So I'd like to be able to take a video file and create a still picture from all of its frames superimposed upon each other, like a time-lapse photo.  Does anyone know of a good program to do this in?  Preferably a scripting program since I'd also like to be able to tweak the formula: only include frames during which the volume is above X and weight them accordingly, etc.  I've been reading about Avisynth, but I'm not sure if it'll do that type of thing.  Any advice?

Thanks.