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 eras-
ing 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 key-
frames are later filled in with transitional frames in a pro-
cess 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 anima-
tion 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 morph-
ing, 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, ani-
mals, 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 posi-
tions 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 wear-
able 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. Computation-
ally 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 consider-
ation: many desktop PCs sold today have 256MB or more of
video memory.
Applications
Computer animation is used extensively in many fea-
ture films, such as for creating realistic dinosaurs (Juras-
sic Park) or buglike aliens (Starship Troopers). Computer
games combine animation techniques with other tech-
niques to provide smooth
action within a vivid 3D landscape. Simpler forms of ani-
mation 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 com-
puter animation suggests that the ability to fascinate the
human eye that allowed Walt Disney to build an empire is
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