The optical flow cannot be computed at a point in the image independently of neighboring points without introducing additional constraints, because the velocity field at each image point has two components while the change in image brightness at a point in the image plane due to motion yields only one constraint. Consider, for example, a patch of a pattern where brightness' varies as a function of one image coordinate but not the other. Movement of the pattern in one direction alters' the brightness at a particular point, but motion in the other direction yields no change. Thus components of movement in the latter direction cannot be determined locally.