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

http://www.youtube.com/watch?v=seHnJ-6BxSY At the very least NPC reactions can seem

completely unrealistic Ex: Oblivious AI in Oblivion

http://www.youtube.com/watch?v=6KN7cKO8-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

Openness Appreciation for art, emotion and

adventure Unusual ideas Imagination Curiosity

Conscientiousness

Conscientiousness Tendency to show self-discipline, act

dutifully and aim for achievement Planned rather than spontaneous

behavior

Extraversion

Extraversion Energy Positive emotions Tendency to seek stimulation and the

company of others

Neuroticism

Neuroticism A tendency to experience unpleasant

emotions easily Anger Anxiety Depression Vulnerability

General emotional instability

Agreeableness

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

http://youtube.com/watch?v=GmuLV9eMTkg

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