JPEG
DCT
Subtract the previous DCT coefficient from the original picture data and calculate the average value. Multiply by +1 or -1, depending on whether the square is black or white.{(+1) x (105-100)}
+ {(-1) x (75-100)}
+ {(+1) x (135 - 100)}
+ {(-1) x (85 - 100)} / 4
= 20