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Chapter 2 Motion Tracking 51
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.
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.
When the tracker’s position and correlation values for a given frame have been
determined, Motion records this information in keyframes. This process is then
repeated for every frame, until the end of the track range has been reached.
The recorded data is stored as keyframes in the tracking behavior. The data allows for
you to quickly apply the tracks to many project elements.
Note: The Stabilize behavior uses an advanced technology that analyzes the motion of
the entire frame of a clip, without the use of trackers.
There are six tracking behaviors in Motion, four in the Motion Tracking behaviors
subcategory, one in the Shapes behavior subcategory, and one in the Parameter
behaviors subcategory.
Tracker