Highlight-shading relationship as a cue for the perception of translucent and transparent materials

Natural surfaces, such as those of food and drink, have translucent properties. Translucent materials involve complex optics, such as sub-surface scattering and refraction, but humans can easily distinguish them from opaque materials. Here, we investigated image features that are diagnostic of the perceived translucency and transparency, focusing on the fact that variations in the opacity of a surface affect largely the non-specular component (shading pattern) of an image and little the specular component (highlights). In a simple rating experiment with computer-generated objects, we show that the non-specular image component tends to be blurred, faint, and even partially contrast-reversed for objects that appear more translucent or transparent. A subsequent experiment further demonstrated that manipulation of the contrast and blur of the non-specular image component dramatically alters the apparent translucency of an opaque object. The results support the notion that the spatial and contrast relationship between specular highlights and non-specular shading patterns is a robust cue for the perceived translucency and transparency of three-dimensional objects.