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398 Chapter 12: Animation
expression. Since the w ired pair can be animated
through either parameter, the choice of master
parameter is essentially just a convent ion.
Transfer Expression boxes
Underneath the parameter trees are the transfer
expression text boxes. These expressions
determine how changes to each parameter affect
the other and are usually i nverses of one another.
For more information on expression syntax, see
the topic “ Script Controllers” in the
MAXScript
Reference
.
When the p arameters are first wired, the default
expressions are simple 1-to-1 links between the
parameters. These expressions can be edited
into any valid script fragment that will yield a
result of the correct type for its parameter. For
example, if you link a parameter such as height
(which contains a float v alue) to a parameter such
as position (which cont ains a point3 value), your
expressions must include conversions that produce
the same output value type.
For one-way wires, the Expression box for the
controlling parameter is unavailable, since there is
no W ire controller assigned to it.
For two-way wiring, both transfer expression text
boxes will be enabled. It is possible for the user to
supply transfer expressions for the two parameters
that are not inverses of one another, but this is of
course discouraged, since the relationship of the
parameters will be different depending on which
one is changed first.
Hier ar chies and Ki nemati cs
When animating characters, mechanical
assemblies, or complex motion, you can simplify
the process by lin king objects together to form
a
hierarchy (page 3–1045) or chain. In a linked
chain, the animation of one member can affect
some or all of the others, ma king it possible to
animate a number of objects or bones at once.
The term
kinematics (page 3–1054) describes the
move ment or anim ation of the chain. There a re
two types of kinematics:
With
forward kinematics (page 3–1038) (FK) ,
you manipulate the top of the hierarchy to animate
theentirechain.
With
inverse kinematics (page 3–1052) (IK),
youmanipulateanobjectatthebottomofthe
hierarchy to animate the entire chain. Frequently,
IK is also used to make an object “stick” to the
ground or another surface, while allowing the
chain to rotate off the pivot of that object.
Forward kinematics is the most straightforward
method for animating h ierarchies. Inverse
kinematics requires more setup than forward
kinematics, but is more intuitive for complex tasks
such as character animation .
Hierar chies
Bipedal figure with part of its hierarchy diagrammed in the
backgroun d