8

402 Chapter 12: Animation
1 and 2 each represent the root of the characters.
Both struc tures are suitable for forward kinematics.
The structure on the r ight is best for most inverse
kinematics.
The figure above shows two approaches to lin k ing
a skeletal structure. Either str ucture is suitable for
working with forward kinematics. The structure
ontheright,however,isabetterchoiceforworking
with inverse kinematics.
• The root object is lo cated 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 rn in g : Be sure to avoid using non-uniform scaling on
objects in a hierarchy that will be animated using IK. You
will see stretching and skewing if you do. Instead 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.
Linki ng Objects Af ter Animation
When you link an object to another, the link
relationship between the child and its parent is
determined by the position, rotation and scale of
the p arent and child objects when the link is made.
Imagine linking a stationar y 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 unli nked and stationary
while the box moves.
Linking the sphere to the box causes the sphere
to move with the box. The distance between the
sphere and the box depends on the frame when
the link is made. Linking the sphere on different
frames has t he following effects:
• Link on frame 0, and the sphere stays next to
the b ox as it moves.
• Link on frame 50, and the sphere stays a
distance 20 units away from the bo x as it moves.