Motion 5 - How a Tracker Works

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How a Tracker Works

In Motion, you apply a tracking behavior to an object—typically a movie clip—to record
and analyze its motion. The result of this analysis is a track—recorded movement
data—that can be applied to any other object in the project, transferring the motion of
a source object to a destination object. Tracking behaviors analyze an area of pixels known
as a reference pattern over a range of frames in a movie clip to “lock onto” the pattern as
it moves across the Canvas. You specify the reference pattern to be tracked (a specific
swatch or snapshot of pixels in the clip) by dragging one or more onscreen trackers (a
red crosshair in a circle, shown below) to the area of the clip you want to analyze. Motion
then proceeds to track the designated reference pattern 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.

Ideally, the reference pattern should be a consistent, easily identifiable detail with high
contrast. This makes the pattern easier to track.

During the analysis, the tracker advances to each subsequent frame, sampling many
positions in the search region around the center point of the tracker. Some of those
positions fit the designated reference pattern more closely than others; 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 rotates. The tracker
looks for the reference pattern and any shifts in that pattern’s scale or rotation.

When the tracker’s position and correlation values for a given frame are determined,
Motion records this information in keyframes. This process is repeated for every frame,
until the end of the track range is reached.


Chapter 22

Motion Tracking

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The recorded data is stored as keyframes in the tracking behavior. This data allows you
to apply the recorded motion 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 Shape behavior subcategory, and one in the Parameter behaviors
subcategory. For a general overview of Motion behaviors, see

Using Behaviors