hog.m

function ohist = hog(I)
%
% compute orientation histograms over 8x8 blocks of pixels
% orientations are binned into 9 possible bins
%
% I : grayscale image of dimension HxW
% ohist : orinetation histograms for each block. ohist is of dimension (H/8)x(W/8)x9
%

[h,w] = size(I); %size of the input
h2 = ceil(h/8); %the size of the output
w2 = ceil(w/8);
nori = 9;       %number of orientation bins

[mag,ori] = mygradient(I);
ori = (ori./180).*pi;
thresh = 0.1*max(mag(:));  %threshold for edges


% separate out pixels into orientation channels
% you should divide up the range of edge orientations [-pi/2 , pi/2]
% into 9 equal sized bins.

% as a sanity check, we will make sure that every pixel gets
% assigned to at most one orientation bin in the for loop below
pixelbincount = zeros(size(I));
ohist = zeros(h2,w2,nori);

currOri = -pi/2;
stepOri = pi/9;

for i = 1:nori
  % create a binary image containing 1's for the pixels that are edges at this orientation
  B = zeros(h, w);
  for j = 1:(h*w)
      if mag(j) >= thresh && ori(j) >= currOri && ori(j) < (currOri+stepOri)
          B(j) = 1;
      else
          B(j) = 0;      
      end
  end
  currOri = currOri + stepOri;

  % this is just for checking correctness
  pixelbincount = pixelbincount + B;

  % record which pix
  % sum up the values over 8x8 pixel blocks.
  chblock = im2col(B,[8 8],'distinct');  %useful function for grabbing blocks
                                         %sum over each block and store result in ohist
  ohist(:,:,i) = reshape(sum(chblock,1),h2,w2);
end

% At this point, if each pixel has been included in at most 
% one orientation bin, then all the entries in pixelbincount
% should be <= 1. If this assertion fails then you should 
% visualize pixelbincount, ori etc. to help debug.
assert(all(pixelbincount(:)<=1),'every pixel should appear in only a single bin');

% normalize the histogram so that sum over orientation bins is 1 for each block
%   NOTE: Don't divide by 0! If there are no edges in a block (ie. this counts sums to 0 for the block) then just leave all the values 0. 

for i = 1:nori
    n = sum(ohist(:,:,i), 2);
    if n ~= 0
        ohist(:,:,i) = ohist(:,:,i)./n;
    end
end