MOBILE CHASE

Towards a Framework for Location-Based Gaming

Mirko Fetter

Faculty of Media, Bauhaus University Weimar, Bauhausstr. 11, Weimar, Germany

Markus Etz, Heiko Blechschmied

Graphic Information Systems Department, Fraunhofer Institute for Computer Graphics

Fraunhoferstr.5, Darmstadt, Germany

Keywords: Location-based Gaming, Pervasive Gaming, Mobile Computing, Computer Supported Cooperative Play.

Abstract: Pervasive Gaming and Location-based Games in particular have gained more and more attention recently.

Researchers from a variety of fields, media artists, mobile service providers as well as the entertainment

industry all seem to have their specific interests in this area. Today a couple of different games exist from

basic applications that are already available to the consumer market to bleeding edge research projects. In

this paper we introduce a framework for Location-based Gaming that on the one hand helps with the

development of market ready games. On the other hand it serves as a toolkit for researchers aiming to

rapidly develop Location-based Games, not having to deal with implementation details far away from their

research interests in order to focus on their specific research aspects.

1 INTRODUCTION

Modern information and telecommunication

technologies offer more and more possibilities for

interaction, entertainment and recreational activities.

In Pervasive Games the physical world merges with

the virtual one and thus offers new adventures and

experiences for the players by utilizing these

technologies like mobile devices, tangible user

interfaces or augmented reality. To increase the level

of immersion these games incorporate the players’

context by means of e.g. time, place, or personal

data.

Location-based Games are one of the many areas

where the concepts of Pervasive Gaming come to

life. Thereby single players or teams perform tasks

in specified scenarios using mobile computers like

laptops, personal digital assistants or mobile phones

in combination with wireless communication and

location-sensing technologies, having the real world

as their game board.

There are many reasons to be interested in the

current development of Location-based Games. On

the one hand there are clear market issues. As

current video and computer games are already

selling to a billion dollar market and the analyst

Forrester (Forrester Research Inc., 2000) stated that

Pervasive Gaming will gain a big market share there

are also other interested parties besides the game

development companies. The mobile service

providers might see the possibility that Location-

based Games could be a real driver for Location-

based Services and technologies like 3G. On the

other hand Pervasive Games may help researchers to

test and analyze the use of a wide spectrum of

technologies in game scenarios. The findings and

things learned could then be transferred to other

ubiquitous computing application fields.

With the framework presented in this paper we

introduce a tool which can serve as a solid base for

developers to implement their own gaming ideas.

Therefore the framework not only takes care of data

management and communication tasks but also

delivers a pattern for the modelling of Location-

based game content on server- and client-side.

The remainder of the paper is organized as

follows: In section 2 we present work which has

been done on the area of Location-based Games in

several fields. In section 3 we outline the basic

Fetter M., Etz M. and Blechschmied H. (2007).

MOBILE CHASE - Towards a Framework for Location-Based Gaming.

In Proceedings of the Second International Conference on Computer Graphics Theory and Applications - AS/IE, pages 98-105

DOI: 10.5220/0002077400980105

 SciTePress

requirements of the framework and give a detailed

overview of the technical concepts. Further on, in

section 4, we describe “Mobile Chase”, a

prototypically-implemented game based on the

framework. In the final section we outline future

work and draw conclusions.

2 RELATED WORK

Pervasive Games can appear in many forms as

described by Magerkurth et al. (Magerkurth, Cheok,

Mandryk, & Nilsen, 2005), the genres span from

smart toys over augmented tabletop games to a form

that he calls Location-Aware Gaming. Thereby the

approaches from different genres sometimes are

mixed in one system and bring up interesting

combinations that are hard to differentiate.

Location-based Games can be distinguished by

the fact that they use the position of the player in the

real world as a major input to the game process. In

these games the real world is seen as a game board

so they can be considered as the consequent answer

to ideas like Live Action Role Playing Games -

which bring classic Pen and Paper Role Playing

Games to life in events lasting several days -

Alternate Reality Games like “The Beast”

(McGonigal, 2003) - that use the full bandwidth of

modern media and communication technologies to

blur the borders between game and reality - and

modern video games.

A couple of games and systems have already

been developed coming from fields like research,

media art, or the industry. While last mentioned

according to Jegers (Jegers, 2004) only use a small

part of what the Pervasive Gaming paradigm offers

and are technically reduced to fit today’s user

devices, some of them are already quite successful

from a market perspective. For example the Tokyo

based game “Mogi, Item hunt” (KDDI, 2003) where

it is the player’s task to collect virtual items spread

over the city and to complete different collections of

items. Or the first commercial Location-based Game

“Botfighters” (Botfighters.com, 2005) developed by

“It’s alive” which appeared in 2001 and is about to

be released in version 2.0 where the user plays a

robot and has to solve missions hunting other

players. Both games can be run on standard mobile

phones supporting J2ME, using Cell-ID based

positioning methods.

The approach of Blast Theory, a group of

London based artists seems in contrast very

different. They developed a couple of Location-

aware Games like “Can You See Me Now?”

(Flintham et al., 2003) or “Uncle Roy All Around

You” (Benford et al., 2004) that come to life on the

streets as a sort of artistic event or interactive

performance. In contrast to the commercial games

mentioned before, these events take place for a fixed

time only with a limited number of participants. This

is somehow reflected in many details of the

realisation. Professional actors interact with the

players. In addition, as the devices are provided to

the players for the time they play, the technological

constraints to cope with are fewer than the

constraints commercial game developers have to

take care of, which leads to more sophisticated game

interfaces and a gain in the level of immersion

during play time.

Though Blast Theory collaborates with different

research institutes their interests seem to derive from

their artistic background in the first instance.

Therefore we also want to mention three

contributions made by the research community.

In “Pirates!” (Björk, Falk, Hansson, &

Ljungstrand, 2001) the homonymous classic

computer game from the renowned game designer

Sid Meier was brought to real life. The task of the

player is to “sail” between different locations,

representing islands, to search treasures and battle

near players all by using handheld computers,

WLAN and proximity sensors.

To prevent a virus from escaping from a campus,

the players of “Epidemic Menace” (Lindt,

Ohlenburg, Pankoke-Babatz, Prinz, & Ghellal,

2006), a pervasive crossmedia game, are equipped

with a variety of mobile devices - some of them

offering mobile Augmented Reality (AR).

Another research project dealing in the area of

Location-based Games was the publically funded

German research project GEIST (Holweg,

Schneider, & Göbel, 2004). The goal of the project

was to provide pupils with historical information via

means of edutainment. Virtual ghosts from the past

gave the pupils tasks by which they could learn facts

about the town history. Therefore the children were

provided with a wearable computer connected to

various positioning devices and a semi-transparent

AR-display on which digital reconstructions of

historical buildings were shown on their original

position together with the virtual ghosts.

As we can see, a lot of work has been put into

the field of Location-based Gaming, and a lot of

good ideas and systems have been developed.

However it seems that many implementations are

tightly bound to the implemented game concept.

Therefore our approach is a more generic one, not

focusing on the game idea, but on the underlying

MOBILE CHASE - Towards a Framework for Location-Based Gaming

concepts that are common to every Location-based

Game. In the following we present our proposal for

that generic approach.

3 A FRAMEWORK FOR

LOCATION-BASED GAMES

In this section we describe the requirements we had

concerning the framework, which will directly lead

us to the concepts and implementation details.

3.1 Requirements

As mentioned it was our aim to define a generic

platform for Location-based Games. Therefore a

major requirement was to define the design of the

framework as openly as possible in order to allow a

variety of different game ideas. The framework

should give the game designers the freedom to

implement their concepts without being constrained

to technical shortcomings or bound to specific

devices or services. For example the selection of the

best fitting positioning technology should be in the

hands of the game developer not limiting him to a

specific accuracy or the fact that the game can only

be played outdoor, etc.

Therefore we defined a J2ME enabled mobile

device supporting MIDP 2.0

(SUN Microsystems Inc., 2002a) and CLDC 1.1

(SUN Microsystems Inc., 2003) with network access

and some positioning mechanism - which can be at a

low level like an address input - as the minimal

specification for a mobile player.

Though there already exist some great

frameworks, middleware, and platforms for mobile

multiplayer games like Exit Games Neutron 3.5

(Exit Games, 2006), Nokia’s SNAP Mobile (Nokia,

2006), or the proposal of the Mobile Games

Interoperability Forum (Mobile Games

Interoperability Forum, 2002) , their focus does not

lay on supporting Location-based Games. Therefore

another requirement while planning the framework

was to identify those concepts, which are specific to

Location-based Games, and to concentrate on these.

On the other side, this means to clearly omit services

and concepts that can also be found in normal

multiplayer game platforms and often just surround

the actual game like e.g. Competition Management,

Buddy Management or Match Making which are

suggested by the Mobile Games Interoperability

Forum.

Figure 1: Schematic overview of the framework and its interfaces.

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100

3.2 The Framework

The framework is completely implemented in Java

whereby the basic concepts are implemented in a

way that they can be run on devices supporting

J2ME, J2SE or J2EE. The base of the framework

builds a domain model that maps the concepts,

entities and their relationships that can be found in

Location-based Games.

Therefore we have identified four basic entities

that are common to most of the Location-based

Games:

User, Marker, Job and Message. The

class

User represents a player, which would

normally be a human, mobile player. But the

concept also allows to model e.g. virtual players,

controlled by stationary players or the CPU, that

interact with players in the real world. The

Marker

class models places or objects with a position

relevant to the game. Thus a marker can be an actual

object like a house, a monument, or a public display

or just mark a position where a virtual object can be

found like a waypoint or some virtual item that can

be collected by passing by. As

User and Marker

both have a spatial element, their position is

manifested in the class

PositionableObject both

inherit from. Besides the current and last position

stored in the WGS 84 format (NIMA, 1991) the

class holds information like the name, team name, or

an image that can be used for multiple purposes like

an avatar, additional information like the photo of a

place, or visual tags to identify a player or marker

via computer vision.

Job describes a single objective that has to be

accomplished. Each

Job has inter alia an owner who

has to complete the task and a subject. To fulfil the

requirement, to restrict the game design as less as

possible, a

Job could comprise and be assigned to

either a

User or a Marker. This offers the

possibilities to design games in which it can be the

job of a player to catch another player, or to find a

waypoint. It is even possible to design games where

Marker is in charge of fulfilling a job. For

example some sort of digital artefact like a

Bluetooth beacon could identify players passing by.

The class

Message is modelled in the same open

way to give players the possibility to send messages

to their team mates, to public displays represented

by a

Marker, and so on. It is in the hands of the

game designer whether the text field of the class is

just used to send plain text messages or to utilize this

system and for example XML-Serialization to let the

players exchange collected items, etc. via messages.

Besides these four basic concepts we have

defined one central abstract class, the

GameWorld,

which finds its manifestation on the server and the

client side and has to be implemented by the game

designer. The most important task of the

ServerGameWorld and the ClientGameWorld is

to provide the basic game logic and rules and act as

a Mediator between the single instances. Besides the

methods addressing the entities like e.g.

getJobsByUserID, getLocations, or setJob,

etc. which are already implemented the game

designer has to realize a couple of methods dealing

with the game logic and presentation. Depending on

where the computation can be done these methods

have to be implemented on the client or the server

side in the specific

GameWorld class. These

methods are dealing with the provision of the

playing field as a mobile 3D model or 2D graphic or

the effects it has to the game process when players

update their position or identify other

Users or

Markers.

3.3 Technical Details of the Framework

As mentioned before the framework has been

implemented in Java designed to be as less

demanding on resources as possible. In addition the

use of open standards makes it easier for the later

game developer to make modifications and use

standard tools. Therefore the communication

between the server and the clients is realized as a

web service using SOAP (Gudgin, 2003) instead of

defining a proprietary low-level exchange protocol

of its own.

The server side offers all the methods that are

intended for the client encapsulated in the

ServerFacade utilizing Apache Axis

(The Apache Software Foundation, 2006). The

J2ME clients use kSoap (Haustein, Morgan, Beatty,

& Mcdaniel, 2003) to consume the web service and

also make use of the Façade pattern in combination

with the Proxy pattern. In this way the flow of the

framework-relevant data is tightly packaged and

offered at clear endpoints.

If it is necessary to access other services e.g.

looking up street addresses on GIS servers for

positioning, applications identifying visual tags, or

billing services this has to be done bypassing the

framework’s communication flow - as can be seen in

Figure 1.

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101

4 THE GAME “MOBILE CHASE”

Based on the principle of the good old paper chase,

we all will remember from our childhood days, we

implemented the game “Mobile Chase” in order to

test and refine the functionality of the framework. In

the following we explain how we adapted the

principle of the classic game and adapted it to a

Location-based Game and give some detailed

information on the implementation.

4.1 The Game Principle

Exactly as in the classic archetype a team of players

runs through town marking their route each time

they change direction. A team of pursuers starts with

a several-minute delay in order to track down the

first group. Instead of using chalk arrows, in

“Mobile Chase” the first team has to mark each shift

in direction with a photo, which is then geo-coded

using GPS and uploaded to the server by the cellular

phone.

Figure 2: Students Playing “Mobile Chase”.

The pursuing team can use GPS to verify if they

have reached the next ‘marking’. Once the pursuers

are at the right spot, the phone fetches the next

picture from the server, which is superimposed in

correct positional arrangement on a three-

dimensional playing field on the phone’s display

(see Figure 3). This way the pursuing team follows

the route of the first team trying to catch up. When

the first team of players is finally in sight, the

pursuing team can ‘catch’ them via Bluetooth, and

the time is taken. The teams can now change role,

and compete for shorter times.

4.2 Implementation

The client-side implementation of “Mobile Chase”

was done in Java ME, thus allowing, on the one side,

the use of the technologies provided by the

framework and, on the other side, the porting of the

game to a variety of devices.

As mentioned above various tasks had to be

fulfilled in order to implement the game play

planned for “Mobile Chase”. One of the key tasks

was to retrieve the players’ location during the

game. This location was used, on the one side, to

georeference the taken images and, on the other side,

to compare the positions of the pursuing team with

the markings. Here two methods were evaluated

during the implementation phase, one based on the

Location API (JSR 179) (SUN Microsystems Inc.,

2005) provided by cellphones in combination with

positioning data retrieved from the cellular network,

the other one using a satellite-based localization

technology. While from a theoretical point of view

the location data retrieved from the cellular network

could have been used for a first rough localization of

the players, tests showed that the usage of this data

depended on a collaboration with network providers,

a prerequisite which was not met during this project

phase. Therefore the localization in the final

implementation was done using GPS. Technical

obstacles connected to GPS like the impreciseness

related to shadowing effects in city areas were

overcome by allowing a reasonable degree of

inexactness during the game play.

The second main technical task in the

implementation of “Mobile Chase” was the

visualization of the playing field. Here one of the

main tasks, which had to be fulfilled, was the

playability of the game and the satisfaction of the

players. On the one side, the locating of the markers

should not be made too easy for the pursuing team

as this would reduce the “fun factor” of the game

and make it too boring for the players. On the other

side the team should be provided with enough hints

allowing to find the markers’ position in the real

world and to catch the first team of players.

We decided to use a three-dimensional playing

field, an adaptation of the “Mobile 3D Viewer”-

Technology developed at Fraunhofer-IGD

(Blechschmied, Coors, & Etz, 2006), which is based

on the Mobile 3D Graphics-API (JSR 184)

(SUN Microsystems Inc., 2002b) and allows the

visualization of three-dimensional virtual worlds on

mobile devices. As described in chapter 4.1, the

markers are located in correct position relative to

each other on the playing field, allowing the players

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102

to deduce information out of the playing field like

the distance and the direction from one shown place

to the next one. Technically the visualization of the

markers was realized by using textured cuboids

which were added to the 3D scene. By that it was

possible, on the one side, to support the pursuing

team in their task without, on the other side,

providing the players’ with too much information

like in a regular street map where the additional

information like the road names would have affected

the complexity of the players’ task.

Figure 3: Zoomed view of the next Marker and a view of

the 3D display of the playing field showing four Markers

and the game menu.

Another main technical element of the client-side

application was the implementation of a module,

which allows the first team to use the camera

integrated in the mobile phone to generate the

markers and to transfer the pictures to the game

server. For this task the Mobile Media API (JSR

135) (SUN Microsystems Inc., 2006b) was used.

Here the central class

Manager of the API allows

the access to the resources of the mobile device like

the video stream of the integrated camera. After

having made a photography, the user is shown the

picture on the screen, giving him the possibility to

accept the picture and send it to the game server

using a HTTP-connection. As the photos which are

taken by the first team are to be integrated into the

playing field as textured cuboids, the transferred

photos are afterwards scaled to a size of 128 x 128

pixels on the server. This is done due to the

prerequisite of the Mobile 3D Graphics-API only

allowing textures with a side length of 2

To geo-reference the taken photos, an external

GPS device was connected to the mobile phone

using the Bluetooth API (JSR 82)

(SUN Microsystems Inc., 2006a). Here the data

retrieved from the GPS device in form of the

NMEA-protocol - a protocol containing amongst

others the retrieved position - were parsed on the

mobile device and used to determine the position of

the players. The identification of the first team at the

end of the game was another task where the

Bluetooth-technology was used. By using the class

BTDeviceDiscovery the pursuing team starts a so

called discovery process scanning for other

Bluetooth-enabled devices in its range, usually a few

meters. The scanned Bluetooth-addresses are sent to

the game server, where the addresses from

ServerGameWorld and ClientGameWorld are

compared. If one of the scanned addresses

corresponds to the Bluetooth-address of the first

team’s device, the job is closed and the pursuing

team wins the game.

Figure 4: Web-interface for the game-player.

To demonstrate the flexibility of the framework

an additional web-interface was realized. This

interface based on the functions offered by the

ServerFacade allows the retrieval of the current

markers, users and their jobs and their representation

in table form and the reset of the game world. While

the different teams are playing “Mobile Chase” on

their mobile devices, this web-interface allows the

online administration of the game via a game master

or the inclusion of online players watching the

progress of their teams (see Figure 4).

MOBILE CHASE - Towards a Framework for Location-Based Gaming

103

5 FUTURE WORK

Of course one major future task will be to implement

a variety of different games that will focus on

different research aspects and get them played.

Based on ethnographic studies, like first-hand

observations or interviews etc. of people playing

these games we expect insights in different fields

like playability and usability on the one side but also

effects on heterogeneous research topics.

One of these topics is how digital tools can help

mobile people working together and collaborating in

different environments and how these tools can be

improved. Today we have to deal with uncertainties

like e.g. unsatisfactory network coverage or with

mobile devices that are not specialized to specific

tasks, and therefore are highly complex to use.

Having people play these games, we will probably

see what they will accept and where current lacks in

acceptance have to be addressed.

Another main research focus is the usage of the

developed technologies in “serious” location-based

technologies like locating and navigating systems on

mobile devices. As an example, the visualization of

markers on the playing field seem to be a good base

for representing Points-of-Interest in a Location-

based Service (LBS) while the visualization of a

three-dimensional map and the usage of this map

inside of the game could provide us with

information about the usability and the navigational

help provided by 3D map representations.

Concerning the localization of an LBS-user, future

work will be done in order to overcome the

challenges resulting from restricting factors like the

inexactness of cell-based location data or the

absence of satellite-based locating data inside of a

closed building. These insights will also give us a

deeper understanding on how the physical world and

electronic spaces can be better matched as described

by (Gross & Specht, 2001) in the field of context-

aware applications (Schilit, Adams, & Want, 1994)

to provide a seamless movement between these

worlds.

In the end, all this knowledge will of course help

us refine the framework and integrate these new

findings. In this way, we want to extend the area of

application of the framework from supporting games

to support a variety of different location-aware,

cooperative applications.

6 CONCLUSIONS

As suggested by Starner et al. (Starner, Leibe,

Singletary, & Pair, 2000) games seem to be an ideal

arena to test new concepts and prototypes from the

field of human-computer interaction. And as

described by Davies et al. in the introduction of

“Rapid Prototyping for Ubiquitous Computing”

(Davies, Landay, & Hudson, 2005) we also see the

dilemma concerning the implementation of ubicomp

scenarios where researchers have to deal with the

implementation of broad systems even if they just

want to focus on a specific area. In this spirit we

wanted to make our contribution by providing our

framework. In our eyes the development and things

learned from Location-based Games today might

give us insights and ready-to-use technologies for a

variety of application fields tomorrow. Our

framework is designed to help researchers to

implement technology and scenarios in order to get

early insights in feasibility and the acceptance and

usage by the potential users.

ACKNOWLEDGEMENTS

The work presented in this paper has been realized

during the stay of Mirko Fetter at the Fraunhofer

IGD under the supervision of Professor Horst Grauer

from the University of Applied Science Wuerzburg.

We want to thank everybody involved in facilitating

this cooperation.

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