Graphics Programs Reference
In-Depth Information
This method resembles stop-motion animation, in which a puppet, along with its underly-
ing armature, is posed frame by frame. With FK, the animator moves the character into
position by rotating the joints that run the geometry.
The rotation of a joint affects the position of the
bones and joints beneath it in the hierarchy (see
Figure 9.2). If you rotate the hip up, the knee and
ankle swing up as if the character is kicking. If you
rotate the knee down, the ankle pivots down as if
this character is seated. This form of motion moves
the way you would expect it to move in hierarchies
and is, therefore, called Forward Kinematics.
IK uses a more complex, but often easier, system
of IK handles that are attached to the tip of a joint
system. The corresponding base of the IK system
is attached further up the skeleton hierarchy to a
joint determined to be the root of that IK segment.
It need not be the root joint of the entire skeleton,
though.
The bones and joints in the IK chain are affected only by movement of the IK handle.
When the handle moves, an IK solver figures out how to rotate all the joints to accommo-
date the new position of the IK
handle. Moving an IK handle
causes the bones to rotate
around their joints to accom-
modate a new position.
The effect is as if someone
grabbed your hand and moved
it. The person holding your
hand is similar to an IK han-
dle. Moving your hand causes
the bones in your arm to rotate
around the shoulder, elbow,
and wrist. As you can see in
Figure 9.3, the animation flows
up the hierarchy and is, there-
fore, called Inverse Kinematics.
Figure 9.2
In Forward Kinemat-
ics, the joints are
rotated directly.
Figure 9.3
In Inverse Kinemat-
ics, the joints rotate
in response to the IK
handle's position.
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