Tangible A

Tangible Augmented Reality
Mark Billinghurst Hirokazu Kato Ivan Poupyrev
HIT Laboratory Faculty of Information Sciences Interaction Lab
University of Washington Hiroshima City University Sony CSL
Box 352-142, Seattle 3-4-1, Ozaka-higashi, Asaminami-ku 3-14-13 Higashi-Gotanda
WA 98195, USA Hiroshima 731-3194, JAPAN Tokyo 141-0022, JAPAN
grof@hitl.washington.edu kato@sys.im.hiroshima-cu.ac.jp poup@csl.sony.co.

ABSTRACT In this paper we advocate a new approach to designing AR
This paper advocates a new metaphor for designing three- interfaces that we refer to as Tangible Augmented Reality
dimensional Augmented Reality (AR) applications, Tangible (Tangible AR). Tangible AR interfaces are those in which 1)
Augmented Reality (Tangible AR). Tangible AR interfaces each virtual object is registered to a physical object (Figure
combine the enhanced display possibilities of AR with the 1) and 2) the user interacts with virtual objects by
intuitive manipulation and interaction of physical objects or manipulating the corresponding tangible objects. In the
Tangible User Interfaces. We define what Tangible AR Tangible AR approach the physical objects and interactions
interfaces are, present some design guidelines and are equally as important as the virtual imagery and provide a
prototype interfaces based on these guidelines. Experiences very intuitive way to interact with the AR interface.
with these interfaces show that the Tangible AR metaphor
supports seamless interaction between the real and virtual
worlds, and provides a range of natural interactions that are
difficult to find in other AR interfaces.
CR Categories: H.5.2 [User Interfaces] Input devices and
strategies; H.5.1 [Multimedia Information Systems]
Artificial, augmented , and virtual realities.
Keywords: Augmented Reality, Collaboration, CSCW.
Additional Keywords: Tangible User Interfaces
1 INTRODUCTION
In 1965 Ivan Sutherland built the first head mounted display
and used it to show a simple wireframe cube overlaid on the
real world [1], creating the first Augmented Reality (AR)
interface. We use the term Augmented Reality to mean
interfaces in which three-dimensional computer graphics are Figure 1: The user interacts with 3D virtual objects by
superimposed over real objects, typically viewed through manipulating a tangible object; such as a simple paper card.
headmounted or handheld displays. Our work derives large part of its inspiration from Ishii’s
Today, computers are incomparably faster and graphics are Tangible Media group [3]. The many projects developed by
almost lifelike, but in many ways AR interfaces are still in this group are developed around the notion of the Tangible
their infancy. As Ishii says, the AR field has been primarily User Interface (TUI) in which real world objects are used as
concerned with “..considering purely visual augmentations” computer input and output devices, or as Ishii puts it “by
[2] and while great advances have been made in AR display coupling digital information to everyday physical objects
technologies and tracking techniques, interaction with AR and environments” [2]. The Tangible AR approach builds
environments has been usually limited to either passive on the principles suggested by TUI by coupling an AR
viewing or simple browsing of virtual information registered visual display to a tangible physical interface.
to the real world. Few systems provide tools that let the In the remainder of this paper we first review related work
user interact, request or modify this information effectively and describe the need for new AR interface metaphors. We
and in real time. Furthermore, even basic interaction tasks, then describe our notion of Tangible AR in more detail and
such as manipulation, copying, annotating, dynamically outline some Tangible AR design principles. Next we show
adding and deleting virtual objects to the AR scene have how these design principles have been applied in a variety
been poorly addressed. of prototype applications and describe the technology
involved.

Note that some of the interfaces described have previously everyday physical objects for effective manipulation of
been written about in other publications. However this is virtual objects.
the first time that they have been used together as examples
For over a decade researchers have been investigating
of the Tangible AR design approach. This paper aims to
computer interfaces based on real objects. We have seen
generalize scattered results, report new work, and present
the development of the Digital Desktop [ 16], ubiquitous
the conceptual vision that has been driving our exploration.
computing [17] and Tangible User Interfaces (TUI) [2]
2 BACKGROUND among others. The goal of these efforts is to make the
When a new interface medium is developed it typically computer vanish into familiar real world objects or the
progresses through the following stages: environment. As with VR interfaces, recent Tangible User
1. Prototype Demonstration Interfaces employ unique interface metaphors in areas such
2. Adoption of Interaction techniques from as 3D content creation [18], and work has begun on
other interface metaphors developing and testing models of user interaction [19].
3. Development of new interface metaphors Tangible interfaces are extremely intuitive to use because
appropriate to the medium physical object manipulations are mapped one-to-one to
4. Development of formal theoretical models for virtual object operations, and they follow a space-
predicting and modeling user interactions multiplexed input design [19]. In general input devices can
For example, the earliest immersive Virtual Reality (VR) be classified as either space- or time-multiplexed. With a
systems were used to just view virtual scenes. Then space-multiplexed interface each function has a single
interfaces such 3DM [ ] explored how elements of the
4 physical device occupying its own space. In contrast, in a
desktop WIMP metaphor could be used to enable users to time-multiplexed design a single device controls different
model immersively and support more complex interactions. functions as different points in time. The mouse in a WIMP
Next, interaction techniques such as the Go -go [5] or World interface is a good example of a time-multiplexed device.
in Miniature [6] were developed which are unique to Virtual Space-multiplexed devices are faster to use than time-
Reality. Most recently, researchers are attempting to arrive multiplexed devices because users do not have to make the
at a formal taxonomy for characterizing interaction in virtual extra step of mapping the physical device input to one of
worlds that would allow developers to build 3D virtual several logical functions. In most manual tasks space-
interfaces in a systematic manner [7]. multiplexed devices are used to interact with the
surrounding physical environment.
In many ways AR interfaces have barely moved beyond the
first stage. The earliest AR systems were used to view Although intuitive to use, with TUI interfaces information
virtual models in a variety of application domains such as display can be a challenge. It is difficult to dynamically
medicine [8] and machine maintenance [9]. These interfaces change an object’s physical properties, so most information
provided a very intuitive method for viewing three- display is confined to image projection on objects or
dimensional information, but little support for creating or augmented surfaces. In those Tangible interfaces that use
modifying the AR content. More recently, researchers have three-dimensional graphics there is also often a disconnect
begun to address this deficiency. The AR modeler of between the task space and display space. For example, in
Kiyokawa [10] uses a magnetic tracker to allow people to the Triangles work [20], physical triangles are assembled to
create AR content, while the Studierstube [11] and EMMIE tell stories, but the visual representations of the stories are
[12] projects use tracked pens and tablets for selecting and shown on a separate monitor distinct from the physical
modifying AR objects. More traditional input devices, such interface. Presentation and manipulation of 3D virtual
as a hand-held mouse or tablet [13][14], as well as intelligent objects on projection surfaces is difficult [21], particularly
agents [15] have also been investigated. However these when trying to support multiple users each with
attempts have largely been based on existing 2D and 3D independent viewpoints. Most importantly, because the
interface metaphors from desktop or immersive virtual information display is limited to a projection surface, users
environments. This means that in order to interact with are not able to pick virtual images off the surface and
virtual content the user uses a special purpose input device manipulate them in 3D space as they would a real object.
and the number of physical input devices are limited.
So we see that current Tangible interfaces provide very
In our work we are trying to develop interface metaphors intuitive manipulation of digital data, but limited support for
that enable AR applications to move beyond being primarily viewing 3D virtual objects. In contrast current AR interfaces
3D information browsers. In Augmented Reality there is an provide an excellent interface for viewing virtual mo dels, but
intimate relationship between 3D virtual models and limited support for interaction and space-multiplexed input
physical objects these models are attached to. This devices. We believe that a promising new AR interface
suggests that one promising research direction may arise metaphor can arise from combining the enhanced display
from taking advantage of the immediacy and familiarity of possibilities of Augmented Reality with the intuitive

manipulation of Tangible User Interfaces. We call this 4 TANGIBLE AR INTERFACES
comb ination Tangible Augmented Reality. In the next In order to explore the Tangible AR design space we have
section we show how Tangible AR supports seamless developed the following prototype interfaces:
interaction, and provide some design guidelines. Space-Multiplexed Interfaces
Shared Space: A collaborative game
3 TANGIBLE AUGMENTED REALITY ARgroove: A music performance interface
The goal of computer interfaces is to facilitate seamless Tiles: A virtual prototyping application
interaction between a user and their computer-supported
task. In this context, Ishii defines a seam as a discontinuity Time-Multiplexed Interfaces
or constraint in interaction that forces the user to shift VOMAR: A scene assembly application
among a variety of spaces or modes of operation [22]. In this section we briefly describe these interfaces, showing
Seams that force a user to move between interaction spaces how the Tangible AR design principles have been applied.
are called functional seams, while those that force the user Although each of this interfaces has been developed for a
to learn new modes of operation are cognitive seams. particular application the interaction principles that we have
In the previous section we described how Tangible User explored in them are generic and can be broadly applied.
Interfaces provide seamless interaction with objects, but SHARED SPACE Space-Multiplexed Interaction in AR
may introduce a discontinuity or functional seam between The Shared Space interface is an example of how Tangible
the interaction space and display space. In contrast most AR principles can be used to design simple yet effective
AR interfaces overlay graphics on the real world interaction multiple user space-multiplexed AR interfaces [23]. The
space and so provide a spatially seamless display. However Shared Space was designed to create a collaborative AR
they often force the user to learn different techniques for game that could be used with no training. Several people
manipulating virtual content than from normal physical stand around a table wearing Olympus HMDs with cameras
object manipulation or use a different set of tools for attached (figure 2). The video from the camera is feedback
interacting with real and virtual objects. So AR interfaces into the HMD to give them a video-mediated view of the
may introduce a cognitive seam. world with 3D graphics overlaid. On the table are cards and
A Tangible AR interface provides true spatial registration when these are turned over, in their HMDs the users see
and presentation of 3D virtual objects anywhere in the different 3D virtual objects appearing on top of them. The
physical environment, while at the same time allowing users users are free to pick up the cards and look at the models
to interact with this virtual content using the same from any viewpoint.
techniques as they would with a real physical object. So an
ideal Tangible AR interface facilitates seamless display and
interaction, removing the functional and cognitive seams
found in traditional AR and Tangible User Interfaces. This
is achieved by using the design principles learned from TUI
interfaces, including:
• The use of physical controllers for
manipulating virtual content.
• Support for spatial 3D interaction techniques
(such as using object proximity).
• Support for both time-multiplexed and space-
multiplexed interaction.
• Support for multi-handed interaction. Figure 2: Shared Space: the insert shows user view
• Support for Matching the physical
constraints of the object to the requirements The goal of the game is to collaboratively match objects
of the interaction task. that logically belonged together. When cards containing
• The ability to support parallel activity where correct matches are placed side by side an animation is
multiple objects are being manipulated. triggered (figure 3). For example, when the card with the
• Collaboration between multiple participants UFO on it is placed next to the alien, the alien appears to
jump into the UFO and start to fly around the Earth.
Our central hypothesis is that AR interfaces that follow
these design principles will provide completely seamless
interaction with virtual content and so will be extremely
intuitive to use. In the next section we describe some
prototype interfaces that support this hypothesis.

When users were asked to comment on what they liked
most about the exhibit, interactivity, how fun it was, and
ease of user were the most common responses. Perhaps
more interestingly, when asked what could be improved,
people thought that reducing the tracking latency,
improving image quality and improving HMD quality were
most important.
ARGROOVE: 3D AR Interface for Interactive Music
ARGroove is a tangible AR musical interface that allows
Fig 3: The Spatial Interaction Based on Proximity people to control interactive electronic musical
compositions individually or in a group. Unlike the previous
Although is a very simp le application it provides a good
interface ARGroove uses more complex 3D physical motion
test of the usefulness of the tangible interface metaphor for
to control a non-visual modality, and it uses 3D widgets for
manipulating virtual models. Shared Space is essentially
AR displays.
virtual model viewing software that support six degree of
freedom viewing with a simple proximity based interaction. In the ARGroove music is constructed from a collection of
However, rather than using a mouse or magnetic tracker to short looped elements, each carefully composed to fit
manipulate the model, users just hold cards. The form of the others so they can be interactively mixed. For each
cards encourage people to manipulate them the same way individual loop a composer assigns filters and effects, to
they would use normal playing cards, such as turning them allow the user to interactively modulate them.
over, rotating them, holding them in the hands, passing The interface consists of a number of real LP records with
them to each other and placing them next to each other. markers placed on a table in front of a projection screen. An
They can hold them in either hand and many cards can be overhead camera captures the image of the table and shows
uncovered at once to show a number of different objects. it on the screen with additional 3D widgets overlaid on each
Shared Space has been shown at Siggraph 99 and Imagina card (figure 7). The user can play, remix and modulate
2000 and the thousands of people that have tried it have musical elements by picking up and manipulating the real
had no difficulty with the interface. Users did not need to records in space which serve both as physical musical
learn any complicated computer interface or command set containers, grouping together related elements of musical
and they found it natural to pick up and manipulate the composition, and tangible AR 3D controllers, that allow
physical cards to view the virtual objects from every angle. users to interactively modulate music, mix and fade between
Players would often spontaneously collaborate with music pieces. To start playing a musical element, the user
strangers who had the matching card they needed. They simply flips over the associated record so that the overhead
would pass cards between each other, and collaboratively camera can identify it and start playing corresponding
view objects and completed animations. By combining a musical sequence. The user then modifies the sound by
tangible object with virtual image we found that even young translating the record up and down, or rotating and tilting it
children could play and enjoy the game. (Figure 8), these 3D motions are mapped into corresponding
modulation functions, e.g. pitch, distortion, amplitude, filter
After the Imagina 2000 experience 157 people filled out a cut-off frequency, and delay mix. Since the system tracks
short user survey, answering the following questions on a and recognizes several records at the same time, users can
scale of one to seven (1=not very easily, 7=very easily): play several musical elements simultaneously and
Q1: How easily could you play with other people? collaboratively.
Q2: How easily could you interact with the virtual objects?
Table 2 summarizes the results. Users felt that they could
very easily play with the other people (5.64) and interact
with the virtual objects (5.62). Both of these are significantly
higher than the neutral value of 3.5; the t-test value row
showing the results from a one-tailed t-test.
Q1 (n=132) Q2 (n=157)
Average 5.64 5.62
Std Dev. 1.19 1.20
t-test val. 237.09, p<0.01 278.74, p<0.01
Table 2: Shared Space Survey Results Figure 7: Two users are playing music in the Augmented
Groove: the insert shows the view on the projector screen

A three-dimensional virtual controller is overlaid on top of across tangible AR widgets (that we called tiles) letting the
each of the records providing the user with instant visual user to choose operation simply by picking a needed tile.
feedback on the state and progression of musical The application domain is rapid prototyping for aircraft
performance. For each control dimension a corresponding instrument panels. The interface consists of a metal
graphical element changes depending on the value of the whiteboard, a book, and two stacks of magnetic tiles
control (figure 8). For example, as the user raises the record (approximately 15cm x 15cm). Sitting in front of the
a pyramid in the middle also goes up and when the musical whiteboard the user wears a lightweight high resolution
control reaches limit a small, animated character pops up Sony Glasstron HMD with a video camera attached (fig. 4).
cuing the user. Although virtual controls are not absolutely
necessary to control music, they are essential to make the
system easier and more enjoyable to use.

Figure 4: Using the Tiles Interface
The various tangible elements of the interface serve a
different purpose. The whiteboard is the working space
where users can layout virtual aircraft instruments. The
book serves as a menu object, and when the user looks
through its pages they will see a different virtual instrument
model on each page. One stack of tiles serve as data tiles
and shows no virtual content until virtual objects are copied
onto them. The remaining tiles are operator tiles and are
Figure 8: Gestural musical interface in Augmented Groove used to perform basic operations on the data tiles. There is
a unique tile for each operation and currently supported
ARGroove was demonstrated at SIGGRAPH 2000 where
operations include deletion, copying and a help function.
users could perform an electronic composition using three
Each of the operations tiles has a different three-
records. Over 1500 people tried the experience and the
dimensional virtual icon on them to show what their
majority found it very enjoyable. One of the reasons what
function is and tell them apart from the data tiles (fig 5).
was that the range of possible musical variations afforded
was very large and the ability to simultaneously control
multiple sound effects and filters resulted in a very
expressive and enjoyable interface. Only a short explanation
was usually sufficient for them to be able to effectively
control the musical performance. We interviewed 25 users
and of these 92% rated the experience as “Excellent”. In
terms of ease of learning 40% rated it as “Excellent”, while
52% said it was “Okay” and only 8% as “Poor”.

TILES: Putting Everything Together Trashcan delete widget Talking head help widget
Tiles is an AR authoring interface that explores how more
Figure 5: Virtual Widgets on Operator Tiles
complicated behaviors can be supported, including
copying, pasting, deleting, and browsing virtual information As with the Shared Space interface, virtual images appear
in AR settings. In Shared Space all of the cards possessed attached to the physical objects and can be picked up and
the same functionality, whereas in Tiles we further explore looked at from any viewpoint. Interaction between objects
space-multiplexed control by assigning different behaviors is also based on physical proximity, however the operation
to different objects, creating tangible 3D widgets that we that is invoked by bringing objects next to each other
first explored in ARGroove. We distribute functionality depends on their semantic. For example, to copy a virtual
instrument from the menu book to an empty data tile, the tile

is just placed by the appropriate book page. However, The application is layout of virtual furniture in a room,
touching a data tile that contains a virtual instrument with although the same interface can be applied to many
the trashcan delete tile, removes the virtual n strument,
i domains. When the user opens the book on each of its
while putting the help tile beside it displays a help message pages they see a different set of virtual furniture, such as a
(fig 5). Once virtual instruments have been placed on the set of chairs, rugs etc (fig 10 (b)). The 3D virtual models
data tiles, these can be attached to the whiteboard to layout appear exactly superimposed over the real book pages.
a prototype virtual instrument panel (figure 6). Looking at the large piece of paper they see an empty
virtual room. They can then copy and transfer objects from
the book to the virtual room using the paddle (fig 10 (c,d)).
The paddle is the main interaction device and it is a simple
object with an attached tracking symbol. It is designed to be
used by either hand and allows the user to make static and
dynamic gestures to interact with the virtual objects:
Static Dynamic
1. Paddle proximity to 1. Shaking (side to side
object motion of paddle)
2. Paddle tilt/inclination (e.g. 2. Hitting (up and down
Figure 6: Virtual Instrument Panel view by the user fig 10 (d)) motion of paddle)
3. Pushing object (fig 10 (e))
The main difference between this interface and the previous
interfaces is the use of different shaped physical objects for
different interface properties, and assigning different
semantics to different objects. Supporting one interface
function per object is similar to the interface models of
desktop GUI interfaces, where each icon and tool has
unique functionality. Despite this added functionality, the
physical interactions are still based on object manipulation
and proximity, showing that quite complex AR interfaces
can be built from simple physical interactions. The use of
(a) The VOMAR interface (b) A virtual furniture menu
different objects for different functions further emphasizes
the space-multiplexed nature of the interface.
We have not evaluated the Tiles interface in a formal
manner, however we have demonstrated it at the ISAR 2000
conference were over 70 people tried it. Once again these
users had no difficult with the interface, although the wider
range of interface functions typically meant that they
needed to be shown how the different operator tiles worked
before they tried it themselves.
(c) Picking virtual furniture (d) Placing object in a room by
VOMAR – Time-Multiplexed Interaction in AR object with a paddle sliding it from the paddle
The VOMAR project explores how a time-multiplexed
Tangible AR interface could be designed. VOMAR uses a
single input device that allows the user to perform multiple
different tasks in a virtual scene assembly application. To
achieve this we explored how complex physical gestures
can be used to support natural and effective interaction.
The physical components of the interface comprise a real
book, a cardboard paddle the user holds in their hand, a
large piece of paper and a lightweight HMD the user wears
(e) Moving virtual objects (f) Constructed scene
(figure 10 (a)). As before the form of each of these objects
reflects their function; the book serves as a container Figure 10: The VOMAR interface
holding all the virtual models, the paddle is the main To copy an object from the object book onto the paddle the
interaction device, and the large piece of paper the user simple places the paddle beside the desired object and
workspace. the close proximity is detected and the object copied onto

the paddle (fig 10 (c)). Once a model is on the paddle it can the dynamic changes in their configuration, and how this
be picked up and viewed from any viewpoint. To drop a can be achieved are interesting future research directions.
model into the virtual scene the paddle is placed at the Another advantage is the use of physical form-factor to
desired location and tilted until the model slides off (figure support interface functions. In our interfaces the physical
10 (d)). Models in the scene can be pushed around by design of the tangible interfaces elements provide
pushing motions of the paddle (fig. 10 (e)). A shaking affordances that suggest how they are to be used. So for
motion is used to delete an object from the paddle, while example, in the Shared Space interface users discover the
models in the virtual room can be removed by hitting them. proximity based interactions because placing cards together
As can be seen these interactions are very natural to is a natural behaviour and suggested by the shape of the
perform with a real paddle, so in a matter of a few moments a cards. Naturally the physical form factor and the computer
user can assemble a fairly complex arrangement of virtual graphics design of the virtual images attached to the
furniture (figure 10 ( Of course what the user is really
f)). interfaces is important and should correspond to each other.
doing is interacting with a simple CAD program, but instead
Finally, Tangible AR interfaces naturally support face-to-
of using a mouse or keyboard they are just manipulating a
face collaboration as shown by the Shared Space and
cardboard paddle in very intuitive ways.
ARgroove interfaces. People commonly use the resources
5 DISCUSSION of the physical world to establish a socially shared meaning
Tangible AR interfaces couple AR displays with tangible [24]. Physical objects support collaboration both by their
user interface controllers and 3D spatial interaction to appearance, the physical affordances they have, their use
design a wide variety of powerful AR interfaces. From our as semantic representations, their spatial relationships, and
prototype interfaces we have found that there are several their ability to help focus attention. In a Tangible AR
advantages of Tangible AR interfaces. First, Tangible AR interface the physical objects can further be enhanced in
interfaces are transparent interfaces that provide for ways not normally possible such as providing dynamic
seamless two-handed 3D interaction with both virtual and information overlay, private and public data display, context
physical objects. They do not require participants to use or sensitive visual cues, and physically based interactions.
wear any special purpose input devices and tools , such as One of the reasons why our interfaces are successful is that
magnetic 3D trackers, to interact with virtual objects. the virtual models appear to be attached to the physical
Instead users can manipulate virtual objects using the same objects they are associated with. In the next section we
input devices they use in physical world – their own hands describe the tracking technology that makes this possible.
- leading to seamless interaction with digital and physical
6 TRACKING TECHNOLOGY
worlds. This property also allows the user to easily use
both digital and conventional tools in the same working Precise registration of real and virtual objects anywhere in
space. space is a significant research problem in the AR field.
Tangible AR allows seamless spatial interaction with Azuma provides a good review of the issues faced in AR
virtual objects anywhere in their physical workspace. The tracking and registration [25] and there are a number of
user is not confined to a certain workspace but can pick up possible tracking approaches that could be used in
and manipulate virtual data anywhere just as real objects, as developing Tangible AR interfaces. We have developed a
well as arrange them on any working surface, such as a computer-vision based method in which a virtual model can
table or whiteboard. The digital and physical workspaces be fixed in space relative to one or more tracking markers
are therefore continuous, naturally blending together. [26]. These markers are simple black squares with a unique
pattern inside them. Our approach is summarized in figure
Tangible AR interfaces can allow the design of a simple yet
13. After thresholding an input image, square markers are
effective and consistent AR interface model, providing a set
extracted and identified. Then pose and position of markers
of basic tools and operations that allow users, for example,
are estimated from coordinates of the 4 vertices. Finally
to add, remove, copy, duplicate and annotate virtual objects
virtual images are drawn on the input image. A more
in AR environments.
complete explanation is given in Appendix A.
An interesting property of Tangible AR interfaces is their One of the advantages of this method is that in a video-see
ad-hoc, highly re-configurable nature. Unlike traditional through AR system, the video frame that is shown in the
GUI and 3D VR interfaces, Tangible AR interfaces are in users display is the same frame that is used to track their
some sense designed by user as they are carrying on with viewpoint and so the virtual models can appear exactly
their work. In these interfaces the users are free to put overlaid on a real object. These markers are also simple and
interface elements anywhere they want: tables, whiteboards, cheap and can be attached to any flat surface. The tracking
in boxes and folders, arrange them in stacks or group them works in real time (30 fps on a 766 Mhz Pentium III) and is
together. How the interface components should be robust provided the square marker is in view.
designed for such environments, if they should be aware of

Figure 13: Vision based AR Tracking Process
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In the future we plan to conduct more rigorous user studies
Shared Virtual Reality and a Shared Augmented Reality",
to quantify the benefits of Tangible AR interface, as well as
Proceedings of the IEEE International Conference on
developing a wider range of interaction techniques such as
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14. Hollerer, T., S. Feiner, et al. (1999). “Exploring MARS: tracking process. A size-known square marker is used as a
developing indoor and outdoor user interfaces to a base of the marker coordinates in which virtual objects are
mobile augmented reality system.” Computers & represented. The goal is to find the transformation matrix
Graphics 23: 779-785. from the marker coordinates to the camera coordinates (Tcm),
represented in eq.1. This matrix consists of the rotation R3x3
15. Anabuki, M., H. Kakuta, et al. (2000). Welbo: An
and the translation T3x1. These values are found by
Embodied Conversational Agent Living in Mixed Reality
detecting the markers in the camera image plane and using
Spaces. CHI'2000, Extended Abstracts, ACM.
the perspective transformation matrix.
16. Wellner, P. Interactions with Paper on the DigitalDesk.
Communications of the ACM, Vol. 36, no. 7, July 1993,
pp. 87-96.
17. Weiser, M. The Computer for the Twenty-First Century.
Scientific American, 1991, 265 (3), pp. 94-104.
18. Anderson, D., Frankel, J., et. al. Tangible Interaction +
Graphical Interpretation: A New Approach to 3D
Modeling. In Proceedings of SIGGRAPH 2000, August
2000, New Orleans, ACM Press, pp. 393-402.
19. Fitzmaurice, G. and Buxton, W. (1997). An Empirical
Evaluation of Graspable User Interfaces: towards
specialized, space-multiplexed input. Proceedings of the
ACM Conference on Human Factors in Computing Figure 14: Coordinates System for Tracking
Systems (CHI'97). pp. 43-50. New York: ACM.
20. Gorbet, M., Orth, M., Ishii, H. Triangles: Tangible
Interface for Manipulation and Exploration of Digital
Information Topography. In Proceedings of CHI 98, Los
Angeles, CA, 1998.
21. Fjeld, M., Voorhorst, F., Bichsel, M., Lauche, K., Rauter-
berg, M., H., K. (1999). Exploring Brick-Based Navigation
and Composition in an Augmented Reality. In
Proceedings of HUC’ 99, pp. 102-116.
22. Ishii, H., Kobayashi, M., Arita, K. (1994). Iterative (eq.1)
design of seamless collaborative media. CACM 37(8),
83-97.
1) Perspective Transformation Matrix
23. Billinghurst, M., Poupyrev, I., Kato, H., May, R. (2000)
The camera coordinate frame is transformed to the ideal
Mixing Realities in Shared Space: An Augmented
screen coordinates by the perspective matrix P (eq.2). This
Reality Interface for Collaborative Computing. In
perspective transformation matrix is found from an initial
Proceedings of the IEEE International Conference on
off-line calibration process.
Multimedia and Expo (ICME2000), July 30th - August 2,
New York.
24. Gav, G., Lentini, M. Use of Communication Resources in
a Networked Collaborative Design Environment.
http://www.osu.edu/units/jcmc/IMG_JCMC/ResourceUs
e.html , (eq.2)

25. Azuma, R. "A Survey of Augmented Reality", Presence, 2) Marker Detection
6, 4, pp.355-385, 1997
Step 1: Low-level Image Processing

Input images are thresholded by the constant value. Then vertices coordinates of the marker in the marker coordinate
all regions in the image are labeled. frame and those coordinates in the ideal screen coordinate
Step 2: Marker Extraction frame, eight equations including translation component Tx
Ty Tz are generated and the value of these translation
The regions whose outline contour can be fitted by four line component Tx Ty Tz can be obtained from these equations.
segments are extracted. Parameters of these four line
segments and coordinates of the four vertices of the Step 3: Modification of R3x3 and T3x1
regions found from the intersections of the line segments The transformation matrix found from the method mentioned
are calculated in the ideal screen coordinates and stored. above may include error. However this can be reduced
3) Pose and Position Estimation through the following process. The vertex coordinates of
the markers in the marker coordinate frame can be
Step 1: Estimation of R3x3 transformed to coordinates in the ideal screen coordinate
When two parallel sides of a marker are projected on the frame by using the transformation matrix obtained. Then the
image screen, the equations of those line segments in the transformation matrix is optimized as sum of the difference
ideal screen coordinates are the following: between these transformed coordinates and the coordinates
(eq. 3) measured from the image goes to a minimum. Though there
are six independent variables in the transformation matrix,
only the rotation components are optimized and then the
For each of markers, the value of these parameters has been translation components are re-estimated by using the
already obtained. The equations of the planes that include method in step 2. By iteration of this process a number of
these two sides respectively can be represented as (eq.4) in times the transformation matrix is more accurately found. It
the camera coordinates frame by substituting x and yc in
c would be possible to deal with all of six independent
(eq.2) for x and y in (eq.3). variables in the optimization process. However,
computational cost has to be considered.
(eq.4) Evaluation of Tracking Accuracy
Given that normal vectors of these planes are n1 and n2 In order to evaluate accuracy of the marker detection, the
respectively, the direction vector of the parallel two sides of detected position and pose were recorded while a square
the marker is given by the outer product. Given that two marker 80[mm] wide was moved perpendicular to the camera
unit direction vectors that are obtained from two sets of two and tilted at different angle. Figure 16 shows errors of
parallel sides of the marker is u1 and u2, these vectors position. Accuracy decreases the further the cards are from
should be perpendicular. However, image processing errors the camera and the further they are tilted from the camera.
mean that the vectors won't be exactly perpendicular. To
compensate for this two perpendicular unit direction
vectors are defined by v1 and v2 in the plane that includes u1
and u2 as shown in figure 15. Given that the unit direction
vector which is perpendicular to both v1 and v2 is v3, the
rotation component R3x3 in the transformation matrix Tcm
from marker coordinates to camera coordinates specified in
eq.1 is [V1t V2t V3t].

Figure 16: Errors of position

Figure 15: Two perpendicular unit direction vectors
Step 2: Estimation of T3x1
Since the rotation component R3x3 in the transformation
matrix Tcm was given, by using (eq.1), (eq.2), the four

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