In this fig, Is the point (i,j) the one in blue or the one in yellow?
and if the point is the one in blue then the point green should be (x,j) and not (x,j-1)
Similarly, the purple point should be (i,y) and not (i-1,y)
Also at the end when we add that common area it should be (0,0) to (i,j) and not (0,0) to (i-1,j-1)
Please explain!