PURPOSE: Lambertian surfaces, which are used to model matte appearance, have been reported to present illusory gloss under special conditions: the surface is of high-magnitude RMS height deviation and is viewed fronto-planar while being illuminated with collimated light originating from directly behind the observer (Wijntjes et al, 2010, Journal of Vision, 10(9), 13). We conducted two psychophysical experiments to examine whether this illusory gloss is perceived in the same manner as those of real glossy surfaces, and whether observers can recognize the surfaces are of unordinary RMS height.
METHODS: In the first experiment, observers adjusted the parameters of a glossy reflection model applied on surfaces with ordinary RMS height, and they successfully matched the perceived gloss of these real glossy surfaces to the perceived gloss of the high-RMS-height Lambertian surfaces. Then the observers participated in a 2AFC experiment, in which they were asked to choose the bumpier one from a pair of surfaces used in the first experiment.
RESULTS: The results support the previous finding that glossiness and bumpiness inter-influence the perception of each other (Ho et al, 2008, Psychological Science, 19, 196-204). We also find that the perceived bumpiness of the high-RMS-height Lambertian surfaces are not significantly different from those of the ordinary-RMS-height glossy surfaces.
CONCLUSIONS: This indicates that the high-RMS-height Lambertian surfaces are perceived by observers as low-RMS-height glossy surfaces. Similar to the bas-relief ambiguity, the literature and our current work show that under certain surface geometry, lighting and viewing conditions, the perception of surface property is puzzled by a kind of “bump-gloss” ambiguity.