Estimating local shape from shading in the presence of global shading.

In theory, global shading may help with the estimation of local surface structure from shading (e.g., in specifying the illuminant direction). Empirically, we do not know whether human observers combine the information given by the local and global shading to estimate local shape. Observers had to indicate the orientation of a local elongated perturbation with or without global shading information provided by a background surface. Our psychophysical results show the following: 1. Observers do not estimate the orientation of the local perturbation more accurately with global shading information than they do in the absence of such information. 2. Responses depend dramatically on the inclination between the illuminant direction and the viewing direction. For an inclination of 20 degrees, observers indicate more or less the orientation of the local ridge; however, for an inclination of 40 degrees, they indicate either the direction of the illuminant or an orientation close to the shadow edge of the perturbation. Most subjects show some combination of these behaviors. This behavior is not altered by global shading information. We conclude that in our paradigm, global shading information does not aid the estimation of local shape.