3D Compositing with Action > Action: Masks >
Tracking a Mask

You can track a mask in your scene from the GMask object's Tracking tab. You can track the mask using the Action module Tracker to track mask vertices, or the GMask object's shape tracker.

Tracker button
  Opens the Tracker menu where you apply tracking data to selected vertices. See Other Tracking Options.

Tracking View box
  When the current node is attached to a Source node, select to track the media in the Source Front, Source Matte or Source Result view.

Shape Track button

  Generate tracking data. Tracking information is stored in the Axis node to which the mask is attached.

Use Media field
  Displays the layer number (as defined in the Media list) of the media that will be tracked.

Track Axis Hierarchy button
  Enable to overwrite and store tracking information in the topmost Axis node in the schematic hierarchy.

Perspective Tracking button
  Lock the scaling factor of the x and y axes on the Axis node to which the mask is attached.

Forward and Backward box
  Select to track the image sequence forward or backward from the current frame.

Size field
  Displays the area in pixels of each tracker.

Maximum Number field
  Displays the number of trackers to use in the analysis.

Motion Tolerance field
  Set a value to determine the tolerance level of acceptable changes in the pixel environment. If an external object passes in front of the tracked object, a low tolerance value ensures that the motion of an external object does not get tracked through the analysis.

To apply shape tracking to a mask:

1. Select the mask you want to track.
2. Open the Tracking tab.
3. In the Shape Tracking panel, set the Use Media field to assign the front and matte clip.
4. In the Tracking menu, adjust the fields in the Trackers panel as needed.
5. Select an option from the Forward and Backward box to determine if the frames before or after the current frame will be used for analysis.
   TipWhen tracking a shape that cuts off during analysis (for example, person walking into or out of the frame), begin shape tracking when the whole shape is apparent, and track in the direction of the frames at which the shape begins to disappear.
6. Click the Shape Tracking button to perform the shape tracking analysis.
7. To perform additional analysis in a different direction from the first frame of the analysis, toggle the Forward and Backward box and click Shape Tracking again.