Immersive Autostereoscopic Display for Mutual Telexistence: TWISTER I(Telexistence Wide-angle Immersive STEReoscope model I)
Yutaka Kunita

Naoko Ogawa Atsushi Sakuma Masahiko Inami Taro Maeda Susumu Tachi
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656
Japan
kuni@star.t.u-tokyo.ac.jp

Abstract
We present an autostereoscopic display named
TWISTER I (Telexistence Wide-view-angle ImmersiveSTEReoscope, Model I), which is designed for a faceto-face tele-communication system called ”mutualtelexistence.” By rotating display units that consist of LED
arrays and a barrier around a viewer, TWISTER I candisplay panoramic stereoscopic images that can be observed without the use of special eyewear. This ”glassless”
feature is essential for applying this apparatus to mutual
telexistence because eye contact is important in non-verbal
communication.

1

Introduction

Development of a system that allows for face-to-facecommunication between remote users has long been a challenging concept. Our team is also studying such a systemcalled ”mutual telexistence.” [7],[8],[13]. The concept isshown in Figure 1. Although there are still some technicalproblems to be solved, this paper focuses on a visual display.
The visual display is required to provide realistic threedimensional images that enables users to feel as if they weresharing an environment. Recently, some studies employ immersive projection technology (IPT) such as CAVE[3] thatcan generate wide view-angle stereoscopic views. However, this kind of display demands that viewers wear special eyeglasses that obscure the viewer’s eyes and preventseye contact between its users. This situation is undesirable,
since facial expressions, particularly around the eyes, arean essential part of face-to-face communication. On theother hand, conventional autostereoscopic (glassless stereoscopic) displays do not have a viewing-angle that is sufﬁcient for mutual telexistence in a situation such as, for ex-

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Figure 1. Concept of Mutual Telexistence

ample, a cocktail party. Not unlike a round-table discussion,
many other participants may surround a participant. Thus,a wide-angle, stereoscopic display that is not dependent onspecial eyewear is optimal for mutual telexistence.
To satisfy the demands noted above, a novel autostereoscopic display system was constructed.
It is called
TWISTER I (Telexistence Wide-angle Immersive STEReoscope, Model I). By mechanically rotating display units
that consist of LED arrays and a barrier around a viewer,
TWISTER I can display very wide-angle stereoscopic im-ages that can be perceived without the use of special eyewear.

2Related Work

Parallax barrier is one of the most common methods forautostereoscopic display. This method seems to have been
invented in the early 20th century by F. E. Ives and has
been studied by several researchers[10]. The method makes
use of striped images for each eye. The images are spatially multiplexed on a screen, and very ﬁne vertical slits
are located between the observer and the screen. By viewing these images through vertical slits, only the left images
can be observed with the left eye and the right images withthe right eye. Displays that use lenticular lenses instead ofslits [6],[9] and place slits between a backlight and a liquidcrystal panel [5] are also in this category.
However, there are two well-known disadvantages tosuch displays. First, the eye position of the observer isstrictly limited; thus, head motion is restricted to a few ﬁxed
positions or a relatively narrow region. Secondly, horizontal spatial resolution of the screen is divided so that each eye
perceives only one half of the screen.
To address the ﬁrst problem, some displays adjust the positions of lenticular lenses [2] or light sources [11] according to the observer’s head motion; however, there seems to
be room for improvement of the tracking speed.
On the other hand, some display systems employ a periodic barrier motion that is not similar to the tracking motion mentioned above. Since a dynamic barrier motion
works as a time-multiplexor, each eye perceives the wholescreen. Records say the cyclostereoscope by F. Savoye wasdemonstrated at Luna Park in Paris in 1949. Recently, thecyclostereoscope has been revived[1]. The apparatus consists of a screen with a rotating fence around it, and theleft and right images are projected from outside. Because afence horizontally interlaces both of the projected images,
the viewers outside the rotation can see stereoscopic images through the apertures in the fence. If the rotating speed
were fast enough, the viewers would perceive no ﬂicker. Although the cyclostereoscope provides only a pair of parallax images, TAO (Telecommunications Advancement Organization of Japan) constructed a similar display capable ofmultiple parallax images [4]. Also, Homer B. Tilton produced the parallactiscope in which a vertical slit scans horizontally in front of a CRT (cathode-ray tube) [14]. Furthermore, some recent experiments employ an electronic barrier
instead of a mechanical one [12].

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Figure 2. Principle of Movable Parallax Barrier
Method (Top View)

3Principle

3.1

Movable Parallax Barrier

Figure 2 shows the method movable parallax barrier that
we have used. The basic idea is to scan a pair of directional
light sources, each of which is visible to one eye only. Toproduce directional light sources, a pair of vertical LED arrays and a vertical barrier are combined. By rotating themaround the observer as a single display unit, the apparatus
can present cylindrical images that are separately perceived
by each eye. Thus, the observer can experience wide-angle
stereoscopic images without special eyewear.
In Figure 2, the right and left eyes need to be in region
I and II, respectively, for stereoscopic viewing. Region III
is a ”cross-talk” region, where both images are visible, andboth images are visible in region IV. By design, regions I, II,II, and IV intersect at the center of the circle so that the apparatus permits a relatively wide range of head movement.

3.2Viewing Areas

Each eye should be located in a speciﬁc area as the display unit rotates. First, let R be the radius of a display, ωthe beam angle of a light source, γ the viewing-angle of abarrier from the center, and θ the rotating angle of a display
unit, as shown in Figure 3. The condition in which only theFurthermore, by using
0Rl(0, φ) = R ·sin ωsin( π2 +φ+γ−ω)0−π ≤ φ < − π2 + γ− π2 + γ ≤ φ < − π2 + 2ω − γ− π2 + 2ω − γ ≤ φ < π2 − γ
π2 −γ ≤ φ< π(5)(when ω > γ), we obtain the region shown in Figure 4 thatsatisﬁes equation (4). In Figure 4, the actual parametersof TWISTER I (R = 600[mm], ω = 15◦ , γ = 3.63◦,∆θ = 30◦ ) are applied. The black dots indicate typical eye
positions (interocular distance: 6.25cm).

Figure 3. Variables for Calculation of Viewing
Areas90

3.3600120Time-Space Tradeoff

60

Let f[Hz] be the frame rate of the images, n the pixels
per round, F [Hz] the driving frequency of the light source,
N the number of the display units and ν[rad/s] the angularvelocity of the display units. f1 seconds after one display
unit passes at a point on the circle and refreshes the pixel
there, the next display unit passes the same point and refreshes the pixel. As the angle between the display units is
2πN ,12π 1=· .
(6)
fN ν400
150

30200

180

0210

Also, the display unit takes 2π
ν seconds to go around and thelight source refreshes F times per second. Therefore,

330

240

2π.
ν(7)

f ·n=F ·N(8)n=F ·300
270

Then, by equation (6),(7),Figure 4. Calculated Viewing Areas. The
black dots indicate typical eye positions (interocular distance: 6.25cm).

light for the right eye is visible at the position (r0 , φ0) is
r0 ≤ l(θ, φ0 )(1),where l(θ, φ) is the length between the center of the circle
and region I at angle φ. Then, if a display unit moves within
the range −∆θ ≤ θ ≤ ∆θ, r0 must satisfy the followingcondition:r0 ≤minl(θ, φ0 )(2)−∆θ≤θ≤∆θis established. This indicates a tradeoff relationship in
which the product of the resolutions of time and spaceis constant. The right side indicates a situation in which
the display units are static. When we use an LED as alight source, F can be much bigger than the frame rate required by humans; whereas, very ﬁne slit processing suchas that used in a conventional parallax barrier is required toget enough N . However, sufﬁcient time resolution can becompensated for deﬁcient space resolution by dynamicallyscanning the display units.

4Implementation

4.1

Speciﬁcation

By substituting l(θ, φ) = l(0, φ − θ),r0≤=min−∆θ≤θ≤∆θ

minl(0, φ0 − θ)

φ0 −∆θ≤φ≤φ0 +∆θ(3)l(0, φ) (whereφ ≡ φ0 − θ).(4)Proceedings of the Virtual Reality 2001 Conference (VR’01)
0-7695-0948-7/01 $17.00 © 2001 IEEEFigure 5 shows an overview of the ﬁrst prototype
TWISTER I (Telexistence Wide-angle Immersive STEReoscope, Model I). Display units are attached to a hexagonalframe that is rotated by an AC servomotor. The hexagonal

Figure 6. Layout of the LED arrays and thebarrier

Figure 5. Overview of TWISTER I (Telexistence Wide-angle Immersive STEReoscope,Model I)

frame is made of aluminum, which is light and solid enoughto rotate about 1 ∼ 3 rounds per second. To each vertex ofthis frame, a display unit could be attached; however, threeof them have been attached in every other vertex to date.
The AC servomotor (Oriental Motor KBLM460GD-A)
rotates this frame at a rate of about one round per second(60rpm). Since the rated speed is 3000rpm, a 10:1 gearbox
is attached to slow the speed.
Each display unit consists of two columns of LED arrays
and a thin aluminum barrier. One vertical line has 128 red
LEDs, 32 of which are controlled by one microcomputer.
A vertical view angle is 57◦ when the observer is locatedat the center. Horizontally, 960 dots can be displayed perround, although the view-angle and visible regions mentioned previously have a tradeoff relationship. To date,
160x128 monochrome images (60◦ × 57◦ ) are restored in
the EEPROM on the microcomputer and displayed. Three
display units are rotated at one round per second. Thus,
the refresh rate of the images is three frames per second byequation 6.
The layout of the LED arrays and the barrier is shownin Figure 6. The line connecting the edges of the barrier
and the LED intersects at the center of the apparatus. This
arrangement contributes to minimizing a cross stalk region
where both LED arrays are visible.

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Figure 7. Photograph of TWISTER I

4.2

Experiment

In Figure 7, an observer views stereoscopic images without eyewear. A helmet is worn for safety and does notcontribute to the stereoscopic view; however, a transparent
acrylic guard will be used in the future. Due to mechanicalbarrier scanning, the left and right images are completely
isolated; thus, a so-called ghosting artifact that is common in polarization-based stereoscopic displays is hardlyobserved. Moreover, the barrier is not perceivable becauseof its speed. Also, the rotation of the frame is quite stable and quiet so that observers can enjoy noticeably clearstereoscopic images.
As an experiment, the images shown in Figure 8 weredisplayed. Since the disparity of the lower square is larger
than that of the upper one, the lower one should be perceived as deeper. When we tested the 10-male/female subjects, all of them correctly answered which square is thearound the user are obtained, a new view can be constructedfrom an arbitrary viewpoint in real-time by our previously
proposed technique[7],[8]. When each booth provides a reconstructed view from a viewpoint in a virtual environment,
this situation achieves mutual telexistence.

6
Figure 8. Stereoscopic test pattern. The left
image for the left eye, the right image for theright eye, respectively.

deeper.

5Future Work

There are short- and long-term plans for the continuationof this study.
Short-term work will focus on improving the basic quality of TWISTER as a three-dimensional display. First, fullcolor displays are indispensable for realism. This might beachieved by replacing the current monochrome red LEDs
with full-color LEDs; however, rotating separate red, green,
and blue LEDs would be a lower-cost implementation. If
the rotation speed is fast enough, the observer will perceive
mixed color. Second, the display of images in motion is alsoessential. To achieve this, we have to transmit image data
to rotating display units instead of restoring them in the onchip memory of a microcomputer. For our next prototype
TWISTER II, we will employ wired data transmission using
a slip ring, although wireless transmission is an attractive
choice. Third, head tracking would be necessary to widen avisible area and provide motion parallax. Although the visible area of the current system is relatively wide compared
to conventional parallax barrier displays, an observer’s motion is restricted near the center of the booth. Also, motion
parallax is crucial for providing three-dimensional space aswell as a stereoscopic view. Lastly, of course, we cannot neglect safety. Currently, some users would be frightened byaluminum barriers rotating at relatively fast speeds in frontof their faces without a guard. Therefore, we will place alarge acrylic cylinder in-between the observer and the rotator.
Besides all these speciﬁc improvements, we intend toadd a new element to this apparatus. In our mutual telexistence, one user sees other users and is simultaneously seenby them. To satisfy this bi-directional nature, the booth
will display stereoscopic ﬁgures of the other users and capture the image of the user within. This can be achieved byplacing cameras between display units and rotating them.
Furthermore, once these images from various viewpoints

Proceedings of the Virtual Reality 2001 Conference (VR’01)
0-7695-0948-7/01 $17.00 © 2001 IEEEConclusion

This paper presented a new autostereoscopic display
TWISTER I (Telexistence Wide-angle Immersive STEReosope, Model I), which can provide wide-angle stereoscopic
images without requiring the use of eyewear. The implementation of this apparatus is simple. It requires rotating
display units consisting of two columns of LEDs and a barrier around the observer. In addition, if cameras are rotated
together, the human ﬁgure can be captured simultaneously
from all directions. Although the current prototype is immature, the feasibility of mutual telexistence can be demonstrated.

Acknowledgement
The authors have beneﬁted from the technical advice ofcolleagues from the Tachi Lab. Yasuyuki Yanagida, Naoki
Kawakami, and Dairoku Sekiguchi have been especiallyhelpful. Also, thanks are due to T. K. Treadwell of The Institute for Photographic Research for providing the authors
with a copy of an article about the cyclostereoscope.

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