Computer animation combines the arts of computer graphics and hand-drawn, two-dimensional (2D) animation to create computer-generated characters, settings and surroundings. The two common uses of computer animation in films are fully computer-animated projects (such as the Pixar films) and the use of computer-generated imagery in live-action films (such as the cyborgs in the Terminator films.)
How Animation Is Used
Animated subjects appear to move because images appear on screen and are quickly replaced with a series of similar, slightly altered images in a sequence that suggests walking, waving, jousting or any action the viewer’s eyes perceive to be occurring. Computer animation is favored precisely because it speeds up the process of creating the many images needed for such a sequence.
How Computer Animation Is Used
Computer animation can originate from 2D drawings or be drawn in computer programs. A character will be scanned in to a computer animation program or a virtual skeleton of the character represents them. Once the virtual skeleton is in the program, the animator moves key features, such as limbs and mouth, to key frames, the next major movement of the character. The programs “tween” the differences between these key images and know what movements to fill in to produce the desired movements between key frame A and B. The program then renders the images, enabling it to present a fluid final form. The same process goes for any moving objects or backgrounds around the characters.
How Computer Animation is Used in Live Action
Computer animation in otherwise live-action films is commonly known as computer-generated imagery (CGI), or CG in the motion picture industry. CG is used as a way to facilitate something that would be costly and time-consuming to produce physically. This method can save the producers on large set pieces, scenes and backdrops. It is also used to create special effects that might otherwise not be possible or that would detract from the visual “feel” of the film.
CGI effects in live action are created the same way as in completely computer-animated projects. The live-action segments are filmed near a blue or green screen backdrop that is then removed in the editing process and replaced with the created CGI in a process commonly known as chroma keying. Sometimes live-action actors and crews are working entirely with a CG backdrop, such as in "Sin City" and the Star Wars prequel trilogy.
CGI is also used to blend live-action characters and backdrops with animated characters and vice versa. "Who Framed Roger Rabbit?" "Lord of the Rings" and "Cool World" are notable examples. Another form of computer animation in films is projects that use motion-capture technology to film a live actor’s performance and digitize it into a computer-animation program. The performance is rendered into an often photorealistic animated character against an animated backdrop. Examples include the films of Robert Zemeckis and James Cameron`s "Avatar."