A cel is a transparent sheet of acetate less than one quarter of an inch thick, 10 inches long and 12 inches wide. Holes along the top of the cel correspond to pegs on the baseboard of a device called an animation stand, which holds the drawings while they are being filmed.
Animated films begin life as a story sketched out in a rough form similar to a comic strip called a storyboard. The storyboard depicts the major actions of each scene, dialog and sound-effects cue.
After the story is finalized, the dialogue, sound effects and music are recorded. A technician called a track reader takes careful measurement of each sound in the dialogue and creates an exposure sheet, which represents one frame of film. The exposure sheet records the timing for each word, helping the animators synchronize their character's mouth movements with the dialog.
The animators work out model sheets for each character. The model sheets help keep the characters' appearances uniform and give examples of facial expressions and body language. Meanwhile, the layout artists create line drawings that help the animators develop action and background artists create backgrounds for each scene.
The head animator sketches out a character's major actions on translucent paper that is lit from below. After one drawing is completed, a second piece of paper is placed over it and the movement is varied slightly, creating the illusion of movement. After the major actions are completed, an assistant animator fills in the details, making the action seem fluid and lifelike.
After the sketches are finished, they are checked for accuracy and then cleaned up and enhanced. The drawings are then videotaped and checked for correct facial expressions and smooth movement.
Using the pencil layouts as a guide, background artists create detailed background paintings that function like the sets in films made with actors. Originally painted by hand, backgrounds are now colored by computers.
The finished drawings are transferred to cels using a pen and ink, a technique similar to photocopying called xerography. The cels are then carefully painted with paint applied to the back of the cels. Once painstakingly done by hand, much of this work is now done on computer. For example, Disney's 2007 short film, "How to Hook Up Your Home Theater," combined hand-drawn elements with elements drawn directly on a computer with a device called a Wacom tablet.
The finished cels are carefully laid on top of the background painting and the scene is filmed one frame at a time by a special camera. After filming is complete, the exposed film is sent to a photo lab where negatives and prints are made. If the film has been created on computer, it is transferred directly to digital video tape. When the film is projected the characters seem to come to life.
The separate sound, music and dialog tracks recorded earlier are combined into one soundtrack. The soundtrack and the film are then combined. If the animation is for TV, it is then recorded onto video.