iOS image comparison algorithm to find the differences in two very similar but different photos

My use case is that I have two images (photos) that are almost identical but do have some differences throughout. They won't be perfectly 开发者_C百科aligned, but they will be pretty darn close to each other. As a rough estimate, I'd expect to find a half dozen or less differences scattered randomly throughout a pair of 640x480 photos. The size of each difference would be roughly 20x20 px but there may be some odd shaped ones in there like 200x30 or so.

The algorithm will probably need to do some kind of blurring or fuzzy comparison or use "shaders" and "filters". Unfortunately, this is nowhere near my area of expertise. I've seen some libraries in my google searches, but nothing that looks like it would run on an iPhone.

Ideally, I'm looking for a working library, tutorial, or example code that can run on iOS.

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The basic comparison may be the eigenvector comparison. But there is nice tutorial for java devs, here. May be helpful.