The visual ambiguity of a moving plane

It is shown that the optic flow field arising from motion relative to a visually textured plane may be characterized by eight parameters that depend on the observer's linear and angular velocity and the coordinate vector of the plane. These three vectors are not, however, uniquely determined by the values of the eight parameters. First, the optic flow field does not supply independent values for the observer's speed and distance from the plane; it only gives the ratio of these two quantities. But more unexpectedly, the equations relating the observer's linear velocity and the plane's coordinate vector to the eight parameters are still satisfied if the two vectors are interchanged or reversed in direction, or both. So in addition to the veridical interpretation of the optic flow field there exist three spurious interpretations to be considered and if possible excluded. This purpose is served by the condition that an interpretation can be seriously entertained only if it attributes every image element to a light source in the observer's field of view. This condition immediately eliminates one of the spurious interpretations, and exhibits the other two as mutually inconsistent: one of them is tenable only if all the visible sources lie on the forward half of the plane (relative to the observer's linear velocity); the other only if they all lie on the backward half-plane. If the sources are distributed over both halves of the plane, only the veridical interpretation survives. Its computation involves solving a 3 $\times $ 3 eigenvalue problem derived from the flow field. If the upper two eigenvalues coincide, the observer must be moving directly towards the plane; if the lower two eigenvalues coincide, his motion must be directly away from it; in both cases the spurious interpretation merges with the veridical one. If all three eigenvalues are equal, it may be inferred that either the observer's linear velocity vanishes or the plane is infinitely distant.