[ALUG] Webcam as security monitor

Just found an old webcam at the back of a cupboard, plugged it in to my Ubuntu box, installed Camorama, and woot! it works. Which set me to wondering. Camorama can be to record a picture from the webcam every (say) 20 minutes. What I was wondering was is there is any software out there that records the picture at a set interval, but only if the webcam image changes (you'd presumably need some sort of change threshold, to cope with minor changes). Any thoughts, anyone? Greg