Visual information for the identification of nonrigid motion.

van Zuijlen et al. (2025) have recently argued that the perceptual distinction between rigid and non-rigid motions of a torus knot can be based on patterns of optical flow produced by systematic changes in image shading. The present commentary will present theoretical analyses and empirical demonstrations to show that the 2D vector flow field produced by smoothly shaded surfaces is relatively useless as a source of information about rigid and nonrigid motion when an object moves relative to its sources of illumination. The evidence to support this conclusion is that the nonrigid distortions can easily be identified from deforming smooth occlusion contours when all variations in shading are removed. Although a rotating shaded surface without smooth occlusions can be perceived as rigid, that perception is eliminated entirely if its flow field is presented as a pattern of moving lines without any gradients of shading. The computation of 2D image velocities is often taken for granted by researchers in visual perception, but the theoretical underpinnings of that approach are highly questionable when applied to untextured shaded surfaces that move relative to their sources of illumination.