comparing values of nodes inside grid

Hi friends ,,, i just beginner in matlab and have some task ,, i making grid on image and trying to check the nodes of each segment of image then change it's values to 0 or 1 equal to the condition ,,, i made a loop but there is a mistake .. all image showed as black // please check and help ?? I = imread('lena64.bmp'); figure,imshow(I); hold on M = size(I,1); N = size(I,2); a=4; b=4; for k = 1:a:M x = [1 N]; y = [k k]; plot(x,y,'Color','black','LineStyle','-'); set(findobj('Tag','MyGrid'),'Visible','on') end for k = 1:b:N x = [k k]; y = [1 M]; plot(x,y,'Color','black','LineStyle','-'); set(findobj('Tag','MyGrid'),'Visible','on') end

for p=1:a:M for q=1:b:N

     for h=4:a:M 
       for z=4:b:N

 if (I(p,q)==I(h,z)==I(p,h)==I(z,q))
    for k= p:h
       for k2= q:z 
           I(k,k2)=0;
       end
    end
       else
      for k= p:h
       for k2 = q:z
              I(k,k2)=1;
       end
      end
  end
       end
     end
   end
end
        figure,imshow(I);
hold off