User Manual

In addition to locating high contrast, defined patterns, watch for the frames where the pattern
moves the most. Identifying the maximum range of a pattern’s motion will help to determine the
correct size for the pattern search area.
It is not uncommon to have a scene that requires the use of several different patterns to
generate a single path. This most often occurs because the pattern moves out of frame or is
temporarily obscured by another scene element. Combining patterns into a single pattern is
described later in the chapter.
Selecting the Pattern’s Image Channels
When a pattern of pixels is selected, the Tracker automatically selects the color channel used
for tracking the pattern based on an analysis of each channel for contrast, clarity, and reliability.
The channels selected are highlighted in the bars to the right of the Pattern display window in
the node controls.
Highlighted channel bars indicate which
channel is selected for tracking.
You can override the automatic channel selection by clicking the buttons beneath the bars for
each channel to determine the channel used for tracking.
You can choose any one of the color channels, the luminance channels, or the alpha channel to
track a pattern.
When choosing a channel, the goal is to choose the cleanest, highest contrast channel for use
in the track. Channels that contain large amounts of grain or noise should be avoided. Bright
objects against dark backgrounds often track best using the luminance channel.
Selecting Patterns for Stabilization
Selecting patterns for stabilization can be a tricky business. The location of the pattern, when it
is selected, is used to determine precisely how the image will be stabilized. At least two
patterns are required to correct for rotation; using three patterns will correct for scaling, and
more will usually improve the quality of the solution.
Try not to select just any potentially valid pattern in the sequence, as some patterns will make
the solution worse rather than better. To help with your selection, use the following guidelines
when selecting patterns for stabilization.
 Locate patterns at the same relative depth in the image. Objects further in the
background will move in greater amounts compared to objects in the foreground due
to perspective distortion. This can confuse the stabilization calculations, which do not
compensate for depth.
 Locate patterns that are fixed in position relative to each other. Patterns should not be
capable of moving with reference to each other. The four corners of a sign would be
excellent candidates, while the faces of two different people in the scene would be
extremely poor choices for patterns.
Chapter – 73 Using the Tracker Node 1499