By David Salomon
In 1829, Louis Braille, a tender organist in a Paris church, blind due to the fact that age three, invented the well known code for the blind, nonetheless in universal use at the present time around the world and named after him. Braille himself transformed his code in 1834, and there were a number of differences considering the fact that. even if, the elemental layout of this code, the place each one personality is represented via a gaggle of three x 2 dots, has remained intact. The dots are embossed on thick paper and every should be raised or flat (i. e. , current or absent). each one dot is as a result reminiscent of one little bit of info. for that reason, the Braille code (Figure 1) is a 6-bit code and will hence symbolize sixty four symbols (the code of six flat dots shows a clean space). Braille's fans prolonged the services of his code in different methods. One im portant extension is contractions. those are letters that, once they stand by myself, suggest phrases. for instance, the letter "b" status by myself (or with punctuation) ability the be aware "but," the letter "e" status on my own potential "every," and "p" potential "people. " one other extension is short-form phrases. those are combos of 2 or extra codes that suggest a whole be aware (short-form phrases may well include contractions). for instance, "ab" capacity "about," "rcv" skill "receive," and "(the)mvs" ability "themselves. " (The "the" in parentheses is a contraction, dots 2-3-4-6. ) determine 2 exhibits a few examples of those distinctive codes.
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This may change a Huffman tree to one that does not satisfy the sibling property. 15h. 15i. The latter tree, however, is no longer a Huffman tree, since the counts are no longer in sorted order. The solution is to rebuild the tree each time the counts are rescaled, which does not happen very often. A Huffman data compression program intended for general use should thus have large count fields that would not overflow very often. 3 X 109 . It should be noted that after rescaling the counts, the new symbols being read and compressed have more effect on the counts than the old symbols (those counted before the rescaling).
Pointers bl , b2 are set to point to the start of the first and second runs on the reference line, respectively. 22, aD is updated as shown in the flowchart, and the other four pointers are updated relative to the new aD. The process continues until the end of the coding line is reached. The encoder assumes an extra pelon the right of the line, with a color opposite that of the last pel. 22 is used to abort the encoding process prematurely, before reaching the end of the page. This is necessary if the rest of the page is transmitted in a different code or even in uncompressed form.
2 Two-Dimensional Coding Two-dimensional coding was developed because one-dimensional coding does not produce good results for images with gray areas. Two-dimensional coding is optional on fax machines that use Group 3 but is the only method used by machines intended to work on a digital network. When a fax machine using Group 3 supports two-dimensional coding as an option, each EOL is followed by one extra bit to indicate the compression method used for the next scan line. That bit is 1 if the next line is encoded with one-dimensional coding, 0 if it is encoded with two-dimensional coding.
A Guide to Data Compression Methods by David Salomon