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R. Robilotto, Q. Zaidi; Perceptual Strategies for the Identification of Patterned 3–D Real Objects Across Illuminants . Invest. Ophthalmol. Vis. Sci. 2005;46(13):4688.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Lightness constancy studies show that material matches across illuminants cannot be explained by physical matches of reflectance or luminance. The perceptual qualities underlying lightness judgments have not been identified. Almost all studies have used flat plain stimuli, but real objects are often 3–D and patterned, giving additional cues for identification. We examine the perceptual strategies that underlie material identification of real objects. Methods: The stimuli were randomly crumpled papers printed with achromatic patterns with precisely calibrated mean–reflectance and reflectance–contrast. Pairs of objects were placed in two adjacent compartments. Three of the objects had identical patterns, while the fourth object differed in either mean–reflectance or reflectance–contrast in a method of constant stimuli. Illumination in one compartment was four times the radiance of the other. The observer was asked to identify the object physically different from the other three. Psychometric curves and thresholds for reflectance identification across illuminants as functions of mean–reflectance and reflectance–contrast were calculated for eight standard patterns. In two control experiments, using identical conditions, observers were asked to pick the object most dissimilar in contrast or brightness respectively. Results: For all observers, the reflectance identification functions were systematically skewed depending on the illumination on the test object and its mean or contrast relative to the standards. In the achromatic domain, the simplest percepts are perceived brightness and perceived contrast. A model of perceived–brightness dissimilarity (luminance–dissimilarity modified by light–adaptation) fit the psychometric curves for mean–reflectance, and a model of perceived–contrast dissimilarity (contrast–dissimilarity modified by surface scatter) fit the psychometric curves for reflectance–contrast. In addition, from the results of the control experiments, identification thresholds for mean–reflectance could be predicted from observers’ independent judgments of brightness–dissimilarity, and identification thresholds for reflectance–contrast could be predicted from observers’ independent judgments of contrast–dissimilarity. Conclusions: Two types of evidence show that, for achromatic material identification, observers use simple perceptual strategies based on brightness and contrast. These results argue against a reverse optics model, where observers first estimate illuminant intensity and then extract relative lightness by discounting the illuminant.
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