Apple Motion 4 - Manual (Page 1249)

Note: Although Motion provides a 3D workspace, tracking in Motion is planar. In other
words, tracking does not occur in Z space. For example, if you are analyzing two features
in a clip—and that clip is moving in 3D space—you are recording the changes in position,
scale, or rotation over time in the clip but not its actual 3D transformation.
The object that is tracked is called the background or source element. The object to which
the tracking data is applied is called the foreground or destination element.
How a Tracker Works
A tracker analyzes an area of pixels over a range of frames in a movie clip in order to “lock
onto” a pattern as it moves across the Canvas. You specify the snapshot of pixels in one
or more reference frames, and Motion proceeds to track that snapshot for a specified
duration of time. This duration of time is based on the length of the tracking behavior,
the length of the defined play range, or the length of the clip. In Motion, that snapshot
is known as a reference pattern, and its area is automatically defined around the onscreen
tracker.
Tracker
Ideally, the reference pattern should be a consistent, easily identifiable detail with high
contrast—this makes the pattern easier to track.
The tracker advances to each subsequent frame, sampling many positions within the
search region around the center point of the tracker. Some of those positions “fit” the
previously designated reference pattern more closely than others, and the tracker finds
the position where the search region most closely matches the reference pattern (with
subpixel accuracy). For every frame analyzed, the tracker assigns a correlation value by
measuring how close the best match is.
In addition to searching for the reference pattern’s position, the tracker identifies how
the pattern transforms (scales, rotates, or shears) from one frame to the next. Imagine
you are tracking a logo on the shirt sleeve of a person walking past the camera. If the
person turns slightly as he passes the camera, the reference pattern becomes rotated.
The tracker looks not only for the reference pattern, but also for any shifts in that pattern’s
scale or rotation.
1249 Chapter 19 Motion Tracking