Extracts a SIFT descriptor each STEP pixels.
</p></dd><dt>
Size
<span class="defaults">3</span></dt><dd><p>
A spatial bin covers SIZE pixels.
</p></dd><dt>
Bounds
<span class="defaults">[whole image]</span></dt><dd><p>
Specifies a rectangular area where descriptors should be
extracted. The format is [XMIN, YMIN, XMAX, YMAX]. If this
option is not specified, the entiere image is used.  The
bounding box is clipped to the image boundaries.
</p></dd><dt>
Norm
</dt><dd><p>
If specified, adds to the FRAMES ouptut