Apple Motion 5.1.1 - Manual (Page 956)

Chapter 22 Motion tracking 956
•
Multiple-point tracking: Track as many reference patterns in a clip as you like. You can manually
add trackers in the Analyze Motion behavior. When you apply a Track Points behavior from
the Shape behaviors subcategory to a shape or mask, a tracker is assigned to each shape
control point.
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 tracked is called the background or source element. The object to which the tracking
data is applied is called the foreground or destination element.
To learn more about tracking, choose a topic in the Help table of contents (the sidebar to the left
of this window).
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.
67% resize factor