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The spread value controls how much the mask is expanded, after the coverage checks above. Use the spread control to account for the motion of the occluding object from one frame to the next: the mask shows where the object is occluding on frame 100, say, yet the mask is needed for frame 101, and the object and plane will have moved between frame 100 and 101. The mask from 100 is just an approximation of the mask needed for 101; the spread control makes it a bit bigger so that the object is still masked out.
While a larger spread accommodates more object motion, a larger mask leaves that many fewer pixels for the actual tracking, putting the track at risk. And a larger spread does increase processing time.
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