I discuss the idea of timing in Chapter 8, “Timing,” and lay out the fundamentals behind creating strong arcs in the previous section. It’s now time to consider both aspects of motion and begin to unravel how the two co-exist in the same scene. To do this, I look to the in-between.

Referring back to the Frisbee throw from the previous section, I remember the computer’s inability to create ideal arcs. The next step was setting a few extra keys so the hand would have a natural feel as it propelled the disc forward. Although the hand now traveled on a more favorable path, it lacked the necessary force to give it a sense of weight.

In-betweens, if properly placed, can create both a circular path and a believable amount of force. Characters need a certain amount of time to begin and end an action. This particular scene doesn’t call for an excessive amount of time to move the arm forward, but will most certainly require a “weighting” of the keys at the beginning of the motion.

The in-betweens need to be placed so that the accelerating arm will ease out of its initial starting position before thrusting forward and releasing the Frisbee. The current keys are set at frame 1 and frame 32. I’m going to place an in-between key at frame 16. However, instead of placing the arm directly between the first and last positions, I’m going to place it toward frame 1.

The key will result in the arm taking half of the 32 frames to move 1/3 the total distance. This will create an easing out of the move and will give the perception of the body actually projecting an object that has some sense of mass and a deliberate force behind it. How much force will depend on the character, of course. A different character of the same size, throwing the same object will have a new set of in-betweens, arcs, and timings. This example is not intended to be a timing lesson; rather, it is intended to show how the in-between keys will have an effect on both the timing and arcs in a motion. Although this one additional key has an impression on the action, a few more are required.

The shot is now approaching a somewhat desirable result. However, the computer still has too much influence on the throw. The arcs aren’t quite right, and the timing is a bit flat. The next in-between I’ll set will occur at frame 6. I’m placing this key closer to frame 1 to continue with the concept of easing out, and to help watch the arc that’s required. I’ll also need to place another key between frames 16 and 32 for the same timing and arc control.

This trend would continue until the desired animation was achieved. The coupling of both arcs and timing is a constant factor when animating a scene. Both must constantly be monitored in order to achieve the best results. By approaching a more complicated action from start to finish, I can get a complete picture of the importance these fundamentals have. The next section takes a look at such an example.

Exercise: The Arm Swing
One thing you can always count on in animation is the character walk cycle. One of the most difficult parts of a walk is the arm swing. It’s a complex motion that relies on a complete understanding of the principles already discussed in order to be successful. Proper arcs, an understanding of timing, and appropriate movement along multiple axes are all equally important.

Although there are many variations on how an arm can swing, I’m going to explain a technique that I often rely on. It’s referred to as the “figure-eight” due to the resulting arc generated from the hand traveling along a path of this shape. (See Figure 7.) This is a generic approach, but can be modified to fit most any walk.

[Figure 7] The path of an arm swing. The wrist travels in a squashed figure-eight pattern.

The example is being presented as a cycle in that the arm starts and stops at the same position. When played, the arm will appear to make a smooth transition from front to back for as long as the animator desires. This allows for proper dissection of the motion and lets the reader see both directions of the swing completely. It begins by setting a few simple keys.

The first poses that will get key frames are the two extremes. These two keys will define the farthest distance forward and backward that the arm will reach. Keeping the arms extended in these positions helps sell the “breaking” of the elbow as the arm moves in the opposite direction. In addition, these keys provide a framework for the subsequent in-between poses. (See Figure 8.)

The example is being presented as a cycle in that the arm starts and stops at the same position. When played, the arm will appear to make a smooth transition from front to back for as long as the animator desires. This allows for proper dissection of the motion and lets the reader see both directions of the swing completely. It begins by setting a few simple keys.

The first poses that will get key frames are the two extremes. These two keys will define the farthest distance forward and backward that the arm will reach. Keeping the arms extended in these positions helps sell the “breaking” of the elbow as the arm moves in the opposite direction. In addition, these keys provide a framework for the subsequent in-between poses. (See Figure 8.)