Computing the difference between images

Question

Do you guys know of any algorithms that can be used to compute difference between images?

Take this webpage for example http://tineye.com/ You give it a link or uplo

Answer 1

Correlation techniques will make a match jump out. If they're JPEGs you could compare the dominant coefficients for each 8x8 block and get a decent match. This isn't exactly correlation but it's based on a cosine transfore, so it's a first cousin.