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446 Chapter 12: Animation
His tor y -Independent (HI ) IK
Solver
The HI (History-Independent) Solver does
not rely on IK s olutions calculated in previous
keyframes in the timeline, so it is just as fast to use
at fr ame 2000 as it is at frame 20.
The HI Solver uses a goal to animate a chain. You
animate the goal and the IK solver attempts to
move the end effector (the pivot point of the last
joint of the chain) to match the position of the
goal. Often the go al is parented to other control
objectssuchaspointsordummies,splinesor
bones, and these control objects in turn are wired
to viewport or rollout sliders.
The IK solution takes place in a plane, kno wn as
the solver plane. The angle of the solver plane in
world space is controlled by a parameter ca l led
the sw ivel angle (page 2–449).Theswivelangleis
animatable.Youcanadjustitdirectly,orwitha
manipulator.
TheHISolverallowsforthecreationofmultiple
or overlapping chains. This allows you to create
multiple goals for additional controls. By linking
the goals to points, splines, bones or dummies,
you can create simple controls to animate complex
chains or hierarchies. You can also use constraints
on these goals or control objects, as another
animation tool.
Applying an HI Solver
To a pply a n H I S olve r to a ny p a r t o f a h ier archy
selecttheboneorobjectwhereyouwanttothe
solver to start. Then choose Animation menu > IK
Solvers > HI Solver. In the active viewport move
your cursor to the bone where you want the chain
to end. When you click to select that bone, the
goal is drawn at the pivot point of that bone. If
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 b ones, a sma ll "nub" bone is
automatically created at the end of the chain to
assist in t his process.
Setting Up Multiple Chains
To r ig 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 s pace. This means
it will keep the foot planted on the ground as the
character’s body moves. All three IK chains in this
hip-to-toe setup place goals at key posit ions i n 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, ball of foot, and toe. Use the IK goals to raise
theheel,bendthetoe,moveandrotatetheentire
foot, and maintain the foot’s position in space.
Over l appi ng Cha ins
The IK solver system allows you to create
overlapping IK chains in a single hierarchy. In a
human leg, for example, you could create a chain
running from the hip to the ankle, then a second
chain from the kn ee to the bal l of the foot, and a
third from the ankle to the toes. Use overlapping
chains, when you want to apply goals to sequential
bones, but you don’t want to refine the bones.