Skew correction of engineering drawings by digital-geometric analysis of Farey ranks

A novel algorithm for detection and correction of skews present in scanned engineering drawings is proposed. The novelty of the algorithm lies in the usage of certain periodic properties of digital straightness directly on gray-scale images, in tandem with the ranks of fractions in a Farey sequence. Straight edges derived in this manner are binned by their Farey ranks, which, in turn, are analyzed to obtain the principal bin from the sums of lengths of the edges in a sequence of bins. The principal bin corresponds to the principal direction, from which the skew angle is estimated to finally correct the skew. Owing to primitive operations in the integer domain and a linear-time clustering procedure, the algorithm runs significantly fast with the desired level of precision, even for document pages with text-graphics mix or containing tabular structures with boundary lines. Experimental results on several datasets demonstrate its elegance, efficiency, and robustness.