Modeling tension and relaxation for computer animation

The use of tension and relaxation in the muscles of real creatures gives rise to nuanced motion that conveys emotion or intent. Artists have long exploited knowledge of this in traditional animation and other areas, but it has been both overlooked and difficult to achieve in physically based animation. The robotically stiff motion that has come to typify physically based approaches belies the fact that dynamics has much to offer in facilitating far more subtle motion in which animators could freely "shape" a motion. We demonstrate that tension and relaxation can be introduced into joint-level, posture based animation. While we show that these modalities can be efficiently incorporated into traditional proportional-derivative control models, we instead formulate a more flexible and better-behaved model based on antagonistic control. This approach is more biomechanically sound, but more importantly it permits the separation of stiffness control from position control, achieving better posture interpolation, better error control, and passive and active dynamics. We introduce effective mechanisms to control the shape of a motion and describe an animation system that efficiently integrates relaxation and tension control in a physically based simulation environment.