ILLUMINATION, SHADING AND THE PERCEPTION OF LOCAL ORIENTATION

We investigated the perception of local surface orientation on a simpl e smooth object, under several different illumination conditions. The perceived local orientation was determined for several points on the s urface 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 th at subjects were less biased at estimating the surface orientation whe n the shape was locally egg-shaped rather than saddle-shaped or cylind rical. In order to investigate the relationship between perceived shap e and light source direction, we developed a method to compute the lig ht source direction most consistent with an observer's settings. Also we compared human errors with those of an ''ideal observer'' which mak es explicit assumptions about the illuminations, shapes and materials in its world. From converging evidence based on (i) the light directio n most consistent with the observer's settings; (ii) a supplementary e xperiment where the object is displayed as a silhouette, and (iii) the computer simulations of the ideal observer, we conclude that the obse rvers used the occluding contour of the object rather than shading to estimate the local surface orientation. Copyright (C) 1996 Elsevier Sc ience Ltd.