Abstract
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 betterbehaved 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.