Apple Motion 4 - Manual (Page 1270)

Note: As an alternative to using masks, you can also crop an object with applied tracking
data using the Crop controls in the Properties tab of the Inspector.
You can track the control points of a mask to a clip or apply existing tracking data to the
control points of a mask. For more information, see
Track Points Workflow .
Using Mimic Source for Four-Corner Pinning
Depending on your source footage, you may need to corner-pin an element using
reference points that are in a different position from the final “pinned” size of the
foreground element. You do this by offsetting the trackers using the Mimic Source option.
In the basic example below, the reference patterns (the “markers”) to be tracked are
located inside a frame, rather than at the corners.
The orange visual aid (the orange
outline) represents the resulting
size of the corner-pinned image. The orange visual aids circle the
tracking reference points.
Because the tracking reference points (markers) are not flush with the inside edge of the
frame, you must offset the image from the four trackers. Otherwise, the final corner-pinned
image will appear too small, as in the example below.
Note: In this specific example, you could track to the corners of the frame. However, the
steps in the following section demonstrate a simple example of how to use the Mimic
Source parameter in order to four-corner pin an object when your predefined tracking
markers (markers placed on the objects in the physical set) are offset from the reference
points to which you want to track.
1270 Chapter 19 Motion Tracking