Image deformation as a cue to material category judgment

Human observers easily recognize complex natural phenomena, such as flowing water, which often generate highly chaotic dynamic arrays of light on the retina. It has not been clarified how the visual system discerns the source of a fluid flow. Here we show that the magnitude of image deformation caused by light refraction is a critical factor for the visual system to determine the perceptual category of fluid flows. Employing a physics engine, we created computer-rendered scenes of water and hot air flows. For each flow, we manipulated the rendering parameters (distortion factors and the index of refraction) that strongly influence the magnitude of image deformation. The observers rated how strongly they felt impressions of water and hot air in the video clips of the flows. The ratings showed that the water and hot air impressions were positively and negatively related to the magnitude of image deformation. Based on the results, we discuss how the visual system heuristically utilizes image deformation to discern non-rigid materials such as water and hot air flows.