The preferred angle of illumination in shape from shading

Abstract

Recovering 3D shape from shading is an ill-posed problem which the visual system can resolve only after making assumptions about the lighting, surface material, and/or viewpoint. Previous research has shown that people tend to assume light is above and slightly to the left [Sun and Perona 1998], but it is not known whether there is also an assumed elevation angle of the light with respect to the viewpoint. Consider a plane containing the eye, the point of interest, and the assumed light position. In this paper, we investigate whether there is an assumed elevation of light within this plane by measuring observer percepts of surface orientation with varying angles of light above or below the viewpoint. The estimations of surface orientation were made on smooth, virtual 3D shapes displayed monocularly using local Lambertian shading. We varied the elevation angle of light within a range +/66 deg (above/below) from the viewpoint, and subjects indicated local surface orientation at specified points on the shape by rotating a gauge figure to appear normal to the surface [Koenderink et al. 1992]. Observer settings were more accurate when the light was positioned above rather than below the viewpoint. Additionally, the angular difference between the indicated and actual surface normal was minimized when the light was 20-30 deg above the viewpoint. Measurements of slant and tilt error, as well as the variance of settings, support this result. These findings confirm that the human visual system has an aboveand-left prior for light position and demonstrate that the preferred position is 20-30 deg above the viewpoint.