Illumination, shading and the perception of local orientation.

We investigated the perception of local surface orientation on a simple smooth object, under several different illumination conditions. The perceived local orientation was determined for several points on the surface and quantified as slant and tilt of the local tangent plane. We found an underestimation of the perceived slant and a larger variance for the perceived tilt than for the perceived slant. We found also that subjects were less biased at estimating the surface orientation when the shape was locally egg-shaped rather than saddle-shaped or cylindrical. In order to investigate the relationship between perceived shape and light source direction, we developed a method to compute the light source direction most consistent with an observer's settings. Also we compared human errors with those of an "ideal observer" which makes explicit assumptions about the illuminations, shapes and materials in its world. From converging evidence based on (i) the light direction most consistent with the observer's settings; (ii) a supplementary experiment where the object is displayed as a silhouette, and (iii) the computer simulations of the ideal observer, we conclude that the observers used the occluding contour of the object rather than shading to estimate the local surface orientation.