< image height.
< number of octaves.
< current GSS data.
< k-smoothing
< peak threshold.
< current octave height.
< current Gaussian filter
< current Gaussian filter std
< size of Gaussian window (in spatial bins)
< GSS gradient data.
< image width.
< current Gaussian filter width
< number of detected keypoints.
< norm threshold.
< detected keypoints.
< edge threshold.
< maximum level index.
< number of levels per octave.
< temporary pixel buffer.
< current octave width.
< current DoG data.
< size of the keys buffer.
< minimum level index.
< minimum octave index.
< nominal image smoothing.
< smoothing of pyramid base.
< current octave.
< delta-smoothing.
< magnification factor.
** ** This filter implements the SIFT detector and descriptor.