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652 Chapter 12: Animation
high-weight tr ackers. Use weights of two to six to
get a good lock.
Ma x M ove/Frame—Sets the maximum number
of pixels that the tracked feature moves from
frame-to-f r a me over the entire tracking range
and establishes an initial motion search b ounds
box for newly creat ed trackers. You can also set
thisdynamicallybyadjustingthemotionsearch
bounds box directly in the Movie window.
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 tracking 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 chang ing 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 frames. 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 f rame and continues tracking f rom that frame.
Subpi xel Track i ng—Usually, an image feature
won’t move an integral number of pixels f rom
frame-to-frame. With pixel-level tracking,
however, the nearest estimate for a feature center
is always at a pixel boundar y. This can lead to
camera match errors. With subpixel tracking you
canzoominonafeatureandpositionitsgizmo
within a pixel. This makes the tracker attempt to
trackatthatresolutionbyup-samplingthetarget
image and search bounds, thereby improving
match accuracy.
You can turn on subpixel tr acking select ively for
each tracker b y choosing a level in the Subpixel
Tra ck ing drop- dow n m enu on the Mot ion
Trackers rollout. This defines the resolution at
which feature tracking is performed in the tracker
up to 1/32 of a pixel.
Twothingshappenwhenyouselectasubpixel
level:
• The grid over which y ou can position a tracker
gizmo in the image window is adjusted to the
newsubpixellevel.Youcanpositionthegizmo
can be positioned inside pixels in zoomed
views. You can zo om in on a tracker g i zmo by
selecting it and pressing the “I” or “i” key (“O”
or “o” for zooming out).
• 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 keyframes,
it’s crucial that y ou zoom in enough so you can
place the g iz mo in the center of the feature to the
degree of accuracy allowed 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 tracked to the
nearest pixel at whole pixel increments and then
tracked at the subpixel level within that one pixel
range, the track ing time can increase significantly
in proportion to the in verse square of the subpixel
level.Forthisreasonit’ssuggestedthatyousetthe
subpixel le vel to as low as possible. Usually 1/8
of a pixel is adequate for features well distr ibuted
around the scene. Use greater levels if the features
are fairly close together, or move very slowly
within the frame.