Tracking Multiple Points

When you track multiple control points, you apply the motion of each reference feature to control points on the foreground object.

The shape and content of the object changes to reflect the motion of the reference features it is tracking. For this reason, a precision track is required, as even small drifts will cause any lettering or graphics on the front clip to "swim" or "crawl".

For example, you can put a new label on a pair of jeans. You create a new label, then have each control point on the new label track one of the reference features of the old label. The shape of the new label changes with the shape changes of the original. This example is similar to the example for tracking position (see Tracking Position) except that the new label is a layer instead of a Paint object, and the four corners of the label are tracked for more precision.

To track multiple control points:

1. Play the clip a few times to evaluate what reference feature you want to use and if there are any foreseeable problems for the tracking. See Finding a Good Reference Feature.
2. Determine a good reference frame and position the clip at that frame.



3. Select the object to which you want to apply the motion of the reference features (in this example, the new label), and position it over the reference feature (the old label).
4. With the object still selected, click the Composite Controls tab, and then click the Layer button.
5. In the Layer controls, click Four-Corner.
6. In the Toolbar, click the Arrow tool and position the corners of the new label to fit neatly on the old label.



7. In the viewport, select all the control points. To do so, Shift-click each one.
8. On the Tracker panel, select a source from the Source list (in this example, the Jeans layer).
9. Click Position.

A Tracker appears for each selected control point.

10. Position each reference box over a reference feature. (In this example, each of the corners of the old label.)
11. On the Tracker panel, click Select All Trackers.

All the tracker and reference boxes are selected.

12. On the Tracker panel, adjust the settings for all Trackers at once. In this example, the following settings are used:
• Mode is set to Relative since minor adjustments need to be made to the positions of the reference boxes. The vertices of the label layer should change their current position when tracking starts.
• Axis is set to X & Y so that each control point follows both the horizontal and vertical motion of the reference feature to which it is locked.
• Reference is set to Fixed since the reference feature does not undergo any significant changes that would prevent the Tracker from finding a match.
• Tolerance is at its default value given that each reference feature should be easily matched throughout the clip.
13. Click the Analyze Forward button on the Tracker panel to start the analysis.

The Trackers follow the reference features.



The new label now follows the old label as it moves toward the upper edge of the screen. In the following illustration, notice the difference in the shape of the polygon between the first frame and the last frame.



14. Save the Tracker data if necessary. Note that it has already been applied to the target object (the new label), so resave the data only if you have another use for it or for safe-keeping. For more information, see Importing and Exporting Tracker Data.
15. Click the Off button in the Tracker panel when tracking is complete.