Very few living organisms are capable of moves that have a mechanical in and out or up and down precision. The action of a woodpecker might be an exception. and. because of the restrictions of an external skeleton, there are undoubtedly some examples in the insect world, but the movements of most living creatures will follow a slightly circular path. The head seldom thrusts straight out. then back again; it lifts slightly, or drops as it returns. Perhaps this has to do with weight or maybe with the inner structure of the higher forms of life. but. whatever the reason, most movements will describe an arc of some kind.
The action of a hand gesture with a pointing finger follows a circular path. The animator charts the position of his drawings along this arc. He makes his key drawings, indicating where inhetweens should be placed to keep the line of action on this arc. Inhetweens done without following this arc change the action radically.
This discovery made a major change in the type of movements animators designed for their characters, breaking w ith the rigid and stiff actions that had gone before. In a walk, the characters had popped up and down like mechanical gadgets on an engine; now they "arced" over at the top of their steps and "arced" under at the bottom position. A hit or a throw could be on a completely straight line, but the beginning of the action came sweeping in on an arc and the Follow Through started a corkscrew action.
As this principle was better understood, scenes were plotted out with charts and dots, as well as rough poses, to determine just how high and how low the
character should go in any action. Arcs were sketched in. as the key actions were planned, to guide the eventual drawings along this curved path. When the final drawings were being made, more ways would become apparent for the character to go even farther in the action, especially using Squash and Stretch and Overlapping Action to good advantage.
One of the major problems for the inbetweeners is that it is much more difficult to make a drawing on an arc than one halfway between two other drawings. Even when the position has been indicated, or a stern warning written on the extremes. "Watch arcs!" there is a strong inclination to pull back toward a more normal inbetween. It is only as a series of drawings is
"rolled" on the pegs that the proper location for the drawing becomes evident. No one has ever found a way of insuring that the drawings will all be placed accurately on the arcs, even when experienced people are inbetweening the action, and it is one of the most basic requirements for the scene. Drawings made as straight inbetweens completely kill the essence of the action.
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