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Learning to animate is a little more than just learning how to draw and how to make things move. There are ranges of movement to take into account. How much squash to add, how much secondary action to leave out. We can identify a lot of these fundamental issues because of the founding work done in the 30’s when Walt Disney set up classes for his animators, under the instruction of Don Graham, and developed the 12 basic principles of animation. In this chapter of the study of these principles using Flash 8, Terri Dentry and Steve Piscopo put Arcs of Motion under the microscope and find that woodpeckers probably do come from outer space.
Header images: Edward Muybridge (run sequence), and Piscopo using Muybridge sequence to create fluppet parts for Flashman character
Original publication: Desktop Magazine 2008
Republished with permission
The best route from point A to point B is a straight line – right? Not so when it comes to movement in a biological entity. Nature has determined that the best route from point A to point B is to detour via point C in a swooping arced path of motion.

Almost everything organic moves in an arced path, with the noted exception of the motion of a woodpecker (more on these tricky birds later). A head turn from left to right to look at a passing car moves in a gentle arc that dips slightly to the ground. A hand that drops from your lap to the floor to pick up a piece of paper arcs outwards from the body before reaching the target. Bees circling a bush for honey move in seemingly random arcs as they dart between the flowers and each other. No matter how large or small the movement is, if it’s a living object the chances are it will be moving in an arc of motion.

head turn in straight line
head turn with arc of movement

Why do we move in arcs? Physicists tell us this is due to a complex energy conservation system of energy out, energy in, to conduct the flow of movement through the muscles, and gravity accounts for the presence of arced motion when an object is thrown creating a parabolic path on its fall to the ground. Biologists simply point to our joint structure which makes it almost impossible to move without causing an arc. You can try this for yourself by holding out your arm and trying to move it in a straight line. Lets not get into the Sociologists account – something to do with dancing around the true facts I believe (or was that another argument?)

So whats up with the woodpecker? No one really understands why, but not only does this mainland American bird tap endlessly on anything available in a sharp rap tap tap motion that moves in a straight line, they are also known to fly a direct path from A to B without going past C, for about 90% of their journey. Cartoon animations of these critters often have them portrayed in a mechanical manner because of this lack of organic arc movement. Scientists point out that the cartoon joke however, of the endless headaches these poor birds must endure is only a myth.

So, unless you are animating a woodpecker, you are going to need to know how and when to create an arc of motion. When working in Flash8 most movement is done using tweens, which tends your animation towards straight lines. To counteract this inbetweening is used to animate this movement instead of tweening. This process requires drawings (images) to be produced between two extremes or key positions. If more drawings are included the movement produced will be slower and if fewer drawings are included the illusion of faster movement will be created.

Creating an Arc of Motion using Inbetweening

Steve Piscopo, of Nectarine (www.nectarine.com.au), has prepared a file based on our earlier articles in this series, using his character Flashman. For this example a motion arc will be added to Flashman’s head turn movement using an inbetweening count of 9, as shown in the following chart:

In this chart frame-1 and frame-9 represent the key positions. Frame-5 represents the first inbetween half way between the keypositions. Frame-3 indicates the need for an inbetween halfway between key frame-1 and the first inbetween. Similarly, frame-7 is the inbetween half way between frame-5 and key frame-9.
In Steve’s diagram, left, you can see how these key frames and inbetweens relate to the head shape of our Flashman character as he moves his head from left to right.

Steve also recommends adding additional keyframes at the start and end of the motion tween to work with the ease in ease out of movement to make the head turn more believable. On the mid point of the arc the eyes and mouth parts should also be altered to give the head turn expression. The eyes most likely blink at this point, adding to the downward look of the head, while the mouth would close or form a more condensed shape in concentration or anticipation of the result of the head turn.

The additional inbetweens above create the illusion of the action speeding up and then slowing down by adding more drawings at the beginning and end of the movment.

What Needs to be Arced?

Key movements of a character need to be arced to make them more life-like. In nature almost every movement of the body moves in an arc, but in an animation tricks are used to minimise the amount of work required in creating these movements.

By adding arc’s to the movements that we use as keys our eye assumes the other movements are following, even if they are moving in a straight line. Keith Lango in his tutorial “Arc D’Triumph!” recommends, “building arcs into your hips for sure. A key ingredient to selling the weight of your character is to build arcs into the hips. A key to convincing and pleasing moving holds is the arc in the hips, specifically the 'settle back' from an extreme. Additionally the 'settle up' into an extreme needs to flow along an arcing path. If there's one part of your character that absolutely must have arcs it's the hips.”

When we view basic human motion the exaggeration of this arced movement tells us a lot about the weight of the character. The same reference points are used when we view the arced motion of liquids, which in turn gives us information on its viscosity and purity. Non-arced motion comes across as sinister, restricted or robotic.

Keith says “But that's just the beginning. Your arms need arcs as well. When the arm moves it does so as the result of various joint rotations. There's no excuse for not arcing your arms. Really, any part of the body that moves should move in an arcing fashion. Track your wrists, finger ends, nose top of the head, elbows, knees, feet/ankles, toe tips, sternum. Look at these body parts in motion, track their progression frame by frame right on your screen with a dry erase marker- take care to see just how these parts move.”

Back to our Flashman example, Steve has worked on each one to create an exaggerated head turn, which gives this character elements of effort and personality.

In the final animation Steve has exaggerated the extreme points (the extreme points being the characters highest position and lowest position) just pushing the two a little further out. He has also added some “Squash and stretch” on the character’s movement. And finally added some “Follow Through” to his cape.


Terri Dentry is an independent film journalist, animation producer, and the Director of thinkRED film & media in Melbourne, Australia.

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Steve Piscopo, Nectarine
Steve Piscopo, Nectarine