F = VL_SIFT(I) computes the SIFT frames [1] (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 the SIFT descriptors [1] as well. Each column of D is the descriptor of the corresponding frame in F. A descriptor is a 128-dimensional vector of 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. The default value is 3.
- FirstOctave
Set the index of the first octave of the DoG scale space. The default value is 0.
- PeakThresh
Set the peak selection threshold. The default value is 0.
- EdgeThresh
Set the non-edge selection threshold. The default value is 10.
- 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, if there are there are 4 spatial bins along each spatial direction, the ``diameter'' of the descriptor is approximatively 4 * MAGNIF. The default value is 3.
- WindowSize
Set the variance of the Gaussian window that determines the descriptor support. It is expressend in units of spatial bins and the default value is 2.
- 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).