Recovering 3D shape from shading is an ill-posed problem that the visual system can solve only by making use of additional informa- tion such as the position of the light source. Previous research has shown that people tend to assume light is above and slightly to the left of the object [Sun and Perona 1998]. We present a study to in- vestigate whether the visual systemalso assumes the angle between the light direction and the viewing direction. We conducted a shape perception experiment in which subjects estimated surface orien- tation on smooth, virtual 3D shapes displayed monocularly using local Lambertian shading without cast shadows. We varied the an- gle between the viewing direction and the light direction within a range +/- 66 deg (above/below), and subjects indicated local sur- face orientation by rotating a gauge figure to appear normal to the surface [Koenderink et al. 1992]. Observer settings were more ac- curate and precise when the light was positioned above rather than below the viewpoint. Additionally, errors were minimized when the angle between the light direction and the viewing direction was 20-30 deg. Measurements of surface slant and tilt error support this result. These findings confirm the light-from-above prior and pro- vide evidence that the angle between the viewing direction and the light direction is assumed to be 20-30 deg above the viewpoint.
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