Graphics Reference
In-Depth Information
as long as it is somewhere down the center line of the
armature and is easily accessible, away from others. In
Figure 9.11
, you can see where I've placed mine: the
tail of the bone behind the character at waist level
with the head meeting the base of the spine. Select
the lower spine bone, the pelvis, and both upper leg
bones, then Shift-RMB select this new control bone.
Use Ctrl-P (with Keep Offset) to make all of these
selected bones the disconnected children of the control
bone. Name
the bone
something
like “body_
control.”
Jump into Pose mode (Ctrl-Tab), select the bone, and
transform it. The entire armature should happily
follow along.
Rigging with Constraints
While you're in Pose mode, bring up the tool shelf
and enable
Auto IK
in the
Pose Options
section.
Select one of the lower arm bones and use the G key
to move it. Because the lower arm is the connected
child of the upper arm, it shouldn't be able to trans-
late—only rotate. However, using IK (which stands
for inverse kinematics) allows you to manipulate a target and have other bones automatically adjust them-
selves to follow that target. If you spent any time playing with our rudimentary armature in Pose mode
(which you should have done, because playing around is a huge part of learning), you'll see that this is a
significantly different approach to armature manipulation.
Figure 9.11
The body control bone.
Without IK involved, you are using what is called forward kinematics, FK. With FK, a pose is built from
the base of the chain upward, the user defining the rotation of each bone by hand along the way. IK
works in reverse. The
last
bone in the chain tries to reach the IK target, which can be another bone or
some object outside of the armature, and that motion propagates
down
the chain, eventually reaching the
head of the base bone.
When animating, certain types of motion are more easily and realistically achieved with IK, and some
are better suited to FK. Here's the rule: Limbs that bear weight or support the rest of the armature
use IK; everything else uses FK. For a human, the solution appears simple: IK for the legs and feet
(they support the body), and FK for everything else. True enough. But what if our character leans on a
table, or puts his or her hand against a wall? In those cases, the hand would also be bearing weight, so
you would want to use IK. So, we'll work toward constructing a rig that uses IK for the legs and feet
and FK for the main body and arms, but gives us the option of using IK on the arms and hands if we
need to.
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