Combining active and passive simulations for secondary motion

Varied, realistic motion in a complex environment can bring an animated scene to life. While much of the required motion comes from the characters, an important contribution also comes from the passive motion of other objects in the scene. We use the term secondary motion to refer to passive motions that are generated in response to environmental forces or the movements of characters and other objects. For example, the movement of clothing and hair adds visual complexity to an animated scene of a jogging gure. In this paper, we describe how secondary motion can be generated by coupling physically based simulations of passive systems to active simulations of the main characters. We discuss three coupling methods for the interface between the passive and active systems: two-way, one-way, and hybrid. These three methods allow the animator to make an appropriate tradeo between accuracy and computational speed. We use a basketball passing through a net as an illustrative example to demonstrate each of the three coupling methods. To provide guidance as to when each method is most appropriate, we present additional examples including a gymnast on a trampoline, a man on a bungee cord, a stunt kite, a gymnast landing on a exible mat, a diver entering the water, and several human gures wearing clothing. The information gained from analyzing these examples is summarized in a decision tree and a set of guidelines for coupling active and passive systems.