Optical material properties affect detection of deformation of non-rigid rotating objects, but only slightly.

Although rigid three-dimensional (3D) motion perception has been extensively studied, the visual detection of non-rigid 3D motion remains underexplored, particularly with regard to its interactions with material perception. In natural environments with various materials, image movements produced by geometry-dependent optical effects, such as diffuse shadings, specular highlights, and transparent glitters, impose computational challenges for accurately perceiving object deformation. This study examines how optical material properties influence human perception of non-rigid deformations. In a two-interval forced choice task, observers were shown a pair of rigid and non-rigid objects and asked to select the one that appeared more deformed. The object deformation varied across six intensity levels, and the stimuli included four materials (dotted matte, glossy, mirror, and transparent). We found that the material has only a small effect on deformation detection, with the threshold being slightly higher for transparent than other materials. The results remained the same regardless of the viewing angles, light field conditions (Experiment 1), and the deformation type (Experiment 2). These results show the robust capacity of the human visual system to perceive non-rigid object motion in complex natural visual environments.