Intelligent virtual environments for virtual reality art

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  • Computers & Graphics 29 (2005

    mea, Saco

    Tees

    rmita

    Churc

    y of P

    objectives. This makes the prospect of generic tools

    rather unrealistic. Another approach consists in obser-

    ARTICLE IN PRESS

    Corresponding author. Tel.: +44 1642 342 657;

    ving that often-artistic concepts revisit fundamental

    aspects of interactivity, or question essential concepts

    0097-8493/$ - see front matter r 2005 Elsevier Ltd. All rights reserved.

    doi:10.1016/j.cag.2005.09.002

    fax: +44 1642 230 527.

    E-mail address: m.o.cavazza@tees.ac.uk (M. Cavazza).1. Introduction and objectives

    Virtual reality (VR) art has emerged in the last decade

    as an unexpected application for high-end VR systems

    as well as a new direction for digital arts [1,2].

    However, the development of VR art installations is

    an extremely complex process. Leading VR artists have

    often beneted from a supportive technical environment

    for the development of their major installations. Some of

    them were able to hire teams of systems developers,

    while others were afliated to academic institutions,

    which brought together artists and scientists or engi-

    neers. The level of complexity and cost of such

    development is certainly a limitation to the development

    of VR art. As such there is a rationale for new tools that

    would facilitate the development of VR art installations.

    However, the strategy for creating such tools has to be

    carefully considered, as one can only feel bemused at

    how diverse the relation to technology is among various

    artists. Some advocate a strong technical involvement

    and even participation in programming tasks while

    others tend to follow a production model in which

    technical developments are subordinated to the artisticAbstract

    The development of virtual reality (VR) art installations is faced with considerable difculties, especially when one

    wishes to explore complex notions related to user interaction. We describe the development of a VR platform, which

    supports the development of such installations, from an art+science perspective. The system is based on a CAVETM-

    like immersive display using a game engine to support visualisation and interaction, which has been adapted for

    stereoscopic visualisation and real-time tracking. In addition, some architectural elements of game engines, such as their

    reliance on event-based systems have been used to support the principled denition of alternative laws of Physics. We

    illustrate this research through the development of a fully implemented artistic brief that explores the notion of causality

    in a virtual environment. After describing the hardware architecture supporting immersive visualisation we show how

    causality can be redened using articial intelligence technologies inspired from action representation in planning and

    how this symbolic denition of behaviour can support new forms of user experience in VR.

    r 2005 Elsevier Ltd. All rights reserved.

    Keywords: Virtual reality art; Articial intelligence; Causal perception; Immersive displaysIntelligent virtual environ

    Marc Cavazzaa,, Jean-Luc LugrinAlok Nandic, Jeffrey JaSchool of Computing, University of

    bCLARTE, 4 Rue de lEcCommediastra, 182, av. W.

    dDepartment of Information Sciences, Universit) 852861

    nts for virtual reality art

    imon Hartleya, Marc Le Renardb,bsond, Sean Crooksa

    side, Middlesbrough, TS1 3BA, UK

    ge, 53000 Laval, France

    hill, 1180 Brussels, Belgium

    ittsburg 135, North Bellefield, PA 15260, USA

    www.elsevier.com/locate/cag

  • animate the world objects. This process could be

    laws of Physics [5]. The Ego.geo.Graphies brief is

    ARTICLE IN PRESSM. Cavazza et al. / Computers & Graphics 29 (2005) 852861 853facilitated if behaviours could be described at a more

    abstract, conceptual level, in the VR system itself. The

    creation of alternative behaviours could take place

    directly in this representation layer, which would also

    support iterative explorations of initial ideas. The use of

    an AI layer to dene the behaviour of a virtual

    environment implements the notion of an intelligent

    virtual environment [4]. This experimental technology

    should bring numerous benets to the development of

    VR art installations: it supports the redenition of non-

    realistic and alternative behaviour from rst principles,

    it allows rapid prototyping and experimentation and,

    nally it is well adapted to an art+science approach as it

    explicitly represents those concepts that are the object of

    artistic or scientic experimentation.such as reality, physical experience or even the perceived

    nature of life. In other words, as these interrogations

    also happen to be scientic ones, they open the way to

    what has been recently described as the art+science

    approach, in which VR artists have otherwise played a

    prominent role. In this paper, we describe such research,

    whose aim is to facilitate the development of VR art

    installations in an art+science context [3].

    This is why, rather than simply developing a toolkit

    to lower the accessibility threshold of VR art technol-

    ogy, we propose a system where artistic and scientic

    simulation can meet at the level of conceptual repre-

    sentations, while still generating technical output in the

    form of implemented VR installations.

    2. Intelligent virtual environment: knowledge layer and

    programming principles

    The notion of behaviour of a virtual environment

    normally encompasses all reactions of the environment

    to the users physical intervention. This in turn

    corresponds to the physical processes triggered by the

    user, when for instance s/he grasps, then drops an

    object. More often, it will consist of all devices

    behaviour that are ultimately not derived from physical

    simulations (for obvious reasons related to optimal

    levels of description), but scripted within the systems

    implementation. In both cases, such behaviour is

    encoded procedurally and the concepts underlying

    behaviours (e.g., patterns of motion, physical concepts,

    etc.) are not explicitly represented other than through

    variables embedded in equations or scripts. VR art is

    often concerned with the creation of virtual worlds that

    exhibit idiosyncratic behaviours, which might violate the

    traditional laws of Physics, such behaviours often being

    described in the installation briefs in abstract or

    metaphorical terms only. This makes it rather tedious

    to implement non-standard behaviours directly in terms

    of the low-level primitives (physical or procedural) thatexploring interaction and navigation in a non-anthro-

    pomorphic world, blurring the boundaries between

    organic and inorganic. Its installation involves an

    immersive VR world with which the user can interact.

    The virtual world comprises of a landscape in which the

    user can navigate, populated by autonomous entities

    (oating spheres), which are actually all part of the same

    organism. In this world, two sorts of interaction take

    place: those involving elements of the world (spheres and

    landscape) and those involving the user. The rst type of

    interaction is essentially mediated by collisions and will

    be perceived in terms of causality. The second is based

    on navigation and position and will be sensed by the

    world in terms of empathy, as a high-level, emotional

    translation of the user exploration.

    Through the staging of the Ego.geo.Graphies installa-

    tion, we are interested in exploring aspects related to

    predictability/non-predictability and hence some kind of

    narrative accessibility, from the perspective of user

    interaction. On one hand, this brief is an exploration of

    the notion of context through the variable behaviour of the

    environment which itself responds to the user involvement.

    But on the other hand, it constitutes an exploration of

    causality. As such, it requires mechanisms varying the

    physical effects of collisions (bouncing, merging, bursting,

    exploding, altering neighbouring objects, etc.), taking into

    account the semantics of the environment.

    This also implies that we explore how the user can be

    affected by causality. The spontaneous movements of

    the spheres focus the user attention, within the

    constraints of his/her visual and physical exploration

    of the landscape. The user will perceive consequences of

    spheres colliding with each other, which are equivalent

    to an emotional state of the world (as these multiple

    spheres still constitute one single organism) responding

    to perceived user empathy.

    As a consequence, a dialogue should emerge from this

    situation: user exploration will affect world behaviour

    through levels of perceived empathy, and in return the

    kind of observed causality will inuence user exploration

    and navigation.

    4. System overview

    The system presents itself as an immersive installation

    supporting alternative worlds with which the user can

    interact and, through this interaction, experience the

    nature of the fantasy worlds created by the artistic brief.3. The illustrative briefs

    To illustrate the technical presentation we will use

    examples from a fully implemented artistic installation,

    Ego.geo.Graphies by Alok Nandi. This brief is

    situated in an imaginary world governed by alternative

  • ARTICLE IN PRESSM. Cavazza et al. / Computers & Graphics 29 (2005) 852861854The choice of an immersive hardware platform was

    dictated by the necessity to match state-of-the-art VR

    installations. The vast majority of them are based on

    CAVETM-like systems [6], which are multi-screen im-

    mersive projection displays. The advantages of CA-

    VETM-like systems for VR art are well established: they

    constitute an optimal compromise between user immer-

    sion in visual content and the ability for physical

    navigation (although in a limited space) and interaction.

    In addition, CAVETM-based installations can be explored

    by a small audience of up to four spectators (Fig. 1).

    The software architecture implements the notion of an

    intelligent virtual environment, in which alternative

    reality can be dened through a symbolic description

    of the virtual worlds behaviour. This software archi-

    tecture is based on an integration layer, which consists in

    an event-based system, relating the visualisation engine

    to the behavioural layer. We use a state-of-the-art game

    engine, Unreal Tournament 2003TM (UT), as a visua-

    lisation engine. Game engines provide sophisticated

    Fig. 1. Immersive visualisation in the SAS CubeTM.visualisation features and most importantly constitute a

    software development environment in which further

    components can be integrated. This aspect explains that

    game engines are increasingly used in VR research [7].

    The behavioural layer is in turn composed of two

    modules, one for alternative Physics (using qualitative

    Physics) and another for articial causality. In this paper

    we shall concentrate on the latter component. Through

    this event-based system, real-time interaction with the

    visualisation engine can trigger alternative behaviours

    calculated by the intelligent virtual environment.

    5. The VR architecture: stereoscopic visualisation in the

    SAS cubeTM

    The immersive display we have used for this research

    is known as the SAS CubeTM (Fig. 2) and is a four-sidedCAVETM-like projection system in which the front, left,

    right and oor sides (each 3m wide) are used as

    projection screens, receiving a back-projected image

    produced by four BarcoTM projectors.

    This immersive display supports the use of a game

    engine as a visualisation engine through specic soft-

    ware known as CaveUTTM [8]. A multi-screen display

    based on CaveUTTM requires a server computer

    connected by a standard LAN to a number of client

    computers, at least one for each screen in the display.

    Stereo visualisation is an essential feature of immer-

    sive displays and CaveUTTM supports stereographic

    display by using two computers per screen, one to render

    the left eye view and one to render the right eye view,

    with an average frame rate of 60 frames/s per eye in most

    experiments reported here. The camera view can be

    offset from the viewers default conguration by a set

    value equal to half the inter-pupillary distance. Active

    stereo requires a single stereographic projector that will

    alternate between the left and right eye views at 120

    frames per second. The user wears shutter glasses on

    where each lens alternates between black and clear, also

    at 120 frames per second. The glasses switch in time with

    the display, and the result is that each eye gets the view it

    is supposed to at 60 fpsthe left view for the left eye and

    the right view for the right eye. All the screens in the

    composite display must also switch view at exactly the

    same time, a desirable state called genlock.

    The CaveUTTM installation in the SAS CubeTM

    platform uses two computers for each screen, one for

    Fig. 2. The SAS CubeTM installation.each eye view, and uses the DVG (video) cards in their

    ORADTM (PC) cluster to mix the two video signals and

    send the combined signal to a single stereographic

    projector. The DVG cards also handle the genlock

    synchronisation across all screens of the composite

    display. The overall hardware/software architecture

    supporting CaveUTTM in the SAS CubeTM is depicted

    on Fig. 3.

    CaveUTTM supports real-time tracking in physical

    space, using the IntersenseTM IS900 system or any

    similar devices. Tracking the players head allows

    CaveUTTM to generate a stable view of the virtual

    world, while the player is free to move around inside the

  • ARTICLE IN PRESS

    in th

    M. Cavazza et al. / Computers & Graphics 29 (2005) 852861 855Fig. 3. Stereoscopic visualisationdisplay (which has the size of a traditional CAVETM).

    From a system integration perspective, CaveUTTM uses

    another freeware package, Virtual Reality Peripheral

    Network (VRPN)1 to handle input from all control

    peripherals such as joysticks, buttons, gamepads and the

    tracking system itself. All controllers are physically

    attached to the server machine, and data from the

    peripherals are collected by the VRPN server, which

    runs in parallel to the UT game server. The VRPN

    server converts data from the control peripherals into a

    generic normalised form and sends it to the CaveUTTM

    code in the UT game server, via a UDP port. The

    modied UT game server uses this information to

    update the users location in the virtual world from the

    head tracker and to process commands from the other

    control peripherals. The VRPN server also broadcasts

    the users new location to each one of the UT clients,

    and the information is received by a VRPN client. Then,

    the VRPN client sends the tracking information via

    another UDP port to the VRGL code attached to the

    UT client. VRGL uses this information to adjust

    the perspective correction, in real-time, to preserve the

    perspective depth illusion. The overall result is that the

    users view into the virtual world looks stable to him and

    1Released by the Department of Computer Science at the

    University of North Carolina at Chapel Hill.e SAS CubeTM with CaveUTTM.the correspondence between the virtual world and the

    real one is maintained.

    6. Software architecture: the event interception system

    The choice made for the software architecture also

    reects our philosophy of relating technical implementa-

    tion to high-level concepts of interactivity. This is why

    the software architecture, which integrates the visualisa-

    tion components with those in charge of interactivity

    and world behaviour, is based on the notion of event

    as a basic unit of interaction. The role of events as

    formalism for VR is well established [9] and, in addition,

    it plays a crucial role in the implementation of

    interactivity in game engines. Event systems are

    generally developed on top of graphic engines primitive

    that detect collisions between objects or between objects

    and graphic volumes. In particular, and this aspect is

    central to our own use of the concept of event, events

    tend to be used to discretise behaviours taking place in

    the virtual world. This can be illustrated on a simple

    example: moving objects see their behaviour dictated by

    Physics until they interact (e.g. collide) with other

    objects or surfaces. Upon collision, behaviour ceases

    to be determined by physical calculations (such as

    continuous mechanics); instead a collision event is

  • created using impact velocities as input parameters. The

    pre-calculated outcome of this collision is directly

    associated to the event and triggered upon event

    activation. This approach saves considerable computing

    power in current game engines. More importantly, the

    mechanisms behind event systems constitute an ideal

    API for the integration of the kind of behavioural layers

    we have developed. In this context, the overall software

    architecture is represented in Fig. 4.

    In standard event-based virtual environments, beha-

    viours tend to be encoded directly from low-level events.

    Event systems are generally derived from the low-level

    graphical event systems for collision detection (between

    objects, between objects and volumes). In this domain,

    the UT native event system proposes a large collection

    of events (called native events), such as Bump ( ),

    Landed ( ), Hitwall ( ), Encroachedby ( ), etc. For each

    object class and/or states, event-effect relations are

    embedded in native event procedures (call-back system)

    associated to one or many effect procedures. When a

    native event is detected by the visualisation engine, its

    effects procedures are immediately instantiated and

    triggered, generating animations or object movements,

    as a response. Moreover, to obtain realistic animations,

    most of the virtual environments are coupled to power-

    relations are dispersed in the code, their identications

    request expertise of the environment and of its platform

    (visualisation/Physics engines). Secondly, such hard-

    coded associations cannot support dynamic alterations

    of causality. As a result, in its default implementation,

    causality is static, basic and hardly accessible. The Event

    Interception System (EIS), we have developed on top of

    the UT event system, proposes to correct this limitation

    of native formalisms. In addition, it provides a complete

    interface between the event formalism, where causal

    relations are expressed through context event (CE)

    structures, and the UT visualisation/Physics engines,

    which is central to our software architecture. In our

    system, native low-level engine events are not directly

    linked to effect functions. The EIS module processes

    occurrences of the game engines low-level native events,

    to produce intermediate-level events, such as Hit( ),

    Push( ), Touch( ), Press( ), Enter( ), Exit( ), etc. For

    instance, the magnitude of the colliding object momen-

    tum in a colliding event can be used to instantiate a Hit

    (?obj, ?surface) event from the system-level Bump(?obj,

    ?surface) event. Basic events constitute a base from

    which the derivation of higher-level events is possible.

    On the other hand, CEs provide a proper semantic

    description of events, which clearly identies actions and

    ARTICLE IN PRESS

    for a

    M. Cavazza et al. / Computers & Graphics 29 (2005) 852861856ful Physics/Particle engine, as the case of the KarmaTM

    engine used by the UT engine.

    Such an ad-hoc denition of causality (cause-effect

    association) in a virtual environment raises a certain

    number of problems. Firstly, as the event-effect

    Fig. 4. The software architecturetheir consequences and therefore supports the modica-

    tion of such actions to generate alternative effects. Such

    high-level events explicitly encode default object beha-

    viours in the environment. This module constitutes one

    of the most innovative aspects of our approach, in which

    n intelligent virtual environment.

  • an ontology for actions serves as a representation layer

    for the virtual world.

    Typically, a CE is represented using an action

    formalism inspired from those serving similar functions

    in planning and robotics. These representations origin-

    ally describe operators responsible for transforming

    state of affairs in the world. They tend to be organised

    around pre-conditions, i.e. conditions that should be

    satised for them to take place and effects or post-

    conditions, i.e. those world changes induced by their

    application. Our CE formalism has been inspired from

    the SIPE planning representation [10], which clearly

    distinguishes the triggering conditions of an action and

    its effects. Fig. 5 shows an example of CE formalism

    used in the Ego.geo.Graphies artistic installation. The

    triggering conditions correspond to basic events detected

    by the EIS (for instance, the collision between two

    spheres), while the effects eld contains procedures

    corresponding to the consequences of the collision. The

    set of CE denes ontology of possible events in the

    virtual worlds. This ontology will be authored as part of

    the artistic installations to be developed. The CE

    represented corresponds to the default consequence of

    a collision between two spheres, which consists of these

    spheres merging. Dynamic modications of this CE can

    produce new consequences and create new forms of

    causality.

    event-based architecture described above and operate

    ARTICLE IN PRESSM. Cavazza et al. / Computers & Graphics 29 (2005) 852861 857Fig. 5. The recognition of high-level actions from low-level

    system events.interactively in user real-time.

    The concept of causality is central to our under-

    standing of the physical world and this is why it has been

    for many years a topic of discussion for physicists,

    psychologists and philosophers alike. Because we use

    causality to make sense of the conjunction of events

    taking place in our environments, any system that could

    create an illusion of causality would be a powerful tool

    for the creation of alternative realities.

    This environment specically supports the elicitation

    of causal perception by supporting the creation of event

    co-occurrences, in real time, in the virtual world. These

    co-occurrences can be generated from high-level princi-

    ples, such as analogies between object physical proper-

    ties. The original idea behind this research was that suchFig. 5 also demonstrates the instantiation of a CE

    from the stream of low-level events intercepted by the

    system. The collision between two spheres is recognised

    as a Hit(?sphere1, ?sphere2) Event (step 1). This low-

    level event being part of the trigger eld of the CE, it can

    prompt its instantiation by the system, provided the

    objects involved satisfy conditions dened in the CE

    (step 2). Upon recognition of the CE, the control of the

    objects behaviours depends on the effect eld. The

    default effects can be applied to the colliding spheres

    (step 3). Alternatively, these effects can be modied

    during CE instantiation, which will result in a new

    cause-effect association, perceived as alternative caus-

    ality by the user.

    7. The techniques of alternative reality

    Our concept of alternative reality, which is at the

    heart of the ALTERNE Project [11], encompasses all

    descriptions of fantasy worlds in which the elements of

    behaviour underlying alternative laws of Physics or

    imaginary life forms have been described from rst

    principles, using precisely those conceptual representa-

    tions common to art+science. In the search for

    techniques supporting the implementation of alternative

    reality, we have focused our effort on two aspects. The

    rst one is the use of qualitative reasoning, which can

    generate interactive behaviours from the description of

    qualitative laws as generic principles. While qualitative

    physics in itself can address the consequences of user

    interaction in an alternative world, we have indepen-

    dently identied the perception of causality [12] as an

    important element of user experience, already the target

    of contemporary art experiments despite the difculties

    attached with its exploration [13]. This is why we have

    developed, independently of the qualitative physics

    system, a causal engine, whose goal is to support specic

    installations in which the user can be faced with causal

    illusions. Both systems are integrated in the overall

  • high-level principles could be used to implement the

    artistic intentions described in artistic briefs.

    The technical approach for this articial causality

    can be described as follows: as the behaviour of objects

    in virtual environment is under the control of event

    systems, we can use these event systems to associate

    arbitrary outcomes to a given action. This in turn

    generates event co-occurrences that would be perceived

    as causally related by human subjects. In that sense,

    articial causality is potentially a powerful tool to create

    VR experiences, including specic illusions.

    The causal engine operates continuously through a

    sampling cycle, during which it receives low-level events

    and parses them into candidate action representations.

    The essential point is that these action representations

    are frozen during any given cycle, i.e. their conse-

    quences are not enacted in the virtual world. During this

    period of time (unnoticed to the user), the engine can

    substitute new outcomes for the action, prior to its

    reactivation. This substitution is performed by macro-

    operators (MOp) which are knowledge structures,

    which, applied to a CE representation modify the effect

    part of that CE so as to generate a new outcome for the

    frozen action.

    We can now illustrate this, more specically, through

    several examples involving collisions between spheres in

    phenomenon in causal perception. In the world of

    Ego.geo.Graphies, sphere-shaped object-actors may

    collide with one another or with elements of the

    landscape. The effects of a collision between spheres is

    normally expected to be felt on the spheres themselves

    and the nature of the effect will depend on visual cues as

    to their physical properties (i.e. soft/hard, deformable,

    etc.), which can be conveyed to some extent by their

    textures and animations. Because the spheres are all part

    of the same organism, when they collide the basic effect

    should be that they coalesce into a bigger sphere. This is

    represented as the baseline action for spheresphere

    collision (Fig. 6).

    The causal engine can apply various transformations

    to this baseline action. It can for instance replace the

    merging effect with the explosion of one or both spheres

    (by applying a swap effect MOp). As an alternative,

    both spheres can also bounce back from each other

    (Fig. 6). Another way of inducing causal perception is to

    propagate effects to elements of the landscape itself (a

    specic class of operators exists in the system for

    propagating effects). In that instance, the collision

    between two spheres will result in the explosion of

    landscape elements (Fig. 6).

    Fig. 7 details the operation of the causal engine on

    the collision event between two spheres at the level of the

    ARTICLE IN PRESS

    ault e

    M. Cavazza et al. / Computers & Graphics 29 (2005) 852861858the Ego.geo.Graphies brief. It can be noted that

    (although the brief was in no way inuenced by this

    fact) collision between moving objects is the best-studied

    Fig. 6. Alternative effects-inducing causal perception. The def

    disruption of causality correspond to alternative effects.CE formalism [5]. First, the causal engine recognises the

    collision event and instantiates the default action repre-

    sentation for merging spheres (the default consequence),

    ffect consists for colliding spheres to merge. Various levels of

  • ARTICLE IN PRESS

    ir de

    n int

    M. Cavazza et al. / Computers & Graphics 29 (2005) 852861 859while at the same time it freezes its execution. This

    representation can thus be modied to create alternative

    Fig. 7. The causal engine operates by dissociating actions and the

    operators which alter the action parameters while these have bee

    an alternative effect.outcomes for that collision: the nature of this modica-

    tion derives from some parameters of the user interac-

    tion history, thus implementing the dialogue between

    empathy and causality discussed above.

    8. The nal installation: user experience

    The user experience obtained with our platform

    compares favourably with state-of-the-art VR art

    installations in terms of visual aesthetics and user

    interaction. The VR experience can be described as

    resulting from interactive visualisation, from physical

    interaction triggering environmental responses and from

    the observation of autonomous behaviours (of the

    environment or agents that populate it). The world of

    Ego.geo.Graphies blurs the boundaries between the

    organic and the inorganic: the sphere-shaped creatures

    that populate it are constantly generated in various

    regions of the world: they navigate the environment and,

    as they reach a certain density, start colliding with each

    other. The consequences of these collisions correspond

    to levels of causality, which in turn are affected by the

    user interaction.

    The overall user experience of the Ego.geo.Graphies

    installation consists in navigating in the environment

    and perceiving its responses to his/her exploration, in theform of variations of causality induced by the environ-

    ments perceived empathy. The concept of empathy

    fault effects. Actions represented as CE are modied by macro-

    ercepted by the EIS layer. Upon re-activation the action triggerscaptures the relation between the user and the world on

    the basis of his/her interaction with the worlds

    creatures. An empathy value is computed as a function

    of different parameters, measuring the amount of time

    spent in close contact with spheres and the number of

    spheres interacted with. The presence of explicit paths

    (Fig. 8) inside the world facilitates user navigation and

    localisation. They also direct the user to potential

    action zones, like creature emission/collision zones.

    The user navigation is not limited to paths; the user can

    also freely explore the whole terrain, including swamp

    areas. At a human scale, the surface of the map would

    be equivalent to 17,000m2(approximately 130 130m), supporting signicant navigation and explora-

    tion of the environment.

    User interaction consists of navigation and also direct

    physical intervention, as the user can push creatures

    moving around him/her (pressing some controls on the

    tracker), prompting further reactions from the environ-

    ment.

    The behaviour of the environment reects an overall

    level of causality, which manifests itself in the con-

    sequences of collisions between spheres that the user can

    observe. Examples of these consequences include:

    spheres merging (the default world behaviour), spheres

    bouncing away, spheres exploding, spheres collisions

    affecting other elements of the landscape (Fig. 6). The

  • ARTICLE IN PRESS

    in the

    M. Cavazza et al. / Computers & Graphics 29 (2005) 852861860Fig. 8. Explicit navigation pathsactual consequences are computed dynamically by

    modifying the default CE describing the spheres

    behaviour. The user experience and the world behaviour

    are related through the notion of level of disruption,

    which denes how different the perceived causality

    should be from the default behaviour. This level of

    disruption directly controls the kind of transformations

    applied to intercepted collisions in the causal engine.

    In the Ego.geo.Graphies world, the level of causality

    disruption is dynamically updated in relation to the

    perceived empathy, which is calculated from two

    objective parameters that are: user-creature proximity

    and user-agitation (movement amplitude and fre-

    quency).

    The user-creature proximity is a value in [0,1] interval

    corresponding to the average distance between the user

    and the creatures present in a specic radius around

    him/her. The smaller the value, the closer the user is to

    the creatures.

    Fig. 9. Perceived empathy affect the laws of cEgo.geo.Graphies virtual world.The user-agitation represents an appreciation of the

    user movement (velocity and/or rotation speed), ex-

    pressed again as a [0,1] value. A value of 0 means that

    the user is immobile; a value close to 1 denotes a user

    moving fast.

    This level of disruption is frequently updated (every

    25 s). We use a simple matrix (depicted in Fig. 9) to

    determine the amplitude of the causality transformation

    in relation to the user empathy (i.e. user-creature

    (proximity) and user-agitation (movement)). In turn,

    the level of disruption affects causality by determining

    the kind of MOps that will transform CE representa-

    tions associated to ongoing actions. Each type of MOp

    uses the current level of disruption to constraint their

    action, and so the amplitude of the transformation they

    produce.

    We can illustrate this with a MOp that swaps

    effects from the default effect of a collision to an

    alternative one (that is still compatible with the type of

    ausality in the Ego.geo.Graphies world.

  • object considered). As this list has been classied

    regular intervals and is used to compute the level of

    context the systems sophistication is not an obstacle to

    2002;45(1):3942.

    [9] Jiang H, Kessler GD, Nonnemaker J. DEMIS: a dynamic

    symposium on Virtual Reality Software and Technology

    (VRST2003). Osaka, Japan: 2003. p. 1008.

    ARTICLE IN PRESSM. Cavazza et al. / Computers & Graphics 29 (2005) 852861 861disruption using a matrix representation, which associ-

    ates levels of causality to empathy scores. Low empathy

    scores are associated with signicant alterations of

    causality, which translates into the selection and

    application of MOp. This provides a unied principle

    to relate user interaction to user experience through the

    concept of causality.

    9. Conclusions

    One of the objectives of this research was to facilitate

    the development of art+science experiments, or VR art

    installations whose briefs address fundamental concepts

    of interaction. This can only be achieved by providing

    systems that support high-level representations whose

    concepts can be as close as possible to those used in the

    early steps of brief creation. In other words, we have

    tried to evolve the development of VR art installation

    from a software engineering process, in which the artistic

    specications have to be interpreted by a team of

    developers and compiled into low-level representa-

    tions, to a knowledge engineering process, in which the

    system representations remain closer to the original

    abstractions of the artistic brief. This prototype envir-

    onment remains of a signicant complexity and is only

    usable within art+science approaches where VR artists

    have a genuine concern about philosophical issues in

    interaction, realism or articial life. However, in thisplausible to the less plausible alternative effect), the level

    of disruption determines the position of the effect to

    swap with. In the Ego.geo.Graphies brief, alternative

    affects are used to be an expression of an emotional

    state of the actor. Thus, alternative effects classication

    translates an escalation of emotional state, from calm to

    aggressive. For instance, if we consider the collision

    between two spheres, the default effect merge will be

    replaced by expand in a low disruption conguration

    and by explode in a very high level of disruption.

    By relating the level of disruption to perceived

    empathy, we obtain a complex interaction loop in which

    the whole world reacts to the users interaction history.

    The users approach to the world, in terms of navigation

    and interaction modes, will determine the overall world

    behaviour, which will be perceived by the user as more

    or less predictable, and more or less agitated environ-

    ment. Fig. 9 illustrates the processing cycle with its

    updating of the disruption level as a function of

    perceived empathy, and the implications in terms of

    alternative causality. The empathy score is updated at[12] Michotte A. The perception of causality [Translated from

    the French by T. R. and E. Miles]. New York: Basic

    Books; 1963.

    [13] Sato M, Makiura N. Amplitude of chance: the horizon of

    occurrences. Kawasaki, Japan: Kinyosya Printing Co.;

    2001.event model for interactive systems. Hong Kong: ACM

    Virtual Reality Software Technology; 2002.

    [10] Wilkins DE. Causal reasoning in planning. Computational

    Intelligence 1988;4(4):37380.

    [11] Cavazza M, Hartley S, Lugrin J-L, Le Bras M. Alternative

    reality: a new platform for digital arts. In: ACMwe have formalised can be elicited from simple natural

    language descriptions of tables. In addition, we have

    recently developed authoring tools that enable artists to

    browse and modify the conceptual representations

    underlying the system.

    Acknowledgements

    This research has been funded in part by the

    European Commission through the ALTERNE project,

    IST-38575.

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    Intelligent virtual environments for virtual reality artIntroduction and objectivesIntelligent virtual environment: knowledge layer and programming principlesThe illustrative briefsSystem overviewThe VR architecture: stereoscopic visualisation in the SAS cubetradeSoftware architecture: the event interception systemThe techniques of alternative realityThe final installation: user experienceConclusionsAcknowledgementsReferences

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