Resolving the conflict between player choice and narrative in video games

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Resolving the conflict between player choice and narrative in video games

SUBMITTED IN PARTIAL FULLFILLMENT FOR THE DEGREE OF MASTER OF SCIENCE

C

OLIN

R

ULLKOTTER

11119004

M

ASTER

I

NFORMATION

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TUDIES

G

AME

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TUDIES

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ACULTY OF

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CIENCE

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NIVERSITY OF

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MSTERDAM

August 16, 2016

1st_Supervisor ₂nd_Supervisor

Dr. Frank Nack Dr. Tibor Bosse

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Resolving the conflict between player choice and

narrative in video games

Colin Rullkotter

Universiteit van Amsterdam

11119004 colinrullko@gmail.com

Abstract

A perennial impediment to the capacity of video games to tell complex, involving stories to compete with literature and film is that the writer cannot anticipate the actions of the player. A common means of dealing with this issue is to prevent the player from carrying out certain actions, but this undermines the potential of games as a ludic medium. Conversely, player freedom can be maintained by simplifying the story, but that has similar implications for the medium's narrative effectiveness. A possible way of squaring this circle is for the game world to be reshaped in response to player behaviour which threatens to thwart the writer's intentions, such that the planned storyline can continue regardless. Using an AI which serves this purpose, and an interactive scenario which demonstrates its capabilities, this study compares the conventional solution of limiting player freedom with two methods of manipulating the world to accommodate the player's actions, and measures user response to each. The findings suggest that such approaches can improve the player's engagement with both a game's narrative and its mechanics without significantly increasing his or her frustration.

Categories and subject descriptors

E.1 [Data Structures] Lists, stacks and queues; Trees I.2.1 [Applications and Expert Systems] Games

I.2.4 [Knowledge Representation Formalisms and Methods] Representations (procedural and rule-based)

I.2.8 [Problem Solving, Control Methods, and Search] Graph and tree search strategies; Plan execution, formation, and generation

General terms

Algorithms, design, experimentation

Keywords

Video games, interactive stories, narrative, autonomy, Fate Manager, AI, GOAP, virtual storyteller, director agent, immersion, flow, frustration

1 Background

Video games are the one medium in which truly interactive storytelling is possible.1_{Although other media can incorporate}

some degree of choice – as in the once-popular “choose your own adventure” genre of books2_{– this is very limited, and by}

necessity only allows the user to select from a series of existing storyline branches. The procedural nature of digital games, in theory, allows an infinite number of variations on a single story to be created via the interaction of characters with the player and each other.

However, with greater interactivity comes greater uncertainty over the direction of the plot: every opportunity granted to the player to effect a change to the story comes with an exponential increase in the complexity of the task facing the

writer. As a result, there tends to be an inverse relationship between the degree of choice afforded the player and the game’s capacity to tell a well-structured, compelling story. Among others, this dichotomy has been noted by (and underpinned the work of) Michael Mateas and Andrew Stern, creators of the groundbreaking “interactive drama” Façade:3

The ephemeral quality of gameplay, the experience of manipulating elements within a responsive, rule-driven world, is still the raison d’être of games, perhaps the primary phenomenological feature that uniquely identifies the computer game as a medium. Where gameplay is all about interactivity, narrative is all about predestination. There is a pervasive feeling in the game design community that narrative and interactivity are antithetical.4

Or as Steve Gaynor, developer of 2013’s Gone Home, puts it, from the writer's perspective the player is an “agent of chaos”.5

It is difficult to construct a complex, coherent and meaningful storyline when such a wild card exists. It has been argued6_that

this inherent interactivity will forever prevent games from qualifying as an art form: the writer’s voice is muffled if control over the direction of his or her work is surrendered to the consumer of that work. If it were possible for Romeo to decide not to commit suicide, and to live happily ever after with Juliet, would it not completely undermine what Shakespeare intends to say about the world?

This study will examine a number of approaches that have been taken in the past to allowing a writer’s vision to coexist alongside player intervention. Subsequently, another method will be proposed, involving the use of a high-level AI to adapt a game world dynamically in response to player actions which conflict with an intended story. Finally, the reactions of users to an implementation of that concept will be compared with their attitudes to more conventional approaches. This will indicate whether this new, procedural approach has the potential to ease the choice/narrative tension more successfully than has been achieved to date.

1.1 Story, plot and narrative

Before proceeding, it will be helpful briefly to clarify the senses in which this paper uses the terms “story”, “plot” and “narrative”, and the distinction between these. Although they are sometimes used interchangeably and a close relationship exists between them, they are not quite synonymous.

The story, or fabula in the words of folklorist Vladimir Propp7_,

is simply a sequence of events, or the progression of the world from an initial state through subsequent states.2_{It is the raw}

stuff which is used to construct plot. A plot, called mythos by Aristotle and sjuzhet by Propp, is the structure imposed on the story by its teller, and the relationship between the events within it. The essential elements of a plot constitute a separate field of study of its own (narratology), but there are two prevailing models: the basic three-act structure described by

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Aristotle (beginning, middle and end),8_{and the more modern}

five-act model which amplifies its predecessor, and whose components are most commonly referred to in terms coined by Gustav Freytag: exposition, rising action, climax, falling action, and dénouement. 9

Narrative, as used in this paper, is an adjectival term referring

to the dimension of (in this case) a game which is concerned with stories and plots: the static, creator-directed aspect of the medium which, as outlined above, has traditionally been diametric to the dynamic, player-directed ludic dimension. The extent to which these dimensions are fundamentally incompatible is the main concern of this study.

1.2 Related work

This section will examine some current and past approaches to addressing the choice/narrative paradox using procedural (i.e. algorithmic) methods, as well as existing analysis of why freedom and narrative are actually desirable in games.

1.2.1 Procedural techniques in game narrative

Research into the use of procedural methods to generate or support narrative in games is barely younger than the medium itself,10_{though the extensive use of such techniques outside of}

academia remains scarce.11 12 _{In 1983's Murder on the}

Zinderneuf, developers Free Fall Associates built an adventure

game based around a murder mystery whose victims and perpetrators are randomly determined upon start-up, resulting definin high replay value.13 _{This interchangeability of}

characters, however, lends the storyline a rather mechanistic feel, demonstrating the weakness of computer-generated story. Over the last fifteen years or so, a trend has been for game designers to provide a premade world for the player to explore, while offering the player relative freedom within that world.14

These are so-called open world games, major contemporary examples being Grand Theft Auto V and Skyrim.15 16_While

manually authored stories are usually also incorporated into games of this type, players are at liberty to ignore this “main quest”, and instead produce whatever emergent stories they wish within the world provided. As with entirely procedurally generated ones, these player-created stories tend to be quite basic: in Skyrim, the player may steal something from a shop, be driven from the town by guards, commandeer a ship in order to escape, and later become the target of bounty hunters.17_{This sequence of events qualifies as a story (and}

possibly even as a plot) per the definition in Section 1.1,2_{but it}

lacks the potential for thematic resonance and character development which is possible in pre-scripted stories. It is simply a character wandering around doing things, without undergoing any kind of meaningful internal change.

The standard approach to accommodating player freedom within a pre-written story is to restrict that freedom.18

Characters who are scheduled to play a future role in the storyline in The Elder Scrolls simply cannot be killed, no matter how hard the player tries (Fig 1). In addition to inducing a jarring effect in the player, these games involve more work for the developers, requiring asset creators or level designers manually to flag such characters, objects or actions.19

A procedural approach would initially shift that burden to the programmer; but unlike in the manual approach, the workload required is independent of the story's scale and complexity. The notion of an AI-controlled manager whose job is to reconcile events in the world with a pre-written script has existed, at least on a theoretical level, for more than a decade,

and some implementations have been produced. In these, the manager is referred to variously as a “virtual storyteller”,20_a

“director agent”,21_{a “drama manager”}3_{and others, but all}

describe a similar fundamental concept: an AI which, instead of controlling an individual NPC, manages the world itself (which may include the management of individual NPC-controlling agents within that world, making this a type of supra-agent).

Fig 1. A story-critical NPC in TES: Oblivion, indicated by a

crown icon. Attempts to kill such a character will be blocked.

As part of the process of developing a system for the training of medical students in simulated emergency situations, Marieke Peeters22 _{examines a number of prior approaches to}

implementing a high-level world manager.

One such supra-agent is the “virtual storyteller” developed at the University of Twente.20_{This story manager has the ability}

to intervene in the world in three ways: environmentally, by introducing new objects or characters; proscriptively, by forbidding an agent from performing its intended action; and

motivationally, by assigning goals to agents. As will be

discussed in Section 3.2, this study will employ an approach using techniques closely related to the first two of these. Peeters also considers the enduring example of Mateas and Stern's Façade,3_{whose narrative management system operates}

primarily by allowing and disallowing subsets of actions in a character's repertoire, depending on the current “context” (roughly equivalent to world state). This is a variant of the proscriptive approach used in the “virtual storyteller”,20_but

operates at a much lower level. The narrative controller in

Façade (referred to as a “drama manager”) maintains a

particularly tight grip on agent behaviour, intervening in practically every action by an NPC, even at the level of sentence construction. The drama manager focuses less on enforcing a predetermined storyline than on ensuring that the behaviour and speech of NPCs seems naturalistic, and that story fragments (or “beats”) are sequenced in such a way as to produce engaging arcs, in compliance with Aristotelian dramatic theory.23_{Paradoxically, this rigid control actually}

allows for a more fluid high-level story, which is procedurally generated as the user progresses through the scenario.3_{This is}

not quite comparable with this study’s interest, which is in ensuring that the story attains mandatory world states, specified offline by the writer. In addition, because there are no immutable story nodes, the drama manager in Façade requires no influence over the physical layout of the world itself: its remit is entirely concerned with the behaviour of NPCs. By contrast, a director agent which is forced to adhere strictly to a scripted story will often have no alternative but to add, remove or alter the properties of inanimate objects in the world.

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Because of the tremendous depth of Façade, its breadth is necessarily limited – the entire experience lasts around fifteen minutes, and is confined to a single location. This lack of scalability is largely a function of the tiny size and huge number of “beats” from which it constructs its story.24

Developing a feature-length game whose NPC behaviour is micromanaged to the same degree would entail an exponential increase in required man hours, expense and storage space (despite its brevity, Façade's audio alone, after compression, consumes around a gigabyte). If resource limitation were no obstacle, the ultimate narrative management system would combine its drama manager with a higher-level director agent: the former making immediate interactions with NPCs as believable as possible, while the latter ensures that major story points occur as intended.

1.2.2 User experience of choice and narrative

Central to the importance of player autonomy and storytelling power – what it is which makes these things worth preserving and maximising – is the concept of immersion: the sensation of not merely controlling an avatar in a virtual world, but becoming a part of that world; the feeling that the interface between player and game has dissolved.25

While the concept of immersion is commonly intuitively understood, it is non-trivial to deconstruct and explain precisely, and several definitions have been offered.26_A

number of factors have a bearing on how absorbed a player feels by a game – some inherent to the game itself, some environmental, and some related to the player's own disposition – but there is a consensus that a critical ingredient is autonomy, or the level of control that the player perceives him or herself to be exercising over in-game events 3527 28_{. In}

theory, if user immersion is partly a function of the level of that user's active participation in a medium, and if games are capable of replicating the narrative apparatus of comparable media such as film, then games have the potential to be more immersive than those non-interactive forms of storytelling. If that has not been achieved yet, then the blame likely lies with stories in games having to be adulterated in order to accommodate player autonomy.

Immersion is a fairly expansive species, however, and its individual genera can be identified by the aspect of a game from which that immersion arises. The literature on the psychological effects of games provides multiple names for these distinct, though related, phenomena. Marie-Laure Ryan distinguishes between narrative immersion and ludic immersion:29_{the level of preoccupation of a player in a game}

induced respectively by its storytelling and by its mechanics (including the degree of autonomy these mechanics afford). It should be noted that, while narrative immersion may be experienced when consuming any storytelling media, ludic immersion is unique to games. Lennart Nacke et al. refer to the former as imaginative immersion, and the latter as

challenge-based immersion.30 _{Names assigned to the same general}

concepts by other researchers include presence and flow. 30 31

One of the most widely used tools for assessing user experience is the Game Experience Questionnaire, a self-report tool developed at Eindhoven University to measure the psychological effects of video games. 32 _{Given its long track}

record in academia, 33_{this study will use it as the basis for user}

feedback, and consequently, will borrow its terminology: narrative immersion will be referred to simply as immersion, and ludic immersion as flow.

1.3 Proposal

This paper proposes a new approach to breaking (or at least loosening) the conflict between player choice and narrative by employing the concept of fate – the notion that certain events are destined to occur, regardless of the actions of free-willed agents. Although the concept properly belongs to the field of ontology, it is relevant to the problem under discussion in that it posits a means by which predetermination and human freedom can coexist.34

The player (to whom, for the sake of uncluttered prose and because most gamers are male, this paper will henceforth refer using masculine pronouns) will have a high degree of freedom to act as he wishes in the game world, but if his actions threaten to derail the planned storyline – say, by destroying an important object – an overarching AI (a Fate Manager, or FM) will dynamically adapt the low-level details of the story so that the high-level story points remain intact.

Principally, the paper will seek to answer the following:

Can the concept of fate be applied to games to improve player choice while preserving a scripted narrative?

2 Methodology

The above question was addressed in three stages: an interactive scenario was developed which demonstrates the principles of the Fate Manager; a group of users were invited to play through various versions of the scenario, both with the FM enabled and without, and to complete a quantitative survey comparing the effectiveness of each; and finally, one-way ANOVA was performed on the results to determine whether the FM-enabled versions outperform the others.

If the FM works as intended, users should report an increase in autonomy (flow), and no change or an increase in engagement in the narrative (immersion), when playing through the FM-enabled scenario. The means employed by the FM to achieve this may have the side effect of frustrating the user, and so this will be tested for as well. If the FM is successful, the results will reject H0 while accepting H1, as follows:

H0: µf1 = µf2, µi1 = µi2, µr1 = µr2

against

H1: µf1 > µf2, µi1 ≥ µi2, µr1 ≤ µr2

where µf1, µi1 and µr1 are, respectively, the mean levels of

flow, immersion and frustration evoked by a scenario using the Fate Manager, and µf2, µi2 and µr2 are the corresponding levels

evoked by a scenario which does not use it.

2.1 The demo scenario

To allow users to evaluate the success of the Fate Manager, an application was developed to apply its principles in a very simple interactive scenario. The application and other required files can be downloaded from tinyurl.com/fate-manager-test, while the complete C++ source code is at tinyurl.com/fate-manager-test-src. Building the executable requires two third-party libraries: Allegro (liballeg.org/download.html), which is used for handling user input, text display and timing, and SOIL (lonesock.net/soil.html), for importing image files from disk. The scenario depicts a small segment of a plot, which in narratological terms corresponds to the rising action area of

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Freytag’s pyramid.9_{A budding robber organises a heist on a}

bank, and the police, who have been tipped off about these plans, prepare the bank's defence. The involvement of the writer (i.e. scenario designer) is limited to supplying both of these agents with goals – respectively, “acquire money” and “defend the bank” – and to specifying that these must be fulfilled simultaneously, in order to set up the story’s climax (the bank heist). It is left to the FM planner procedurally to find the optimal route for each, as well as synchronising them with each other so that both agents are ultimately at the bank at the same time. Preconditions for robbing the bank are that the robber is armed and in disguise. The sole precondition for defending the bank is that the cop is armed.

Fig 2. the initial state of the Fate Manager Test demo,

depicting the robber, cop, police station, ski mask shop, bank and gun shop. The player character is in the centre.

The unpredictable element in the scenario is a player-controlled character, who can be issued commands by clicking on a building and selecting an action to perform there. For the sake of broadening the player's options (and thereby increasing his autonomy),35_{some of the actions which the two NPCs can}

perform – such as “buy gun” – are also available to the player character; but the latter’s main power lies in its unique ability to burn buildings to the ground, making literal Gaynor’s description of the player as an “agent of chaos”.5_{This quite}

clearly gives the player the power to interfere with the storyline. Destroying the ski mask shop before the robber has visited will prevent him from disguising himself, thus making it impossible for the robbery to occur (though the player can also derail the story in more subtle ways, such as buying up every ski mask in the shop). The scenario thereby demonstrates the FM's capacity both to recognise player-initiated actions which jeopardise the intended story, and to adapt the world to accommodate them.

2.2 Gathering user feedback

As mentioned in Section 1.2.1, the study will use Eindhoven University's Game Experience Questionnaire as the foundation for its surveying of users. Although the GEQ is one of the most widely used such tools,33_{there are a number of reasons why it}

would not have been appropriate to use in its entirety, without modification. Most obviously, some of its questions are concerned with factors which are irrelevant to this experiment, such as challenge and competence.

Because the sessions are unlikely to be particularly immersive due to their simplicity, the questionnaire does not adopt the GEQ's response mechanism of inviting users to indicate their agreement with each statement on a Likert scale. Instead, users are invited to assess their relative experience of the sessions,

and to indicate which of the four comes closest to satisfying a given statement.

Before the demo was distributed, a qualitative pre-test of the survey was carried out among three users. The consensus arising from this was that a couple of the questions taken from the GEQ were inappropriate: “I lost track of time” and “I lost connection with the outside world” were deemed to be slightly grandiose claims to make about such a basic scenario (which tends to support the decision not to use a Likert scale); and there was felt to be some ambiguity around the meaning of “I was deeply concentrated in the game”, which one user took to suggest that the game was challenging and required deep concentration, though it is intended as a solely flow-related question. These three were replaced with “I felt engaged with the experience”, “I felt like I had a lot of freedom” and “I was deeply involved in the game” respectively.

Ultimately, all but questions 5, 6, 8 and 10 were taken intact from the GEQ. The complete original GEQ is included in Appendix B for comparison.

Fig 3. A screen from the user experience survey.

The questions can be used to obtain three KPIs: immersion (questions 1, 2, 3, 7), flow (4, 5, 6, 8), and frustration (9, 10). Immersion is used here to measure the user's engagement with the narrative, and flow to measure his feeling of autonomy. Finally, frustration is measured in order to test for the possibility that adjustments made to the world by the FM may arouse negative emotions in the player.

Respondents were obtained anonymously online, primarily through the /r/truegaming subreddit, over a period of 22 days. Some demographic information was collected (gender, age, level of education, frequency of game play), and users could optionally submit comments to assist with qualitative analysis of the results.

The complete survey (with results) is in Appendix A.

3 The Fate Manager

The primary concern of this study is the efficacy (or otherwise) of the Fate Manager narrative management system in improving player experience by reconciling a game’s narrative

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and ludic dimensions. In this section, the means by which the FM strives to do this will be discussed.

For a more in-depth look at the implementation of the FM, its source code is included in that of the demo scenario, which as mentioned in Section 2.1 is available online.

3.1 Concepts used by the Fate Manager

Before discussing the internals of the FM, some of the concepts employed by it and its accompanying demo scenario, and the terminology used to refer to them, should be outlined.

Object refers to every individual entity in the game world.

These can be divided into agents and buildings. The primary difference between the two is that agents have a repertoire of

actions and a plan consisting of a sequence of some or all of

these actions, while buildings are inanimate. The player object is also an agent, but its plan is determined by the user rather than an AI, and at any given time consists only of a single action. All objects additionally have their own mutable state.

States and actions are the building blocks of agents' plans. The

use of these terms in this paper is based upon that in the “vanilla” implementation of Goal Oriented Action Planning (or GOAP, more on which in Section 3.2) as described by Orkin et al.,36 37 _{but with some extensions.}

A state is a set of state attributes, each one a key-value pair. No two attributes with the same key can exist within a state. Every object, agent or building, has its own state. For example, the robber's initial state is:

{WillingToBreakLaw : 1, HasGunLicence : 1,

CanMove : 1, HasMoney : 150}

which indicates that this agent is willing to break the law, has a gun licence, can move, and has 150 units of money. As the above implies, each value can be interpreted either as a boolean or as an integer, depending on the associated key. An attribute whose value is 0 is treated as equivalent to the absence of that attribute in the set, so adding the attribute HasGuns : 0 to the above state does not actually change it: whether that attribute is present or not, the robber has no guns. Each agent has its own set of available actions, which are the means by which an object transitions from one state to another. An action consists of preconditions, effects, a cost or duration (measured in in-game clock ticks, 60 to a second), and optionally a target object.

Preconditions and effects can apply to both the agent carrying out the action, and the target of the action (if any). They are stored as states, but unlike in states belonging to an object, each attribute is associated with a mathematical operation in addition to its key and value. This allows for the expression of relationships between that key and value beyond the mere equality or assignment supported by standard GOAP. Preconditions and effects can apply to both the agent carrying out the action, and the target of the action (if any). This is the agent precondition state of the action BuyGun:

{HasGunLicence = 1, HasGuns = 0, HasMoney >= 100} and its agent effect state is:

{HasGuns + 1, HasMoney - 100}

This means that the action can only be performed by an agent who has a gun licence, no guns, and at least 100 units of money. Once the agent has performed the action, 1 is added to its HasGuns attribute and 100 is subtracted from its HasMoney attribute (BuyGun also has preconditions and effects which apply to its target object, which will be ignored here for brevity's sake).

Finally, each agent can optionally be assigned a goal state. The aim of the planner is to devise a sequence of actions which will move the agent from its current state to that goal. The robber's goal state is:

{HasMoney >= 5000}

It can be seen that, given a target state and a set of available actions, the planner can (by attempting every combination) produce a tree representing every possible sequence of actions which leads to that goal.

An interfering action is one whose effects invalidate any agent's existing route to the goal state. The Fate Manager, built on top of agents' GOAP planners, has two tasks to perform: determine whether a proposed action is interfering, and if so, decide how to respond to this.

The detection of an interfering action resembles some previous approaches in its results,18_{though the actual implementation is}

somewhat different. The real purpose of the Fate Manager system, however, is its capacity to adapt the world state in response to interfering actions in order to allow agents' goals to be realised regardless of the player's behaviour. Its effectiveness in doing so without evoking a negative reaction in the user is the main concern of this paper.

3.2 Fate Manager system design

In order to make the application as versatile and demonstrative of the Fate Manager's potential as possible, it operates entirely procedurally. There are no hard-coded behaviours or events, and its design is modular. Potentially, new objects and actions could be added to the scenario without altering the internal code of the FM itself.

As mentioned in Section 3.1, the FM is built upon a modified Goal Oriented Action Planning algorithm, which has been among the most popular solutions for agent-based planning in games throughout the last decade or so.38_{This particular}

implementation is largely indebted to the specification outlined by Jeff Orkin et al,36 _{with the only significant modifications} being to extend the semantic scope of agent states (see Section 3.1) and to introduce synchronisation between actions (see later in this section).

Briefly, GOAP manages agent behaviour by taking as input one or more goals (such as “sate hunger” or “obtain money”), leaving an independent controller associated with each agent, or planner, to determine the details of how those goals will be attained. This gives the impression of relatively naturalistic behaviour in agents, who act in a fluid manner and react to changing circumstances rather than simply following a script laid down by the writer.39_{It also means that these relationships}

between actions do not need to be stated explicitly by the designer, but will be determined procedurally by the planner, which permits a modularity lacking in alternatives such as

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Finite State Machines.40_{Additional actions and their properties}

can be added to the system without worrying about the implications for other actions. For example, given the information that money can be obtained by robbing a bank, and that robbing a bank requires weapons and a disguise, and that these items can be acquired at shops, the planner can devise a clear sequence of actions which will result in the agent fulfilling its goal of obtaining money.

Fig 4. A player attempts to burn the gun shop (top) and the

FM responds in (clockwise from top-left) BLOCK ACTION, STOP, FORCE SUCCESS and CHANGE WORLD modes. Note that only in the latter two does the action finish and the session continue.

The Fate Manager can run in four main modes (see Fig 4), and the demo requires the user to play through the scenario once in each (plus a “practice” mode in which there are no NPCs). In the first two sessions, the FM's only role is to identify potentially interfering actions by the player. In the latter two, however, following the identification of an interfering action, the FM has the additionally ability to intervene in the world itself in order to allow that action to proceed without disruption to agents' goals.

 The first session runs in BLOCK ACTION mode. As soon as the player selects an action, the FM checks whether it will interfere with agents' goals; in all other modes, the player action is allowed to complete before the FM determines whether it interferes. If the action is found to be interfering, the player is notified to that effect and the scenario proceeds as before. This mode is comparable to the method used to prevent the player derailing the story in most open-world games, and to the “proscriptive” approach of the University of Twente's virtual storyteller,20_both

of which were discussed in Section 1.1.

 The second session uses STOP mode. This runs similarly to BLOCK ACTION, except that interfering actions will not be blocked; instead, upon their completion the player is notified that the session cannot continue, and the next begins.  In this third session, FORCE SUCCESS mode is

activated. This time, once an interfering action completes, the FM determines which intermediate states in the agent's plan can no longer be attained, and assigns those states to the agent regardless. For example, burning the gun shop prior to the robber's arming himself will result in the FM immediately

assigning {HasGuns, 1} to the robber, even though no BuyGun/StealGun action has been performed. This is effectively a form of narrative omission: the acquisition of the gun becomes implied, rather than explicitly shown.41

 The final session uses CHANGE WORLD mode: the FM adds object(s) to the world, or changes the state of existing ones, so that the agent's original plan can proceed. This mode resembles the “environmental” approach of UT's virtual storyteller.

Although the FM performs the function of identifying interfering actions in all four modes, the feature this study will examine is its attempt to reconcile the game world with those actions, which occurs only in the latter two modes, FORCE SUCCESS and CHANGE WORLD. For simplicity, therefore, those modes will be referred to as the FM-enabled modes. To determine whether a proposed action, a, will interfere with agents' plans, the FM follows this process (for a graphical depiction of which, see Fig 5):

1. Cache the current world state in U. This is the “pre-action” state.

2. Apply the effects of a to the current world state, giving us a “post-action” state, V.

3. For every non-player agent, step through its existing action plan and flag any actions which can no longer be performed under the new world state, V. Let {b} be the set of these actions.

4. Restore U as the current world state.

5. If {b} is empty, the player's action is allowed to be performed unimpeded.

6. If {b} contains at least one action, the next step depends on the FM mode.

a. In STOP mode, the player agent is permitted to perform the action, and the session ends as soon as it completes.

b. In BLOCK ACTION mode, a message informs the user that his attempt to perform action a was blocked.

c. In FORCE SUCCESS or CHANGE WORLD mode: for each action in {b} – those which can be performed under world state U, but not V – step through the corresponding agent's plan and determine which subsequent actions in the plan have preconditions which depend on the effects of b. Those preconditions are collected in a new state, T. Action a is allowed to be completed by the player, making V the new “real” world state. Once again, the next step depends on the mode. i. In FORCE SUCCESS mode, the FM adds

the precondition state T to the current state: i.e. forces every object affected by b to comply with that action’s preconditions. This means that b can be removed from that agent's plan and all subsequent actions will be able to proceed as before.

ii. In CHANGE WORLD mode, the FM finds all actions in the agent's repertoire which will fulfil the precondition state T, and then selects one of these at random (c). This action is inserted into the agent's plan in place of b. If c requires a particular object to exist which does not exist under V, that object is created at a random location, with a state which fulfils the preconditions of c. If such an object already exists, but its state

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does not fulfil c's preconditions, its state is amended accordingly.

Note that this final step may result in the world being returned to a state very similar to U, the world state before a was performed. For example, the cop can acquire weapons from either the police station or the gun shop. If the player destroys the police station, and has already destroyed the gun shop, there is a 50% chance that a new police station will be created. This could engender a feeling in the player that his actions are ultimately pointless – which, of course, in terms of the larger story, they are, though concealing this is preferable. Possible ways of addressing this issue are discussed in Section 4. The process of identifying interfering actions has a complexity of O(mn), where m is the number of non-player agents and n is the number of actions in that agent's existing plan. This is negligible compared to the re-planning stage which will have to take place if an interfering action is found to occur – it runs in O(mn!), where n is the average number of actions available to an agent, though there are possible optimisations which could be applied to this process to improve scalability. See Section 5 for more on this.

The primary way in which the GOAP variant underlying the FM extends the vanilla implementation is in its capacity to synchronise actions. The FM is concerned with ensuring not only that agents' goals are reached, but that they are reached at a particular time. This is essential to the management of story points: the heist requires the cop and the robber to be at the bank at the same time.

As in standard GOAP, when multiple routes to an agent's goal are found, it is the most efficient one which is chosen to be enacted. Thereafter, however, the temporal cost of each action becomes flexible, and may be padded or truncated as required to align with another agent's plan. After all, the user is unlikely to notice that an agent takes twice as long to buy a gun as usual. Even if he does, there is no actual reason why this should be objectionable: indeed, it would be less realistic for every type of action always to take exactly the same amount of time. Ideally, there would be limits to this flexibility: taking 940 ticks to buy a gun rather than the usual 470 is acceptable, but 9000 (or indeed 10) may not be. In the demo, though, action costs can be padded without limit, and truncated to a minimum of 1 tick.

Fig 5. When the player attempts an action, a, the FM executes

the above process for every agent in order to determine whether a interferes with that agent’s plan, and if so, how to deal with it.

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4 Results

44 respondents completed the survey, 25 of whom (56.8%) are female. Their ages range from 15 to 66, with a mean of 29. This is comparable to the average of 31 among gamers as a whole in 2008.42

By grouping together the four questions on immersion, the four on flow and the two on frustration (as discussed in Section 2.2), the individual results for those categories can be compared. Fig 6 shows the results of the questions from each category, as well as the means of those results.

Immersion Flow Frustration

BLOCK ACTION 11, 4, 1, 1 4, 3, 3, 1 15, 6 M = 4.25 M = 2.75 M = 10.5 STOP 5, 4, 5, 2 4, 6, 7, 1 16, 5 M = 4 M = 4.5 M = 10.5 FORCE SUCCESS 11, 20, 13, 17 14, 10, 14, 23 4, 20 M = 15.25 M = 15.25 M = 12 CHANGE WORLD 17, 16, 25, 24 22, 25, 20, 19 9, 13 M = 20.5 M = 21.5 M = 11

Fig 6. The individual results and mean for each category.

The data in each category were successfully tested for normal distribution and homogeneity of variance, with the standard error in the immersion category ranging from STOP (SE=0.707) to BLOCK ACTION (SE=2.3585), and in the flow category from BLOCK ACTION (SE=0.62925) to FORCE SUCCESS (SE=2.75). The error in the frustration category was higher owing to the smaller number of samples.

It is obvious from a glance that, in terms of the two positive KPIs measured (immersion and flow), the FM-enabled modes (FORCE SUCCESS and CHANGE WORLD) outperform the others. One-way ANOVA confirms this, with statistically significant differences existing between the four modes in terms of immersion (F(3,12) = 17.4, p = 0.000115) as well as flow (F(3,12) = 27.77, p < 0.0001).

Comparing the sessions individually, a post hoc Tukey test shows that, in terms of those categories, there are significant differences between the FM-enabled sessions and the others (all at p < 0.01), but not individually between BLOCK ACTION and STOP, or between FORCE SUCCESS and CHANGE WORLD.

In the case of frustration, meanwhile, although the FM-enabled modes score slightly more highly (i.e. poorly), the difference is not statistically significant (F(3,4) = 0.02, p = 0.99548). In other words, the null hypothesis in Section 2 can be rejected: using the Fate Manager to manipulate the world in

response to interfering actions results in a statistically significant improvement in immersion and flow, while incurring no significant increase in terms of frustration.

Which particular method of world manipulation is preferable, however, cannot be determined from these figures.

A number of points of note emerge from the results. The most startling of these is the increase in immersion which users reported in the FM-enabled modes. As discussed in Section 2, these modes were designed principally to increase perceived

player autonomy (i.e. flow), with the hope that doing so would not concomitantly reduce narrative engagement (i.e. immersion). For immersion to undergo almost as much improvement as flow is therefore highly unexpected.

Any attempt to explain this from the evidence available will necessarily be conjecture. With that said, it may be that increasing flow so significantly has a knock-on positive effect on immersion. As long as the increase in flow does not actively undermine immersion – as can happen in freeform games which maximise player choice at the expense of narrative – the player becomes freer to focus on what is going on in the story. This effect is perhaps more pronounced in such a simple story, so the application of the FM concepts in an environment more closely resembling an actual game would be advantageous for future study.

Less charitably, it could be that a weakness in survey design is responsible. Statements such as “I felt that I could explore things”, although marked by the GEQ as pertaining to imaginative immersion, could be interpreted as relating to player autonomy (and therefore flow). Counting against this possibility, however, is the fact that “I was interested in the story”, a statement quite explicitly concerned with imaginative immersion, also recorded stronger results for the FM-enabled modes. In fact, in every question relating to immersion – 1, 2, 3 and 7 – FORCE SUCCESS and CHANGE WORLD either equalled or outperformed the other two modes.

Respondents were asked which types of games they were inclined to play. Of the genres listed, the one to which an FM would be most suited is likely the western RPG, which tends to be narrative-driven, yet open-ended. Among the 58% of users who play such games, the FM-enabled sessions did indeed perform slightly better in terms of flow (M = 10.64 vs M = 9.625), but not significantly so (F(1,14) = 0.41, p = 0.532). A majority of respondents (25) state that they play games only “occasionally” or “never”. One might expect this to have a substantial effect on the validity of the results: perhaps more inveterate gamers would be more sensitive to the FM's manipulation of the world, and consequently more frustrated by it. However, comparing the results on frustration with the FM-enabled modes as a whole with those from respondents who play “frequently” or “practically non-stop”, there is no significant difference (F(1,6) = 0.05, p = 0.830482). In fact, after scaling for the different number of responses, avid gamers actually reported a slightly lower mean frustration with the FM-enabled sessions (M = 10.42) than did respondents as a whole (M = 11.5).

Some of the user comments provide insight into why this may be. One user, a 23-year-old woman who frequently plays games, noted of the CHANGE WORLD session: “When I burned the police station...it just reappeared in front of me. I can see why you do this but it could be distracting in a game.” Another, a 29-year-old woman who occasionally plays, said: “The way buildings would appear as soon as I'd destroyed them in Session 4 made my actions feel pointless.”

This may be the principal objection of the minority of users who chose CHANGE WORLD as the mode which was most “frustrating” (20.45%) or “jarring” (29.55%). The distracting effect is likely to be worsened when the new object is identical to one the player just destroyed (as will usually be the case in the demo scenario), making it quite plain that, for all his apparent freedom, the player lacks any ability permanently to alter the world in a meaningful way.

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This problem can arguably be attributed to the nature of the demo more than to the concept itself. The demo grants the user constant omniscience in terms of which objects exist in the world, and deliberately so: it is desirable for the purposes of the experiment that the user understand as far as possible what the FM is doing to handle interfering actions. However, in most games which are sufficiently complex to benefit from an FM, only a portion of the world is visible at any time. If the replacement police station were to appear off-camera – especially distantly off-camera, so that when the user revisits that location he is less likely to recall its prior layout – it would go some way to addressing this objection.

Another possible device which may lessen the dissonance of the CHANGE WORLD solution – if not the threat to perceived player empowerment – would be to consider time as well as space: in place of the instantaneous creation of a new building, an agent is spawned off-screen, moves to a position in the display, and constructs a new building there itself. This would complicate the FM's planning algorithm, but is certainly worth considering for future revisions. It is fair to surmise that these improvements would reduce CHANGE WORLD's frustration score below that of the non-FM sessions. It may be that experienced players are more likely to be conscious of such possible solutions, and are therefore less bothered by the crudities of that mode.

Perhaps surprisingly, more users found STOP to be “jarring or hard to believe in” (45.45%) than thought that of CHANGE WORLD (29.55%). This could be to do with the opacity of what is actually taking place in STOP mode: the user is not privy to an agent's internal state, so it is not clear exactly why an NPC suddenly stops moving towards one building and goes to another instead. A 26-year-old female user who occasionally plays games said that, while she “felt most free” in that session, it was also the most frustrating as she “wasn't really sure what the characters were doing”.

A few shortcomings in the experiment should be noted:  There were some outliers within categories. In the

first question (“I was interested in the story”), 11 users (25%) selected BLOCK ACTION, far more than did so in the other three immersion questions (4, 1 and 1 respectively). There were multiple comments which hinted that the reason for this may simply be because of its novelty value, such as this from a 23-year-old man: “I was most engaged by the first session just because it was the first!” See Appendix A for more such comments. A more sophisticated future experiment would randomise the order of sessions for each user. This was not done here in order to simplify the data collection process.  66% of respondents are university graduates. For

comparison, in 2013 only 38% of the UK population had completed higher education.43 _{Users who are}

more familiar with this kind of work may be biased towards the answers they suspect the researcher desires, which could be partly responsible for the favourable results for the FM-enabled modes. It would be interesting to compare this experiment with one conducted among a more educationally representative sample.

The full results can be found in Appendix A.

5 Conclusion & future work

The results of the experiment suggest that the Fate Manager system has significant potential in addressing the choice/narrative tension in games, and that the research question in Section 1.3 may be answered with a tentative affirmative: the concept of fate, or predestination, may indeed offer a route to enforcing a scripted story without impinging on player freedom in the way that more conventional methods do. This must be qualified, however, with a couple of points. The scenario used to test responses to the FM is a lot more simplistic than a real-world game, and its “story” really only a story fragment. In addition, the conspicuously high performance of the FM-enabled sessions in the immersion category raises some questions, as outlined in Section 4. For these reasons, future study in more overtly game-like situations is merited.

In addition to the suggestions in the foregoing sections, there are a number of areas in which the experiment could be expanded upon.

 A 3D version of the demo, or at least more sophisticated 2D graphics, would improve immersion generally. A more complex story, with a greater number of characters, would also help with this.  While the BLOCK ACTION and CHANGE

WORLD modes resemble the “proscriptive” and “environmental” approaches of UT's virtual storyteller (see Section 1.1), it would also be worth exploring a mode approximating the VS's “motivational” method, which adjusts NPCs' goals in response to player behaviour.

 Currently, the player can interact directly only with inanimate objects (buildings). CHANGE WORLD mode in particular would benefit from the ability to interact with NPCs, as it is the sole mode which could cope with the killing of an essential character (via respawning).

 Allowing multiple simultaneous players would introduce a new dimension to the experiment. It may be speculated that this would increase frustration, as changes to the world would arise not only from one's own interfering actions, but from one's fellow players.

 One impediment to increasing the complexity of the scenario is that, as mentioned in Section 3.2, the existing planning algorithm uses a brute force method which executes in factorial time. This makes it unsuitable for even a medium-scale project. A more sophisticated algorithm, which selects actions to attempt based on some intelligent metric rather than iterating blindly through all of them at each node of the planning tree, would go some way to resolving this.

6 Acknowledgements

My thanks to the University of Amsterdam for an enjoyable and enlightening year, and in particular to Dr Frank Nack for his perseverance throughout the thesis process. Dank je wel also to Dr Tibor Bosse for agreeing to assess this paper, and to Bethesda for Fallout 4, which helped to fill the lonely summer nights when working on this project became too much.

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25.00% 11

11.36% 5

25.00% 11

38.64% 17

Q1

I was interested in the story

Answered: 44 Skipped: 0

Total 44

Session 1(actions which interfere with the story are...

Session 2(actions which interfere with the story end...

Session 3 (Fate Manager allows AI charactersto complete thei... Session 4 (Fate Manager creates new objects to replace cruci... 0% 20% 40% 60% 80% 100%

Answer Choices Responses

Session 1(actions which interfere with the story are blocked)

Session 2(actions which interfere with the story end the session)

Session 3 (Fate Manager allows AI charactersto complete their goals regardless of crucial objects being removed)

Session 4 (Fate Manager creates new objects to replace crucial ones which have been removed)

1 / 19

Fate Manager survey

Appendix A:

Appendix A - results

SurveyMonkey

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9.09% 4 9.09% 4 45.45% 20 36.36% 16

Q2

I felt imaginative

Answered: 44 Skipped: 0 Total 44 Session 1(actions which interfere with the story are...

Session 3 (Fate Manager allows AI characters to complete thei... Session 4 (Fate Manager creates new objects to replace cruci... 0% 20% 40% 60% 80% 100%

Session 3 (Fate Manager allows AI characters to complete their goals regardless of crucial objects being removed)

2 / 19

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2.27% 1

11.36% 5

29.55% 13

56.82% 25

Q3

It felt like a rich experience

Total 44

3 / 19

(16)

9.09% 4

31.82% 14

50.00% 22

Q4

I was fully occupied with the game

Total 44

4 / 19

(17)

6.82% 3

13.64% 6

22.73% 10

56.82% 25

Q5

I felt engaged with the experience

Total 44

5 / 19

(18)

6.82% 3

15.91% 7

31.82% 14

45.45% 20

Q6

I was deeply involved in the game

Total 44

6 / 19

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2.27% 1

4.55% 2

38.64% 17

54.55% 24

Q7

I felt that I could explore things

Total 44

7 / 19

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2.27% 1

52.27% 23

43.18% 19

Q8

I felt like I had a lot of freedom

Total 44