W
ith the advent of commer-
cially available affordable
wearable computers and
head-mounted displays
(HMDs), it is possible to
develop augmented reality entertainment appli-
cations suitable for an outdoor environment.
We extended an existing desktop game and
developed it into the ARQuake system [4], one
of the first systems that allows users to play aug-
mented reality games outdoors—allowing them
to move in the physical world, and at the same
time experience computer-generated graphical
monsters and objects.
The game we extended was Quake from id
Software (see www.idsoftware.com), a first-
person-perspective, shoot-em-up game ini-
tially released in 1996. In Quake, the player
runs around a virtual world, shooting at mon-
sters, collecting objects, and completing objec-
tives. The game is desktop-based, with the
user interacting with it using a monitor, key-
board, and mouse. Although the game is rela-
tively old, the graphics engine is very powerful
and runs on a wide range of computing hard-
ware. Recently, id Software released the source
code for Quake, making it possible for enthu-
siasts and researchers to modify the game for
their own purposes.
Augmented reality (AR) is the process of
overlaying and aligning computer-generated
images over a user’s view of the physical world.
Using a transparent HMD placed on the user’s
head, an internal half-silvered mirror com-
bines images from an LCD display with the
user’s vision of the world (see Figure 1). By
combining this display technology with a
wearable computer, it is possible for the user
to walk outdoors and visualize graphical
objects that are not normally visible. A com-
prehensive survey article [1] discusses most
aspects of this research area.
Wa yne Piekarski and Bruce Thomas
THE
O
UTDOOR
AUGMENTED
REALITY
GAMING
SYSTEM
36 January 2002/Vol. 45, No. 1 COMMUNICATIONS OF THE ACM
ARQUAKE:
In Communications of the ACM, Vol 45 No 1
January, 2002 - Copyright (C) 2002 ACM
Please visit http://wearables.unisa.edu.au for more info
The aim of the ARQuake project was to con-
struct a first-person-perspective game with the fol-
lowing attributes:
• The game is played in the physical world, with
the user able to freely move about the world.
• The view is deter-
mined solely by the
orientation and
position of the user’s
head.
• The game is experi-
enced as augmented
reality using a trans-
parent HMD.
• The game is con-
trolled using easy to
understand real-life
props and
metaphors.
We wanted to build
a system that allowed
the user to play the
game in a natural way.
The user can see the
game monsters at their
virtual locations, and
use real-life props such
as a plastic gun with
simulated recoil to
shoot at the monsters.
Our experience with
test subjects has shown
they find the game very
natural, since the haptic feedback gun operates in
a way people are used to from other experiences,
such as movies. Moving and looking around in the
game world is simple, the user just has to walk in
the appropriate direction for the required distance,
or look in the correct direction. To shoot the
weapon, the user presses the trigger on the gun
prop. The mobile nature of the game prohibits the
use of traditional desktop input devices such as a
mouse and keyboard.
As previously mentioned, the user’s position and
orientation are updated by physically walking
around, and we developed Quake worlds where the
user was not required to swim or fly. Also, we had
to carefully chose monsters that would not be too
powerful for the user to play against—in traditional
Quake the user has superhuman powers and there-
fore fighting these monsters is easier. In order to be
visible outdoors to the user, we had to modify the
skin textures of the monsters, which are normally
quite dark and not visible on a transparent HMD.
We chose seven differ-
ent types of monsters,
with our choices
restricted to land-based
creatures in order to
enhance playability.
In order to walk
around the campus and
play a Quake game, a
complete model of all
the buildings was created
and entered as a Quake
map. The buildings
were all created as solid
black objects, so they
do not render to the
display—in real life we
have the actual build-
ings to provide both
visible and haptic feed-
back on their locations.
By having the buildings
modeled in this way,
this becomes one of our
rendering techniques,
and any monsters that
are located around a
corner or behind a
building will not be vis-
ible until the user would be able to see them in
real life. Items such as secret or locked doors are
not used in our maps as they can not be synchro-
nized between the real and virtual worlds. The
campus model contained 30 buildings and was
approximately 350 meters
x 450 meters in size. Fig-
ure 2 shows an example of ARQuake being played
outdoors, taken with a camera through the HMD.
A monster with adjusted colors is shown in front of
the user, and another is in the distance to the right.
Quake Modifications
Desktop-based Quake uses the keyboard or mouse
in order to move the player through the world, and
is relatively controlled. By pressing an arrow key or
COMMUNICATIONS OF THE ACM January 2002/Vol. 45, No. 1 37
LCD
HMD User
Virtual Image
Real World
Half-Silvered
Mirror
Figure 1. Augmented reality HMD.
Figure 2. Outdoor ARquake example.
the mouse, the user moves in the specified direction,
but when walking around outdoors, the player’s cur-
rent position must be matched against that inside
Quake. To achieve this, we modified the Quake game
so it would accept absolute tracker information for
the user’s position and orientation. We used a TCM2
digital compass for orientation
and a Garmin GPS for position.
Our previous research into
AR has led to the development
of the Tinmith software system
[3]. Components of this sys-
tem were designed to handle
trackers for AR systems, and so
we reused some software mod-
ules as well as the communica-
tions infrastructure. Apart
from just trackers, other inter-
faces were also written to con-
trol weapons firing, and also to
allow external programs to
monitor Quake’s progress by
sending out UDP status pack-
ets as well. Using these inter-
faces, the gun trigger can fire
off Quake weapons, and the
haptic feedback can be driven
by the Quake status packets.
It should be possible to play ARQuake in most
locations, assuming reasonably accurate maps have
been created. The simplest example would be to
play in a large open field, with no obstacles and just
monsters—in this case problems such as position
inaccuracies will be less noticeable because the user
has no reference objects. In our example, we used
our university campus buildings to provide occlud-
ing objects, but due to the accuracy of our tracking
devices, the alignment is not perfect—monsters
appear to walk through walls or pop out of thin air.
Hardware
The ARQuake system previously discussed [4] runs
on inexpensive and easy to acquire hardware, as the
software does not require much resources. We use a
Toshiba Pentium-233 laptop running Linux, driving
an I-Glasses color PAL display, along with a Preci-
sion Navigation TCM2-80 orientation sensor, and
Garmin GPS12XL with DGPS for positioning.
Recently, we have upgraded our backpack system
(shown in Figure 3) to use much newer and more
accurate hardware, which is used for a variety of
other AR user interface research in [2] and [3].
To support the ARQuake game, a haptic feedback
gun was constructed. A toy gun was used as the base,
with a solenoid and large weight attached used to
generate feedback when the user fires the weapon or
is hit by a monster. Appropriate interfacing hard-
ware was also built to allow this gun to connect to
the laptop. Currently, the gun does not have any
tracking devices so it cannot be used by the user as a
pointing device, the aim is
made by adjusting the head
orientation.
Conclusion
We have presented a new and
novel entertainment applica-
tion that uses AR and is based
on an existing freely available
game engine. We use physical
props as well as real-world
motion of the user to interact
with the game engine so that
the rendered display approxi-
mately aligns with that of the
real world. The entertainment
industry is a major driver of
technological progress in the
computer industry, and cur-
rently there are only a small
number of games designed for
AR systems, ARQuake being one of the first.
References
1. Azuma, R. A survey of augmented reality. Presence: Teleoperators and
Virtual Environments 6, 4 (Apr. 1997).
2. Piekarski, W., Gunther, B., and Thomas, B. Integrating virtual and aug-
mented realities in an outdoor application. In Proceedings of the Second
International Workshop on Augmented Reality (San Francisco, CA, Oct.
1999), 45–54.
3. Piekarski, W. and Thomas, B. Tinmith-Metro: New outdoor techniques
for creating city models with an augmented reality wearable computer.
In Proceedings of the Fifth International Symposium on Wearable Comput-
ers (Zurich, Switzerland, Oct. 2001).
4. Thomas, B., Close, B., Donoghue, J., Squires, J., De Bondi, P., Morris,
M., and Piekarski, W. ARQuake: An outdoor/indoor augmented reality
first-person application. In Proceedings of the Fourth International Sym-
posium on Wearable Computers, (Atlanta, GA, Oct. 2000), 139–146.
Wayne Piekarski (wayne@cs.unisa.edu.au) is a Ph.D. research
student in the School of Computer and Information Science at the
University of South Australia.
Bruce Thomas (thomas@cs.unisa.edu.au) is an associate professor
in the School of Computer and Information Science at the University
of South Australia.
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38
January 2002/Vol. 45, No. 1 COMMUNICATIONS OF THE ACM
Figure 3. Tinmith outdoor
backpack computer.