Object lightness constancy: effects of object pose and shape

Abstract

Purpose. Most studies of lightness constancy have considered how light source intensity affects perceived object lightness. Here we report experiments designed to help unravel how perceived lightness depends on object pose and shape. Methods. Subjects viewed gray test objects placed in an illuminated experimental chamber. The chamber illumination was provided from above by a single theater stage lamp. Each test object was uniformly painted. Across the set of test objects, both object reflectance and shape varied. The chamber also contained a palette of 36 grayscale samples. On each trial of the experiment, subjects viewed a test object and indicated which palette sample had the same paint. The arrangement of the samples on the palette was varied from trial to trial. Objective instructions (match the physical paint rather than the appearance) were used. Results and Conclusions. When object reflectance is held fixed, constancy may be assessed by examining the stability of subject's matches across variations of pose and shape. This stability may be compared to the variation expected if subjects showed no constancy and based their matches directly on some simple property of the retinal image (e.g. mean luminance of the test object). We used this idea to develop a constancy index (CI) that takes on a value of 1 for perfect constancy and 0 for no constancy. A) We studied the effect of the pose (45 rotation) of a flat smooth object. On average, subjects showed substantial constancy with respect to the change of pose (mean CI 0.54, 21 naïve subjects). There were, however, enormous individual differences. Some observers showed complete constancy, while others showed none at all. Different observers may have employed different conscious strategies to perform the objective task. B) Varying the shape of the flat surfaces by introducing relief texture had no effect on subjects' matches. Relief texture does not provide useful cues about the reflectance of matte objects.