9

744 Chapter 13: reactor
constraints like Hinge (page 2–747) or Point-Point
(page 2–750) constraints.
Because they represent complex joints, the
rotational limits u se a complex parameterization
that can benefit from some explanation. This
sect ion covers the different axes, planes, and cones
used for that p ar ameterization. We’ll begin w ith a
simple picture of two objects connected by a rag
doll constraint; you can think of a shoulder joint,
wheretheparentbodyisthetorsoandthechild
body the upper arm:
A. Twist Axis
B. Plane Axis
C. Twist X Plane Axis (out w ard)
1. Parent Body
2. Child Body
The il lustration shows three axes of importance:
the Tw ist Axis,thePlane Axis and the Tw i st X
Plane Axis.Thetwistaxisisprobablytheeasiestto
visualize and you can do so by simply stretching
out your arm and rotating it so that your thumb
changes from pointing upwards to downwards.
Your movement is just a series of rotations along
the twist axis of the shoulder and elbow. In
general, the twist axis should follow the length of
thechildbodyinajoint.Inreactoryoucanspecify
the extent to either side of the neutral position the
constraint can tw ist; this need not be symmetrical.
A. Twist Axis (outward)
B. Plane Axis
C. Twist X Plane Axis
1. Twist Min.
2. Twist Max.
3. Twist Range
A. Twist Axis
B. Plane Axis
C. Twist X Plane Axis
1. Twist Range
The othe r two axes are interlinked a nd together
control the volume the child body may occupy. So
first we’ll look at the cone angles that allow you
to specify the volume that the child may move
through: