[fatboy]

One place I've seen it used was a lib by a guy called DHoerl for reading images that are too big to fit in memory (this was years ago on iOS).

A very over-simplified and probably a bit incorrect description of what it did was to create a smaller version of the image - one that could fit in memory - by sub sampling every nth pixel, which was addressed via mmap.

It actually dealt with jpegs so I have no idea how that bit worked as they are not bitmaps.