The influence of object orientation and shading on pictorial relief of Lambertian surfaces

We studied the influence of object orientation and lighting direction on pictorial relief of 2D images of generic 3D objects. We used two objects, namely a globally convex object with a furrow in it and a globally convex object with a dimple in it. Participants adjusted a local surface attitude probe at 200-250 different locations in the image such that it seems to lie on the pictorial surface. In a 2×2 orthogonal design we manipulated the object orientation and the lighting direction. Lighting direction was always with respect to the view frame. The orientations were sufficiently altered to be qualitatively different from each other; that is, different kinds of shading singularities and contour singularities were present in the images. It was found that changes in lighting condition induced systematic changes on the settings for both orientations of the objects. First, the results showed that the reconstructed depth range was altered by a change in lighting direction. Second, we looked at the parabolic lines in the surface reconstructions; parabolic lines are curves on a surface where at least one of the principle curvatures equals zero. It was found that the parabolic lines shift due to a change in light direction. This is a particularly interesting finding since the actual location of the parabolic lines on the 3D object is invariant for changes in light direction and object orientation.