Rotoscoping

Rotoscoping is an animation technique used by animators to trace over motion picture footage, frame by frame, when realistic action is required. Originally, photographed live-action movie images were projected onto a glass panel and re-drawn by an animator. This projection equipment is referred to as a Rotoscope. Although this device was eventually replaced by computers, the process is still referred to as Rotoscoping. 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.

Rotoscoping (often abbreviated as "roto") has often been used as a tool for visual effects in live-action movies. By tracing an object, the moviemaker creates a silhouette (called a matte) that can be used to extract that object from a scene for use on a different background. While blue and green screen techniques have made the process of layering subjects in scenes easier, rotoscoping still plays a large role in the production of visual effects imagery. Rotoscoping in the digital domain is often aided by motion tracking and onion-skinning software. Rotoscoping is often used in the preparation of garbage mattes for other matte-pulling processes.

Rotoscoping has also been used to allow a special visual effect (such as a glow, for example) to be guided by the matte or rotoscoped line. One classic use of traditional rotoscoping was in the original three Star Wars movies, where it was used to create the glowing lightsaber effect, by creating a matte based on sticks held by the actors. To achieve this, effects technicians traced a line over each frame with the prop, then enlarged each line and added the glow.

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