Efficiently compute Felzenszwalb's HOG (FHOG) features.


function H = fhog( I, binSize, nOrients, clip, crop )


 Efficiently compute Felzenszwalb's HOG (FHOG) features.

 A fast implementation of the HOG variant used by Felzenszwalb et al.
 in their work on discriminatively trained deformable part models.
 Gives nearly identical results to features.cc in code release version 5
 but runs 4x faster (over 125 fps on VGA color images).

 The computed HOG features are 3*nOrients+5 dimensional. There are
 2*nOrients contrast sensitive orientation channels, nOrients contrast
 insensitive orientation channels, 4 texture channels and 1 all zeros
 channel (used as a 'truncation' feature). Using the standard value of
 nOrients=9 gives a 32 dimensional feature vector at each cell. This
 variant of HOG, refered to as FHOG, has been shown to achieve superior
 performance to the original HOG features. For details please refer to
 work by Felzenszwalb et al. (see link above).

 This function is essentially a wrapper for calls to gradientMag()
 and gradientHist(). Specifically, it is equivalent to the following:
  [M,O] = gradientMag( I,0,0,0,1 ); softBin = -1; useHog = 2;
  H = gradientHist(M,O,binSize,nOrients,softBin,useHog,clip);
 See gradientHist() for more general usage.

 This code requires SSE2 to compile and run (most modern Intel and AMD
 processors support SSE2). Please see: http://en.wikipedia.org/wiki/SSE2.

  H = fhog( I, [binSize], [nOrients], [clip], [crop] )

  I        - [hxw] color or grayscale input image (must have type single)
  binSize  - [8] spatial bin size
  nOrients - [9] number of orientation bins
  clip     - [.2] value at which to clip histogram bins
  crop     - [0] if true crop boundaries

  H        - [h/binSize w/binSize nOrients*3+5] computed hog features

  I=imResample(single(imread('peppers.png'))/255,[480 640]);
  tic, for i=1:100, H=fhog(I,8,9); end; disp(100/toc) % >125 fps
  figure(1); im(I); V=hogDraw(H,25,1); figure(2); im(V)

  % comparison to features.cc (requires DPM code release version 5)
  I=imResample(single(imread('peppers.png'))/255,[480 640]); Id=double(I);
  tic, for i=1:100, H1=features(Id,8); end; disp(100/toc)
  tic, for i=1:100, H2=fhog(I,8,9,.2,1); end; disp(100/toc)
  figure(1); montage2(H1); figure(2); montage2(H2);
  D=abs(H1-H2); mean(D(:))

 See also hog, hogDraw, gradientHist

 Piotr's Computer Vision Matlab Toolbox      Version 3.23
 Copyright 2014 Piotr Dollar.  [pdollar-at-gmail.com]
 Licensed under the Simplified BSD License [see external/bsd.txt]

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