Mark O. Riedl

Narrative planning: balancing plot and character

Narrative, and in particular storytelling, is an important part of the human experience. Consequently, computational systems that can reason about narrative can be more effective communicators, entertainers, educators, and trainers. One of the central challenges in computational narrative reasoning is narrative generation, the automated creation of meaningful event sequences. There are many factors - logical and aesthetic - that contribute to the success of a narrative artifact. Central to this success is its understandability. We argue that the following two attributes of narratives are universal: (a) the logical causal progression of plot, and (b) character believability. Character believability is the perception by the audience that the actions performed by characters do not negatively impact the audiences suspension of disbelief. Specifically, characters must be perceived by the audience to be intentional agents. In this article, we explore the use of refinement search as a technique for solving the narrative generation problem - to find a sound and believable sequence of character actions that transforms an initial world state into a world state in which goal propositions hold. We describe a novel refinement search planning algorithm - the Intent-based Partial Order Causal Link (IPOCL) planner - that, in addition to creating causally sound plot progression, reasons about character intentionality by identifying possible character goals that explain their actions and creating plan structures that explain why those characters commit to their goals. We present the results of an empirical evaluation that demonstrates that narrative plans generated by the IPOCL algorithm support audience comprehension of character intentions better than plans generated by conventional partial-order planners.

Believable agents and intelligent story adaptation for interactive storytelling

Interactive Narrative is an approach to interactive entertainment that enables the player to make decisions that directly affect the direction and/or outcome of the narrative experience being delivered by the computer system. Interactive narrative requires two seemingly conflicting requirements: coherent narrative and user agency. We present an interactive narrative system that uses a combination of narrative control and autonomous believable character agents to augment a story world simulation in which the user has a high degree of agency with narrative plot control. A drama manager called the Automated Story Director gives plot-based guidance to believable agents. The believable agents are endowed with the autonomy necessary to carry out directives in the most believable fashion possible. Agents also handle interaction with the user. When the user performs actions that change the world in such a way that the Automated Story Director can no longer drive the intended narrative forward, it is able to adapt the plot to incorporate the users changes and still achieve dramatic goals.

Towards an architecture for intelligent control of narrative in interactive virtual worlds

The creation of novel, engaging and dynamic interactive stories presents a unique challenge to the designers of systems for interactive entertainment, education and training. Unlike conventional narrative media, an interactive narrative-based system may be required to generate its own story structure, determine the appropriate interface elements to use to convey the storys action and manage the effective interaction of a user within the story as it plays out. Here we describe the architecture of the Mimesis system, which integrates a 3D graphical gaming environment with intelligent techniques for generating and controlling interaction with and within a narrative in order to create an engaging and coherent user experience

A perception/action substrate for cognitive modeling in HCI

This article describes a general-purpose programmable substrate designed to allow cognitive modeling systems to interact with off-the-shelf interactive applications. The substrate, called VisMap, improves on conventional approaches, in which a cognitive model interacts with a hand-constructed abstraction, an artificial simulation or an interface tailored specifically to a modeling system. VisMap can be used to construct static scenarios for input to a cognitive model, without requiring its internal modification; alternatively, the system can be integrated with a cognitive model to support direct control of an application.

Toward supporting stories with procedurally generated game worlds

Computer role playing games engage players through interleaved story and open-ended game play. We present an approach to procedurally generating, rendering, and making playable novel games based on a priori unknown story structures. These stories may be authored by humans or by computational story generation systems. Our approach couples player, designer, and algorithm to generate a novel game using preferences for game play style, general design aesthetics, and a novel story structure. Our approach is implemented in Game Forge, a system that uses search-based optimization to find and render a novel game world configuration that supports a sequence of plot points plus play style preferences. Additionally, Game Forge supports execution of the game through reactive control of game world logic and non-player character behavior.

A lightweight intelligent virtual cinematography system for machinima production

Machinima is a low-cost alternative to full production filmmaking. However, creating quality cinematic visualizations with existing machinima techniques still requires a high degree of talent and effort. We introduce a lightweight artificial intelligence system, Cambot, that can be used to assist in machinima production. Cambot takes a script as input and produces a cinematic visualization. Unlike other virtual cinematography systems, Cambot favors an offline algorithm coupled with an extensible library of specific modular and reusable facets of cinematic knowledge. One of the advantages of this approach to virtual cinematography is a tight coordination between the positions and movements of the camera and the actors.

Story planning as exploratory creativity: Techniques for expanding the narrative search space

The authoring of fictional stories is considered a creative process. The purpose of most story authoring is not to invent a new style or genre of story that will be accepted by the population but to invent a single narrative that is novel enough to be tellable. Computational story generation systems are more limited than human authors in the space of narratives that can be considered because it is often the case that story generation systems are constrained to operate within a fixed representation of the story world. These limitations can impact whether a story generation system is considered creative or not. In this paper, we describe a story planning system, Fabulist. Fabulist however is constrained by the world model input by the system user. We present two algorithms that enable story planning systems such as Fabulist to break outside the bounds of the initial world model in order to search a larger space of narratives.

Automated scenario generation: toward tailored and optimized military training in virtual environments

Scenario-based training exemplifies the learning-by-doing approach to human performance improvement. In this paper, we enumerate the advantages of incorporating automated scenario generation technologies into the traditional scenario development pipeline. An automated scenario generator is a system that creates training scenarios from scratch, augmenting human authoring to rapidly develop new scenarios, providing a richer diversity of tailored training opportunities, and delivering training scenarios on demand. We introduce a combinatorial optimization approach to scenario generation to deliver the requisite diversity and quality of scenarios while tailoring the scenarios to a particular learners needs and abilities. We propose a set of evaluation metrics appropriate to scenario generation technologies and present preliminary evidence for the suitability of our approach compared to other scenario generation approaches.

An objective character believability evaluation procedure for multi-agent story generation systems

The ability to generate narrative is of importance to computer systems that wish to use story effectively for entertainment, training, or education. One of the focuses of intelligent virtual agent research in general and story generation research in particular is how to make agents/characters more lifelike and compelling. However, one question that invariably comes up is: Is the generated story good? An easier question to tackle is whether a reader/viewer of a generated story perceives certain essential attributes such as causal coherence and character believability. Character believability is the perception that story world characters are acting according to their own beliefs, desires, and intentions. We present a novel procedure for objectively evaluating stories generated for multiple agents/characters with regard to character intentionality - an important aspect of character believability. The process transforms generated stories into a standardized model of story comprehension and then indirectly compares that representation to reader/viewer mental perceptions about the story. The procedure is illustrated by evaluating a narrative planning system, Fabulist.

Character-focused narrative generation for execution in virtual worlds

Because narrative plays such a key role in the understanding of events in our daily lives, the ability to generate narrative can be of great use in virtual reality systems whose purpose is to entertain, train, or educate their users. Narrative generation, however, is complicated by the conflicting goals of plot coherence – the appearance that events in the narrative lead towards the narrative’s outcome – and character believability – the appearance that events in the narrative are driven by the traits of the story world characters. Many systems are capable of achieving either one or the other; we present a new approach to narrative generation in the Actor Conference system, which is capable of generating narratives with both plot coherence and character believability. These narratives are declarative in nature, readily lending themselves to execution by embodied agents in virtual reality environments.