Creating Motion Paths

To begin tracking

1. In the FxTree, double-click the Tracking operator to open its property editor and preview it in the Fx Viewer.

2. Click the Options button to display the Options page.

3. Activate the following tracking options, as necessary:

   Option	Function
   Show Points	Displays the motion paths as points.
   Show Lines	Displays the motion paths as lines.
   Retrack on Process	Lets you track or retrack while rendering. You don't have to set this option manually because it is automatically activated when you begin tracking and deactivated when the tracking process is complete.
   Crop When Tracking	Applies the crop region while tracking.
   Field Mode	Tracks by frames or, if you are using field-based material, by fields. Frame: tracks templates from frame to frame. Field1: tracks templates in Field 1 mode if you are using field-based material. Field2: tracks templates in Field 2 mode if you are using field-based material.

4. Click the Trackers button to display the Trackers page.

5. From the Mode tab, set the Track Mode to Edit Tracks.

6. Do one of the following:

   • Click the Track From Start button to begin tracking from the beginning of the sequence.

   • Click the From Current button to begin tracking from the current frame.

   • Click the Step button to track for a single frame.

   All of these options automatically activate the Retrack on Process option (on the Options page) during tracking, and deactivate it when the tracking process is complete.

   The tracking process begins, and keyframes are automatically set for all activated trackers.

Repositioning the Template and/or Search Area

When tracking, you'll often come across a situation where the region you would like to follow can't be tracked directly. For example, you might want to track something that goes behind a tree for part of the sequence. You can reposition the template in the Fx Viewer on something you can track, while the track point remains on what you would like to have tracked.

At frames where you position the template by hand, the separation between the template and track point is preserved in subsequent frames.

An example of this kind of offsetting occurs when you change your choice of template halfway through the track. In the example of the template going behind the tree for a few frames, you would:

• Track normally until just before it goes behind the tree.

• Find some other part of the object you are following that isn't obscured by the tree, and move the template to follow that.

• Track using this template until the tree is no longer in the way, and then move the template back to the original object and continue tracking.

The tracking operator adjusts for all the template movements to produce a single unbroken final track.

The principle described above is true for the search area as well. For example, when the template region is moving quickly, you can reposition or resize the search area at a given frame to ensure that the template doesn't lose the region that it's tracking. The search area's new position and/or size relative to the template is preserved in subsequent frames, until you change it again.

Managing Tracking Errors

The properties on the Accuracy tab of the Trackers page allow you to monitor and set thresholds for tracking errors. The following properties are available:

• Current Frame Error: for each track, this describes the error between the template for which the Tracking operator is searching and the area it actually finds.

  The tracking algorithm compares regions on a pixel-by-pixel basis, calculating all the small differences to produce an overall error value. This error value is more relative than absolute — sometimes the error is greater than 2 and the track works correctly; however, sometimes the error value can be low as 0.1 and the track can go off course.

  This information is for reference only — you cannot modify it.

• Error Warning Threshold: for each track, this parameter defines the maximum acceptable error. The Tracking operator takes the defined area, then moves into the search area to compare the pixels in the template with those in the image at this frame. All occurrences of small errors are added together to find overall error.

• Auto Template Change Threshold: for each track, if the error that is calculated is over this value, the Tracking operator will switch templates at that frame. Instead of following the original template, it follows the new region at the frame where the error became bigger than the threshold.

  This function is useful when a template is gradually changing over a sequence of frames, so that the feature is significantly different at the last frame from the first frame, such as when the tracker is following a point that is being rotated.

Offsetting Track Points

You can reposition any track point using the controls on the Area Shift tab of the Trackers page of the Tracking property editor. This tab provides sliders that you can use to position each track point in X and Y. If you reposition a track point after its motion path has been calculated, the entire motion path is offset accordingly.