t descriptor is placed at (XMIN, YMIN). The next
three bins to the right are at XMIN + SIZE, XMIN + 2*SIZE, XMIN +
3*SIZE. The X coordiante of the center of the first descriptor is
therefore at (XMIN + XMIN + 3*SIZE) / 2 = XMIN + 3/2 * SIZE.  For
instance, if XMIN = 1 and SIZE = 3 (default values), the X
coordinate of the center of the first descriptor is at 1 + 3/2 * 3
= 5.5. For the second descriptor immediately to its right this is
5.5 + STEP, and so on.
</p><p>
See also: <a href="VL_SIFT.html">