Modified Huffman coding is used infax machines to encode black-on-white images (bitmaps). It combines the variable-length codes ofHuffman coding with the coding of repetitive data inrun-length encoding.
The basic Huffman coding provides a way to compress files with much repeating data, like a file containing text, where the alphabet letters are the repeating objects. However, a single scan line contains only two kinds of elements – white pixels and black pixels – which can be represented directly as 0 and 1. This "alphabet" of only twosymbols is too small to apply theHuffman coding directly. But if we first use run-length encoding, we can have more objects to encode. Here is an example taken from the article onrun-length encoding:
A hypothetical scan line, with B representing a black pixel and W representing white, might read as follows:
WWWWWWWWWWWWBWWWWWWWWWWWWBBBWWWWWWWWWWWWWWWWWWWWWWWWBWWWWWWWWWWWWWW
With a run-length encoding (RLE) data compression algorithm applied to the above hypothetical scan line, it can be rendered as follows:
12W1B12W3B24W1B14W
Here we see that we have several different numbers in addition to the two items "white" and "black." These numbers provide plenty of additional items to use, so the Huffman coding can be directly applied to the sequence above to reduce the size even more.
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