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+\title{Portable Documents: Why Use SGML?}
+\author[David Barron]{David Barron\\
+Department of Electronics and Computer Science\\
+University of Southampton}
+\begin{Article}
+\section{Introduction}
+In this article we present a few ideas as a framework for the
+discussion of portable documents. We address a number of questions:
+\begin{itemize}
+\itemsep 0pt plus .4pt
+\item What are portable documents?
+\item Who needs them, and why?
+\item How to produce them, now and in the future
+\end{itemize}
+
+\section{Documents}
+Traditionally, a document was a file (or a deck of cards),
+and consisted solely of text.
+Today, documents are typically {\em compound}, a
+mixture of text and graphics (bit-map or line art) that can be
+rendered on paper or screen. Additionally, they may include
+hypertext links (in which case they can only be viewed on screen).
+A recent development is the ability to incorporate video
+and sound in a compound document, either embedded
+within the document or linked by a pointer:
+such a document is a {\em multimedia} document. Hypertext-style links
+may also be included to form a {\em hypermedia} document: evidently,
+multimedia and hypermedia documents can only be `read' on a suitably
+equipped computer system.
+
+World Wide Web (WWW) documents are a special case of
+compound hypermedia documents
+where the links are to other documents elsewhere on the Internet.
+They can be regarded as virtual documents, in the sense that the
+whole document never exists as a single identifiable object.
+More generally, we can define a {\em virtual document} as
+a structured collection of information from which instances of
+documents and other resources can be derived. Examples include:
+\begin{itemize}
+\item The Oxford English Dictionary which exists as a database
+from which are derived various printed editions (Shorter, Concise,
+Pocket etc.), as well as the CD-ROM version
+\item Critical editions of a literary text, where a single source
+`document' contains all the variations, and can be printed out using
+different variants as the base text
+\end{itemize}
+
+\section{Portability}
+The definition of portability that we shall use in this discussion is
+the ability to transmit the document digitally (over a network,
+or on a disk or CD-ROM) and re-create a faithful rendering of the document
+after transmission, if need be
+on a different hardware and/or software platform
+from that on which the document was originally created.
+It is important to observe that there are three different forms in
+which the text and graphics in a document might be re-created:
+\begin{itemize}
+\item with absolute visual fidelity
+\item with approximate visual fidelity
+\item retaining content only
+\end{itemize}
+
+\section{Who needs portable documents, and why?}
+
+Three different needs for portable documents can be adduced
+\begin{enumerate}
+\item Publishers need them in order to distribute electronic
+books and journals
+\item Communities with common interests
+who need to share information need them. An example is a
+scientific research community whose members use diverse hardware and
+software
+\item Librarians responsible for digital archives need
+portable documents, since they cannot assume that a particular
+hardware/software platform will exist in perpetuity
+\end{enumerate}
+
+\section{Examples of successful portability}
+\begin{itemize}
+\item Computer science researchers and software manufacturers
+distribute documents as PostScript files. This works well
+if the fonts employed are restricted to the basic 35, and
+the use of Adobe Acrobat (pdf files) increases portability when other
+fonts are used.
+
+\item The Physics pre-print library at Los Alamos National Laboratory
+is used by many physicists world-wide: over 10,000 retrievals per day
+are reported. The archive holds pre-prints in \LaTeX\ and PostScript
+formats (figures in PostScript only). This is successful because the
+Physics community has for some years used \TeX\ as its preferred means
+of exchanging information.
+
+\item WWW documents are highly portable, since
+their rendering is (almost entirely) determined by the
+browser software, and the use of a common mark-up language (HTML)
+ensures portability
+\end{itemize}
+
+\section{Achieving portability}
+At first sight it appears that portability might be achieved by
+agreeing standards (e.g. \LaTeX, PostScript, ODA, HTML). At present
+there is too much choice, and no obvious winner, especially in
+hypermedia documents. This is a sign of an immature technology.
+Another important fact to take into account is that it is difficult to
+impose standards in some environments (e.g., academia) where personal
+preferences lead to the equivalent of religious wars.
+
+Particular problems in achieving portability arise from varying
+fonts and character codes e.g. in handling European languages.
+Unicode will go a long way towards solving the character
+codes problem.
+
+\section{Why use SGML?}
+SGML provides a formal and portable definition of document structure.
+SGML syntax can define a hierarchical structure of embedded document parts,
+and can associate a type with each component in the hierarchy.
+By associating a rendering definition with each type of component,
+it is possible to achieve a portable document. In particular,
+SGML provides a uniform archive format for a library of portable
+documents.
+
+\subsection{An example}
+Suppose it is required to maintain a library of technical
+documents in an environment where some authors use \LaTeX,
+whilst others use Microsoft Word. We can define an SGML
+DTD for the document structure, together with \LaTeX\ and Word
+styles to define the rendering.
+This opens up three possibilities:
+\begin{enumerate}
+\item Author in SGML and use a tool to produce a \LaTeX\ or
+Word version from which the printed version can be produced
+\item Author in \LaTeX\ and use a tool to translate to SGML to
+produce the archive copy
+\item Author in Word and use a tool to translate the RTF form to
+SGML to produce the archive copy.
+\end{enumerate}
+
+In addition to the SGML version of the documents, the
+archive must contain the Word and \LaTeX\ style files and the
+translation tools. Once this is done, anyone can collect a
+document, the required style files and tools and produce a copy of
+the document.
+This will of course only work for text documents. For any
+document with graphics content, and for hypermedia
+documents, more is required. This is possible in
+principle, but much remains to be done
+
+\section{The future}
+A combination of SGML and OpenDoc is probably the best way forward.
+OpenDoc provides an architecture for portable documents:
+it treats a
+document as a container for a collection of `parts', each of which can
+have other parts embedded within it. Each type of part has
+associated programs to edit and render it, so that
+documents can be re-created with varying degrees of fidelity
+depending on the availability of rendering software
+for the particular varieties of parts that it includes.
+
+OpenDoc is a dynamic architecture, and assumes that a new type
+of part may occur at any time.
+In principle SGML can be used to describe the static
+structure of an OpenDoc document, providing the final link in the
+portability chain.
+\end{Article}