Enhanced JPEG compression of documents

The JPEG standard was designed for compression of photographic digital images, but it also works well on digitized documents with only a limited number of shades of gray. For documents in which compression and legibility are more important than preserving all of the intermediate values, preprocessing the images to reduce their dynamic range can enhance JPEG compression, as it selectively discards some noise. If modified quantization tables are substituted for the encoding quantization tables in the JPEG compressed data stream, an unchanged JPEG decoder can restore the dynamic range and increase image contrast. Graphs of the compressed size in bytes vs. dynamic range after the application of different dynamic-range-reduction techniques are given for both Huffman coding and arithmetic coding. Examples of the reconstructed front and back sides of a check with normal processing and enhanced compression are shown