SIFT descriptor is a spatial histogram with 4 x 4 bins).
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The sampling density is controlled by the option STEP, which is
the horizontal and vertical displacement of each feature cetner to
the next.
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The sampled image area is controlled by the option BOUNDS,
defining a rectangle in which features are comptued. A descriptor
is included in the rectangle if all the centers of the spatial
bins are included. The upper-left descriptor is placed so that the
uppler-left spatial bin center is algined w