[FRAMES, DESCRS] = VL_PHOW(IM) extracts PHOW features  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.
- Verbose false
Set to true to turn on verbose output.
- Sizes [4 6 8 10]
Scales at which the dense SIFT features are extracted. Each value is used as bin size for the VL_DSIFT() function.
- Fast true
Set to false to turn off the fast SIFT features computation by VL_DSIFT().
- Step 2
Step (in pixels) of the grid at which the dense SIFT features are extracted.
- Color 'gray'
Choose between 'gray' (PHOW-gray), 'rgb', 'hsv', and 'opponent' (PHOW-color).
- ContrastThreshold 0.005
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().
- WindowSize 1.5
Size of the Gaussian window in units of spatial bins.
- Magnif 6
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.
- FloatDescriptors false
If set to TRUE, the descriptors are returned in floating point format.