INBETWEENS: 2D Animation principle

17 December, 2008

INBETWEENS: Animation principle

The drawings that come between
key drawings are called inbetweens. As the inbetweens are added the action looks
more complete and appealing.


Notices that drawings numbered
2 to 16 are inserted between the key drawings 1 and 17 these are the inbetweens
for this action.

What Are Inbetweens? 

‘Inbetweens’ are those drawings
that define the type of movement and the time that passes between each key pose
drawing and position. How you arrange their spacing greatly influence the look
of the resulting movement. These intermediate drawings are called ‘tweens’ in
USA cartoon animation studio jargon that makes an invented verb, ‘tweening’.

Timing and Spacing

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.

Motion Arcs – Paths of

Machines may move in straight
lines but animal or human characters rarely do. Their inbetweens are very often
placed along a path of action that describes 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.

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