The Wave Equation And Optic Flow

The standard optic-flow gradient constraint equation relates temporal and spatial derivatives of the image to the image-plane velocities. Differentiating this equation with respect to time produces a non-standard wave equation. Since the resulting equation involves second spatial and temporal derivatives of the intensity, it is probably too noise sensitive for use in computing the velocity field. However, it is likely to be of use in checking the results of a velocity computation for consistency. Most optic-flow computations assume that the intensity of a scene point remains unchanged as it moves around. The wave equation constraint can be used to check this assumption. If the equation is strongly violated, then it is a signal that the velocity field should not be believed, probably because the illumination is changing.