Ordering techniques for facsimile coding: A review

Abstract

This paper presents a review of ordering techniques for efficient digital coding of two-level facsimile signals. Ordering techniques use the two-dimensional correlation present in spatially close picture elements (pels) to change the relative order of transmission of elements in a scan line so as to increase the average length of the runs of consecutive black or white elements in the ordered line. This makes the ordered data more amenable to run-length coding. Several extensions of this basic technique are discussed along with simulation results using the 8 standard CCITT images. Only schemes which allow an exact reproduction of the picture at the receiver are considered. Simulations show that on the average one-dimensional run-length coding requires 445 316 bits/document, whereas, one of the variations of the ordering technique requires 264 632 bits/document. A total of 3 702 720 bits/ document would be required if no compression is attempted. Thus the ordering techniques reduce the number of coded bits by about 41 percent compared to one-dimensional coding, and by about 93 percent compared to uncompressed data.