Rotoscoping is an animation technique in which animators trace over footage, frame by frame, for use in live-action and animated films. Originally, recorded live-action film images were projected onto a frosted glass panel and re-drawn by an animator. This projection equipment is called a rotoscope, although this device was eventually replaced by computers.
In the visual effects industry, the term rotoscoping refers to the technique of manually creating a matte for an element on a live-action plate so it may be composited over another background.
The technique was invented by cartoonist/illustrator/writer/inventor Max Fleischer, who used it in his technologically groundbreaking, Out of the Inkwell (debut: 1915) animated series. The live-film reference for the series' main cartoon/animated character, Koko the Clown, was supplied by one of his brothers (Dave Fleischer) -- performing choreographed movements while dressed in a clown outfit. Max Fleischer patented the rotoscope method in 1917.