I suggest indexing all images in the database by a "perceptual hash" . See here : http://www.hackerfactor.com/blog/?/arch ... ke-It.html it's extremely simple and fast. There is a library here to do it : https://www.phash.org/
I use it for a professional project and it's incredible : with a 32 bits key, you can instantaneously (well O(log N) ...) find duplicates in hundreds of thousands of images. And very quickly find similar images.
Since XnView already builds a database, I think this feature would make it the fastest duplicates and similar image finder on Earth.
