Furthermore, lighting conditions generally vary across spatial locations within a scene, sometimes gradually as when moving through a large, evenly lit space, and sometimes suddenly as when crossing a shadow boundary. The lighting conditions at any point in space therefore depend on both primary sources and on the geometry and optical properties of surrounding materials (Gershun, 1939 Moon & Spencer, 1981). It can include primary light sources such as the sun or electric lights, as well as reflective surfaces and light-scattering volumes that function as secondary light sources. In contrast, however, lighting in everyday environments is complex. We conclude that, rather than relying on a single estimated illumination direction, the human visual system can accommodate illumination that varies substantially and rapidly across a surface.
Furthermore, we show that despite being able to accommodate rapid changes in lighting direction when judging shape, observers are generally unable to detect these changes. Here we show that the human visual system can successfully judge 3D surface shape even when the lighting direction varies from place to place over a surface, provided the scale at which these lighting changes occur is similar to, or larger than, the size of the shape features being judged. In natural scenes, however, lighting can be much more complex than this, with multiple nearby light sources. Theories about how we accomplish this often assume that the human visual system estimates a single lighting direction and interprets shading cues in accord with that estimate. People are able to perceive the 3D shape of illuminated surfaces using image shading cues.
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