An example of dense sampling and SIFT feature point description.
(a) SIFT descriptors are computed at regular grid points with a spacing of 15 pixels, as represented by the red circle; (b
e) At each point, SIFT descriptors are calculated on four different scales using four different radii: r = 4, 6, 8, and 10 pixels. The descriptor of each scale has 16 patches, represented by the red rectangles, which are rotated to the dominant orientation of the feature point. Each patch is described in gradient magnitudes of eight directions (red bins inside the red rectangles).