Dividing an image into block and calculate difference between pixel

Hey, I have an image of size 512*521 .I divided into blocks of size 2 by 2. which means four pixels are defined for each block. now the my question is : How can I compute the difference between pixel pairs in each block? .e.g if U1,u2,u3, and u4 are pixels in the first block how can I generate a random value from each block then use it to compute the difference in the corresponding block . like for block1: diff0 =u1, diff1= u2-rand value, diff2 = u3-rand value, diff3=u4- Then do it for all blocks and store the results for each block in a vector . Your help will be highly appreciated.