Vector run-length coding of Bi-level images

Abstract

Run-length coding (RC) is a simple and yet quite effective technique for bi-level image coding. A problem with the conventional RC which describes an image by alternating runs of white and black pixels is that it only exploits the redundancy within the same scan line. The modified relative address run-length coding (MRC) used in Group III facsimile transmission is more efficient by making use of the correlation between adjacent lines. The paper presents a vector run-length coding (VRC) technique which exploits the spatial redundancy more thoroughly by representing images with vector or black patterns and vector run-lengths. Depending on the coding method for the block patterns, various algorithms have been developed, including single run-length VRC (SVRC), double run-length VRC (DVRC), and block VRC (BVRC). The conventional RC is a special case of BVRC with block size of 1*1. The proposed methods have been applied to the CCITT standard test documents and the best result has been obtained with the BVRC method. With a block dimension of 4*4, it has yielded compression gains higher than the MRC with k=4 by 15.5% and 22.7%, when using a single and multiple run-length codebooks, respectively.<>