story-line and drama in video games by greg brander
TRANSCRIPT
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