A Grunge Map is another name for a dirt map, used to add dirt, grime, rust, dust and other imperfections to a virtual object to make it seem more realistic whilst still keeping the actual polygon count down.
Unlike a normal dirt map which is typically a grey scale map (typically a light map) repurposed as a semi-random repeating dirt pattern overlaid on a base texture, a grunge map is usually more complex.
Physical world examples of rust or grime, rot, mould, or whatever nastiness is desired, are sought out, and photographed to give the basis for a grunge map. This is then turned to grey scale, any irrelevant parts edited out, and heavily manipulated to enhance the contrast.
The result is a texture that is applied in the same way as any other dirt map, but rather than being semi-random, actually contains realistic – often photorealistic – data of the type of dirt or grime being sought after.
Sometimes multiple grunge maps are composted together. For example a grunge map of rust to be applied to metallic surfaces, will then have a second grunge map of oily filth overlaid onto it, to give a final result of oil-smeared rust, that is then applied as a second texturemap layer to the metallic surface, the same as any other dirt map would be.
In most cases, this approach still creates the most realistic rot and grime on a surface.
See Also: Dirt Map, Light Map, Ambient Occlusion Map, Multiple Textures, Photo Realism, Baking, Texturemap
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