Subsequently, in order of change of gradation, namely in order of change of saturation, the table generating unit 18c obtains a ratio of radii, which have been obtained at Step S137, between the two adjacent empty circles 45, namely an adjacent-empty-circle-radius ratio (Step S138). Consequently, in the Voronoi diagram 40 illustrated in FIG. 10, the table generating unit 18c obtains the adjacent-empty-circle-radius ratio of the Voronoi seed 44A and the Voronoi seed 44B and the adjacent-empty-circle-radius ratio of the Voronoi seed 44B and the Voronoi seed 44C.

After the process of Step S138, the table generating unit 18c determines whether all the adjacent-empty-circle-radius ratios between the two adjacent empty circles 45 in order of change of gradation are within the radius-ratio acceptable range indicated by the radius-ratio-acceptable-range information 17d or not (Step S139).

When determining that at least a part of the adjacent-empty-circle-radius ratios are not within the radius-ratio acceptable range (that is, outside the radius-ratio acceptable range) at Step S139, the table generating unit 18c targets the two adjacent empty circles 45 the adjacent-empty-circle-radius ratio of which is not within the radius-ratio acceptable range (Step S140).

Subsequently, the table generating unit 18c sets one of the generatrices 41 that the two empty circles 45 targeted at Step S140 share as a target point (Step S141).