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Bitmap printing from digital sources is commonly termed "digital printing"; however, machine retrieval of information content from printed images is typically inaccurate and unreliable in terms of digital information systems standards. For example, good optical character recognition systems provide error rates on the order of 1%. In another example, digital color copying, the loss of information compared to the original digital print file is much higher. Fundamentally, hardcopy printed for the needs of human readers does not provide a reliable digital data channel. In this sense these hardcopies are analog rather than digital documents. Embedded data technologies provide the means for encoding and retrieving digital information marked on hardcopy documents; they enable reliable information exchange between paper and electronic domains. A distinctive embedded data technology called self-clocking glyph codes has been developed by Xerox. Glyph codes provide digital data recording on hardcopy documents with high data density, robustness, pleasing appearance, and allowance for natural graphical integration with other printing. A good example of this code is composed of a rectangular lattice of mark centers with a linear or elliptical mark on each center oriented in one of two directions for recording one bit pei mark or one of four directions for recording two bits per mark. This code has the property of uniform visual texture and appears as a uniform gray or uniform color when printed at suitable scale compared to the viewing distance; for example, 5x5 pixel glyph cells at 300 dpi. Two-dimensional codes for data, synchronization, error correction, and other functions proide remarkable digital system robustness and functionality. Arbitrary digital data may be encoded to represent a vast variety of information. Examples include data representing visible printed content such as text files or color information and nonprinted document aspects such as formatting, revision histories, mathematical relations in spread sheets, and network file references. Thus, glyph codes provide high capacity digital data channels enabling document systems to bridge electronic and hardcopy domains with the functionality and reliability that users have come to expect of digital technologies. The format and appearance of glyph codes facilitate their graphical integration in documents. Together these features enable truly digital hardcopy documents that combine excellence in appearance and function.
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