Abstract
The colors of transparent objects, such as gemstones, have a unique quality closer to light than surfaces, which is both beautiful and an enigma. While viewing a transparent object, our eyes receive light that is a multiplicative composite of surface, illumination, and filter spectra. Yet, when we observe the color of a transparent object, we simultaneously perceive surface and overlay layers in two different colors at the one retinal locations. How our brains separate the information and extract the transparency color remains a mystery. Physical characteristics of transmitting layers could provide cues for transparency separation by generating images with stereotypical geometrical and color features, such as X-junctions, multiplicative contrast changes, 3-D diagonal color transforms in cone space, and revealing motion. We compared the importance of such cues, by estimating a perceptual scale for transparency with stimuli where X or T junctions, revealing or occluding motions, and consistent or inconsistent colors, combine or compete in forced-preference psychophysics experiments. We found that motion works as a facilitator by separating layers for geometric cues to assign as transparent or opaque, and that geometric cues can override color inconsistency. A probabilistic graphical model provides a quantitative estimate of the influence of different factors on perceived transparency.