Game Development Reference
In-Depth Information
the children joints will go with it. Be sure to only move using the X handle
(red cone) to maintain the positioning in Y and Z.
Why?
By building the leg joints in the side view, we were sure that they
were straight up and down when looking at the character from the
front (this also makes the future IK calculations cleaner). However,
creating the joints from the side also ensured that the joints were
along the X-axis. Moving them over is a pretty quick and painless
process though.
Leg IK Chain
Kinematics refers to the movement of objects or groups of objects. In
animation terms there are two types of kinematics that are used. The first is
forward kinematics
(FK), which is the process of animating a chain of joints
by rotating a parent joint, then rotating the next joint down the hierarchy,
then the next, and so on. The benefit of FK animation is that it moves body
parts in nice arches (which is how real anatomy tends to move). However, it
can sure be a slow process having to rotate each joint separately. Still, for the
upper body, FK (because of its nice arches) is a great way to animate.
The second method is
inverse kinematics
(IK). IK works by moving a child
joint (note, moving as in translating, not rotating) and the joints above it
rotating to accommodate that movement. Using IK, you could move the foot
and the hip and knee would rotate to allow the foot to get there (as opposed
to manually rotating the hip, then the knee, then the ankle to get the pose
desired). This can speed up the animation process considerably.
Of further power to the IK system is the ability to have
sticky IK
. Sticky IK
actually has a fairly intuitive, although goofy, name. What it does is allows for
the end of an IK chain to stick to a location. So when a foot at the end of an IK
chain is set on the ground, it doesn't move even if joints above it do. This means
that moving the hips up and down will automatically bend the knees, and that a
character can walk with feet that plant on the ground without sliding.
For Aegis, we will set up IK for the legs.
Step 22:
Create the leg IK chain. Select Animation>Skeleton>IK Handle Tool.
In the side-view panel click first on the hip joint (AC_L_Hip) and then a second
time on the ankle joint (AC_L_Ankle). The results should show a line between
the hip and ankle and a very large IK handle at the ankle (
Figure 10.8
). Note
that there is also a new object in the Outliner called ikHandle1.
Why?
Why just to the ankle? Why not to the toe? IK chains can be powerful, but
can be tough to control if there are too many joints along the chain. Maya
can have a hard time deciding how much to bend the joints along the
chain to solve for the IK or get to the location of the IK handle. By going
