Rotoscoping

Roto is the manual version of a green screen. An artist draws a mask around the subject, then keeps adjusting that mask as the subject moves, so the software can separate them from the background for the whole shot. It gets used for compositing, for putting a graphic behind a presenter, for isolating something to grade or blur it, and for anything shot on location where keying was never an option. The name comes from the rotoscope, a device that projected film one frame at a time so animators could trace over it. Modern tools handle much of the tracking, and it is still the slowest job in post. Hair, motion blur and semi-transparent edges are where the hours disappear.