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Animation Notes #7
Inbetweens - what do they do?
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‘Inbetweens’ are those drawings which define the type of movement and the time that passes between each key pose drawing or position. How you arrange their spacing greatly influences the look of the resulting movement. These intermediate drawings are called 'tweens' in USA cartoon animation studio jargon which makes an invented verb, 'tweening'.

One of the questions I'm most frequently asked by students is "Why is my animation so jerky?" Students often spend a lot of time creating drawings only to be disappointed by the results. Smooth animation is not necessarily a result of doing lots of inbetweens, although as we know, this will slow down the action so that it may appear smoother.

It is more to do with the considered placement of inbetweens upon a path of action or 'motion arc'. It can even be greatly aided by strong intelligently worked out key poses in which two drawings account for how the various forms, masses and shapes will animate in a way that has some logic behind it.

Figure 1 shows the key poses or 'extremes' of action involved in the following example.

Figure 2 is an example of ill-considered inbetweening. Students with a background in fine art, design or graphic art often have no problem in drawing great looking characters. As a set of static drawings, every thing seems to work fine. But it is how a sequence of drawings work together across time which is important to the illusion of animation. Within a sequence, the forms and shapes need to be impelled with a sense of purposeful direction. The chaotic quality of the example in Figure 2 could be useful in some circumstances, however. For example when a frail old lady reaches out for a glass of water with an unsteady hand. Or for animating Peter Sellers' extraordinary performance of Dr Strangelove and his errant uncontrollable arm. The example of Figure 3 seeks to use more logic in the arrangement of inbetweens.

Broadly, inbetweens which are closely spaced will move slower than those spaced further apart. If you space most of these drawings close to the start of an action and progressively space them further and further apart towards the end, the action will start slowly and build to a punch. The opposite will be true if most of the inbetweens are spaced close to the end - the action will come to a gentle halt. This variation in spacing is called 'fairing' the movement, or 'slow in' and 'slow out' or 'ease in' and 'ease out' and became one of the 12 principles of animation developed by Disney studios.

Fast or slow, straight or curved, smooth or jerky, more than any other factor, timing via the placement of inbetweens defines the weight of an object and the inertia required to get it moving or to slow it down. Two objects of identical size and shape can appear to have vastly different weights simply by manipulating the spacing of their inbetweens. A heavy goods train with massive inertia, might take several kilometers of railway track to build up to its final running speed. This acceleration is long and slow. A mash mellow on the other hand, with practically no weight at all, might be shot from a gun and attain full speed within a few micro seconds. We can artificially represent these two types of movement through the way we use slow-in and slow-out.

The default setting for most time-based software packages designed to manipulate visual elements is to create strict mathematically even divisions between key positions. The result is very unnatural motion in that all objects instantaneously achieve full speed, or stop instantaneously. This works against almost everything we observe in nature and we read this type of motion as a 'bump' when its starts and a 'bump' when it finishes. This also applies to digital camera moves which can look particularly unnatural when no fairings have been used to start and stop the movement.

Machines may move in straight lines but animal or human characters rarely do. Their inbetweens are very often placed along a paths of action that describe curves or arcs. In fact moving things in arcs was considered so important to the look of naturalistic animation that it became one of the 12 guiding principles of Disney Studios. Motion arcs describe the path of action (travel) that things plot out when they move. When an animal moves, various parts of its body will move in sweeping arced paths of motion rather than in straight lines. When animating from one pose to another it is vitally important that we consider how the inbetween action is arranged in order to create a sense of flow, which is at the heart of all good animation.

Figure 3 has used both a 'path of action', (see Figure 4), and a considered arrangement of the inbetween drawing to produce smooth action. Since there is exactly the same number of drawings in both examples, smooth animation does not necessarily mean more work. There was just a little bit of thinking required to create a path of action for the hand, which gave it a purposeful direction, and the arrangement of inbetweens to depict the forces of acceleration and deceleration.

While key poses describe WHAT happens, inbetweens describe HOW it happens - the nature and qualities of the movement between the key poses or the 'extremes' of an action.

If you want to see what inbetweens can do, then add some considered inbetweens to your key pose assignment. Key Poses

"Being a dinosaur
of action, I wonder
what path I'll take
in life?"

figure 1
Roll back and forth over the above image to see the two key poses or 'extremes' animate.
figure 2
An example of inbetweening which has no logic behind it.
figure 3
Thoughtful inbetweening. This has exactly the same number of drawings as the above example, but they are arranged in such a way as to produce smooth movement.
figure 4
This is the 'path of action' (or motion arc) and arrangement of inbetweens used in the above example. Note the fairings used at either end of the path to accelerate and decelerate the action which progressively slows down the movement towards the end.
figure 5
These are the three keys involved in the following example.
figure 6
Inbetweening is used here to convey the notion that the club is very heavy. The action of the club lags behind the character's body giving the impression that it takes considerable effort to raise it above his head. Once the club is at its maximum height, gravity starts to work in its favour and its descent to earth is very quick.

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