Adobe After Effects CS4 - Manual (Page 272)

266 USING AFTER EFFECTS CS4
Animation and keyframes
Last updated 12/21/2009
Attach point offset from feature region
5. Adjust the feature region, search region, and tracking options
Place each feature region control tightly around its tracked feature, completely enclosing the tracked feature, but
including as little of the surrounding image as possible.
The size and position of the search region depend on the movement of the feature you want to track. The search region
must accommodate the movement of the tracked feature, but only the frame-to-frame movement, not its movement
throughout the shot. As After Effects locates the tracked feature in a frame, both the feature region and search region
move to the new location. Therefore, if the frame-to-frame movement of the tracked feature is gradual, then the search
region needs to be only slightly larger than the feature region. If the feature changes position and direction quickly,
then the search region needs to be big enough to encompass the largest position and direction change in any pair of
frames.
You can also set tracking options that determine such things as which color channels are compared to find a match to
the feature region.
6. Analyze
You perform the actual motion tracking step by clicking one of the Analyze buttons in the Tracker panel. When
tracking a tricky set of features, you may want to analyze a frame at a time.
7. Repeat as necessary
Because of the changing nature of an image in motion, automatic tracking is rarely perfect. In moving footage, the
shape of a feature changes, along with the lighting and surrounding objects. Even with careful preparation, a feature
generally changes during a shot and at some point no longer matches the original feature. If the change is too great,
After Effects may not be able to track the feature, and the track point will wander or drift.
When the analysis begins to fail, return to the frame where tracking was still accurate and repeat steps 5 and 6: adjust
and analyze.
8. Apply tracking data
If you’re using any Track Type setting other than Raw, you apply tracking data by clicking Apply, after making sure
that the correct target is shown for Motion Target. You apply tracking data from a Raw tracking operation by copying
keyframes from the trackers to other properties or by linking properties with expressions.
You can also adjust the Attach Point or Attach Point Offset property after tracking in the Timeline panel, which can
be useful when applying the same tracking data to multiple targets that you want to distribute around the tracked
feature.
Note: If the layer that you’re attaching has motion blur enabled, make sure that the Shutter Phase value is set to -1/2
times the Shutter Angle value. This combination of settings centers the motion blur on the attach point. Otherwise, the
attached object may appear to lead or lag the object that it’s attached to.