Improving Narrative Consistency in Planning-Based Interactive Storytelling

Interactive Storytelling (IS) systems are an emerging class of interactive entertainment applications with emphasis in narrative aspects. This paper deals with stories that have plots centered on a riddle, and describes a mechanism that can be introduced to planning-based IS in order to make the generated stories follow a specified tension arc. Results from experiments with users suggest that the proposed method is capable to bring improvements to these systems. Introduction and Related Work Games for computers and consoles are established as the leading form of interactive digital entertainment. Technologically, the development of the field of computer games is impressive. A more conceptual analysis, though, shows that most games developed in the past decades have explored fundamentally the same concepts: hand-eye coordination, puzzle-solving and resource management (Crawford 2004). Countering this reality, several researchers have been exploring aspects normally treated as marginal by the majority of today’s games, like the dramatic and narrative aspects. The field that aims to create interactive applications capable to generate consistent narratives is called Interactive Storytelling (IS). Researchers on this field are exploring different approaches in order to achieve the goal of seamless conciliation of interactivity and narrative consistency. One of these approaches is based on the use of planning algorithms to generate the sequence of events that compose the story. Planning-based IS has characteristics that help to satisfy the interactivity requirements of IS systems, but has some problems concerning the narrative consistency. In this paper, we propose a mechanism that intends to improve these systems by making the generated stories follow a tension arc defined by the storyworld author. This mechanism is focused on stories that follow the Riddle master plot (Tobias 1993). Planning has been used to develop narrative-related systems since the seminal works on non-interactive story generation, like UNIVERSE (Lebowitz 1985). In this work, Lebowitz automatically generates plot outlines that resemble soap operas through the use of planning techniques and libraries of plot fragments and characters. Copyright c © 2007, Association for the Advancement of Artificial Intelligence (www.aaai.org). All rights reserved. The present work is based on the use of STRIPS-like planning systems, an approach introduced by Charles et al. (2003). In a previous work (Barros & Musse 2005), we built on these ideas, focusing on ways to make the generated stories more narratively consistent. The design of the interactive drama Façade, by Mateas and Stern (2003), shows a lot of care in aspects related to generating highly consistent narratives. In particular, they try to ensure that the generated stories follow a certain tension arc. Our paper also presents a mechanism that aims to make the generated stories follow an author-defined tension arc, but our work (which is in the context of planning-based IS and centered on riddles) is considerably different. Architecture Overview Before delving into the method we propose for respecting tension arcs, we present a brief overview of our IS architecture (details can be found in (Barros & Musse 2005)). We see the story as a sequence of actions performed by characters that can transform the world in a way imagined by the storyworld author. The player controls the story protagonist. Actions performed by the player may render the current plan invalid. Whenever this happens, the planning algorithm is used to create a new plan, thus adapting the story to the player’s actions. Our current implementation uses the Metric FF planning system (Hoffmann 2003). World state, represented as a set of predicates, is changed whenever characters perform actions. This includes both actions performed by NPCs (Non-Player Characters) and by the player-controlled story protagonist. Our prototype displays the story to the player with animated characters in a 3D environment (Figure 2). The player executes actions with the mouse: clicking in an object (or character) brings a list of actions related to it. Riddles and Tension Arcs The level of tension in a story in function of time results in a curve called tension arc. The goal of our work is to introduce a mechanism that makes the generated stories follow an author-defined tension arc. We are dealing with stories that follow the Riddle master plot (Tobias 1993), i.e., stories in which there is a mystery to be solved, like “whodunits”. Unfortunately, Tobias is very brief when discussing tension