Story-Line and Drama in Video Games By Greg Brander

Outline of the Presentation Problems in video game drama Ideal Models Techniques for creating realistic NPC’s HTN Planning for game story lines Several examples of interactive drama in video games

Problems with Plot Development in Games Mostly linear story progressions Non-Reactive, scripted sequences Boundaries on player actions Lack of re-playability Unbelievable NPC interactions

Problems with Scripting and Non-Responsive NPCs If a game fails to pre-empt a players actions the player may be able to “break” the game  Ex: Dark Messiah Scripted Sequence Glitch Bug At the very least NPC reactions can seem completely unrealistic  Ex: Oblivious AI in Oblivion P0 P0 We will discuss solutions to these problems later in the presentation

Ideal Model D&D  Dungeon Master starts with basic story outline in mind  Players can choose to delve into this story or go in completely different directions  Dungeon Master must be able to dynamically change the story on the fly and push the characters in the right direction

Another Ideal Model The Holodeck in Star Trek Very High level  “Computer, create a 1930’s, film-noir mystery story” Completely immersive  Simulates all five senses  Reacts to user actions within the confines of the described story, settings

Aspects of a Good Video Game Story Realistic NPC interactions  Non-scripted conversations Player actions reverberating in the game world Dynamically changing story line Lack of obvious “boundary conditions”

Conversational Agents Model of personality and emotion that affects conversation Adds to the sense of game world with intelligent characters Helps avoid repetitive or unrealistic dialogue Main functions of an NPC are to relay information and assign tasks to the player  Conversational agents make these exchanges more intriguing by adding an emotional element

Creating a Conversational Agent Personality and emotion  Five Factor Model (FFM) Openness to experience, Conscientiousness, Extraversion, Agreeableness, Neuroticism  Each factor is assigned a weight and is combined to form a personality model  Set emotional thresholds based on personality “traits”

Openness  Appreciation for art, emotion and adventure  Unusual ideas  Imagination  Curiosity

Conscientiousness  Tendency to show self-discipline, act dutifully and aim for achievement  Planned rather than spontaneous behavior

Extraversion  Energy  Positive emotions  Tendency to seek stimulation and the company of others

Neuroticism  A tendency to experience unpleasant emotions easily Anger Anxiety Depression Vulnerability  General emotional instability

Agreeableness  Traits include: Compassionate Cooperative Not Suspicious Not Antagonistic

Representing Emotion Emotion can easily displayed in the conversation choices an NPC chooses to engage in with a character Many emotions can be represented by just a few base ones using fuzzy logic  Low Happiness = Content  Medium Happiness = Happy  High Happiness = Estatic Give predetermined events that could occur in the game world a weighted significance if they occur Reduce the emotional significance of events over time

General Example Of FFM A character has a low score in agreeableness This dictates a low anger threshold value One or two in game events the character finds disagreeable would trigger the emotion of anger Dialogue options and reactions to the player would change to register this emotion

NPC Input Create sensory honesty  NPC are aware of events in their immediate area E.g. Characters should run or be frightened when they witness others getting killed at their table in Oblivion  News of events spreads to NPC’s over a given time period and distance  Computationally expensive to do for NPC in the game Only model NPC’s in players immediate area

NPC Output Conversation choices defined by personality, emotional state, and knowledge base and purpose of the particular NPC Knowledge base  List of everything the character knows about the world  Can be time consuming and difficult to design  Better to use a subset of a main knowledge base for each in game character Model surprises into NPC conversations NPC knowledge is filtered through its personality and emotion to determine its reaction/willingness to help the player

HTN Planning for Interactive Storytelling HTN – each task is decomposed into subtasks until they can be described in terms of primitive actions Top level node – Story Goal First Layer nodes – Scenes Lower Layer nodes – tactics for achieving scene goals

Example HTN

Creating the Story NPC reactions derived by searching through their plan based on game state If an NPC cannot execute an action then it uses a top-down left-to-right search with backtracking on tree Assumes player actions will follow the basic outline Rival characters may follow a plan that tries to disrupt the players plan based on the current game state Drama or humor can be derived when a plan fails  Ex: Friends Ross gets dressed to go on a date with Rachael, but gets in a fight with Joey and must backtrack to perform the task of getting redressed

Problems with HTN Planning Not much variability in actions per scene Actions cannot be undone Long distance dependencies not modeled into design Now a better solution…

Intelligent Story Direction Interactive Drama  Offers player a large amount dramatically relevant choices in the game world  Player directly influences how the story unfolds Story Director  Coordinates the choices the player makes in the game world, the actions and behaviors of the synthetic characters and the story content authored by a game designer

Story Coordination Game designer or author must create a story space which can react to any of the actions a player can take in a game When a player takes an action such as shooting a main character vital to the plot later on this action is outside the story space and creates a boundary problem  Ex: The Elder Scrolls: Morrowind  'With this character's death, the thread of prophecy is severed. Restore a saved game to restore the weave of fate or persist in the doomed world you have created.'

Story Director AI coordinator Input:  Player interaction with game world  Authored story content Output:  Change behavior of synthetic characters  Modify the game world

MOE First major system to implement a story director Story content written as partially ordered set of USER-MOVES, or actions a player can take to move the story along MOE executes actions in the game world that attempt to guide the player towards the next “scene” in the story progression  Heuristically searches through space of possible stories (e.g. the story events in the past plus the combination of possible future events)  Stories are rated based on a author defined heuristic and the next scene is chosen from the highest rated story line

Façade Uses a later version of the MOE technology to create an interactive drama called Façade Makes use drama manager that dynamically sequences units of story (dramatic beats) in response to the history of the player's interaction Also uses several text recognition models to react to player comments and maintain conversation threads

MINMESIS Project Observes player behavior Used to avoid boundary problems Story contained in partially ordered plans  Player actions can: Fulfill a plan’s precondition Have no bearing on the plan Threaten a plan’s precondition Results in boundary problems that are solved with accommodation and intervention in the MINMESIS implementation

Accommodation and Intervention Accommodation  Replanning algorithm attempts to change the story plan to resolve the threat introduced by the player  Player’s action is incorporated into the set of story operators and story goals used to build the plan  Only works if a consistent plan can be built Intervention  Results when a consistent plan cannot be built as a result of the player’s action  Prevents effects of a player’s actions from occurring E.G. A gun jamming, instead of being used to kill a main character

Interactive Drama Architecture (IDA) Five different elements

Author Responsible for synthetic character behaviors, a description of story events, any domain-dependent functions of the director, and the environment and art assets Story Content  Description of the events that occur in the world and the details of those events Story Structure  Chronological order of story content

Plot Points Story is represented by partially ordered plot points, with any linearization of these points possibly making a complete story Each point is considered active if its parents, or the node behind it, has executed

Synthetic Characters Semiautonomous agents  Defined by long-term (a hierarchical set of agent goals) and short-term knowledge  Working Memory Elements Basic knowledge of environment Know how to speak, navigate and use objects

The Director Central AI agent Roles:  Knowledge maintenance  Plot monitoring  Story direction  Reactive direction  Predictive direction

Knowledge Maintenance Omniscient view of the world Observes how player actions affect the story world Domain specific rules such as social conventions or how relationships change

Plot Monitoring Checks available knowledge against plot representation Checks if changes in the story world contribute to or harm the story In IDA threats to plot point preconditions are ignored unless a player is in danger of violating a timing constraint

Story Direction When a plot point is active and all of its precondition have been met the story director may choose to execute it Occurs by giving commands to synthetic character or the environment

Reactive Direction Directors responsibility to guide the player to specific plot points Used to react to the previous mentioned violation of a timing constraint Subtlety valued

Predictive Direction IDA’s response to using reactive direction as the sole means of solving boundary problems Attempts to foresee future boundary problems based on observing past player actions Uses less heavy handed techniques to guide the player

Affecting Story Content Story content is primarily supposed to consist of player chosen plot points, signified by the preconditions of an active plot point being met If a timing condition is in danger of being violated the director picks a plot point based on an author defined heuristic that takes into account the attributes of the current plot point such as tension a brevity

Summary There are many techniques being developed for creating believable interactive story-lines in video games Games have yet to completely capitalize on these innovations