Several researchers have investigated the role of texture as a basis for the recovery of surface orientation. Gibson 1 was the first to address the problem of recovering the orientation of a plane covered with textural elements. He assumed that the density (number of elements per unit area) of these elements is uniform. Under this assumption he observed that the density gradient in the image specifies surface orientation. More recently , Witkin addresses the problem of recovering the slant a and tilt r of a planar surface viewed under orthographic projection. Let /? be the angle which a tangent at a point of a surface marking makes with the projection of a fixed coordinate axis of the image plane and a the angle the projection of that tangent makes with the coordinate axis. Under orthographic projection then Witkin develops a geometric model which relates a to /?: