The Influence of Shape Cues on the Perception of Lighting Direction
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 ca nnot 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 unkno wn 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 observer s use 3D shape information when estimating lighting direction. In the second experiment, we investigated h ow different shape cues affect lighting direction estimates. Observers can accurately determine lighting d irection 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 estima te of lighting direction when the shape is globally, but not locally, consistent with convexity.