Lossless and lossy compression of text images by soft pattern matching

Abstract

We present a method for both lossless and lossy compression of bilevel images that consist mostly of printed or typed text. The key feature of the method is soft pattern matching, a way of making use of the information in previously encountered characters without risking the introduction of character substitution errors. We can obtain lossless compression about 20 percent better than that of the JBIG standard by direct application of this method. By allowing some loss in a controlled manner based partly on the pattern matching, we can obtain compression ratios about two to four times the compression ratios of JBIG and three to eight times those of G3 facsimile with no visible loss of quality.