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.

The spectral shape, irradiance, direction, and diffuseness of daylight vary regularly throughout the day. The variations in illumination and their effect on the light reflected from objects may in turn provide visual information as to the time of day. We suggest that artists' color choices for paintings of outdoor scenes might convey this information and that therefore the time of day might be decoded from the colors of paintings. Here we investigate whether human viewers' estimates of the depicted time of day in paintings correlate with their image statistics, specifically chromaticity and luminance variations. We tested time-of-day perception in 17th- to 20th-century Western European paintings via two online rating experiments. In Experiment 1, viewers' ratings from seven time choices varied significantly and largely consistently across paintings but with some ambiguity between morning and evening depictions. Analysis of the relationship between image statistics and ratings revealed correlations with the perceived time of day: higher "morningness" ratings associated with higher brightness, contrast, and saturation and darker yellow/brighter blue hues; "eveningness" with lower brightness, contrast, and saturation and darker blue/brighter yellow hues. Multiple linear regressions of extracted principal components yielded a predictive model that explained 76% of the variance in time-of-day perception. In Experiment 2, viewers rated paintings as morning or evening only; rating distributions differed significantly across paintings, and image statistics predicted people's perceptions. These results suggest that artists used different color palettes and patterns to depict different times of day, and the human visual system holds consistent assumptions about the variation of natural light depicted in paintings.