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function feat = phog(featOpts, im)
% PHOG Extract PHOG features
% FEAT = PHOG(FEATOPTS, IM) returns the PHOG features of
% image IM. IM is an image in the standard MATLAB format. WORDS is
% the word map image, and WEIGHTS is the weight map.
%
% Author:: Andrea Vedaldi
conf_.period = 360 ;
conf_.vocabSize = 16 ;
conf_.gradientThresh = 1 / 256 ;
conf = override(conf_, featOpts) ;
% make gray scale
im = im2double(im) ;
if size(im, 3) > 1, im = rgb2gray(im) ; end
% sanity check
if 0
im = 0 * double(im) ;
[v,u] = meshgrid(1:size(im,1), 1:size(im,2)) ;
im = (v - size(im,1)/2).^2 + (u - size(im,2)/2).^2 < 100*100 ;
im = 256*double(im) ;
end
E = edge(im, 'canny') ;
[imx,imy] = gradient(imsmooth(double(im),.5)) ;
wgt = sqrt(imx.^2 + imy.^2) ;
period = conf.period / 180 ;
angle = mod(atan2(imy, imx) / pi, period) / period ;
vocabSize = conf.vocabSize ;
angle = angle * vocabSize ;
pre = floor(angle) ;
postw = angle - pre ;
prew = 1 - postw ;
post = pre + 1 ;
pre = mod(pre, vocabSize) + 1 ;
post = mod(post, vocabSize) + 1 ;
% convert to sparse format for uniformity
sel = find(wgt .* E >= conf.gradientThresh) ;
[i, j] = ind2sub(size(im), sel) ;
feat.frames = [j(:)' j(:)'; i(:)' i(:)'] ;
feat.descrs = [pre(sel); post(sel)]' ;
feat.weights = [wgt(sel) .* E(sel) .* prew(sel) ; wgt(sel) .* E(sel) .* postw(sel)]' ;
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