a cell-based shared virtual world management mechanism in the cyber mall system

Ž .Computer Networks and ISDN Systems 30 1998 1865–1874

A cell-based shared virtual world management mechanism in thecyber mall system

Jeongdan Choi ), Jinseong Choi 1, Chanjong Park 2, Donghyun Kim 3

VR Team, ETRI, 1 Ueun-Dong Yusong-Gu, Taejon, 305-333, South Korea

Abstract

This paper deals with the problem of reducing the volume of network traffic when broadcasting all events generated in ashared virtual world. The proposed mechanism relies on a cell-based management system. The user may adjust the size of acell, depending on the level of VR reality he wishes. We describe a hybrid client–server design and implementation. Wealso describe experimental results for a system that supports the real-time visual exploration of information about 3D objects,and the interaction between multi-participants in the shared virtual worlds. It is expected that this method will be applicableto distributed VR systems such as Internet shopping malls. q 1998 Elsevier Science B.V. All rights reserved.

Keywords: Shared virtual world; Cell-based mechanism; Cyber mall system

1. Introduction

Ž .Recently, the virtual reality VR technology orig-inating from a remote robot enabled the creation ofPathfinder to explore Mars. With the aid of acceler-ated graphics technology, a large portion of the realworld can be expressed in terms of virtual space.VRML — the Virtual Reality Modeling Language

) Corresponding author. Tel.: q82-42-869-1597; Fax: q82-42-869-1951; E-mail: [email protected], http:rrvr3.seri.re.krr; jdchoi

1 E-mail: [email protected] E-mail: [email protected] E-mail: [email protected].

— is established as the standard 3D format for thedistribution of virtual worlds on the Internet. There-fore, the multi-user and distributed VR systems cansupport a large number of participants simultane-ously connected to the Internet.

The primary idea of the distributed VR system isvery simple: a simulated virtual world runs on not asingle system but several systems connected over a

w xnetwork 1 . The distributed VR system allows usersto meet, interact and collaborate with each other in

w xreal time, sharing the common virtual world 2 . ToŽ .implement a shared virtual world SVW in a dis-

Ž .tributed virtual environment DVE , two major as-pects are considered. One is the graphics perfor-mance to display the complex and various 3D scenes

Ž .that represent a number of world objects WOs inreal time. The other is the communication perfor-

0169-7552r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved.Ž .PII: S0169-7552 98 00217-7

( )J. Choi et al.rComputer Networks and ISDN Systems 30 1998 1865–18741866

mance to maintain a shared scene and the associatedinteraction information among participants.

In this paper, to implement an example of thedistributed VR system, we designed the cyber mallsystem on the Internet. It allows the Internet shop-pers to gain information of products such as shape,property and operation, as well as to interact withother users. This paper especially focuses on a cell-based SVW management mechanism in the cybermall system to improve the graphics and communi-cation performances. The remainder of this paperdescribes the design and implementation of the cybermall system. Section 2 describes the network modelsto manage the SVW data in the distributed VRsystem. Section 3 describes a set of componentscomposing the cyber mall system. Section 4 presentsour proposed cell-based SVW management mecha-nism to improve the graphics and communicationperformances. Section 5 discusses performance re-sults. Finally, Section 6 draws conclusions from ourresearch and describes future works.

2. Network models

A number of experimental approaches have beentried to support real-time interaction between playersand simulation of a virtual world. So, this sectiondiscusses a set of network models used in the previ-ous VR systems.

Fig. 1. Peer-to-peer model.

Fig. 2. Client–server model.

2.1. Peer-to-peer model

w x w xMR Toolkit 3 and VEOS 4 adopted the peer-to-peer model. In this model, all players are respon-sible for the maintenance of the downloaded SVWdata from the data server. Whenever any entity in anSVW data is changed, the update message must bebroadcast to all players as shown in Fig. 1. Thismodel is difficult to support large scaled distributedVR system due to heavy network traffic as thenumber of players is increased.

2.2. Client–serÕer model

The client–server model is a typical model usedin many military simulations and training systems.

w x w x w xSIMNET 5 , NPSNET 6 and VERN 7 adoptedthis model, which consists of one single data serverand many players as shown in Fig. 2. The data servermaintains the SVW data for the connected players,and is responsible for its consistency. On the otherhand, each player must process update messagesfrom the data server as well as simulate behaviorsfor all other players. However, this model suffersfrom the bottleneck of the centralized data server.

2.3. Hybrid client–serÕer model

In the hybrid client–server model, a data serverconnects to players in a client–server manner, while

( )J. Choi et al.rComputer Networks and ISDN Systems 30 1998 1865–1874 1867

Fig. 3. Hybrid client–server model.

connects to other data servers in a peer-to-peer man-w xner as shown in Fig. 3. RING 8 adopted this model.

The virtual world may be partitioned into severalregions with respect to the area of interest or zonew x9 . In this case, each server maintains several re-gions simultaneously based on its performance andmembership of a multicast group for message send-ing. The player gets only the message on the virtualworld of a subset of zone. This model seems to havemore advantages in performance and scalability thanthe former discussed models, i.e., several serversdisperse the load of a centralized server.

Our developed cyber mall system is based on ahybrid client–server model. The entire cyber mall ispartitioned into several rooms, each room have dif-ferent purposes. For example, each shopping room isexclusive, and also its interaction is limited to room

w xusing the occlusion filtering method 10,11 . We usebarriers such as walls or segments for occlusionfiltering. Note that a data server maintains one ormore rooms with connected multiple players simulta-neously.

3. The cyber mall system

3.1. System oÕerÕiew

The cyber mall system consists of four majorŽ .components: the Cyber Mall Creator CMC , the

Ž .Cyber Mall Server CMS , the Cyber Mall Player

Ž .CMP and the WWW server. Fig. 4 shows thesecomponents and procedures to construct an SVWdata.

In the pre-processing step, templates, world ob-jects, and avatars are initially created andror col-lected using the conventional 3D modelling tools.Note that the template represents static object suchas walls, utilities, and doors in the cyber mall, whilethe avatar is a moving object which serves as astand-in for the Internet shopper who controls it.These generated objects are converted into a VRMLfile by the CMC.

In the authoring step, the CMC constructs anSVW data by importing the generated objects in theprevious step. The SVW data should be registered tothe CMS and posted to the WWW server.

In the browsing step, an Internet shopper whowants to shop in the cyber mall, first of all, connectsto the CMS using the web browser and then down-loads an SVW data from the CMS. And the Internetshopper selects an avatar, then she enters into theSVW to shop using the CMP.

3.2. The cyber mall creator

These days, many authoring tools have been re-leased. However, a novice has difficulty in buildinga virtual world with these authoring tools. The CMC

Žas an authoring tool enables the IPs information.providers and end users to build a virtual world

Fig. 4. Overview of the cyber mall system.


Fig. 5. The cyber mall creator.

without understanding the virtual reality and pro-gramming technique.

Fig. 5 shows that the CMC consists of fourmodules. The object library is used to store VRMLfiles for the objects such as avatar, template andWO. The resource manager is in charge of customiz-ing objects. That is, it endows attributes to thetemplate, designates the information of each WO,and defines and stores scripts. The object manager isresponsible for the efficient management of the addi-tion or deletion of objects or information that isbeing changed, by constructing them in a hierarchi-cal structure. On the other hand, the viewing man-ager manages three windows for author who designs

Fig. 6. The Avatar View, the Plane View, the Front View and thefunctional icons.

a cyber mall: the Avatar View, the Plane View andthe Front View. Fig. 6 shows these windows andsome icons for authoring. The Avatar View windowrepresents the scenes that an Internet shopper’s avataris looking at. The Plane View, top view of the room,gives the overview of objects located in the virtualworld. Finally, the Front View represents a verticalcutting view of the world and provides an Internetshopper with height adjustment of the object. That is,the just imported object is always represented simul-taneously in these three windows. For example, inFig. 6, three

’s of each window indicate the same object. Also,the navigation through the cyber mall can be easilyperformed by dragging the avatar indicator marked

in Fig. 6. It consists of two circles: large one meansthe user’s position and the small one means the viewdirection. Finally, the tool bar at the bottom-right inFig. 6 shows the functional icons for window activa-

Fig. 7. The cyber mall server.


Fig. 8. The cyber mall server’s homepage.

tion, dialog box pop up, and object manipulation,etc.

3.3. The cyber mall serÕer

The SVW data in the object library represents avirtual world including avatars, templates and worldobjects. It is managed by the CMS that consists offour modules as illustrated in Fig. 7.

Fig. 9. The CMP management dialog.

Fig. 10. Cyber mall player.

The client manager is in charge of the initiationand the maintenance of the connection with CMPs,and the event serializer processes the requested eventsfrom CMPs by using a timestamp in the eventmessage. The event processor selects the CMPs thatcan multicast a message based on the cell resolutionof their own. Finally, the server adapter is used toshare the common information with other servers,such as the IP address, the current player list, etc. Ifa CMP wants to get out from one to another roomleaving this server, the server adapter references this

Fig. 11. The avatar selector window.


Fig. 12. An example of the Internet health market.

information then supports the connection to thepointed server.

Fig. 8 shows the WWW server page supported bythe CMS. This figure shows the cyber mall partitionsand links from each room to the CMS. And Fig. 9shows the CMP lists connected to one CMS. Thisdialog gives the various communication methods tothe associated CMPs, such as homepage connection,calling a telephone and sending e-mail.

3.4. The cyber mall player

The CMP consists of five modules as shown inFig. 10.

At the first time of connecting to a cyber mall, anInternet shopper can choose his preference of avatarshape using the avatar selector. Fig. 11 shows theavatar selector window that includes six avatar shapesmodelled on Korean traditional masks. The face ofan avatar betrays the player’s feeling such as joy,anger and grief.

The event manager classifies the events into twogroups, emergency and non-emergency. And then,the event manager transmits them to data serverthrough the network manager. The template vieweris a component that affects all clients connected tothe identical SVW, and displays a modified virtualspace after obtaining approval from the CMS throughthe network manager. On the other hand, the WOviewer is a component that displays the manipulation

of each object, and is itself manipulated indepen-dently from other clients. This method is effective tominimize the network traffic. Finally, the networkmanager assumes the functions of managing theconnection to the CMS, transmitting messages whenthe virtual space is modified, and maintaining theconformity of the SVW.

Fig. 12 shows an example of the health marketwith two other participants, who are conversing andexploring some information for the bicycle throughthe lespo company’s homepage.

4. Cell-based SVW management mechanism

4.1. Cells in the SVW

This paper divide the SVW into regular cells toimprove the graphics and communication perfor-mances. Each client downloads the SVW data from aserver then adjusts the size of a cell, taking intoconsideration both the numbers of shoppers in thecyber mall and own graphic processing capacity. Thecell size is a logical resolution of the virtual space.The smaller cell size, the higher the rendering perfor-mance.

4.2. Classification of eÕents

An event is any manipulation to the SVW by aclient. However, it is not necessary to broadcastevery manipulation to all clients sharing the SVWdata. In this paper, events from the client are classi-fied as follows.Ø Exclusive event: The result of this event does not

affect the other clients. It includes the change ofattribute information such as the color or textureof an object, or the searching information on the

Ž . Ž .Fig. 13. Weak-shared event. a Intra-cell event. b Inter-cellevent.


object, or the manipulation function according tothe script, etc.

Ø Weak-shared event: This event is distributed toachieve weak synchronization of user communi-cation. It is not necessary for other clients torenew the event immediately. In our approach,the position of avatars is regarded as this. Weak-shared events are further divided into two sub-events: the intra-cell event, where the event isprocessed only by local processes, and the inter-cell event, where an event is delivered to a serverin order that it be processed by other clients.

Ø Tight-shared event: All clients should update theirSVW data immediately when this type of eventoccurs. This event includes downloading or up-loading of the SVW data as well as the registra-tion or moving of the shared WO.As shown in Fig. 13, the intra-cell events occur

when an avatar moves within the cell, while theinter-cell events occur when an avatar moves fromthe cell A to cell B.

4.3. Processing of eÕents

As described previously, the event manager in theCMP, classifies events from the users. Exclusiveevents are only performed on the local system. Whena weak-shared event occurs, the cell checker deter-mines whether it is an intra-cell event or an inter-cellevent. The intra-cell event is regarded as a localinteractivity, like an exclusive event that does notrequire renewal of the SVW. But the inter-cell eventis processed by the local viewer, and also transmittedto the server at the same time. In addition, a tight-

Fig. 14. The event flow.

Table 1Cell table

Client Cell size A A A A1 2 3 4

ID

C 2 = 2 1932 3300 6450 128751

C 4 = 4 454 818 1625 32372

C 8 = 8 99 281 396 8183

C 16 = 16 17 60 102 2014

shared event is a message requiring the most urgentrenewal, thus it is transmitted to a server immedi-ately. All events transferred to the server are propa-gated to other clients. This method enables eachclient to focus on a local interactivity rather than amessage processing. Fig. 14 shows the event flowbased on a cell-based mechanism.

The cell table contains information on the cellsize and the position of objects for each client. Table1 shows a snapshot of a cell table, where four objects

ŽA –A in the SVW data are at the coordinate 25,1 4. Ž . Ž . Ž .30 , 200, 50 , 100, 100 , and 150, 200 , respec-

tively.The cell number, C , is calculated as fol-NUMBER

lows:

C x , y s y diÕC ) S diÕCŽ . Ž .Ž .NUMBER y x x

q x diÕC ,Ž .x

where x and y are the coordinate of an object in theSVW, C and C are the size of the cells, and Sx y x

and S are the magnitude of the SVW.y

For example, suppose that object A on client C3 3Ž . Ž .moves from 100, 100 to 103, 103 . Since the

Ž .coordinate 103, 103 is still located inside cellnumber 396 in C , it belongs to an intra-cell event.3

Table 2Cell table after movement A 3

Client Cell size A A A A1 2 3 4

ID

C 2 = 2 1932 3300 6453 128751

C 4 = 4 454 818 1626 32372

C 8 = 8 99 281 397 8183

C 16 = 16 17 60 102 2014


In this case, C renews object A in its own viewer,3 3

but does not transmit a message to the server.ŽWhereas, when moving to the adjacent cell 106,

.100 , an inter-cell event occurs in client C , and this3

event is notified to the server. The server determineswhether to change object A with respect to all3

clients. Table 2 shows that cell numbers are changedat C as well as C and C . In this case, the CMS3 1 2

transmits coordinate for A to C and C . In the3 1 2

meantime, since the cell number at C is not changed,4

the coordinate for A is not transmitted to C . In3 4

addition, each client who has received such an eventrenews the position of object A .3

5. Implementations and performance evaluations

We have implemented the cyber mall system as aŽ .prototype for a low-end PC Pentium 166 MHz

Ž .platforms on a LAN Local Area Network , usingŽ .Microsoft Visual Cqq v 4.2 and Open Inventor

Ž . w xv2.2.0 graphics library 12 .In order to investigate the performance of our

proposed cell-based SVW management mechanism,the frame rate is measured. Also, the avatar naviga-tion scenario simulates a shopping situation wheresix users move in the room as shown in Fig. 15.

Fig. 16 shows the comparison of performanceusing the three models: one is noncell-based model,and the others are cell-based model with cell size of

Fig. 15. The experiment system.

Fig. 16. Comparison of the performance with different cell mod-els.

10=10 and 30=30, respectively. When a walkingŽ .action is produced, Fig. 16 a shows a case to which

this method is not applied. So the system applied allupdate messages that occurred from the other clients.

Ž .The mean frame rate is 5.40. Fig. 16 b shows a casein which an inter-cell event is produced by the clientof a coarser cell size. The system does not adopt allmessages every time. The mean frame rate is 7.40, a


Ž .little improved. Finally, Fig. 16 c shows a case inwhich the clients of smaller cell size produce allevents. The system adopts only its own outgoingmessages. In this case, the mean frame rate is 7.70.

6. Conclusions and future works

We showed how the proposed cell-based modelcould be used to drive a fine-grained group commu-nication mechanism, allowing the exploitation ofmulticast communication to further reduce messagetraffic. The increasing demands of VW realism — interms of both scene complexity and network dataloads — are placing excessive strain on currentlow-level PC, especially when computationally ex-pensive graphics, such as drawing operations, arerequired.

In our future work, we will further explore themechanisms required to support large distributed vir-tual world without a significant loss of consistencyor interactivity on the Internet. We will further con-sider the requirements for time-critical rendering tosupport the rendering performance monitoring sys-tem on PCs. After this, we will explore how novicescan model the WOs. Finally, we will further examinehow different types of distributed behaviors can bebetter supported by minimizing the effort of theindividual world author when allowing other users toshare the virtual world.

Acknowledgements

This work is supported partially by the MinistryŽ .of Information and Communication MIC of the

Republic of Korea Grant No. 9P00460.

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Jeongdan Choi received the B.E. andM.E. degrees in computer science andengineering from Chung-Ang Univer-sity, Seoul, in 1993 and 1995, respec-tively. She is currently a researcher ofVR team, ETRIyCSTL. Her researchinterests include virtual reality, geome-try modeling and GIS.

Jinseong Choi is a research scientist atthe virtual reality laboratory in ETRIsince 1994. He works in the field ofauthoring tools for virtual environmentin the Internet. His major interests are inthe study of immersive VR system andauthoring software for largeyscaled vir-tual world.

Chanjong Park is the manager of VRTeam at ETRI, Korea. He is also aPh.D. student in computer science atKAIST. His interest include Avatar, dis-tributed VR, VR authoring tools, VRMLand computer graphics.


Donghyun Kim is team leader at thevirtual reality laboratory in ETRI since1991. He works in the field of computergraphics and virtual reality. His majorinterests are in the study of augmentedreality, haptic interface and 3D games.

a cell-based shared virtual world management mechanism in the cyber mall system

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