After using Picasa’s face detection tool from the time it was unveiled, I have always wondered why there is no simple api for it to be used across urls/local storage?
The way it extracts features and does the clustering even if picture is rotated/fuzzy is just awesome. There are very few false positives. How they extract feature and normalize across rotation and fuzziness is something I would love to understand. All of this without ever training with “best picture(s)” comes across very refreshing. If this api which I am sure all search engines have in certain degree, is made public, it will help solve lot of challenges existing today. I have tried using few xrays, mri scans without much success though.
If you want to try/understand face detection/clustering and related stuff
One of the many research behind the technology in OpenCV-
At some point Photosynth too has to do this to accomplish sticthing across images and creating walkthroughs, point cloud A little different from face detection and matching, but nonetheless similar thing.
Update: 16 July 2010:
Paper – http://portal.acm.org/citation.cfm?id=1290121 (Eigenfaces representation)