Contours produced by internal specular interreflections provide visual information for the perception of glass materials

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Abstract

Two experiments are reported that investigated how the perceptual identification of glass is influenced by banding contours formed by internal specular interreflections within glass materials. Observers made material categorization judgments for images depicting glass, chrome, shiny black and shiny white objects, and for contour drawings that were created by edge filtering images of glass, chrome or textured objects. Observers rated each stimulus by adjusting four sliders to indicate their confidence that the depicted material was glass, metal, shiny black, or something else, and these adjustments were constrained so that the sum of all four settings was always 100%. The results revealed that the rendered images were all categorized correctly with a high level of confidence. The contour drawings of glass and textured materials were also categorized correctly with a high level of confidence. However, the contour drawings of chrome materials were miscategorized as glass, with an average confidence rating that was significantly lower than those obtained for the glass contours. It is hypothesized that these different contour types are perceptually distinguished from one another based on how they align with the pattern of surface curvature on an object and the smoothness of the contours.

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Specular reflections and the perception of shape.

Roland Fleming, Edward Adelson, Antonio I Torralba (2004)

Many materials, including leaves, water, plastic, and chrome exhibit specular reflections. It seems reasonable that the visual system can somehow exploit specular reflections to recover three-dimensional (3D) shape. Previous studies (e.g., J. T. Todd & E. Mingolla, 1983; J. F. Norman, J. T. Todd, & G. A. Orban, 2004) have shown that specular reflections aid shape estimation, but the relevant image information has not yet been isolated. Here we explain how specular reflections can provide reliable and accurate constraints on 3D shape. We argue that the visual system can treat specularities somewhat like textures, by using the systematic patterns of distortion across the image of a specular surface to recover 3D shape. However, there is a crucial difference between textures and specularities: In the case of textures, the image compressions depend on the first derivative of the surface depth (i.e., surface orientation), whereas in the case of specularities, the image compressions depend on the second derivative (i.e., surfaces curvatures). We suggest that this difference provides a cue that can help the visual system distinguish between textures and specularities, even when present simultaneously. More importantly, we show that the dependency of specular distortions on the second derivative of the surface leads to distinctive fields of image orientation as the reflected world is warped across the surface. We find that these "orientation fields" are (i) diagnostic of 3D shape, (ii) remain surprisingly stable when the world reflected in the surface is changed, and (iii) can be extracted from the image by populations of simple oriented filters. Thus the use of specular reflections for 3D shape perception is both easier and more reliable than previous computational work would suggest.