Abstract: An image obtained by imaging an object having a solid shape, such as that of a circular column or prism, develops a distribution in brightness since the angle of each portion of the surface of the object with respect to a lighting device and image pick-up device differs depending upon the location. The distribution of brightness is substantially uniform along the longitudinal direction of the object but varies transversely of the object. One line is designated on the captured image of the object in a direction that cuts across the object, and the distribution of brightness on this line is obtained. A two-dimensional shading pattern is obtained by expanding this brightness distribution in the longitudinal direction of the object. The shading pattern represents the distribution of the brightness in the image of the surface of the object.