The.Algorithm.Design.Manual.Springer-Verlag.1998

War Story: Text Compression for Bar Codes
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War Story: Text Compression for Bar
Codes
Ynjiun waved his laser wand over the torn and crumpled fragments of a bar code label. The system
hesitated for a few seconds, then responded with a pleasant blip sound. He smiled at me in triumph.
``Virtually indestructible.''
I was visiting the research laboratories of Symbol Technologies, of Bohemia NY, the world's leading
manufacturer of bar code scanning equipment. Next time you are in the checkout line at a grocery store,
check to see what type of scanning equipment they are using. Likely it will say Symbol on the housing.
Although we take bar codes for granted, there is a surprising amount of technology behind them. Bar
codes exist primarily because conventional optical character recognition (OCR) systems are not
sufficiently reliable for inventory operations. The bar code symbology familiar to us on each box of
cereal or pack of gum encodes a ten-digit number with sufficient error correction such that it is virtually
impossible to scan the wrong number, even if the can is upside-down or dented. Occasionally, the cashier
won't be able to get a label to scan at all, but once you hear that blip you know it was read correctly.
The ten-digit capacity of conventional bar code labels means that there is only room to store a single ID
number in a label. Thus any application of supermarket bar codes must have a database mapping 11141-
47011 to a particular size and brand of soy sauce. The holy grail of the bar code world has long been the
development of higher-capacity bar code symbologies that can store entire documents, yet still be read
reliably. Largely through the efforts of Theo Pavlidis and Ynjiun Wang at Stony Brook [PSW92],
Symbol Technologies was ready to introduce the first such product.
Figure: A two-dimensional bar-code label of the Gettysburg Address using PDF-417
``PDF-417 is our new, two-dimensional bar code symbology,'' Ynjiun explained. A sample label is
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