Visual Perception of Materials that Transmit Light

Many materials that we commonly encounter, such as ice, marmalade and wax, transmit some proportion of incident light. Broadly, these can be separated into transparent and translucent materials. Transparent materials (e.g. gemstones, water) are dominated by specular reflection and refraction, leading to a characteristic glistening, pellucid appearance. Translucent materials (e.g. marble, cheese) exhibit sub-surface light scattering, in which light bleeds diffusely through the object creating a distinctive soft or glowing appearance. Importantly, both types of material are poorly approximated by Metelli’s episcotister or other models of thin neutral density filters that have shaped our understanding of transparency to date. I will present various psychophysical and theoretical studies that we have performed using physically based computer simulations of light transport through solid transmissive objects. One important observations is that these materials do not exhibit many image features traditionally thought to be central to transparency perception (e.g. X-junctions). However, they compensate with a host of novel cues, which I will describe. I will discuss the perceptual scales of refractive index and translucency and report systematic failures of constancy across changes in illumination, 3D shape and context. I will discuss conditions under which various low-level image statistics succeed and fail to predict material appearance. I will also discuss the difficulties posed by transmissive materials for the estimation of 3D shape. Under many conditions, human vision appears to use simple image heuristics rather than correctly inverting the physics. I will show how this can be exploited to create illusions of material appearance