2009

Linking Strategy
Before you begin linking any but the simplest hierarchy you should take a
few minutes to plan your linking strategy. Your choices for the root of the
hierarchy and how the branches grow out to the leaf objects will have
important effects on the usability of your model.
The strategy 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 flexibility 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.
Progression from Parent 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 strategy later. The effort to figure out how to transform objects
linked in such a strange 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 a toe, you can make one
chain from the hip to the ankle, and then 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. However, the way you animate them treats each chain separately,
allowing fine control over the parts.
With this type of arrangement 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.
Hierarchies | 3335