A shading method and computer graphics system are provided in which a path of light is reflected at a given object from a virtual light source. The path of light reaches a given eye point is sampled. A distribution function for a surface of the object is obtained using the sampled path of light, and arithmetic processing is performed such that a maximum value of the distribution function is less than or equal to a predetermined value. In this regard, an estimated specular roughness of the surface of the object is obtained by setting the distribution function to be less than or equal to the predetermined value.