Giving Motion a Hint by Looking Ahead
When using the Analyze Motion behavior, you can direct a tracker where to look in a later
frame for its reference pattern. This tool is ideal for the following types of clips:
• A clip that contains fast-moving features
• A clip with subjects moving in a relatively straight vector (with or without obstructions)
• A clip with swish pans (but you might need to reset the “look-ahead” tracker at each
panning change)
Important:
If your track fails and you reposition your tracker, you must also reset the
look-ahead tracker in the Canvas to provide a new motion vector from the new reference
point.
To define look-ahead frames
1
Apply an Analyze Motion behavior to a clip.
2
In the Canvas, position the tracker on a reference pattern.
1328
Chapter 22
Motion Tracking
In the following image, the tracker is positioned on a reference pattern on the front
bumper of the car.
Tracker
3
In the Behaviors Inspector, use the Look Ahead Frames slider or value slider to specify
how many frames you want the tracker to look ahead.
Note: The maximum default for the Look Ahead Frames slider is 10 frames. However, you
can enter a larger frame amount using the adjacent value slider.
4
While holding down the Command key, click the tracker in the Canvas, then drag in the
direction the reference pattern is moving in the clip.
As you Command-drag the track point, an inset displays a magnified view of the frame
specified in the Look Ahead Frames parameter.
5
When the look-ahead tracker is positioned on the reference pattern, release the mouse
button.
When you click the Analyze button, the new reference point is used as the tracking pattern.
Note: Look Ahead Frames can be used when tracking in reverse. When the Reverse
checkbox is selected in the tracking behavior’s parameters and you use the Look Ahead
Frames parameter, you are looking at previous frames rather than future frames. The
Reverse checkbox is only available for the Analyze Motion, Track (in the Parameter
behaviors category), and Track Points (in the Shape behaviors category) behaviors.
1329
Chapter 22
Motion Tracking