Principles Of Animation

Between the late 1920's and the late 1930's animation grew from a novelty to an art form at theWalt Disney Studios. With every picture, actions became more convincing, and characters wereemerging as true personalities. Audiences were enthusiastic and many of the animators weresatisfied, however it was clear to Walt Disney that the level of animation and existingcharacters were not adequate to pursue new story lines-- characters were limited to certaintypes of action and audience acceptance notwithstanding, they were not appealing to the eye. Itwas apparent to Walt Disney that no one could successfully animate a humanized figure or alife-like animal; a new drawing approach was necessary to improve the level of animationexemplified by the "Three Little Pigs".

Disney set up drawing classes for his animators at the Chouinard Art Institute in Los Angelesunder Instructor Don Graham. When the classes were started, most of the animators were drawingusing the old cartoon formula of standardized shapes, sizes, actions and gestures, with littleor no reference to nature. Out of these classes grew a way of drawing moving human figures andanimals. The students studied models in motion as well as live action film, playing certainactions over and over. The analysis of action became important to the development of animation.

Some of the animators began to apply the lessons of these classes to production animation, whichbecame more sophisticated and realistic. The animators continually searched for better ways tocommunicate to one another the ideas learned from these lessons. Gradually, procedures wereisolated and named, analyzed and perfected, and new artists were taught these practices as rulesof the trade.

They became the fundamental principles of traditional animation:

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Squash and Stretch - Defining the rigidity & mass of an object by distorting its shape during an action.
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Timing - Spacing actions to define the weight & size of objects & the personality of characters.
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Anticipation - The preparation for an action.
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Staging - Presenting an idea so that it is unmistakably clear.
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Follow Through & Overlapping Action - The termination of an action & establishing its relationship to the next action.
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Straight Ahead Action & Pose-To-Pose Action - The two contrasting approaches to the creation of movement.
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Slow In and Out - The spacing of in-between frames to achieve subtlety of timing & movements.
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Arcs - The visual path of action for natural movement.
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Exaggeration - Accentuating the essence of an idea via the design & the action.
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Secondary Action - The Action of an object resulting from another action.
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Appeal - Creating a design or an action that the audience enjoys watching.

The application of some of these principles mean the same regardless of the medium of animation.2D hand drawn animation deals with a sequence of two dimensional drawings that simulate motion.3D computer animation involves creating a three dimensional model in the computer. Motion isachieved by setting keyframe poses and having the computer generate the inbetween frames.Timing, anticipation, staging, follow through, overlap, exaggeration, and secondary action applyin the same way for both types of animation. While the meanings of squash and stretch, slow inand out, arcs, appeal, straight ahead action, and pose-to-pose action remain the same, theirapplication changes due to the difference in medium.
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Squash N Stretch

The most important principle is called Squash and Stretch. When an object is moved, the movement emphasizes any rigidity in the object. In real life, only the most rigid shapes (such as chairs, dishes and pans) remain so during motion. Anything composed of living flesh, no matter how bony, will show considerable movement in its shape during an action. For example, when a bent arm with swelling biceps straightens out, only the long sinews are apparent. A face, whether chewing, smiling, talking, or just showing a change of expression, is alive with changing shapes in the cheeks, the lips, and the eyes. The squashed position depicts the form either flattened out by an external pressure or constricted by its own power. The stretched position always shows the same form in a very extended condition. The most important rule to squash and stretch is that, no matter how squashed or stretched out a particular object gets, its volume remains constant. If an object squashed down without its sides stretching, it would appear to shrink; if it stretched up without is sides squeezing in it would appear to grow. Consider the shape and volume of a half filled flour sack when dropped on the floor, it squashed out to its fullest shape. If picked up by the top corners, it stretched out to its longest shape. It never changes volume. The standard animation test for all beginners is drawing a bouncing ball. The assignment is to represent the ball by a simple circle, and then have it drop, hit the ground, and bounce back up into the air. A simple test, but it teaches the basic mechanics of animating a scene, introducing timing as well as squash and stretch. If the bottom drawing is flattened, it gives the appearance of bouncing. Elongating the drawings before and after the bounce increases the sense of speed, makes it easier to follow and gives more snap to the action.

Squash & stretch in bouncing ball Squash and stretch also defines the rigidity of the material making up an object. When an object is squashed flat and stretches out drastically, it gives the sense that the object is made out of a soft, pliable material and vice versa. When the parts of an object are of different materials, they should respond differently: flexible parts should squash more and rigid parts less. An object need not deform in order to squash and stretch. For instance, a hinged object like Luxo Jr. (from the film Luxo Jr.) squashes by folding over on itself, and stretches by extending out fully.

Squash & stretch in Luxo Jr.'s hop Squash and stretch is very important in facial animation, not only for showing the flexibility of the flesh and muscle, but also for showing the relationship of between the parts of the face. When a face smiles broadly, the corners of the mouth push up into the cheeks. The cheeks squash and push up into the eyes, making the eyes squint, which brings down the eyebrows and stretches the forehead. When the face adopts a surprised expression, the mouth opens, stretching down the cheeks. The wide open eyes push the eyebrows up, squashing and wrinkling the forehead. Another use of squash and stretch is to help relieve the disturbing effect of strobing that happens with very fast motion because sequential positions of an object become spaced far apart. When the action is slow enough, the object's positions overlap, and the eye smoothes the motion out. However, as the speed of the action increases, so does the distance between positions. When the distance becomes far enough that the object does not overlap from frame to frame, the eye then begins to perceive separate images. Accurate motion blur is the most realistic solution to this problem of strobing, [8,9] but when motion blur is not available, squash and stretch is an alternative: the object should be stretched enough so that its positions do overlap from frame to frame (or nearly so), and the eye will smooth the action out again.

In 3D keyframe computer animation, the scale transformation can be used for squash and stretch. When scaling up in Z, the object should be scaled down in X and Y to keep the volume the same. Since the direction of the stretch should be along the path of action, a rotational transformation may be required to align the object along an appropriate axis.
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Timing

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Timing, or the speed of an action, is an important principle because it gives meaning to movement- the speed of an action defines how well the idea behind the action will read to an audience. It reflects the weight and size of an object, and can even carry emotional meaning.
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Proper timing is critical to making ideas readable. It is important to spend enough time (but no more) preparing the audience for: the anticipation of an action; the action itself; and the reaction to the action. If too much time is spent on any of these, the audience's attention will wander. If too Little time is spent. the movement may be finished before the audience notices it, thus wasting the idea.
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The faster the movement, the more important it is to make sure the audience can follow what is happening. The action must not be so fast that the audience cannot read it and understand the meaning of it.
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More than any other principle, timing defines the weight of an object. Two objects, identical in size and shape, can appear to be two vastly different weights by manipulating timing alone. The heavier an object is, the greater its mass, and the more force is required to change its motion. A heavy body is slower to accelerate and decelerate than a light one. It takes a large force to get a cannonball moving, but once moving, it tends to keep moving a the same speed and requires some force to stop it. When dealing with heavy objects, one must allow plenty of time and force to start, stop or change their movements, in order to make their weight look convincing.
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Light objects have much less resistance to change of movement and so need much less time to start moving. The flick of a finger is enough to make a balloon accelerate quickly away. When moving, it has little momentum and even the friction of the air quickly slows it up.
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Timing can also contribute greatly to the feeling of size or scale of m object or character. A giant has much more weight, more mass, more inertia than a normal man; therefore he moves more slowly. Like the cannonball, he takes more time to get started and, once moving, takes more time to stop. Any changes of movement take place more slowly. Conversely, a tiny character has less inertia than normal, so his movements tend to be quicker.
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The way an object behaves on the screen, the effect of weight that it gives, depend entirely on the spacing of the poses and not on the poses themselves. No matter how well rendered a cannonball may be, it does not look like a cannonball if it does not behave like one when animated. The same applies to any object or character.
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The emotional state of a character can also be defined more by its movement than by its appearance, and the varying speed of those movements indicates whether the character is lethargic, excited, nervous or relaxed.
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Thomas and Johnston describe how changing the timing of an action gives it new meaning: Just two drawings of a head, the first showing it leaning toward the right shoulder and the second with it over on the left and its chin slightly raised, can be made to communicate a multitude of ideas, depending entirely on the timing used.
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Each in-between drawing added between these two extremes gives a new meaning to the action.

NO inbetweens
The Character has been hit by a tremendous force. His head is nearly snapped off.

ONE inbetween
The Character has been hit by a brick, rolling pin, frying pan.

TWO inbetweens
The Character has a nervous tic, a muscle spasm, an uncontrollable switch

THREE inbetweens
The Character is dodging a brick, rolling pin, frying pan

FOUR inbetweens
The Character is giving a crisp order, "Get going!" "Move it!"

FIVE inbetweens
The Character is more friendly, "Over here." "Come on-hurry!"

SIX inbetweens
The Character sees a good looking girl, or the sports car he has always wanted

SEVEN inbetweens
The Character tries to get a better look at something

EIGHT inbetweens
The Character searches for the peanut butter on the kitchen shelf

NINE inbetweens
The Character appraises, considering thoughtfully

TEN inbetweens
The Character stretches a sore muscle

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Anticipation

An action occurs in three parts: the preparation for the action, the action proper, and the termination of the action. Anticipation is the preparation for the action;

There are several facets to Anticipation. In one sense, it is the anatomical provision for an action. Since muscles in the body function through contraction, each must first be extended before it can contract. A foot must be pulled back before it can be swung forward to kick a ball. Without anticipation many actions are abrupt, stiff and unnatural.

Anticipation is also a device to catch the audience's eye, to prepare them for the next movement and lead them to especially before it actually occurs. Anticipation is often used to explain what the following action is going to be. Before a character reaches to grab an object, he first raises his arms as he stares at the article, broadcasting the fact that he is going to do something with that particular object. The anticipatory moves may not show why he is doing something, but there is no question about what he is going to do next.

Anticipation is also used to direct the attention of the audience to the right part of the screen al the right moment This is essential for preventing the audience from missing some vital action. In the very beginning of Luxo Jr., Dad is on screen alone looking offstage. He then reacts, anticipating something happening there. When Jr. does hop in. the audience is prepared for the action.

The amount of anticipation used considerably affects the speed of the action which follows it If the audience expects something happen, then it can be much faster without losing them. If they are not properly prepared for a very fast action, they may miss it completely; the anticipation must be made larger or the action slower. In a slow action the anticipation is often minimized and the meaning carried in the action proper. In one shot in The Adventures of André and Wally B., Wally B. zips off to the right. The actual action of the zip off is only 3 or 4 frames long, but he anticipates the zip long enough for the audience to know exactly what is coming next.

Anticipation can also emphasize heavy weight. As for a character picking up an object that is very heavy. An exaggerated anticipation, like bending way down before picking up the object, helps the momentum of the character to lift the heavy weight. Likewise for a fat character standing up from a seated position: he will bend his upper body forward, with his hands on the armrests of the chair, before pushing up with his arms and using the momentum of his body.

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Staging

Staging is the presentation of an idea so it is completely and unmistakably clear, this principle translates directly from 2D hand drawn animation. An action is staged so that it is understood; a personality is staged so that it is recognizable; an expression so that it can be seen: a mood so that it will affect the audience.

To stage an idea clearly, the audience's eye must be led to exactly where it needs to be al the right moment, so that they will not miss the idea. Staging, anticipation and timing are all integral to directing the eye. A well-timed anticipation will be wasted if it is not staged clearly.

It is important, when staging an action. that only one idea be seen by the audience at a time. If a lot of action is happening at once, the eye does not know where to look and the main idea of the action will be "upstaged" and overlooked. The object of interest should contrast from the rest of the scene. In a still scene, the eye will be attracted to movement. In a very busy scene, the eye will be attracted to something that is still. Each idea or action must be staged in the strongest and the simplest way before going on to the next idea or action. The animator is saying. in effect, "Look at this, now look at this. and now look at this."

In Luxo Jr., it was very important that the audience was looking in the right place at the right time, because the story, acting and emotion was being put across with movement alone, in pantomime, and sometimes the movement was very subtle. If the audience missed an action, an emotion would be missed, and the story would suffer. So the action had to be paced so that only Dad or Jr. was doing an important action at any one time, never both. In the beginning of the film, Dad is on screen alone your eye was on him. But as soon as Jr. hops on-screen, he is moving faster than Dad. therefore the audience's eyes immediately goes to him and stays there.

Most of the time Jr. was on-screen, Dad's actions were very subtle, so the attention of the audience was always on Jr. where most of the story was being told. If Dad's actions were important, Jr.'s actions were toned down and Dad's movements were emphasized and the attention of the audience would transfer to Dad. For example, when Jr. looks up to Dad after he's popped the ball and Dad shakes his head, all eyes are on him.

Another idea developed in the early days at Disney was the importance of staging an action in silhouette. In those days, all the characters were black and white, With no gray values to soften the contrast or delineate a form. Bodies, arms and hands were all black, so there was no way to stage an action clearly except in silhouette. A hand in front of a chest would simply disappear. Out of this limitation, the animators realized that it is always better to show an action in silhouette. Charlie Chaplin maintained that if an actor knew his emotion thoroughly, he could show it in silhouette.

In The Adventures of André and Wally B., André awakes and sits up, then scratches his side. If he were to scratch his stomach instead of his side, the action would happen in front of his body and would be unclear what was happening.
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Follow Through

Just as the anticipation is the preparation of an action, follow through is the termination of an action. Actions very rarely come to a sudden and complete stop, but are generally carried past their termination point. For example, a hand, after releasing a thrown ball, continues past the actual point of release.

In the movement of any object or figure, the actions of the parts are not simultaneous some part must initiate the move, like the engine of a train. This is called the lead. In walking, the action starts with the hips. As the hip swings forward, it sets a leg in motion. The hip "leads", the leg "follows." As the hip twists, the torso follows, then the shoulder, the arm, the wrist, and finally the fingers. Although most large body actions start in the hips, the wrist will lead the fingers in a hand gesture, and the eyes will usually lead the head in an action.

Appendages or loose parts of a character or object will move at a slower speed and 'drag' behind the leading part of the figure. Then as the leading part of the figure slows to a stop. These appendages will continue to move and will take longer to settle down. As with squash and stretch, the object's mass is shown in the way the object slows down. The degree that the appendages drag behind and the time it takes for them to stop is directly proportional to their weight. The heavier they are the farther behind they drag and the longer they take to settle to a stop. Conversely, if they are lighter, they will drag less and stop more quickly.

In The Adventures of André and Wally B., this principle was used extensively on Wally B.'s feet, antennae and stinger. They all dragged behind his head and body. and continued to move well after the body had stopped. To convey that these loose appendages were made of different materials and different masses, the rate of the follow through was different for each type. His antennae were fairly light, so they dragged behind just slightly. His stinger was like stainless steel, so it dragged behind the action more than the antennae. And his feet were heavy and very flexible, as though they were water balloons; therefore, they always followed far behind the main action.

Often, slight variations are added to the timing and speed of the loose parts of objects. This overlapping action makes the object seem natural, the action more interesting. In Wally's zip off (figure 5), his feet zipped off, one after the other, about one or two frames apart. The action was so fast that it was difficult to see each foot going off separately, but it made the action as a whole more interesting.

Perhaps more important, overlapping is critical to conveying main ideas of the story. An action should never be brought to a complete stop before starting another action, and the second action should overlap the first. Overlapping maintains a continual flow and continuity between whole phrases of actions.

Walt Disney once explained overlapping this way, "It is not necessary for an animator to take a character to one point, complete that action completely, and then turn to the following action as if the had never given it a thought until after completing the first action. When a character knows what his is going to do he doesn't have to stop before each individual action and think to do it. He has it planned in advance in his mind. For example, the mind thinks, ' I'll close the door - lock it then I'm going to undress and go to bed.' Well, you walk over to the door before the walk is finished you're reaching for the door - before the door is closed you reach for the key - before the door is locked you're turning away - while you're walking away you undo your tie - and before you reach the bureau you have your tie off. In other words, before you know it you're undressed and you've done it in one thought, "I'm going to bed."
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Slow In and Out

Slow in and out deals with the spacing of the inbetween drawings between the extreme poses. Mathematically, the term refers to second- and third-order continuity of motion.

In early animation, the action was limited to mainly fast and slow moves, the spacing from one drawing to the next fairly even. But when the poses of pose-to-pose animation became more expressive, animators wanted the audience to see them. They found that by grouping the inbetweens closer to each extreme, with only one fleeting drawing halfway between, they could achieve a very spirited result, with the character zipping from one attitude to another. "Slowing out" of one pose, then "slowing in" to the next pose simply refers to the timing of the inbetweens.

The animator indicates the placement of the inbetweens, the slow in or slow out, with a "timing chart" drawn on the side of the drawing. This tells himself, or his assistant who will be doing the inbetweens later, how he wanted the timing to be and where he wanted the inbetween drawings placed.

Timing chart for ball bounce In most 3D keyframe computer animation systems, the inbetweening is done automatically using spline interpolation. Slow in and slow out is achieved by adjusting the tension, direction or bias, and continuity of the splines. This works well to give the affect of slow in and out, but a graphical representation of the spline is required to see the effect of tension, direction, and continuity have on its shape.

With this type of spline interpolation, a common problem is the spline overshooting at extremes when there is a large change in value between them, especially over a small number of frames. This also happens when the direction control of an extreme is adjusted. The danger is that, depending on the variable the spline controls (translate, rotate, or scale), the value will shoot in the wrong direction just before (or just after) the large change in value. Sometimes this effect works out well when it occurs just before a large movement it may appear to be an anticipation. However, more often than not, it gives an undesirable effect.

In Luxo Jr., there was an example of this problem of overshooting splines, Jr.'s base was very heavy and when he hopped, we wanted the base to start stationary, then pop up in the air from the momentum of his jump, arc over, then land with a thud, suddenly stationary again. For the up translation, there were three keyframes, the two stationary positions and the highest point of his jump. The spline software forced continuity, so that his base would move down under the surface of the floor just before and after the jump. The solution was to put two new extremes, equal to the two stationary extremes, on the frames just before and just after the extremes. This "locked" down the spline, so that the up translation stayed the same value, popped up in the air, landed and then stayed the same value again. This gave the desired feeling of weight to his little base.

Types of Animation

Cel Animation

Drawings are made on transparent sheets (celluloid) which may then be laid on top of each other to combine characters and backgrounds

Stop Motion Animation

A model or puppet (shadow puppets were used in early stop motion) is shot a frame at a time, with tiny changes in position being made between each frame

Computer animation

Computers can be used to entirely create the shapes and colours of animated action, working from a series of mathematical codes, or they can be used to enhance hand-drawn characters.

Introduction

Early research in computer animation developed 2D animation techniques based on traditionalanimation. Techniques such as storyboarding, keyframe animation, inbetweening, scan/paint, and multiplane backgrounds attempted to apply the cel animation process to the computer. As 3D computer animation research matured, more resources were devoted to image rendering than to animation. Because 3D computer animation uses 3D models instead of 2D drawings, fewer techniques from traditional animation were applied. Early 3D animation systems were script based, followed by a few spline-interpolated keyframe systems. But these systems were developed by companies for internal use, and so very few traditionally trained animators found their way into 3D computer animation.

The last two years have seen the appearance of reliable, user friendly, keyframe animationsystems from such companies as Wavefront Technologies Inc., Abel Research (RIP), Vertigo Systems Inc., Symbolics Inc., and others. These systems will enable people to produce more high quality computer animation. Unfortunately, these systems will also enable people to produce more bad computer animation.

Much of this bad animation will be due to unfamiliarity with the fundamental principles thathave been used for hand drawn character animation for over 50 years. Understanding theseprinciples should also be important to the designers of the systems used by these animators.