3. The computer-readable medium according to claim 2 , wherein the extracting is adapted to select a graphic that makes the element parameter to be found within the corresponding parameter range from the document image and consolidate a plurality of ruled line elements that make the consolidation parameter to be found within the corresponding parameter range to generate the ruled line candidate.
5. The computer-readable medium according to claim 3 , wherein the ruled line is a broken ruled line; the parameter range of the consolidation parameter is defined by a threshold value for a gap separating two ruled line elements, a threshold value for a drift in the direction perpendicular to the direction of consolidating two ruled line elements and/or a threshold value for the number of consolidated ruled line elements; and the extracting is adapted to select a plurality of ruled line elements that make the consolidation parameter to be found within the parameter range as ruled line candidate.