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3. With the spaceship selected, choose Animation⇒Constraints⇒Path Constraint. Then click the animation path to select it as the path to follow. Select the Follow option in the Path Parameters rollout, and choose the Y-Axis option.

4. Click the Play Animation button in the Time Controls to see the spaceship follow the path.

Figure 22.3 shows the spaceship as it moves between the asteroids.

FIGURE 22.3

The spaceship object has been attached to a spline path that it follows.

Position constraint

You can use the Position constraint to tie the position of an object to the weighted position of several target objects. For example, you could animate a formation of fighter jets by animating one of the jets and using Position constraints on all adjacent jets.

The Position constraint menu option lets you select a single target object, enabling you to place the pivot points of the two objects on top of one another. To add another target object, click the Add Position Target button in the Position Constraint rollout in the Motion panel. This button enables you to select another target object in the viewports; the target name appears within the target list in the rollout.

If you select a target name in the target list, you can assign a weight to the target. The constrained object is positioned close to the object with the higher weighted value. The Weight value provides a way to center objects between several other objects. The Keep Initial Offset option lets the object stay in its current location, but centers it relative to this position.

Figure 22.4 shows a sled positioned between four tree objects using the Position constraint. Notice how the weight of the downhill tree object is weighted higher than the other targets and the sled is close to it.

FIGURE 22.4

You can use the Position constraint to control the position of an object in relation to its targets.

Link constraint

The Link constraint can transfer hierarchical links between objects. This constraint can cause a child's link to be switched during an animation. Any time you animate a complex model with a dummy object, the Link constraint makes it possible to switch control from one dummy object to another during the animation sequence. This keeps the motions of the dummy objects simple.

The Link Params rollout includes Add Link and Delete Link buttons, a list of linked objects, and the Start Time field. To switch the link of an object, enter for the Start Time the frame where you want the link to switch, or drag the Time Slider and click the Add Link button. Then select the new parent object. The Delete key becomes active when you select a link in the list.

Note

If you create a link using the Link constraint, the object is not recognized as a child in any hierarchies. •

All links are kept in a list in the Link Params rollout. You can add links to this list with the Add Link button, create a link to the world with the Link to World button, or delete links with the Delete Link button. The Start Time field specifies when the selected object takes control of the link. The object listed in the list is the parent object, so the Start Time setting determines when each parent object takes control.

The Key Mode section lets you choose a No Key option. This option does not write any keyframes for the object. If you want to set keys, you can choose the Key Nodes options and set keys for the object itself (Child option) or for the entire hierarchy (Parent option). The Key Entire Hierarchy sets keys for the object and its parents (Child option) or for the object and its targets and their hierarchies (Parent option).

This constraint also includes the PRS Parameters and Key Info rollouts.

Caution

You cannot use Link constraints with Inverse Kinematics systems. •

Tutorial: Skating a figure eight

For an animated object to switch its link from one parent to another halfway through an animation, you need to use the Link constraint. Rotating an object about a static point is easy enough: Simply