O Combining Active and Passive Simulations for Secondary Motion

bjects that move in response to the actions of a main character often make an important contribution to the visual richness of an animated scene. We use the term secondary motion to refer to passive motions generated in response to the movements of characters and other objects or environmental forces. Secondary motion may be created by background elements or by objects interacting with an active character. The flags shown in Figure 1 are examples of secondary motion generated by environmental forces, while the trampoline and skirt in Figure 2 are objects that exhibit secondary motion in response to the actions of active characters. Secondary motions aren't normally the main focus of an animated scene, yet their absence can distract or disturb the viewer, destroying the illusion of reality created by the scene. For example, if the skirt in Figure 2 were rigid, the scene would be less believable; with painted-on, skintight clothing, the scene would be less interesting. While the viewer may not always be explicitly aware of secondary motions, they're an important part of many animated scenes. Much of the research in computer animation has focused on the difficult problem of animating the primary characters. Because objects that exhibit secondary motions tend to be complex, deformable objects with many degrees of freedom, the techniques that have been developed for character animation are usually not appropriate for animating secondary motion. In particular , methods based on motion capture or key-framing are often impractical for secondary motion. As a result, researchers have developed specialized procedural methods for many of these objects. While procedural models may be derived in a number of ways, physically based simulation has proven to be both a highly effective and an elegant solution, particularly for passive systems with many degrees of freedom. One advantage of simulation is that the motion is generated automatically from the initial specification of the environment and the applicable physical laws. For some applications, such as character animation, this automation results in an undesirable loss of direct control over the details of the motion. However, for secondary motion this lack of control is usually not a significant problem because these motions are passive, dictated only by forces from the environment or the actions of the primary characters. Even in situations where aesthetic considerations call for an exaggerated or otherwise unrealistic motion, often the movement of the actor is exaggerated while the passive secondary …