What do reflections tell us about the shape of a mirror?

Three-dimensional shape may be perceived from static images. Contours, shading, texture gradients, perspective and occlusion are well-studied cues to this percept. When looking at a picture of a specular object, such as a silver vase, one additional cue is potentially available: a deformed picture of the reflected environment is seen in the surface of the object and the amount and type of deformation depend on its shape. Can specular reflections be used as a visual cue for shape perception? Our experiments show that our subjects are very poor at judging the shape of mirror surfaces in absence of other visual cues. However, for a considerable subset of the stimuli, subjects are highly consistent in their (most often wrong) perception. This observation leads us to the hypothesis that our subjects rather than 'computing' a percept from each image based on geometrical considerations, may be associating a shape to each pattern in a stereotypical way, akin to pattern-matching. This behavior is reasonable since, as suggested by our ideal observer analysis, the information available from specular reflections is ambiguous when the surrounding world is (partially) unknown.