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720 Chapter 25 Image Tracking, Stabilization, and SmoothCam
For this reason, most trackers don’t handle significant rotational movement very well—
they (Shake’s included) only test for panning changes, not rotational. If they did, they
would have to multiply the amount of panning samples by the amount of degrees for
the number of samples to take, which would be prohibitively costly at this stage. If you
are tracking an object with rotational movement, try using a referenceBehavior set to
update every frame. This means that the reference pattern is updated at every frame,
so you are only comparing a frame with the frame before it, and not the first frame.
Also keep in mind that manual adjustments are a standard solution for many tracking
problems.
This section discusses tracking in depth, including interface features, workflow issues,
and tips on successful tracking and manipulation of tracking data. For specific formats
of each node, see the functions listing at the end of this chapter. For a tutorial on how
to use the Tracker, see Tutorial 7, “Tracking and Stabilization,” in the Shake 4 Tutorials.
Image Tracking Workflow
The following is a general overview of the steps required to generate a track. The steps
are further detailed in subsequent sections.
To generate a track:
1 Apply a motion tracking node to an image.
2 Double-click the tracking node to load its image into the Viewer and its parameters
into the Parameters tab.
Note: If you don’t load the motion tracking node into the Viewer, the track will not be
performed.
3 Play your background clip several times to determine a good tracking point.
4 Make sure that the onscreen controls are visible in the Viewer.
5 Go to the frame that you want to start the track.
6 Position the tracker on the point you want to track, then adjust the reference pattern
and the search region boxes used to identify the desired tracking point.
7 Ensure the trackRange parameter reflects the frame range you want to track.
For FileIn nodes, the default is the range of the clip. For Shake-generated elements, you
must provide a frame range, for example, 1-50.
8 Click the Track Forward or Track Backward button (in the Viewer shelf) to begin
processing.
The track generates animation curves.
9 To stop a track, click the mouse button.