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436 Chapter 12: Animation
goal and the root, without keys being applied to
the individual chain objects.
With inverse kinematics you can quickly set
up and animate complex motions. The basic
procedure involves these tasks:
• Building a mo del. It could be a jointed structure
or man y pieces or a single continuous surface.
• Linking the jointed model together and defining
pivot points, as described in Hierarchies (page
2–416).
For a continuous-surface model, create a Bone
(page 1–404) structuretoanimatetheskinof
the character.
•ApplyingIK solvers (page 2–440) to either the
join ted hierarch y or to the bones. You will
probably create several IK chains throughout
the hierarchy, r ather than just one. You might
also create several independent hierarchies,
rather than linking everything together in one
large hierarchy.
•Definingjoint behavior (page 2–483) at the
pivot points, setting limits or preferred angles,
depending on the type of IK solvers you are
using. Here you can set up sliding joints or
rotating joints. You might also need to move
the root of the hierarchy, as well. You might also
wanttoaddcontrolobjectssuchasdummies
at this point.
• Animatingthegoal(inthecaseofanHI Solver
(page 2–446) or IK Limb solver (page 2–472))
or the end effector (in the case of the HD
Solver (page 2–461)). This animates all the
components of the IK chain.
Yo u c a n a p p l y constraints (page 2–392) to the
goals or control objects or to the root of a chain.
You can externally reference IK chains in your
scene. An XRef IK chain behaves the same as a
non-XRef chain, except that you cannot retarget
its XRef controller (page 2–383) once it is in your
master scene. For more information, see XRef
Objects (page 3–394).
Control Objects to Assist IK
Yo u c a n l i n k a g o a l o r a n e n d e f f e c t o r t o p o i n t s ,
splines, or dummy objects that serve as quick
controls to translate or rotate the end of the chain.
These control objects can be linked together as
well, or they can be controlled with constraints.
You can also use parameter wiring to build
relationships between these control objects.
You can wire control objects to manipulator helpers
(page 2–27) or to custom attributes (page 1–129),
creating an easily accessible interface for your
animatable model.
You can add further controls to manipulate the
elements in the m iddle of the chain.
Note: In the HI Solver (page 2–446),thesw ivel angle
(page 2–449) has its own manipulator, w hich can
be animated or linked to another target object.
Dif fer ences B etween Forwar d a nd
Inverse K inematics
Forward kinematics uses a top-down method,
where you begin by posit ioning and rotating
parent objects and work down the hierarchy
positioning and rotating each child object.
Basic principles of forward kinematics include:
•Hierarchicallinkingfromparenttochild.
• Pivot points defining joints be tween objects.
• Children inheriting the transforms of their
parents.
These principles are fairly forgiving. As long as
everything is linked together and the pivots are
loca ted at joint locations, you can successfully
animate the structure.
Inverse kinematics (IK ) uses a goal-directed
method, where you position a goal object and the