View Squash & Stretch
Squash & Stretch is a basic technique in animation that requires the effects of motion, weight, gravity and so on to be exaggerated onto the animated object. In reality, objects can deform (sometimes quite a bit) as a force is applied to them. A great example is watching a slow motion sequence of a slap on a face (YouTube). You will see the shock wave of the slap initially roll through the soft tissue of the face, while the hard bones will react by rotation and position shifts. The harder the slap, the more the soft tissue in particular will deform. It is a wonder, when watching this in slow motion, why more damage does not occur as a result of this type of physical contact. When played at speed, the effects so prevalent in slow motion are hardly noticed, though if you were to animate a slap without much of a reaction, it will likely look “off…”.
In the animated realm, we can take this effect and push it to the extreme. You likely have seen the pervasive bouncing ball example. It is the most classic presentation of this animation concept, regardless of the animation technique. In 2D animation, you will also see the shapes of objects distorted to account for the lack of motion blur (even if brushstroke blur is used). These distortions are often just a frame in length, but tend to be exaggerated to the extreme. Search for Keith Lango’s blog for great discussion on classic animation techniques, and how they can be applied to CG animation.
Even if photorealism is the target, squash & stretch can be used to “cheat” a pose as needed, beyond the constraints of realistic anatomy. You would be surprised at how much this cheat is used. As an animator, don’t be afraid to really push it. You can always back it off as needed if you have gone to far. That’s better than not going far enough, and not getting the results your looking for.