In any scene where the illumination model is not perfectly diffuse, the amount of light reflected from a small achromatic surface patch may vary as the patch changes orientation. To the extent that a visual system actively discounts orientation in estimating lightness ('perceived albedo'), we would expect that errors in perceived orientation are paired with compensating errors in perceived surface lightness. We test whether human observers, asked to judge both the lightness and the orientation of achromatic surfaces, exhibit such tradeoffs in complex scenes designed to induce large errors in perceived surface orientation. The stimuli were images of complex scenes, all rendered with identical punctate+diffuse lighting models, all containing identical specular and matte objects in the same locations. Each also contained a trapezoidal, achromatic, matte test patch. Only the shape of test patch and its albedo changed from scene to scene. The shape was altered so as to lead the observer to misperceive the orientation of the test patch (false perspective cues). We constructed 14 different scenes that differed only in the shape of the test patch (7 levels) and its albedo (2 levels). On each trial, observers first estimated the orientation of the gray rectangle by adjusting a gradient probe superimposed on the test patch and then matched the lightness of the patch to a standard gray scale. Observers made these two judgments 20 times for each of the 14 scenes in random order (280 trials). Five observers participated in the study. Observers made large errors in judging the orientation of the test patch (up to 40 degrees) as anticipated. Their judgments of lightness were highly reliable but did not covary with perceived surface orientation, suggesting that, even in complex scenes, human observers do not correct perceived lightness for perceived orientation.
ASJC Scopus subject areas
- Sensory Systems