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428 Chapter 12: Animation
you want a goal at the far end of the bone, refine
thebonewhereyouwanttogoaltobeplaced.An
extrabonewillbeadded,andthenchoosingthat
bone allows you place the goal at the end.
When you create bones, a small "nub" bone is
automatically created at the end of the chain to
assist in this process.
Setting Up Multiple Chains
To rig a skeleton for a human leg you could use
three chains in one leg, as follows:
• The first chain is created from the hip to the
ankle. This chain controls the overall leg
motion including bending of the knee.
• The second chain is created from the ankle to
the ball of the foot. This chain controls the
heel’s up and down motion.
• The third chain is created from the ball of the
foot to the toe.
When the three chains work together they help to
maintain the foot’s position in space. This means
it w ill keep the foot planted on the ground as the
character’s bo dy moves. All three IK chains in this
hip-to-toe setup place goals at key positions in the
foot that mimic natural foot behavior. In real life,
thetoe,ballofthefoot,andheelcanbeplanted
on the ground or raised.
Each chain has an goal that drives motion on the
heel, bal l of foot, and toe. Use the IK goals to raise
theheel,bendthetoe,moveandrotatetheentire
foot, and maintain the foot’s posit ion in space.
Over la pping Chains
The IK solver system allows y ou to create
overlapping IK chains in a single hierarchy. In a
human leg, for example, you could create a chain
runningfromthehiptotheankle,thenasecond
chain from the knee to the b all of the foot, a nd a
third from the ankle to the toes. Use overlapping
chains, when yo u want to apply goals to sequential
bones, but you don’t want to refine the bones.
Another good use for overlapping HI IK chains is
to keep the goa ls at the bottom of the hierarchy
firmly rooted in place. In the case of the leg
example, the an kle, ball and toe of the foot would
notmoveuntiltheupperportionofthehierarchy
had reached its full stretch.