DETECTING ILLUMINATION CHANGES IN THREE-DIMENSIONAL SCENES
Craven & Foster (1992) found that observers could readily distinguish illuminant chromaticity changes from surface color changes in Mondrian scenes. Changes in illuminant did not affect the photoreceptor excitation ratios across edges; they concluded that the visual system detected surface changes as ratio constancy violations. In 3D scenes, with multiple illuminants and matte surfaces at many orientations, changes in the positions or chromaticities of light sources need not leave edge ratios invariant. We examine how well observers can distinguish changes in the direction of a collimated light source in rendered 3D scenes viewed binocularly. Observers were asked to distinguish illumination changes from matched changes in achromatic surface colors. We find that, in 3D scenes with non- homogeneous illumination, observers can discriminate very small changes (at 2-4 degrees, dâ€™ exceeds 1) in the spatial distribution of light from matched surface changes despite the lack of ratio constancy between adjacent surfaces.