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Chapter 25 Image Tracking, Stabilization, and SmoothCam 751
Parameters
This node displays the following controls in the Parameters tab:
trackRange
The trackRange parameter is the potential frame range limit of your track. By default,
the range is set to the clip range. For Shake-generated elements such as RGrad, this
parameter takes a range of 1. You can set new limits using Shake’s standard range
description, for example, 10-30x2. If you stop tracking and start again, the analysis
starts from the current frame until it reaches the lower or upper limit of your
trackRange, depending on whether you are tracking forward or backward.
subPixelResolution
The resolution of your track. The smaller the number, the more precise and slower your
tracking analysis. The possible values are:
• 1: Area is sampled at every pixel. Not very accurate or smooth, but very fast.
• 1/4: Area is sampled at every .25 pixels (16 times more than with a sampling of 1).
• 1/16: Area is sampled at every .0625 pixels (256 times more than with a sampling of 1).
• 1/32: Area is sampled at every .03125 pixels (1024 times more than with a sampling
of 1).
• 1/64: Area is sampled at every .015625 pixels (4096 times more than with a sampling
of 1).
matchSpace
The pixels are matched according to the correlation between the selected color
space—luminance, hue, or saturation. When an image has roughly the same
luminance, but contrasting hues, you should switch to hue-based tracking.
referenceTolerance
A tracking correlation of 1 is a perfect score—there is an exact match between the
original reference frame and the sampled area. When the referenceTolerance is
lowered, you accept greater inaccuracy in your track. If tracked keyframes are between
the referenceTolerance and the failureTolerance, they are highlighted in the Viewer.
Also, in some cases, referenceBehavior is triggered if the tracking correlation is below
the referenceTolerance.
referenceBehavior
This behavior dictates the tracking area reference sample. By default, the reference
pattern is the first frame at which the track analysis begins, not necessarily the first
frame of the trackRange. The last two behaviors in the referenceBehavior list measure
the tracking correlation and match it to the referenceTolerance to decide an action.
The referenceBehavior pop-up menu contains the following options:
• use start frame: The new samples are compared to the reference pattern from the
first frame of the track. If you stop tracking midway, and start again at a later frame,
the later frame is used as the reference sample.