Graphics Reference
In-Depth Information
parts, would not possess the capacity for such a hierarchy of action. We may
consider those subjects that do have articulation and are capable of using
their different parts independently of one another as having the potential
for a hierarchy of action. These I have divided into three areas:
primary action,
secondary action,
and
tertiary action
. Each of the separate parts of a subject
may possess its own range of movements, some of them capable of primary
motion, others only having the ability to move as tertiary action. Arms are
usually able to lift objects; the tails of some animals can't. Each of the different
elements of an object or a living being may also demonstrate different kinds
of movement as they react to the external forces that act on them. Consider
the effect that a light breeze has on hair; this effect will be very different from
the breeze's effect on the head on which the hair grows.
The hierarchy of actions that I set out here is not necessarily set or fixed; it can
shift throughout a particular movement. During one phase the primary action
may be located in one part of a figure. As the action progresses and changes,
the source of the primary action may also shift and change.
Primary Action
The term primary action is an attempt to describe the types of actions and the
sources of those actions that
drive
a particular movement. For some types of
movement, the primary source of the action may be constant throughout the
action; for other actions the primary action may change and shift throughout
the movement.
Let's take a look at a couple of examples. First, consider a figure walking
in a regular fashion on a flat and even surface. The primary action and the
source of the movement in this walk are located in the figure's legs and
hips. A human figure walking could be described as controlled falling. The
balance of the figure is thrown forward (for forward motion) and each leg
in turn is swung forward from the hip until the foot is placed on the floor,
which prevents the figure from falling to the ground. Parts of the figure other
than the legs may be in motion during a walk, but they are not the source
of this form of locomotion. It is only the legs swinging forward and the hips
rotating that provide the primary source of action. Certainly motion in other
parts of the anatomy during the walk may be considered primary, but for the
animator's purposes it might be useful to limit the attention to the legs. The
primary source of action throughout a regular walk cycle is located in the hips
and legs and for the most part will not change or shift throughout the action.
Now let's consider the action of a human figure lifting a heavy object from
the floor. The principle action will shift throughout the animation of the lift.
Initially it may be located in the legs and hips as the figure bends down to
prepare for the lift. As the dynamics of the lift progress, this action will shift
to the arms as the object is picked up. This action may then transfer back to
the legs as the weight is lifted, though it may be transferred to the spine as
