Textual Image Compression: Two-Stage Lossy/Lossless Encoding of Textual Images

Abstract

A two-stage method for compressing bilevel images is described that is particularly effective for images containing repeated subimages, notably text. In the first stage, connected groups of pixels, corresponding approximately to individual characters, are extracted from the image. These are matched against an adaptively constructed library of patterns seen so far, and the resulting sequence of symbol identification numbers is coded and transmitted. From this information, along with the library itself and the offsets from one mark to the next, an approximate image can be reconstructed. The result is a lossy method of compression that outperforms other schemes. The second stage employs the rec..