Rotoscoping
Tracing a mask around a subject frame by frame so you can cut it out of footage that never had a green screen.
See it
What it is
Rotoscoping is drawing a mask around a subject and updating that mask on every frame (or on keyframes the software interpolates between) so you end up with a moving cutout. The name comes from Max Fleischer's 1915 rotoscope, a projector rig for tracing live-action footage into cartoons, which is how Koko the Clown moved so smoothly. The same idea later drove the 'Take On Me' video and 'A Scanner Darkly'.
You reach for it when there was no green screen: putting a title behind a walking person, grading only the subject, removing a stray boom mic, or compositing someone into a different plate. Modern tools do most of the grunt work: After Effects Roto Brush, DaVinci Resolve's Magic Mask, and Mocha's planar tracking all propagate a shape forward, then you clean up the frames they get wrong.
Gotcha: edges are where roto dies. Hair, motion blur, and semi-transparent things (glass, smoke, a chiffon sleeve) have no crisp boundary to trace, so a hard mask looks cut out with scissors. Feather the edge, match the shot's motion blur, and watch playback at speed, not frame by frame: a matte that looks perfect on stills can boil and chatter in motion.
Ask AI for it
Rotoscope the main subject out of this footage: create an animated mask that follows the person's outline across every frame, keeping the edge soft (2 to 3 px feather) and matching the shot's existing motion blur so the cutout doesn't look pasted on. Output the subject on a transparent alpha channel, and keep the matte stable frame to frame with no edge chatter or boiling.