Local histogram equalization manually

Question

I am implementing local histogram equalization manually but the result is not satisfactory What I want and what I am getting is in picture, I have implemented the code according to given expression but still results are not same.

Here is the code:

I=rgb2gray(imread('peppers.png'));
if (isa(I,'uint8'))
  I=double(I)/255;
end
if (size(I,3)==3)
  I=(I(:,:,1)+I(:,:,2)+I(:,:,3))/3; % average the RGB channels
end
windowsize=17;
% Create an empty array
Ieq = zeros(size(I,1),size(I,2));
% Apply this over a NxN region around each pixel (N is odd)
n = floor(windowsize/2); % <-- N is the half size of the region ie. [-N:N,-N:N]
for r=1+n:size(I,1)-n
  for c=1+n:size(I,2)-n
    % -- INSERT YOUR CODE BELOW ------------------------------------------
    % NOTE: For pixels near the boundary, ensure the NxN neighbourhood is still
    % inside the image (this means for pixels near the boundary the pixel may
    % not be at the centre of the NxN neighbourhood).
      if r-n <=1
      fromrow=1;
      torow=r+n;
      else
          fromrow=abs(r-n);
          if n+r >= size(I,1)
              torow=size(I,1);
          else
              torow=r+n;
          end
      end
      if c-n <= 1
          fromcol=1;
          tocol=c+n;
      else
          fromcol=abs(c-n);
          if c+n > size(I,2);
              tocol=size(I,2);
          else
              tocol=c+n;
          end
      end
      neighbour = I(fromrow:torow,fromcol:tocol);
      lessoreq=neighbour(neighbour<=I(r,c));
      sumofval=sum(lessoreq);
      pixval=sumofval/(size(neighbour,1)*size(neighbour,2));
      Ieq(r,c)=pixval;
      % -- INSERT YOUR CODE ABOVE ------------------------------------------
    end
  end
  imshow(Ieq);

Answer 1

I guess there is more than one formula for "Local Histogram Equalization" - you implemented a different formula than your wonted result.

I found an implementation here: http://angeljohnsy.blogspot.com/2011/06/local-histogram-equalization.html

I modified the code to use you input.

%http://angeljohnsy.blogspot.com/2011/06/local-histogram-equalization.html

A=rgb2gray(imread('peppers.png'));
figure,imshow(A);
Img=A;


%WINDOW SIZE
M=17;
N=17;


mid_val=round((M*N)/2);

%FIND THE NUMBER OF ROWS AND COLUMNS TO BE PADDED WITH ZERO
in=0;
for i=1:M
    for j=1:N
        in=in+1;
        if(in==mid_val)
            PadM=i-1;
            PadN=j-1;
            break;
        end
    end
end
%PADDING THE IMAGE WITH ZERO ON ALL SIDES
B=padarray(A,[PadM,PadN]);

for i= 1:size(B,1)-((PadM*2)+1)

    for j=1:size(B,2)-((PadN*2)+1)
        cdf=zeros(256,1);
        inc=1;
        for x=1:M
            for y=1:N
  %FIND THE MIDDLE ELEMENT IN THE WINDOW          
                if(inc==mid_val)
                    ele=B(i+x-1,j+y-1)+1;
                end
                    pos=B(i+x-1,j+y-1)+1;
                    cdf(pos)=cdf(pos)+1;
                   inc=inc+1;
            end
        end

        %COMPUTE THE CDF FOR THE VALUES IN THE WINDOW
        for l=2:256
            cdf(l)=cdf(l)+cdf(l-1);
        end
            Img(i,j)=round(cdf(ele)/(M*N)*255);
     end
end
figure,imshow(Img);
figure,
subplot(2,1,1);title('Before Local Histogram Equalization'); imhist(A);
subplot(2,1,2);title('After Local Histogram Equalization'); imhist(Img);

The result looks like the one you wanted: