Searching Your Footage; Selecting A Good Feature Region; Adjusting The Search Region - Adobe AFTER EFFECTS 7.0 Manual

For best results, first prepare the object or region you are tracking before you begin filming. Because After Effects
compares pixels from one frame to the next to produce an accurate track, attaching high-contrast markers to the
object or region lets After Effects more easily follow the motion from frame to frame. Lightweight, brightly colored
balls, such as ping-pong balls, placed on the feature work particularly well to preserve the shape of the track point.
The number of markers you use corresponds to the number of points you are tracking. For example, if you're
tracking four points using the Perspective Corner Pinning option, you need at least four features, to correspond with
the four corners of the layer you want to attach to your filmed footage. The more markers you add to your subject
before shooting, the more features you'll have for tracking later.

2. Searching your footage

Before you begin tracking, it is important to view the entire shot to determine the best feature to track. What is clearly
identifiable in the first frame may later blend into the background because the angle, lighting, or surrounding
elements have changed. A feature may disappear offscreen or be obscured by another element at some point in the
scene. While After Effects can extrapolate the motion of disappearing items, your chances for a successful track are
higher if you step through the entire shot and select the best candidates for tracking.

3. Selecting a good feature region

You need to locate and size the feature region carefully. For the smoothest tracking, the feature you select should meet
the following criteria:
•
Visible for the entire shot
•
A contrasting color from the surrounding area
•
A distinct shape (at least within the search region)
•
A consistent shape and color throughout the shot
Because of the changing nature of an image in motion, digital tracking is rarely perfect. In moving footage, the shape
of an image inevitably changes, along with the lighting and surrounding objects. Even with careful preparation, a
feature generally evolves 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 appear to wander or drift.
It can take time to develop an eye for good features to track. Because of the many variables interacting in moving
footage, regardless of your experience, you often need to make adjustments and track again. After Effects includes
several options to help improve your success in tracking. For example, you can track using different aspects of color
(RGB, luminance, or saturation), change the adaptiveness of the feature (allowing the feature region to change along
with the footage), or have After Effects extrapolate or estimate motion if necessary. You can track the layer as many
times as needed and then apply the best tracking results.

4. Adjusting the search region

The size and position of the search region depends on the movement of the feature you want to track. The search
region must accommodate the movement of the feature, but only the frame-to-frame movement, not its movement
throughout the shot. As After Effects locates the feature in a frame, both the feature and search regions move to the
new location. Therefore, if the frame-to-frame movement is gradual, the search region needs to be only slightly larger
than the feature region. If the feature changes position and direction quickly, the search region needs to be big
enough to encompass the largest position and direction change in any pair of frames.
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