[FRAMES, DESCRS] = VL_PHOW(IM) extracts PHOW features [1] from the image IM. PHOW is simply dense SIFT applied at several resolutions. This function is a commodity interface to VL_DSIFT() and VL_IMSMOOTH().
DESCRS has the same format of VL_SIFT() and VL_DSIFT(). FRAMES(1:2,:) are the x,y coordinates of the center of each descriptor, FRAMES(3,:) is the contrast of the descriptor, as returned by VL_DSIFT() (for colour variant, contranst is computed on the intensity channel). FRAMES(4,:) is the size of the bin of the descriptor.
By default, VL_PHOW() computes the gray-scale variant of the descriptor. The COLOR option can be used to compute the color variant instead.
Set to true to turn on verbose output.
Scales at which the dense SIFT features are extracted. Each value is used as bin size for the VL_DSIFT() function.
Set to false to turn off the fast SIFT features computation by VL_DSIFT().
Step (in pixels) of the grid at which the dense SIFT features are extracted.
Choose between 'gray' (PHOW-gray), 'rgb', 'hsv', and 'opponent' (PHOW-color).
Contrast threshold below which SIFT features are mapped to zero. The input image is scaled to have intensity range in [0,1] (rather than [0,255]) and this value is compared to the descriptor norm as returned by VL_DSIFT().
Size of the Gaussian window in units of spatial bins.
The image is smoothed by a Gaussian kernel of standard deviation SIZE / MAGNIF. Note that, in the standard SIFT descriptor, the magnification value is 3; here the default one is 6 as it seems to perform better in applications.
If set to TRUE, the descriptors are returned in floating point format.