The role of self-occluding contours in material perception.

The human visual system extracts both the three-dimensional (3D) shape and the material properties of surfaces from single images. Understanding this remarkable ability is difficult because the problem of extracting both shape and material is formally ill posed: information about one appears to be needed to recover the other. Recent work has suggested that a particular class of image contours formed by a surface curving smoothly out of sight (self-occluding contours) contains information that co-specifies both surface shape and material for opaque surfaces. However, many natural materials are light permeable (translucent); it is unknown whether there is information along self-occluding contours that can be used to distinguish opaque and translucent materials. Here, we present physical simulations, which demonstrate that variations in intensity generated by opaque and translucent materials are linked to different shape attributes of self-occluding contours. Psychophysical experiments demonstrate that the human visual system exploits the different forms of intensity-shape covariation along self-occluding contours to distinguish opaque and translucent materials. These results provide insight into how the visual system manages to solve the putatively ill-posed problem of extracting both the shape and material properties of 3D surfaces from images.