Water Ball Z: An Augmented Fighting Game Using Water as

Water Ball Z: An Augmented Fighting Game Using
Water as Tactile Feedback
Lode Hoste and Beat Signer
Web & Information Systems Engineering Lab
Vrije Universiteit Brussel
Pleinlaan 2, 1050 Brussels, Belgium
{lhoste,bsigner}@vub.ac.be
ABSTRACT
Water Ball Z is a novel interactive two-player water game
that allows kids and young adults to “fight” in a virtual world
with actual physical feedback. The body movement of a
player is captured via an RGB-D sensor and analysed by
a 3D gesture recognition engine. In order to enable tactile
feedback without the need for wearable devices, a number
of water nozzles are positioned around each user’s play area.
The idea is to translate the input gesture of one player to
the corresponding water spray hitting the other player. Besides severely reducing the risk of injury in a fight, Water
Ball Z engages people in a real and fun experience where a
hit is physically manifested via a water spray. Furthermore,
power up moves and a live scoreboard extension bring the
virtual world of Dragon Ball Z and Mortal Kombat cartoons
into real (augmented) life. In addition to a detailed description of the physically augmented game, we discuss two new
control parameters for mapping gesture input to haptic output which, to the best of our knowledge, are not present in
existing 3D gesture recognition approaches.
Author Keywords
Gaming; haptic feedback; gestures; water; entertainment;
augmented reality
ACM Classification Keywords
H.5.2 Information Interfaces and Presentation: Haptic I/O
INTRODUCTION
With the introduction of inexpensive controller-free sensors
such as Microsoft’s Kinect controller and new skeleton tracking algorithms, a wide variety of new applications became
possible to build and explore. We present Water Ball Z, a
free-air fighting game using water as a mean for tactile feedback. Water Ball Z is designed for entertainment purposes
and could find a place at theme parks, fun fairs or subtropical
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http://dx.doi.org/10.1145/2540930.2540946.
swimming pools. The concept is loosely based on the popular Dragon Ball Z1 cartoon where intense power up moves
are key to defeat the enemy in a fight. Furthermore, computer games such as Mortal Kombat2 illustrate how a virtual
battle can look like between two players.
Virtual fighting games have always been popular. Since the
introduction of the Microsoft Kinect, the transformation of
the player into a powerful character has never been greater.
However, one major component missing in free-air virtual
fighting games is haptic feedback. In this paper, we introduce the use of multiple water nozzles to simulate the interaction with an opponent. The attacker can see the physical effects of their punches and kicks on their opponent and
might be hit themselves if the opponent performs a counter
attack. The haptic feedback makes the fighting game much
more realistic than existing virtual counterparts.
We discuss the design of our Water Ball Z augmented fighting game and illustrate what kind of hardware can be used
to construct such an augmented game using water as tactile
feedback. Furthermore, we propose two new control parameters, shoot-and-continue and holding-pose, to control and
simplify the gesture recognition process for the application
developer. We start by presenting some related work and
continue by describing the general setup and functionality
offered by Water Ball Z. We proceed by providing detailed
information about the necessary gesture recognition process
as well as the hardware design. Last but not least, we outline
future work and provide some concluding remarks.
BACKGROUND
Sodhi et al. [8] highlight that a physical feeling for virtual
entities is a missing part in the emerging field of controllerfree augmented reality. They introduce an air-based haptic
technology called AIREAL to impart physical forces to a
user in free air. Their solution generates an air vortex by
aligning a number of subwoofers. However, the range from
8.5 centimetres up to one metre is not enough to be used
in our fighting scenario. More traditional air compressors
might represent an alternative solution for our application
domain but have not been tested.
1
2
Dragon Ball Z: http://www.dragonballz.com
Mortal Kombat: http://www.mortalkombat.com
Richter et al. [5] recently presented LiquiTouch as a mean
for tactile feedback on touch surfaces. In their work, a nozzle mounted on a robot arm can be programmed to spray
water with different pressure, frequency and rhythm. Unfortunately, the configuration of their system does not allow for
large range water output. Additionally, different safety measures have to be taken in account when using much more
water and a wider spray area.
The AquaTop display [3] uses speakers to agitate the water
to form a tactile experience for tabletop displays. The water
is mixed with white bath milk in order that a projector can
display images on top of the water. The hands are tracked
by a Microsoft Kinect sensor positioned above the surface.
Based on the presented related work it is clear that new ways
for providing haptic feedback are finding their way into interesting application domains.
Last but not least, Parés et al. [4] demonstrate a successful
public installation of a water game for with they got very
positive user feedback. In their installation, a camera was
used to recognise people standing in a circle and holding
their hands to trigger artistic fountains.
WATER BALL Z SETUP
Our proposed two-player Water Ball Z setup is illustrated in
Figure 1. It consists of two circular play areas called battle
stations. The size of these stations is defined by the range
of the Kinect sensor denoted by (1) in Figure 1 and the water nozzle configuration (2). Each battle station requires one
Kinect sensor tracking the movement of the player. The water nozzles are activated and deactivated by using solenoid
valves, a wireless Arduino board and a dedicated computer
running Linux (located near 3).
tion gesture. The last gesture is triggered by raising the hand
to head level pointing up and performing a circular motion.
All gestures result in a water spray on the opponent’s battle station. For instance, a regular jab with the right hand
will inflict a short water spray (100 ms) on the front left
nozzle aiming to the head of the opponent. As the spray
is rather short, a double jab will have the effect of spraying
twice. Similarly, a haymaker is bound to the two sideway
water nozzles at the opposite site. The foot kicks trigger a
nozzle aiming to the back of the opponent. Finally, the circular motion gesture is a special gesture where all nozzles
are enabled one-by-one in the same direction the gesture is
performed. However, this action is only activated after one
full circular motion has been performed. Note that we call
this type of gesture a shoot-and-continue gesture.
Potential Extensions
Due to budget constraints our prototype is limited to the
above specification. However, we foresee a number of extensions to the game:
Live Scoreboard
By providing a live scoreboard at the side of the battle stations, a real championship could be set up. A health bar
showing the remaining life energy, similar to Mortal Kombat games, can be used and more complex attacking gestures
could result in a large drop of the life energy shown in the
health bar.
Power Up Moves
Several special gestures can be used to power up an attack.
The Dragon Ball Z cartoon is famous for their power up
moves and we think that they could further enrich the game.
While player A can charge their next move by using a special pose, player B can launch any attack they want resulting in quite some loss of life energy for player A. On the
other hand, player A is investing in a powered up attack that
might cost player B a lot of energy in one hit. Visualising
the progress of such a power up move could be done on the
live scoreboard or by subsequently enabling the LEDs surrounding the battle station.
Defensive Gestures
Figure 1. Water Ball Z setup
In our setup, the water sprayed out of the water nozzles
shown in Figure 4 might easily reach the upper body of a
player. For safety reasons, we opted for a setup where the
solenoid module with the 12V electronics can operate wirelessly and fully disconnected from the electricity network
by using a battery. The Kinect sensor and the computer responsible for the gesture recognition and wireless activation
of water spays are protected against water by covering them
with plexiglas.
We currently distinguish seven key input gestures, including
a regular jab and a haymaker (wide punch) with the left and
right arm, left and right foot kicks as well as a circular mo-
The detection of defensive gestures could be useful to balance the scoreboard. In future work, we might experiment
with defensive gestures to block the activation of water nozzles. We are not sure whether blocking the activation of
a water nozzle is more engaging than the current format,
where water is being hold off by the hands, arms and evasive moves.
Voice Input
A final extension to our game is the incorporation of multimodal input where a roar could result in a power up for the
current move or a reduction of the opponent’s spray time.
IMPLEMENTATION
The implementation of our Water Ball Z prototype touches
three different domains, namely gesture recognition, elec-
tronic design and water plumbing. In the following we describe our work in these three domains in more detail.
as part of a larger gesture. As long as the player holds the
pose, a callback will be triggered every x milliseconds (with
parameter x defined by the developer). For example, a user
can hold their hands to form a virtual energy ball and when
it is powerful enough (based on the elapsed time) they can
unleash it towards their opponent by using a throw gesture.
Gesture Recognition
In order to detect different moves of a player, we use the
existing Mudra [1] 3D gesture recognition engine. Mudra
is the multimodal successor of the Midas [7] multi-touch
framework and offers online gesture processing without explicit segmentation based on a gesture spotting approach [2].
The implicit segmentation of a real-time input data stream
has two advantages. First, players are not forced to perform
explicit segmentation poses and can therefore try to evade
water sprays in many ways while trying to attack their opponent at the same time. Second, by using a precise declarative
gesture definition, sprays will only be triggered when a valid
move is executed. Declarative reasoning over movement in
time makes the recognition system more robust to false positives. The sprays only trigger for the correct movements
which improves the feeling of a real battle compared to a
system that reacts to less focused movements. If necessary,
the gesture rules can be modified to support more variation.
A partial implementation of a clockwise (CW) circular motion gesture in Mudra is illustrated in Listing 1. The first
few lines declare a rule that will be triggered whenever the
right hand (id 15) relative to the right shoulder (id 12) enters a virtual ellipsoid. The coordinates (lines 3 to 8) are obtained by transforming, rotating and scaling the ellipsoid in
a graphical editor [6]. A boolean matchEllipsoid function checks whether a relative joint falls inside the given ellipsoid coordinates. In case the conditions are met, on lines 9
and 10 we assert a new fact called EnterEllipsoid that
represents a control point (characteristic point in the movement) and can be used in composite rules to form a gesture.
The following rule (lines 13 to 21) consists of a combination of four EnterEllipsoid control points to define a
circular motion. The gesture is defined by a sequence of relative positions of the hand compared to the shoulder moving
in a clockwise circle. To be more precise, line 14 to 17 expresses that a combination of four EnterEllipsoid facts
(i.e. front, right, back and left) should be found. Further,
line 18 describes that the control points should be matched
in a sequential temporal relation with a maximum timeout
parameter. A full implementation of the gesture set is available on our website3 .
Listing 1. Circular motion gesture
+12V
rule enter circular g0HandR0
r = RelativeJoint { parent joint == 12, child joint == 15 }
no { test matchEllipsoid(0.09754365, -0.45139461,
0.00097420, 0.1444336, 0.5310212, 0.4756404,
0, 0, 0, r.x, r.y, r.z) }
test matchEllipsoid(0.09754365, -0.45139461,
0.00097420, 0.1444336, 0.5310212, 0.4756404,
0, 0, 0, r.previous x, r.previous y, r.previous z)
assert EnterEllipsoid {
id => ”cHandR0”, time=> r.time }
end
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Finally, the application logic can be added using a callback
mechanism. In this case, the option :sac is used to express
that the gesture should be completed at least once (i.e. shootand-continue). Subsequent intermediate steps of the gesture
trigger the callback and make it behave like an online gesture. This results in a direct mapping between the location
of the hand and the activation of the respective water nozzles
installed at the battle station.
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Z: http://wise.vub.ac.be/water-ball-z
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While developing Water Ball Z, we identified the necessity
for three different types of 3D gestures: one-shot, shootand-continue and a holding pose. The one-shot gesture is
a commonly used type of gesture, meaning that a single particular movement results in the corresponding single action.
On the other hand, a shoot-and-continue (sac) means that
a gesture should be matched at least once and that following movements which are in line with the gesture definition
should be processed in an online manner. For instance, the
gesture based on the circular motion of the hand requires
one full rotation and from then on every consecutive online
step in the gesture can trigger an action as defined in the rule
provided in Listing 1. Finally, the holding pose type allows
developers to register an event handler to a particular pose
D0
D1
D2
D3
D4
D5
D6
D7
UNO R3
V1
V2
V3
V4
V5
V6
V7
V8
ARDUINO
rule(:circularcw).register(:sac, 500.ms) { |p|
spray(p / 100 ∗ sprays, 100.ms)
}
+12V
1
+12V
U$7
V3
GND
STPS140U
3
R30
10K
3
4
R29
D
E1 D
10K
V15
S
STPS140U
VV15
N-ch
1
D10
E3
G
R8
10K
U$1
1
STPS140U
5
E1 D
G
1
10K
4
R19
2
10K
V10
4
R7
VV10
D
N-ch
10K
V4
GND
V16
V15
V14
E2
UNO R3
ARDUINO
VIN
GND1
GND2
5V
3V
/RESET
IOREF
RESERVED
R20
S
STPS140U
D5
5
G
E3
G
E2
4
R9
D
E1 D
10K
10K
V5
N-ch
R10
GND
GND
X1-1
D8
VV5D9
D10
D11
D12
D13
GND
AREF
SDA
SCL
A5/SCL
A4/SDA
A3
A2
A1
A0
3
V9
V10
V11
V12
V13
X1-2
D11
1
STPS140U
F1
1A/F-9943749
D0
D1
D2
D3
D4
D5
D6
D7
2
+12V
V1
V2
V3
V4
V5
V6
V7
V8
+12V
1
U$7
rule circularcw
e0 = EnterEllipsoid { id == ”cHandR0” }
e1 = EnterEllipsoid { id == ”cHandR1” }
e2 = EnterEllipsoid { id == ”cHandR2” }
e3 = EnterEllipsoid { id == ”cHandR3” }
test time:before(e0, e1, e2, e3, 1.5.seconds)
assert CircularCW {
start time => e0.time, stop time => e3.time }
end
10K
V5
R10
F1
1A/F-9943749
GND
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Electronic Schema
CONCLUSION
We used a wireless XinoRF Arduino board (similar to an Arduino UNO R3 with wireless connectivity) to digitally interface the solenoid valves with our computer. However, the
digital pins of an Arduino board do not provide the necessary power to control the solenoid valves. In order to
solve this problem, we designed a PCB using N-channel
power MOSFETs (40V/23mΩ) and 1N4001 Diodes. We can
then draw 12V from the Arduino board using an external
battery to couple it to the ten plastic water solenoid valves
(12V-1/2” Nominal). Figure 2 shows the Water Ball Z electronic schematics. Furthermore, the complete Water Ball Z
solenoid module is highlighted in Figure 3.
We have provided an in-depth description of our novel augmented fighting game using body motion input and water
sprays as haptic feedback. By using an RGB-D sensor for
the tracking, there is no need for calibration or special tracking suits. In addition, the use of well-positioned water nozzles allows us to simulate the effect of a real hit without causing pain or injury to the player. Future work includes the
experimentation and study of various options such as a live
scoreboard, power up moves, defensive gestures or voice input. Further, we encourage gesture recognition approaches
to incorporate our new shoot-and-continue and holding pose
control parameters, offering software developers fine grained
control over their action handlers. After various Water Ball Z
sessions, we are confident that the presented ideas contribute
to the blending of the virtual and the real world and that the
presented water-based free air feedback can be applied to
other games or application domains. Water Ball Z with its
water-based tactile feedback might further stimulate novel
interaction paradigms for augmented reality games and entertainment systems.
ACKNOWLEDGEMENTS
Figure 3. Water Ball Z solenoid module
Solenoid Valves and Nozzles
For our prototype, we used off-the-shelf gardening hoses in
combination with plastic solenoid valves. The water input is
split into multiple hoses and is then switched by the solenoid
valves. The final output of each water hose is attached to a
nozzle aiming at a player’s front, head, back or other parts.
An installation of our prototype is shown in Figure 4 and
requires about 11 metres in length and 5 metres in width for
a single battle station.
We would like to thank everybody who helped in realising
Water Ball Z, in particular Christophe Scholliers, Hannes
De Bondt, Wouter Heyse, Sandra Trullemans and Kevin Van
Vaerenbergh. The work of Lode Hoste is funded by an IWT
doctoral scholarship.
REFERENCES
1. L. Hoste, B. Dumas, and B. Signer. Mudra: A Unified
Multimodal Interaction Framework. In Proc. of ICMI
2011, Alicante, Spain, November 2011.
2. L. Hoste, B. D. Rooms, and B. Signer. Declarative
Gesture Spotting Using Inferred and Refined Control
Points. In Proc. of ICPRAM 2013, Barcelona, Spain,
February 2013.
3. Y. Matoba, Y. Takahashi, T. Tokui, S. Phuong, and
H. Koike. AquaTop Display. In Proc. of VRIC 2013,
Laval, France, March 2013.
4. N. Parés, J. Durany, and A. Carreras. Massive Flux
Design for an Interactive Water Installation: Water
Games. In Proc. of ACE 2005, Valencia, Spain, June
2005.
5. H. Richter, F. Manke, and M. Seror. LiquiTouch: Liquid
as a Medium for Versatile Tactile Feedback on Touch
Surfaces. In Proc. of TEI 2013, Barcelona, Spain,
February 2013.
Figure 4. Tests with an early Water Ball Z prototype
Limitations
We currently use a Microsoft Kinect sensor which is not
suitable for use in direct sunlight. However, more expensive models such as the Panasonic D-IMager EKL3106 can
serve as a direct replacement for this hardware component.
Further, in future work we plan to evaluate the effects of several options such as the live scoreboard, power up moves and
blocking gestures.
6. B. D. Rooms. VolTra: A Development Environment for
Extracting and Defining 3D Full-Body Gestures.
Master’s thesis, Vrije Universiteit Brussel, 2012.
7. C. Scholliers, L. Hoste, B. Signer, and W. De Meuter.
Midas: A Declarative Multi-Touch Interaction
Framework. In Proc. of TEI 2011, Funchal, Portugal,
January 2011.
8. R. Sodhi, I. Poupyrev, M. Glisson, and A. Israr.
AIREAL: Interactive Tactile Experiences in Free Air. In
Proc. of SIGGRAPH 2013, Anaheim, USA, July 2013.