animation, computer

Ever since the first hand-drawn cartoon features entertained moviegoers in the 1930s, animation has been an important part of the popular culture. Traditional animation uses a series of hand-drawn frames that, when shown in rapid succession, create the illusion of lifelike movement.

comPuter animation techniques

The simplest form of computer animation (illustrated in games such as Pong) involves drawing an object, then erasing it and redrawing it in a different location. A somewhat more sophisticated approach can create motion in a scene by displaying a series of pre-drawn images called sprites—for example, there could be a series of sprites showing a sword-wielding troll in different positions.

   Since there are only a few intermediate images, the use of sprites doesn’t convey truly lifelike motion. modern animation uses a modern version of the traditional drawn animation technique. The drawings are “keyframes” that capture significant movements by the characters. The keyframes are later filled in with transitional frames in a process called tweening. Since it is possible to create algorithms that describe the optimal in-between frames, the advent of sufficiently powerful computers has made computer animation both possible and desirable. Today computer animation is used not only for cartoons but also for video games and movies. The most striking use of this technique is morphing, where the creation of plausible intermediate images between two strikingly different faces creates the illusion of one face being transformed into the other.

   Algorithms that can realistically animate people, animals, and other complex objects require the ability to create a model that includes the parts of the object that can move separately (such as a person’s arms and legs). Because the movement of one part of the model often affects the positions of other parts, a treelike structure is often used to describe these relationships. (For example, an elbow moves an arm, the arm in turn moves the hand, which in turn moves the fingers). Alternatively, live actors performing a repertoire of actions or poses can be digitized using wearable sensors and then combined to portray situations, such as in a video game.

   Less complex objects (such as clouds or rainfall) can be treated in a simpler way, as a collection of “particles” that move together following basic laws of motion and gravity. Of course when different models come into contact (for example, a person walking in the rain), the interaction between the two must also be taken into consideration.

   While realism is always desirable, there is inevitably a tradeoff between the resources available. Computationally intensive physics models might portray a very realistic spray of water using a high-end graphics workstation, but simplified models have to be used for a program that runs on a game console or desktop PC. The key variables are the frame rate (higher is smoother) and the display resolution. The amount of available video memory is also a consideration: many desktop PCs sold today have 256mB or more of video memory.

aPPLications

Computer animation is used extensively in many feature films, such as for creating realistic dinosaurs (Jurassic Park) or buglike aliens (Starship Troopers). Computer games combine animation techniques with other techniques (see computeR gRaphics) to provide smooth action within a vivid 3D landscape. Simpler forms of animation are now a staple of Web site design, often written in Java or with the aid of animation scripting programs such as Adobe Flash.

   The intensive effort that goes into contemporary computer animation suggests that the ability to fascinate the human eye that allowed Walt Disney to build an empire is just as compelling today.