LibPuzzle was developed by Jedi / Franck Denis, mr PureFTPd (or mr SkyBlog according to some people that worked with me at Qualys).

Excerpt from the README :

The Puzzle library is designed to quickly find visually similar images (gif,png, jpg), even if they have been resized, recompressed, recolored or slightly modified. ... The Puzzle library is a implementation of "An image signature for any kind of image", by H. CHI WONG, Marschall BERN and David GOLDBERG.

For the moment, as I don't understand all the subtleties of the vector fixed-length splitting (I have discovered libpuzzle 2 days ago...), ftwin is mainly computing vectors for every files it checks, then, compares all the vector with each others.

That's pretty overkill, but for a desktop usage, it is sufficient.