squash and stretch

Squash and stretch entire body for attitudes

From: Animation Notes by Disney animator, Ollie Johnston

Squash and stretch, is perhaps the most important aspect of to how a character moves. 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.

A rule of thumb is that no matter how squashy or stretchy something becomes, its volume remains relatively the same. 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. Objects, such as bouncing ball, will compress when they hit an immovable object, such as the floor, but they soon come back to their original shape. If a character or object is in motion it will undergo certain changes within its overall shape. For example, a cat character falling through space stretches in the direction of the fall and squashes, or 'splats', when it reaches the ground. The scaling may seem extreme when viewed in a single frame, but in motion it is remarkable how much the squashing and stretching can be exaggerated while still retaining a natural look.

This elasticity can be used to imply weight, mass or other physical qualities. For example, the shape of an iron balloon would not be affected by a drop to the ground, whereas a balloon full of water undergoes dramatic shape changes both as it dropped and when it impacts the ground.

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.

Complex models present complex problems. If a hierarchically defined character lands with a thud on his bum, perhaps a global scale in Y would not be appropriate, as this would also squish his legs, feet and everything else. This implies equal weight and mass among all his parts, when in fact his bum and torso would be the part squishing the most. Scaling the peripheral body parts back up by a percentage of the original scale usually keeps the visual weight on the body part with the most implied mass. Ideally, a flexible model would be used, in which the shape of various parts can be appropriately changed. This would allow the character to take on much a pear-shaped squash, more convincing in implying weight.

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.

figure 1.

figure 2.

figure 3.

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. (figure 1) 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. (figure 2) Accurate motion blur is the most realistic solution to this problem of strobing, 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. (figure 3)

Ask yourself: What is the character made of? How much muscle, fat,bone, hair, cloth? How fast is the character moving, accelerating, or decelerating? Try to maintain an objects original volume and its material integrity. ("truth to materials"). Remember that the material does not have to be a bouncing ball to have squash and stretch. Human and animal skeletons have an enormous amount of squash and stretch going as they move. Think about the words "collapsed" and "sustained" when animating a character.

Image by Pixar Studios "Luxo Jnr" (1986)

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