You assign a
Link constraint to
an object to animate links from one parent to another. You use a
link constraint instead of using the regular Select and Link and
Unlink Selection buttons on the toolbar. (See
An example of using a
link constraint is to pass a ball from one hand to another. Assume
that at frame 0 the ball is in the first hand. The hands are animated
to meet at frame 50 and then spread apart until frame 100.
To animate the links for the ball:
- On the Motion
panel, assign a Link constraint as the ball's Transform controller.
You can also put a Link constraint on the ball from the Animation
menu by choosing Constraints Link Constraint.
- Go to
frame 0, then on the Motion panel, click Add Link, and click the
hand holding the ball. The ball will now move along with the hand,
as if it were linked to it.
- Drag the time slider to frame 50, where
you want the second hand to pick up the ball, click Add Link, then
click the second hand. From this frame on, it's as if the ball were
linked to the second hand.
When you play back the
animation, the ball travels with the first hand until frame 50,
where you added the second link, then the ball is passed to the
second hand for the rest of the animation.
Robot arms pass a ball
from one hand to the other.
Adding and Deleting Links
You add and delete links
on the Motion panel. Expand the Link Parameters rollout and click
Add Link or Delete Link.
- Click Add Link then click the object
that you want to link to as a parent. The frame at which you add
the link is the frame at which control is passed. You can change
the link frame with the Start Time parameter.
- Click the name of a parent object in
the list, and then click Delete Link to remove the link.
Properties of the Link
- The Link constraint respects the link
inheritance settings applied to the child object.
- The object using a Link constraint is
not a true child object. It does not appear in the subtree of any
linked parent objects.
- Objects with Link constraint do not participate
in IK solutions.
Link to World
You can also link an
object to the world using the Link to World button. This will keep
the object stationary without the use of a dummy object. Just click
Link to World and the world is automatically entered as a Target.
You can choose between
three different key modes, which determine how
keyframes are written on the linked objects as part of the link
constraint. These options provide the following:
- No Key Mode
No keys are created any of
the objects involved. No keys will be visible in the track bar.
- Key Nodes
Sets keys for some of
the objects. Child applies keys to the child
object only. Parents applies keys to both parents
and the child object.
- Key Entire Hierarchy
This applies keyframes
to the chosen nodes and their entire hierarchies. Child keys
the chosen object and the nodes in its hierarchy up to the world. Parents keys
both parents and the child and all three hierarchies up to the world.
Side Effects of the Link
The Link constraint works
to keep a child object from jumping position at the time when the
link changes from one parent to another parent.
Considering the previous
example, the following should hold true:
- During frames 0 to 50 the ball remains
constant relative to the first hand.
- During frames 50 to 100 the ball remains
constant relative to the second hand.
- At frame 50, the time when link control
changes, the ball does not jump.
If you change the animation
of the second hand at frame 75, it affects the position of the hand
relative to the ball at the time of the link (frame 50). This change
in relative position affects the ball over all frames where it is
linked to the second hand. Therefore, as you change the position of
the hand at frame 75, the child's position will also change, possibly
in a counter-intuitive way. However, when playing back the animation
the above three rules will hold true.