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motion-logo.jpgMotion's motion trackers are relatively sophisticated, as 2D trackers go.  In the general case, motion trackers can useful to give you a head start on basic compositing tasks like corner-pinning, to extrapolate simple camera motion to guide match-moves, and to extract elements of natural camera motion (see also the Stabilize Behavior).  And in Motion's case, its trackers are not only quite good at what they do, they're easy to use quickly.

To play with a basic motion tracker, load some kind of footage into Motion.  Then, apply an Analyze Motion behavior to it (Behaviors -> Motion Tracking -> Analyze Motion).  You should see something that looks like a circle with crosshairs:

trackers1.png

This is your tracker.  Basically, you want click and drag to position this on top of the feature you want to track, then click the "Analyze" button in the Tracker Inspector.

Finding Features to Track

If you've never used trackers before, it's time for a whirlwind course in good features to track.  Motion uses Optical Flow math instead of straight correlation like more elderly tracking strategies (I'm looking at you, Color), but the principle is much the same.  It's on you to pick the a point that, to the software, looks the same from frame to frame.  Sharp corners between areas of high contrast work well, and edges work well. 

If you need advice on what are mathematically the "best" points to track in your current frame, hold down the Option key as you click and drag.  You'll get a field of little crosshairs, which Motion thinks are the ideal places to drop motion trackers.  As you drag your tracker, it will gladly snap to any of these crosshairs.  But as a practical matter, don't lean on the software's recommendations too much: it doesn't know anything about your shot's content, and you'll be better off in the long run if you train your own eye to find good track points.

trackers10.png
Executing the Motion Track
To start the motion tracking process, click the Analyze button in the Inspector.  Motion will show you in the Canvas as it analyzes frame by frame, placing a yellow path with white keyframes at each point along its calculated track.  At any point, you can click Stop in the Analyzing dialog box, and you'll keep the calculated keyframes up to the current point -- this is useful when the motion you need to track only occupies a relatively small portion of the clip.

trackerfailed.pngSometimes (often), you'll lose your track: in other words, Motion will decide that it isn't certain enough where the feature has gone since the last frame.  You'll know this because the tracking process will suddenly quit, and you'll see a red X at the end of the tracking path.  There are plenty of strategies to recover a track in situations like this, but one of the least obvious and most helpful is to set a Look Ahead track point.

Once the track fails and you get a red X, manually move the tracker back to the feature you're tracking.  Then, hold down on the Command key, and click and drag it again -- you'll see a tracker in a square box "peel off," and you'll notice that the preview in your zoom box doesn't correspond to the original frame anymore.  That's because it's a preview of what the frame will look like 3 frames into the future.  Now, you're setting your Look Ahead point by placing the square tracker: you're essentially giving the tracker a strong hint about the correct path that it should follow.  Once you've replaced the tracker and set a Look Ahead point, simply click Analyze again to pick up where you left off calculating track points.

trackers20.pngUsing the motion track

As of now, your motion track is simply an abstract path in the brain of the computer.  To actually use it, you'll need to associate it with a behavior.  One way to do this is to apply a Match Move behavior to the item you want to follow the newly-tracked path, and select the Analyze Motion behavior that you used to get your original motion track.  And there are too many other variations on these techniques and applications to cover in this particular article -- but if you're looking for places to start, Behaviors -> Parameter -> Track is a good one as applications go.

trackers30.png


There are many more ways to use good tracks and refine persnickety ones, which I may write about here, but all of which we cover in our various Motion classes both at our classrooms and on-site at your offices.  866-566-1881's the number, if you're interested.

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