9

Hierarchies 417
Common U ses f or Hier a rchies
• Link a large collection of objects to a single
parent so they c an be easily animated and
transformed by moving, rotat ing , or scaling the
parent.
• Link the target of a camera or lig ht to another
object s o 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 rchy
The relationship between objects linked together
in a hierarchy is analogous to a fam ily tree.
Par ent—Object that controls one or more children.
Aparentobjectisoftencontrolledbyanother
superior parent object. In t he follow ing figure,
objects 1 and 2 are p arent objects.
Child—Object controlled by its parent. A child
object can also be a parent to other children. In the
followingfigure,objects2and3(thesupportand
hub) are children of object 1. Objects 5 (the seats)
are children of object 4, the Ferris wheel.
Ancestors—Parent and all of the parent’s p arents of
a child object. In the following figure, objects 1
and 2 are ancestors of object 3.
TheseatsoftheFerriswheelarechildrenofthewheel,which
is in turn a child of the base and support objects, as shown in
the following hierarchy.
Descendants—Children and all 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 st ructure.
Root —Single parent object that is superior to all
other objects in the hierarchy. A ll 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.