ARCS: 2D Animation principle

17 December, 2008

ARCS: 2D Animation principle

Arcs bring life to a movement, thus avoiding a
mechanical look. Arcs play an important role in bringing appeal to an action.
Movements without arcs will end up in a boring pose-to-pose animation. Applying
arcs also serves the purpose of applying anticipation, as the position in the
middle of the arc gets squashed to fit in the path of action. Concept of Arcs is
very simple. Since human anatomy is made up of joints the body parts will follow
the shape of an arc in movement.

An arc always describes the visual path of action
from one extreme to another. Arcs in nature are the most economical routes by
which a form can move from one position to another. In animation, such arcs are
used extensively, for they make animation much smoother and less stiff than a
straight line for the path of action. In certain cases, an arc may resolve
itself into a straight path, as for a failing object, but usually, even in a
straight-line action, the object rotates.

In most 3D key frame computer animation systems,
the path of action from one extreme to another is controlled by the same spline
that controls the timing (slow in and out) of the inbetween values. This may
simplify computations the inbetweens but it has unfortunate effects. When a
motion is slow, with many inbetweens, the arc of the path of action is curved,
as desired. But when the action is fast, the arc flattens out: the faster the
action, the flatter the arc. Sometimes this is desirable, but more often, the
path of even a fast motion should be curved or arced. Straight inbetweens can
completely kill the essence of an action.

The spline that defines the path of action should
be separate from the spline that defines the timing or spacing of the inbetweens
for several reasons: so that the arc of a fast action doesn’t flatten out. So
that you can adjust the timing of the inbetweens without effecting the path of
action, so that you can use different splines to define the path of action and
the timing (a Cat mull – Rom spline so you can adjust it’s tension and direction
controls to get slow in and out). This technique is not common, but research is
being done in this area.

Almost everything in the natural world moves in
arcs. There are two major reasons for this:

1: Rotational Joints

Your body is made up of a series of rotational
joints, so when you move your body, it’s actually the result of your various
limbs rotating around your joints. Because of this, our movements tend to follow
arcs.  A human walk cycle is full of arcs.

2: Gravity

Gravity also causes objects to move along Arcs.
Take the example of the bouncing ball. If you throw it forward, it is also
pushed down by gravity, making it move in an arc.

Using arcs
to animate the movements of characters helps achieve a natural look because most
living creatures move in curved paths, never in perfectly straight lines.
Non-arc motion comes across as sinister, restricted or robotic. In
three-dimensional computer animation we can use software constraints to force
all or some of the motion within arcs. Even motion-captured performances can be
fine-tuned with curve editors, as long as the motion is not flattened. Arcs give animation a more natural action and better
flow. Think of natural movements in the terms of a pendulum swinging. All arm
movement; head turns and even eye movements are executed on an arc.

Arcs describe the path of movement for animation
of most creatures. It is the use of arcs that gives a character the feeling that
he is moving in harmony with the restrictions of an actual musculoskeletal
system, not some robotic linear construct. Using arcs to animate your characters
makes it easier for the eye to follow their movements by avoiding abrupt, stiff
transitions from one movement to the next.

Expressive Motion:

The visual path of action from one extreme to
another is always described by an arc. In nature, arcs are the most economical
routes by which a form can move from one position to another.

Such arcs are used extensively in animation,
since they create motion that is more expressive and less stiff than action
along a straight path.

Relevance in CGI

In Computer Animation, motion is usually
represented in a timeline view using splines (arcs). The arcs represent the
values of objects parameters at a specific moment in time. The method used for
calculating interpolated keyframe values determines the characteristic of the
arc (motion).

A linear interpolation creates
motion that is rather dull and stiff. While a spline interpolation creates
motion that is more expressive.

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