Perceptual transparency from image deformation

Significance The perception of liquids, particularly water, is a vital sensory function for survival, but little is known about the visual perception of transparent liquids. Here we show that human vision has excellent ability to perceive a transparent liquid solely from dynamic image deformation. No other known image cues are needed for the perception of transparent surfaces. Static deformation is not effective for perceiving transparent liquids. Human vision interprets dynamic image deformation as caused by light refraction at the moving liquid’s surface. Transparent liquid is well perceived from artificial image deformations, which share only basic flow features with image deformations caused by physically correct light refraction. Human vision has a remarkable ability to perceive two layers at the same retinal locations, a transparent layer in front of a background surface. Critical image cues to perceptual transparency, studied extensively in the past, are changes in luminance or color that could be caused by light absorptions and reflections by the front layer, but such image changes may not be clearly visible when the front layer consists of a pure transparent material such as water. Our daily experiences with transparent materials of this kind suggest that an alternative potential cue of visual transparency is image deformations of a background pattern caused by light refraction. Although previous studies have indicated that these image deformations, at least static ones, play little role in perceptual transparency, here we show that dynamic image deformations of the background pattern, which could be produced by light refraction on a moving liquid’s surface, can produce a vivid impression of a transparent liquid layer without the aid of any other visual cues as to the presence of a transparent layer. Furthermore, a transparent liquid layer perceptually emerges even from a randomly generated dynamic image deformation as long as it is similar to real liquid deformations in its spatiotemporal frequency profile. Our findings indicate that the brain can perceptually infer the presence of “invisible” transparent liquids by analyzing the spatiotemporal structure of dynamic image deformation, for which it uses a relatively simple computation that does not require high-level knowledge about the detailed physics of liquid deformation.