Measurements of the effect of surface slant on perceived lightness

Abstract

When a planar object is rotated with respect to a directional light source, the reflected luminance changes. If surface lightness is to be a reliable guide to surface identity, observers must compensate for such changes. To the extent they do, observers are said to be lightness constant. We report data from a lightness matching task that assesses lightness constancy with respect to changes in object slant. On each trial, observers viewed an achromatic standard object and indicated the best match from a palette of 36 grayscale samples. The standard object and the palette were visible simultaneously within an experimental chamber. The chamber illumination was provided from above by a theater stage lamp. The standard objects were uniformly-painted flat cards. Different groups of naive observers made matches under two sets of instructions. In the Neutral Instructions, observers were asked to match the appearance of the standard and palette sample. In the Paint Instructions, observers were asked to choose the palette sample that was painted the same as the standard. Several broad conclusions may be drawn from the results. First, data for most observers were neither luminance matches nor lightness constant matches. Second, there were large and reliable individual differences. To characterize these, a constancy index was obtained for each observer by comparing how well the data were accounted for by both luminance matching and lightness constancy. The index could take on values between 0 (luminance matching) and 1 (lightness constancy). Individual observer indices ranged between 0.17 and 0.63 with mean 0.40 and median 0.40. An auxiliary slant-matching experiment rules out variation in perceived slant as the source of the individual variability. Third, the effect of instructions was small compared to the inter-observer variability. Implications of the data for models of lightness perception are discussed.

Optimal sampling of visual information for lightness judgments

“…Along these lines, we propose that the brightness of the most illuminated region within the object is the source from which our visual system extracts a global impression of lightness. The use of the simple heuristic described here agrees with the finding that observers are impaired when they have to judge the lightness of a flat object, because they are unable to discount the effect of surface orientation (24). Flat surfaces show a reduced gradient of illumination generated by variations in the angle of the incident light, and the assumption that some portion of the object is always optimally illuminated does not hold.…”

“…One way the system could use to solve this task is to produce separate representations of the illuminant on the surface and its albedo (10). However, there is evidence that lightness judgments are, at least to some degree, directly based on brightness (24,26). Along these lines, we propose that the brightness of the most illuminated region within the object is the source from which our visual system extracts a global impression of lightness.…”

“…Flat surfaces show a reduced gradient of illumination generated by variations in the angle of the incident light, and the assumption that some portion of the object is always optimally illuminated does not hold. In such a situation, the heuristic we propose would produce a lightness estimate highly contaminated by object orientation, as observed (24).…”

“…The piece of paper cut out from the cone, mounted at the location of the matching box, appears much darker than the cone, because the surface of the computer monitor was oriented nearly parallel to the light source. Observers are known to be far from perfect in taking such geometrical aspects into account (24). The match is also much brighter than the mean luminance across the whole object, which raises the question as to why observers match the brightest parts of the objects, and whether this strategy is of advantage.…”

Color perception of 3D objects

“…Recently, there have been studies of how the scene geometry affects the perception of lightness and color of such surfaces [Kraft and Brainard 1999;Ripamonti et al 2004;Yang and Maloney 2001;Boyaci et al 2003;Doerschner et al 2004;Boyaci et al 2004]. In these experiments, the observer does not need to integrate a range of test object chromaticities and luminannces, as the reflected light is essentially uniform across the flat object.…”