9

420 Chapter 12: Animation
1 and 2 each represent the root of the characters.
Both structures are suitable for forward kine matics.
The structure on the right is b est for most inverse
kinematics.
The fig ure above shows two approaches to linking
a skeletal structure. Either str ucture is suitable for
working with forward kinematics. The str uc ture
ontheright,however,isabetterchoiceforworking
with inverse kinematics.
• The root object is located near the body’s center
of mass.
• The link order more closely simulates the
connections of a real body.
The structure on the left has the arms and torso
linked to the neck. The st ructure on the right lin ks
the arms and neck to the torso, a more realistic
approach.
Wa rning: Be sure to avoid using non-uniform scaling on
objects in a hierarchy t hat will be animated using IK. You
will see stretching and skewing if you do. I nstead do
all non-uniform scaling at the sub-object level, to avoid
this problem. Use Reset XForm if you have objects that
exhibit this behavior.
Linking Objects Af ter A nimation
When you link an object to another, the link
relationship between the child and its paren t is
determined by the position, rotation and scale of
the parent and child objects when the link is made.
Imagine linking a stationary sphere to an animated
box.
• At frame 0 the box is beside the sphere.
• At frame 50 the box is 20 units away.
Original animation, with ball unlinked and stationary
while the box moves.
Linking the sphere to the box causes the sphere
to move w ith the b ox. The dist ance between the
sphere and the box depends on the frame when
the link is made. Linking the sphere on different
frames has the following e ffects:
• Link on f rame 0, and the sphere stays n ext to
the b ox as it moves.
• Link on frame 50, and the sphere stays a
distance 20 units away fro m the box as it moves.