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From: goetz@cs.buffalo.edu (Phil Goetz)
Subject: Review of IF
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Keywords: interactive fiction, adventure
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Organization: State University of New York at Buffalo/Comp Sci
Date: Fri, 3 Dec 1993 17:38:30 GMT
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This is a paper I wrote for my dissertation committee,
to introduce them to interactive fiction.
It consists of these sections:

	History:	Hypertext fiction
			Adventures
	Issues in IF:	Freedom vs. drama, open-endedness vs. control
			IF != adventures
			The path not taken: Can we "play to lose"?
	Computer science problems:  Physical simulation
			Character simulation
			Why IF is interesting to AI
	Future directions:  Virtual reality IF
			Multireader IF

It's in LaTeX format.  To see it done prettily, you _w_rite it to a file
called "review.tex", delete eveything before the line "\documentstyle[...",
and type
        latex review.tex
then, to view on-screen under X-windows, type
        xdvi review.dvi &
to print out, type
        texprint -h -Pprinter review.dvi

You may have some other command than "texprint".
If you can't figure out how to print or view the dvi file,
convert it to a postscript file by typing
        dvips -o review.ps review.dvi

and then
        ghostview review.ps &    to view using X-windows
or      lpr -h review.ps        to print

Remember, printing is environmentally unsound!

- Phil goetz@cs.buffalo.edu
-----------------------------

\documentstyle[11pt]{article}

%The following incantations give you a 1" margin on all 4 sides
\topmargin -0.5in
\textheight 9.0in
\textwidth 6.5in
\hoffset -0.875in
\parindent 0.5in

\parskip 0.5em
\pretolerance=10000

\title{\bf Interactive Fiction}
\author{\bf Phil Goetz\\Dept. of Computer Science, SUNY, Buffalo NY 14260}
%\date{December 2, 1993}

\begin{document}
\maketitle

\section{Introduction}

A definition, as always, is hard to come by.
All fiction is interactive in that each reader brings a different
perspective to the story.  Interactive fiction (IF) is fiction where
the experiences of different readers are objectively, measurably
different.  Usually the reader can influence the outcome of the story.
The degree of interactivity in IF ranges from
movies where the audience votes on one of two endings to live role-playing
games where the participants are given characters to play and placed in a
situation of conflict, and each try to steer the outcome to their advantage.
I'm going to focus on forms of IF which are enhanced or made possible by
computers.

\section{History}
\subsection{Hypertext fiction}

Hypertext is text with links.  Links take you from one text to another.
Sometimes there is a default linear path which the reader can follow
through the narrative, and the links are optional.

For instance, say you were reading the hypertext version of {\it Hamlet}
on a Macintosh.
After reading Act II, you might be prompted, ``Should Hamlet (A) kill
his uncle, (B) leave the country, or (C) mope about life and death?"
You type ``A", and read
a considerably shortened version of {\it Hamlet}.  (This exhibits one problem
with interactive fiction - sometimes the action which builds up to a
more dramatic climax is not the action which a goal-oriented reader
would take.)

It is possible to do this on paper by letting the reader decide at each
crisis what the protagonist would do next,
and telling her a page to turn to depending on her decision.
This is like the programmed learning textbooks from the 1960s,
e.g. \cite{schagrin}.
Now there are many juvenile novels written this way \cite{brust}.
Jorge Luis Borges described such a book
(though he did not write one)
in ``El jardin de senderos que se bifurca" (``The garden of forking paths")
in 1941: \cite{fishburn}

\begin{quote}
In all fiction, when a man is faced with alternatives he chooses one at the
expense of the others.  In the almost unfathomable Ts'ui Pen, he chooses
-- simultaneously -- all of them...

Fang, let us say, has a secret.  A stranger knocks at his door.  Fang makes
up his mind to kill him.  Naturally there are various possible outcomes.
Fang can kill the intruder, the intruder can kill Fang, both can be saved,
both can die and so on and so on.  In Ts'ui Pen's work, all the possible
solutions occur, each one being the point of departure for other bifurcations.
Sometimes the pathways of this labyrinth converge.  For example, you come
to this house; but in some possible pasts you are my enemy; in others
my friend.
\cite{borges}
\end{quote}

In the same year, Borges described a backwards hypertext fiction, the likes of
which has never been written, in ``An examination of the work of Herbert
Quain." \cite{borges}  Herbert Quain's supposed book {\it April March} was
a backwards branching hypertext:  The first chapter described the events of an
evening.  The next three chapters describe three alternate preceding evenings.
The next nine chapters describe nine alternate evenings before those in
the second through fourth chapters, with three possible preludes to each
of those three chapters.  (There never was any such book; Borges often
pretended to review an imaginary book in order to explain the principles
he had in mind for a book without actually writing it.)

Julio Cortazar wrote the novel {\it Rayuela} ({\it Hopscotch}) in 1963,
which is a simple non-interactive type of hypertext.  He provides two
ways of reading it:  with or without a set of optional chapters between
the required chapters \cite{cortazar}.  To my knowledge,
the only interactive fiction written on paper before it had been
demonstrated on a computer was ``Norman vs. America", a 20-frame
cartoon by Charles Platt
based on an idea by John Sladek, published in an underground comic in
1971 \cite{platt}.
Interactive drama had been experimented with;
two early examples are the first British science-fiction TV show,
{\it Stranger from Space} (1951), and a movie shown in the
Czechoslovak pavilion at Expo 67 in Montreal.  The next week's
installment of the TV show was based on suggestions in viewer mail
\cite{ford}.  Viewers of the Czechoslovakian movie voted on the spot
to choose between possible, previously-filmed continuations \cite{time}.

A computer is useful for hypertext fiction because a
reader wants to move through the story without filling his book
with bookmarks of points to return to, without keeping a
mental subordinate-text (see below) calling stack in his head,
and without constantly searching for the next part of text.

Another type of link does not alter the course of the plot, but is
a digression.  When you read ``When he himselfe might his Quietus
make With a bare Bodkin", you might click on the
word ``Bodkin", and see a window come up that says, ``Bodkin:
A short pointed weapon; a dagger, poniard, stiletto, lancet."
A hypertext annotation of James Joyce's {\it Ulysses} is being
assembled at the State University of New York at Buffalo, which
should make that book more readable.

The most straightforward type of hypertext novel would
be a plot tree through which the reader chooses one path
which takes her along a traditional narrative, or a non-branching
narrative from which she may take minor digressions.

Unfortunately, the people who write hypertext fiction using computers today
are people who want to be very cutting-edge, and to use
this new medium to communicate a fundamentally new reading experience.
The computer hypertext stories that have been written, such as ``Afternoon",
try to replace the straightforward following of a narrative with a stochastic
sampling of the story that leads you through a maze of links until
you (hopefully) finally have a feel for the entire set of interrelated
people and events that populate this piece of fiction
\cite{joycehere,coover}.
Glen Hartley has proposed that the ``ultimate participatory novel"
may resemble the ``Tralfamadorian novel" in Kurt Vonnegut's
{\it Slaughterhouse Five:}

\begin{quote}
Each clump of symbols is a brief, urgent
message - describing a situation, a scene.  We Tralfamadorians read
them all at once, not one after the other.
There isn't any particular relationship
between all the messages, except that the author has chosen them carefully,
so that, when seen all at once, they produce an image of life that is
beautiful and surprising and deep.
\cite{hartley}
\end{quote}

My reaction to these types of works is that interactivity is actually
very low.  They are more like the computer game {\it Portal} by
Activision than like IF:  rather than affecting the story, the reader
merely searches through the hypertext until he understands what's going on.

I believe that before trying to create entirely new means of communicating
fiction, we should extend traditional narratives with hypertext, especially
since that is the only practical way to interest most people in hypertext.

\subsection{Computer adventures}

Suppose that, instead of giving the reader two or three choices at
every branch point, you gave him hundreds.  And suppose that branch
points came not every page, but every sentence.  The resulting hypertext
would be too large to list in a tree fashion.  Instead, the effects of
each choice must be computable.  This means that the fictional world
must have a representation which can be altered in detail and in
ways not foreseen by the author.  Furthermore, the list of possible
choices is too large to present as a menu; it must be presented implicitly;
for instance, by allowing choices to be specified using a subset of English.
The resulting hypertext is an {\it adventure.}

\subsubsection{Crowther and Woods:  The Original}

\begin{quote}
You are standing at the end of a road before a small brick building.
Around you is a forest.  A small stream flows out of the building and
down a gully.
\end{quote}

That is the first line that greets you upon running {\it Adventure},
which was finished in early 1977 by Willie Crowther and Don Woods.
The first version, in 1975, was simply a map of Mammoth Cave, KY, which
let the player walk from room to room.  Commands were added to pick up
carry, and use items in various ways.  The player's goal was to
find treasure.  Various problems presented themselves, ranging from
the obvious (a fierce green dragon bars the way) to the subtle
(a gold nugget is too heavy to carry up the stairs to the treasure
room).  Objects or information that can be used to overcome these
obstacles are also waiting to be found.  Each treasure gained
or problem solved added to the player's score.

Since {\it Adventure} was written in Fortran, which everyone had, it spread
rapidly over the Arpanet.  It may have set the entire computer industry back
two weeks:  when it reached a site, work was suspended until everyone
had solved it \cite{anderson}.

The way this world was constructed has remained the same in all
adventures:  The world consists of things contained in other things.
For instance, at the start, you are contained in a location described
in the above quote.  If you enter the building, you will find a lantern
in the building.  Pick it up, and it is in you.  The world is discrete,
not allowing you to be ``in transit" between locations, nor (generally)
for an item to be in two locations at the same time, even if it should
be (e.g. a rope).  Each command you issue takes one unit of time;
events between moves occur all at once rather than continuously.
Your commands are issued by typing a sentence (in {\it Adventure's} case,
a verb and a noun) at the start of each turn.

\begin{quote}
$>$down steps

You are at one end of a vast hall stretching forward out of sight to
the west.  There are openings to either side.  Nearby, a wide stone
staircase leads downward.  The hall is filled with wisps of white mist
swaying to and fro almost as if alive.  A cold wind blows up the
staircase.  There is a passage at the top of a dome behind you.

$>$north

You are in the hall of the mountain king, with passages off in all
directions.
A huge green fierce snake bars the way!

$>$release bird

The little bird attacks the green snake, and in an astounding flurry
drives the snake away.
\end{quote}

Unlike almost all traditional fiction, adventures use second person present.
This is because they are immersive:  The player projects himself into
the role of the protagonist with an immediacy not possible in static fiction.
Years later, Brian Moriarty designed {\it Trinity} so that the player
had to kill a lizard.  In an interview, he said,

\begin{quote}
I was amazed to see how many people were actually
bothered by the scene with the lizard, because it was them doing it. It's
nice to know that interactive fiction could do that, make you feel
uncomfortable about killing things.  In no other media could I make you feel
bad about killing something.  Because there is only one medium where I can
make {\it you} do it, and make you feel empathy for a thing that doesn't exist.
It's only with interactive fiction that you can explore these emotions.
\cite{moriarty}
\end{quote}

\subsubsection{Zork}

After playing {\it Adventure}, many people wanted to write their own.
In a few months Tim Anderson, Marc Blank, Bruce Daniels, and Dave Lebling
of the Dynamic Modelling Group in the MIT Artificial Intelligence Lab
created {\it Zork} \cite{anderson}, which was famous for its sense of humor,
its anticipation
of actions the player might try, the cleverness of its puzzles,
and (eventually) the complexity of its parser.
{\it Zork} was the first adventure which could parse complete imperative
sentences, plus a few questions.  {\it Zork} was also the first adventure
whose non-player characters had personality.  The Thief was a gentleman
gone wrong, with good manners, a cynical sense of humor,
and the willingness to slit your throat in a moment.

{\it Zork}, like Apple Computer, got its name because no one came up
with another.  ``Zork" was a nonsense word; the Dynamic Modelling Group
usually called its programs ``zork" until they were ready to install.
Since {\it Zork} never was officially installed, it was never named
\cite{anderson}.

{\it Zork} was written not in Fortran, but in MUDDLE, a LISP variant which
was not very widespread.  {\it Zork} gained fame because, although people
couldn't distribute it widely, anyone could log onto the MIT Artificial
Intelligence Lab computers and run it.  They ran an MIT-grown OS
called ITS (Incompatible Time-sharing System), which had no security
\cite{anderson}.
(In fact, ITS had a command KILL SYSTEM which would do just that, so that it
wouldn't pose a challenge to crackers \cite{levy}.)

The Implementors (as they were known) later formed Infocom (below), which
gained fame as the best (some would say the only) publisher of interactive
fiction.

\subsubsection{Scott Adams}

But it wasn't Infocom that first brought adventures to the masses.  {\it Zork},
like {\it Adventure}, originally ran only on mainframes, since it took a
megabyte of RAM \cite{anderson} ({\it Adventure} took 300K \cite{adams}).
In 1977, few believed that a personal
computer (which then had 16K RAM if you were lucky) had enough memory for
an adventure.  A systems programmer named Scott Adams thought they did.
He worked so hard to prove this that his wife, feeling neglected,
once put his floppies full of source code in the oven \cite{adams}.
But after a year, he amazed the personal computer world with
an adventure interpreter which allowed adventures (though not
nearly as large as {\it Adventure} or {\it Zork}) to run in 16K.
They were distributed on cassette tape because they didn't leave enough RAM
for a disk operating system to load them in.  Adams quickly
wrote twelve text adventures which became the standard literature.
The simple verb-noun parser he borrowed from {\it Adventure}
is still often referred to as an ``Adams parser," since he was the last
major author to get away with using it.

Scott Adams touched on all the genres.  He published Tolkienesque, pirate,
mystery, gothic horror, spy, science fiction, and western adventures.
To this day, adventures generally remain genre vehicles.

\subsubsection{Infocom}

In 1979, various people from the MIT Dynamic Modelling Group formed Infocom.
They cut {\it Zork} in half, squeezed the first half onto one 140K floppy
disk for personal computers, and called it {\it Zork I}.
They licensed it to Personal Software Inc.
PSI sold about 10,000 copies, then gave up on {\it Zork I} since that was all
they expected from a game.  Infocom regained the rights to market {\it Zork I}
themselves, and by 1986 had sold 250,000 copies \cite{anderson,lebling}.
{\it BYTE} magazine said,

\begin{quote}
That the program is
entertaining, eloquent, witty, and precisely written is almost beside the
point. Unlike the kingdoms of the Adventures for machines with 16K bytes of
memory and far from the classic counter-earthiness of the Colossal Cave in
the original Adventure, Zork can be felt and touched -- experienced, if you
will -- through the care and attention to detail the authors have rendered.
.. . . [A] most excellent and memorable work of computerized fiction.
\cite{zorkreview}
\end{quote}

Infocom's games kept getting better.  They updated their game
engine to simulate the world more realistically, and to parse more sentences.
After {\it Zork}, their games always followed a central narrative in a
unified setting.  They learned to avoid the cardinal sins of mentioning
something in a description, but not understanding references to it; requiring
a specific sentence to accomplish something so that the game degenerated to
synonym guessing (``Move curtain?  Examine curtain?  Open curtains?");
presenting problems that were simply exercises in combinatorial search;
or selling buggy software.

In 1983 they released {\it Deadline}, a murder mystery which advanced the
state of the art in several ways.  Characters in the story played their
parts out, moving throughout the mansion and its grounds on their own
business.  But if you interfered with them, you could thwart their plans.
Simply following someone around could cause them to do something more
innocent than what they had in mind.  {\it Deadline} was also the first
adventure to use the plot tree form of hypertext:
it had about 30 possible endings \cite{hartley}.

\subsubsection{Graphic adventures}

Roberta Williams, co-founder of On-Line Systems (now Sierra Software),
was hooked on {\it Adventure}, and wrote her own adventure which took place
in a Victorian mansion with a killer on the loose.  Her husband and co-founder
Ken told her she needed a new angle to sell it, and she thought it would be
great if a game had pictures as well as words.

The result, {\it Mystery House}, was released in 1980.  It had a picture for
every location.  Despite the fact that its pictures were monochrome
line-drawings with stick figures for people, it was an instant success
\cite{levy}.

From then on, the trend was towards graphics adventures.  Even Scott Adams
rereleased all his adventures with accompanying pictures, and found he could
charge twice as much as he did for the text-only versions.
Fans of text-only adventures complained about the smaller scenarios,
concentration on graphics to the exclusion of other issues
such as plot and ease of use, and the limitation of the imagination.
But pictures sold programs.  Infocom included some in {\it Zork
Zero} shortly before the company was dissolved in 1989.

Publishers used Infocom's failure as proof that text adventures were dead
\cite{goetz}.  But Infocom's failure was not because their text adventures
weren't selling, but because their relational database {\it Cornerstone},
an expensive long-term development project, didn't sell \cite{forbes}.

Now commercial software publishers deal only with graphic adventures,
claiming they have more mass appeal \cite{goetz}.
But the large group of amateurs who write and swap their own adventures
write only text adventures \cite{ftp}.

\section{Issues in IF}

\subsection{Freedom vs. drama}

A fundamental problem with interactive fiction concerns open-ended
interactivity vs. drama.
A dramatic story is one crafted by a writer to be so.  Though there
may be 30 possible endings to an IF, that is still a finite number.
The player does not have true freedom.  Yet if you let the player
wander outside the storyline, the author cannot provide a dramatic
experience.

Jim Gasperini, author of both a text adventure ({\it Star Trek:  The
Promethean Prophecy}) and a simulation ({\it Hidden Agenda}, a narrative
simulation of Central American politics), contrasts the closed-endedness
of an adventure to the open-endedness of a simulation.  His comments
on adventures apply equally to IF if we substitute ``plot has been
played out" for ``puzzles have been solved":

\begin{quote}
Even in the best ``interactive fiction," once all the puzzles have
been solved the plot is revealed in all its naked linearity. A
finished ``closed-ended" work is like a punctured balloon, emptied
of all ambiguity. There is little reason for anyone to go through
it again.

By contrast, an ``open-ended" work becomes more ambiguous, not
less, the more it is played. It is through repeated playings,
comparing different plots chosen through the same web of
potential plots, that the experience becomes most meaningful.
This can be most clearly seen in the genre known as
``simulations."...

Each subsequent time the player enters the election
campaign, comparisons naturally arise between what happens this
time and what happened other times. This serves to deepen the
player's awareness of the range of structural possibilities.
\cite{gasperini}
\end{quote}

How can we experience open-ended interactivity that isn't boring?
If we set up the characters in the story and let them work out their
problems around the player, they are likely to find sudden and
unsatisfying resolutions.  (A con man comes to Dave's town to trick
old ladies out of their retirement funds.  Dave threatens to expose
him.  The con man shrugs his shoulders and moves on to the next town.)
James Meehan wrote a program called TALESPIN in 1976 that generated
simple stories based on rules about how characters interact.  Many
of them were boring:  He set up goals for the characters, and they
satisfied them.  End of story.

Stories rely on conflict.  Conflict is implicit in a simulation
of a battle, a dogfight, or an economic system (survival vs. collapse).
When the conflict is resolved, the simulation is over.  But a novel
is constructed by sustaining a major conflict, continually introducing
new complications that prevent the protagonist from resolving the situation.
The gradual escalation of conflict we find dramatic is unnatural, a
failure on the parts of both protagonist and antagonist.  It needs
artifice to maintain it.
Furthermore, the consequences of the resolution must be
commensurate to the magnitude of the conflict.  In real life,
the war may be lost for want of a nail, but in IF, the protagonist
had better have to work harder than to provide someone a nail.

Brenda Laurel, in her 1986 dissertation, proposed the development of a
computational theory of drama, possibly based on Aristotle's theory of
dramatic structure, which would be a sort of grammar for drama \cite{laurel}.
This would allow a computer to construct a dramatic turn of events
whatever the participant does.

Writers often complain that ``everything has been written before," meaning
that there is a small number of basic plots.  Georges Polti claimed in 1921
that there are 36 dramatic situations \cite{polti}, and others have tried
to find similar ``basic plots".
Joseph Bates of CMU, David Graves of Hewlett-Packard, and Jurgen Appelo
all advocate compiling a library of standard plot fragments and writing
a computer composer capable of combining them in sensible ways
\cite{bates,gravesfaq,appelo}.
This calls to mind Mozart's dice minuets, in which before performing
you would roll dice to choose which phrases to play when; or, on a
more mundane level, Mr. Potato-Head.  Which of these two the products
of such an automated playwright would more resemble remains to be seen.

An automated playwright would have an enumeration of plots,
match the current state of events and past history to one of them,
and be responsible for the other parts in the current plot
besides the protagonist.

Good fiction takes creativity on the part of the author.
No AI program in the next twenty years is likely to be able to
choose good descriptive details, or to provide humor, pathos, or
provocative ideas.  Writing is {\bf AI-complete}\cite{shapiro}:
we'd have to solve all the problems of AI before writing a computer author.
The systems these people advocate don't require full intelligence because
they will be hack writers, at best able to churn out westerns, space opera,
and romances.  Mysteries and sitcoms will remain beyond them.
As for me, I will not abandon closed-ended, human-controlled fiction.

\subsection{IF $\neq$ Adventures}

{\it Adventure} was not really a story, since it suffered from a lack of
a plot (other than ``gather treasure") or motivation (magic wands, lanterns,
and gold nuggets were just laying around for the taking).
The final point needed for a perfect score of 350
was infamous for its arbitrariness: you had to take a certain item
(among hundreds) and drop it off in a certain room (among hundreds).
Bruce Daniels, one of the authors of {\it Zork}, had to disassemble
the {\it Adventure} object code to discover the solution \cite{anderson}.

Infocom preferred to distinguish between adventures, such as {\it Zork},
and interactive fiction, such as {\it Deadline.}  In an adventure,
players solve puzzles.  Interactive fiction requires plot and characterization
\cite{hartley}.  We want to do in IF the things we do in traditional
fiction:  make readers care about the characters, create suspense
and concern, and a feeling of dramatic completion.

I'm even squeamish about the term ``player," because of its connotation
of frivolity.  Since reading fiction is entertainment, and
interactive entertainment is a game (alert Wittgenstein!), the term
``player" is justified.  Please understand that this does not imply
that all IF will be like adventure games, played to win.

\subsection{The path not taken}

In role-playing games, there are two types of players.  Some, who are
found playing {\it Dungeons and Dragons}, are very goal-oriented; they
will do only that which increases the power of their character.
They play to win.
Others devised their own games, such as {\it Paranoia} or {\it Toon},
to emphasize the role-playing aspects.  In {\it Paranoia}, you have
six lives; in {\it Toon}, an infinite number.  This frees the player
to take actions which lead to their characters' deaths if those actions
are in character.  To these players, playing is winning.

The former class will never be able to appreciate many forms of IF.
The development of a satisfactory story depends on both the author
and the participant.  If the participant cannot take on another persona,
he cannot enter into the world the author has devised, and cannot explore
the nature of that world.  Ibsen's {\it A Doll's House} would jump straight
to the final act, and  Kafka's {\it The Trial} would turn into {\it 1984},
because the participant would never take the actions, dictated by the
character of the protagonists, which make those stories what they are.

Unless players can find reasons to play other than to win, IF will not
escape the literary ghettos of genre fiction.
Even some traditional adventure-genre stories
would lose their charm under the imposition of a different character;
imagine {\it The Hobbit} with a self-confident and aggressive Bilbo Baggins,
or an interactive Father Brown mystery played by a Humphrey Bogart fan.

In particular, truly tragic fiction might never work in IF.  I'm not
referring to ``tragedies" such as {\it Hamlet}, which are mereley sad.
I'm referring to works such as {\it 1984}, {\it Brave New World},
{\it Lord of the Flies}, {\it Heart of Darkness}, or {\it Deliverance},
in which it is dramatically necessary for the main
character to be psychically crushed.  The IF participant might feel that
giving him the freedom to choose how to act had been a cruel farce.

One way to keep the player from identifying too closely with the
protagonist might be to have the player interact with several characters.
She might change viewpoints, or she might simply have a display panel
with a point-and-click interface controlling the emotional responses
of each character (level of anger, contentment, fear, urgency, etc.)
and see how the story unfolds.  But this defeats the intimacy of IF.

\section{Computer Science Problems}

\subsection{Physical simulation}

In linear fiction, the author creates a suspension of disbelief only
with great care.  In interactive fiction, there are more opportunities
to shatter this illusion.  The world and the characters in it must
respond realistically to the player, even in situations the author
has not foreseen.  If the player drops a crystal vase, it should shatter,
and he should be able to cut a plastic wrapper with the shards.
Open-ended stories cannot begin to be developed unless the entire
simulated world is complex enough to run on its own without authorial
control.  Special problems include liquids, fire, transparent items, and
accessibility to sight, sound, and touch.

One problem is a result of time passing in discrete steps.
If, in time interval I, character X decides to leave the room and
character Y decides to shut the door, X may successfully leave the
room (if he acts first), or he may run into a shut door.

The different types of entities in the world (people, mountains,
candy bar wrappers) require different types of simulation.
It may take elaborate calculations to decide how a pile of leaves
will blow in the wind \cite{wejchert}; these aerodynamic computations
should not be applied to a falling safe.
Saying one representation should handle all situations is like saying
text, photographs, movies, and music should all be stored with the same
representation.  A context mechanism must be found for deciding
when a particular level of abstraction is appropriate -- see, for example,
\cite{guha}.

Methods for limiting computation will be important.
For example, areas outside the current deictic (narrative) center might be
given less processor time, and be simulated at a cruder grain.

\subsection{Simulated characters}

Joseph Bates says we don't need to create intelligent characters,
just ones that aren't obviously stupid \cite{bates91}.
He calls them shallow but broad agents.  They need some knowledge
in many areas, to avoid acting unbelievably, e.g. standing in the path
of a steamroller, or being happy to see you even though you always
beat them up.  This brings to mind reactive agents as popularized by
Rodney Brooks' subsumption architecture \cite{brooks}.
SNePS, a {\bf S}emantic {\bf Ne}twork {\bf P}rocessing {\bf S}ystem,
has a component called GLAIR, a {\bf G}rounded {\bf L}ayered
{\bf A}rchitecture with {\bf I}ntegrated {\bf R}easoning, which is intended
to integrate reflexive behavior with knowledge-level reasoning
\cite{glair}.  This may be useful for creating broad and shallow (reactive)
agents with particular deep and narrow (symbolic) capabilities.

Doug Lenat \cite{lenat} proposed the use of Cyc to maintain simulated worlds
because it has a lot of commonsense knowledge about the physical behavior
of objects.  Such knowledge would also help characters reason.
James Meehan's 1976 dissertation, using his program {\it Tale-Spin},
explored the type of knowledge needed to simulate realistic interaction
between characters \cite{meehan}.
Joseph Bates of Carnegie Mellon plans to use Cyc in Oz, the CMU
interactive fiction platform, to provide such knowledge, as well as
drawing on cognitive structures such as Soar \cite{bates}.

\subsection{Why IF is interesting to AI}

When you have an idea for a representation or technique and write test
cases, your imagination is restricted by the techniques you have in mind.
IF forces you out of the blocks world.  You have to bring things in for
the story, and you quickly find the weaknesses in your system.
Players are much more thorough testers than you can be.

\section{Future Directions}

\subsection{Virtual reality IF}

Real-time 3D rendering is still beyond the capabilities of personal
computers, as is thorough real-time 3D physical simulation.  Silicon Graphics
claims they will provide realtime rendering in late 1995 for around \$5000
(as opposed to \$100,000 today) \cite{simerman}.
Real-time polygon-based 3D is already available for personal computers,
and some systems, such as Autodesk's {\it Cyberspace Developer's Kit},
Sense8's {\it WorldToolKit}, and Robert Grant's {\it Multiverse}, provide
some aspects of physics simulation (friction, gravity, and elasticity)
in real-time \cite{autodesk,cgw,multiverse}.
Knowledge Revolution's {\it Working Model} is detailed real-time 2D graphical
simulation
of physics on the Macintosh, taking into account velocity, mass, inertia,
gravity, collisions, static and kinematic friction, elasticity,
electrical charge, and torsion, among other things \cite{schaff}.

As it becomes easier to render good 3D graphics on personal computers
and to simulate physics for a 3D world,
graphical IF will approach a detailed physical model of the world.
We can imagine an interface which is
more like a virtual reality system than a text adventure.
You are looking at a 3D world on your computer screen, with colors,
shadows, reflections, surface reflectivity, and textures.

The command parser is used only for speaking with other characters.
Your physical interaction with this world is done
entirely with an arm.  This arm has no joints.  It sticks out straight
in front of you.  You can pull it in or push it out, and control the
speed at which this is done.  You can make it sticky or un-sticky.
You can also rotate it (roll, for you aerospacers).

If you want
to pick up an object from the floor, you bend over until the object is in
the center of the screen at the end of the arm.
Then you extend the arm, change it to sticky, and retract it.
You can ``drop" items from the arm into your inventory,
and pop them back onto the arm from your inventory list.
Say you want to throw a ball.  You pick it up as described, then
rapidly extend the arm.  At the end of the arm's movement, the ball flies
off the arm.

With the head-mounted display and body motion sensors used in W
Incorporated's 1992 game {\it Dactyl Nightmare}, we can envision
this system being fully immersive virtual reality.

This type of IF may be produced by Hollywood movie moguls.
Real-life actors may have their images copyrighted and licensed to
be used to generate actors in VR IF games.
Despite all the interface changes, this type of IF still has the same
problems as all-text IF:  offering the player freedom while keeping him
within the plot, or generating a plot on the fly; creating believable
characters; and escaping from genres.
(VR IF will probably run in continuous time, eliminating the
discrete time-interval problem.)

Textual IF will survive, just as text
novels haven't been entirely replaced by movies.  It is a matter of
time investment:  A graphical presentation takes longer to ``play",
just as a 2-hour movie can't communicate as much as 2 hours of reading.
It also takes much longer to create.  Individual authors simply don't have
the time to stop every time they write a scene, and create every object
in that scene as a 3D object, as well as the background.

Some aspects of textual IF will change.  Time may run continuously,
rather than always waiting for the player's next move.  A more detailed
physical model may lie behind the text (which can only communicate so
much detail).

\subsection{Multireader IF}

Many computer games have developed from single-player to multiplayer,
such as {\it Nettrek} and {\it Conquest} (space war games),
{\it Maze} (a tank-war game), and many multiplayer dungeons.
But having multiple readers in an IF is not as simple as introducing
another person into the same scenario.  If the two people act
independently, how can they both experience a dramatic unfolding of
events?  How can they both even understand what is happening in the
world?  In order for a reader to experience drama, what unfolds before
him must be important to future events.  But if what unfolds before
each reader is central to the plot, then each sees only one-half of
what he needs to.  If they find each other and exchange information,
the plot will not unfold but be thrown on them in disordered chunks.

Writers using the third person limited point of view must be careful
to ensure that their central character has an appropriate view of the
story:  no events should occur without explanation, but when suspense is
intended, the outcome should not be known in advance.  Can these
restrictions be ignored safely, as they are in live-action role-playing games?
Perhaps the solution is that each participant experiences a different drama:
one may be the protagonist, and the other the antagonist.

For adventures, the task is difficult, but different.  Since
the primary purpose in an adventure is puzzle-solving, players can
interview each other.  It doesn't matter in what order information
is revealed.  Half of the fun might be divining which players are
telling the truth and which are lying.  Some might pretend to be
computer-controlled, so as not to be feared as competitors.
A different problem for adventures is that in traditional
puzzles, particular items are often necessary.  If
there is only one key to the attic, and one player keeps it,
what can the rest do?  Perhaps the scenario can be designed so that
each participant has a different set of tasks.

What if one player reads for hours on end, and the other only an hour
a day?  Can a narrative be such that one can drop in and out of it
and still enjoy it?  In a narrative with competing players, perhaps
the players can take turns, with large sections of narrative between
each turn -- a play-by-mail (or email) format.

\section{Conclusion}

It is inevitable that future IF will have
more real-world knowledge and more realistic interfaces.
It is not clear whether authors and players cooperating can communicate
the same range of emotions and thoughts to the players as in traditional
fiction, whether a theory of drama would enable the player to have
an exploratory literary experience rather than a controlled one,
or if IF will escape from genres.

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