Graphics Programs Reference
In-Depth Information
given character is directly proportional to
the complexity of the animation you want to
achieve. Sound confusing? Think of it this
way: A single bone can be used to animate a
hand waving back and forth. But if you want
that hand to perform sign language, you'll
need a lot more bones.
Complex character animation can often
yield dozens (and in some cases hundreds)
of bones. These bones can be arranged in
any number of ways. The person responsi-
ble for their arrangement is known as a
rigger
and the set of controls used to manip-
ulate them is known as a
rig
.
Inverse Kinematics,
Forward Kinematics, and IK
Booster
Every character rig employs a form of
motion control known as
kinematics
.So
what the heck is a “kinematic” and why
would anyone care whether it's “forward,”
“backward,” or somewhere in between?
Kinematics refers to the study of
mechanics concerned with motion. Where
3D animation is concerned, kinematics
defines the way in which you manipulate a
system of hierarchical (parent/child) items.
FK (Forward Kinematics)
Forward kinematics
(most often referred to
simply as FK) is the “old reliable” way of
animating. It is labor-intensive, but there
are no surprises — everything that's in an
animation must be put there, on purpose,
by the animator.
With FK, if you wanted to make the tip
of the character's “forearm” reach the null
object (represented as the “+” in Figure
16-1), you'd have to first rotate the bicep
and then the forearm, evaluate how close
you got to your objective, and then refine.
As you can see in the figure, the tip of
the forearm overshoots the null. I'd have to
go back and tweak the rotations of the bicep
and forearm again and again (and probably
again).
As you can see, there are no surprises in
this kind of animation — everything that's
done must be done on purpose by the ani-
mator, and as such, it is a painfully slow way
to work with a character's rig.
Figure 16-1: Forward kinematics. Here, you see a hierarchical set of bones that could easily be a
character's bicep, forearm, and hand.
