Before anyone jumps in with "Why do all that coding just to do what I can do with my hand and chest?"...this is a cool hack that shows practical code for breaking down a video file and measuring changes. The same concept could be used to, for example, count number of unique faces that pass through a room/hallway. Or, for a C-SPAN clip, gauge when who speaks when (depending on whose face is center-frame during a debate). And other less pedantic ideas.