You could possibly use a perceptual hash[0] to compare images. While building an image processing application I used the pHashion library to compare thumbnails

You could possibly use a perceptual hash[0] to compare images. While building an image processing application I used the pHashion library to compare thumbnails to their originals to make sure it still looked "the same" to humans. It worked very well and might be useful in the case of comparing "nude" photos to a reference. [0] http://phash.org/ http://phash.org/ [1] https://github.com/westonplatter/phashion https://github.com/westonplatter/phashion