Sun and sky: Does human vision assume a mixture of point and diffuse illumination when interpreting shape-from-shading?

• Published in: Vision research (Oxford) Vol. 51; no. 21; pp. 2317 - 2330
• Main Authors: Schofield, Andrew J., Rock, Paul B., Georgeson, Mark A.
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier Ltd 01.11.2011; Elsevier
• Subjects: 3D surface and shape perception; Biological and medical sciences; Cues; Dark-is-deep; Depth Perception - physiology; Eye and associated structures. Visual pathways and centers. Vision; Form Perception - physiology; Fundamental and applied biological sciences. Psychology; Humans; Illumination; Lighting; Lighting-from-above; Models, Theoretical; Pattern Recognition, Visual - physiology; Photic Stimulation - methods; Shading; Vertebrates: nervous system and sense organs; Dark-is-deep; 3D surface and shape perception; Shading; Illumination; Lighting-from-above; Human; Stimulus shape; Lighting; Vision; Shadow; Surface
• ISSN: 0042-6989; 1878-5646
• DOI: 10.1016/j.visres.2011.09.004

► Shape-from-shading was assessed using sine wave grating stimuli. ► Offsets between luminance peaks and surface peaks varied with orientation. ► This effect is modelled by shape-from-shading under a mixed lighting assumption. ► Humans adopt a mixture of point and diffuse lighting for shape-from-shading. People readily perceive smooth luminance variations as being due to the shading produced by undulations of a 3-D surface (shape-from-shading). In doing so, the visual system must simultaneously estimate the shape of the surface and the nature of the illumination. Remarkably, 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. A widely held view is that the default illuminant is a point source located above the observer’s head. However, some have argued instead that the default illuminant is a diffuse source. We now present evidence that humans may adopt a flexible illumination model that includes both diffuse and point source elements. Our model estimates a direction for the point source and then weights the contribution of this source according to a bias function. For most people the preferred illuminant direction is overhead with a strong diffuse component.