Animation Alley - Arcs

Hello Everybody. This week we are going to go over one of the elements that helps us create more realistic animations: Arcs. In nature nothing moves in a linear way, but rather in a curved way. Whether it's a tennis player swinging his racket, or a bunch of leaves falling, they all follow a circular path. Let's take a look at an example. Say a character is looking at the front and then he turns his head to the right. If you were to animate the vertical rotation axis only, you would end up with a very mechanical and linear animation. On the other hand, if you tilt your head a little during the movement, you would get a more realistic result.

When a character walks, you will notice that the body parts follow a curved path (we will discuss more about walks in the future). On every step, the hips move side to side, switching the weight on the leg that is supporting the body. At the same time, it moves up and down in a "bouncing ball" trajectory.

Every animator has come across the "bouncing ball" exercise. Even this infamous ball moves in arcs (as a matter of fact, this ball is the most basic way to learn some of the principles of animation, including arcs). Add arms and legs and you have a jumping character. Continuing with our walking character ... If you watch the hands and feet you will notice that they also move in curves. The arms follow an horizontal "eight" path. The legs follow almost the same path, but the eight is flat on one side. Most 3D apps have the feature to show your object's trajectories. They display your "keys" information, and draw the actual interpolation generated for the object between 2 keys. This is especially useful, because you can see how the objects are moving and tweak the animation if it's too linear. You can also use it to decide whether or not you need more keys in a particular section of your animation to make it look more organic.

Even if curves are usually applied to the objects themselves, they can also be used in Graph Editors to refine the otherwise linear interpolations between keys. This is especially useful when you are fine-tuning your animation, but also when you want to tweak the lead-in and lead-outs for your keys. A falling object, for example, should not be keyed linearly. However, this is more related to forces. We will discuss forces in a future article.

Well, this is it. Arcs, is a very simple principle but if used correctly it can improve the look of animations incredibly. We'll see Arcs again in the next article. Happy posing, and keep on animating! nemirc

Animation Alley is a regular featured column with Renderosity Staff Writer Sergio Rosa [nemirc].
June 20, 2005

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