drama manager overseeing semi-autonomous agents

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Interactive Storytelling Essey

DRAMA MANAGER OVERSEEING

SEMI-AUTONOMOUS AGENTS

Interactive Storytelling Essey, topics: drama manager, autonomous agents.

University of Turku

Department of Information Technology

Information and Computation Sciences

Interactive Storytelling

Tommi Laine 84367

28.10.2010


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Table of Contents

1 Introduction ........................................................... 1

2 Stories and characters ............................................ 12.1 Story ........................................................................................ 2

2.2 Non-player Characters .............................................................. 3

3 Restricting the drama management ....................... 4

3.1 Drama manager ....................................................................... 4

3.2 Character behaviour ................................................................. 6

3.3 Character actions ..................................................................... 73.4 Failing believably ..................................................................... 8

4 Conclusions ............................................................ 9

5 References ........................................................... 10


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1 Introduction

Storytelling is a major component in modern computer entertainment in one way

or another. Interactive storytelling enables the user to make decisions that direcly

affect the direction or outcome of the story. Approaches to creating a story that

develops according to the choices of a user can be divided roughly into two

categories: author-centric and character-centric.

In the author-centric approach the plot is woven by providing a drama manager

with a non-branching or a predefined set of braching storylines. The drama

manager is an intelligent, mostly all-knowing agent that manages the virtual world

in order to bring forth a narrative storyline which has been provided beforehand.

The drama manager bends the world according to its predefined will and thus

lowers the believability of the characters. A character might change his personal

goals according to the drama manager's instructions in a way which can be

perceived as schizophrenic behaviour. For example the drama manager might

forbid the player from assassinating an important character in a role-playing game

because that particular character is needed further in the plot.

In the character-centric approach, the characters are autonomous agents, and the

plot rises from interaction between the characters, thus improving believabilityand immersion. All characters act and interact with each other only according to

their personal goals and preferences, without involvement of any god-like level. But

unrestricted character-centric approach also means losing plot coherence, thus

making the content, quality or even the whole genre of the story uncertain.

In this paper I will discuss a way assembled from [1], [2] and [3] to merge the best

characteristics from all approaches by the use of a drama manager and semi-

autonomous agents, while trying to minimize the disadvantages of these

approaches. The goal is to maintain control over the plot while allowing thecharacters to act as autonomously as possible, thus increasing character

believability.

2 Stories and characters

Before character believability and controlling the plot can be discussed in more, the

definitions of story and character need to be determined.

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2.1 Story

All stories can be divided into three levels and I won't be using the terms in the

narrative theory because I find them misleading. I will use the terminology used in

[1].

The first level is the plotwhich contains events that occur in the world, that is, the

content of the story. Not any set of events is considered a plot. A plot must be

consistent so that the sequence of events is natural and in accordance with the

story world. The events should also have an intriguing structure so that the plot is

formed into the form of a conventional story, i.e. a beginning, a climax and an

ending.

The second level is a narrative that is an abstract representation of the plot from a

particular point of view, e.g. from the eyes of a playable character. Narration in a

chronological order of events is called linear narrative whereas the sequence of

events told in non-chronological order is nonlinear narrative.

The third level of story is the presentation. Presentation determines the medium

through which the narration is realized e.g. text, speech or animation.

Figure 1 contains example with a one thrilling story about a lumberjack, two bears,

revenge and misunderstanding. The example shows that the narrative andpresentation levels are similar, especially in character-centric action where

characters' actions make the plot, but the presentation can be done differently

from the narration like in the lowest cutscene element in the presentation pile.

The example also shows situations where the narrative does not have an element

of the first confrontation with the bear but presentation includes a immersive

sneakpeek to the past. This is done with a cutscene while another avatar tells the

lumberjack avatar what happened. In both narrative and presentation levels the

lumberjack has the same knowledge, but presentation level implements anelement of non-linear storytelling.

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2.2 Non-player Characters

Simulation of social interaction can be modeled through a collection of non-player

characters (NPCs) that appear intelligent, motivated and reactive [2]. Usually the

underlying goal storytelling systems try to achieve is not to have full artificial

intelligence reasoning agents, but agents that appear to be intelligent, motivated,

emotional and capable of learning.

This approach is called "broad but shallow" approach e.g. character might have a

goal to go and eat food but eating food would not be a requirement for the

character to stay alive. This approach will also be used in this paper. If left alone

without the direct control from the drama manager, characters should be able to

act lifelike without external instructions, pursuing their own goals to maintain high

level of character believability.

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Figure 1: Example of the story levels.

Lumberjack fighting

with the bear

Lumberjack knocked

unconscious

Lumberjack wakes

up with amnesia

Lumberjack decides

for revenge

Lumberjack fights

again with wrong bear

Lumberjack kills

the wrong bear

A bear sees

a lumberjack

Lumberjack is told

what has happened

Lumberjack wakes

up with amnesia

Lumberjack learns

what has happened

Lumberjack decides

for revenge

Lumberjack fights

with a bear

3D world

renders lumberjack

avatar waking up

Lumberjack avatar speaks

with another avatar

Lumberjack avatar speaks

speaks with himself

Lumberjack kills

the bear

Cutscene from the

eyes of the dying bear

Lumberjack avatar fights

with a bear avatar

Plot Narrative Presentation

Cutscene of the past events

told by another avatar


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But, in this method of overseeing, the drama manager is still needed to ensure that

a non-player character does not perform actions that might totally ruin the plot,

that will be discussed further in the essey.

3 Restricting the drama management

To achieve both a coherent plot and believable characters, a drama manager can be

controlling the behaviour of semi-autonomous agents at some degree, making

them follow the plot.

3.1 Drama manager

The drama manager can be set to three different level of detail [1]. Firstly you can

predefine all branches of actions the characters can choose from to the drama

manager. Secondly you can have the drama manager to ensure the occurence of

some essential events. Or thirdly you can tell the drama manager to make sure

small narrative fragments are regularly inserted.

As we are using a drama manager adopted from the author-centric approach, the

main rising concern is the belieavability of semi-autonomous agents that must be

dealt with.

There are total of three ways the drama manager should be allowed control the

plot in this method [1] summarized in figure 2. To begin with, the drama manager

can subtly introduce a new enviromental element like a new character or object

into the story world. Subtly meaning that items do not appear out of thin air. We

will call this enviromental control.

Secondly the drama manager can use motivational control, used in example of the

next paragraph i.e. enforce a new goal to a particular character.

And thirdly the drama manager can use proscriptive control, disallowing an action.

When a character wants to initiate an action, it will always ask the drama manager

is it a legal action. I should also be noted that the drama manager has no

prescriptive control, meaning it can not force a character to act instantly, just

change the underlying goals of the characters so the believability does not suffer

from the overseeing.

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To support proscriptive control of the drama manager, there should also be createdcausal links between the events [2]. The causal links are used to spot a sequence of

events made by the interacting player that may threaten the plot coherence. e.g. if

an assassin has a mission to kill another character by stabbing, the assassin needs

to have a bladed weapon. In this kind of situation, the causal links come into the

play. The inconsistency of the assassin not having a bladed weapon can be circled

around by generating new event where the bladed weapon is not needed.

For each point of inconsistency that will arise, firstly the drama manager will try to

repair the causal link in question by its three ways of control defined above e.g.giving the assassin a knife in a natural way.

If the repair by fullfilling the event requirements is not possible, the drama

manager attempts to remove any events that were dependent on the threatened

causal link and then repair the plot by filling in events the drama manager feels

required to restore the plot coherence e.g. assassination by strangling and this

change in the events is taken into account in all future events in the plot.

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Figure 2: Three ways a drama manager of this method can control the flow

of the events.

Character

Character

Character

Drama manager

'I am feeling

hungry...

'Can I eat this?' 'Yes.'

Enviromental

control

Motivational

control

User

User's point of view

Proscriptive

control


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Lastly, if all else fails, the drama manager selects new goals and rebuilds the plot

e.g. the assassination never happens and the outcome is different.

All in all, if the user performs an action that causes an inconsistency that threatens

the plot coherence, the drama manager looks up the appropriate branch in the tree

of events and seamlessly begins directing the semi-autonomous agents based on

the new plot. See figure 3 for example where the red arrows mark the plot

reconstructions that the drama manager does and black arrows are causal links. It

should also be noted that the inconsistency of casual links can only happen fromthe actions of the interacting user, because NPCs always ask the drama manager

before they act.

3.2 Character behaviour

I propose using the method used in [2] and [3] for the model of character

behaviour while introducing a level of priotiy to the NDBs. Character motivations

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Figure 3: Plot reconstructions and causal links.

has(bladed weapon)

can not obtain(bladed weapon)

Initial state

Stab(target)

does not have(bladed weapon)

Strangle(target)

Acquire(knife)

Intermediate stateOutcome(death by stabbing)

Outcome(death by strangling)

Intermediate state

Outcome(nobody dies)

knows(martial arts)

has(bladed weapon)

can not know

(martial arts)


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are divided into two kinds of behaviour: local autonomous behaviours and

narrative directive behaviours.

Local autonomous behaviours (LAB) are sequences of actions that make the

character seem humanlike. e.g. eating lunch, chatting with people, working. The

idea is same as of objects in object-oriented programming. The character itself only

knows the contents of his LABs and others don't have to know the implementation

of the actions. Even if the LAB contains interaction with another agent, the target of

interaction must only provide an applicable interface for the interaction. Character

will continue doing these, at some degree repetitive and generic, tasks to create

illusion of lifelikeness.

Narrative directive behaviours (NDB) are sequences of actions that advance the

plot. They are behaviours enforced to the characters by the drama manager and are

initiated when the change of underlying goals does not cause a stir in the

believability.

For example if a character is eating (a LAB), the playable character is present and

in the plot there is an event for this particular character to commit a assassination

(a NDB), the character would not suddenly stop eating and run to the murder

scene. The character would either finish eating or, if the priority of NDB is high,

provide an excuse or request an excuse from the drama manager to stop eating e.g.

make a personal remark: "Oh! I'm so full already!" or receive an incoming call

generated by the drama manager, allowing him an excuse to leave the scene.

On the other hand, if the priority of the NDB would be low, the behaviour would

only be initiated when the character is figuring out a new goal because the

previous goal was achieved.

Both LABs and NDBs need to have requirements i.e. resources they require.

Requirements can be location, object, emotional state, player not present and so

on. When the requirements are met and if the drama manager is giving a green

light according to postscriptive control, the action sequence of the behavior can be

initiated.

3.3 Character actions

Non-player characters have their own set of actions they can execute. Actions can

be for instance picking up an object, using an object to another object, eating an

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object i.e. destroying an object with animation and so on. When a character has

behaviour provided by motivational control of the drama manager (NDB) or a

behaviour in character's of repertoire of generic believability supporting

behaviours (LAB) to pursue some goal, the character wil try to accomplish the goalwith actions it can do.

For example there is a character that has a LAB with requirements of player being

present, the character not having eat in a while and the character having food like

banana. If these conditions are met the character will ask the drama manager if it is

legal to eat the banana. If the action is confirmed by proscriptive control of the

drama manager, character can eat the banana accompanied by a munching sound

and an animation.

3.4 Failing believably

This all is making sense on a paper but because we are discussing interactive

stories, the drama manager does not always know every possible thing yet what

the characters should be forbid to do. Proscriptive limits may become activated

only after some particular event in the plot. Next we will discuss how to handle

situations where a character has adopted a behaviour and initiated some actions

towards a now illegal goal.

I will be adopting a concept of failing believably from [3] to tackle this problem. In

this kind of situation, we have a character with a sequence of now illegal actions

but has not yet reached the goal e.g. an assassin character has located himself to

the murder scene but now the drama manager's current plot structure requires the

target of the assassin to stay alive a while longer. As the assassin ask the drama

manager if it is legal to jump from the shadows and strike the dagger to the heart of

the target, the drama manager gives the assassin a behaviour to fail the

assassination and flee the scene.

Of course there would not be any problem in the first case if the user experiencing

the story does not know anything about the assassination, but for the same of the

example lets determine that the user is a spectator in some way. Even if the user

knows about the assassin leaving for a mission to assasinate another character,

there is need to generate a backup story why the assassintion did not occur.

If this kind of method of automated failing would be implemented in an interactive

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storytelling system, it could have alternative uses too. For example the failing

believably can be used as a tool to boost the believability of a particular character

as humans make errors. Althrough even if does not differ from a prescripted

sequence of actions much, a template for the believable failing could turn outfruitful.

4 Conclusions

This paper describes a way to allow a drama manager to control agents of an

interactive story so that the character believability stays still reasonable. The

method is assembled by merging the works [1], [2] and [3] with some own twists.

The agents won't be fully autonomous and the drama manager won't have absolutecontrol of the events in this approach but there is alot more flexibility than the

extremes of author- and character-centric approaches. The agents can not be fully

autonomous if there is a desire to have some control over the plot and the drama

manager can not have absolute control if you want to maintain majority of the

character's believability. If the characters are allowed to do anything they desire,

they might break links in between the events of the predefined plot so we allow

some control tools for the drama manager.

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5 References

[1] Theune, M.; Faas, S.; Nijholt, A.; Heylen, D.; (2003)

The Virtual Storyteller: Story Creation by Intelligent Agents.

Proc. of the 1stInt. Conf. for Interactive Digital Storytelling and Entertainment.

[2] Riedl, M.O.; Stern, A.; (2006)

Believable Agents and Intelligent Story Adaptation for Interactive

Storytelling,

Proc. of the 3rdInt. Conf. For on Technologies for Interactive Digital

Storytelling and Entertainment

[3] Riedl, M.O.; Stern A.; (2006)

Failing Believably: Toward Drama Management with Autonomous Actors in

Interactive Narratives

Proc. of the 3rdInt. Conf. For on Technologies for Interactive Digital

Storytelling and Entertainment

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