DTIC ADA499027: An Event-Based Data Distribution Mechanism for Collaborative Mobile Augmented Reality and Virtual Environments

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An Event-Based Data Distribution Mechanism for Collaborative Mobile
Augmented Reality and Virtual Environments
DennisBrown,SimonJulier,YohanBaillot MarkA.Livingston
ITTAdvancedEngineering&Sciences NavalResearchLaboratory
2560HuntingtonAve 4555OverlookAveSW
Alexandria,VA22303 Washington,DC20375
fdbrown,julier,[email protected] [email protected]
Abstract The problem of distributing data to such users is some-
whatunusual. Mostresearchintodistributedvirtualoraug-
The full power of mobile augmented and virtual real- mented reality focuses on the support of common, consis-
itysystemsisrealizedwhenthesesystemsareconnectedto tentvirtualworlds,typicallymaintainedbyasmallnumber
one another, to immersive virtual environments, and to re- of users. However, the objective of BARS is to support a
mote information servers. Connections are usually made consistentinformationspace. Therefore,objectstendtobe
through wireless networks. However, wireless networks less complicated and updates occur less frequently than in
cannot guarantee connectivity and their bandwidth can be virtualenvironments. Furthermore,itrequiresaunifiedar-
highlyconstrained. Inthispaperwepresentarobustevent- chitecturethatallowstransportofgeneralstateinformation
based data distribution mechanism for mobile augmented thatcanbeusedformanypurposes,suchastheobvioustask
reality and virtual environments. It is based on replicated of distributing the virtual object database, as well as more
databases, pluggablenetworkingprotocols, andcommuni- generalusessuchas“remotecontrol”ofapplications. This
cationchannels. WedemonstratethemechanismintheBat- systemmustalsohandlethepoornetworkconnectivitythat
tlefieldAugmentedRealitySystem(BARS)situationaware- cansometimesbeencounteredinmilitaryoperations.Given
ness system, which is composed of several mobile aug- theseparameters,wehavedevelopedarobust,flexible,and
mented reality systems, immersive and desktop-based vir- generalevent-basednetworkinginfrastructurefordatadis-
tual reality systems, a 2D map-based multi-modal system, tribution. The mechanism builds upon three techniques:
handheldPCs,andothersourcesofinformation. distributed databases, pluggable transport protocols, and a
high-levelmanagementtechniqueknownaschannels.
The structure of this paper is as follows. The problem
statementandasurveyofexistingliteratureisgiveninSec-
1.Introduction
tion 2. Section 3 describes the event distribution system.
ConclusionsandfutureworkarediscussedinSection4.
Ascomputershavebecomesmallerandmorepowerful,
theconceptofportable,highperformancesystemcomputer
2.ProblemStatement
systemforaugmentedandvirtualrealityhasbecomefeasi-
ble. State-of-the-art (2002) laptops boast processor, mem-
ory, disk, andgraphicshardwarethatrivalsupercomputers TheBattlefieldAugmentedRealitySystem(BARS)isa
from only five years ago. The full power of these systems collaborativemobileaugmentedrealitysystemdesignedto
isrealizedwhenthesesystemsarenetworkedwithonean- improve the situation awareness and the coordination be-
otherandwithremoteinformationservers. Forexample,a tween a team of mobile users. By situation awareness, we
user walking down a street might be able to see the loca- mean that each user obtains a better understanding of the
tionsofotherusers, up-to-dateinformationaboutpricesin environment. Thetypesofdataincludethenamesofbuild-
shop windows, and even email [4]. In our research on the ings, routes, objectives, and the locations of other users.
BattlefieldAugmentedRealitySystem(BARS)[9][11],we While short-range radio communications can accomplish
arefocusedontheproblemofdevelopinginformationsys- much of this, the passive and natural display paradigm of
tems able to provide users with “situation awareness” — augmented reality (AR) makes distribution of this type of
dataabouttheirenvironmentanditscontents. informationimportantformobileARsystems.
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Thehardwareofoneofourprototypesystemsisshown
inFigure1. Itconsistsofamobilecomputer,eitheranem-
bedded PC with a high-end graphics card or a laptop with
high-end graphics built in. A see-through SVGA display
is worn by the user; we have built systems with the Sony
GlasstronTM andtheMicrovisionNomadTM retinaldisplay.
Thesystemsupportsspatializedaudiothroughsoftware,us-
ingcommoditysoundcardsandheadphones. Theuserop-
erates the system using a cordless mouse and a wrist key-
board. A Global Positioning System (GPS) receiver han-
dles position tracking while an inertial device handles ori-
entationtracking. Acameracanbeusedfortrackingorfor
sending video reports to a base station. Wireless 802.11b
Figure2.ASampleAugmentation
networking is used for data distribution and GPS correc-
tions1. TheBARSmobileuserseesgraphicsonornextto
the real objects they are intended to augment, as well as (cid:15) Datadistributionbetweenuserscanbeheterogeneous.
status information such as a compass and messages from Differentusersmightcarryoutdifferenttasksandthus
other users. Figure 2 shows an output from the system — havedifferentinformationrequirements.
the parking lot is augmented to highlight the neighboring
(cid:15) Thedistributionsystemshouldfacilitatecollaboration
buildingandafellowBARSuser.
between different users. In addition to environmental
data,thedistributionsystemmustsupportthepropaga-
tionofmeta-datasuchastaskassignments,objectives,
andmessages.
(cid:15) Usersshouldhavetheabilitytocreatereportsandup-
dateentitiesinthedatabase.Forexample,ausermight
observethatanenvironmentalfeature(suchasavehi-
cle) is not where the database indicates it should be.
Theusershouldhavetheabilitytomovetheobjectto
itscorrectlocation.
(cid:15) Network connectivity is unreliable. As a user moves
fromlocationtolocation, receptionstrengthwillvary
and the bandwidth can be limited and can be time
Figure1.TheBARSWearable varying (depending on signal strength and number of
users).
This application introduces number of characteristics
that impact the distribution of information and events be- Given its potential importance, a great deal of research
tweenusers: hasbeencarriedoutintodistributedsystemsforvirtualand
augmentedreality.
(cid:15) The objective is to provide information, not a consis- The Naval Postgraduate School has been working on
large-scale distributed virtual environments for over a
tent virtual world. The environment can be viewed
decade, their first version of NPSNET being shown in
to be populated by a set of objects which are self-
1991.Thedatadistributioninthecurrentversion,NPSNET
contained entities and other types of discrete data.
V [3], uses replicated data and the model-view-controller
Each object can be relatively simple. It is not neces-
paradigmtomaintainit. Usersofthesystemseeaviewof
sary to transmit complicated geometric objects or be-
the entities (models) drawn by a rendering system. These
haviors(suchasanimation). Thelatencyintheupdate
entities are modified by the protocols (controllers) that
ofanobjectoranentityisasecondaryconsideration.
translate network messages into state changes. New pro-
1WhensomefutureversionofBARSisusedinrealoperations,com- tocolscanbeloadedatrun-timetoextendthebehaviorsof
municationwouldlikelyhappenovertheUSmilitary’scommunications entities. Thepacketsaresentviamulticast,andthesystem
system of that time, its construction probably influenced by the current allows many protocols to share a single multicast socket.
researchdescribedbyNorthet.al.[13]. However,itislikelythatanyde-
Network traffic is controlled with an area-of-interest man-
ployedsystemwillstillhaveproblemswithconnectivityandbandwidthin
built-upareas. ager.
TheDistributedInteractiveVirtualEnvironment(DIVE)
system [5] is designed to scale to many participants while
maintaining a high level of interactivity at each site. It is
basedonapeer-to-peerarchitecturethatusesreliablemul-
ticasttomaintainadatabasethatisreplicatedateachpeer.
Usingaheirarchicalpartitioningofthedatabase, onlypart
ofthedatabasemaybereplicatedatsomepeers. Tomain-
tain highly interactive rates, some copies of the database
maybeupdatedfasterthanothers,buttherearemechanisms
toensureequalityovertime.
MASSIVE-2 [6] uses the concept of a local object, or Figure3.Eventdistributionwithinanapplica-
artifact,andremoteviewsofthatartifact.Theseartifactsare tion: Arrowsshoweventmovement.
pagedinandoutofremoteapplications. Aspatialmodelof
interaction is used in that a remote application will have a
focusthatdefinesasubgroupoftheartifactstopageinand
sponsible for dispatching events within an application and
out, instead of copying the entire database. Artifact state
distributingthoseeventstoremoteapplications.
changesaredistributedusingreliablemulticast.
WhentheObjectandEventManagerreceivesanevent,
Notallworkhasbeensolelyforimmersivevirtualenvi-
itplacesitonanasynchronouseventqueue. Theeventdis-
ronments. SomecollaborativeARsystemshavebeenbuilt,
patchingthreaddeliverstheeventtoallthelistenersthatare
typicallyforsmallgroupsofco-locatedusers.Oneexample
subscribed to receive the specified event type. The event
isSharedSpacesystem[2]forcomputer-supportedcollabo-
dispatching mechanism maintains two sets of data — the
rativeworkusingAR.Typically,usersareoperatingclosely
setofvalidevent/listenerpairs,andthesetoflistenersreg-
with one another and latency requirements are extremely
istered for each event type. Because the event system has
high. Another system designed for augmented and virtual
itsrootsintheJavaAWTeventmodel[17]weleveragethe
reality is COTERIE [12], which provides useful semantics
Reflection API to achieve these steps. The type of each
throughitsnotionsofDistributedSharedMemoryandCall-
eventisspecifiedbyitsclass.Foreacheventtype,alistener
backObjects. DistributedSharedMemoryallowsmanyap-
is defined as well. When a listener is registered with the
plications to shared the same data objects, while Callback
object and event manager, its interface is queries and it is
Objectsallowapplicationstobenotifiedofchangestothose
registeredtoreceivealleventtypesforwhichitsinterfaceis
dataobjects—anevent-likemechanism. TheStudierstube
compatible. Therefore,itispossibletodynamicallyextend
system[16]extendstheconceptthroughitsuseofanexten-
thesetofeventandlistenertypesatruntime.
siblescenegraphofapplicationobjectsthatisdistributedto
Thefollowingisanexampleofthelifeofaneventwithin
allusers.
an application. A thread handling the position and orien-
tation trackers will update the user’s position by calling a
3.TheBARSEventDistributionSystem
method in the user object to set its attitude based on data
gatheredfromthetrackers. Thismethodinturncreatesan
The BARS distribution system is based entirely on the event encapsulating the change to the attitude and sends it
concept of events. Events are used both to instantiate ob- to the dispatcher. The event arrives in the dispatcher’s in-
jects(ineffect, transmitaviewofadatabasebetweensys- comingqueueandissoonprocessed. Thedispatchersends
tems), to send information to update preexisting objects, theeventtoalllisteners,includingtheobjectitself,aswell
andtoprovideothernon-databasestatusinformation(such asthegraphicssystem(toupdatetheviewpoint)andthefil-
as a new objective for an individual user). The event dis- ter(toupdatewhatobjectsarebeingdrawn),andanyother
tributionsystemisbasedonthreecomponents: eventtrans- registeredlisteners. Notethattheobject’sattitudeisn’tset
porters, replicated object repositories, and communication untilitreceivestheeventbackfromthedispatcher(theal-
channels. We will describe these components, as well as ternativeistosetthepositionatthesametimetheeventis
theuseofbridgeapplicationstocommunicatewithoutside set) — this way, the order of events is retained by going
virtualandaugmentedrealitysystems,andsomeotheruses through the event queue in the dispatcher. Figure 3 shows
oftheeventdistributionsystem. theflowofeventswithinanapplication.
Wehavedescribedhoweventspropagatewithinasingle
3.1.EventTransportation application instance. We extended this event mechanism
to allow many separate BARS applications to trade events
TheheartoftheeventtransportationsystemistheObject bycreatingEventTransportersthatallowObjectandEvent
andEventManager. TheObjectandEventManagerisre- Managers in different application instances to send events
to and receive events from each other using Internet Pro- system is self-contained, the user will have a working AR
tocol (IP). If an event is tagged as distributed, an Event systemevenifallnetworkconnectivityislostduringanop-
Transporter serializes the event and broadcasts it to other eration.
applications. The Event Transporters in remote applica- Although network limitations may hamper wireless
tions synthesize the event object, and dispatch it on those communications for the mobile users, there are very few
applications’ event queues. Our system uses several types limitationsonthebaseusers. Theirapplicationsrunonsta-
oftransporters,basedonIPmulticast,theLightweightRe- tionaryVRsystemssuchasadesktopcomputers,3Dwork-
liable Multicast Protocol (LRMP) from INRIA [10], and benches,andimmersiveVRrooms. Usingthesamedistri-
a combination protocol we call the Selectively Unreliable butionsystem,theywillhavehighlevelsofdetailandinter-
MulticastProtocol(SUMP)thatcombinesIPmulticastand actionbytakingadvantageofthebetterbandwidthforrepli-
LRMP. Typically, application instances use SUMP on the cating more objects and seeing change events at a higher
localnetwork.Tocommunicateoutsideofthelocalnetwork frequency.
(where multicast is typically filtered out) a TCP/IP trans-
porter and bridge are used (described later in the Bridges 3.3.Channels
section). BecauseoftheconnectionlessnatureofIPmulti-
cast,thedistributionisrobustinthatthenetworkconnection
Ifsimplyimplementedasdescribedabove,theeventdis-
cancomeandgoandtheuserapplicationstillfunctions,al-
tribution mechanism will send all events to every applica-
thoughwithoutnetworkupdatesatsometimes.
tion,whichforexamplewouldreplicatetheentiredatabase
Aseventsarecreated,theyaretagged“reliable”or“un-
insideeveryapplication. Asdemonstratedbytheworksof
reliable”designatinghowtheyshouldbesent. Objectcre-
othersdescribedearlier,creatingcopiesofeveryobjectfor
ationanddeletionarealwayssentreliably. Objectchanges
every user and updating those replicas would soon swamp
are sent reliably or unreliably based first on whether the
the network with information many users would not care
change is relative or absolute. Relative changes have an
about anyway. So, like those systems, we have devised a
ordering and each one is important, so those are sent re-
way of partially replicating the database to minimize the
liably. Absolute changes, such as the constant updates of
amountofunwanteddatadistributedtoeachapplication.
a user position, are mostly sent unreliably since if one is
In creating this replication mechanism, we first looked
missed,thenextwouldoverwriteitanyway—periodically
at the uses of BARS that should drive our policies. One
these changes are sent reliably. This policy makes the as-
condition to consider is that a mobile user can only see so
sumptionthattheimplementationofIPnetworkinginareal
muchanddealwithinformationinarelativelysmallradius,
operationmaydropIPpacketsoften,makingreliablemulti-
so we looked at a spatial area-of-interest mechanism. It is
castexpensive,andsowedon’tsendeventsreliablyunless
notnecessarilythecasethatamobileuseronlycaresabout
we think it is truly necessary. Figure 4 shows the flow of
objectsthatcanbeseenfromhisorhercurrentpositionin
eventsbetweenapplications.
therealworld;forexample,ourmobileapplicationincludes
an overhead map mode in which the user can zoom out to
3.2.ReplicatedObjectRepositories
an arbitrary height and would want to see objects within a
possiblyhugeradiusaroundthecurrentposition. However,
An object repository inside a BARS application holds itseemsthattherewouldbefewsituationsinwhichamo-
the data for that application. This data consists of the bileuserwouldcareaboutobjectsveryfaraway,soformost
mostly static models of the physical surroundings (build- situations,asimplearea-of-interestmechanismcouldwork.
ings, streets, points of interest, etc), dynamic avatars for However, another condition is the type of information
users and entities, and objects created as communications, beingdistributed. Evenifsomeobjectsareneartoamobile
suchasreportsofenemylocations, routesforuserstofol- user, they may have no importance and could be distract-
low,anddigitalink. Thisdatabaseisreplicatedinwholeor ing. Or, they may indeed be too far away to be seen, but
inpartforeveryBARSapplication. veryimportant,suchaswithpossiblesniperlocations. For
When an application starts, it loads an initial set of ob- thesecases,asimplearea-of-interestmechanismisn’tsuffi-
jects from a number of sources, including data files, other cient. Inanearlierpaper[8],wedescribedafilteringmech-
applicationsalreadyrunningonthenetwork,andfilesspec- anism for mobile augmented reality. This filtering mecha-
ified on the command line. This initial set of objects typi- nism works on the object repository within an application.
callyconsistsofstreetlabels,landmarks,buildinginforma- Itissufficientforitsoriginalgoalofnotshowingusersob-
tion,andotherterrain-likeinformation,aswellasaninitial jects in which they have no interest in order to reduce dis-
setofobjectives, routes, andphasemarkersforthecurrent playclutter. However,itsimplyhidesobjectsfromtheuser
task. Therefore, since the user will have some form of the — it does not actually control whether or not the applica-
databasetostart,andeverythingelseinthewearableBARS tion holds replicas of these objects, or whether or not the
Figure4.Eventdistributionbetweenapplications: Arrowsshoweventmovement.
canjoinandleavechannelsbasedonspatialareas. Another
exampleisthesetofhazardousobjects;whileintheprevi-
ous example, the application would replicate most objects
nearby, thehazardousobjectschannelcouldcoveralarger
area, but only include those hazards. BARS also has sev-
eralinteractionmodulesthatproducemanyobjectsasthey
areused. Oneinteractorisforconstructioninwhichusers
cancollaboratetoplacepointsandbuildobjectsfromthose
points [1]; in this case, the intermediate points would be
in a channel only joined by the constructing users so that
otheruserswouldonlyseethefinalobjects. Anotherinter-
actorallowsausertodrawdigitalinkforinterpretationby
a multimodal interaction system — this ink is turned into
new objects or user interface commands. In this case, the
user drawing the ink and the application to interface with
the multimodal system would place the ink in a separate
channelsothatotheruserswouldnotseethesesketchesout
ofcontext.
Figure5.Applicationjoinedtotwochannels:
3.4.Bridges
Arrowsshoweventmovement.
Aswealludedtointheprevioussectionwiththemulti-
modal interaction example, some of our applications com-
applicationstillreceiveseventsrelatedtotheseobjects. municate with other systems. We call these applications
Keeping these situations in mind, we have developed bridges.TheseapplicationsjoinboththeBARSdistribution
channels. The term has many uses in many systems, but system and an outside system. They translate object cre-
in our system, a channel is a set of related objects. It is ationandchangesbetweenBARSandoutsidesystems. By
implemented as an instance of an event transporter and a maintainingmapsbetweenBARSobjectsandtheseoutside
multicast group designated for that transporter. An appli- objects, wecanrepresenttheoutsideobjectsinBARSand
cation can join an arbitrary number of channels and create vice-versa. Two systems with which we can communicate
new ones, until all available multicast groups are used up. are the Columbia Mobile Augmented Reality System [7]
Figure5showsasingleapplicationusingtwochannels. and the Oregon Graduate Institute’s Quickset multimodal
Oneexampleofachannelisasetofobjectsinacertain interface[14].
spatialarea. Asusersmovefromlocationtolocation,they We also mentioned that a TCP/IP transporter is avail-
able in the BARS distribution system. Although the sys- Finally, we would like to implement some performance
temisdesignedprimarilyformulticast,sometimesweneed evaluationcodeandrunteststocompareagainstotherdis-
to connect to networks that block multicast. In this case tribution systems. While we’ve found the performance to
we have a simple TCP/IP bridge application that joins lo- besufficientforourneeds,wehavenotactuallyquantified
calmulticastgroupsandcreatessocketconnectionstoother ittocomparedagainstothersystems.
BARSapplicationsthroughTCP/IPeventtransporters.This
canbeabitofabottleneck, butwecancreateoneTCP/IP
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