Graphics Programs Reference
In-Depth Information
Flipping
Note
In changing the rotation of an item's pivot,
you must do so on that item's rest pose, the
frame where your character exists without
any changes to his default rigging position
whatsoever.
IK relies on a heavy-duty set of calcula-
tions. Even the best IK solvers still have
issues when things get really complicated.
The biggest problem with even the best
IK is
flipping
, where a joint will spin 180°
over the course of one frame.
Foresight and planning in both your rig-
ging and animation, coupled with a strong
IK solver such as LightWave's, will usually
keep flipping to a minimum.
I find that I can avoid most flipping by:
•
Keeping the goal object at a distance of
at least one-third the length of the bicep or
thigh away from the bicep or thigh's point of
rotation (the shoulder or hip, respectively).
•
Keeping the bicep or thigh from near-
ing/exceeding +/- 90° in the pitch axis.
•
Keeping the goal object well away from
the area
behind
the bicep or thigh as that
bone would lie in its rest pose.
•
Not trying to have my character
assume a pose that would be painful to do in
real life.
Note
You can also use this technique of altering
the Bank setting of an item's pivot rotation
to help control IK instabilities, which happen
most often when an item using IK to solve
for both heading and pitch approaches or
exceeds +/- 90° in its pitch axis.
If you notice that your character requires a
greater range of movement in the pitch axis
of an item that uses IK for both heading and
pitch, and you're getting headaches from
the IK misbehaving, you can set the item's
pivot rotation for its bank axis to +/- 90°, so
then the greater range of movement occurs
on the heading axis, which now falls in the
same plane that used to be pitch!
Figure 16-28
