a first rectangular region, a second rectangular region adjacent to the first rectangular region, a third rectangular region diagonal relative to the first rectangular region and adjacent to the second rectangular region, and a fourth rectangular region diagonal relative to the second rectangular region and adjacent to both the first rectangular region and the third rectangular region, wherein the first rectangular region is parallel to the third rectangular region, of and the second rectangular region is perpendicular to the first rectangular region and parallel to the fourth rectangular region, the first rectangular region and the third rectangular region respectively have a first pattern thereon, and the second rectangular region and the fourth region respectively have thereon a second pattern which is distinct from the first pattern.