Geometric hashing.

Students in last semester's cs 306 may find this

  http://nedbatchelder.com/blog/200902/fast_geometric_hashing_for_automated_astrometry.html

interesting: it's an example of using geometric hashing (a simpler version of
which was the subject of the first 306 programming assignment) to find
constellations in space pictures.  The algorithm's been hooked into flickr.