That's a whole field of study on its own, called perceptual hashing. I surveyed these a while for amusement and the TL;DR is that all immediately obvious approa

That's a whole field of study on its own, called perceptual hashing. I surveyed these a while for amusement and the TL;DR is that all immediately obvious approaches tend to have particularly bad corner cases. https://en.wikipedia.org/wiki/Perceptual_hashing https://en.wikipedia.org/wiki/Perceptual_hashing