We examined in which way gradual changes in the geometric structure of the illumination affect the perceived glossiness of a surface. The test stimuli were computer-generated three-dimensional scenes with a single test object that was illuminated by three point light sources, whose relative positions in space were systematically varied. In the first experiment, the subjects were asked to adjust the microscale smoothness of a match object illuminated by a single light source such that it has the same perceived glossiness as the test stimulus. We found that small changes in the structure of the light field can induce dramatic changes in perceived glossiness and that this effect is modulated by the microscale smoothness of the test object. The results of a second experiment indicate that the degree of overlap of nearby highlights plays a major role in this effect: Whenever the degree of overlap in a group of highlights is so large that they perceptually merge into a single highlight, the glossiness of the surface is systematically underestimated. In addition, we examined the predictability of the smoothness settings by a linear model that is based on a set of four different global image statistics.
Keywords: gloss constancy; gloss perception; light; material perception.