Uniqueness, the minimum norm constraint, and analog networks for optical flow along contours

E. Hildreth´s (1984) method of computing optical flow along contours cannot resolve the aperture problem for a rigidly translating straight line contour. The authors propose an additional constraint, which they call the minimum norm constraint, as a means of resolving the ambiguity for such a contour. The minimum norm constraint tends to drive the velocity estimate towards the direction normal to the contour everywhere along the contour, i.e., it counters the effect of the smoothness constraint. This is in accord with recent psychophysical studies of K. Nakayama and G. Silverman (1988) which have revealed the presence of such a tendency in the human visual system. The authors propose an analog network for computing contour based optical flow in real-time. They illustrate the minimum norm constraint through experiments