Dual stage compression of bit mapped image data using refined run length and LZ compression

Abstract

A dual stage data lossless compressor for optimally compressing bit mapped imaged data. The first stage run length compresses data bits representing pixel positions along a scan line of a video image to data units of fixed length. The units alternate to represent runs of alternate video image data values. The run length compressed data units are subject to second stage compression using a sliding window Lempel-Ziv compressor. The output from the Lempel-Ziv compressor includes raw tokens of fixed length and compressed tokens of varying lengths. The combination of a run length precompressor and a sliding window Lempel-Ziv post compressor, in which the run length compressor output is a succession of data units of fixed length, provides an optimum match between the capabilities and idiosyncracies of the two compressors, and related decompressors, when processing business form data images. Furthermore, the asymmetric simplicity of Lempel-Ziv sliding window decompression and run length decompression simplicity leads to a decompression speed compatible with contemporary applications.

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