9

Camera Tracker: Motion Trackers Rollout 675
Resample on error—
Yo u c a n m a k e t h e t r a c k e r
identifypossibletrackingerrorsandyoucanuse
this during a manual t racking review and during
actual tracking to correct errors. Typical sources of
error include frame-to-frame jumps greater than
the search bounds, image contrast changes, or a
feature changing shape over a sequence of frames,
suchasacornermightifthecameramovesbyit.
The error detector is controlled by measurement
thresholds set in the Error Thresholds rollout.
When you place a gizmo on some frame, a
key f r ame is created and a new target feature
image is sampled at that frame for searching in
subsequent fr ames. Turning on Resample On
Errorcausesthetrackertodetecterrorsduring
tracking. If one is found, it backs up to the last
good frame and places a new keyframe there
causing a new target feature image to be sampled at
that fr ame and continues tracking from that frame.
Subpi xel Track i ng—Usually , an image feature
won’t move an integ ra l number of pixels f rom
frame-to-fr ame. With pixel-level tracking,
however, the nearest est imate for a feature center
is always at a pixel boundary. This can lead to
camera match errors. With subpixel t racking you
canzoominonafeatureandpositionitsgizmo
within a pixel. This makes the tracker attempt to
trackatthatresolutionbyup-samplingthetarget
image and search bounds, thereby improving
match accuracy.
You c an turn on subpixel tracking selectively for
each tracker by choosing a level in the Subpixel
Tracking drop-down menu on the Motion
Trackers rollout. This defines the resolution at
which feature tracking is performed in the tracker
up to 1/32 of a pixel.
Two things happen when you select a subpixel
level:
• The grid over which you can position a tracker
gizmo in the image window is adjusted to the
new subpixel level. You can position the gizmo
can be positioned inside pixels in zoomed
views. You can zoom in on a tracker g i zmo by
selecting it and pressing the
I or O key to
zoom in or out, respectively.
• Feature searching is performed on internally
scaled-up versions of the target image feature
and portions of the search window. These
versions are scaled up in inverse proportion to
the subpixel level using bicubic interpolation,
which allows the feature tracker to search at this
scaled pixel level.
When manually positioning gizmos at keyfr ames,
it’s crucial that you zoom in enough so you can
place the gizmo in the center of the feature to the
degree of accuracy al lowed by the selected subpixel
level. If you don’t do this, the tracker won’t be
able to track the correct center at the selected
resolution.
Tip: Even though the feature is first t racked to the
nearest pixel at whole pixel increments and then
tracked at the subpixel level within that one pixel
range, the tracking t ime can increase significantly
in proportion to the inverse square of the subpixel
level.Forthisreasonit’ssuggestedthatyousetthe
subpixel level to as low as po ssible. Usually 1/8
of a pixel is adequate for features well distributed
around the scene. Use greater levels if the features
are fairly close together, or move very slowly
within the frame.
You can also reduce feature tracking times by
makingtheinnerfeatureboundsboxinthegizmo
as t ig ht as you can while still retaining a couple of
pixels worth of surrounding contrast. Search times
areproportionaltothefeatureboundsboxsize.
Track Range
Use these w hen a feature move s off-screen or is not
visibleintheMoviewindow.
Set Start—Sets the st art of the active frame range
for a tracker .