Manual/Tracking

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Contents [hide] 1 Tracking 1.1 Description 1.2 Options 1.2.1 TrackTo Constraint 1.2.2 LockTrack Constraint 1.2.3 Old Track 1.3 Hints 1.3.1 Invalid Tracking or settings

Tracking

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

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

This options causes the tracker to point a local "To" axis at the target with an "Up" axis always maintaining a vertical orientation. This tracking is similar to billboard tracking in 3D. This is the preferred method over Old Track. As mentioned TrackTo constraint is the preferred tracking constraint because it has a more easily controlled constraining mechanism. It also can act as a constraint on the constraint stack and can be moved up and down in the stack.

The controls for changing the "Tracking" and "Up" axis of a tracker object are located in the Constraints panel. This panel is located in the same place as the Anim settings and Draw panels (in Buttons Window, Object(F7) context, Object buttons sub-context); see (Constraints panel). The constraint fields are:

• Target - The name of the target that the tracking object tracks.
• To - The tracking axis. It shouldn't be the same as the "Up" axis.
• Up - The "Up" oriented axis relative to the global coordinate system.
• 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 TrackTo constraint applied and have input driving the influence channel.

If you select an invalid combination of "Tracking" and "Up" axis the field labeled "AutoTrack" will turn red. In addition, the tracking object will stop tracking the target until you choose a valid combination. This behavior is different than Old Track where Old Track would continue to track using a previous valid combination.

(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" and "Up" axis. Cube "A"'s constraint setting are reflected in (Constraints panel). +X is the "Tracking" axis and +Z is the "Up" axis.

You can also see in (Constraints panel) 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 "OB:Cube". You can redirect tracking to another object simply by entering in the name of another object.

LockTrack Constraint

This options causes the tracker to point a local "To" axis at the target with a "Lock" axis always fixed and unable to rotate. This tracking is similar to billboard tracking in 2D. This constraint always has one axis locked such that it can not rotate. An example of billboarding is to have a Plane object textured with a tree image that always faces the camera.

The controls for changing the "Tracking" and "Lock" axis are located in the Constraints panel. This panel is located in the same place as the Anim settings and Draw panels; see (Constraints panel). The constraint fields are:

• Target - The name of the target that the tracking object tracks.
• To - The tracking axis. It shouldn't be the same as the "Lock" axis.
• Lock - The locked axis relative to the global coordinate system. The tracking object's local axis will snap to a global 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.
• 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 constraint works well for billboarding 2D trees.

For further details see TrackTo constraint. LockTrack and TrackTo are very similar where the former has a locked axis verses an "Up" axis.

Old Track

This is an older algorithm prior to version 2.30 and is similar to TrackTo 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 Alt R when applying or changing the tracking or "Up" axis. However, the preferred method to use is TrackTo Constraint. Let's assume you have 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 (Alt R).

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

If you want to change this you need to get to the "Anim settings" panel where Old Track's setting are accessed. First select the tracking object (not the target) and change the Button Window to Object Context by clicking the icon (), 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 (Alt R) 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 a track constraint, select the tracking object and press Alt T. As with clearing a parent constraint, you must choose whether to lose or save the rotation imposed by the tracking. Note: (Alt R) only works with the Old Track constraint.

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 axis" 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 tracking axis and also choose the +Z axis as the "Up" axis you have choosen 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 "Tracking" 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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