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Activating Joint Axes 485
paste them. This is very useful when pasting from
one side of an object to another, such as left arm
joints to a right ar m.
You can also copy joint settings from a non-IK
controller to an IK controller, but you can’t copy
from an IK controller to a non-IK controller.
Activati ng J oint A xes
You set whether an object can move or rotate
aboutagivenaxisbyusingtheActivecheckbox
in the joint rollouts. Joints have a maximum of
six possible axes: three for rotation and three for
position. You constrain the motion of a joint by
setting w hich axes are act ive.
• A joint with all axes active can move and rotate
freely, independent of its parent.
• A joint with all ax es inactive is locked to its
parent and cannot move independently.
• ThesettingofIKjointaxesoverridesInherit
and Lock settings on the Link Info rollout.
Unders tanding Joint Ax is Or ientation
IK joint axes for an object are defined by the Lo c a l
axes of the object’s p arent. That means if you
activate the X axis of an object’s rotational joint
parameters, the object rotates about its parent ’s X
axis, not its ow n X axis.
You might have a problem when you set joint
parameters for an object whose local coordinate
system is oriented 90 degrees from its parent’s
coordinate system. In such a c ase, the rotation
angleaboutoneaxisbecomesindeterminate.The
resultisthatoneaxiswilloftencauserotation
about one of the other two axes.
The follow ing techniques will make setting joint
parameters easier and more successful:
• Assemble IK hierarchies with objects aligned
along the World axes.
• Consider using the Adjust Pivot (page 2–488)
functions to align your object pivots with
either the World axes or the root object of the
hierarchy.
•SetthetransformmanagerstoParent
coordinate system and Use Pivot Point Center,
while setting joint parameters. This helps
you see the orientation of the joint axes by
displaying the parent axis icon at the selected
object’s pivot point.
• When activating joint axes, drag the From
spinner up and down. This causes the object
to mov e or rotate about the active axis and is a
quick check that you chose t he right axis.
Activating R otati onal J oints
1. Rotational links
When you turn on Active for one of the X, Y, Z
axes of a rotational joint, the object can rotate
about that axis of its parent’s coordinate system.
Joints that rotate about multiple axes are very
common. A ba ll joint, like your shoulder, is a
rotational joint active about all three axes. A pin
joint, like your elbow, is a rotational joint active
onasingleaxis.