I = imread('after-384x288.jpg');
subplot(2,3,1);imshow(I, []);  xlabel('input')

C = zeros(256);


x = 1
y = 1
nx = 200
ny = 300

xoffset = 20
yoffset = 12


A=I(x:nx-xoffset, y:ny-yoffset);
B=I(x+xoffset:nx, y+yoffset:ny);

subplot(2,3,2);imshow(A, []);  xlabel('A')
subplot(2,3,3);imshow(B, []);  xlabel('B')

i = sub2ind(size(C), double(A(:)+1), double(B(:)+1) );

u = unique(i);
C(u) = hist(i, u);


subplot(2,3,4); imshow(C,[]);  xlabel('C')
subplot(2,3,5); imshow(log(C+1),[]); xlabel('log(C+1)')
subplot(2,3,6); imshow(C>0,[]); xlabel('C>0')