Squash and Stretch

Squash and stretch is a way of deforming an object such that it shows how rigid the object is.

No matter how an object deforms, it should still appear to retain it's volume.

The squash and stretch of an object can be thought of as the springiness of an object or material. This can be related to Hooke’s Law F=-kx Where the springiness or rigidity of an object is, k the force constant and the amount the object was deformed would be x.

Just as Hooke’s Law implies the object must retain its volume or at least its density. If an object reduces in size in the Y direction it should increase in size in the X direction. If this were not so the object would appear to shrink or grow.

If a ball flattens down 2 times as much as normal, it will have to get 2 times as wide to keep its volume.

Example

Anything composed of living flesh, no matter how bony, will show considerable movement in its shape during an action. For example,  a bending arm with swelling biceps.....

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

Rigidity

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.

When an object moves, its movement indicates the rigidity of the object. Many real world objects have little flexibility, such as wood furniture, however most organic objects have some level of flexibility in their shape.

Squash and Stretch help to define how rigid an object is. All objects change shape when a force is applied to them. Solid objects have so little that it can be ignored when you throw a bowling ball you won't see it change form or when you sit on a concrete bench it does not change shape. These are rigid objects and in animation they do not change shape when force is applied to them.

Take for example a bouncing ball. A rubber ball bounces higher and squashes more upon impact than a hard league ball. The ease with which an object squashes and stretches defines the rigidity of the material making up an object.

Volume

• If a character or part of a character doesn't maintain volume with squash and stretch, believability will be lost.

• The most obvious usage in character animation is muscles. When a muscle is contracted it will squash and when extended, it stretches.

When a person smiles, the shape of the face is determined by the movement of muscles underneath a layer of skin. During a smile, though the head seems to increase in size, with the widening of the mouth and jaw, it does not. The object is simply displacing its matter into the stretched shape. The most important rule to squash and stretch is that no matter how squashed or stretched out an object gets, its volume remains constant.

Our muscles are constantly squashing and stretching with every movement that we make. Our feet squash and spread out when we put our body weight down on them. An animator could not make a person or an animal look real with out deforming the shape of the object that has force acting upon it.

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.

An Animator can also stretch an object that is moving fast to make its motion seem smoother.

When animating a baseball that has been thrown fast if the ball were not stretched it would look like a strobe effect not a smooth motion. When the ball is drawn so that each image overlaps the motion looks smooth (in real life a ball does not stretch, this is just one of the freedoms granted to an animator to help create the illusion of movement.).

However even in cases where motion blur can be used, there may still be reasons to use it.

Tex Avery marrying the nymphomaniac grandmother and the Wolf. Removed end of the film Red-Hot Riding Hood.
From the black and white picture found in the Patrick Brion's Tex Avery (Ed du Chêne)

Strobing

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 4a) 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, 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 4c)

In slow action, an object's position overlaps from frame to frame which gives the action a smooth appearance to the eye

Strobing occurs in a faster action when the object's positions do not overlap and the eye perceives separate images

Stretching the object so that it's positions overlap again will relieve the strobing effect
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.