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Electronic paper describes the use of publishing technologies to provide a similar appearance and functionality to traditional print publications. The two main technologies used for this purpose are page images and PDF.
Early e-journal projects (CORE, TULIP) used scanned page images stored as TIFF files as one (or the main) format for viewing. The problems with this approach are numerous. The page images are large (1 Megabyte/page for uncompressed A4 pages at 1 bit/pixel, more for higher bit-depths in greyscale or colour) and therefore take significant amounts of bandwidth to move around the network. Displaying such images also takes an appreciable time on all but the most high-end workstations. 'Flipping' through such pages is like swimming through treacle. Finally, the text of the images is not directly searchable, as it is represented internally only through a pattern of bits.
The most widely used current technology for providing electronic page images is Adobe's PDF/Acrobat. PDF has the significant advantage over HTML that it can cope easily with complex page layout (such as multiple columns and text wrapping around graphics), and large amounts of mathematical formulæ. It is however often hard to read on screen, particularly if the PDF has been created for portrait orientation pages and is being viewed on a landscape orientation screen. [Kasdorf, 1998] argues that PDF alone is an inadequate solution and the e-journal publishers should use both PDF and SGML.
Acrobat is an appropriate way to provide fairly low-bandwidth delivery of page images that will print at the highest available resolution of the user's output device. It is often possible to generate the PDF files at little extra expense as part of the p-journal production process, particularly if this process already creates Postscript files on the way to the print device. The Postscript files can be automatically converted into PDF using Adobe's Distiller program.
PDF also has advantages in other areas. It can be used to 'print' electronically things that would be prohibitive to produce on paper, or that change frequently. Publications containing large amounts of colour graphics, or long publications with a limited market (such as conference proceedings) are good examples of the former. Catalogues and internal policy manuals are good examples of the latter. PDF in this case can be used to support 'print on demand'.
PDF also allows publishers to produce exactly parallel electronic and print versions of a journal. This may well prove to be quite important while we remain with parallel versions. From the user's perspective receiving a PDF file just means that they now have the option to print it out rather than being required to get it in paper.
An example of a stable of journals provided only in PDF format is the nine journals being made available by the CAJUN (CD-ROM Acrobat Journals Using Networks) project and available on-line at < http://cajun.cs.nott.ac.uk/ > and described in [Smith et al., 1993].
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