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418 Chapter 12: Animation

These control objects can be linked together as

well, or they can be cont rolled with constraints.

You can also use parameter w iring to build

relationships between these cont rol objects.

You can wire control objects to

manipulator

helpers (page 2–26)

or to

custom attributes (page

1–124)

, creating an easily accessible interface for

your animatable model.

You can add further controls to manipulate the

elements in the middle of the chain.

Note: In the

HI Solver (page 2–427)

,the

swivel

angle (page 2–430)

has its own manipulator, which

canbeanimatedorlinkedtoanothertargetobject.

Dif fer ences Between For war d and

Inverse K inematics

Forward kinematics uses a top-down method,

where you begin by positioning and rotating

parent objects and work down the hierarchy

positioning and rotating each child object.

Basic princ iples of forward kinematics include:

• Hierarchical linking from parent to child.

• Pivot points defining joints between objects.

• Children inheriting the transforms of their

parents.

These principles are fairly forgiving. As long as

everything is lin ked together and the pivots are

located at joint locations, you can successfully

animate the structure.

Inversekinematics(IK)usesagoal-directed

method, w here you position a goal object and the

program calculates the position and orientation

of the end of the chain. The final position of the

hierarchy, after all of the calculations ha v e been

solved, is called the IK solution. There are a variet y

of IK solvers that can be applied to a hierarchy.

Inverse kinematics starts with linking and pivot

placement as its foundation and then adds the

following principles:

• Jointscanbelimitedwithspecificpositional

and rotational properties.

• Position and orientation of parent objects are

determined b y the position and orientation of

child objects.

Because of these additions, IK requires greater

thought about how you link your objects and place

pivots. Where many different solutions for linking

objects may be suitable for forward kinematics,

there are usually just a few good solutions for any

given IK approach.

Inverse kinematics is often easier to use than

forward kinematics, and you can quickly create

complex motions. If you need to edit those

motions later, it can be simpler to rev ise the

animation if you are using IK. It also is the best

waytosimulateweightinananimation.