The influence of shape cues on the perception of lighting direction.
James P. O’Shea, Maneesh Agrawala, Martin S. Banks.
Three scene properties determine the luminances in the image of a shaded object: the material reflectance, the illuminant position, and the object's shape. Because all three properties determine the image, one cannot solve for any one property without knowing the other two. Nevertheless, people perceive consistent 3D shape and consistent lighting in shaded images; they must therefore be making assumptions about the unknown properties. We conducted two psychophysical experiments to determine how viewers use shape information to estimate the lighting direction from shaded images. In the first experiment, we confirmed that observers use 3D shape information when estimating lighting direction. In the second experiment, we investigated how different shape cues affect lighting direction estimates. Observers can accurately determine lighting direction when a host of shape cues specify the objects. When shading is the only cue, observers always set lighting direction to be from above. We modeled the results in a Bayesian framework that included a prior distribution describing the assumed lighting direction. The estimated prior was slightly counterclockwise from above at a 30deg slant. Our model showed that an assumption of convexity provides an accurate estimate of lighting direction when the shape is globally, but not locally, consistent with convexity.
Sample stimuli from the main experiment. The shape of the test object was specified by different sets of cues in each condition (listed on the left). The shape of the response object was specified by the same set of cues in all four conditions (disparity, texture gradient, global convexity, occluding contour, and shading). Observers adjusted the lighting direction on the response object until it matched the perceived direction on the test object.