Apple Shake 4 - Manual (Page 730)

730 Chapter 25 Image Tracking, Stabilization, and SmoothCam
The following example shows a track marker placed on a TV screen so the client could
place an image on the TV. The default tracker reference pattern is unnecessarily large.
The only interesting detail is the black cross in the middle. Otherwise, most of the
pattern matches up with most of the rest of the image—green. To adjust for this, limit
the reference pattern to more closely match the black crosshairs.
Picking a Good Search Region
You should also suit your search region to match the clip’s movement and the patterns
near the reference pattern. With the default settings, the bus example clip does not
track the lower-left corner of the sign very well. This is because the middle black
horizontal line can easily be matched with the black line at the very base of the search
region in later frames of the clip. The X axis is squeezed so much that vertical details
disappear. Additionally, since the bus is moving to the right, there is no point in
wasting cycles to the left of the point. Remember, the larger the search region, the
more samples the tracker must take.
The corrected search region, illustrated below, is now high enough to not include the
lower black line, and extends minimally to the left.
Adjusted tracker
reference pattern Default tracker reference pattern