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 chair it does not change shape.
These are rigid objects and in animation they do not change shape when
force is applied to them. Objects such as a bouncing ball, or a pillow
will change shape when a force is applied to them for example a rubber
ball will flatten or squash against the floor when thrown against it
and will stretch back up when it bounces off the floor.
This is number one on the list because in animation any thing that
is alive will squash and stretch. 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.
The squash and stretch of an object can be thought of as the springiness
of an object or material.Just as the laws of physics implies, the object must retain its volume
or at least its density. If an object reduces in size in the one direction
it should increase in size in the other direction. If this were not so the
object would appear to shrink or grow.
An Animator can also stretch and 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.) you can see this effect to an extent in the image
six images, from the movie The Emperor's New Groove (2000) produced
by Walt Disney Studios, are an excellent example of the usage of Squash
and Stretch. The bag Squashes to the shape of the steps followed by
Cronk who dropped the bag when he stepped on a cat sleeping on the steps.
In the last part you see the cat coming down after. The amount that
each object deforms gives you an idea as to the rigidity of each object
in the clip.