8
Hierarchies 399
One of the most useful tools in producing
computer animation is the ability to link objects
together to form a
chain
.Bylinkingoneobjectto
another, you create a parent-child relationship.
Transforms applied to the parent are also
transmitted to child objects. A chain is also
referred to as a
hierarchy
.
You can find the commands to build and
manipulate hierarchies in the fo llowing places in
the interface:
•The
Bones system (page 1–381)
on the Create
panel is used to create a hierarchy of bones.
Yo u c a n a l s o c r e a t e b o n e s b y c h o o s i n g
Bone
Tools (page 1–388)
from the
Character menu
(page 3–729)
.Youcanturnanyhierarchyof
objects into bones by selecting the hierarchy
and turning on Bone Tools dialog > Object
Properties rollout > Bone On.
•The
Hierarchy panel (page 3–816)
contains
commands to control how links behave.
•The
Motion panel (page 2–445)
contains
commands to control how links behave when
usinganHistoryDependent(HD)Solver
Common Us es for H ier ar chies
• Link a large collection of objects to a single
parent so they can be easily animated and
transformed by moving, rotating, or scaling the
parent.
• Link the target of a camera or light to another
object so it tracks the object through the scene.
• Link objects to dummy objects to create
complex motions by combining multiple simple
motions.
• Link objects to simulate jointed structures to
animate characters or mechanical assemblies.
Parts of a Hier a r chy
The relationship between objects linked together
in a hierarchy is analogous to a family tree.
Parent —Object that controls one or more children.
Aparentobjectisoftencontrolledbyanother
superior parent object. In the following figure,
objects 1 and 2 are parent objects.
Child—Object controlled by its parent. A child
object can also be a parent to other children. In the
following figure, objects 2 and 3 (the support and
hub) are children of object 1. Objects 5 (the seats)
are chi ldren of object 4, t he Ferris wheel.
Ancestors—Parent and all of the parent’s parents of
a child object. In the following figure, object s 1
and 2 are ancestors of object 3.
TheseatsoftheFerriswheelarechildrenofthewheel,
whichisinturnachildofthebaseandsupportobjects,as
shown in the following hierarchy.
Descendants—Children and a ll of the children’s
children of a parent object. In the figures, all the
objectsaredescendantsofobject1.
Hiera rchy—Collection of all parents and children
linked together in a single structure.
Root —Single parent object that is superior to al l
other objects in the hierarchy. All other objects
are descendants of the root object. In the figures,
Object 1 is the root.
Subtr ee—All the descendants of a selected parent.
In the figure below, the Rotational Hub, Ferris
Wheel, and Seats represent the subtree under the
Support object.