Story-Line and Drama in Video Games

1. Story-Line and Drama in Video Games By Greg Brander

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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”

8. 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

9. 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”

10. Openness • Openness • Appreciation for art, emotion and adventure • Unusual ideas • Imagination • Curiosity

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

12. Extraversion • Extraversion • Energy • Positive emotions • Tendency to seek stimulation and the company of others

13. Neuroticism • Neuroticism • A tendency to experience unpleasant emotions easily • Anger • Anxiety • Depression • Vulnerability • General emotional instability

14. Agreeableness • Agreeableness • Traits include: • Compassionate • Cooperative • Not Suspicious • Not Antagonistic

15. 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

16. 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

17. 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

18. 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

19. 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

20. Example HTN

21. 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

22. 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…

23. 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

24. 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.'

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

26. 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

27. 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

28. 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

29. 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

30. Interactive Drama Architecture (IDA) • Five different elements

31. 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

32. 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

33. 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

34. The Director • Central AI agent • Roles: • Knowledge maintenance • Plot monitoring • Story direction • Reactive direction • Predictive direction

35. 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

36. 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

37. 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

38. 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

39. 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

40. 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

41. 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