Perceptual hashing could have been the technique used. For example, see: https://github.com/JohannesBuchner/imagehash https://github.com/JohannesBuchner/imageha

Perceptual hashing could have been the technique used. For example, see: https://github.com/JohannesBuchner/imagehash https://github.com/JohannesBuchner/imagehash I use this for deduplicating my, er, 'home movies' collection. EDIT: Here's a good explanation of one specific technique that should give you the general idea: http://www.hackerfactor.com/blog/?/archives/432-Looks-Like-It.html http://www.hackerfactor.com/blog/?/archives/432-Looks-Like-I...