Apple Shake 4 Tutorials - Manual (Page 211)

Chapter 7 Tracking and Stabilization 211
Gee, everything was going fine until around frame 34, right? At that frame (results
vary), the tracker spitefully decided that the words on the sign were the best match to
the original reference pattern at frame 1.
The tracker continually compares the new images back to frame 1 (or whichever frame
you used to start the track). As the bus moves away, the sign gets smaller. At a certain
point, the words match the original thickness of the sign border from frame 1 better
than the sign border itself.
This matching behavior is controlled by the referenceBehavior setting in the tolerances
subtree. The referenceBehavior pop-up menu is set to “use start frame” by default—it
compares the new samples to the original start frame (not necessarily frame 1 if you
start at a later frame).
Two settings in the referenceBehavior pop-up menu can help with scaling changes in
the image sequence. The settings are “update every frame,” which uses the sample of
the previous frame for the reference pattern, and “update if below reference tolerance,”
which checks the correlation of every frame. If the correlation falls below the
referenceTolerance value (the parameter directly above referenceBehavior), Shake uses
the previous frame as the reference pattern. Shake continues to use that pattern until
the correlation once again dips below the referenceTolerance. The second setting
(“update if below reference tolerance”) is more accurate because you get inherent drift
with “update every frame” as tiny errors accumulate.