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I'm trying to think of some reasons why there might be more than one UV map on a mesh. Usually I just unwrap the model, and the same UV map works for both the diffuse color texture, the bump map texture, and any other textures I chose to add. Are there any examples of when a model might need more than one UV map?

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One example is for light maps. Usually texture space is maximized by overlapping faces which have the same diffuse texture, like the six sides of a crate. With light mapping this would mean that all six sides get the same light and shadow, not to mention it would almost assuredly confuse the light mapper because it can't tell which point in space it should be sampling light from. Obviously this is not what you want so a second set of UV coordinates is used for the light map which do not overlap.

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Lightmaps are what I came here to point out. There are a few other more esoteric uses of multiple UVs that I've seen, but lightmaps are far and away the most common use. A distant second may be terrain splatting, while everything else would be the long tail. –  jhocking Apr 12 '13 at 19:20

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