Modified Huffman coding

Modified Huffman coding is the scheme used in fax machines to compress black-and-white page images. A scan line of a fax contains only two kinds of element, white pixels and black pixels, so an alphabet of two symbols leaves plain Huffman coding nothing to shorten. The method first applies run-length encoding, which turns a line into alternating counts of white and black pixels, and then assigns Huffman codes to those run lengths so that common lengths get short codes. The result combines the variable-length codes of Huffman coding with the repeated-data handling of run-length encoding, and it shrinks the mostly empty lines of a typical document by a large factor. It is a standard example of how two simple compression ideas can be chained to suit one kind of data.