How does the perception of shape interact with the perception of shiny material?

Intensity variations within the image boundaries of an object are a function of variations in illumination, surface reflectance, and shape. Even if the shape is known, there is an inherent ambiguity in distinguishing whether variations are due to pigment change or reflections. This ambiguity is particularly acute for highly specular shiny surfaces. The traditional view is that specular highlights are crucial to the perception of shiny material; however, other more subtle information may be important to perceiving material. Hartung & Kersten (2002) showed that reflections that are "stuck" to a rotating object are perceived to be paint, presumably because they move like paint. Dror, Leung, Adelson & Willsky (2001) showed that natural illumination maps exhibit statistical regularity similar to that of natural images. Fleming, Dror & Adelson (2001) showed that subjects more accurately estimate material under natural rather than unnatural illumination. This suggests that the visual system has knowledge of how natural illumination is warped by the shape of the reflecting object. But how does the perception of shape interact with the perception of material? We investigated the roles of local surface curvature and global contour on the perception of material. Because areas of high curvature "see" more of the illuminating environment, reflections are compressed in the direction of high curvature and stretched in the direction of low curvature. We show luminance features consistent with this stretching are perceived as reflections. When rotated across the object's surface so as to be inconsistent with this stretching, these features are perceived to be paint. We also show that when global contour affects perceived local shape, the perceived shininess of the material is also affected. These studies show that the visual system uses prior knowledge of the interaction of shape and illumination to distinguish reflections from pigment changes.