Optical Camouflage Using Retro-reflective Projection Technology*Masahiko INAMI*, Naoki KAWAKAMI† and Susumu TACHI†The University of Electro-Communications, †The University of Tokyo
inami@computer.org, {kawakami, tachi}@star.t.u-tokyo.ac.jp
Abstract

This paper describes a kind of active camouflage
system named Optical Camouflage. Optical Camouflage
uses the Retro-reflective Projection Technology, aprojection-based augmented–reality system composed ofa projector with a small iris and a retroreflective screen.
The object that needs to be made transparent ispainted or covered with retroreflective material. Then aprojector projects the background image on it making themasking object virtually transparent.

1. Introduction
Various methods have been proposed to integrate thevisual space. In the field of Mixed Reality, one of themost popular topics is about displaying a virtual object
into real world [1].
However making objects virtually transparent, like in
HG. Wells’ “Invisible Man” can also be seen as dream of
human being. In this paper, we describe what could becalled a camouflage technique named OpticalCamouflage.

2.

To place a projector into a position opticallyconjugated with the observer's eye by using a halfmirror;
3. To make the projector's iris as small as possible (byusing a pinhole).
Each of these points provides the following
advantages, respectively:
Fig. 1 shows the principles of RPT. The image of avirtual object is projected through a pinhole. The
projected image is reflected by the half-mirror on a rightangle and then retro-reflected by the retro-reflective
screen.

Projector
Retroreflector

Small IrisOperator

2. Retro-reflective Projection Technology(RPT)
Optical Camouflage is a type of active camouflage thatuses technology named "Retro-reflective Projection
Technology (RPT) / X'tal Vision," [2], which uses opticalprojection. Before describing the Optical Camouflage, wewill first illustrate RPT. When using a See-ThroughHead-mounted Display (STHMD) to merge virtual andreal environments, the operator may see the image of avirtual object that is meant to be located behind a realobject. This contradicts our intuition of depth, since theprojected image of an object located behind anotherobject in one's field of view will be obstructed at least
partially. This depth cue is called occlusion, and iscritical for the effectiveness of the presentation of virtual
objects in three dimensions. To solve the occlusion
contradiction problem, we developed RPT.
The three key techniques of RPT are the followings:
1. To use an object covered by retro-reflective material
as a screen;

Fig. 1 X'tal Vision

3. Optical Camouflage
Optical Camouflage is developed with the simplicityof Columbus’ egg. A projector projects the image of thebackground onto the masked object. (Fig. 2)

Proceedings of the Second IEEE and ACM International Symposium on Mixed and Augmented Reality (ISMAR ’03)
0-7695-2006-5/03 $17.00 © 2003 IEEEA camera behind the masked object takes the image ofthe background. The object that needs to be transparent ispainted or covered with retroreflective material. The
projected image is composed by computer using animage-based rendering method to create the image thatshould be seen from the viewpoint of the user from theimage from the viewpoint of the camera.

Projector
ObserverFig. 4 “Invisible Cloak”

Combiner
Background Camouflaged object(covered with retro-reflector)

Fig. 2 Configuration of Optical Camouflage

4.

Results

Fig.3 shows the haptic display (real object) hiding thevirtual object, but Optical Camouflage techniques permit
to make the haptic display to become transparent.
However, the operator’s hand is not made transparent,
which implies that it is possible to use this techniqueselectively.
Fig.4 shows a demonstration of ”Invisible Cloak”. It
looks as if a red truck can be seen through the body of aman who wear a retro-reflective coat. Actually, he does
not become transparent perfectly. The shape of the coat isobserved clearly. Nevertheless, it looks like a very low
refractive index glasswork, which is enough to observe
the background.

5. Conclusions
We have developed an Optical Camouflage system.
Optical Camouflage can be used on surgical globes or
equipments so they don’t block surgeon’s view during
delicate operations. In aviation, cockpit floors could
become 'invisible' to assist pilots during landing.
The weak point of this technique is that the observerneeds to look through a half-mirror. The current systemneeds a half-mirror and projectors, which were fixed on
the ground.
In the next step of our research, an observer would be
able to observe the background image from various
viewpoint with H.M.P. (Head-Mounted Projector)[3].

References[1] R. Azuma and J. Rolland, A Survey of Augmented
Reality,
Presence:
Teleoperators
andVirtual
Environments, Vol.6, No.4, pp.355 – 385, 1997
[2] N. Kawakami, M. Inami, D. Sekiguchi, Y. Yanagida,T. Maeda and S. Tachi, "Object-Oriented Displays: A
New Type of Display Systems -From immersive display
to Object-Oriented Displays-", IEEE InternationalConference on Systems, Man, and Cybernetics '99
Abstracts, p.493, 1999
[3] M. Inami, N. Kawakami, D. Sekiguchi, Y. Yanagida,
T. Maeda and S. Tachi, "Visuo-Haptic Display Using
Head-Mounted Projector", Proceedings of IEEE VirtualReality 2000, pp.233-240, 2000

Fig. 3 Optical camouflaged haptic display

Proceedings of the Second IEEE and ACM International Symposium on Mixed and Augmented Reality (ISMAR ’03)
0-7695-2006-5/03 $17.00 © 2003 IEEE