Non uniform mip maps are called rip-maps.You will be hard pressed to find any GPU that supports them in any way....

You don't need a square image to mip map it, but the results will be stored square and have areas with non used information in them.

Mip mapping doesn't increase total memory consumption when using rgb, which many people do not realize. Throw in alpha, and memory consumption goes up.

Mip mapping can process less data and yet consume more memory, if that makes any sense.

See this. http://en.wikipedia.org/wiki/Mipmap

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