Difference between two blocks of images?

How can I extract a block of pixels from an image and subtract it with another block of different image to find minimum disparity? my block size is 8. How do I proceed with this?

iteration = 0;

subsub = zeros(8);
subblock = zeros(8);
x = rgb2gray(imread('testimage.jpg'));
J = imresize(x, 0.7); 
x0 = imrotate(J,30);

 r = randi(225-7);
 c = randi(225-7); //random block of size 8 from original image

subblock = x(r:r+7, c:c+7);
imshow(subblock);

block = size(x0);
k =1; m= 8;

for i =1:8: (block(1)-7) 
    for j = k:m
     submatrix = x0(i,j);  // creating a block from second image

      end

    for w = 1 : 8
        for y = 1:8
        diff = (subblock(w,y) - submatrix(w,y))^2; // subtracting two blocks
        end
    end
    k = m+1; // to move to the next block
    m = m+8;
  end

When I run the code, my submatrix is always zero. Why is that? please help me