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Manual - Blender 2.44

[edit] Locked Track Constraint

Mode: Object Mode and Pose Mode

Panel: Object Context → Constraints

Hotkey: F7

[edit] Description

Locked Track is a difficult constraint to explain, both graphically and verbally. The best real-world example would have to be a compass. A compass can rotate to point in the general direction of its target, but it can’t point directly at the target, because it spins like a wheel on an axle. If a compass is sitting on a table and there is a magnet directly above it, the compass can’t point to it. If we move the magnet more to one side of the compass, it still can’t point at the target, but it can point in the general direction of the target, and still obey its restrictions of the axle.

When using a Locked Track constraint, you can think of the target object as a magnet, and the affected object as a compass. The “Lock” axis will function as the axle about which the object spins, and the “To” axis will function as the compass needle. Which axis does what is up to you! If you have trouble understanding the buttons of this constraint, read the tool-tips; they’re pretty good. If you don’t know where your object’s axes are, turn on the Axis button in the Draw panel, Object context (F7). Or, if you’re working with bones, turn on the Draw Axes button, Armature panel, Editing context (F9).

This constraint was designed to work cooperatively with the Track To constraint. If you set the axes buttons right for these two constraints, Track To can be used to point the axle at a target object, and Locked Track can spin the object around that axle to a secondary target.

This constraints also works very well for 2D billboarding.

This is all related to the topic discussed at length in the tracking tutorial.

[edit] Options

The Locked Track Constraints panel.

Target

The name of the target object that the tracking object tracks.

VG: This is only available when tracking to a mesh object. If you type in the name of a vertex group of the target, it will be the center of this vertex group that will be the “physical” target, not the object’s center.
BO: This is only available when tracking to an armature object. If you type in the name of a bone of the target, it will be this bone that will be the “physical” target, not the armature’s center.

To: The tracking axis. It shouldn’t be the same as the Lock axis.

Lock: The locked local axis.

Influence: This controls how accurately the tracking object tracks the target. 0 means that the constraint is turned off. The tracking object will remain locked in orientation. 1 means tracking is completely on and the tracking axis will stay tightly focused on the target.

Show: This adds an influence Ipo channel to the constraint if one is not present. You can then add keys to the channel.

Key: This adds animation keys to the influence Ipo channel. This is a very powerful combination. For example, you could have a camera with a Locked Track constraint applied and have input driving the influence channel.