9

Using Dummy Ob jects 429
Manipulating the hierarchy of a leg.
Consider the linked man nequin in the figure. If
you want to position the mannequin’s right foot to
rest on top of the soccer ball beside it, you perform
the following steps:
1. Rotate the r ight thigh s o the entire leg is above
the soccer ball.
2. Rotate the right shin so the foot is near the top
of the soccer ball.
3. Rotate the rig ht foot so it is parallel with the top.
4. Repeat steps 1 through 3 until the foot is
properly placed.
Youalwaysstarttransformingobjectsatthe
highest-level parent affected by the mot ion and
workyourwaydownthehierarchytothelastchild.
You have considerable control over the exact
placement of every object in the hierarchy using
forward kinematics. However, the process
can become tedious with large and complex
hierarchies. In such situations, you might wan t to
use inverse kinematics (page 2–435) .
Usi ng Dummy Objects
Theprimaryuseofdummy helper objects (page
2–16) is to assist in creating complex motions and
building complex hierarchies. B ecause dummies
are invisible when rendered, they are an excellent
choice for offset joints, connectors between
objects, and handles for complex hierarchies.
Dummies and Points (page 2–23) can act as null
objects that function as controls for transforming
parts of an IK chain.
Usi ng a Dummy t o Contr ol M oti on
Breaking complex motions into simple
components often makes it easier to go back and
edit your animations.
Consider a bouncing ball that moves along a path.
Yo u c o u l d a n i m a t e t h e b a l l b y p o s i t i o n i n g i t o n
many f r ames, but it would b e very difficult for you
to go back and adjust the height of the bounce or
the path of the ball. You have to edit the mot ion
of the ball on many frames to make even a simple
change.
Usingadummyobjectsolvesthisproblemby
breaking the motion into simple components. One
component is the up and down bounce of the ball.
The other is movement on the path.
Combining the bouncing motion of a ball with the forward
motion of a dum my results in a moving bouncing ball.