The crowd in the cloud: moving beyond traditional boundaries forlarge scale experiences in the cloud

Adam Roughton1 John Downs1,2 Beryl Plimmer1 Ian Warren1

1 Department of Computer ScienceUniversity of Auckland, New Zealand

Email: arou012@aucklanduni.ac.nz {beryl|ian-w}@cs.auckland.ac.nz2 Department of Information Systems

University of Melbourne, AustraliaEmail: j.downs@pgrad.unimelb.edu.au

Abstract

In this paper we propose a taxonomy for crowd basedinteraction paradigms, and categorise the literatureaccording to this taxonomy. The conventional defini-tion of crowds needs to be reconsidered in the light ofadvances in communication technology such as smartphones and cloud based infrastructures. We have ex-tended the definition to encompass virtual dispersedcrowds by considering the core components of crowdbased activities. We found that much of the exist-ing work offers simplistic reactive control in exchangefor economical, highly synchronous, co-operative ac-tivities. We argue that the same co-operative compo-nent and economy can be obtained with rich reflectivecontrol. By combining the cloud, smart phones, andtools, this gap can be exploited to create a new classof rich, thought provoking, economical, crowd com-puting.

Keywords: Crowd computer interaction, crowd com-puting, crowd and the cloud, collective behaviour

1 Introduction

Humans are an inherently community-based species;we associate with nations, societies, and clubs. Ourinteraction with society at large is often at times ofshared activity: events like the Olympics are able toengage the passions of entire nations. There existslittle technology that attempts to engage communi-ties in crowd activity. Work in the emerging field ofcrowd computer interaction attempts to address thisshortcoming.

Crowd computer interaction first came to promi-nence in 1991 during the SIGGRAPH Electronic The-atre show with the deployment of Cinematrix Interac-tive Entertainment System (Carpenter, 1993). Eachaudience member was given a double sided paddlewith a green face on one side and a red one on theother; cameras tracked the paddles in real-time allow-ing for collaborative audience activities. In this waythe audience was able to collectively control a singlepaddle in a game of pong, vote on an issue, or movethrough a maze (Maynes-Aminzade et al., 2002; Bre-gler et al., 2005). The system was invented to getaround the expense and the required preparedness ofwiring seats with controllers, or the need to hand outexpensive wireless controllers to audience members

Copyright c©2011, Australian Computer Society, Inc. This pa-per appeared at the 12th Australasian User Interface Confer-ence (AUIC2011), Perth, Australia, January 2011. Conferencesin Research and Practice in Information Technology (CRPIT),Vol. 117, Christof Lutteroth and Haifeng Shen, Ed. Reproduc-tion for academic, not-for profit purposes permitted providedthis text is included.

(Carpenter, 1993). Much of the subsequent work inthis area has built on this foundation of throwawaysensors deployed to a co-located crowd (or in somework no controller at all) and a large shared screenfor feedback.

Crowds present a unique problem space for thedeployment of technology. This is because of the or-ganisation and composition of individuals within thecrowd, and the sheer scale presented by the possiblenumbers of participants. The organisational struc-ture of the crowd takes on a looser form than groups(Turner and Killian, 1972): crowds are far more ad-hoc in their nature, and in most cases there is no clearleadership. Crowd membership is not as stringentlyregulated as groups with participation often open toanyone who is able to take part. The relaxed formof membership and the large number of people leadsto a collection of many different cultures and iden-tities. Within the larger crowd aggregate are manysmall sub-crowds and groups with their own idiosyn-crasies and traditons (Reicher, 2002). Behaviours ap-propriate to the traditions of one sub-crowd may notbe appropriate for another. How the crowd works asa whole to work around these differences, or how itmight attenuate or exaggerate issues remains a newarea of inquiry for technology solutions deployed intothis arena.

In this paper we considered how two new technolo-gies, smart phones and cloud computing, can changecrowd participation.

The recent proliferation of smart phones equippedwith accelerometers, compasses, microphones, andgyroscopes eliminates the need to build, develop, ordistribute sensors. This paired with the rich inter-faces provided by the devices opens up the possibil-ity of richer crowd experiences. Application market-places have provided the ability to quickly distributeapplications to client phones, eliminating the originalproblem of sensor deployment cost.

Cloud computing provides the second piece of thepuzzle: the cloud computing paradigm reduces entrybarriers with reduced infrastructure costs and quickaccess to computing power and storage (Greengard,2010). The potential benefits of cloud computingto crowd computing are vast. By their very nature,crowd applications benefit from the elasticity offeredby the cloud (Owens, 2010): the ability to scale upa modest infrastructure for short durations at a verymodest cost fits the typical crowd application scenariobeautifully.

The combination of smart phones and cloud com-puting opens the possibility for rich, economicalcrowd computer interaction without the need for co-location.

This paper first proposes a new definition ofcrowds for the purpose of crowd computer applica-

tions based on the developments in communicationtechnologies. A taxonomy based on the key compo-nents of crowd activity is then presented, followed bya categorisation of applications in the literature. Adiscussion of the trends and gaps found in the cate-gorisation follows before the conclusion of the paper.

2 Definitions

Some definitions are in order before a taxonomy canbe approached.

First and foremost, what is meant by the ‘crowd’needs to be defined. Formally, the American Psychol-ogy Association defines a crowd as “a sizable gather-ing of people who temporarily share a common focusand a single location” (VandenBos, 2007, pg 247).This definition is rather ambiguous for the purposesof designing applications for crowds. What is a ‘siz-able gathering’? How long is ‘temporarily’? Withthe advent of technologies that support large num-bers of people in a shared activity without spatialconstraints (Twitter, 2010; Blizzard Entertainment,2010), the boundaries of what has traditionally beendeemed a crowd are challenged.

A novelty for research into crowd based interactionis the unique form of behaviour found in crowd basedsettings. Turner and Killian (1972) define collectivebehaviour as all those behaviours found in collectivesthat are not based on the traditional behaviour of so-ciety (Turner and Killian, 1972). In conventional set-tings an individual is subject to the social norms ofsociety, the workplace, or teams: established operat-ing procedures guide what behaviour is appropriate.They go on to suggest crowd behaviour is similar tothat of groups in that members interact with one an-other with a sense that they constitute a unit; thebehaviours of members are clearly influenced by theother members, and social norms are arrived at by all.Where crowd behaviour differs from group behaviouris the formation of these social norms. Unlike groups,crowds have a looser structure with regards to mem-bership and leadership: member participation is un-regulated by any existing structure, and leaders arethose that the crowd chooses to follow at a given mo-ment. While groups have procedures that are basedon society at large, crowds have a far more sponta-neous form of behaviour that stems from the loosenessof structure; some norms may reflect those in society,others might outright reject them. There may in factbe competing social norms in a crowd. The interestof crowds for technology deployment are supportingcrowd activities for large numbers of people with thiscollective behaviour.

For the purposes of this paper, a crowd is definedas a large collective of people under a synchronousshared influence, where each member is connected toone another via some communication medium, andall members have the potential to participate in thecollective activity.

This definition raises points that need to be ad-dressed:

• collective activity: refers to the defining activityof the crowd; viewed as the focal point for a givencrowd. This is linked to both explicit membershipidentity (I am part of the crowd watching the AllBlacks vs Wallabies game at Eden Park), and im-plicit membership identity (I am part of the crowdwatching the All Blacks vs Wallabies game at EdenPark singing the New Zealand national anthem).Sub-crowds are often identified by specifying theactivity with greater precision. In the preceding ex-ample, Australian supporters in the stadium crowdare often not able to participate in singing the an-them due to social pressure; they are hence not

part of the singing the New Zealand national an-them sub-crowd, though they still spectate its out-put. They still retain membership in the greaterstadium crowd along with those participating inthe anthem.

• large collective: the crowd size has been definedambiguously on purpose: existing work in this areahas crowds ranging from less than a hundred par-ticipants through to the tens of thousands. Whatdistinguishes crowds from traditional groups is thepresence of collective behaviour. The size of thecollective must therefore be large enough for thisbehaviour to be a property of the crowd. The mod-ifier large also hints at the requirements of any po-tential system being able to cope with tens of thou-sands of members, while also being able to supportsmaller gatherings.

• communication medium: each crowd member isconnected in some way to every other crowd mem-ber via some medium. In this way, each member isable to have some input into the collective activity.In co-located settings, this medium is most oftenmade up of visual and aural channels: crowd mem-bers can be seen and heard by others in the crowd,and their participation forms a part of what is seenas the crowd.

• synchronous shared influence: the influencerefers to the output of the collective activity; eachmember has an awareness of their role in the crowdand is affected in some way by the output of thecrowd. All crowd members semantically share thesame influence simultaneously, but perhaps via dif-ferent mediums.

• potential to participate: all crowd membersmust be able to contribute input towards the activ-ity’s output; any choice to not actively participateis based solely on the individual’s choice not to par-ticipate. Other limiting factors including physical(e.g. no means of participating), social (e.g. peerpressure), and temporal (e.g. outside of the activ-ity’s timeframe) limitations may prevent an indi-vidual from being considered part of the crowd.

Communication Medium

Spectators

Participants

Output (∑ action effects)

Shar

ed In

flu

ence

Actions

Crowd

Figure 1: Components of a crowd activity.

The above definition specifies that those with thepotential for participation are members of the crowd.Spectators of a crowd activity already satisfy the cri-teria for shared influence (spectators are influencedby the output). Hence, it is implied that all specta-tors of a crowd activity are also members of the crowdas long as they have the potential for involvement inthe activity. This potential for participation implies

that spectators share the communication medium ofparticipants and so are able to input into the activ-ity. Every crowd activity (collective activity) involvesa set of actions made by participants. The completeaction set defines the crowd activity. Each action hasat least one consequential action effect. Figure 1 putsthe components of a crowd activity together accord-ing to the above definition.

As an example, the well known stadium activityof the “Mexican wave” has two actions: standing upwith one’s arms outstretched, and remaining seated.Both of these actions are crucial to the recognisableactivity: participants who are not part of the wavemust remain seated until the wave reaches their partof the crowd, and stand only for the instant at whichthe wave reaches their seat section. The effect of eachaction is visible in the output of the activity, namelythe movement of the wave around the stadium. Allcrowd members are able to spectate the output of thecrowd by watching the participant constructed wavemove around the stadium.

3 Taxonomy

A taxonomy provides a useful method of exploringthe domain space: prior and ongoing work can berelated to the domain as a whole. This is especiallyuseful in the crowd computer interaction domain asthere has been little work in the area of classificationof applications in the domain space.

The taxonomy presented here builds on the ob-servations made from existing work, with the help ofsociological and psychological research into the phe-nomenon of crowds.

3.1 Intention

In classifying the interaction individuals have bothwith and within the crowd, an important distinctionmust be made between the actions and the associatedintentions, due to the occurrence of collective inten-tion in crowd activity.

In the case of a single individual, an intention leadsneatly into an associated action. However, in the caseof crowd or group intentionality, experience tells usthat a shared intention comes into play. One proposi-tion terms this shared intention ‘collective intention’(Searle, 1990). Searle argues that a collective inten-tion is not the mere result of individual intentions, butthat it is a distinct primitive construct of its own. Anindividual specifies the intention with the form ‘weintend’ to do such and such, as opposed to ‘I intend’.Searle further stipulates that ‘I-intentions’ then de-rive from the collective ‘we-intentions’: a collectionof soccer players has the intention ‘we intend to per-form play X’, with each player having an individualintention of the form ‘we intend to perform play X bymeans of my action Y’.

Although this is still an ongoing discussion in phi-losophy, the concept of ‘we-intentions’ is useful indescribing the intuition that collective intentionalitymanifests different behaviour in group settings thana mere collection of single individual intentions.

At the top level, crowd activities can be separatedbased on the intention makeup of the crowd duringthe activity:

• Individual: crowd members have many differentI-intentions

• Shared: crowd members share the same intention

Activities that are made up from members of thecrowd with different I-intentions (or in some cases we-intentions) are treated first. In this class of activ-ity, individuals have their own goals and objectives.

These objectives are performed inside the activity do-main: members recognise themselves as actors withinsome common activity. Street crowds are an exampleof this form of activity that many of us are famil-iar with; as actors within the crowd, each person hastheir own intentions to get to their intended desti-nations, or perhaps even to just wander the streets.On entering the crowd, they enter into the shared in-fluence of the crowd activity, namely the flow of thecrowd. Whether or not they wish to be in the crowd,each actor recognises themselves as part of the crowdas they pass through it.

Activities of this type are by no means alwaysmade up incidentally in the same way as streetcrowds; participants may wish to engage in someshared activity by which the presence of others in theactivity is required for the experience, but the inten-tions of participants is individual. Within a stadiumcrowd a member might wave a banner of support fortheir team: this action requires the presence of othercrowd members for the action to have the intendedeffect, but does not require all crowd members havethe same intention.

Shared intention activities are defined by crowdmembers sharing the same intention. The activityclass can be further divided based on the type of theshared intention:

• Co-operative intentioned activities are those whereevery participant (or most participants) of thecrowd have the same we-intention. These activ-ities involve all participants working together tosolve some common objective.

• Separate intentioned activities involve every par-ticipant having the same I-intention. An exam-ple of this class of activity would be a competi-tive pull on some shared resource. Voting systemsalso fall into this class; although individual mem-bers may vote for separate outcomes, the activ-ity of voting is a shared I-intention. Activities ofthis class might also appear to be shared activitiesfrom observation: crowd technologies that aggre-gate the input of all participants to control someshared control (an example here would be the ponggame from (Carpenter, 1993)), might result in in-dividual crowd members having I-intentions (I in-tend to move the paddle left) if the crowd doesnot communicate within itself. However, if crowdmembers co-ordinate themselves through commu-nication such as yelling, and cheering, the activitybecomes a co-operative one.

3.2 Interaction setting

The interaction form describes the configuration ofthe actions contained within a single interaction withthe system. A single interaction is defined as startingfrom the point of first input until the point of out-put presentation. Each interaction setting describesboth the origin and the destination for the action(s)(Whitworth and Plimmer, 1997):

• One-to-many (1:N): each interaction involvesthe input of a single crowd member with output(action effects) presented to many crowd members.In this way the action is amplified. An example isthe activity of hitting a beach ball around a crowd:each strike of the beach ball is made by a single (ora marginally small subsection) of the crowd withthe action effects of the raised ball visible by all.

• Many-to-many (N:M): each interaction involvesthe input of many crowd members with output pre-sented to many other crowd members. An exam-ple of this is crowd pong (Maynes-Aminzade et al.,

2002), where all crowd members act together tocontrol a single paddle. In this instance a singleinteraction involves the input of all participants,with the output directed to all crowd members.

• Many-to-one (N:1): for each interaction, the in-put of many is presented to a single crowd member(or a very small subset of the crowd). The ac-tion effects are therefore aggregated. This can beseen in stadiums where a crowd member is singledout on a large display, and adjusts their behaviourbased on the feedback of the crowd.

3.3 Interaction style

Describes the style of interaction encouraged or re-quired by the activity.

• Reflective: the input into the activity is basedon reflection and real creative input; the activityencourages participants to reflect before they con-tribute to the output. Reflective actions allow pur-poseful and creative input into the system: mem-bers might use strategy to plan their action, or theinput might require creativity in expression or con-tent. An example of this can be found in the use ofTwitter during events: each ‘tweet’ requires care-ful reflection to properly express the thoughts ofthe participant before the expression is presentedto the crowd.

• Reactive: the input is based on quick reactionto some stimuli: crowd members are encouragedto act quickly for their contribution to the out-put. Reactive actions are based on coordinationand speed: members need to quickly process thestimuli and react accordingly. Activities like crowdpong (Maynes-Aminzade et al., 2002) require thatall participants lean in time to control the onscreenpaddle; participants must time their lean carefullyto move the paddle into the path of the oncomingball.

3.4 Social Exposure

The degree to which an individual in the crowd is‘socially exposed’ depends on three factors:

• Visibility of the action effects: this factor de-scribes how visible the action effect is to othermembers of the crowd. Highly visible effects arethose that are accessible by all members, while ef-fects with low visibility are accessible by only a fewmembers.

• Strength of the association between an in-dividual and their action effects: a strong as-sociation allows crowd members who can see theeffect to be able to always find the individual whoperformed the action. A weak association requiressome effort to be made by crowd members who canperceive the effect, while no association means thatan individual can never be linked to their actions.

• Duration of the action effects: a long durationmeans that the effects of the action are visible for along period of time, while a short duration meansthat the effects of an action are quickly removedfrom the activity.

3.5 Crowd member connectivity

This describes the ‘closeness’ of crowd members interms of communication channels: crowd membersare close if they share a large amount of informationamongst themselves. The information relayed by one

member to another depends on three factors relatedto the communication channels between them (Whit-worth and Plimmer, 1997):

• Channel type: describes the media type of thechannel. Primary channels relate to the senses;these include visual, audial, and tactual amongstothers. Symbology is a secondary channel thatrelates to the symbology found in communicationsuch as language.

• Number of channels: the number of communi-cation channels in the media space.

• Channel bandwidth: describes the informationcontent capacity and speed of the channel. Highbandwidth channels convey a large amount of in-formation quickly, while low bandwidth channelsconvey information at a slower rate.

Co-located crowds have a large number of highbandwidth channels of different media types due tothe richness of the shared media space. Dispersedcrowds may also have these same channels, but at alower bandwidth. Alternatively, they might only havea subset of these channels, and some channels mightrequire mediation by technology.

4 Categorisation of existing research

With the lens of the taxonomy in place, existing workin the crowd computer interaction domain can be cat-egorised and compared. This section will place exist-ing work into each of the identified dimensions of thetaxonomy with justification.

4.1 Intentionality

Figure 2: A game of crowd pong is played using crowdleaning as the input to control the paddle (Maynes-Aminzade et al., 2002). This is a classic example of ashared input system: co-operative intentions are cre-ated by requiring the crowd to co-ordinate themselvesas if a single unit.

Co-operative intentions can be classified intothree themes.

First, Aggregate control: the Cinematrix system(Carpenter, 1993) and its derivatives (Fisher et al.,1997; Dannenberg and Fisher, 2001) have applicationsthat aggregate the input of all participants (using theaverage) and map this to a single control. Applica-tions like crowd pong and collectively steering a car(Carpenter, 1993; Maynes-Aminzade et al., 2002) re-quire the crowd to co-ordinate themselves by turningtheir paddles to either the red or green side in somerequired proportion: the system reads the input of allpaddles in real-time and moves the paddle or vehicle

based on the aggregate of the crowd. The cell mem-brane game of Fisher et al. (1997), based on the sametechnology of Cinematrix, requires that the audiencebalance the ion content of cells by keeping the aggre-gate value of the paddles at some equilibrium. Thesame idea is found in the work of Maynes-Aminzadeet al. (2002) and derivatives (O2, 2009; Terdiman,2007) who instead aggregate crowd input using ma-chine vision to capture crowd leaning (see figure 2).

Second, Shared control: requires the crowd co-ordinate a single control around crowd members. Thegames of Missile command (Maynes-Aminzade et al.,2002), Time Bomb (Sieber et al., 2008), and Squid-ball (Bregler et al., 2005) use large inflatable ballsas the input of the system. The spatial property ofthe crowd is utilised to encourage crowd members toco-ordinate control: members act to move the ballaround the crowd to achieve the required input. TheMissile Command game uses the shadow of a beachball as the cursor for the game: crowd members musthit the ball with a certain velocity and direction tosuccessfully destroy the missiles shown on screen. TheTime Bomb game divides the audience in two andrequires that members closest to the virtual beachball (shown on a screen that projects a mirror imageof the audience) hit it to the opposing side. Squid-ball requires that the crowd move sixteen helium filledweather balloons to different locations in the crowdto score points. In each of these instances, the crowdneeds to work co-operatively to co-ordinate control ofthe input.

Third, Power-by-numbers: the numbers found inthe crowd are utilised to achieve a shared goal. TheRed Nose Game (O’Hara et al., 2008) requires thatparticipants push together virtual red blobs shown ona projection of the crowd displayed on a large pub-lic screen (see figure 3). Crowds of one city com-pete with those in others by attempting to put thered nose back together as quickly as possible. Co-operative intentions are created here as more par-ticipants join in to complete the shared goal: mem-bers work together to bring the pieces back into awhole as quickly as possible. In a similar way themulti-user connect-the-dots game (Maynes-Aminzadeet al., 2002) requires that audience members positiondots emitted by lasers in the required configurations;members must work together to complete the task.“Connect-the-dots required each audience member toposition his laser over a different dot, and since itrequired the audience to cooperate in order to suc-ceed it was a more social game” (Maynes-Aminzadeet al., 2002, pg 5). The traditional Mexican Wave hasbeen augmented with technology to encourage crowdco-operation. The crowd system created by Uplause(Uplause, 2009) requires that members work togetherto create an effective wave.

In each of these approaches the crowd is encour-aged to work co-operatively by requiring the co-ordination of all members to achieve the shared goal.Properties of the crowd such as the number of crowdmembers and the spatial layout of the crowd are usedeffectively to encourage the shared we-intentions.

Separate intention is evident primarily in votingsystems. Participants all have the same intentionduring each interaction: vote for some preference.Although the preference may differ between crowdmembers, the intention for interaction is the same(I-intend to vote), as opposed to individuals havingdifferent intentions during each interaction. The lit-erature presents variations on the voting theme. TheCinematrix system (Carpenter, 1993) and the work ofMaynes-Aminzade et al. (2002) each present methodsby which the crowd can vote on selected issues, usingpaddles and lasers respectively. The cheering systemof Barkhuus and Jørgensen (2008) allows crowd mem-

bers to vote with applause and cheering. The vot-ing metaphor can be extended to other systems likethe Hewlett-Packard DJ (HPDJ) system of Graham-Rowe (2001) whereby the DJ system automaticallypicks songs based on how the crowd reacts to them;a genetic algorithm is used to judge which songs havebeen successful based on who remains on the dancefloor among other metrics. During each interactionwith the DJ system, crowd members vote with theirreaction to the system. Likewise the dance temposystem of Feldmeier (2003) allows crowd members tovote for the tempo of the song by the amount of activ-ity they engage in during their dancing (a sensor mea-sures the amount of force in the dancing). Althoughcrowd members might have their own intentions dur-ing the activity, they share the same I-intention whenthey interact with the system.

Other systems whereby the participants are pre-sented with a goal, but not required to co-operateto achieve it are also shared intentioned activities.Shared I-intentions can be seen in activities like theWhack-a-Mole and picture reveal games of Maynes-Aminzade et al. (2002): “In a game like Whack-a-Mole, each audience member is involved in the activ-ity for himself, much like a game of soccer played byyoung children in which everone clusters around theball” (Maynes-Aminzade et al., 2002, pg 5). The for-mer game requires that crowd members shine a laseron an image of a mole that shows on screen, while thelatter has crowd members removing the black layerobscuring a hidden image using their laser pointers.

Individual intention, the final type of intention,is represented by crowd activities with no unifyinggoal or objective. A pattern seen in all the existingwork classified with individual intentions is the openended input each activity provides.

The collective paint application of Maynes-Aminzade et al. (2002) allows participants to drawwith laser pointers on a shared canvas. In thisinstance, crowd members might create co-operativesub-activities if they engage with one another; how-ever, in the general case without system interven-tion, participants form their own creative intentions(I-intend to draw such and such). The distinctionis made from shared intentioned voting activities ascrowd members engage with freedom as to the goalof the action, as opposed to voting systems where thegoal is singular (to vote).

The interactive dance club (Ulyate and Bianciardi,2002) allows participants to influence the music andlighting of the dance club environment by using ‘in-teractive zones’ situated around the club. Each zoneencourages crowd members to engage with unique in-terfaces to influence the various parts of the environ-ment. Each individual has their own intentions asto how they will influence the shared environment:much like the canvas in the paint application, the en-vironment around the crowd becomes the canvas andthe interaction zones the paintbrush.

The Pirates! game (Falk et al., 2001) is a collectiveactivity very much in the spirit of traditional gameslike hide and seek: players act as ship captains andmove around a shared arena with their own questsand objectives. The pervasive game augments thephysical arena with a virtual environment (Falk et al.,2001). Although individuals have their own goals andobjectives throughout the game, the activity itself iscreated by the involvement of all crowd members.

4.2 Interaction setting

Many-to-many interactions, whereby many partici-pants contribute to the output in a given interaction,are the most prolific in the identified existing work.The aggregated input systems of Cinematrix based

applications (Carpenter, 1993; Fisher et al., 1997;Dannenberg and Fisher, 2001) and crowd leaning ap-plications (Maynes-Aminzade et al., 2002; O2, 2009;Terdiman, 2007) take input from many members fora given interaction and display this on a shared dis-play. The same is true for others of Maynes-Aminzadeet al. (2002): connect-the-dots, whack-a-mole, picturereveal, paint; and the Red Nose Game (O’Hara et al.,2008), which use individual participant input insteadof aggregation. The interactive dance club (Ulyateand Bianciardi, 2002) operates in a similar way withthe environment (lights, music) as the shared outputinstead: participants interact with different stationssimultaneously, and in many cases share a stationwith others.

The voting systems require the input of all par-ticipants before the output is created during a giveninteraction: therefore the cheering meter (Barkhuusand Jørgensen, 2008), real-time voting systems (Car-penter, 1993; Maynes-Aminzade et al., 2002), dancetempo (Feldmeier, 2003), and the HPDJ (Graham-Rowe, 2001) all have the many to many interactionstyle.

The Pirates! game (Falk et al., 2001) requires theinput of many participants during each interaction;although it can be played as a single player, the ac-tivity is only considered a crowd activity with manyparticipants (which is the intended usage (Falk et al.,2001)). The input of many participants, both the vir-tual manipulation as well as the physical presence, istaken in within each interaction with the system: theoutput (game environment as well as the physical en-vironment created by the presence and movement ofthe players) is then given to all participants.

One-to-many interaction is most represented inco-located applications where crowd members shareda single input mechanism amongst themselves. Sin-gle crowd member actions are broadcast to the crowdas a whole during one interaction with the system.The missile command game (Maynes-Aminzade et al.,2002), the time bomb game (Sieber et al., 2008), andSquidball (Bregler et al., 2005) all use an inflated ball(virtual in the case of the time bomb game) to controlthe application displayed on the large screen; mem-bers in the crowd control the application one at atime (or in small isolated groups) while the rest ofthe crowd spectates. Although the crowd may actas one unit (shared we-intention), the interaction ismade by a single crowd member.

Twitter (Twitter, 2010), when used in a real-timecapacity, also uses the one to many interaction set-ting. Participants post their message to the rest ofthe crowd during a single interaction with the activ-ity; the crowd output (message stream) is manipu-lated by single individuals during a given interactionwith the system.

4.3 Interaction style

Reflective style applications allow participants to re-flect on the system state before they interact. Reflec-tion allows strategy, expression, and/or creativity foreach interaction with the system.

The Pirates! game (Falk et al., 2001) gives eachparticipant objectives that they must complete tomove forward in the game. Players search for trea-sure, compete in battles, and trade with other playersto increase both their rank in the game (measuredwith experience points), and the amount of moneythey have for upgrading ships. Participants are ableto reflect on the system state during every interactionwith the system by observing their individual displayand interacting accordingly: there is no pressure toreact to a stimuli without consideration of the state.Even in the case of sea battles, whereby one player

engages another in proximity, participants are able toreflect on the state of their ship and resources whilethey engage in the battle, and strategically plan theirattack or defence.

The voting applications using the Cinematrix sys-tem (Carpenter, 1993), or laser pointers (Maynes-Aminzade et al., 2002) are reflective in that they al-low participants to consider and reflect on the op-tions available during each interaction: in the work ofMaynes-Aminzade et al. (2002) participants are givena topic to vote on and those with laser pointers in thecrowd shine their laser beams on their choice, bring-ing in prior knowledge from their own experiences aswell as the knowledge presented by those yelling inthe crowd. In a similar way, the cheering system ofBarkhuus and Jørgensen (2008) allows participants toreflect during the performance of the artist that theyare to judge, and then bring this knowledge into theirinteraction with the system.

The remaining applications identified with reflec-tive interfaces use expression as their primary reflec-tive mechanism. The use of Twitter (Twitter, 2010)during real-time events allows participants to reflecton the current thread of conversation on the topic andrespond accordingly with their own thoughts and feel-ings. Again there is no pressure to immediately inter-act without first reflecting on the crowd output. Theinteractive dance club of Ulyate and Bianciardi (2002)allows participants to manipulate the music and light-ing in the dance environment in a very creative way:participants are free to explore the musical and visualcanvas by modifying the various components of themusic and lighting. The paint application of Maynes-Aminzade et al. (2002) allows creativity in much thesame way: participants are free to express themselvesthrough the images they draw on screen.

Reactive interfaces, in contrast, require partici-pants react quickly with speed and/or co-ordinationto the system state. Work classically identified ascrowd computer interaction mostly centre around re-active interfaces, with interaction made in the mo-ment. Games like crowd pong (Carpenter, 1993;Maynes-Aminzade et al., 2002); collectively drivinga car, connect-the-dots, whack-a-mole, picture reveal(Maynes-Aminzade et al., 2002); and the Mexicanwave (Uplause, 2009) require participants react to theoutput shown on screen (and with the crowd itself forthe Mexican wave), and react accordingly for success-ful input.

The activities that use an inflated ball for control(Bregler et al., 2005; Maynes-Aminzade et al., 2002;Sieber et al., 2008) all require the participant(s) cur-rently engaging with the interface to react to the on-coming ball and steer it in the required direction.

In the Red Nose game (O’Hara et al., 2008) partic-ipants are presented with a fragmented red blob thatthey have to piece back together by pushing the vari-ous sections into one. Although participants are ableto observe and perhaps reflect on the system stateon-screen at any time, the primary interaction modeinvolves quickly reacting to the position of the largestblob, and moving the closest blob towards it: the sim-plicity of the task means that the reflection time isminimal at best as other participants will quickly actto move the blob if it is not moved immediately bythe player.

The dance tempo system of Feldmeier (2003) al-lows participants to influence the music and lightingusing distributed sensors that detect force: the sys-tem measures the amount of activity in the dancingand adjusts the music and lighting accordingly. Par-ticipants react to the current state of the music andlighting as they dance, and attempt to influence theintensity up or down with their sensor movement.

The HPDJ system (Graham-Rowe, 2001) involves

participants reacting to the music in an indirect way.The HPDJ system measures the response to the var-ious music tracks it attempts in engaging the crowd.Participants are presented with a stimuli (the mu-sic) and evaluate this in real-time by reacting withtheir activity. The dancers don’t reflect on the in-put and judge, as with the other voting systems, asthe reactions are mostly involuntary: if the track hasa beat that suits a given participant, they will reactpositively with their dancing; otherwise they mightdecrease their activity or leave the dance floor alto-gether.

4.4 Social exposure

Figure 3: The Red Nose game (O’Hara et al., 2008)pictured here has the participants highly socially ex-posed as their actions are clearly visible and highlyassociated to the actions they make.

Participants in the Time Bomb game (Sieber et al.,2008) are highly socially exposed as their actions arehighly visible, and a clear association can be made be-tween the action and the participant. A large screenin front of the audience displays a mirror image ofthe crowd; overlaid on top of this is a virtual timebomb (with behaviour like a beach ball) that can bemanipulated by the audience using gestures. The au-dience is divided into two competing teams with theobjective to have the bomb explode on the opposingteams side of the tiered seating. During each interac-tion with the system, a single crowd member becomeshighly visible as they attempt to swat the bomb away.The member is able to be readily linked with theiraction as they are displayed on the large screen nearthe focal point of the crowd as they hit the bomb.Although the duration of the actual action is short,the effects of the action are potentially longer lastingas members remember fumbles or successful saves. Ina similar way participants in the missile commandgame (Maynes-Aminzade et al., 2002) are also con-sidered highly socially exposed as crowd members hita real beach ball around the audience to control theaction on-screen. This game requires more in the wayof precision as the player must hit the ball with boththe right force and direction to successfully preventthe missile from hitting the city.

Real-time tweets expose the participants due tothe nature of the Twitter service (Twitter, 2010):each tweet is identified by the user who created it andbecomes a highly visible part of the event’s Twitterstream; the action effects are a highly visible compo-nent of the crowd output. The duration of the tweetin the event’s Twitter stream can vary based on thenumber of participants actively engaging at any giventime; however, even when the stream is quickly flow-ing due to a high volume of tweets, crowd memberscan still choose to focus in on a particular tweet atany given moment as the lifetime of the tweet is much

longer than the lifetime of the activity.The Red Nose game (O’Hara et al., 2008) is con-

sidered to have high social exposure due to the config-uration of the crowd. The proportion of participantsis much lower than the number of spectators as theactivity has limitations on the amount of space infront of the screen, as well as the reluctance of othersto participate. The exposure plays a big role in thisreluctance, with by-standers noting an evaluation ap-prehension based on a fear that they would be judgedby others as they interacted with the system (O’Haraet al., 2008). The low number of participants makesactions highly visible and easily associated with thoseplaying the game.

Like the Time Bomb (Sieber et al., 2008) and mis-sile command games (Maynes-Aminzade et al., 2002),Squidball (Bregler et al., 2005) has participants so-cially exposed as they hit the helium filled balls.However, as the activity uses sixteen balls that aredispersed throughout the crowd, the focus of crowdmembers is divergent. The visibility of the action ef-fects is therefore somewhat diminished compared tothe other ball based activities.

The interactive dance club (Ulyate and Bianciardi,2002) has a number of ‘interactive zones’ that allowsmall numbers of participants to interact with thesystem. The small clusters of participants togetherinfluence the environment of the dance club. The vis-ibility of the manipulations is moderate as the focusof the crowd is divergent due to the many zones anddisplays situated around the club. The association ofaction effects to participants is moderate to high de-pending on the interaction made and the zone in use:zones like the ‘tweak zone’ allow only one participantto engage at a time with a high association betweenthem and the effect, while others like the ‘infraredzone’ track a large group of participants.

Applications with low social exposure make up thebulk of the existing work. Many of the applicationshave large numbers of participants interacting simul-taneously with the effects of a single individual onlya minor part of the crowd output. This diminishesthe association of the action effects to the participantthus causing only low social exposure. This is true ofthe Cinematrix system and its derivatives (Carpen-ter, 1993; Fisher et al., 1997; Dannenberg and Fisher,2001), the dance tempo system of Feldmeier (2003),HPDJ (Graham-Rowe, 2001), the cheering meter ofBarkhuus and Jørgensen (2008), and the Mexicanwave (Uplause, 2009). The use of lasers in connect-the-dots, whack-a-mole, picture reveal, paint, and thevoting systems of Maynes-Aminzade et al. (2002) al-most eliminate the link between participants and theaction effects.

The Pirates! game (Falk et al., 2001) has its par-ticipants spread throughout the game arena: as suchmany actions are only visible to those in the directvicinity of the participant. The visibility of actions istherefore reduced as only small subsets of the crowdsee the effects of the participant at a given time.

4.5 Crowd member connectivity

The majority of the existing work is based on the co-location of crowd members. In these settings, the fullspectrum of communication channels are available athigh bandwidth due to crowd members being in closephysical proximity.

The real-time use of Twitter (Twitter, 2010) is theexception to other identified work; it is considered tohave low member connectivity as only a single highbandwidth symbolic channel is provided between par-ticipants. The utilisation of this bandwidth dependsentirely on the ability of participants to structure thecontent of their messages as succinctly as possible.

5 Discussion

This section discusses the categorisation developed inthe previous section, and builds on observations madefrom existing work.

A trend seen in activities identified as having co-operative intentions is the use of crowd properties tocreate the co-operative environment. Three mech-anisms have been identified from the existing workthat utilise these properties in engaging crowds co-operatively:

• Work-as-one: crowd members are treated by thesystem as a single entity. This forces the crowdto work together as if a single person to achievethe shared goal. Examples of this are found in thesystems that aggregate crowd input, such as thecrowd leaning system of Maynes-Aminzade et al.(2002); and those that use a shared input device,like the beach ball system of Sieber et al. (2008).The success of this mechanism is due to the needto co-ordinate and communicate in order to achievethe required unity.

• Power-by-numbers: the activity is made easieror possible as more members participate. Crowdmembers must pool their collective resources toachieve the shared goal: this requires communi-cation and team based thinking.

• Division-of-labour: resources or propertiesunique to individual crowd members must be ex-ploited by the crowd to achieve the shared goal. Inthe existing work, the spatial properties of crowdmembers reflects this mechanism: in the MissileCommand game (Maynes-Aminzade et al., 2002),crowd members closest to the ball are required tohit it to prevent the oncoming missiles from reach-ing the cities. The layout of the crowd is exploitedhere to fully engage and unify the crowd; at anygiven moment a participant may be required toparticipate for the benefit of the crowd.

A second trend seen in much of the existing workis the use of reactive interface styles: the visceral im-mersion provided by this form of interaction createsactivities that are able to engage all crowd membersregardless of their background or expertise. One ofthe primary motivations for simplistic interfaces isthe need to avoid crowd segregation based on technol-ogy or system knowledge: the requirement to engagemany co-located individuals in an activity they havehad no previous time to explore demands the learningcurve be minor for full crowd involvement. The useof reactive interfaces in audience configurations is en-tirely appropriate due to the requirement for simple,economical, and engaging activities.

Visceral interaction works well to break social bar-riers; crowd members are able to become fully im-mersed in the activity and relate to one another ata low common social denominator, the shared expe-rience. The co-operative intentions seen in the ac-tivities with reactive interaction styles suggests thisstrategy works well to unify the crowd in co-locatedsettings.

However, limiting ourselves to this form of crowdactivity alone prevents further exploration of the do-main. What might the creativity of a thousand mindsworking in unison create? What is the consensusof the crowd on that offside play? These questionsamong others provide ample motivation for exploringreflective interaction styles further.

Of the activities identified with a reflective inter-action style in the literature, all have associated withI-intentionality; crowd members reflect on their inputwith individual goals without regard for the crowd as

a whole. Observing the reactive applications identi-fied with co-operative intentions, we see that all ofthe work relies on crowd members being highly syn-chronous. In order to allow reflection, each individ-ual would need to be able to observe the view of theoutput for variable periods of time (reflection time).Reducing the high synchronicity requirement raisesissues that should be explored in this space: how willthe crowd maintain unity if members are out-of-sync,or individual action times are elongated to supportthe required reflection? The application of the co-operative mechanisms identified from reactive activi-ties might prove a good starting point for this avenueof enquiry.

Low social exposure is a third trend seen in ex-isting work: crowd members and their actions arekept hidden within the crowd as a whole. This isespecially true of shared input activities where eachmember plays only a small role in the collective in-put. Other activities like the Red Nose game (O’Haraet al., 2008) and BallBouncer (Sieber et al., 2008)have highly visible actions that are strongly linked toparticipants which provide strong motivating forcesfor involvement.

Part of the thrill of engaging in crowd activities isthe sense of being a part of something big; in mostcases this is simply the knowledge that participationled to some effect in the activity and/or the associated‘kudos’ of having been at an event (e.g. attending theWorld Cup final). In other cases this does not suf-fice: a criticism of the Cinematrix system (Carpenter,1993) was that the “approaches bury the individualin a crowd where the person’s connection with theaction on the screen is marginal at best and held indisbelief at worst” (Dannenberg and Fisher, 2001, pg3).

Social exposure can act to incentivise crowd mem-bers to participate in both positive and negative ways.Crowd activities like BallBouncer (Sieber et al., 2008)encourage participation by accountability: each mem-ber is clearly visible on shared display, and so mem-bers are held accountable if they don’t attempt to hitthe virtual beach ball shown on screen. Conversely,crowd members are seen by their team in a positivelight when they work hard for the team, or help scorea critical point. The Red Nose game (figure 3) pro-vides incentive for participation through the noveltyof the activity, as well as the potential embarrassmentif called out by the compere (O’Hara et al., 2008). Inthese instances both positive and negative forces actto incentivise participation and engagement.

Conversely, unwanted social exposure can preventsome crowd members from participating. Care mustbe taken with activities that have high social expo-sure to limit embarrassment and allow for differingdegrees of participation (Reeves et al., 2010). Givingthe choice of social exposure to crowd participants al-lows the best of both worlds as crowd members areable to choose to bask in the kudos of their achieve-ments, or squash their fumble into obscurity.

Almost all of the identified work bases itself on thetraditional definition of crowds, with members shar-ing a single location: as members share the same me-dia space, the connectivity between each of them isvery high. This benefits applications in the literatureas the crowd is able to co-ordinate itself well with therich communication medium provided by co-location.However, the high connectivity also has drawbacksin that the channels between participants are uncon-trolled. Reducing the connectivity between crowdmembers by slowing the bandwidth of channels, orreducing the number of channels, allows a reductionin the synchronicity constraint of the system. Thisleads to a number of advantages. The reduced loadon crowd members allows for richer reflective input

systems: there is now time to reflect on the systemstate and so a greater degree of complexity can beintroduced. Twitter (Twitter, 2010) allows reflectivesymbolic communication to occur while also allowingsynchronised activity as observed during ‘tweeting’about natural disasters (Sakaki et al., 2010). How-ever, the greatest benefit is the ability for crowd mem-bers to determine how much involvement they havein the activity and which part of the crowd they con-sider themselves a part of. This flexibility fits theheterogeneity of crowd members well (Reeves et al.,2010).

Applications built for virtual crowds have the po-tential to also offer similar synchronous activity foundin co-located settings, while engaging a larger anda potentially more diverse crowd; the facilitation ofcommunication channels over technology solutions isa potential direction of enquiry for crowd applica-tions. Can we create compelling activities that al-low members from a diverse range of backgrounds,nationalities, and locations to work together?

From the categorisation of existing work, two ap-plication class gaps are apparent:

• individually intentioned reactive interac-tions: this class of application requires that crowdmembers react to some stimuli with speed and co-ordination to solve their own goals. This could po-tentially be achieved by having crowd members re-act to the shared influence in different ways to meettheir goals.

• shared intentioned reflective interactions:applications of this class allow crowd members toreflect on their input into a co-operative goal, oract to compete against others with the same sharedgoal.

The latter application class is a good fit for thevirtual crowd, owing to the ability to control the com-munication medium between participants. Individualviews of the output could allow crowd members toreflect on their contribution to some shared activity,or perhaps strategise on their own goals in a compet-itive one. The move away from co-located settingsopens up the potential for richer and more complexinterfaces: crowd members have time to reflect andup-skill before they fully participate. Members couldbe presented with their own personal training pro-gramme or sandbox before engaging in the activitywith others. The ability to control social exposurecould allow for a greater degree of experimentationon the part of crowd participants; an important prop-erty for reflective interaction styles as interaction nowhas purpose, as opposed to the reactive interactionsof games like crowd pong (Carpenter, 1993; Maynes-Aminzade et al., 2002), and crowd members are moreaccountable for their interactions.

The virtual crowd provides the possibility of richerreflective interaction: there is no longer the issue ofcrowd segregation due to technology as each memberis equipped with the required interface for the system,and the synchronicity constraints of the system canbe reduced to provide reflection time.

The virtual crowd presents the following require-ments for any potential supporting technology infras-tructure:

• The ability to connect a large number of par-ticipants simultaneously: crowd members haveto rely on the system to facilitate the connectionspreviously created by virtue of co-location. Elim-inating the requirement for co-location increasesthe potential number of participants; the spatialboundaries of typical crowd settings no longer ap-ply in the virtual domain. In addition, the interface

that participants use to interact with the systemneeds to be pervasive enough that a sizeable crowdcan form.

• Rich communication channels: the systemwould need to replace communication channels lostfrom the move away from co-location. Rich com-munication channels would be needed to supportco-operative intentions as they require that thecrowd communicate within itself.

• Real-time: any potential system would need tosupport the necessary synchronicity of the crowdso that all members are under the same influencesimultaneously.

• Economy: one of the major benefits providedby existing solutions like Cinematrix (Carpenter,1993) and the machine vision system of Maynes-Aminzade et al. (2002) is the economical way inwhich they engage the crowd. This benefit needsto be carried over to virtual crowds to justify theircreation and support.

• Distribution: mechanisms for distributing thecrowd application to potential participants are re-quired.

The requirements above dictate a solid infrastruc-ture that is able to meet the demands of large numberof connected participants, while remaining economi-cal enough to justify the supported activities. Thecloud computing paradigm provides two features thataddress this requirement: large scale economical com-puting, and elasticity. A solid infrastructure is attain-able through cloud service providers at a very modestcost; one of the biggest benefits of the cloud is the re-duced entry barriers it brings to large scale comput-ing (Greengard, 2010). Elasticity allows applicationsto be scaled up and down rapidly as demand on theapplication grows or shrinks (Owens, 2010). The util-ity model of the cloud fits the virtual crowd well ascrowds tend to meet only temporarily.

Modern smart phones provide a number of advan-tages as the interface for crowd based applications:they are pervasive; they contain a myriad of sensorssuch as accelerometers, microphones, with modernphones also sporting compasses and gyroscopes; theyhave a large amount of computing power; and theyare tightly linked to their owners, providing a privateinterface. The application marketplaces associatedwith modern phones provide strong distribution net-works for applications on the devices.

The combination of the cloud and modern smartphones has the potential to allow the realisation ofco-operative reflective interaction in the crowd com-puting domain.

6 Conclusion

The traditional definition of crowd is challenged byemerging communication technologies. The use ofcloud based technologies such as Twitter (Twitter,2010) in real-time capacities shows that the co-location of participants is no longer necessary forthe collective behaviour commonly associated withthe traditional form of crowd. This paper proposesthat the crowd be defined by the core components ofcollective behaviour, synchronicity, shared influence,communication channels, and potential for participa-tion in a shared activity. The activity is proposed asthe identity of the collective for the purposes of anycrowd based application. The proposed taxonomypresents five dimensions considered important for thecategorisation of crowd applications: intentionality,interaction setting, interaction style, social exposure,

and crowd member connectivity. Using these dimen-sions, a gap was identified in the existing work: allof the identified work that encourages shared inten-tions is based around reactive interfaces, utilising theco-location of crowd members. The virtual crowd mo-tivates the exploration of reflective interactions for co-operative and competitive crowd based applications;participants can be given more reflective time by con-trolling the communication channels between mem-bers. The cloud computing paradigm provides an in-frastructure that is able to support the virtual crowdeconomically. Combining the cloud with sensor-richmodern smart phones provides the necessary ingre-dients to create compelling reflective activities thatallow members from a diverse range of backgrounds,nationalities, and locations to play together.

7 Acknowledgements

This work has been partially funded by Microsoft Re-search Asia, through project “Crowd and the Cloud:Event Central Social Networking with Mobile De-vices”.

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