How the visual system infers the shape of opaque objects has already been investigated in a large number of theoretical and empirical works, and several essential shape cues have been identified-for example, contours and edges, texture, shading, mirror images, and highlights (see Figure 1a). Perceiving the spatial extent and shape of objects is a fundamental ability that allows us to identify objects and successfully interact with them. 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. Furthermore, the influence of individual image regularities varied considerably depending on the properties of both object and scene. Our results show that the shape of transparent objects was perceived both less accurately and less precisely than in the opaque case. We further investigate experimentally how shape perception depends on the availability of these potential cues in realistic scenes under natural viewing conditions. Using computer simulations, we first analyze under which conditions these regularities may be used as shape cues. 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. 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. In order to estimate the shape of objects, the visual system must refer to shape-related regularities in the (retinal) image.
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