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===Background===

ISRST (Interactive Storytelling Model using RST, a recent evolution of
SRST) proposes an interactive computational model wherein reasoning
over an instantiated conceptual framework is informed by interactive
input, which is not directly mapped to model constituents, but rather
mapped to dynamically changing global and local (i.e., agent-specific)
criteria for the selection of the next presentation unit of the story
(and thus also its evolution). ISRST promotes a strict separation of
concerns, keeping the visual presentation aspects distinct from event
sequencing determined by story reasoning carried out over an
ontological structure. The design of this ontological structure draws
heavily upon RST (Rhetorical Structure Theory) but deviates based on
experiences gathered with SRST to (better) accommodate desiderata of
interactive storytelling.

===Description===

ISRST proposes what the authors term a "full semantic conditioned"
storytelling model based on the organisation of events using a
selection of rhetorical relations identified by Rhetorical Structure
Theory and narrative principles.  The authors distinguish the "full
semantic conditioned" storytelling model from the "timed conditioned"
and "pseudo-semantic conditioned" applications models, where explicit
(wall-clock) timing information is included in the modelling and user
manipulations are mapped directly to reified interaction variables that
are constituent part of the model (see figure below).

Quality of experience provided by "timed conditioned" models is
prone to issues arising because of the inherently only contingent
relation of timing information and interaction variables (as opposed
to semantically grounding devices). These issues are addressed, but
only to a limited extent, in so-called "pseudo-semantic conditioned"
models, where (typically: rhetorical) semantic event relations are
mapped to spatio-temporal relations by means of a fixed set of rules.
Thus, "pseudo-semantic conditioned" models on the one side eliminate
the use of timing information attributes, if with limited means; but
on the other they still do include interaction variables directly in
the definition of events, thereby interfering with the purpose of
explicit semantic relations.


[[Image:ISRST-12-sm.png]]

In contrast, "full semantic conditioned" models are characterised by
allocating interaction variables defined in the story events within
the very story engine, eliminating altogether their potential to
establish temporal relations (even if implicitly). This comes at the
cost common to any forsaking of "shallow modelling", in particular the
renouncing of the possibility to patch limitations of the model (which
includes not only making up for fundamental limitations of scope of
the modeling approach, but also the option to provide cheaper "good
enough" solutions for parts of the model of very limited overall
impact). In addition, deployment of "full semantic conditioned" models
requires also to provide alternative interaction devices, in terms of
both, the identification of useful qualities defining the coupling of
the user to the story's evolution, and the providing of practical
solutions, including the ensuring of users' understanding of the
capabilities and responsibilities they are endowed with and assigned
to, and the providing of suitable engineered solutions in terms of
user interface devices.  For this latter challenge, the authors of
ISRST build upon the concept of user interest as proposed by Ed Tan as
key performance evaluation criterion of story experience.

The ISRST modelling approach comprises formal specifications of the
central notions of: story event (a minimum unit by which a story or
part of it can be told; due to presumed high inter-application
variation, any more detailed formalisation of this entity is
considered out of scope of the model) and related semantic relations
selected from the set relation types covered in rhetorical structure
theory (RST); story concept networks defining the directed dependency
links between scenes or episodes (allowing to identify existing
logical transitions required for a smooth flow of knowledge along a
dynamically generated concept path); agents and related points of view
(capturing the possibly differing subjective relevancies of events for
different agents); and nucleus and satellite events: necessary and
optional elements of the interactive presentation of the story.

The ISRST ontology model comprises the classes of "Concept" (a
specific topic that a story or a part of it may refer to); "Event"
(the unit of information presentation); "Relation" (a rhetorical
binding between two event or scene entities expressing a specific
semantics); "Origin" and "Destination" defining the position of an
event or scene in a given relation; "Agent" (a story entity used as
target of user interest, see the concluding critical notes); "Scene"
(a minimal organisation of relations, expressed as a graph); "Episode"
(the object associating a scene and a concept); and "Story" (the
framing of a whole story in terms of one or more episodes).
Additional attributes and annotations are provided by auxiliary
classes: the single "EntryPoint" and possibly multiple "ExitPoint"s of
a scene; the "RelationType" name of a relation; and the
"InformationType" of the content of origin and destiny object of a
relation (in terms of nucleus or satellite).

[[Image:ISRST-3-sm.png]]

The scene class is one of the main changes introduced with ISRST over
its predecessor SRST. It marks the departure from the tree-based
organisation of related events inherited from RST to overcome
experienced issues in both, content creation (regarding flexibility
and clarity for the author) and support for interactivity (in terms of
variety of even selection, in particular regarding the possibility of
having multiple entry and exit points).

===Application to Interactive Storytelling===


In addition to the organisational aspects covered so far, ISRST also
implements two narrative principles describing how a story is to be
presented. The concept of "Advancing the Story" identifies locally
whereto the current position of a story can change, based on
"advancing" relation instances anchored to the current story position
(in particular, the "CAUSE" relation from RST does not figure among
the advancing relations, since disclosure of the cause of a given
state does not by itself necessarily advance the story).  This concept
is applied in a backward chaining manner to a given graph structure
created by the relation objects of a particular scene, to establish
the conditions by which the scenes may be traversed.

At a global level, ISRST follows a fixed narrative structure
articulated in terms of story phases, each of which may be introduced
only by a specific rhetoric relation: story introduction (initiated by
the non-advancing BACKGROUND relation), story conflict (initiated by
the advancing CONTRAST relation), story resolution (initiated by the
advancing SOLUTIONHOOD resolution), and story conclusion (initiated by
the advancing EVALUATION relation).

Event sequencing

Reasoning on instantiated ISRST models is informed by loosely coupled
user feedback regarding their currently perceived interestingness of
the story (cf. Tan). Such input is computationally mapped to two
derived values used to direct ontological reasoning: estimated overall
interest, which relates to the degree of "user stimulation" in terms
of density of present events (which are themselves classified by
whether they are story-advancing or not), and identification of user's
sympathy in specific characters. These values are used by "Interest
based tasks" in the event selection process, as depicted in the
following figure.

[[Image:ISRST-4-sm.png]]

===Examples===

The website http://ninfort.com/isrst/isrst.html provides online access to
"When Your Heart Takes Over", a full-length implementation of
an ISRST-based interactive story that is playable on a Java enabled Web browser.

===Critique===

The view of storiness implemented in ISRST is strongly centred on
individual characters, as clearly exposed by the interpretation of
user interest feedback in terms of agent empathy and the related
promotion or demotion of candidates in the selection of events to be
presented. Such a stance misses out on the large narrative potential
of aesthetic emotions related to dynamic configurations of multiple
characters, as argued in Greg Smith's analyses of TV series, such as
Seinfeld or Ally McBeal. Therein, Smith explains how user interest
need not necessarily be rooted in any individual character, and
conversely how even and exactly the performance of characters
considered uninteresting can instrumental for the staging of
encompassing situations which the user may very well be interested
in. Furthermore it is shown how the traditional characterisation of
the realm of satisfactory user experiences in terms of "heroes
actively pursuit and reaching of their goal states" can be expanded in
a principled fashion.  

===References===

*Nakasone, A. & Ishizuka, M. (2006). SRST: A Storytelling Model using Rhetorical Relations. Third International Conference on Technologies for Interactive Digital Storytelling and Entertainment, TIDSE 2006, Stringer LNCS 4326, Darmstadt, Germany, pp. 127-138.

*Nakasone, A. & Ishizuka, M. (2006). ISRST: An Interest based Storytelling Model using Rhetorical Relations. Proc. Edutainment 2007, Springer LNCS 4469, Hong Kong, China, pp. 324-335, 2007

*Nakasone, A., Prendinger, H. & Ishizuka, M. ISRST: Generating interesting multimedia stories on the Web, Applied Artificial Intelligence, forthcoming.