Augmented Invaders: A Mixed Reality Multiplayer Outdoor Game

Abstract

Many virtual and mixed reality games focus on single player experiences. In this paper, we describe the concept and prototype implementation of a mixed reality multiplayer game that can be played with a smartphone and an HMD in outdoor environments. Players can team up to fight against attacking alien drones. The relative positions between the players are tracked using GPS, and the rear camera of the smartphone is used to augment the environment and teammates with virtual objects. The combination of multiplayer, mixed reality, the use of geographical location and outdoor action together with affordable, mobile equipment enables a novel strategic and social game experience.

Full text

Augmented Invaders: A Mixed Reality Multiplayer
Outdoor Game
Michael Bonfert
University of Bremen
Bremen, Germany
bonfert@uni-bremen.de
Inga Lehne
University of Bremen
Bremen, Germany
ilehne@uni-bremen.de
Ralf Morawe
University of Bremen
Bremen, Germany
r.morawe@uni-bremen.de
Melina Cahnbley
University of Bremen
Bremen, Germany
mac6@informatik.uni-bremen.de
Gabriel Zachmann
University of Bremen
Bremen, Germany
zachmann@uni-bremen.de
Johannes Schöning
University of Bremen
Bremen, Germany
schoening@uni-bremen.de
ABSTRACT
Many virtual and mixed reality games focus on single player ex-
periences. In this paper, we describe the concept and prototype
implementation of a mixed reality multiplayer game that can be
played with a smartphone and an HMD in outdoor environments.
Players can team up to ght against attacking alien drones. The
relative positions between the players are tracked using GPS, and
the rear camera of the smartphone is used to augment the envi-
ronment and teammates with virtual objects. The combination of
multiplayer, mixed reality, the use of geographical location and
outdoor action together with aordable, mobile equipment enables
a novel strategic and social game experience.
CCS CONCEPTS
•Applied computing →Computer games;
KEYWORDS
Augmented Reality, AR, Mixed Reality, Games, GPS, Multiplayer
ACM Reference format:
Michael Bonfert, Inga Lehne, Ralf Morawe, Melina Cahnbley, Gabriel Zach-
mann, and Johannes Schöning. 2017. Augmented Invaders: A Mixed Reality
Multiplayer Outdoor Game. In Proceedings of VRST ’17, Gothenburg, Sweden,
November 8–10, 2017, 2 pages.
https://doi.org/10.1145/3139131.3141208
1 INTRODUCTION & RELATED WORK
Last year, the hype around Pokémon Go [Niantic Inc. 2016] pushed
the popularity of augmented reality (AR) and mixed reality (MR)
games. People searched the streets for rare Pokémon often in small
groups [Colley et al
.
2017] although players cannot cooperate within
the game. The game uses GPS to determine the location of the play-
ers and map it onto the virtual world. However, the appearance
of virtual objects in MR is only approximate and not at the same
physical location for every player. There are some mixed or virtual
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ACM ISBN 978-1-4503-5548-3/17/11.
https://doi.org/10.1145/3139131.3141208
Figure 1: Illustration of the game setting with three players
shooting virtual drones in a park.
reality (VR) games that oer a multiplayer experience. Most of these
games require specic and expensive hardware (e.g. Oculus Rift or
HTC Vive). One approach is to equip only one player with a head-
mounted display (HMD) while others act in supporting roles by
giving the player in the virtual world instructions or by interacting
with the conventional controllers of the game console (e.g. The Play-
room VR [Japan Studio 2013]). Another approach is that all players
need the same VR hardware and play in dierent rooms, for exam-
ple in Star Trek: Bridge Crew [Red Storm Entertainment 2017]. The
established platforms are incompatible with each other, which leads
to a fragmentation of target audiences. This makes it less attractive
for large game studios who wish to develop across platforms, as well
as for users who can only play with those using the same platform.
Moreover, these devices have a high demand of system resources,
which constrains their usage to stationary indoor computer setups.
In contrast to this, our prototype runs on Android and can be built
for iOS, together serving 99.7% of the smartphone market [IDC
2017]. One example that features no multiplayer mode, but is an
outdoor AR game, is Sky Invaders 3D by [Avery et al
.
2006]. In this
game, the player res at virtual space shuttles. Despite the intrusive
hardware ten years ago, the study results show that people found

VRST ’17, November 8–10, 2017, Gothenburg, Sweden Bonfert, Cahnbley, Lehne, Morawe, Schöning and Zachmann
the augmented version of the game more joyful and intuitive than
the same game on a stationary computer. Father.IO [Proxy42 Inc.
2017], on the other hand, is a multiplayer MR game where the player
can re at opponents, who are visible through the smartphone’s
camera. Despite the capabilities of smartphones, an additional piece
of hardware including several sensors is needed to play the game.
While the described games either feature no direct interaction, do
not use the player’s exact location, equip only one player with an
HMD, only run on certain platforms without interoperability, are
stationary, or require additional hardware, this paper presents a
game prototype that improves on these shortcomings. We aimed to
conceive an inclusive outdoor MR game for multiple players that
gives an immersive rst-person experience to everyone.
2 GAME CONCEPT AND INTERACTION
Augmented Invaders attempts to make MR games more accessible
to the general public by using cheap and mobile VR headsets for
smartphones and by enabling groups of players to indulge in short,
spontaneous multiplayer sessions with low access barrier. The game
is played outdoors due to the need of location services via GPS
and a large play area like a park to move freely (min. 20m x 20m
recommended). This experimental game prototype is an homage to
the arcade classic Space Invaders [Nishikado 1978].
Figure 2: Illustrations of the main menu and shooting a
drone from the player’s perspective.
The players can see their physical environment through the
rear camera and are surrounded by virtual alien drones that are
invading the world (see Figure 1). They ght the intruders as a team
by shooting them with lasers coming from their eyes (see Figure 2).
To shoot, the players pull the magnetic trigger of the HMD. This
main interaction principle is introduced to the players already in
the menu while setting up a game session (see Figure 2). The drones
are displayed in the exact same position of the real world for all
players. There is no avatar representing a player since the team
members are already visible through the camera. Only the laser
beams originating from their eyes are augmented. Finally, after the
timer is elapsed, an overlay with the score of the team indicates the
end of the session.
2.1 Implementation
The game was implemented with the Unity3D game engine [Unity
Technologies 2017]. We used the built-in networking service UNet
to communicate between all players. To create the MR scene, we
used the Vuforia toolkit [PTC Inc. 2017], which handles commu-
nication with the mobile device’s camera and the rendering into
stereoscopic images. The positions of the players are obtained us-
ing GPS and converted from the spherical earth coordinate system
into the cartesian Unity world coordinate system with the help
of an open-source script provided by [Taylor 2013]. At the start
of the game, the player who created the game session determines
the origin of the virtual world space according to the initial GPS
location. This calibration is then synchronized to all other players.
Thereby, the position of drones and lasers in virtual space corre-
sponds directly to a physical location on earth and is displayed in
the same spot for every team member. Also, the players’ movements
are constantly synchronized to determine the starting point and
direction vector of lasers shot by team members.
3 CONCLUSION
In this paper, we presented Augmented Invaders, a mixed reality
multiplayer outdoor game. We showed that it is feasible to develop
an MR game that is location-based and features multiplayer aspects
directly in the augmented world, enabling exciting strategic team-
play within the own physical environment for everyone. One of
our current system’s deciencies is the player’s eld of view that is
strongly limited by the phone’s screen. Albeit walking is still possi-
ble, the most part of the ground is cut o the player’s view. Thus,
the player is moving more carefully and slowly than without the
HMD. Secondly, our approach does not account for occlusion of the
drones and lasers by real world objects. For example, a drone that
is far away would not be covered by another player or a tree. This
aects the immersion and distorts the perceived distance. Further-
more, the tracking accuracy is insucient as GPS signals are noisy
and lack precision. The adjustment of the drones’ and lasers’ posi-
tions according to the movement of the player turned out smoother
and calmer than expected, but should be further stabilized with
e.g. gyroscope data or optical ow analysis. After improving and
completing the prototype, the game concepts will enable a variety
of intriguing studies, e.g. on strategic team play and group dynam-
ics, on playing with many players on a large scale, or on social
acceptance of spontaneous, inclusive game sessions.
REFERENCES
Benjamin Avery, Wayne Piekarski, James Warren, and Bruce H. Thomas. 2006. Evalua-
tion of User Satisfaction and Learnability for Outdoor Augmented Reality Gaming.
In Proceedings of the 7th Australasian User Interface Conference - Volume 50 (AUIC
’06), Vol. 50. Australian Computer Society, Inc., Darlinghurst, Australia, Australia,
17–24. http://dl.acm.org/citation.cfm?id=1151758.1151759
Ashley Colley, Jacob Thebault-Spieker, Allen Yilun Lin, Donald Degraen, Benjamin
Fischman, Jonna Häkkilä, Kate Kuehl, Valentina Nisi, Nuno Jardim Nunes, Nina
Wenig, et al
.
2017. The geography of Pokémon GO: benecial and problematic
eects on places and movement. In Proceedings of the 2017 CHI Conference on Human
Factors in Computing Systems. ACM, 1179 – 1192.
IDC. 2017. Smartphone OS Market Share, 2017 Q1. (2017). Retrieved September 20,
2017 from https://www.idc.com/promo/smartphone-market-share/os
Japan Studio. 2013. The Playroom. Game. (nov 2013). Retrieved August 16, 2017 from
https://www.playstation.com/en-gb/games/the-playroom-vr- ps4/
Niantic Inc. 2016. Pokémon Go. Game. (july 2016). Retrieved July 4, 2017 from
http://www.pokemongo.com/
Tishihiro Nishikado. 1978. Space Invaders. Game. (1978). Retrieved August 16, 2017
from http://spaceinvaders.net/
Proxy42 Inc. 2017. Father.IO. Game. (2017). Retrieved August 17, 2017 from https:
//www.indiegogo.com/projects/father-io-massive- multiplayer-laser-tag- app#/
PTC Inc. 2017. Vuforia. (2017). Retrieved July 4, 2017 from https://developer.vuforia.
com/downloads/sdk
Red Storm Entertainment. 2017. Star Trek: Bridge Crew. Game. (may 2017). Retrieved
July 4, 2017 from https://www.ubisoft.com/de-de/game/star-trek- bridge-crew/
Michael Taylor. 2013. UnityGPSConverter. Unity Package. (2013). Retrieved July 4,
2017 from https://github.com/MichaelTaylor3D/UnityGPSConverter
Unity Technologies. 2017. Unity3D. Game Engine. (2017). Retrieved July 4, 2017 from
https://unity3d.com/