Tracking Objects

Tracking lets you follow and store the motion of an object on-screen so you can apply the stored motion to a second element. creating a action relationship. The Tracker first tracks motion of the reference feature(s), then applies or inverts that motion, depending on if you are tracking or stabilizing.

Once you have set up your trackers, the analysis can begin. This is how the Tracker analyzes and remembers the motion of the reference feature:

• The Tracker stores the position of the reference box on frame 1.
• The Tracker advances to the next frame and positions the tracker box in the same place as in the previous frame.
• It searches the tracker box for an area that matches the contents of the reference box. The Tracker uses luma cues (white versus black) and shape cues to determine what is a match.
• Once it finds a match, the Tracker uses the location of the match to move the center of the tracker box over the center of the match. This change in position is stored as an X and Y shift value.
• On the next frame, and on all future frames, the Tracker gets a little smarter about where it positions the tracker box. It extrapolates the likely position of the reference feature based on the amount and direction of the motion that occurred between the last two frames. The Tracker makes an educated guess of where the feature may be, based on past matches.
  Note This means the Tracker works best when the motion of the reference feature is regular and does not suddenly or erratically change direction or speed.
• Once the Tracker has analyzed the motion of the reference feature, and stored it as Shift values for each frame, it copies the motion of the reference feature to the element that you are tracking, so that the two appear locked together.

In all tracking except for stabilizing, the Tracker applies the shift values without modification.

With stabilizing, the Stabilizer operator inverts the shift values, resulting in a negation (and therefore elimination) of the motion—see Stabilizing.

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