Define Keyframe systems.
Key-frame systems are specialized animation languages designed to generate the in-between frames from user-specified keyframes. Each object in the scene is described as a set of rigid bodies connected at the joints and with a limited number of degree of freedom. In-between frames are generated from the specification of two or more fey frames. Motion paths can be given by kinematic description as a set of spline curves or physically based by specifying the force acting on the object to be animated.
