Timing for Animation, 2nd Edition

Repeat Movements of Inanimate Objects

In straightforward to-and-fro movements, such as those of a piston or pendulum, it may or may not be possible to use the same drawings in the reverse order for the return movement. With a rigid pendulum (Fig. A) it would be possible, but with a cartoon piston (Fig. D) animated with steam pressure in mind, the drawings used for the forward movement would not work in reverse.

In any flexible movement, or that of any object which has something trailing, then a fresh set of inbetweens must be done for the two directions of movement (Figs B, C and E).

In a to-and-fro movement in which the same drawings can be used in reverse, an optical problem arises at the ends of the movements. Suppose the animation is done ...