F = VL_SIFT(I) computes the VL_SIFT frames (keypoints) F of the image I. I is a gray-scale image in single precision. Each column of F is a feature frame and has the format [X;Y;S;TH], where X,Y is the (fractional) center of the frame, S is the scale and TH is the orientation (in radians).

[F,D] = VL_SIFT(I) computes in addition the VL_SIFT descriptors D. Each column of D is the descriptor of the corresponding frame in F. A descriptor is a 128-dimensional vector of storage class UINT8.

Options:

Octaves

Set the number of octave of the DoG scale space.

Levels

Set the number of levels per octave of the DoG scale space.

FirstOctave

Set the index of the first octave of the DoG scale space.

PeakThresh

Set the peak selection threshold.

EdgeThresh

Set the non-edge selection threshold.

NormThresh

Set the minimum l2-norm of the descriptor before normalization. Descriptors below the threshold are set to zero.

Magnif

Set the descriptor magnification factor. The scale of the keypoint is multiplied by this factor to obtain the width (in pixels) of the spatial bins. For instance, since by default there are 4 spatial bins along each spatial direction, the ``diameter'' of the descriptor is approximatively 4 * MAGNIF.

Frames

Set the frames to use (bypass the detector). If frames are not passed in order of increasing scale, they are re-orderded.

Orientations

Compute the orietantions of the frames overriding the orientation specified by the 'Frames' option.

Verbose

Be verbose (may be repeated to increase the verbosity level).