For the curious, this explains a lot of the science in a hands-on, approachable way: http://stackoverflow.com/questions/10168686/algorithm-improv... EDIT: As this is a deep topic, there are also books if that's more your style: http://www.amazon.com/dp/0123725380/?tag=stackoverfl08-20 or maybe you like Wikipedia (Ol' Trusty): http://en.wikipedia.org/wiki/Feature_detection_(computer_vis...
There is a whole chapter on shape contexts in it, which I use with shapecatcher, too.