Computer animation is the process used for digitally generating animated images. The more general term computer-generated imagery (CGI) encompasses both static scenes and dynamic images, while computer animation only refers to moving images. Modern computer animation usually uses 3D computer graphics to generate a two-dimensional picture, although 2D computer graphics are still used for stylistic, low bandwidth, and faster real-time renderings. Sometimes, the target of the animation is the computer itself, but sometimes film as well.
Computer animation is essentially a digital successor to stop motion techniques, but using 3D models, and traditional animation techniques using frame-by-frame animation of 2D illustrations. Computer-generated animations are more controllable than other, more physically based processes, like constructing miniatures for effects shots, or hiring extras for crowd scenes, because it allows the creation of images that would not be feasible using any other technology. It can also allow a single graphic artist to produce such content without the use of actors, expensive set pieces, or props. To create the illusion of movement, an image is displayed on the computer monitor and repeatedly replaced by a new image that is similar to it but advanced slightly in time (usually at a rate of 24, 25, or 30 frames/second). This technique is identical to how the illusion of movement is achieved with television and motion pictures.
For 3D animations, objects (models) are built on the computer monitor (modeled) and 3D figures are rigged with a virtual skeleton. For 2D figure animations, separate objects (illustrations) and separate transparent layers are used with or without that virtual skeleton. Then the limbs, eyes, mouth, clothes, etc. of the figure are moved by the animator on key frames. The differences in appearance between key frames are automatically calculated by the computer in a process known as tweening or morphing. Finally, the animation is rendered.
For 3D animations, all frames must be rendered after the modeling is complete. For 2D vector animations, the rendering process is the key frame illustration process, while tweened frames are rendered as needed. For pre-recorded presentations, the rendered frames are transferred to a different format or medium, like digital video. The frames may also be rendered in real time as they are presented to the end-user audience. Low bandwidth animations transmitted via the internet (e.g. Adobe Flash, X3D) often use software on the end-user's computer to render in real time as an alternative to streaming or pre-loaded high bandwidth animations.
In 2D computer animation, moving objects are often referred to as "sprites." A sprite is an image that has a location associated with it. The location of the sprite is changed slightly, between each displayed frame, to make the sprite appear to move. The following pseudocode makes a sprite move from left to right:
var int x := 0, y := screenHeight / 2; while x < screenWidth drawBackground() drawSpriteAtXY (x, y) // draw on top of the background x := x + 5 // move to the right
Computer animation uses different techniques to produce animations. Most frequently, sophisticated mathematics is used to manipulate complex three-dimensional polygons, apply "textures", lighting and other effects to the polygons and finally rendering the complete image. A sophisticated graphical user interface may be used to create the animation and arrange its choreography. Another technique called constructive solid geometry defines objects by conducting boolean operations on regular shapes, and has the advantage that animations may be accurately produced at any resolution.
To animate means, figuratively, to "give life to". There are two basic methods that animators commonly use to accomplish this.
Computer-assisted animation is usually classed as two-dimensional (2D) animation. Drawings are either hand drawn (pencil to paper) or interactively drawn (on the computer) using different assisting appliances and are positioned into specific software packages. Within the software package, the creator places drawings into different key frames which fundamentally create an outline of the most important movements. The computer then fills in the "in-between frames", a process commonly known as Tweening. Computer-assisted animation employs new technologies to produce content faster than is possible with traditional animation, while still retaining the stylistic elements of traditionally drawn characters or objects.
Examples of films produced using computer-assisted animation are The Little Mermaid, The Rescuers Down Under, Beauty and the Beast, Aladdin, The Lion King, Pocahontas, The Hunchback of Notre Dame, Hercules, Mulan, The Road to El Dorado and Tarzan.
Computer-generated animation is known as three-dimensional (3D) animation. Creators design an object or character with an X, a Y and a Z axis. No pencil-to-paper drawings create the way computer-generated animation works. The object or character created will then be taken into a software. Key-framing and tweening are also carried out in computer-generated animation but so are many techniques unrelated to traditional animation. Animators can break physical laws by using mathematical algorithms to cheat mass, force and gravity rulings. Fundamentally, time scale and quality could be said to be a preferred way to produce animation as they are major aspects enhanced by using computer-generated animation. Another positive aspect of CGA is the fact one can create a flock of creatures to act independently when created as a group. An animal's fur can be programmed to wave in the wind and lie flat when it rains instead of separately programming each strand of hair.
A few examples of computer-generated animation movies are Toy Story, Antz, Ice Age, Happy Feet, Despicable Me, Frozen, and Shrek.