VLFeat - Documentation > MATLAB API > SIFT - vl_phow

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Documentation> MATLAB API> SIFT - vl_phow

[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.

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.

See also: VL_DSIFT(), VL_HELP().