What algorithm could be used to identify if images are the “same” or similar, regardless of size?

Question

TinEye, the \"reverse image search engine\", allows you to upload/link to an image and it is able to search through the billion images it has crawled and it will return link

Answer 1

The Hough Transform is a very old feature extraction algorithm, that you mind find interesting. I doubt it's what tinyeye uses, but it's a good, simple starting place for learning about feature extraction.

There are also slides to a neat talk from some University of Toronto folks about their work at astrometry.net. They developed an algorithm for matching telescoping images of the night sky to locations in star catalogs in order to identify the features in the image. It's a more specific problem than what tinyeye tries to solve, but I'd expect that a lot of the basic ideas that they talk about are applicable to the more general problem.