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Abstract

Many commonly occurring substances are somewhat translucent (e.g. wax, jade, fruit-flesh, and cheese). When light strikes a translucent material, it passes through the surface and scatters a number of times within the body of the object before re-emerging. This causes light to ‘bleed’ through translucent objects, giving them a distinctive visual softness and glow. What image cues are responsible for this characteristic appearance? How do we distinguish between translucent and opaque materials? Here we use a combination of image statistics and psychophysics to study the perception of translucent materials. There has been a large amount of previous work on the perception of materials that transmit light. Almost all of this work is based on simple physical models of transparency, (e.g. the episcotister model of Metelli, 1974), in which the object of interest is a thin filter or screen. However, recent advances in computer graphics (Jensen et al. 2001; Jensen and Buhler, 2002) make it possible to simulate the complex physics of solid translucent objects. We have used this model to study how a wide range of factors influence the perception of translucency, including highlights, colour, contrast, brightness, blurriness, and conditions of illumination. Our main findings are as follows: (1) We have found that it is possible to enjoy a vivid impression of translucency even when many of the cues that were traditionally believed to be important for the perception of transparency (e.g. X-Junctions, contrast conditions) are absent from the display. (2) We argue that sub-surface light scattering is too complex for the visual system to infer translucency by inverse optics. Accordingly we suggest that the visual system tracks low-level image statistics that reliably correlate with changes in translucency. (3)We find that perceived translucency varies dramatically with conditions of illumination. We compare how well a number of candidate cues can predict these variations. In conclusion, there is a wide range of materials that have not been studied before, and which we are only just beginning to understand. Many intuitions that we have about which cues are crucial for recovering opacity turn out to be at best incomplete.