In hand-drawn 2D animation, a cel is a separate piece of artwork drawn on a transparent sheet used to create a scene. Cellulose acetate, or acetate, is a plastic that is used in the process of making a cel; since it was flammable, unstable, and dimensionally unstable, it was later replaced by cellulose acetate.
With the advent of computer-assisted animation production, the use of cels has been all but abandoned in major productions. Disney studios stopped using cels in 1990 when Computer Animation Production System (CAPS) replaced this element in their animation process, and in the next decade and a half, the other major animation studios phased cels out as well.
Generally, the characters are drawn on cels and laid over a static background drawing. This reduces the number of times an image has to be redrawn and enables studios to split up the production process to different specialized teams. Using this assembly line way to animate has made it possible to produce films much more cost-effectively. The invention of the technique is generally attributed to Earl Hurd, who patented the process in 1914.
The outline of the images is drawn on the front of the cel while colors are painted on the back to eliminate brushstrokes. Traditionally, the outlines were hand-inked but since the 1960s they are almost exclusively xerographed on. Another important breakthrough in cel animation was the development of the Animation Photo Transfer Process, first seen in The Black Cauldron, released in 1985.
Typically, an animated feature would require over 100,000 hand-painted cels.