METHOD FOR HIGH QUALITY COMPRESSION OF BINARY TEXT IMAGES

摘要

The invention relates to a method for the compression and decompression of binary text images. The method distinguishes between large low-frequency areas and small high-frequency areas in the original frame. For the low-frequency areas, a scheme for lossy compression is used, whereas for the high-frequency areas, a scheme permitting lossless compression is applied. The compression/ decompression process involves the following five stages - (1) prefiltering to remove all black patches (e.g. by removing all black pixels, except where they belong to a large black segment; (2) fast evaluation of compressibility by partitioning the images into mutually exclusive segments and applying different compression modes to each segment; (3) connectivity oriented subsampling to reduce the resolution in horizontal and vertical directions (the image is segmented into blocks, and a 1-pixel representation for each block is determined); (4) lossless compression and decompression where the reduced file is compressed by conventional techniques; and (5) reconstruction by sequence reversal (lossless decompression will retrieve the subsampled file, expansion of the subsampled file through replacement of each pixel by a block having equal value, postfiltering).