Tracking Motion - Adobe AFTER EFFECTS CS3 PROFESSIONAL User Manual

Tracking motion

Track or stabilize motion
Tracking motion and stabilizing motion are essentially the same process, only with a different target and result. Use
Track Motion to track motion and apply the results to a different layer or effect control point. Use Stabilize Motion
to track motion and apply the results to the tracked layer to compensate for that motion.
To stabilize a layer, After Effects tracks the motion of a feature in the layer that should be stationary in the frame, and
then uses the tracking data to set keyframes to perform the opposite motion. You can stabilize to remove any combi-
nation of changes in position, rotation, and scale, while leaving desired motion unaffected. For example, if the
camera is panning, deselect Position but select Scale and Rotation as the properties to stabilize.
When you select Rotation or Scale in the Tracker Controls panel, you set two track points in the Layer panel. A line
connects the attach points; an arrow points from the first attach point (the base) to the second. If possible, place the
feature regions on opposite sides of the same object, or at least on objects that are the same distance from the camera.
The farther apart the regions, the more accurate the calculations and the better the result.
After Effects calculates rotation by measuring the change of angle of the line between the attach points. When you
apply the tracking data to the target, After Effects creates keyframes for the Rotation property.
After Effects calculates scale by comparing the distance between attach points on each frame with the distance
between the attach points on the start frame. When you apply the tracking data to the target, After Effects creates
keyframes for the Scale property.
When you track motion using either parallel or perspective corner pinning, After Effects applies keyframes for the
Corner Pin effect to the layer to scale and skew the target layer as necessary to fit the four-sided area defined by the
feature regions. The feature regions should lie in a single plane in the real world—for example, on the side of a bus,
on the same wall, or on the floor. The attach points should also all lie in a single plane, but not necessarily the same
plane as the feature regions.
Note: For parallel corner pinning only: To change which point is inactive, Alt-click (Windows) or Option-click (Mac OS)
the feature region of the point to make inactive. (One point must remain inactive to keep the lines parallel.)
Select the layer to track in the Timeline panel.
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2 Do one of the following:
• Click Track Motion in the Tracker Controls panel (or choose Animation > Track Motion), click Edit Target, and
choose the target to apply the tracking data to.
• Click Stabilize Motion in the Tracker Controls panel (or choose Animation > Stabilize Motion). The target layer
is the tracked (source) layer.
Select Position, Rotation, and/or Scale to specify what kinds of keyframes to generate for the target.
3
4 Move the current-time indicator to the frame from which to begin tracking.
Using the Selection tool, adjust the feature region, search region, and attach point for each track point.
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6 In the Tracker Controls panel, click either the Analyze Forward or Analyze Backward button to begin tracking.
If the tracking ceases to be accurate, click the Stop button
track" on page 344, and resume analysis.
When you are satisfied with the position of the feature region and attach point throughout the track, click the
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Apply button to apply the motion to the specified target.
, correct the problem as described in "Correct a motion
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