We are currently using perceptual hashes (e.g. phash.org) to do hundreds of thousands of image comparisons per
day.
As mentioned in another comment, you really have to test different hashing algorithms to find one that suits your needs best. In general though, I think it is in most cases not necessary to develop an algorithm from scratch :)
As mentioned in another comment, you really have to test different hashing algorithms to find one that suits your needs best. In general though, I think it is in most cases not necessary to develop an algorithm from scratch :)
For us, the much more challenging part was/is to develop a system that can find similar images quickly. If you are interested in things like that have a look at VP/MVP data structures (e.g. http://pnylab.com/pny/papers/vptree/vptree/, http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.43.7...).