In describing an object’s motion, it is often useful to relate it to another object. Consider, for example, a coordinate system centered at our sun in which the moon’s motion must be defined. It is much easier to describe the motion of the moon relative to the earth and the earth’s motion relative to the sun than it is to come up with a description of the moon’s motion directly in a sun-centric coordinate system. Such sequences of relative motion are found not only in astronomy but also in robotics, amusement park rides, internal combustion engines, human figure animation, and pogo sticks.
This chapter is concerned with animating objects whose motion is relative to another object, especially when there is a sequence ...