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authorNorbert Preining <norbert@preining.info>2019-09-02 13:46:59 +0900
committerNorbert Preining <norbert@preining.info>2019-09-02 13:46:59 +0900
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+\newcommand{\lispname}[1]{{\sf #1\/}}
+\newtheorem{theorem}{Theorem}
+\newtheorem{algorithm}{algorithm}
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+\newtheorem{definition}{Definition}
+\newtheorem{corollary}{Corollary}
+\newtheorem{conjecture}{Conjecture}
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+\newcommand{\bld}[1]{{\bf #1}}
+\newcommand{\type}[1]{{\tt #1}}
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+\newcommand{\inference}[2]{\frac{#1}{#2}}
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+
+%section reference.
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+\newcommand\cref[1]{Chapter~$\ref{#1}$}
+\newcommand{\aref}[1]{Appendix~$\ref{#1}$}
+%integral d
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+%quantifiers: cs611
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+\newcommand{\dy}{\,dy}
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+\newcommand{\dt}{\,dt}
+
+\newcommand{\naive}{na{\"\i}ve{}}
+\providecommand\AmSTeX{$\cal A\kern-.1667em\lower.5ex\hbox{$\cal
+ M$}\kern-.075emS$-\TeX}
+\providecommand{\amstex}{\AmSTeX{}}
+\newcount\TestCount
+\providecommand{\La}{\TestCount=\the\fam \leavevmode L\raise.42ex
+ \hbox{$\fam\TestCount\scriptstyle\kern-.3em A$}}
+\providecommand\AllTeX{(\La)\TeX}
+\providecommand{\alltex}{\AllTeX{}}
+
+\providecommand{\latex}{\LaTeX{}}
+
+\providecommand{\tex}{\TeX{}}
+
+\providecommand{\macro}[1]{{\cal M}_#1}
+
+\providecommand{\rfb}{{\sc RFB}\footnote{Recordings for the
+Blind}}
+
+%
+\newcommand{\www}[1]{{\it WWW}: {\small #1}}
+\newcommand{\email}[1]{{\it E-mail\/}: $\langle\hbox{\tt#1}\rangle$}
+\newcommand{\phone}[1]{{\it Phone\/}: {\tt #1}}
+
+\newcommand{\voicemail}[1]{{\it Voice-mail\/}: {\tt #1}}
+\newcommand\homepage{\sf http://www.research.digital.com/CRL/personal/raman/raman.html}
+\newcommand{\faxno}[1]{{\it Fax\/}: {\tt #1}}
+
+\newcommand{\textalk}{\rm T\kern -.1667em\lower .5ex\hbox {E}\kern%
+ -.125emXT\kern -.1667em\lower .5ex\hbox {A}\kern -.125em L\kern -.125em K}
+
+
+
+
+\newcommand{\Dectalk}{{\sc dectalk}}
+\newcommand{\Sparc}{{\sc sparc}}
+
+
+\title{An Audio View of \alltex{} Documents}
+\author[T. V. Raman]{T.\ V.\ Raman\\
+Digital Equipment Corporation\\
+ Cambridge Research Lab\\
+ One Kendall Square, Building 650\\
+ Cambridge, MA 02139\\
+\emph{Email:} \texttt{raman@crl.dec.com}
+}
+\begin{Article}
+\begin{abstract}
+ \aster{} \dash Audio System For Technical Readings\dash is a computing
+ system that produces audio renderings from the {\em same\/} \alltex{} source
+ used to produce the printed document. \cite{Raman:TB13-3-372-377}
+ described our preliminary
+ work on this project. At the time, correct handling of user-defined
+ \alltex{} macros was described as one of the key issues in building a fully
+ extensible audio rendering system.
+ \aster{} \cite{raman-phd-thesis} has now been fully implemented.
+ This paper reports on the approach used
+ in \aster{} to handle user-defined macros.
+
+\aster{} treats macro definitions as introducing new object types
+into the document logical structure. The \alltex{} macro consists of two
+parts; a declaration, and a series of \TeX{} commands that the macro expands
+into. The macro expansion is nothing but a visual rendering rule that
+specifies how \TeX{} should display instances of the object represented by the
+macro.
+
+\aster{} provides an equivalent mechanism for extending the class of
+logical structures that are recognized. Once \aster{} has been told about a
+user-defined macro, audio rendering rules for the new object type introduced
+by this macro can be defined in AFL (Audio Formatting Language).
+
+The approach used not only makes \aster{} fully extensible;
+it points out a unique advantage of \alltex\dash the ability of the
+author to encode semantic meaning into the markup by extending the document
+model in ways appropriate to the specific document instance that is being
+encoded.
+ \end{abstract}
+\section{Introduction}\label{s:introduction}
+
+
+\begin{center}
+ \asterlogo
+\end{center}
+
+\aster\dash Audio System For Technical Readings\dash is a computing
+system that aurally renders electronic documents marked up in the \alltex{} family of
+markup languages (see~\cite{raman-phd-thesis} for details). \aster{} uses the
+structural markup present in the electronic source to advantage in producing
+high-quality, interactive audio renderings. This paper focuses on a specific aspect of the
+problem; namely that of flexibly rendering the extended document logical
+structure encapsulated in a \alltex{} document.
+
+
+One primary advantage of \alltex{} is the flexibility it provides the author
+in defining logical structures that are specific to a particular document
+instance. In this sense, the class of logical structures that can be
+encapsulated in a \alltex{} document is extensible. \alltex{} macros allow
+an author to abstract away the layout details. At the same time, they provide
+a powerful mechanism for defining new constructs that are not already present
+in the document style (DTD in SGML parlance) in use. As a consequence,
+ when introducing a new piece of mathematical notation, an author can first define
+a new \alltex{} macro that produces a desired layout, and then use this newly
+defined construct throughout the document.
+
+The flexibility of the \alltex{} macro facility initially proved a major
+stumbling block in building a fully extensible audio rendering system. A
+system that attempts to produce aural renderings by {\em mapping\/} the
+built-in \alltex{} commands to an equivalent aural representation faces the
+severe shortcoming of not being able to render documents that contain
+user-defined macros. At the same time, it is impossible to translate such
+user-defined \alltex{} macros into a suitable aural representation. This is
+because \tex{} in its full glory is a Turing-complete programming language, and
+saying ``we can translate a general \tex{} macro to audio'' is equivalent to
+saying that ``Given a \tex{} program, we can predict the result''. Being able
+to achieve the above without actually running \tex{} on the program (document
+fragment) would amount to being able to solve the Halting Problem!
+
+In the rest of this paper, we describe the solution used in \aster{} to
+circumvent this difficulty. The solution we used in fact turns the presence of
+user-definable \alltex{} macros into an advantage.
+Such user-defined constructs allow \aster{} to glean even more information
+about the document logical structure than would be possible if the document
+were encoded using only the built-in \alltex{} operators; as a consequence,
+the audio renderings produced are also significantly better.
+
+\section{Document Models in \protect\aster{}}\label{s:represent}
+
+
+\aster{} produces audio renderings by first extracting the document
+logical structure. In this model, all forms of rendering, \ie visual,
+aural, etc.\ are regarded as a projection of the structure present in
+the information being conveyed onto the medium being used to
+communicate the information. Thus, typesetting a document requires
+visual formatting\Dash projecting the information structure onto a
+two-dimensional visual tablet; aural rendering requires presenting the
+structure using various features of the auditory display.
+
+The recognizer used in \aster{} extracts logical structure present in
+documents encoded in the \alltex{} family of languages. An important
+feature of this recognizer is that it works on the entire gamut of
+encodings, ranging from plain ASCII documents, \ie no explicit markup,
+up to documents containing completely unambiguous encodings of the
+logical structure.
+
+
+The basic document model used in \aster{} is the attributed tree.
+Each hierarchical level of the document is modeled as a node in this
+tree. Each node can have content, children and attributes. Using
+object-oriented terminology, each different kind of node of the tree
+is called an {\em object\/} and represents a document element. Thus,
+``chapter'', ``section'', ``paragraph'', and ``sentence'' are all
+objects. If a document contained five sections, its representation in
+\aster{} would have five instances of object ``section''. This
+object-oriented terminology is used because \aster{} actually uses
+CLOS objects in this fashion. The use of an object-oriented language
+was instrumental in allowing us to develop and implement the ideas in
+\aster{} incrementally and effectively.
+
+This attributed tree
+structure is augmented to represent mathematical content; we call this
+augmented representation the {\em quasi-prefix form},
+(see figure~\ref{fig:math-object} below).
+Expressions that are completely unambiguous, \eg $x+y$, are captured in their
+prefix form. In addition to linearizing the underlying tree structure,
+mathematical notation uses {\em visual attributes\/} such as superscripts and
+subscripts, whose interpretation is context-dependent. We extend the prefix
+form to capture such visual attributes\Dash hence the name {\em
+ quasi\/}-prefix.
+\begin{minipage}{\linewidth}
+\makeatletter\def\@captype{figure}\makeatletter
+ \begin{center}
+\begin{tabular}[h]{|rcl|}\hline
+left-superscript & accent & superscript \\
+ &$\displaystyle \nwarrow$ \hfill
+ $\displaystyle \uparrow$
+ \hfill $\displaystyle \nearrow$ & \\
+& {\bf math object } & \\
+ & $\displaystyle \swarrow$ \hfill
+ $\displaystyle \downarrow$
+ \hfill $\displaystyle \searrow$ & \\
+left-subscript & underbar & subscript \\ \hline
+\end{tabular}
+\end{center}
+\caption{A math object with attributes. Each of the attributes
+ themselves contain math objects.}
+ \label{fig:math-object}
+\end{minipage}
+
+The next section describes how this model is extended to encapsulate the use
+of user-defined constructs in \alltex.
+\section{Extended Logical Structure}\label{s:macros}
+
+The \alltex{} facility can be used to extend the document logical structure by
+defining new constructs. Thus, an author preparing a manuscript on inference
+logic might define
+\begin{verbatim}
+\newcommand{\inference}[2]{{#1\over#2}}
+\end{verbatim}
+\noindent and write
+\begin{verbatim}
+\inference{x}{y}
+\end{verbatim}
+\noindent and use this construct throughout the document.
+
+Notice that defining the \verb|\inference| as shown above and using it to
+encode inference statements is distinct from and more powerful than just using
+the \tex{} built-in operator \verb|\over| throughout the document.
+A commonly mentioned advantage in this context is that using the newly
+defined construct \verb|\inference| will permit the author to easily change
+the notation used to denote {\it inference}.
+Notice, that this is in fact the same as saying that
+\begin{quote}
+ If distinct elements in a document instance are marked up using distinct
+ constructs, then it is possible to recognize and process these elements
+ in a multiplicity of ways.
+\end{quote}
+In \aster, the \alltex{} facility of defining a second \verb|\inference| macro
+that produces a different layout for {\it inference\/} can be generalized to
+the notion of different {\em audio renderings\/} for {\it inference}.
+
+
+ As explained above (``Document models''), \aster{} achieves its
+ aural renderings by building a rich internal representation of the
+ document content. In this representation, each document
+ element\footnote{We use the term {\em element\/} loosely to mean a
+ logical unit of the document. } $E$ is represented by an instance
+ of object $O_E$. \aster{} provides a predefined type $O_E$ for each
+ of the built-in constructs in \alltex. Thus, we could represent the
+ use of \verb|\inference| defined above in terms of object $O_{\rm
+ over}$. However, notice that this would mean losing valuable
+ information. When building up the internal representation, the
+ additional semantic information provided by the author's use of the
+ \verb|\inference| construct is very useful. In addition, expanding
+ all \alltex{} macros results in a pure layout representation, which
+ is not appropriate for producing aural renderings
+ (see~\cite{Raman:TB13-3-372-377}). If we were to represent
+ instances of \verb|\inference| in terms of $O_{\rm over}$, \aster{}
+ would be forced to render \verb|\inference| the same as the
+ \verb|\over| construct. Though the author in this particular
+ example may have chosen to use the same visual rendering for
+ inferences that is normally used for fractions, the same may not
+ carry over well to the aural domain.
+
+
+\subsection*{Representing Extended Logical Structure}\label{s:extend}
+
+
+ \aster{} solves the problem of representing and rendering the
+ extended logical structure arising from user-definable macros by
+ considering each macro definition as introducing a new object type.
+ Instances of a macro $M$, are represented by instances of object
+ $O_M$. Thus, in the example shown above, the definition of the
+ construct \verb|\inference| introduces a new object type $O_{\rm
+ inference}$. The \alltex{} macro consists of two parts; a
+ declaration, and a series of \TeX{} commands that the macro expands
+ into. The macro expansion is nothing but a visual rendering rule
+ that specifies how \TeX{} should display instances of the object
+ represented by the macro.
+
+
+\aster{} provides an equivalent mechanism for extending the class of logical
+structures that are recognized. Once \aster{} has been told about a
+user-defined macro, audio rendering rules for the new object type introduced
+by this macro can be defined in AFL (Audio Formatting Language). Notice that
+such audio rendering rules have to be defined by the user, just as the
+\alltex{} macro is defined by hand. It is not possible in general to translate
+the \tex{} macro into a set of audio rendering rules. This is because the
+\tex{} macro is capable of performing any arbitrary computation permitted by
+the operators present in the \tex{} language \cite{knuth84}\dash a
+Turing-complete programming language.
+\section{Rendering Information}\label{s:rendering}
+\aster{} renders information by applying {\em rendering rules\/} to the
+internal representation described above (``Document models'').
+ The system of rendering rules used in \aster{}
+and the language in which they are written (AFL\dash Audio Formatting
+Language) are described in detail in~\cite{raman-phd-thesis}. In a sense, AFL
+is to audio formatting as Postscript is to visual formatting, although AFL is
+a much smaller language.
+
+Here, we show a
+small example of such a rendering rule for a user-defined macro. In the
+following, we use \term{CLOS} generic function \term{read-aloud}. For the
+present, let us assume that function \term{read-aloud} executes the necessary
+actions to render its argument.
+
+
+ After extending \aster{} to process the \alltex{} macro
+ \verb|\inference| shown above (``Logical structure''), we can define
+
+{\small
+\begin{verbatim}
+ (defmethod read-aloud((inference inference))
+ "Sample rendering for object inference."
+ (read-aloud (argument 1 inference))
+ (read-aloud "implies")
+ (read-aloud (argument 2 inference)))
+\end{verbatim}
+}
+\noindent Given $\inference{A}{B}$, this produces ``A implies B''.
+
+If we wished to produce a rendering that inverts the order in which the
+arguments to macro \verb|\inference| are rendered, we would define:
+
+{
+\small\begin{verbatim}
+ (defmethod read-aloud((inference inference))
+ "Renders inference with arguments reversed."
+ (read-aloud "We know")
+ (read-aloud (argument 2 inference))
+ (read-aloud "because")
+ (read-aloud (argument 1 inference)))
+\end{verbatim}
+}
+\noindent which produces ``We know B because A''.
+
+Switching between these two rendering rules has the effect of inverting a
+proof-tree!
+Notice that writing a new rendering rule for an object $O_E$ has the same
+effect as redefining the \alltex{} macro that corresponds to $E$.
+
+\aster{} makes it easy to write several rendering rules for the same object
+and also allows rendering rules to be partitioned into rendering {\em
+ styles}. Such {\em styles\/} can be thought of as being analogous to
+\latex{} styles, but with one important difference. Due to the
+non-interactive nature of traditional paper documents, a paper is typically
+typeset in a given style. It is not possible for the reader to change the
+style in which the document is typeset.
+Typically, we do not feel the shortcoming of not being able to change the way
+a mathematical expression is rendered when reading a printed paper because the
+eye is capable of reading the various parts of an expression in any order that
+is convenient. However, when listening to an aural presentation, the listener
+does not have this flexibility. In other words, an active reader peruses a
+printed paper, a passive display, whereas in the case of audio, these roles
+are reversed\dash the aural display scrolls {\em actively\/} past a passive
+listener.
+
+\aster{} overcomes these difficulties by being a fully interactive system.
+It is possible for the listener to interrupt the rendering, change the
+rendering style in use, and listen to the document. In an interactive session
+with \aster{}, switching between rendering styles (a collection of rendering
+rules for different objects) and invoking individual rendering rules can be
+done with a few keystrokes, making it easy for a listener to obtain many
+different views of a document.
+This facility enables {\em active\/} listening.
+
+\aster{} derives its power from representing document content as objects and
+by allowing multiple user-defined rendering rules for individual object types.
+These rules can cause any number of audio events (ranging from speaking a
+simple phrase, to playing a digitized sound). The pitch of the voice, the
+physical head-size of the virtual speaker, the volume, and many other
+parameters can be changed by rendering rules, making it easy to create sound
+cues to help display structure.
+In fact, the design of \aster{} does not restrict the system to producing
+purely aural renderings; there is nothing to preclude us from defining
+renderings that produce truly multimodal output; \ie renderings where the
+traditional visual rendering is augmented with aural feedback. We conjecture
+that such multimodal renderings may prove very useful for persons with
+learning impairments.
+
+To give an example of a multimodal rendering, the logo for \aster{} is
+\begin{center}
+ \asterlogo{}
+ \end{center}
+ \noindent and is produced by \alltex{} macro \verb|\asterlogo|.
+ After appropriately extending \aster{} to recognize this macro, we
+ can define an audio rendering rule for object {\em asterlogo\/} that
+ produces a bark when rendering instances of this macro. Thus, the
+ same piece of markup \verb|\asterlogo| produces the picture of
+ Aster\footnote{Aster is my guide-dog. } when rendered visually, and
+ an appropriate sound\footnote{The bark is that of a generic dog,
+ Aster is too well trained to bark, and could not therefore be
+ recorded.} when rendered aurally.
+
+
+ This feature was exploited to advantage when producing the audio formatted
+ version of the author's thesis. The dedication page of the thesis contains
+ a large picture of Aster, and the audio formatted version\footnote{An audio
+ formatted version of the thesis produced by \aster{} (about 6 hours) is
+ being distributed by RFB\dash Recordings For The Blind\dash as the first fully
+ computer-generated talking book. } contains a verbal description of the
+ picture, accompanied by the sound of Aster panting in the background. You
+ can listen to this example on the WWW\dash visit the \aster{} home
+ page by following the link to the \aster{} demonstration
+ from my home
+ page\footnote{\URL|http://www.research.digital.com/CRL/personal/raman/raman.html|}
+ and clicking on the picture of Aster.
+
+Several ideas come together to make all this possible. First, logical
+structure is of paramount importance\dash not its display on any one
+particular medium. The more a document makes structure explicit,
+the better the document can be displayed on (projected onto) several
+different media.
+
+Next, the use of \alltex{} macros to encode structure makes it
+possible to have a system like \aster, in which the internal
+structure can be extended to fit a document. This allows the encoding
+of the structure in a flexible, uniform, and consistent representation
+such as an attributed tree, with the addition of the quasi-prefix form
+for dealing with mathematics.
+
+Finally, providing different rendering rules and styles and a
+flexible way to switch among them makes it possible to obtain multiple
+views of a document in an interactive fashion.
+
+\section{Conclusion}\label{s:conclusion}
+
+
+ The approach used in \aster{} to exploit the additional semantic
+ information present in the electronic encoding in the form of
+ user-defined constructs points to an important feature of markup
+ systems like \alltex{} that is currently missing to a certain extent
+ in systems like SGML. When \aster{} was at its inception, I firmly
+ believed that one should use a semantic-oriented DTD to encode a
+ document in order to be able to produce high-quality audio
+ renderings. I still believe this; however the work on \aster{} does
+ point out one shortcoming with the fixed document DTD model. Given
+ that mathematical and technical notation is being invented all the
+ time, a fixed DTD forces the author to encode new constructs using
+ {\em only\/} primitives that are provided by the DTD. As a
+ consequence, authors end up using a presentation-oriented encoding
+ even though the DTD in use is one that is semantically oriented.
+
+
+ To make this concrete, consider the case of the {\it inference\/}
+ construct described above (``Logical structure''). If the document
+ were being encoded using a fixed non-extensible DTD that only
+ provides a {\it fraction\/} element, the author would be forced to
+ encode {\it inference\/} using this element.
+
+Since in general it is not possible to define an all-encompassing DTD that
+covers every possible kind of math notation (those currently known and those
+yet to be discovered) extensibility of the DTD as provided by \alltex{} is of
+vital importance.
+
+Another good example of this facility in \alltex{} being put to good
+use is the Hyper\tex{} system \mdash an extension to \tex{} that
+allows the user to view his legacy \alltex{} documents as online
+hypertext. Conceptually, we can think of \verb|\ref| and
+\verb|\label| as being object types; traditionally, these cause
+specific marks to appear on paper when rendered visually by \tex; to a
+system like Hyper\tex{} these turn into {\em active\/} links that a
+user can follow interactively.
+
+The ability to produce multiple renderings of the same object provided by
+\aster{} was introduced in the context of aural presentations. However, such
+multiple presentations become equally relevant when interactively perusing
+online documents visually. For instance, when reading a document that
+presents a complex proof, a user may wish to have the same proof displayed as
+an outline in one window, and as a proof-tree in another
+(see~\cite{lamport:proofs93}). In the case of paper documents, the user has
+to use her imagination to achieve such multiple views \mdash though she is
+aided in this by the visual notation.
+ In the interactive scenario presented by electronic
+documents, the previewer can provide some additional functionality to aid in
+this process.
+
+\begin{thebibliography}{}
+\bibitem[Knuth 1984]{knuth84}
+Knuth, D.~E.
+\newblock {\em The \TeX{}book}, volume~A of {\em Computers and Typesetting}.
+\newblock Addison-Wesley, Reading, Massachusetts, 1984.
+
+\bibitem[Lamport 1993]{lamport:proofs93}
+Lamport, L.
+\newblock ``How to write a proof''.
+\newblock Technical Report~94, DEC Systems Research Center, Palo Alto, {CA},
+ 1993.
+\newblock To appear in {\em American Mathematical Monthly}.
+
+\bibitem[Raman 1992]{Raman:TB13-3-372-377}
+Raman, T.~V.
+\newblock ``An audio view of \TeX\ documents''.
+\newblock {\em TUGBoat} {\bf 13}(3), 372--377, 1992.
+
+\bibitem[Raman 1994]{raman-phd-thesis}
+Raman, T.~V.
+\newblock {\em Audio System for Technical Readings}.
+\newblock Ph.D. thesis, Cornell University, 1994.
+
+\end{thebibliography}
+
+\end{Article}