How a tracker works – Apple Shake 4 User Manual
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Chapter 25
Image Tracking, Stabilization, and SmoothCam
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SmoothCam: This node differs from the others above in that it doesn’t track small
groups of pixels. Instead, it evaluates the entire frame, using motion analysis to
derive the movement of the camera. Once derived, this node has two modes. It can
smooth the shot, eliminating unwanted jitter while maintaining the general motion
of the camera. It can also lock the shot, stabilizing a subject within the frame that’s
isolated with a mask. This node can affect translation, rotation, zoom, and
perspective, making it more flexible for certain operations than the other tracking
nodes. For more information on using the SmoothCam node, see “
How a Tracker Works
A tracker works by analyzing an area of pixels over a range of frames in order to “lock
onto” a pattern as it moves across the screen. You specify the “snapshot” of pixels in
one or more reference frames, then Shake proceeds to “track” that snapshot for a
specified duration of time. In Shake, that snapshot is known as a reference pattern, and
its area is defined by the inner box of the onscreen tracker control:
Ideally, the reference pattern should be some easily identifiable detail with high
contrast—this makes it easier to track.
The tracker advances to each subsequent frame, sampling the area inside the search
region, which is represented by the outer box of the onscreen tracker control. The
tracker positions a box the same size as the reference pattern at the pixel in the first
row, at the first column of the search region, and takes a sample. The tracker then
advances to the next pixel (or subpixel) column in the search region and takes a
second sample. For every sample the tracker takes, it assigns a correlation value by
comparing the current sample to the previously designated reference pattern. When all
of the samples have been taken, Shake assigns the new tracking point to the sample
with the highest correlation value. This process is then repeated, every frame, until the
end of the track range has been reached.
Search region
Track point
Reference pattern