Graphics Reference
In-Depth Information
Normalize
The Normalize option adjusts for changes in light and shadow. For example, imagine a video
where a tile on the floor is perfect for tracking; however, during the video an actor walks by it and casts a
shadow over it. Normalize is slower to calculate, but it will adjust for this change and value and allow the
point to be tracked.
Use Mask
Enables the use of a mask during the tracking process itself. Masking portions of the video will
prevent tracking in the masked-out areas. Later in this chapter, you will learn how to set up a mask that can
be used in this way.
Correlation
Determines how much similarity the algorithm requires from one frame to the next when trying
to match tracking patterns. Lower correlation values will make the algorithm more robust but may reduce
accuracy.Iffootage isblurryormovement isfast, lower values may becalled fortoforce the tracker totryto
continue tracking even when the pattern is hard to recognize.
Frames Limit
This value explicitly sets the number of frames to track at a time. Normally, the tracker will
attempt totrack the whole footage tothe end(orthe beginning, depending onthe direction the tracking isgo-
ing).Ifthefootageisdifficulttotrack,itissometimesbettertotrackbitbybitincasethetrackgetsconfused.
Margin
This is a value in pixels that sets how far from the edge of the frame a track should continue to try
to follow a pattern. Tracks can sometimes get confused at the edge of the frame, particularly if the footage is
difficult to track and correlation is set low. This value gives an explicit cutoff point for the algorithm to stop
trying as it nears the edge of the frame.
Match
Ateachframe,thealgorithm searchesforapatternthatmatches whatithasstoredasthetrackingpat-
tern.Whatthatpatternexactlyisdependsonyourselectioninthismenu.ThedefaultisKeyframe,whichsets
a single pattern when you create or modify a tracking marker and attempts to match the subsequent frames to
that original pattern. The other option is Previous Frame. With this selected the algorithm attempts, at each
frame, to match the pattern most similar to the pattern that was matched in the previous frame. This option
may be more robust in cases when motion blur is heavy or when gradual changes occur to the pattern over
the course of the footage, but it can also make it easier for the track to lose its way and gradually drift away
from the original pattern.
Tracking
Tomaketheselected marker(s)begintracking, click therightorleftarrowontheTrackpanelontheToolshelf.
The right arrow will make the tracking progress forward; the left arrow will make tracking progress backward.
There is no difference whatsoever in how the tracking happens. You can track forward or backward and from
any point in the video sequence. If you start at frame 1, you can track the whole sequence forward. If you start
at the last frame, you can track the whole sequence backward. You can start in the middle and track forward
and then return to the middle and track backward from the same start point. You can track forward from the
beginning until the tracker fails, then track backward from the end until the tracker fails, and then work on the
problem areas in between. Tracking tracks only selected markers, so toggle Select All with the A key before-
hand.
The smaller right and left arrows to the right and left of the bigger arrows enable you to track one frame at a
time in either direction.
Clear After clears all tracked points for the selected track for all frames after the current frame. Clear Before
clears the tracked points for all frames previous to the current frame. Clear clears all frames' tracking inform-
ation for the current track. Join combines two separate tracks. If the two tracks have overlapping tracking in-
