7

has begun to exceed the motion picture industry in consumer revenues.

Today's interactive video

games are being made with the same effort as low-budget movies, incorporating complex scripts

and storylines, well-known actors, and mixing real and computer-generated imagery [Sni95].

2. 2. 2

Dynamics-based Animation

The techniques used to integrate physics into the generation of computer animations can be

divided into two basic approaches, trajectory-basedand controller-based.

Trajectory-based techniques such as [WK88], [BN88], [Coh92] and [LGC94] attempt to find a

physically realistic or near-realistic trajectory from one point in the state space of a creature to

another.

Since the systems are typically

highly

underconstrained,

the

trajectory

is

usually

optimized in some way, for example for smoothness, minimum control energy or minimum time.

A disadvantage of the technique is that a new trajectory must be generated for each new desired

instance of motion.

Also, interactions with the environment such as collisions and friction are

often difficult to properly incorporate into the dynamics specification.

One decided advantage of

trajectory-based techniques is that they relate well to keyframing. The animator can control details

of the end result through the specification of keyframes and other trajectory-based constraints.

Trajectory-based techniques are also able to find the most physically plausible solution, even if no

completely physical solution is possible.

In the controller-based approach, a dedicated controlleror control algorithmis used to activate the

simulated muscles of a creature, causing it to perform some motion or action within a simulated

environment.

The use of such controllers has a number of advantages over the trajectory-based

approach.

In many cases, controllers can be designed to be reusableand composable[van89]

[Hod91] [SC92]. Reusability implies that a controller can be used to achieve a given motion with

a variety of initial states.

Composability implies that a sequence of motions can be generated by