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[edit] Introduction
[edit] What it is
This is single-handedly considered the most important principle of physical animation. Well executed, it is a vital component in making otherwise stiff and dull characters look organic and lively on-screen.
Using it simply means drawing or posing figures in deformed -- squashed or stretched -- poses. It can be applied to objects and creatures, as a whole or restricted to more flexible parts when under the action of gravity or some other pull or push.
A few examples:
- A bouncing rubber ball squashes when it hits the ground, then stretches back upon rebounding.
- Facial expressions:
- squash: in a big smile not only the mouth and cheeks move, we have eyes squinting and more -- the whole face can be involved;
- stretch: yell! With the jawbone open wide, the face gets longer, looking stretched.
- Gravity pushes all things down. Cloth, fat tissue and aged stretched skin show this effect clearly.
- A spring can be compressed; once freed, the released potential energy makes it stretch past its equilibrium point, building some potential energy again. This makes it compress again... and so on, until all the added energy is dissipated and it comes to a rest.
[edit] Why
Rubber Hoses and Circles
In its earlier form, squash and stretch was already extensively used during the 1920's, in cartoon series featuring Felix the Cat or Betty Boop, to name the best known examples.
The reasoning behind this principle is obvious: many real materials are pliable, including but not limited to soft organic matter and anything rubbery or jellylike. This must be accounted for in more realistic drawings, paintings, sculptures and, as animators soon discovered, it's a vital effect when things start moving, because movement emphasizes any rigidity in a silhouette.
So they started squashing and stretching in their sequences of poses. The effect was heavily explored in sight gags (those short visual jokes sometimes involving pies), with character's arms, legs, necks or whole bodies extending almost to no end when pulled.
That could be funny, but it was also unrealistic in a "bad" (primitive, disruptive) way. One that wouldn't work in the new kind of films they were planning to create, with engaging stories, appealing characters and well developed scenes. Summing up, they needed to draw squashed and stretched shapes to represent flesh and other flexible materials, but something was wrong with the technique in use.
Until a simple fact was realized, which will be discussed in the next section.
MAIN BENEFIT
improve the illusion of weight, volume and of the elasticity or hardness of each material.
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[edit] How
[edit] Volume Preservation
The key detail that raised Squash and Stretch to a principle is in fact quite trivial to understand:
- stretching does not mean enlarging
- squashing does not mean shrinking
In clear five words
the volume must be preserved!
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Pulling a cartoony arm to twice its length is fine -- hey, cartoon's natural laws allow that! The problem is doing so without thinning the arm to compensate. If something becomes extended along one of its axes, it must be compressed along the other two. Example: a rubber ball elongated in Z should be squashed in its X-Y plane accordingly, or it will seem that the ball increased in size.
Not preserving the volume is one of the most noticeable mistakes an animator can make.
Everyone will perceive it. For this very reason it's also a simple problem to fix. It's not necessary to calculate the volume of the new shape precisely -- eyes and good sense are enough, in general.
Squash and Stretch proved to work so well on the screen that animators felt like getting to the heart of it. They would use the effect whenever possible and kept on experimenting, overdoing it to amusing results.
But apart from this exaggeration -- in itself another of the principles -- there's a real feature. Studying images and videos of athletes, for instance, they realized that Squash and Stretch in fact was more pronounced in the real world then they imagined, which justifies its wilder use in animations.
[edit] Facial Expressions
One of the most important applications of Squash and Stretch is in facial animation, both realistic and cartoony.
Artists responsible for creating blend shapes for expressions (opening the mouth, smiling, frowning, squinting, etc.) and phonemes or visimes (*) for lip synchronization have to pay close attention to how the skin stretches and bulges and where creases appear in each expression.
Cartoons, as we know, can have very pronounced squashing and stretching in exaggerated expressions. While realistic faces don't go anywhere that far, opening a mouth causes a slight head stretching -- we just need to be careful to do it properly. The mirror and pictures are always good references.
(*)
as used in Jason Osipa's Stop Staring book, a very good resource for head modelling and animation.
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[edit] Joints
Squash and Stretch is not restricted to soft tissue and rubbery things. It's present in a different way in any articulated system, be it mechanical or organic.
Consider a character walking or running. Her joints bend her body, more notably at the knees, as she throws her weight on the planted foot; then they stretch to propel her up and forward. If she jumps high, we clearly see her skeleton “squash” to prepare the move, then stretch to launch her into the air, squash again upon reaching the ground and probably stretch back to her normal height once she has finished the landing.
(Just to consider the reach of squash and stretch, in the above example there's also her breathing, expanding and squeezing the trunk. And, being organic, we can't forget the flesh.)
Therefore skeletons are exhibiting a form of Squash and Stretch when they are bent or straightened.
Joints
stretching translates to straightening and squashing translates to bending. This is the basic building block to apply this principle of animation on articulated creatures.
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The objective is creating believable motion, that gives a good idea about the forces (pulls, pushes, weight) in action through their effect on objects and on the characters.
[edit] Strobing
The Squash and Stretch technique was also introduced back in the silent movies era to remedy strobing (jerkiness), a side-effect of fast motions. When an object moves slowly enough, its shape overlaps from frame to frame and our eyes are able to smooth out the action. At higher speeds these shapes appear spaced apart, breaking the illusion of movement: spectators see them as separate images instead of a single fast object.
Squash and Stretch helps diminishing the distance between sequencial positions of an object, which makes the shapes overlap for a wider speed range.
Motion blur
is a better tool against strobing, since it is basically done for us, more precise and works for rigid objects, too. But they didn't have it in the 1920's...
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Tutorial
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[edit] In Blender
What resources does the program offer for this principle of animation?
[edit] Physics
[edit] Story Development
[edit] Notes
Extremes & Inbetweens
An extreme in animation is a main pose, better crafted and meant to be seen for a longer time (by using Slow In and Slow Out, another principle of animation). Between extremes we have the “inbetween” frames.
[edit] Don't let them see it
A great way to apply squash and stretch that works well in cartoons when only a quick effect is wanted, is to do it right before an extreme. This way the deformation can barely be seen, but it still does its job, helping make the action look crispy and natural.
As an illustration, consider a frightened character. We can stretch his head shortly while he screams (*), but only for a frame or two right before entering the main pose for that action. The deformation is barely noticeable, while the extreme, undeformed and with slow in and slow out, is clearly shown to the viewers. Good exercise: animate this without the stretch, then add it and compare both versions.
(*) Where does the justification for this stretching comes from? Hint: look at yourself screaming.
[edit] What's in a walk
Important example of Squash and Stretch via joints: the classic walk cycle is made up of four poses for each step:
- contact
- recoil
- passing
- high-point
(the next pose is the contact one again, but flipped)
At the second pose, recoil, the body is at its lowest point and at the 4th, high-point, at the highest, due mainly to the knee of the supporting leg, bent at the lowest and stretched at the other.
In his great book “Animator's Survival Toolkit”, Richard Williams illustrates in an amusing passage the basic difference between a female and a male walk: the ups and downs. The feminine way keeps legs closer together and doesn't use much knee bending, while the more masculine one, legs apart, arms away from the trunk, exaggerates on the squashing (2nd pose) and on the stretching (4th) of the walk cycle.
[edit] Resizing
Squash and Stretch requires volume preservation, but the simpler effect of resizing (part of) an object is also useful in cartoons, naturally. The first examples that probably come to mind are:
- To indicate pain: when somebody's hand or foot gets stumped and starts to throb.
- For the balloon effect: e.g. when a character quickly inhales way too much air and becomes inflated like a balloon.
Summer of documentation 2006 -- Willian 07:20, 5 July 2006 (CEST)
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