Illumination, Shading and the Perception of Local Orientation

Abstract

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.

A Perceptual-Statistics Shading Model

“…In our results, these thresholds correspond to approximately 15 • − 25 • of the ground truth slant with the exception of the south sector. Mamassian and Kersten [21] expect this threshold to be approximately 20 • which is consistent with our finding of 15 • − 25 • .…”

“…The division into subintervals was selected on a priori grounds. According to previous evidence [21] and also concluding from our own analysis, the underestimation of slant is zero at ca. 20 • of ground-truth slant and highest for slants 40 • − 60 • (see also Figures 5 and 6).…”

“…Mamassian and Kersten investigated the perception of local surface orientation on a simple smooth object, under various illumination conditions [21]. They analyzed perceived local orientations for several points on the surface and quantified the slant and tilt of the local tangent plane.…”

The assumed light direction for perceiving shape from shading

“…Mamassian and Kersten demonstrated that the occluding contour of an object can be a significant cue to surface orientation [Mamassian and Kersten 1995]. In their experiment, subjects were instructed to orient a gauge figure to estimate local surface orientation for an object that is shaded under various lighting conditions.…”

“…Several studies have investigated how lighting and shading affect shape perception [Mamassian and Goutcher 2001;Mamassian and Kersten 1995;Koenderink et al 1996;Langer and Bulthoff 2001], yet none have specifically examined variations in the slant of light.…”