Dynamic, User-Centered Resolution in Interactive Stories

One of the most important parts of a story is its ending. This is the point in which all open questions and conflicts in the narrative must be resolved. This paper presents a dynamic resolution method for interactive stories. The type of stories we support allows the user to participate as one of the characters and influence dynamically the development of the plot. The rest of the cast consists of discrete computer characters. Our resolution method takes into account the motives of the user character to decide on: (i) the outcome of all character actions and (ii) the presentation sequence for these outcomes. The decision process is based on the current story context. In addition, it addresses the need to avoid ambiguities, to preserve consistency and to create suspense during the end. We have integrated this method in PEGASUS, an interactive story system set in ancient Greece. 1 In t roduc t ion One of the most important parts of a story is its ending. This is the point in which all open questions and conflicts must be resolved in order for the audience to understand the main idea of the story and integrate all the plot developments around it. There are two types of possible endings. The first one resolves all open questions using the information provided in the story so far. The second one, known as deus ex machina, is based on the intervention of a character who has played no part in the story until that moment. Although quite popular in ancient dramatic practice, deus ex machina solutions have been dismissed very often in modern dramaturgy mainly for lack of believability. This paper presents a resolution method for interactive stories that supports the first type of ending. Interactive stories are narratives in which the user takes part as one of the characters and interacts with the rest of the cast to influence dynamically the development of the plot. Cast interaction is based on the execution of appropriate actions by each character. Plot resolution, in this case, refers to the computation of the outcome of all character actions and the presentation of these results in the story. This work describes a resolution method that is dynamic, user-centered and unambiguous. Dynamic means that it does not support a priori specific types of ending, such as the happy end for the user character. Instead it determines the story outcome based on the contents of the current plot. As a result, this method can be used in conjunction with dynamic storyweaving methods to create stories in intelligent games or collaborative virtual worlds. User-centered means that the motives behind the actions of the user character in the story determine the final outcome. Consequently, the system presents the resolution as a consequence of user behavior in the story, thus enhancing the clarity and believability of the final outcome. Unambiguous means that the method decides on the results of all actions in the story, therefore leaving no questions unanswered to the user. This feature corresponds to the norm for closed, unambiguous endings found in classical theories of drama [1] and used in mainstream screen writing [3,4,5]. Furthermore, this method generates a presentation sequence for the final outcome that takes into account the need to create suspense during the conclusion. Finally, the decision on the results for all character actions preserves the causal dependencies between conflicting actions in the story. Possible uses for this research include the development of dynamic storyweaving methods for interactive entertainment systems (e.g., stories, games, collaborative virtual worlds), or the creation of intelligent plot assistants for the design of these systems. The rest of the paper is described as follows. Section 2 describes the input to the plot resolution method. Section 3 presents the steps used in this method. We give an example of the system at work in section 4. Finally, section 5 presents some related work, while section 6 is a conclusions and future work section. 2 System Input Plot resolution accepts as input a symbolic description of the story content. This consists of a temporal sequence of the character actions in the story so far. This sequence is expressed in a special-purpose language consisting of predicates that describe these 990 NATURAL-LANGUAGE PROCESSING AND GRAPHICAL PRESENTATION actions, their motivations and goals, along with the types of cast interaction they implement. Resolution assumes that the behavior of each character is motivated by its goals in the story, the roles that it plays and its relation (e.g., friend, enemy) with the rest of the cast. In general, character behavior consists of either favorable or unfavorable interference between cast members, depending on whether the initiating character wants to harm or cooperate, respectively, with another cast member. More specifically, goals motivate each character to interfere with the rest of the cast in order to achieve them. Roles provide the norms regulating the behavior of each character in the story. Each cast member seeks to protect its role-relevant norms from violations by other characters by interfering unfavorably against the offenders. Analogously, each character is motivated to interfere favorably for cast members with similar normative beliefs. Finally, positive (e.g. friendship) or negative (e.g. enmity) social relations motivate the parties involved to either help or block, respectively, each other in the service of their goals. Table 1 describes the primitives representing the possible character interactions and their outcome. Interference (predicates /+ and Iin Table 1) gives rise to the execution of one of possible actions for achieving it. These can be either actions for satisfying a goal or norm (predicate Exec+ in Table 1), or counter-actions for impeding it (predicate Execin Table 1). For each action the motivating interference is noted (predicate Motivates in Table 1). Finally, action execution and interference can either succeed or fail (predicates Succeeds and Fails in Table 1). For example, a character playing the priest role in the story, w i l l seek to interfere with the behavior of the rest of the cast to impose all the norms associated with worshipping and obeying the gods. Furthermore, a character (e.g., X) with a goal of acquiring a valuable resource (e.g., gold) from some other character (e.g. Y) wi l l seek to interfere favorably or unfavorably with Y to satisfy this goal. This interference wi l l initiate a set of appropriate actions for materializing it. For example a stealing action for the resource in question would materialize an unfavorable interference, while an exchange of this resource would implement a favorable one for Y. These actions and, consequently, their associated interference can either succeed or fail.