visibl)

      lit      is a logical flag indicating whether the surface
               point is illuminated; the point is considered to be
               illuminated if the vector from the point to the
               center of the illumination source doesn't intersect
               the target surface. `lit' takes into account whether
               the target surface casts a shadow on `spoint',
               regardless of the incidence angle at `spoint'. `lit' is
               returned with the value tr