THE EFFECT OF ILLUMINANT POSITION ON PERCEIVED CURVATURE

In shaded scenes surface features can appear either concave or convex, depending upon the viewer's judgement about the direction of the prev ailing illumination. If other curvature cues are added to the image th is ambiguity can be removed, However, it is not clear to what extent, if any, illuminant position exerts an influence on the perceived magni tude of surface curvature. Subjects were presented with pairs of spher ical surface patches in a curvature matching task. The patches were de fined by shading and texture cues. The perceived curvature of a standa rd patch was measured as a function of light source position. We found a clear effect of light source position on apparent curvature. Percei ved curvature decreased as light source tilt increased and as light so urce slant decreased. We also found that the strength of this effect i s determined partly by a surface's reflectance function and partly by the relative weight of the texture cue. When a specular component was added to the stimuli, the effect of light source orientation was weake ned, The weight of the texture cue was manipulated by disrupting the r egular distribution of texture elements. We found an inverse relations hip between the strength of the effect and the weight of the texture c ue: lowering the texture cue weight resulted in an enhancement of the illuminant position effect. Copyright (C) 1996 Elsevier Science Ltd.