The cel is an important innovation to traditional animation, as it allows some parts of each frame to be repeated from frame to frame, thus saving labor. A simple example would be a scene with two characters on screen, one of which is talking and the other standing silently. Since the latter character is not moving, it can be displayed in this scene using only one drawing, on one cel, while multiple drawings on multiple cels are used to animate the speaking character.
For a more complex example, consider a sequence in which a boy sets a plate upon a table. The table stays still for the entire sequence, so it can be drawn as part of the background. The plate can be drawn along with the character as the character places it on the table. However, after the plate is on the table, the plate no longer moves, although the boy continues to move as he draws his arm away from the plate. In this example, after the boy puts the plate down, the plate can then be drawn on a separate cel from the boy. Further frames feature new cels of the boy, but the plate does not have to be redrawn as it is not moving; the same cel of the plate can be used in each remaining frame that it is still upon the table. The cel paints were actually manufactured in shaded versions of each color to compensate for the extra layer of cel added between the image and the camera; in this example the still plate would be painted slightly brighter to compensate for being moved one layer down. In TV and other low-budget productions, cels were often "cycled" (i.e. a sequence of cels was repeated several times), and even archived and reused in other episodes. After the film was completed, the cels were either thrown out or, especially in the early days of animation, washed clean and reused for the next film. Also, after the completion of the film, some of the cels were put into the "archive" to be used again and again for future purposes in order to save money. Some studios saved a portion of the cels and either sold them in studio stores or presented them as gifts to visitors.
In very early cartoons made before the use of the cel, such as Gertie the Dinosaur (1914), the entire frame, including the background and all characters and items, were drawn on a single sheet of paper, then photographed. Everything had to be redrawn for each frame containing movements. This led to a "jittery" appearance; imagine seeing a sequence of drawings of a mountain, each one slightly different from the one preceding it. The pre-cel animation was later improved by using techniques like the slash and tear system invented by Raoul Barre; the background and the animated objects were drawn on separate papers. A frame was made by removing all the blank parts of the papers where the objects were drawn before being placed on top of the backgrounds and finally photographed. The cel animation process was invented by Earl Hurd and John Bray in 1915. In lower-budget productions, shortcuts available through the cel technique are used extensively. For example, in a scene in which a man is sitting in a chair and talking, the chair and the body of the man may be the same in every frame; only his head is redrawn, or perhaps even his head stays the same while only his mouth moves. This is known as limited animation. The process was popularized in theatrical cartoons by United Productions of America and used in most television animation, especially that of Hanna-Barbera. The end result does not look very lifelike, but is inexpensive to produce, and therefore allows cartoons to be made on small television budgets.
Moving characters are often shot "on twos", that is to say, one drawing is shown for every two frames of film (which usually runs at 24 frames per second), meaning there are only 12 drawings per second. Even though the image update rate is low, the fluidity is satisfactory for most subjects. However, when a character is required to perform a quick movement, it is usually necessary to revert to animating "on ones", as "twos" are too slow to convey the motion adequately. A blend of the two techniques keeps the eye fooled without unnecessary production cost. Academy Award-nominated animator Bill Plympton is noted for his style of animation that uses very few inbetweens and sequences that are done on threes or on fours, holding each drawing on the screen from an eighth to a sixth of a second. While Plympton uses near-constant three-frame holds, sometimes animation that simply averages eight drawings per second is also termed "on threes" and is usually done to meet budget constraints, along with other cost-cutting measures like holding the same drawing of a character for a prolonged time or panning over a still image, techniques often used in low-budget TV productions. It is also common in anime, where fluidity is sacrificed in lieu of a shift towards complexity in the designs and shading (in contrast with the more functional and optimized designs in the western tradition); even high-budget theatrical features such as Studio Ghibli's employ the full range: from smooth animation "on ones" in selected shots (usually quick action accents) to common animation "on threes" for regular dialogue and slow-paced shots.
Creating animation loops or animation cycles is a labor-saving technique for animating repetitive motions, such as a character walking or a breeze blowing through the trees. In the case of walking, the character is animated taking a step with his right foot, then a step with his left foot. The loop is created so that, when the sequence repeats, the motion is seamless. However, since an animation loop essentially uses the same bit of animation over and over again, it is easily detected and can in fact become distracting to an audience. In general, they are used only sparingly by productions with moderate or high budgets.
The multiplane camera is a tool used to add depth to scenes in 2D animated movies, called the multiplane effect or the parallax process. The art is placed on different layers of glass plates, and as the camera moves vertically towards or away from the artwork levels, the camera's viewpoint appears to move through the various layers of artwork in 3D space. The panorama views in Pinocchio are examples of the effects a multiplane camera can achieve. Different versions of the camera have been made through time, but the most famous is the one developed by the Walt Disney studio beginning with their 1937 short The Old Mill. Another one, the "Tabletop", was developed by Fleischer Studios. The Tabletop, first used in 1934's Poor Cinderella, used miniature sets made of paper cutouts placed in front of the camera on a rotating platform, with the cels between them. By rotating the entire setup one frame at a time in accordance with the cel animation, realistic panoramas could be created.
Applied to animation by Ub Iwerks at the Walt Disney studio during the late 1950s, the electrostatic copying technique called xerography allowed the drawings to be copied directly onto the cels, eliminating much of the "inking" portion of the ink-and-paint process. This saved time and money, and it also made it possible to put in more details and to control the size of the xeroxed objects and characters (this replaced the little known, and seldom used, photographic lines technique at Disney, used to reduce the size of animation when needed). At first it resulted in a more sketchy look, but the technique was improved upon over time.
Disney animator and engineer Bill Justice had patented a forerunner of the Xerox process in 1944, where drawings made with a special pencil would be transferred to a cel by pressure, and then fixing it. It is not known if the process was ever used in animation. The xerographic method was first tested by Disney in a few scenes of Sleeping Beauty, and was first fully used in the short film Goliath II, while the first feature entirely using this process was One Hundred and One Dalmatians (1961). The graphic style of this film was strongly influenced by the process. Some hand inking was still used together with xerography in this and subsequent films when distinct colored lines were needed. Later, colored toners became available, and several distinct line colors could be used, even simultaneously. For instance, in The Rescuers the characters' outlines are gray. White and blue toners were used for special effects, such as snow and water. Invented by Dave Spencer for the 1985 Disney film The Black Cauldron, the APT (Animation Photo Transfer) process was a technique for transferring the animators' art onto cels. Basically, the process was a modification of a repro-photographic process; the artists' work were photographed on high-contrast "litho" film, and the image on the resulting negative was then transferred to a cel covered with a layer of light sensitive dye. The cel was exposed through the negative. Chemicals were then used to remove the unexposed portion. Small and delicate details were still inked by hand if needed. Spencer received an Academy Award for Technical Achievement for developing this process. A cel overlay is a cel with inanimate objects used to give the impression of a foreground when laid on top of a ready frame. This creates the illusion of depth, but not as much as a multiplane camera would. A special version of cel overlay is called line overlay, made to complete the background instead of making the foreground, and was invented to deal with the sketchy appearance of xeroxed drawings. The background was first painted as shapes and figures in flat colors, containing rather few details. Next, a cel with detailed black lines was laid directly over it, each line drawn to add more information to the underlying shape or figure and give the background the complexity it needed. In this way, the visual style of the background will match that of the xeroxed character cels. As the xerographic process evolved, line overlay was left behind.
The methods mentioned above describe the techniques of an animation process that originally depended on cels in its final stages, but painted cels are rare today as the computer moves into the animation studio, and the outline drawings are usually scanned into the computer and filled with digital paint instead of being transferred to cels and then colored by hand. The drawings are composited in a computer program on many transparent "layers" much the same way as they are with cels, and made into a sequence of images which may then be transferred onto film or converted to a digital video format.
It is now also possible for animators to draw directly into a computer using a graphics tablet, Cintiq or a similar device, where the outline drawings are done in a similar manner as they would be on paper. The Goofy short How To Hook Up Your Home Theater (2007) represented Disney's first project based on the paperless technology available today. Some of the advantages are the possibility and potential of controlling the size of the drawings while working on them, drawing directly on a multiplane background and eliminating the need of photographing line tests and scanning.
Though traditional animation is now commonly done with computers, it is important to differentiate computer-assisted traditional animation from 3D computer animation, such as Toy Story and Ice Age. However, often traditional animation and 3D computer animation will be used together, as in Don Bluth's Titan A.E. and Disney's Tarzan and Treasure Planet. Most anime still use traditional animation today. DreamWorks executive Jeffrey Katzenberg coined the term "tradigital animation" to describe films produced by his studio which incorporated elements of traditional and computer animation equally, such as Spirit: Stallion of the Cimarron
Many video games such as Viewtiful Joe, The Legend of Zelda: The Wind Waker and others use "cel-shading" animation filters or lighting systems to make their full 3D animation appear as though it were drawn in a traditional cel style. This technique was also used in the animated movie Appleseed, and cel-shaded 3D animation is typically integrated with cel animation in Disney films and in many television shows, such as the Fox animated series Futurama.
Rotoscoping is a method of traditional animation invented by Max Fleischer in 1915, in which animation is "traced" over actual film footage of actors and scenery. Traditionally, the live action will be printed out frame by frame and registered. Another piece of paper is then placed over the live action printouts and the action is traced frame by frame using a lightbox. The end result still looks hand drawn but the motion will be remarkably lifelike. The films Waking Life and American Pop are full-length rotoscoped films. Rotoscoped animation also appears in the music videos for A-ha's song "Take On Me" and Kanye West's "Heartless". In most cases, rotoscoping is mainly used to aid the animation of realistically rendered human beings, as in Snow White and the Seven Dwarfs, Peter Pan, and Sleeping Beauty.
A method related to conventional rotoscoping was later invented for the animation of solid inanimate objects, such as cars, boats, or doors. A small live action model of the required object was built and painted white, while the edges of the model were painted with thin black lines. The object was then filmed as required for the animated scene by moving the model, the camera, or a combination of both, in real time or using stop-motion animation. The film frames were then printed on paper, showing a model made up of the painted black lines. After the artists had added details to the object not present in the live-action photography of the model, it was xeroxed onto cels. A notable example is Cruella de Vil's car in Disney's One Hundred and One Dalmatians. The process of transferring 3D objects to cels was greatly improved in the 1980s when computer graphics advanced enough to allow the creation of 3D computer generated objects that could be manipulated in any way the animators wanted, and then printed as outlines on paper before being copied onto cels using Xerography or the APT process. This technique was used in Disney films such as Oliver and Company (1988) and The Little Mermaid (1989). This process has more or less been superseded by the use of cel-shading.
Related to rotoscoping are the methods of vectorizing live-action footage, in order to achieve a very graphical look, like in Richard Linklater's film A Scanner Darkly.
Besides traditional animated characters, objects and backgrounds, many other techniques are used to create special elements such as smoke, lightning and "magic", and to give the animation in general a distinct visual appearance. Today special effects are mostly done with computers, but earlier they had to be done by hand. To produce these effects, the animators used different techniques, such as drybrush, airbrush, charcoal, grease pencil, backlit animation, diffusing screens, filters, or gels. For instance, the Nutcracker Suite segment in Fantasia has a fairy sequence where stippled cels are used, creating a soft pastel look.