively easy. You need to make three textures: a decal, a height map, and a normal map. The decal is simple: just take a well-lit, front-on photo of the surface you want, and process it as you would any other texture (resize, rotate, etc.). The height map is more difficult. You can create a greyscale height map manually, but in a lot of cases, you can use the decal and some paint package operations like "Find Edges" to make it much more easily. The normal map is made from the height map. You can write some code to do this, but there are plugins available which will do it for you (and give you a range of options, too). NVIDIA has a great normal map plugin for Photoshop (it works with Paint Shop Pro too, but only up to version 7, I wish they'd write a dedicated one). You can get that here. The GIMP also has a normal map plugin.

You don't actually need a normal map for parallax mapping, just a greyscale heightmap. Although everyone lights them with proper per-pixel lighting so you need a normal map for that, obviously.

Depending on the engine the format for the additional heightmap may vary. It might be specified as a separate texture, but it could also be packed into the alpha of the colour or normal map.