I think Phantast means changing it from a BUM map BUM to a gray-scale Bump JPG.  Changing the extension is easy.  Converting the specialized two-channel red/green normal map into a gray-scale map is not so easy - requires integration (Calculus).  It can be done, but there is no perfect conversion.

C makes it easy to shoot yourself in the foot. C++ makes it harder, but when you do, you blow your whole leg off.

 -- Bjarne Stroustrup

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