Surface-Normals from Closed Paths

It is now well established that the shading present in an image of an object is a potential source of 3-D shape information. With this in mind, we seek constraints arising out of the assumption that an image portrays an object with a smooth surface. On obtaining a threshold and a depth-invariance constraint (the latter requiring integration around closed curves) we offer a method of shape from shading for the purpose of demonstrating their potential usefulness.