Crowded Collaborative Virtual Environments

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    PAPERS CHI 97 * 22-27 MARCH 1997The remainder of this paper is structured as follows. Thefollowing section considers the motivations forintroducing an explicit crowd mechanism into CVES. Wethen introduce the underlying mechanism of third partyobjects as an extension to the spatial model. Followingthis, we discuss how third party objects can be used tocreate a variety of different kinds of crowds in CVE3.Finally, we present a demonstration application,implemented using the MASSIVE-2 system, called theArena, which combines static and dynamic crowds with astructured space so as to create a venue for on-lineperformances.MOTIVATIONS FOR A CROWD MECHANISMWe begin by considering the motivations for introducingan explicit crowd mechanism into CVEs (i.e. for providingan additional level of technical support for crowdrepresentation and management beyond just the ability forlarge numbers of individual participants to gather in oneplace).Our fmt motivation is scale. Current CVEs support atmost a few tens of simultaneous users, Although there aresome exceptions, such as the NPSNET battle simulatorwhich claims of the order of a hundred simultaneousparticipants [4], these have generally only been achievedby relying on then being highly predictable behaviorsfor objects (e.g. the movements of ships, tanks andmissiIes) and by reducing the potential for communicationbetween participants (especially with regard to real-timeaudio). There are severrd dimensions to the problem ofscale. First, can the network exchange rich informationabout many simultaneous participants sufficiently quicklyand reliably so as to engender a sense of co-presence?This is a major limitation for systems based on unicastnetwork protocols but will eventually become an issueeven for systems which utilise more network efficientmukicast protocols. Second, assuming that the networkcan deliver this information, can the computers involvedprocess and render it? Third, even if the combination ofnetwork and computer can deliver and display theinformation, can individual participants make sense of it?(e.g., could one make sense of a thousand people speakingat once or view a thousand detailed embodiments at thesame time?).This leads us to our second major motivation, that oflegibility and structure. Complex environments might bemade manageable for users by introducing additionalstructures which group objeets together, provide ag~gateviews of them and which might then be unfolded at a latertime (e.g. on entering them). Some initiat evidence for thisis provided by recent work on enhancing the legibility ofinformation visualisations through the introduction ofdistricts and related features such as landmarks, edges,paths and nodes [3].

    Our final motivation involves generating a sense of mass-presence for specific classes of application. Audiencesplay a key role in various real-world events such a theatre,concerts, sports, exhibitions, fairgrounds, trade shows,rallies, demonstrations and even town centres and publicspaces. A crowd mechanism might therefore enhance oreven create a sense of audience presence andparticipation in CVB and might open up opportunities fornew forms of social interaction.THIRD PARTY OBJECTS AND AWARENESSOur framework for realising crowds in CVEs is based onour previous spatial model of interaction and in particular,on a recent extension to the model called third partyobjects. The spatial model defines mechanisms for themanagement of awareness and communication in sharedvirtual spaces [2]. To briefly summarise, the modelconsiders a number of objects in a shared virtual spacecommunicating through different media (e.g. audio,graphics, text and video). Instead of having each objecttransmit its information to all other objects, basicconnectivity is enabled through the concept of aura - avolume of space that delimits the presence of an object ina given medium. Thus, aura collisions lead to connectionsbeing established. The quality of any information which issubsequently transmitted (e.g. the volume of audio or thelevel of detail of graphics) depends upon the level ofawareness that the observer has of the observed(awareness is a quantifiable concept in the model). This inturn is negotiated through fcms and nimbus. Focus is asub-space representing the attention of the observer andnimbus is a sub-space representing the projection ofinformation by the observed. The observers awareness ofthe observed is then some function of the observers focuson the observed and the observeds nimbus on theobserver, Aura, focus and nimbus may be mediumspecific, multi-valued, dynamically changeable and neednot h strictly spatial in their definition (i.e. they need notbe simple discrete volumes of space).This basic model is limited in two main ways: its hi-lateralapproach to interaction does not easily scale to largenumbers of participants and, beyond a limited concept ofadapter objects, it provides no support for introducingcontextual factors into awareness negotiations (e.g. forrepresenting the effects of the environment within whichthe observer and observed find themselves). Third partyobjects have been introduced in order to address theseproblems. A third party object is an independent objectwhich affects the awareness between other objects. Thebasic scenario (in any given medium) is therefore now oneof three objects, each with individual awarenessrelationships to the others (see figure. 1).

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    CHI 97 * 22-27 PV!AW:H 1[>(~ PAPERS

    Figure 1: Introducing third party objectsThree general points should be noted about third partyobjects from the outset. FirsL all aspects of their operationas described below may be medium specific (e.g. theymay operate differently in the audio medium than in thegraphical, textuaf or video media). Second, as they areobjects in their own right they may be embodied, mobileor fixe4 dynamically or statically created and may applytheir effem recursively to one another. Third, althoughthey are most often described in spatiat terms in thispaper, they may operate according to non-spatialawareness relationships (i.e. one could define them interms of arbitrary attributes of objects).There are three key aspects to third party objects: theireffects (i.e. what they do to awareness relationshipsbetween other objects), their activation (i.e. when and howthese effects are brought into operation) and their creationand destruction (i.e. how they are introduced to andremoved from the environment). We now consider each ofthese.The effects of third party objectsThird party objects can have two general kinds of effecton awareness (see figure 2) which may be applied indifferent combinations across different communicationmedia. 635secondary sourcingA Badaptation

    Figure 2: the effects of third party objectsAduprafion involves the manipulation of existingawareness relationships between objects. In this sense,third party objects are a generalised notion of the adaptersthat were defined in the initial spatial model. Thesemanipulations include attenuation (e.g. a bamier betweenobjects) and amplification (e.g. increasing awarenessbetween people who are accessing a common object).Secmda~ sourcing involves the introduction of newindirect awareness relationships between objects in order

    to enable new transformed flows of information betweenthem. Typically, secondary sourcing involves theconsumption of information from an external group ofobjects, its transformation in some way and its subsequentre-transmission in order to provide a common view of thegroup. Various filters may also be applied at differentstages of this process in order to reduce level of detail orto select key information. At the heart of secondarysourcing lies the problem of creating a single aggregateview or stream of information from a number of sources.We propose that there are three approaches to theaggregation problem:q selection - switching between individual views or

    streams in some way (e.g. round robin, loudest winsetc.)

    cornbifwtion - the direct composition of a new view fromexisting views (e.g. tiling multiple video windows).

    abstraction - generating an entirely new representationbased on statistical information describing the sources(e.g. mapping the number of sources into the size of therepresentation, the level of their activity into COIOLUetc.).

    We will provide concrete examples of these classes oftransformation when specifically discussing crowds lateron.The activation of third party objectsNext we consider the circumstances under which differentcombinations of these effeCLs are applied. l%e activationof third party objects is bassed on the awarenessrelationships between the third p,arty and the other objectsinvolved. Thus, referring to figure 1, the activation of Tdepends on four possible awareness relationships: Tsawareness of A and B respectively and their awareness ofit. In figure 3 we identify three particularly interestingcases from among the various possibilities.Gf$fi!

    )mwnhcr.ship h) sharing C) hybridFigure 3: Activating third party objects

    u) mmhmhip - cases where the third party is activatedaccording to how aware it is of other objects. This isanalogous to the idea of membership (i.e. the third partysawareness of an object expresses the degree ofmembership of that object). For example, one mightbecome a member of a room by crossing its boundary.b) sharing - cases where the third party is activatedaccording to how aware other objects are of it. This is

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    PAPERS CHI 97 * 22-27 MARCH 19analogous to the idea of objects sharing the third party insome way and consequently, it having an effect on the