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Tracking

Mode: Object mode

Panel: Object » Constraints

Hotkey: CtrlT

Menu: Object » Track » Make Track

Description

Tracking consists of one object watching another. The watcher is the “Tracker” and the watched is the “Target”. If the target moves the tracker rotates; if the tracker moves the tracker rotates. In both cases the tracker maintains a constant heading towards the target.

Options

Make Track menu.

To make one or more objects track another object (the target), select at least two objects and press CtrlT. The active object becomes the target and the others objects the trackers. The (Make Track menu) provides several options for creating the initial tracking:

TrackTo Constraint

The Track To constraint applies rotations to the object, so that it always points a local “To” axis towards the target object, with another local “Up” axis permanently maintained as vertical as possible. This tracking is similar to the billboard tracking in 3D. This is the preferred tracking constraint, because it has a more easily controlled constraining mechanism.

The “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 weren’t 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…


This constraint shares a close relationship to the IK constraint in some ways. This constraint 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.

The controls for changing the “Tracking” and “Up” axis of a tracker object are located in the Constraints panel, in Buttons window, Object context (F7), Object sub-context. The constraint fields are:

To
The axis of the object that has to point to the target.
Up
The axis of the object that has to be aligned (as much as possible) with the world Z axis. An Align: Target button, when enabled, uses the coordinates of the target object's Z axis, thus tilting or rocking the object as it tracks the target.
Head/Tail - With Bone targets only
A number from 0.0 to 1.0 that represents the place on the target bone to Track to (0.0 = the bone's root; 1.0 = the bone's head)
TrackTo example.
The Constraints panel of the example.

(TrackTo example) is an example of a cube using the TrackTo constraint. Cube “A” is tracking “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” axis. Cube “A”’s constraint setting are reflected in (The Constraints panel of the example). +X is the “To” axis and Z is the “Up” axis.

You can also see in (The Constraints panel of the example) what cube “A” is tracking by looking at the Target field. We can see that cube “A” is tracking cube “B” because cube “B”’s name is “Cube”. You can redirect tracking to another object simply by entering in the name of another object.

LockTrack Constraint

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.

The controls for changing the tracking “To” and “Lock” axis are located in the Constraints panel. This panel is located in the same place as the Anim settings and Draw panels (Object context, F7). The constraint fields are:

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.

For further details see TrackTo constraint. LockTrack and TrackTo are very similar where the former has a “Lock”ed axis verses an “Up” axis.

Old Track

Old Track “constraint”.

This is an older algorithm prior to version 2.30, and is similar to Track To constraint in that no axis is locked. This algorithm merely tries to keep a “To” axis pointed at the target. The tracking object will usually end up in an odd orientation when this constraint is first applied. In order to get correct results use AltR when applying or changing the tracking or “Up” axis. However, the preferred method to use is Track To constraint.

Let’s assume you have nonetheless selected Old Track in the dialog with two cubes selected; see (Old Track “constraint”). By default the inactive object(s) track the active object so that their local +Y axis points to the tracked object. Cube “A” is tracking cube “B” using the Old Track constraint. You can see that “A”’s +Y axis is pointing at “B” but at an odd orientation. This typically happens if the object already has a rotation of its own. You can produce correct tracking by canceling the rotation on the tracking object using AltR.

The orientation of the tracking object is also set such that the chosen “Up” axis is pointing upward.

Setting track axis.

If you want to change this you need to get to the Anim settings panel where Old Track’s settings are accessed. First select the tracking object (not the target) and change the Button window to Object context by clicking the icon (Manual-Part-II-ObjectContext F7.png), or F7; see (Setting track axis).

You then have the option of selecting the Tracking axis from the first column-set of six radio buttons and/or selecting the upward-pointing axis from the second column-set in the Anim Setting panel. Each time you change the “Up” axis you need to apply AltR otherwise the tracking object will continue to track with the old orientation. This is one of the drawbacks to using Old Track.

To clear or remove an old track “constraint”, select the tracking object and press AltT. As with clearing a parent constraint, you must choose whether to lose or save the rotation imposed by the tracking.

Note
AltT command works (and is useful) only for the Old Track “constraint”. To clear the Track To and Locked Track constraints, just delete them directly from the stack at the Constraints panel.


Hints

The active object always becomes the target object to be tracked. In all but Old Track a blue dashed line is drawn between the tracker and target indicating that a tracking constraint is in place between the corresponding objects. If you see an object tracking another object without a dashed blue line then you know the tracking object is using the Old Track “constraint”.

Invalid Tracking or settings

If you choose an invalid tracking “To” and/or “Up” axis, the tracking object keeps it current orientation and ignores the incorrect selections. For example, if you choose the +Z axis as the “To” axis and also choose the +Z axis as the “Up” axis, you have chosen an invalid combination because you can’t have the tracking object’s +Z axis doing two different things at the same time.

If you have problems setting up the correct “To” and “Up” axes you may want to turn on the tracking object’s local axes. You can do this from the Draw panel by clicking on the Axis button. See The Interface chapter for further details on the Draw panel.



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