Often a curve editor is used to interactively edit ease functions and, in combination with spline functions, to allow for an infinite number of possible eases. There are accepted algorithms defining the mathematical specifications of all the myriad eases, but actually seeing the curve dip down to its rest position is sometimes as useful as seeing the animation in preview. The ability to interactively adjust the curve that determines the rate of animation or transition between positions in a key frame is crucial. Rather than linearly interpolating from one key frame to the next, passing a curve through the keys gives a more dynamic look to the animation. Again, a function editor that gives an interactive graphic representation is ideal for defining motion curves.
Most systems have some automatic interpolation functions available to the animator.
One problem with cubic interpolating splines is that although they keep slope continuity from key frame to key frame, they also tend to overshoot when confronted with sudden changes in velocity. Since animators intend key frames to represent extremes in motion, these overshoots can have disastrous results. Not overshooting cubic interpolating splines are necessary in a production animation environment to allow smooth motion without jumping in and out of a curve editor. Secondary motion Secondary motion is the motion of objects or body parts which depend on primary motion. An example of secondary motion would be the motion of clothing over the surface of a moving figure.