13th International Symposium on Measurement and Control in Robotics- Toward Advanced Robots: Design, Sensors, Control and Applications Madrid, Spain, December 11-12,2003

Recent Advance in Telexistence-Retro-reflective Projection Technology based Mutual TelexistenceS.Tachi
The University of Tokyo
Department of Information Physics & Computing
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 Japan
http://www.star.t.u-tokyo.ac.jp/
Abstract
Telexistence is fundamentally a concept named for thetechnology that enables a human being to have a realtime sensation of being at a place other than where he or
she actually exists, and to interact with the remoteenvironment, which may be real, virtual, or a
combination of both. It also refers to an advanced typeof teleoperation system that enables an operator at thecontrol to perform remote tasks dexterously with thefeeling of existing in a surrogate robot. Althoughconventional telexistence systems provide an operator
the real-time sensation of being in a remote environment,
persons in the remote environment have only thesensation that a surrogate robot is present, not theoperator. Mutual telexistence aims to solve this problem
so that the existence of the operator is apparent topersons in the remote environment by providing mutual
sensations of presence. This paper reviews how theoriginal telexistence technology has been developed, anddescribes a newly developed method of mutual
telexistence using projection technology onto retroreflective objects dubbed RPT (Retro-reflective
Projection Technology).

1 Introduction
Telexistence (tele-existence) is a technology that enables
us to control remote objects and communicate withothers in a remote environment with a real-time sensationof presence by using surrogate robots, remote/local
computers, and cybernetic human interfaces.
This
concept has been expanded to include the projection of
ourselves into computer-generated virtual environments,
and also the use of a virtual environment for theaugmentation of the real environment.
Before the concept of telexistence was proposed, there
were several systems that aimed for a similar goal. In the
U.S., Sutherland [1] proposed the first head-mounted
display system, which led to the birth of virtual reality inthe late 1980s. In Italy, C. Mancini et al. [2] developed amobile teleoperated robot system, Mascot, as early as the
1960s. In France, J. Vertut et al. [3] developed ateleoperation system that controlled a submarine for deep
submergence technology in 1977.
Although theseremote robots were not a humanoid type and no sensationof presence was provided in a strict sense, the systems
were closely related to the concept of telexistence, andcan be regarded as its forerunner.
In order to intuitively control a remote humanoid robot, it
is important to locally provide the operator a natural
sensation of presence as if the operator felt directly in the
remote site, by means of visual, auditory, tactile, andforce sensations. The concept of providing an operator
with a natural sensation of presence to facilitate
dexterous remote robotic manipulation tasks was called"telepresence" by Minsky [4] in USA and "telexistence"by Tachi et al. [5] in Japan.
The concept of telexistence was proposed and patented inJapan in 1980 [6], and became the fundamental guiding
principle of the eight year Japanese National Large Scale
Project of "Advanced Robot Technology in Hazardous
Environments," which was initiated in 1983 together
with the concept of Third Generation Robotics. Throughthis project, we made theoretical considerations,established systematic design procedures, developed
experimental hardware telexistence systems, anddemonstrated the feasibility of the concept.
Through the efforts of twenty years of research anddevelopment in the U.S., Europe and Japan [7-23], it has
nearly become possible for humans to use a humanoid
robot in a remote environment as if it was an other self,
i.e. they are able to have the sensation of being just inside
the robot in the remote environment.
Although existing telexistence systems succeeded inproviding an operator a real-time sensation of being in aremote environment, human observers in the remoteenvironment did not have the sensation that the humanoperator is presented, but only a surrogate robot. Mutual
telexistence addresses this problem so that the existenceof the operator is apparent by persons in the remoteenvironment by providing mutual sensations of presence[24-26].
This paper reviews the original telexistence technology,
and introduces a method of mutual telexistence based on
the projection of real-time images of the operator on the

S.Tachisurrogate robot, based on newly developed technology
dubbed RPT (Retro-reflective Projection Technology).

2 Short History of Telexistence
Figure 1 shows the concept of telexistence in realenvironments, virtual environments, and the realenvironment through a virtual environment (augmented
telexistence). The following describes the research anddevelopment conducted in order to realize the concept.
Our first report [5, 8] proposed the principle of thetelexistence sensory display, and explicitly defined its
design procedure. The feasibility of a visual display witha sensation of presence was demonstrated throughpsychophysical measurements using experimental visualtelexistence apparatus.
A method was also proposed to develop a mobile
telexistence system that can be driven remotely with bothan auditory and visual sensation of presence. A
prototype mobile televehicle system was constructed andthe feasibility of the method was evaluated [12].

Augmented telexistence can be effectively used innumerous situations. For instance, to control a slave
robot in a poor visibility environment, an experimental
augmented telexistence system was developed that uses a
virtual environment model constructed from design dataof the real environment. To use augmented reality in thecontrol of a slave robot, a calibration system using image
measurements was proposed for matching the realenvironment and the environment model [16, 17].
The slave robot has an impedance control mechanism forcontact tasks and to compensate for errors that remain
even after calibration. An experimental operation in apoor visibility environment was successfully conductedby using a humanoid robot called TELESAR(TELExistence Surrogate Anthropomorphic Robot)(Fig.2) and its virtual dual. Figure 3 shows the virtual
TELESAR used in the experiment and Figure 4 shows
the master system for the control of both real TELESAR
and virtual TELESAR.
Experimental studies of tracking tasks demonstrated
quantitatively that a human being can telexist in a remote
and/or a computer-generated environment by using thededicated telexistence master slave system [15].
A networked telexistence paradigm called R-cubed(Real-time Remote Robotics) was proposed in 1985, andseveral pertinent ongoing research efforts are being
conducted, including a real-time remote robot
manipulation language dubbed RCML [18, 19].

Fig.1 Concept of Telexistence.

In 1989, a preliminary evaluation experiment oftelexistence was conducted with the first prototype
telexistence master slave system for remote manipulation.
An experimental telexistence system for real and/or
virtual environments was designed and developed, andthe efficacy and superiority of the telexistence masterslave system over conventional master-slave systems wasdemonstrated experimentally [13, 14, 15].

Fig. 3 Virtual TELESAR at Work (1993).

Fig. 4 Telexistence Master (1989).
Fig. 2 Telexistence Surrogate Anthropomorphic Robot (TELESAR)at Work (1988).

13th International Symposium on Measurement and Control in Robotics- Toward Advanced Robots: Design, Sensors, Control and Applications Madrid, Spain, December 11-12,2003
Telexistence technology was adapted in the national fiveyear Humanoid Robotics Project (HRP) sponsored by the
Ministry of Economy, Trade and Industry (METI) todevelop a new type of cockpit system to control ahumanoid bipedal robot, as shown in Figure 5. The
telexistence cockpit was completed for this project inMarch 2000 (Fig 6). It consists of three main subsystems:
an audio/visual display subsystem, a teleoperation master
subsystem, and a communication subsystem between the
cockpit and the humanoid robot [20, 21, 22, 23].

Fig.5 HRP Humanoid Robot at Work (2000).

Various teleoperation experiments using the developedtelexistence master system confirmed that kinesthetic
presentation through the master system with visual
imagery greatly improves both the operator's sensation ofwalking, and dexterity at manipulating objects.
If the operator issued a command to move the robot, therobot actually walked to the goal. As the robot walked
around, real images captured by a wide field of view
multi-camera system were displayed on four screens of
the surrounded visual display. This made the operator
feel as if he or she was inside the robot, walking aroundthe robot site.

Fig.6 Telexistence Cockpit for Humanoid Control (2000).

A CG model of the robot in the virtual environment wasrepresented and updated according to the current locationand orientation received from sensors on the real robot.
The model was displayed on the bottom-right screen of
the surround visual display, and by augmenting real
images captured by the camera system, it supported theoperator's navigation of the robot. Since the series of real
images presented on the visual display are integrated
with the movement of the motion base, the operator feels
the real-time sensation of stepping up and down.
Persons can control the robot by just moving their bodies
naturally, without using verbal commands. The robot
conforms to the person's motion, and through sensors on
board the robot the human can see, hear and feel as if
they sensed the remote environment directly. Persons canvirtually exist in the remote environment without actually
being there.
For observers in the remote environment, however, thesituation is quite different: they see only the robot
moving and speaking. Although they can hear the voice
and witness the behaviour of the human operator throughthe robot, it does not actually look like him or her. Thismeans that the telexistence is not yet mutual. In order torealize mutual telexistence, we have been pursuing theuse of projection technology with retro-reflective
material as a surface, which we call RPT (Retroreflective Projection Technology) [24, 25, 26].

3 Retro-reflective Projection Technology(RPT)
Two classic virtual reality visual display types are theHead Mounted Display (HMD) and IPT (Immersive
Projection Technology), which although quite useful, are
not without their shortcomings, as shown in Figure 7 (C)and (D), respectively. The former has a tradeoff of highresolution and wide field of view, and the latter has
problems concerning the user's body casting shadows on
a virtual environment, and the interaction between theuser's real body and the virtual interface. In addition,
both displays have problems concerning occlusion whenin use under the augmented reality condition, i.e. virtual
objects and real objects are mixed.
Figure 7 (A) shows a virtual vase and a virtual ashtray on
a virtual desk. When a real hand is placed between two
virtual objects, an ideal occlusion should be depicted asin Figure 7 (B), i.e., the real hand occludes the virtualvase and is occluded by the virtual ashtray. However, areal hand cannot occlude the virtual vase nor be occludedby the virtual ashtray when an optical see-through HMD
is used to display virtual objects, and the hand and theashtray look as if they are transparent. This is simply due
to the fact that the physical display position of an HMD
is always just in front of the eyes of the user.

S.TachiA retro-reflective surface reflects back the projected light
only in the direction of projection, while conventional
screens normally used for IPT scatter projected lights inall directions ideally as a Lambertian surface (Fig.9).
Figure 10 shows how a retro-reflective surface behaves.
It is covered with microscopic beads of about 50
micrometers in diameter, which reflect the incident lightback to the incident direction. It can also be realized witha microstructure of prism-shaped retro-reflectors densely
placed on a surface.
Fig. 7 (A) A virtual vase and a virtual ashtray on a virtual desk; (B)An ideal occlusion when a real hand is placed between two virtual
objects; (C) Unfavorable results when optical see-through HMD isused; (D) Unfavorable results when IPT (Immersive Projection
Technology) like CAVE is used.

Fig. 9 Three typical reflections.

Conversely, the virtual ashtray cannot occlude a real
hand when IPT like the CAVE (CAVE Automatic
Virtual Environment) is used, as shown in Figure 7 (D).
This is due to the fact that the display position of virtual
objects is always on the screen surface, which is one totwo meters away from the human user when IPT displays
are used.
In our laboratory at the University of Tokyo, a new typeof visual display is being developed called X'tal(pronounced crystal) vision [27,28,29,30], which uses
retro-reflective material as its projection surface. We callthis type of display technology RPT (Retro-reflective
Projection Technology).
Under the RPT configuration, a projector is arranged at
the axial symmetric position of a user's eye withreference to a half-mirror, with a pinhole placed in frontof the projector to ensure adequate depth of focus, as
shown in Figure 8. Images are projected onto a screenthat is constructed, painted, or covered with retroreflective material [27].

Fig.8 The principle of RPT system.

Fig. 10 Retro-reflective surface densely covered with microscopic
beads with about 50 micrometer diameter. Ideally, the refractive
index should be 2.

The retro-reflector screen, together with the pinhole,ensures that the user always sees images with accurate
occlusion relations. In the construction of an RPT system,
screen shapes are arbitrary, i.e., any shape is possible.
This is due to the characteristics of the retro-reflector,
and the pinhole in the conjugate optical system.
By using the same characteristics of an RPT system,
binocular stereovision becomes possible using only one
screen with an arbitrary shape. Figure 11 shows how
stereovision can be realized using RTP. In the figure, theDisplay Unit is an arbitrarily shaped object covered or
painted with retro-reflective material. The light projectedby the right projector is retro-reflected on the surface ofthe display unit and is observed by the right eye, while
the light projected by the left projector is retro-reflectedalso by the same display surface and can be observed by
the left eye.
By using the same display surface, the right eye observes
the image projected by the right projector and the left eyeobserves the image projected by the left projector. Thusby generating CG images with appropriate disparity, thehuman observer perceives the stereo view of an object at
the position of the display unit. By using measurementsof the position sensor on the display unit, it is possible todisplay a three-dimensional object image, which changes
its appearance according to the position and orientation

13th International Symposium on Measurement and Control in Robotics- Toward Advanced Robots: Design, Sensors, Control and Applications Madrid, Spain, December 11-12,2003
indicated by the motion of the display. This enables theuser to have the sensation that he or she is handling a real
object.

Fig.11 Principle of stereo display using RPT.

RPT is applied to realize the situation of Figure 14. The
coat is made of retro-reflective material so that the
coming light is reflected back to the same direction that itcomes from. Microscopic beads on the surface of the coat
have the function of retro-reflection.
A half mirror makes it possible for a spectator to see
virtually from the position of the projector. An HMP
projects an image of the background scenery captured by
the video camera behind the camouflaged subject. A
computer calculates the appropriate perspective andtransforms the captured image to the image to be
projected on the subject using image-based rendering
techniques. Since the cloak the subject is wearing is
made of a special retro-reflective material, which reflectsback the incident light just the same direction it comes
from, an observer looking through a half mirror sees avery bright image of the scenery so that he is virtuallytransparent.

The projector can be mounted on the head of a user,which we call an HMP (Head Mounted Projector) system.
Figure 12 shows a general view of a prototype HMP.

Fig. 12 General view of a Head Mounted Projector (HMP).

Fig. 14 Another example of application of RPT to opticalcamouflage.

Figure 13 shows an example of an image projected on asphere painted with retro-reflective material. As apparent
in the figure, the projected image looks like a real object,and is partly hindered naturally by human fingers.

Fig. 15 A schematic diagram of the RPT system used for the opticalcamouflage in Fig.14.

4 Mutual Telexistence using RPT
Fig.13 Projected image on a spherical retro-reflective screen.

Figure 14 shows how optical camouflage can be achieved
using real-time video information. Figure 15 shows how

More than twenty years have passed since we initially
proposed the concept of telexistence, and it is now
possible to telexist in the remote and/or virtual

S.Tachienvironment with a sensation of presence. We can workand act with the feeling that we are present in several realplaces at once. However, in the location where the user
telexists, people see only the robot but cannot feel thatthe person is actually present. Simply placing a TV
display on board the robot to show the face of the user is
not very satisfying, since it appears mostly comical andfar from reality.
By using RPT, the problem can be solved as shown in
Figure 16 [24]: suppose a human user A uses his
telexistence robot A' at the remote site where anotherhuman user B is present. The user B in turn uses anothertelexistence robot B', which exists in the site where theuser A works. 3-D images of the remote scenery arecaptured by cameras on board both robots A' and B', andare sent to the HMP's of human users A and B
respectively, both with a sensation of presence. Bothtelexistence robots A' and B' are seen as if they were theirrespective human users by projecting the real image ofthe users onto their respective robots.
Motion Control / Tactile Control
Visual & Auditory Information

HMP

3D image observed by
Telexistence Robot B’is projected and observedby Human User B witha sensation of presence.

3D image observed by
Telexistence Robot A’
is projected and observed by
Human User A with asensation of presence.

Human
User A

Telexistence
Robot A’

Telexistence Robot B’
Telexistence Robot B’ isobserved as if it is Human
User B by the projectionof real image of B on
the robot B’.

Telexistence Robot A’ is HMP
observed as if it is Human
User A by the projectionof real image of A on
the robot A’.

Human
User B

Visual & Auditory Information

Figure 17 presents an example of how mutual
telexistence can be achieved through the use of RPT.
Figure 17(A) shows a miniature of the HONDA
Humanoid Robot, while Figure 17(B) shows the robot
painted with retro-reflective material. Figures 17(C) and(D) show how they appear to a human wearing an HMP.
The telexisted robot looks just like the human operator of
the robot, and telexistence can be naturally performed[24].
However, this preliminary experiment was
conducted off-line, and real-time experiments are yet tobe conducted by constructing and using a mutual
telexistence hardware system.

5 Conclusions
Projection technology on Retro-reflective surfaces iscalled RPT (Retro-reflective Projection technology),
which is a new approach to augmented reality (AR). The
first demonstration of RPT together with HMP (HeadMounted Projector) was made at SIGGRAPH98,
followed by demonstrations at SIGGRAPH99 and
SIGGRAPH2000.
Mutual telexistence is one of the most important
technologies for the realization of networked telexistence,
because users "telexisting" in a robot must know whomthey are working with over the network. A method using
RPT, especially an HMP and a robot with retro-reflective
covering was proposed and proved to be a promising
approach toward the realization of mutual telexistence.
In this paper, a short history of telexistence is reviewed,
mutual telexistencethe principle of RPT is explained, andan experimental mutual telexistence system using RPT
was designed and constructed to demonstrate its
feasibility and efficacy.

Motion Control / Tactile Control

Fig. 16 Concept of Robotic Mutual Telexistence (adopted from [24]).

6 Acknowledgments
The research presented here has been conducted in part
under the CREST Telexistence Communication Systems
Project, supported by JST (Japan Science and Technology
Corporation).

7 References[1] I.E.Sutherland: A Head-Mounted Three Dimensional
Display, Proceedings of the Fall Joint Computer
Conference, pp. 757-764, 1968.
[2] H.A. Ballinger: Machines with Arms, Science
Journal, October, pp.58-65, 1968.

Fig. 17 (A) Miniature of the HONDA Humanoid Robot,(B) Painted with Retro-reflective Material,(C) Example of Projecting a Human onto it,(D) Another Example (adopted from [24]).

[3] J. Charles and J. Vertut: Cable Controlled Deep
Submergence Teleoperator System, Mechanism andMachine Theory, pp.481-492, 1977.
[4] M. Minsky: Telepresenc, Omni, Vol.2, No.9, pp.4452, 1980.

13th International Symposium on Measurement and Control in Robotics- Toward Advanced Robots: Design, Sensors, Control and Applications Madrid, Spain, December 11-12,2003[5] S.Tachi and M.Abe: Study on Tele-Existence (I),
Proceedings of the 21st Annual Conference of theSociety of Instrument and Control Engineers (SICE),
pp.167-168, 1982 (in Japanese).
[6] S.Tachi, K.Tanie and K.Komoriya: Evaluation
Apparatus of Mobility Aids for the Blind, Japanese
Patent 1462696, filed on December 26, 1980. ; An
Operation Method of Manipulators with Functionsof Sensory Information Display, Japanese Patent
1458263, filed on January 11, 1981.
[7] R.L.Pepper, R.E.Cole and E.H.Spain: The Influenceof Camera and Head Movement on Perceptual
Performance under Direct and TV-Displayed
Conditions, Proceedings of the SID, vol.24-1, pp.7380, 1983.

[15] S.Tachi and K.Yasuda: Evaluation Experiments of aTele-existence Manipulation Sysytem, Presence,
vol.3, no.1, pp.35-44, 1994.
[16] Y.Yanagida and S.Tachi: Virtual Reality Systemwith Coherent Kinesthetic and Visual Sensation ofPresence, Proceedings of the 1993 JSMEInternational Conference on Advanced Mechatronics(ICAM), pp.98-103, Tokyo, Japan, August 1993.
[17] K.Oyama, N.Tsunemoto, S.Tachi and T.Inoue:
Experimental study on remote manipulation using
virtual reality, Presence, vol.2, no.2, pp.112-124,
1993.
[18] S.Tachi: Real-time Remote Robotics- Toward
Networked Telexistence : IEEE Computer Graphics
and Applications, vo.18, Nov-Dec98, pp.6-9, 1998.

[8] S.Tachi, K.Tanie, K.Komoriya and M.Kaneko: Teleexistence (I): Design and evaluation of a visualdisplay with sensation of presence, Proceedings of
the 5th Symposium on Theory and Practice of
Robots and Manipulators (RoManSy '84), pp.245254, Udine, Italy, (Published by Kogan Page
London), June 1984.

[19] Y.Yanagida,
N.Kawakami
andS.Tachi:
Development of R-Cubed Manipulation Language Accessing real worlds over the network, Proceedingsof the 7th International Conference on Artificial
Reality and Tele-Existence (ICAT'97), pp.159-164,
Tokyo, Japan, December 1997.

[9] J.D.Hightower, E.H.Spain and R.W.Bowles:
Telepresence: A Hybrid Approach to High
Performance
Robots,ProceedingsoftheInternational Conference on Advanced Robotics(ICAR '87), pp. 563-573, Versailles, France,
October, 1987.

[20] T.Nishiyama, H.Hoshino, K.Suzuki, R.Nakajima,
K.Sawada and S.Tachi, "Development ofSurrounded Audio-Visual Display System for
Humanoid Robot Control", Proc.of 9th International
Conference of Artificial Reality and Tele-existence(ICAT'99), pp. 60-67, Tokyo, Japan, December
1999.

[10] L.Stark et al.: Telerobotics: Display, Control andCommunication Problems, IEEE Journal of Robotics
and Automation, vol. RA-3-1, pp.67-75, February1987.
[11] M. S. Shimamoto, "TeleOperator/telePresence
System (TOPS) Concept Verification Model (CVM)
Depelopment", N. K. Saxena, ed., Recent Advancesin Marine Science and Technology, '92, pp.97-104,
1992.
[12] S.Tachi, H.Arai, I.Morimoto and G.Seet: Feasibility
experiments on a mobile tele-existence system, The
International Symposium and Exposition on Robots(19th ISIR), Sydney, Australia, November 1988.
[13] S.Tachi, H.Arai and T.Maeda: Development of an
Anthropomorphic Tele-existence Slave Robot,
Proceedings of the International Conference on
Advanced Mechatronics (ICAM), pp.385-390,
Tokyo, Japan, May 1989.
[14] S.Tachi, H.Arai and T.Maeda: Tele-existence masterslave system for remote manipulation, IEEE
International Workshop on Intelligent Robots andSystems (IROS'90), pp.343-348, 1990.

[21] T.Nishiyama, H.Hoshino, K.Suzuki, K.Sawada and
S.Tachi, "Development of Visual User Interface
Embedded in Tele-existence Cockpit for Humanoid
Robot Control", Proc. of IMEKO 2000 World
Congress, Vol.XI (TC-17 & ISMCR2000), pp.171176, Vienna, Austria, September 2000.
[22] S.Tachi, K.Komoriya, K.Sawada, T.Nishiyama,
T.Itoko, M.Kobayashi and K.Inoue: Development ofTelexistence Cockpit for Humanoid Robot Control,
Proceedings of 32nd International Symposium on
Robotics (ISR2001), pp.1483-1488, Seoul, Korea,
April 2001.
[23] S.Tachi, K.Komoriya, K.Sawada, T.Nishiyama,
T.Itoko, M.Kobayashi and K.Inoue: Telexistence
Cockpit for Humanoid Robot Control, Advanced
Robotics, Vol 17, No. 3, pp. 199-217 (2003)[24] S.Tachi: Augmented Telexistence, Mixed Reality,
pp.251-260, Published by Springer-Verlag, 1999.
[25] S.Tachi: Toward Next Generation Telexistence,
Proceedings of IMEKO-XV World Congress, vol.X(TC-17 & ISMCR'99), pp.173-178 , Tokyo/Osaka,
Japan, June 1999.
[26] S.Tachi: Toward the Telexistence Next Generation,
Proceedings of the 11th International Conference on

S.Tachi

Artificial Reality and Tele-Existence, (ICAT2001),
pp.1-8, Tokyo, Japan, December 2001.
[27] N.Kawakami, M.Inami, T.Maeda and S.Tachi:
Media X'tal -Projecting virtual environments on
ubiquitous object-oriented retro-reflective screens in
the real environment-, SIGGRAPH'98, Orlando, FL,
July 1998.
[28] M.Inami, N.Kawakami, D.Sekiguchi, Y.Yanagida,
T.Maeda and S.Tachi: Visuo-Haptic Display using
Head-Mounted Projector, Proceedings of IEEE
Virtual Reality 2000, pp. 233-240, New Brunswick,
NJ, March 2000.
[29] M. Inami, N. Kawakami, Y. Yanagida, T. Maeda
and S. Tachi, Method of and Apparatus for
Displaying an Image, US PAT. 6,283,598, 2001.
[30] M. Inami, N. Kawakami, Y. Yanagida, T. Maeda
and S. Tachi, Method and Device for Providing
Information, US PAT. 6,341,869, 2002.
[31] S.Tachi, S.Tachi, T.Maeda, R.Hirata and H.Hoshino:
A construction method of virtual haptic space,
Proceeding of the 4th International Conference on
Artificail Reality and Tele-Existence (ICAT'94),
pp.131-138, Tokyo, Japan, July 1994.
[32] S.Tachi: Two Ways of Mutual Communication:
TELESAR and TWISTER, in S.Tachi ed.
Telecommunication,
Teleimmersion
andTelexistence, IOS Press, ISBN 1-58603-338-7, pp.
3-24, 2003.
[33] S.Tachi: Telexistence and Retro-reflective Projection
Technology (RPT), Proceedings of the 5th Virtual
Reality International Conference (IVRIC2003),
pp.69/1-69/9, Laval Virtual, France, 2003.