This shows that the value of the fl

This shows that the value of the flow velocity (u, v) which minimizes the error g2 lies in the direction towards the constraint line along a line that intersects the constraint line at right angles. This relationship is illustrated geometrically in Fig. 4. The distance from the local average is proportional to the error in the basic formula for rate of change of brightness when fi, fi are substituted for u and v. Finally we can see that aZ plays a significant role only for areas where the brightness gradient is small, preventing haphazard adjustments to the estimated flow velocity occasioned by noise in the estimated derivatives. This parameter should be roughly equal to the expected noise in the estimate of E: + E;