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Parents Move Less Than Descendants
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 typical approach to linking 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 when you are linking jointed structures like robots or
machinery, or intend to use the hierarchy with
inverse kinematics on page
3374.
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 the Root of a Hierarchy
You can find the best candidate for the root of your hierarchy by asking the
following question:
If I move this object, should all of the other objects 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, then 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 candidates 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, the root object is mostly unaffected 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 the correct place in the scene.
3336 | Chapter 15 Animation