9

418 Chapter 12: Animation
1. Root
2. Leaves
3. Subtree
Example of a hierarchical structure
Branch—Path through the hierarchy from a parent
toasingledescendant.Inthefigureabove,the
Support, Rotational Hub , and Ferris Wheel objects
comprise a branch from the root to the leaf objects
(the seats).
Lea f —Child object that has no children. The
lowest object in a branch. In the figure above, the
Seat objects are leaf objects.
Link —Connection between a parent and its child.
A link transmits position, rotation, and scale
information from parent to child.
Pivot—Defines the local center and coordinate
system for each object. You can t hin k of a link as
theconnectionbetweenthepivotofachildobject
and the pivot of its parent.
Link ing Strategy
Before you begin link ing any but the simplest
hierarchy you should tak e a few minutes to plan
your linking strategy. Your choices for the root of
the hier archy and how the branches grow out to
the leaf objects w ill have import ant effects on the
usability of your model.
The st rategy behind linking objects into a
hierarchy can be reduced to two main principles:
• The hierarchy follows a logical progression
from parent to child.
• Parent objects move less than their descendants.
Within these two principles you have almost
unlimited flexibilit y as to how you link your
objects. If you think about how you intend to use
the hierarchy, and link it with that use in mind,
you will rarely have a problem.
Progr ess ion f rom Pa rent to Child
Progression from parent to child means the links
do not erratically jump from object to object.
If two objects touch each other they should
probably be linked as parent and child. There is
nothing to prevent you from linking a body in
the order of: Thigh->Foot->Shin->Waist. You
would probably regret such a linking strateg y
later.Theefforttofigureouthowtotransform
objects lin ked in such a st range way would be
quite difficult. A more logical progression would
be Foot->Shin->Thigh->Waist.
Using Multiple Hierarchies
Rather than build a single bone chain from a hip to
atoe,youcanmakeonechainfromthehiptothe
ankle, and t hen a second independent chain from
the heel to the toe. You would then link the chains
together to form a complete leg assembly.
Because they are linked together, the leg and foot
chains could be considered one chain. Ho wever,
the way you animate them t reats each chain
separately, allowing fine control over the parts.
With this type of ar r angement on leg and foot
chains, the foot could be made to stay on the
ground while the leg bends. It also allows for
independent control of the foot’s rotation, pivoting
on the heel or toe, which would then cause the
knee to bend.