User-Based Character Modelling for Interactive Storytelling

PUC

ISSN 0103-9741

Monografias em Ciência da Computação

n* XX/10

User-Based Character Modelling for Interactive Storytelling

Fabio A. Guilherme da Silva

Departamento de Informática

PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO

RUA MARQUÊS DE SÃO VICENTE, 225 - CEP 22451-900

RIO DE JANEIRO - BRASIL

User-Based Character Modelling for Interactive Storytelling

Fabio A. Guilherme da Silva

faraujo@inf.puc-rio.br

Abstract: Interactive storytelling is a new kind of digital entertainment in which users interact with a story being told. In this context, determining the characters’ behaviour according to their individual preferences can be an interesting way of generating plausible stories where the characters act in a believable manner. Diversity of stories and opportunities for interaction are however other key requirements to be considered by such applications. In this paper, we present the development of a prototype that permits the modelling, by the user, of some characters’ personality traits in an interactive storytelling application.

Keywords: interactive storytelling, user modelling, narrative plots, affective computing, big five personality traits.

Resumo: Narração interativa de estórias é uma nova forma de entretenimento digital na qual os usuários interagem com uma estória sendo contada. Nesse contexto, determinar o comportamento dos personagens de acordo com suas preferências individuais pode ser uma forma interessante de gerar estórias plausíveis onde os personagens agem de forma verossímil. Diversidade de estórias e oportunidades de interação são, entretanto, outros requisitos-chave a serem considerados por tais aplicações. Neste trabalho, apresentamos o desenvolvimento de um protótipo que permite a modelagem, pelo usuário, dos traços de personalidade de alguns personagens em uma aplicação de narração interativa de estórias.

Palavras-chave: narração interativa de estórias, modelagem de usuário, enredos de narrativas, computação afetiva, cinco grandes fatores de personalidade.

1. Introduction

Interactive storytelling systems are computer applications for telling stories that can be modified to some extent by their users [Camanho et al. 2009]. Interactive stories allow the audience to change the narrative course by making their own decisions, thus exploring a multitude of possible storylines and creating a user experience that differs substantially from that provided by conventional, linear stories.

One of the main challenges to be faced when implementing such systems is to conciliate two important aspects which are usually in conflict. On the one hand, we have the coherence of the narrative, allowing the user to understand the relationships between the events in the story [Riedl & Young 2006]; on the other hand, there is the control afforded to the user, i.e. the autonomy he has to interact with the system. Strategies aimed most at user control tend to generate threats to the coherence of the narrative, whereas strategies for ensuring coherence usually reach their goal by depriving to the user a higher degree of freedom in how it is possible to change the course of the narrative. This conflict brings forth two main approaches to the development of interactive storytelling. One of them is character-based, being inspired in games [Cavazza et al. 2002; Charles et al. 2001; Young 2001]. In this case, autonomous agents, each with its own goals, interact with each other, the environment and the user. The story itself is the result of these interactions.

The other approach, plot-based, is influenced by Literature [Propp 1968] and Filmmaking and uses a strong control over the flow of the story being narrated, preventing the user from deviating much from the original intention of the author, who defines a structure for the narrative with well-defined points; the effect of user interaction being usually limited to the way the story reaches these points [Grasbon, & Braun 2001; Spierling et al. 2002]. To combine control with consistency, some solutions integrate both plot-based and character-based characteristics [Mateas & Stern 2005; Crawford 1999].

Some works in interactive storytelling employ a predefined repertoire of operations, defined by their pre- and post-conditions. The execution of these operations is then considered to be the occurrence of the events that constitute the narrative plot [Ciarlini & al. 2008]. In this case, a character trying to achieve a goal will have to execute an appropriate plan, i.e. a sequence of one or more operations able to lead to a target state where the goal will hold, possibly together with a number of other effects which may or may not be to the character's liking [Barbosa et al. 2010]. Plans can either be ready-made or be produced on demand by a plan-generation algorithm [Ciarlini & al].

In order to produce believable plots, character-based storytelling depends on a specification of personality traits that may adequately motivate the behaviour of the acting characters. Barbosa et al. [2010] propose a method to determine the behaviour of the characters participating in a story. This method is based on rules that associate a given situation with a list of different goals. Each character engages in a decision-making process along three steps: goal selection, plan selection, and commitment, and the selection criteria reflect individual preferences originating, respectively, from drives, attitudes and emotions. The model was able to generate coherent plots, according to the characters’ personalities. However, the characters’ personality traits are rigidly embedded in the application, demanding programming effort if one wishes to test some other alternative. In addition, users could feel more engaged to the story if they could choose how the characters are modelled, or even have these characters shaped after their own personalities. A valuable resource to capture the user’s personality traits is the use of some psychological test based on documented studies, such as those concerning the Big-Five factors [Goldberg 1992]. As psychological

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