Visual shape perception in the case of transparent objects

In order to estimate the shape of objects, the visual system must refer to shape-related regularities in the (retinal) image. For opaque objects, many such regularities have already been identified, but most of them cannot simply be transferred to transparent objects, because they are not available there at all or are available only in a substantially modified form. We here consider three potentially relevant regularities specific to transparent objects: optical background distortions due to refraction, changes in chromaticity and brightness due to absorption, and multiple mirror images due to specular reflection. Using computer simulations, we first analyze under which conditions these regularities may be used as shape cues. We further investigate experimentally how shape perception depends on the availability of these potential cues in realistic scenes under natural viewing conditions. Our results show that the shape of transparent objects was perceived both less accurately and less precisely than in the opaque case. Furthermore, the influence of individual image regularities varied considerably depending on the properties of both object and scene. This suggests that in the transparent case, what kind of information is usable as a shape cue depends on a complex interplay of properties of the transparent object and the surrounding scene.