8

Linking Strategy 401
ParentsMoveLessThanDescendants
Because of the way transforms are inherited
from parent to child, small adjustments to a
parent object might require you to adjust all of its
descendants. The ty pical approach to lin k ing is to
choose as your root object the object that moves
the least. Objects close to the root should move
very little, and leaf objects should move the most.
This is especially true w hen you are linking jointed
structures like robots or machinery, or intend to
use the hierarchy with
inverse kinematics (page
2–417)
.
An exception to this rule occurs when you are
using the root object as a handle. All of the
descendants of the root are just along for the
ride. Consider a tray full of objects traveling on a
conveyor belt. All the objects should be children
of the tray even though the tray moves much more
than any of the other objects.
Choosing t he R oot of a H iera r chy
You can find the best c andidate for t he root of your
hierarchybyaskingthefollowingquestion:
IfImovethisobject,shouldalloftheotherobjects
in the hierarchy move with it?
• If the answer is almost always, then you are
looking at a likely candidate for the root object.
Examples of this type of object are a torso, a
lamp base, and a tree trunk.
• If the answer is not often, t hen you are probably
looking at a child object. Examples of this
type of object are hands, lamp shades, and
tree leaves. If you move a character’s hand, for
example, its torso should not move.
Once you have a few c andidates for the root object,
you can examine them in greater detail. Use these
criteria to determine a good root object for your
hierarchy:
• Moving the root object usually has a great effect
on all other objects in the hierarchy.
• Conversely,therootobjectismostlyunaffected
by movement of other objects in the hierarchy.
• The root object is rarely animated, and is moved
or rotated primarily to place the hierarchy at
thecorrectplaceinthescene.
• The root object is a t or near the hierarchy’s
virtual center of mass.
Theobjectthatbestsatisfiesthesecriteriaisyour
root object. You then create your hierarchy with
all of the other objects as descend a nts of th at root
object.
Linki ng Objects for Inverse Kinematics
Inversekinematics(IK)usesthechildobjectasthe
driving force for the animation. IK is less forgiving
and is highly dependent on the linking strategy for
performing calculations.
You need to consider two additional principles
when linking hierarchies for use with inverse
kinematics:
• Links and pivot placement simulate real-world
joint locations.
• Choose an object near the struc ture’s center
of mass, or center of gravity, as the root of the
hierarchy. The center of mass in the real world
is the point on an object about which reactions
to external forces are applied.