P M Max−planck−institut Fü R Biologische Kybernetik Perception of Shape from Shading on a Cloudy Day Perception of Shape from Shading on a Cloudy Day

The human visual system has a remarkable ability to interpret smooth patterns of light and shade on a surface in terms of 3-D surface geometry. Classical studies of shape{ from{shading have assumed that surface luminance depends on the local surface orientation. This classical shading model holds, for example, on a sunny day. A common situation in which the classical model fails to hold, however, is a diiuse lighting condition such as on a cloudy day. Here we report on the rst set of psychophysical experiments that explicitly address perception of shape{from{shading under diiuse lighting. Our main ndings are that depth discrimination under diiuse lighting is superior to that predicted by a classical sunny day model, and by a model in which depth varies with perceived luminance i.e. dark means deep. We nd that performance is correlated with the latter model, however, under both diiuse source and point source lighting. The results imply that the visual system uses multiple models when perceiving shape{from{shading.