Modified Huffman coding is used in
fax
Fax (short for facsimile), sometimes called telecopying or telefax (the latter short for telefacsimile), is the telephonic transmission of scanned printed material (both text and images), normally to a telephone number connected to a printer o ...
machines to encode black-on-white images (
bitmap
In computing, a bitmap is a mapping from some domain (for example, a range of integers) to bits. It is also called a bit array or bitmap index.
As a noun, the term "bitmap" is very often used to refer to a particular bitmapping application: t ...
s). It combines the variable-length codes of
Huffman coding
In computer science and information theory, a Huffman code is a particular type of optimal prefix code that is commonly used for lossless data compression. The process of finding or using such a code proceeds by means of Huffman coding, an algo ...
with the coding of repetitive data in
run-length encoding.
The basic Huffman coding provides a way to compress files that have 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 a 0 and 1. This "alphabet" of only two
symbols is too small to directly apply the
Huffman coding
In computer science and information theory, a Huffman code is a particular type of optimal prefix code that is commonly used for lossless data compression. The process of finding or using such a code proceeds by means of Huffman coding, an algo ...
. But if we first use run-length encoding, we can have more objects to encode. Here is an example taken from the article on
run-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, in addition to the two items "white" and "black", several different numbers. 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.
See also
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Fax compression
External links
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Lossless compression algorithms
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