Estimating local shape from shading in the presence of global shading

Abstract

In theory, global shading may help with the estimation of local surface structure from shading (e.g., in specifying the illuminant direction). Empirically, we do not know whether human observers combine the information given by the local and global shading to estimate local shape.Observers had to indicate the orientation of a local elongated perturbation with or without global shading information provided by a background surface. Our psychophysical results show the following:1. Observers do not estimate the orientation of the local perturbation more accurately with global shading information than they do in the absence of such information.2. Responses depend dramatically on the inclination between the illuminant direction and the viewing direction. For an inclination of 20 0 , observers indicate more or less the orientation of the local ridge; however, for an inclination of 40 0 , they indicate either the direction of the illuminant or an orientation close to the shadow edge ofthe perturbation. Most subjects show some combination of these behaviors. This behavior is not altered by global shading information.We conclude that in our paradigm, global shading information does not aid the estimation of local shape.In most theoretical work on "shape from shading, " the three-dimensional (3-D) structure of a surface is calculated from the local luminance distribution. (Horn, 1975(Horn, , 1977 Ikeuchi & Horn, 1981;Oliensis, 1991;Pentland, 1982Pentland, , 1984Pentland, , 1989. However, the shading of an object is not determined just by the surface orientation but also by the illuminant direction and the nature of the surface material. It is therefore impossible to estimate the local shape uniquely from a local shading pattern. Constraints necessary to find local surface structure from patterns of shading can be given by the global shading of the surrounding background surface. For example, the global shading can be used to estimate the illuminant direction (Oliensis, 1991;Pentland, 1982).Some of these shape-from-shadingtheories are proposed for human vision (Pentland, 1982(Pentland, , 1984(Pentland, , 1989. However, we do not know empirically whether human observers combine both local and global shading to estimate local shape. In our experiments, we report on the extent to which observers can indicate the orientation of a local ridge on a background surface (examples of the set of stimuli are given in the first column of Figure I). The global shading information is controlled by the background surface; a spherical background surface reveals global shading information, whereas a planar one does not (equal gray value). In this paper, we investigate whether the responses of observers are altered by the nature of the background surface.Furthermore, we investigate how the responses of the observers depend on the illurninant direction. What is the influence of the inclination between the illuminant direction and the viewing direction on the ability of observers to indicate the orientation of the local ridge? Do observers resp...

Visual discrimination of local surface depth and orientation

“…The results of this research showed clearly that observers' slant judgments for simple planar surfaces tend to be systematically underestimated, and that they can be highly unreliable as well. More recently, similar findings have been obtained for judgments of local metrical structure (e.g., relative depths or orientations) with the use of more complex scenes of smoothly curved surfaces, and for objects depicted with several different types of optical information such as shading (e.g., Erens, Kappers, & Koenderink, 1993;Koenderink, van Doorn, & Kappers, 1992;Mingolla & Todd, 1986), texture (e.g., Todd & Akerstrom, 1987), motion (e.g., Todd &Bressan, 1990;Todd & Norman, 1991), and binocular disparity (e.g., Foley, 1980;McKee, Levi, & Bowne, 1990).…”

The human visual system's assumption that light comes from above is weak

“…Studies that used complex illuminated objects with strong lighting direction cues concluded that lighting cues guide shape-from-shading (6, 7), whereas studies that used weaker lighting cues concluded that they have little effect (8)(9)(10)(11)(12). We discuss the lighting cues in previous studies in further detail in SI Discussion.…”

Fechner, information, and shape perception

“…When variations in local shape are experimentally isolated from global information about the irradiance distribution and surrounding surface structure, local shape is very poorly resolved (Erens, Kappers, & Koenderink, 1993a, 1993b. In the Erens et al (1993b) study, observers' categorical shape judgments (eight regions of the shape index-Eq.…”