Sun and Sky? Probing the Default Illuminant for Human Shape-from-Shading

People perceive smooth luminance variations as being due to the shading produced by surface undulations: shape-from-shading. To do this the visual system must simultaneously estimate the nature of the illumination and the shape of the surface. Shape-from- shading operates even when both these properties are unknown and neither can be estimated directly from the image. In such circumstances humans are thought to adopt a default illumination model. It is widely held that the default illuminant is a point source located above the observer's head, but some have argued that the default illuminant is a diffuse source. We present evidence that humans adopt an illumination model that includes both diffuse and directional (overhead) elements. This record was migrated from the OpenDepot repository service in June, 2017 before shutting down.