From BlenderWiki

Track To Constraint

Description

The Track To constraint applies rotations to its owner, so that it always points a given “To” axis towards its target, with another “Up” axis permanently maintained as much aligned with the global Z axis (by default) as possible. This tracking is similar to the "billboard tracking" in 3D (see note below).

This is the preferred tracking constraint, as it has a more easily controlled constraining mechanism.

This constraint shares a close relationship to the IK Solver constraint in some ways. It is very important in rig design, and you should be sure to read and understand the tracking tutorial, as it centers around the use of both of these constraints.

Billboard tracking

The term “billboard” has a specific meaning in real-time CG programming (i.e. video games!), where it is used for plane objects always facing the camera (they are indeed “trackers”, the camera being their “target”). Their main usage is as support for tree or mist textures: if they were not permanently facing the camera, you would often see your trees squeezing to nothing, or your mist turning into a millefeuille paste, which would be funny but not so credible…

Options

Targets

This constraint uses one target, and is not functional (red state) when it has none.

(Bone Target)

When using a bone target, you can chose where along this bone lays the target point, using the Head/Tail numeric field.

Target

By default, the owner’s Up axis is (as much as possible) aligned with the global Z axis, during the tracking rotations. When this button is enabled, the Up axis will be (as much as possible) aligned with the target’s local Z axis…

To

The tracking local axis (Y by default), i.e. the owner’s axis to point at the target. The negative options force the relevant axis to point away from the target.

Up

The “upward-most” local axis (Z by default), i.e. the owner’s axis to be aligned (as much as possible) with the global Z axis (or target Z axis, when the Target button is enabled).

CSpace

This constraint allows you to chose in which space evaluate its owner’s and target’s transform properties.

If you chose the same axis for To and Up, the constraint will be no more functional (red state).

Examples

Cubes

(Track To constraint example) is an example of a cube using the TrackTo constraint. Cube A is tracking Cube B, where L is the tracking line. Notice how the tracking object’s local axes are visible by using the Draw panel’s Axis button. You can clearly see the tracking To and Up axes. Cube A’s constraint setting are reflected in (The Constraints panel of the example). X is the To axis, and Z the Up one.

You can also see in (The Constraints panel of the example) what Cube A is tracking by looking at the Target field: Cube B. You can redirect tracking to another object simply by entering in this field the name of another object.

A jack

In (A jack rig created with Track To constraints), the jack animation and position is controlled by two Track To constraints (sample blend file):

• The two empties are parented to the two armature bones, that control the whole arm.
• Each of the two pieces of the jack are parented to an empty, and tracking to the other empty. This way, each piece is stuck to its part of the arm, and always points at the other piece. This is enough to have a simple jack – even though it is not actually an actuator, as this is the armature which controls everything…

Eyes tracking

This constraint is also very useful to rotate the eyes of a character to make it look at a fixed point, or follow a moving object. Obviously, the same can be done for cameras or lights.