The effect of specular highlights on 3D shape discrimination for monocular and binocular images

Abstract

Specular highlights provide a source of information about the 3D shape of a shiny object, which can allow accurate 3D shape discrimination from a monocular image (Norman, Todd & Orban, 2004). However, highlights pose a challenge for analysis of stereo shape information because they have binocular disparity relative to the surface of an object. If highlights were misinterpreted as matte shading or surface texture, highlight disparities would indicate the wrong 3D shape. In this study, we tested whether highlights provide a similar benefit under binocular and monocular viewing conditions, and whether the effect of highlights is modulated by the presence of surface texture, which might help to identify highlights. Observers performed sequential 3D shape discrimination of random, smoothly-curved symmetric objects. To prevent a 2D strategy, comparison objects were rotated in depth by 10° and rendered with a different light source direction. Objects were presented monocularly or binocularly, and rendered with either: diffuse shading only, shading and highlights, shading and texture, or shading with texture and highlights. We found that highlights provided a similar benefit in both monocular and binocular conditions, and this benefit not modulated by the presence of surface texture. Stereo provided a similar benefit with and without highlights. There was no evidence that stereo viewing interfered with use of highlights, even when surface texture was absent, and the presence of highlights did not interfere with use of stereo information. Despite the geometric challenge posed by stereo highlights, the visual system appears to be able to make effective use of both sources of information.