Autostereoscopic Projection display using Rotating Screen

Autostereoscopic Projection display using Rotating Screen
Autostereoscopic Projection Display using Rotating Screen
by
Osman Eldeş
A Thesis Submitted to the
Graduate School of Sciences and Engineering
in Partial Fulfillment of the Requirements for
the Degree of
Master of Science
in
Electrical and Electronics Engineering
Koç University
January, 2016
Koç University
Graduate School of Sciences and Engineering
This is to certify that I have examined this copy of a master’s thesis by
Osman Eldeş
and have found that it is complete and satisfactory in all respects,
and that any and all revisions required by the final
examining committee have been made.
Committee Members:
Prof. Hakan Urey
Assoc. Prof. Göksenin Yaralıoğlu
Asst. Prof. Fehmi Çivitci
Date:
Dedicated to Ali Kangel Beyağabey,
iii
ABSTRACT
A new technique for glasses-free (auto-stereoscopic) and multi-view 3D display is
proposed and demonstrated in this thesis. The technique uses two mobile projectors, a
rotating retro-reflective diffuser screen rotated by a mechanical unit, a pupil-tracking
camera and a computer. Two mobile projectors project stereo images onto a transfer
screen composed of a retro-reflector and a 1D diffuser. The transfer screen forms two
viewing slits through each of which only one image of stereo image pair is seen. The
mechanical unit rotates the transfer screen around its center according to the viewer’s
position obtained by the pupil tracker. As the screen is rotated accordingly, the viewing
slits track the viewer’s eyes. Thus, a single viewer can perceive 3D in a large viewing
field. Images from different perspectives can be presented to the viewer using the
information from the pupil-tracker. Advantages of the proposed technique compared
to conventional autostereoscopic projection displays are as follows: it requires only two
projectors rather than an array of projectors; there is no image registration problem
on the screen once the projectors are aligned; the viewing slits remain aligned with the
viewer’s pupils, thus the viewer never perceives discrete transitions between different
perspectives; the technique can provide high-gain, and sufficient brightness using
low-power mobile projectors. In order to demonstrate capabilities of the proposed
technique, a prototype was developed. The prototype consists of two laser based
pico projectors using MEMS scanners , a retro-reflective diffuser screen as a transfer
screen, pupil-tracker, rotating mechanism which rotates the transfer screen around
its center, and a personal computer to provide content and closed-loop control of the
tracker and the screen. The resulted prototype presents stereo images to a single
viewer without need of any glasses. The screen is circular with 60cm diameter and the
image is a rectangular image that fits on the screen and viewed from about 100cm
iv
distance. The prototype allows the viewer to move approximately 70 cm along the
horizontal axis, and 50 cm along the vertical axis with an average crosstalk below
%5. The display quality in terms of brightness and crosstalk has been measured and
reported for different types of light diffusers.
ÖZETÇE
Bu tez içerisinde çoklu görüntülü gözlüksüz 3 boyutlu ekranlar için yeni bir teknik
geliştirildi. Geliştirilen teknik iki adet taşınabilir projektör, bir adet merkezi etrafında
dönen retro-yansıtıcılı ışık dağıtıcı ekran, göz takip kamerası, ve bir adet bilgisayar
kullanmaktadır. Taşınabilir projektörler retro-yansıtıcı ve ışık dağıtıcıdan oluşan
ekran üzerine 3 boyutlu görüntüyü yansıtmaktadır ve ekran iki adet izleme şeridi
oluşturmaktadır. Bir izleme şeridinden sağ göz için gerekli 3B görüntü görülmekte
iken diğer izleme şeridinden ise sol göz için gerekli 3B görüntü görülebilmektedir.
Mekanik aksam göz takip kamerasından alınan bilgiye göre ekranı merkezi etrafında
döndürmektedir ve bu dönüş sayesinde izleme pencereleri de izleyicinin gözlerinin
üzerine gelecek şekilde izleyiciyi takip etmektedir. Bu şekilde bir adet izleyici geniş
bir alanda gözlük takması gerekmeden 3B görüntüyü izleyebilmektedir. Göz takip
kamerası vasıtasıyla kullanıcının konumuna göre farklı açılardan 3 boyutlu görüntüler
kullanıcıya sunulabilmektedir.
Geliştirilen tekniğin benzer tekniklere göre avantajları şunlardır; bir dizi projektör
kullanmak yerine sadece iki adet projektör kullanması, yansıtılan görüntülerde mekanik
hareketten kaynaklanan herhangi bir bozulma olmaması, izleme şeritleri daima izleyicinin gözlerinin üzerinde oluştuğu için izleyicinin hareketlerinden ötürü görüntü kaybı
yaşanmaması, sadece iki adet düşük güçlü taşınabilir projektör kullanmasına rağmen
yüksek kazanım ve parlaklık sağlaması.
Geliştirilen teknik kullanılarak bir adet prototip üretilmiştir. Üretilen prototip
iki adet lazer tabanlı Mikro-Elektro Mekanik Tarayıcı kullanan piko projektör, retro
yansıtıcı ve ışık dağıtıcıdan oluşan ekran, göz takip kamerası, ekranı merkezi etrafında
döndüren mekanik aksam, projektörlere içerik sağlayan ve kullanıcı konumuna göre
ekranı döndürme komutu veren bir adet bilgisayardan oluşmaktadır. Üretilen prototip
vi
bir adet izleyiciye yatay eksende 70 cm, dikey eksende 50 cm hareket özgürlüğü sağlayarak
ve herhangi bir gözlük takmasını gerektirmeden 3B görüntüyü sunmaktadır. Oluşturulan
ekran 60 cm köşegen uzunluğuna sahip altıgen şeklinde olup üzerine yansıtılan
görüntüler 100 cm uzaklıktan izlenmektedir. İzleme alanı içerisinde sağ göz resmi ve
sol göz resmi arasındaki gürültü değeri %5’in altındadır. Parlaklık ve gürültü değeri
cinsinden farklı ışık dağıtıcılar kullanılarak ekran kalite testleri yapılmıştır ve rapor
edilmiştir.
ACKNOWLEDGEMENTS
I am very grateful to my supervisor, Prof. Hakan Ürey for giving me a chance to
be a OML research member. Through all these years, i have learned so much from
him in research and life perspective. His admirable approach to young researchers
must be acknowledged.
I am thankful to my dear colleagues at OML: Kaan Akşit and Selim Ölçer. They
were always helpful in many ways, and provided a good team spirit with an endless
effort. I am also thankful to previous and current members of OML. I really enjoyed
being in a group with Onur Çakmak, Erhan Ermek, Kıvanç Hedili, Ulaş Adıyan
Başarbatu Can, Uğur Aygün and Sven Holmström.
I am also thankful to Assoc. Prof. Göksenin Yaralıoğlu and Asst. Prof. Fehmi
Çivitci for taking part in my thesis committee.
I acknowledge that the whole research through my MSc studies would not have
been possible without the financial support of TÜBİTAK.
Finally, i am very grateful to my family for endless support they have given me
during my life, and believing in me in all those years. It would be impossible to
complete this thesis without the support of my family.
viii
TABLE OF CONTENTS
List of Tables
xi
List of Figures
xii
Nomenclature
xvi
Chapter 1:
1.1
Introduction
1
Contribution of The Thesis Research . . . . . . . . . . . . . . . . . .
Chapter 2:
Autostereoscopic Projection Displays
3
5
2.1
Parallax Barrier Displays . . . . . . . . . . . . . . . . . . . . . . . . .
5
2.2
Lenticular Screen . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
2.3
Fresnel Lens Based Autostereoscopic Displays . . . . . . . . . . . . .
8
2.4
Retro-reflective Diffusing Material . . . . . . . . . . . . . . . . . . . .
9
2.5
Modular Multi-view Autostereoscopic Display using MEMS Projector
Array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chapter 3:
3.1
3.2
Proposed Display
10
12
System Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12
3.1.1
Formation of Viewing Slits . . . . . . . . . . . . . . . . . . . .
13
3.1.2
Changing Position of Viewing Slits . . . . . . . . . . . . . . .
17
3.1.3
Characterization of Display . . . . . . . . . . . . . . . . . . .
20
Constructed Prototype . . . . . . . . . . . . . . . . . . . . . . . . . .
23
3.2.1
24
Mems Based Laser scanning Pico Projectors . . . . . . . . . .
ix
3.2.2
Retro-reflective Diffusing Screen . . . . . . . . . . . . . . . . .
26
3.2.3
Created Viewing Field . . . . . . . . . . . . . . . . . . . . . .
27
3.2.4
Mechanical Analysis of Screen Rotation . . . . . . . . . . . . .
29
3.2.4.1
Mechanical Design 1 . . . . . . . . . . . . . . . . . .
30
3.2.4.2
Mechanical Design 2 . . . . . . . . . . . . . . . . . .
32
Pupil-Tracker and Camera-Projectors Calibration . . . . . . .
34
3.3
Measurements and Results . . . . . . . . . . . . . . . . . . . . . . . .
36
3.4
Further Applications: Super Stereoscopy Technique for More Realistic
3.2.5
3D Display . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chapter 4:
Conclusion
42
48
Publication record
50
Bibliography
52
x
LIST OF TABLES
3.1
Definitions of Symbols . . . . . . . . . . . . . . . . . . . . . . . . . .
20
3.2
Prototype Parameters . . . . . . . . . . . . . . . . . . . . . . . . . .
24
3.3
Mechanical Design Parameters . . . . . . . . . . . . . . . . . . . . . .
29
xi
LIST OF FIGURES
2.1
A parallax barrier screen [1]. . . . . . . . . . . . . . . . . . . . . . . .
6
2.2
Lenticular screen with a flat display.
6
2.3
Different usage of lenticular screen in a projection based autosteresocopic
. . . . . . . . . . . . . . . . . .
displays. (a) Rear-projection 3D display with double-lenticular screen
(b) Front-projection 3D display with single-lenticular screen (c) Rearprojection system sketch (d) Front projection system sketch. . . . . .
2.4
7
Retro-reflective light diffusing screen used with projector array. (a)
Projector array (b) Retro-reflective light diffusing screen (c) The system
sketch showing created viewing slits. . . . . . . . . . . . . . . . . . .
2.5
9
a) Photographs showing a front view of the overall prototype consisting
of 18 projectors, 3 mobile computers, and a 1D diffuser. b) Close-look
at projector array circled in 2.5a. . . . . . . . . . . . . . . . . . . . .
2.6
10
Content creation. For simplicity, only two perspective images and three
projectors are shown. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.1
System sketch showing the elements and the created viewing field of
the display. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2
12
Working principle of a transfer screen (a) The effect of 1D diffuser on
incident light (b) Imaging with reflective imaging lens such as retroreflector (c) Formation of slit shaped viewing slit using transfer screen,
consists of light diffuser and reflective imaging lens. . . . . . . . . . .
3.3
14
The formation of viewing slit pair through which viewer perceives the
stereo image without need of any special glasses. . . . . . . . . . . . .
xii
16
3.4
Viewing slits for different orientations of the transfer screen. (a) Relative
angular position of viewer’s eyes with respect to projectors (b) Rotation
of transfer screen by 0o of α and w < 2 × IP D, thus h = w < 2 × IP D
(c) Rotation of transfer screen by small α, thus w < h < 2 × IP D (d)
Rotation of transfer screen by large α, thus h > 2 × IP D > w, and
there is crosstalk between viewing slits. . . . . . . . . . . . . . . . . .
3.5
17
Illustration of 6 different cases for the position of viewer and corresponding positions of viewing slits. For the sake of simplicity, only center
lines of the viewing slits are shown. . . . . . . . . . . . . . . . . . . .
19
3.6
Top view of the system showing depth of viewing slits . . . . . . . . . . 21
3.7
The realised prototype. . . . . . . . . . . . . . . . . . . . . . . . . . .
3.8
a) Left picture shows the pico projector and a sketch of interior pho-
23
tonics module. Right bottom picture is a photo of MEMS scanner. b)
Photonics module of a pico projector. . . . . . . . . . . . . . . . . . .
3.9
25
a) The sketch of retro-reflective diffusing screen b) the picture of retroreflective diffusing screen c) SEM picture of aperiodic single axis diffuser
from Luminit, LLC. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
26
3.10 (a) Created viewing slits at different rotation angles: 9 shots are superimposed in order to create the photograph. The two bright spots
in the photograph are pico projectors. (b) A sample picture of viewer,
showing viewing slits on his eyes’ position. . . . . . . . . . . . . . . .
27
3.11 Screen shots taken from different viewing positions: (a) left eye, (b)
between the two eyes, and (c) right eye. . . . . . . . . . . . . . . . . .
28
3.12 A picture of constructed prototype by using timing-belt system for
power transmission. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
30
3.13 a) Closer look at timing-belt system. b) 3D sketch of home-made gear . 31
3.14 A picture of constructed prototype by using two-gear system for power
transmission. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xiii
32
3.15 a) The diameters of screen and motor gear. b) The produced gearshaped back plate for the screen . . . . . . . . . . . . . . . . . . . . .
33
3.16 a) Reference image taken from pupil-tracker camera. All projectors are
off. b)Sample image. Right projector is on and left projector is off. .
34
3.17 a) Difference image between fig 3.16b and fig 3.16a with lower threshold
value. b) Difference image with larger threshold value. . . . . . . . .
35
3.18 (a) and (b) Interpolated crosstalk maps of viewer’s space at the projector
plane for diffusers I and II. (c) and (d) Horizontal cross-sections of
viewing slits for different rotation angle, a for diffusers I and II. . . .
37
3.19 (a) and (b) Interpolated luminance map of viewer’s space at the projector
plane for diffuser 1 and 2. . . . . . . . . . . . . . . . . . . . . . . . .
38
3.20 Crosstalk and luminance variations along the projection axis, z, for
diffuser 1 (a) and 2 (b) at the position (x,y) = (0,9)cm. . . . . . . . .
38
3.21 Images captured from a screen with (a) periodic and (b) aperiodic
diffuser, respectively. Moire artifacts are visible in the periodic screen. . 41
3.22 (a) The proposed display used for testing and (b) created pinhole glass
prototype with pinhole diameter of 0.6 mm and pinhole separation of
1.4 mm [2].
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
43
3.23 Retinal image formations in regular stereoscopic 3D and SS3D systems.
(a) Blurred retinal images occured by accomodation of eyes on the
virtual object plane. (b) Sharp retinal image by accomodation of
eyes on the screen. (c) Sharp retinal images due to pinholes on SS3D
glasses (Accomodation of eyes are on the virtual object plane). (d) Two
seperated sharp parallax images results an approximate blur effect, [2].
44
3.24 Content creation procedure. (a) 4 different parallax images (b) Images
with corresponding colour channel (c) Superimposition of two parallax
images, [2].
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
xiv
45
3.25 (a) Estimated distance by testers with different contents. (b) Right
eye content of the object (pumpkin) at 36 cm (c) Right eye content of
the object (pumpkin) at 48 cm. The image of the cartoon character
appears on the screen at 1 m from the user [3]. . . . . . . . . . . . . .
xv
46
NOMENCLATURE
3D
:
Three dimensional
1D
:
Single Dimensional Diffuser
IP D
:
Interpupillary distance
M EM S
:
Micro electromechanical system
SS3D
:
Super Stereoscopic 3D
OM L
:
Optical Microsystems Laboratory
F W HM
:
Full width at half max
F W ZI
:
Full width at zero intensity
LSD
:
Light Shaping Diffuser
xvi
1
Chapter 1: Introduction
Chapter 1
INTRODUCTION
Autostereoscopic displays present stereoscopic images to viewers without need of
any special user-mounted device. Human brain interprets the three-dimensional (3D)
structure of a stereoscopic image by some perceptual cues, such as stereo parallax,
movement parallax, accomodation, and convergence. 3D displays (autostereoscopic
and stereoscopic) provide at least stereo parallax to present stereoscopic images to the
viewers, [4]. Stereo parallax means that each eye of the viewer sees different perspective
of the 3D scene. Stereoscopic displays requires viewers to wear a special glasses for
providing stereo parallax. This special glasses allows only one perspective to pass
through for each eye. In contrast autostereoscopic displays do not require any special
glasses to provide stereo parallax, but they control the light path so that each eye
of the viewer sees different perspective of the stereoscopic image. By controlling the
light path, autosteresocopic displays form exit pupils, so called viewing slits through
each of which different perspectives are visible. These viewing slits are either fixed
where the viewers’ left/right eyes must be positioned or dynamically follow viewers’
eye pupils under the control of a pupil-tracker.
There are several different transfer screens to control light path and form viewing
slits. These transfer screens are parallax barrier based screens, lenticular screens,
Fresnel lens (pupil forming screen) in front of a light shaping diffuser or retro-reflective
light diffusing screens.
In all transfer screens mentioned so far, except parallax barrier, in order to present
stereoscopic images to the viewer in a comfortable viewing area, either tens of viewing
slits must be formed which requires multiple projectors to be used or viewing slits
Chapter 1: Introduction
2
must dynamically follow the viewers’ pupils which requires projectors to be moved
accordingly. These solutions have some drawbacks. If multiple projectors are used,
using many projectors will cause high cost, complex system structure, and image
registration problem. In the case of moving projectors, image registration problem,
which is the problem of projecting images on the substantially overlapping areas of
the screen, and correcting the image distortions, will occur in every movement of
projectors.
Another problem with the multiple projector method and the parallax barrier
method is that they create discrete viewing slits. Thus, when the viewer changes its
position, viewer experiences discrete transitions between viewing slits, and it causes
unnatural 3D experience.
Motivation of the thesis is to introduce an autostereoscopic display method, which
aims to solve all aforementioned problems and presents a high quality 3D experience.
To do so, we have introduced a novel autostereoscopic projection display method
which employs a pair of projectors, a pupil-tracker and a rotating transfer screen
to form a pair of dynamic viewing slits aligned with the viewers eyes. Any of the
aforementioned transfer screens, except parallax barrier, can be used for the proposed
technique. The viewing slits track the viewers eyes in a large viewing field by rotating
the transfer screen in-plane. The present method has advantages over conventional
autostereoscopic projection displays in terms of number of projectors, transition
between different perspectives, and the image registration on the screen. The method
employs two projectors rather than an array of projectors, which decreases the cost,
and complexity of the display. In the present method, there is no loss of 3D vision
and discrete transition between different perspectives when the viewer changes his
position. In the present method, since neither projectors nor screen make translational
movement, projected images are always overlapped on the screen, and no distortion
correction is required. The method can provide high-gain, and sufficient brightness
even with a pair of mobile projectors. The main limitation of this technique is that it
is suitable for only one viewer.
Chapter 1: Introduction
3
A working prototype of the proposed technique which uses retro-reflective light
diffusing screen as transfer screen has been constructed. Luminance, crosstalk, and
dynamic viewing area analysis have been performed. The prototype provides a viewing
field which has 700 mm horizontal length, and 500 mm vertical length. The average
crosstalk value along the viewing field is below % 5 which is the maximum value in
order to prevent reduced viewing comfort in half of population according to [5].
For a possible new application and solving a well-known problem of accomodationvergence conflict of conventional stereoscopic displays, a new technique named as Super
Stereoscopy (SS3D) which has been developed in our group, Optical Microsystems
Laboratory (OML), are tested with the proposed display, as reported in [3]. By adding
a specially developed glasses to the proposed display, the viewer perceives stereo image
without any accomodation-vergence conflict. The detailed analysis is explained in [2].
In chapter 2, the related works found in the literature will be reviewed. Transfer
screen examples will be presented. In chapter 3, the concept of the proposed method,
characterization of the display, works that have been carried to build a working prototype, and display quality measurements, such as luminance, crosstalk measurements
will be presented.
1.1
Contribution of The Thesis Research
The most important contribution of the thesis is that a novel autostereosopic projection display technique was conceived, developed, and demonstrated for the first
time during this thesis research. Our approach has advantages over conventional
autostereoscopic display approaches as detailed in Chapter 2. A patent, title Method
for Autostereoscopic Displays was filed, [6] during the thesis study as the sole inventor
and it has been licensed to Inventram for commercialization. The technique was first
reported as a journal paper in Optics Express, [7] and the article was promoted by
the Optical Society of America’s Spotlight on Optics, [8]. The proposed technique
was presented in 3 different conferences, [9–11]. Overall, a journal paper, 3 conference
papers and a patent has been published as the outcome of the thesis.
Chapter 1: Introduction
4
The prototype developed in this thesis study provides very good image quality and
competes well with other commercial 3D systems. When our group needed an autostereoscopic display system to demonstrate a new technique called super stereoscopic
multi view, the team used the prototype developed in this research and achieved very
good results, [2].
Chapter 2: Autostereoscopic Projection Displays
5
Chapter 2
AUTOSTEREOSCOPIC PROJECTION DISPLAYS
Autostereoscopic displays create virtual viewing slits, through which intended
perspective images can be presented. Through these viewing slits, right and left eye of
the viewer see different perspective images, and viewer perceives stereo image without
the need of glasses. There are several different methods to create virtual viewing
slits. These methods utilize parallax barrier, lenticular lenslet, Fresnel lens (pupil
forming screen), retro-reflective light diffusing material, or light shaping diffuser (LSD)
technology. The created viewing slits can be either fixed, or can dynamically follow
viewers pupil position under the control of a pupil-tracker. In this chapter, related
autostereoscopic display methods in the literature will be examined.
2.1
Parallax Barrier Displays
In parallax barrier based displays, right and left eye images are interlaced in columns
on the display, typically an LCD display. A parallax barrier is positioned in front of
the display so that it creates vertical viewing slits for right eye and left eye, separately.
Created right and left eye viewing slits are repeated horizontally along the viewers
space. Thus, as long as the viewers right eye is in the right eye viewing slit, and left
eye is in the left eye viewing slit, the viewer perceives 3D. In [1], parallax barrier is
used to build an autostereoscopic display. It must be noted that this is not the only
way of parallax barrier usage.
6
Chapter 2: Autostereoscopic Projection Displays
one column of left image
one column of right image
screen
parallax barrier
Figure 2.1: A parallax barrier screen [1].
2.2
Lenticular Screen
Lenticular screens can be used as in parallax barrier case. The horizontally interlaced
right and left images on the display are diffused by the lenticular screen so that right
eye image pixels can be seen only in the right eye viewing slits and left eye image
pixels can be seen only in the left eye viewing slits. Thus, viewer can perceive 3D as
long as viewers right and left eye are in the corresponding viewing slits. In [12], the
mentioned method is utilized.
left eye viewing windows
right eye viewing windows
lenslets
flat display lenticular
screen
Figure 2.2: Lenticular screen with a flat display.
7
Chapter 2: Autostereoscopic Projection Displays
(a)
(b)
(c)
(d)
Figure 2.3: Different usage of lenticular screen in a projection based autosteresocopic
displays. (a) Rear-projection 3D display with double-lenticular screen (b) Frontprojection 3D display with single-lenticular screen (c) Rear-projection system sketch
(d) Front projection system sketch.
One of lenticular screen usages in autostereoscopic displays is based on the optical
property that lenticular screens can be used to retro-reflect projected light back to
the source, horizontally. [13] describes a lenticular lenslet based display method which
utilizes the mentioned optical property. Since each single lenticule in lenticular screen
focuses light in only one direction, which is horizontal direction in the mentioned
patent, lenticular screen creates vertical viewing slits, in which the image projected by
the corresponding projector is presented. By placing multiple projectors, which are
Chapter 2: Autostereoscopic Projection Displays
8
horizontally apart from each other, not more than an average human interpupillary
distance (approximately 65 mm), multiple vertical viewing slits are created in the
viewers space. Through each vertical viewing slit, the perspective image projected by
the corresponding projector can be seen. Thus, the viewer perceives stereo images
in the viewers space . It must be noted that instead of placing multiple projectors
horizontally apart from each other, two projector could be used and horizontally move
according to the position of viewer.
Another usage of lenticular screens is placing a diffuser between two lenticular
screens, as in [14]. The constructed screen is called as double lenticular screen and it
is used in the rear projection displays. A projector projects image onto the double
lenticular screen. The lenticular screen, in the projector side, focuses light as thin
vertical stripes onto the diffuser. The lenticular screen, in the viewer side, diffuses
vertical stripes on the diffuser, such that the projected image can be seen only in a
narrow vertical viewing slit. Thus, by placing multiple projectors horizontally apart
from each other and creating multiple viewing slits, or by moving two projectors
according to the position of viewer, viewer can perceive 3D.
2.3
Fresnel Lens Based Autostereoscopic Displays
In fresnel lens based displays, a vertically aligned single dimensional (1D) diffuser
is placed onto Fresnel lens, [15]. The Fresnel lens focuses the projected image by
projector, and then diffuser diffuses this focused image into a vertical viewing slit.
By placing multiple projectors horizontally apart from each other or by moving two
projectors according to the position of viewer, viewer can perceive 3D through these
created viewing slits. It must be noted that this is not the only method to make use
of Fresnel lens in autostereoscopic displays.
9
Chapter 2: Autostereoscopic Projection Displays
(a)
(b)
(c)
Figure 2.4: Retro-reflective light diffusing screen used with projector array. (a)
Projector array (b) Retro-reflective light diffusing screen (c) The system sketch
showing created viewing slits.
2.4
Retro-reflective Diffusing Material
A retro-reflective light diffusing material can be used to create viewing slits to present
stereo images as disclosed in [16]. A retro-reflective diffusing material consists of a
retro-reflector sheet and a light diffusing sheet which diffuses light in one direction
much more than the other directions. A retro-reflector reflects incident light back to
the source. If a projector projects image onto a retro-reflector, retro-reflector forms
an exit pupil at the same position as the projection lens. In order to see the projected
10
Chapter 2: Autostereoscopic Projection Displays
image, the viewer must look through the created exit pupil, but it is impossible for two
objects, eye of the viewer and projection lens, to occupy same position in space. By
placing a 1D diffuser before the retro-reflector, exit pupil is expanded in the vertical
direction. Thus, anyone looks through vertically expanded exit pupil, called vertical
viewing slit, can see the projected image. By placing multiple projectors horizontally
apart from each other, as in [17], or by moving two projectors according to the position
of the viewer [18], viewer can perceive 3D in a comfortable viewing area.
projector array
and mounting
fan
screen
(a)
(b)
Figure 2.5: a) Photographs showing a front view of the overall prototype consisting of
18 projectors, 3 mobile computers, and a 1D diffuser. b) Close-look at projector array
circled in 2.5a.
2.5
Modular Multi-view Autostereoscopic Display using MEMS Projector Array
A good example of creating fixed viewing slits is Modular Multi-view Autostereoscopic
Display developed in our group [19]. In this display, an array of 18 Microvision Inc.
MEMS pico-projectors, a 1D diffuser with 40o of diffusing angle, a pupil tracker and 3
mobile computational units are used ??.
Chapter 2: Autostereoscopic Projection Displays
11
Figure 2.6: Content creation. For simplicity, only two perspective images and three
projectors are shown.
Each projector in the projector array projects specially created different images to
the 1D diffuser. Each of these specially created images contains different portions from
different perspectives as seen in 2.6. A viewer looking through the vertical axis light
dffuser screen sees one slit-shaped portion of this projected image. This portion of
projected image is actually one portion from a specific perspective. The same viewer
sees another portion of the same specific perspective from another projector. When
all portions of this specific perspective is summed up together by all projectors in the
projector array, the viewer sees a complete image of this specific perspective.
12
Chapter 3: Proposed Display
Chapter 3
PROPOSED DISPLAY
In this chapter, the methodology of the proposed autostereoscopic display, the
characterization of the display and the constructed prototype with the quality measurements will be presented.
3.1
System Design
Figure 3.1: System sketch showing the elements and the created viewing field of the
display.
The system sketch shown in Fig. 3.1 shows two pico projectors, a rotating transfer
screen, an pupil-tracker unit, and a control unit. The stereo content is projected onto
the transfer screen by two pico projectors which are placed horizontally apart from
Chapter 3: Proposed Display
13
each other by average human interpupillary distance (IPD), 63 mm. Each projector is
assigned to one eye of the viewer. One projector projects the content for the right eye
perspective and the other projects the content for the left eye perspective. On the
plane of projectors, as shown in Fig. 3.1, the transfer screen creates two viewing slits,
each of which is parallel to the major diffusing axis of the 1D diffuser, and crosses
over the position of corresponding pico projector’s micro electromechanical system
(MEMS) based scanner. Each viewing slit contains the image content projected by the
corresponding pico projector. The creation of viewing slits by using a retroreflective
diffuser screen, as transfer screen, will be explained in section 3.1.1. A viewer, who is
standing in the plane of pico projector and looks through the viewing slits, perceives
stereo images.
In order to change the position of viewing slits according to the position of viewer’s
eyes, a pupil-tracker tracks the position of the viewer’s pupils and sends the position
information to the control unit. According to the position information, a motor rotates
the transfer screen in-plane such that viewing slits dynamically follow the viewer’s
pupils in a large viewing field, as depicted in Fig. 3.1.
3.1.1
Formation of Viewing Slits
In order to understand the proposed method, one must know how an exit pupil, so
called viewing slit is formed by a transfer screen, and what properties it has. Thus, in
this section, the formation of viewing slits by the transfer screen will be explained. In
the next section, the proposed method to change the position of viewing slits according
to the viewer’s position will be explained.
Autostereoscopic displays control light path such that they create viewing slits,
through which different perspectives of the stereo image are observed, in different spatial
positions. There are several different ways to control light path for autostereoscopic
displays. One of them is relaying the projected image onto a slit shaped exit pupil,
by a transfer screen which both diffuses light in a single axis and images at the same
time. The transfer screen examples have been mentioned in Chapter 2. The transfer
14
Chapter 3: Proposed Display
screen can be thought as a special imaging system which consists of a 1D diffuser and
an imaging lens.
diffusing angle
in major axis
diffused light
major
diffusing axis
reflective
imaging lens
Light Source
output light
incoming light
incoming light
diffuser
(a)
(b)
major diffusing axis
viewing
window
light
retro-reflector
diffuser
light source
transfer screen
(c)
Figure 3.2: Working principle of a transfer screen (a) The effect of 1D diffuser on
incident light (b) Imaging with reflective imaging lens such as retro-reflector (c)
Formation of slit shaped viewing slit using transfer screen, consists of light diffuser
and reflective imaging lens.
Figure 3.2a illustrates the effect of a 1D diffuser on an incident light ray. The 1D
diffuser is characterized by diffusing angle in major axis, and diffusing angle in minor
axis. Since the diffusing angle in major axis is much greater than the diffusing angle
in minor axis for a 1D diffuser, it diffuses light in major diffusing axis much more than
Chapter 3: Proposed Display
15
its orthogonal axis, minor diffusing axis. Thus, when incident light ray passes through
the 1D diffuser, the output light rays are diffused in line shape, and the diffused light
is parallel to the major diffusing axis of the 1D diffuser.
Figure 3.2b illustrates the imaging of a light source by a reflective imaging lens.
In the illustration, the reflective imaging lens does one to one imaging, thus the light
source and its image are at the same spatial position. In Figure 3.2b, the image of
light source is not illustrated for the sake of simplicity. A retro-reflector is an example
for a one-to-one reflective imaging lens.
Figure 3.2c illustrates the formation of viewing slit by a transfer screen. The
transfer screen consists of a 1D diffuser, illustrated in Figure 3.2a and a reflective
imaging lens , illustrated in Figure 3.2b. Light rays from the light source hits on the
transfer screen, and transfer screen forms a slit shaped viewing slit in which lights
from light source are collected. As seen in Figure 3.2c, the created viewing slit has two
important properties; 1) It is parallel to the major diffusing axis of transfer screen,
2) It is anchored by its center to a virtual anchor point, which is the spatial position
of image of light source. The importance of these properties will be explained in the
next section. Since imaging lens in the transfer screen does one-to-one imaging, the
image of light source and light source itself are in the same spatial positions.
16
Chapter 3: Proposed Display
major
diffusing axis
retro-reflective
diffusing screen
left
projector
right
projector
viewer
viewing
slit pair
Figure 3.3: The formation of viewing slit pair through which viewer perceives the
stereo image without need of any special glasses.
Figure 3.3 illustrates the formation of viewing slit pair, through which stereo
image can be perceived. Instead of light source depicted in Figure 3.2a and 3.2b,
two projectors are used and placed apart from each other by an average human
interpupillary distance, (IPD). One of projectors, right eye projector projects right eye
image of stereo image pair, and the other, left eye projector projects left eye image of
stereo image pair onto the transfer screen. The transfer screen forms two viewing slits,
right and left eye viewing slits through which right and left eye image of stereo image
pair are presented, respectively. Since the projectors are placed apart from each other
by interpupillary distance, the viewer’s eyes can fit into viewing slits correctly.
17
Chapter 3: Proposed Display
3.1.2
Changing Position of Viewing Slits
(a)
(b)
(c)
(d)
Figure 3.4: Viewing slits for different orientations of the transfer screen. (a) Relative
angular position of viewer’s eyes with respect to projectors (b) Rotation of transfer
screen by 0o of α and w < 2 × IP D, thus h = w < 2 × IP D (c) Rotation of transfer
screen by small α, thus w < h < 2 × IP D (d) Rotation of transfer screen by large α,
thus h > 2 × IP D > w, and there is crosstalk between viewing slits.
Figure 3.4b is the front view of the system, illustrated in Figure 3.3, when the major
diffusing axis of transfer screen is parallel to the y axis. Since two projectors are
placed apart from each other by interpupillary distance, d, the distance between the
18
Chapter 3: Proposed Display
center line of viewing slits is interpupillary distance, d. Thus, eyes of the viewer fit
into viewing slits, and viewer perceives stereo images, as long as viewer’s eyes are
aligned on an axis which is parallel to the axis of projectors.
α = arctan(
Xe − Xp
)
Ye − Yp
(3.1)
When the viewer changes its position, the pupil-tracker unit locates eyes of the
viewer, and feeds the position information of viewers eyes to the control electronics.
The control electronics calculates the angular position of eyes relative to the position
of projectors, as illustrated in Figure 3.4a. The positions of projectors (Xp ,Yp ) are
always fixed and they are known by the control electronics. Since the positions of
viewers eyes (Xe ,Ye ) and the position of projectors (Xp ,Yp ) are known by the control
electronics, the control electronics calculates the angular position of the viewer which
is the angle, α, between reference axis and projector-eye line as in Equation 3.1. The
reference axis is the axis which is perpendicular to the axis of projectors, and the
projector-eye line is the axis on which one of viewers eyes and the corresponding
projector are aligned.
19
Chapter 3: Proposed Display
rotation directions
viewer at
position A
viewing field
diffusing axis
direction B
viewer at
position B
diffusing axis
direction B
diffusing axis
direction C
viewer at
position C
transfer screen
viewer at
position C'
viewer at
position B'
viewing field
viewer at
position A'
right and left
viewing slits
Figure 3.5: Illustration of 6 different cases for the position of viewer and corresponding
positions of viewing slits. For the sake of simplicity, only center lines of the viewing
slits are shown.
After the calculation of angle, α, the motor rotates the transfer screen around
its center, such that the angle between major diffusing axis of transfer screen and
reference axis is also angle of α, as in Figure 3.4d. Since the viewing slit is parallel to
the major diffusing axis of transfer screen as explained in the previous section, right
and left eye viewing slits are also rotated and formed on the spatial position of right
and left eye of viewer, respectively. The aformentioned procedure are performed in
real time, and viewing slits dynamically track the viewer in a large viewing field, as
depicted in Figure 3.5. The motion of the viewing slits is analogous to the in-plane
rotation of a beam around an anchor point. The MEMS scanner of the pico projector
is the anchor point of the corresponding viewing slit, as mentioned in the previous
20
Chapter 3: Proposed Display
section. Thus, rotation of transfer screen doesn’t change the interpupillary distance
between the center lines of viewing slits along the axis of projectors and eyes of the
viewer can look through viewing slits.
3.1.3
Characterization of Display
Table 3.1: Definitions of Symbols
Prototype Parameters
Viewing Slits Parameters
Parameter
Symbol
Projection Distance
d
Projection Angle
β
Screen size
s
Diffusing angle in major axis (FWHM)
φ
Diffusing angle in minor axis (FWHM)
ψ
Distance between two projectors
p
Diameter of projector’s MEMS scanner
dp
Interpupillary Distance
IP D
Distance between slits
p
Actual width
w
Horizontal width
h
Rotation angle
α
Length
L
Maximum rotation angle
θ
Depth
∆
The viewing field of the proposed autostereoscopic display technique is characterized
by the length, L, depth, ∆, and the maximum rotation angle, θ, of viewing slits, as
depicted in Fig. 3.1. For the proposed technique, the viewing field has been defined as
the three-dimensional space in which the crosstalk value of the display is low enough
to perceive stereo images.
L = 2 × d × tan(φ/2)
(3.2)
The length, L, of viewing slits characterizes the luminance of the display across the
projector plane. It is determined by the projection distance, d, and diffusing angle,
φ, of 1D diffuser in major axis, as in Eq. 3.2. As the viewer moves away from the
21
Chapter 3: Proposed Display
center of the viewing slit, which is the position of the pico projector, the luminance
of the display decreases, according to the diffusing properties of the diffuser. If the
diffusing angle, φ, is expressed as full-width-at-half-maximum (FWHM) value, then
the luminance of the display is less than 50% of the maximum luminance beyond the
length, L, of viewing slits.
Figure 3.6: Top view of the system showing depth of viewing slits
s
d − ∆f ront
d + ∆back
=
=
w
∆f ront
∆back
∆ = ∆f ront + ∆back =
s
w
d
+
+1
s
w
(3.3)
d
−1
(3.4)
The depth, ∆, of the viewing slits characterizes the luminance of the display along
the projection axis. As the viewer moves away from the projector plane along the
projection axis, the luminance decreases at the edges of the transfer screen. Figure
3.6 illustrates a top view of the proposed system. In Fig. 3.6, the projection angle of
the projector is shown as β, projectors to screen distance is shown as d, the width of
viewing slit is shown as ω, and the depth of viewing slits is shown as ∆. By using
22
Chapter 3: Proposed Display
the triangle similarity expressed in Eq. 3.3, the depth, ∆, of viewing slits can be
calculated as in Eq. 3.4. If the width, ω is FWHM value, the luminance at the edges
of the transfer screen is less than 50% of the maximum luminance beyond the depth,
∆, of viewing slits.
h=
w
cos(α)
h < 2 × IP D
α < θ = arccos(
w
)
2 × IP D
(3.5)
(3.6)
(3.7)
The maximum rotation angle, Θ, of the viewing slits is the rotation angle, α,
beyond which the horizontal width of the viewing slits, h, is larger than 2 × IP D, as
in Fig. 3.4d, and crosstalk results in inseparable stereo images. The horizontal width
of viewing slits, h, is the full-width-at-zero-intensity (FWZI) width of viewing slits
along the horizontal axis, x-axis. In order to avoid crosstalk between stereo images,
following two conditions must be satisfied in the proposed system design; (1) the
horizontal distance between viewing slits, p, must be equal to interpupillary distance
of the viewer, IPD, and (2) the horizontal width of viewing slits, h, must be smaller
than 2 × IP D, as in Figs. 3.4b and 3.4c.
The transfer screen does one-to-one imaging to form exit pupils of pico projectors.
Placing two projectors horizontally apart from each other by IPD makes the horizontal
distance between viewing slits, p, equal to the interpupillary distance of the viewer,
IPD. Thus, the aforementioned first condition to avoid crosstalk is satisfied, as long as
the viewer’s eyes are aligned along the horizontal axis.
The horizontal width of viewing slits, h, increases with the increase in rotation
angle, α, of viewing slits, as seen in Figs. 3.4b-3.4d. The relationship between the
horizontal width, h, and rotation angle, α, of viewing slits is stated in Eq. 3.5, where
ω is the actual width of viewing slits. It is the FWZI width of the viewing slit, which
Chapter 3: Proposed Display
23
is measured across the viewing slit at right angles of its length. It is a fixed system
parameter which is proportional to the projection distance, d, the diffusing angle of
1D diffuser in minor axis, ψ, and the diameter of MEMS scanner of projector, dp . By
placing Eq. 3.5 into Eq. 3.6, the second condition to avoid crosstalk, stated in Eq. 3.6,
can be restated as in Eq. 3.7. Equation 3.7 implies that there is a maximum rotation
angle, α, of viewing slits, beyond which crosstalk results in inseparable stereo images.
Figure 3.7: The realised prototype.
3.2
Constructed Prototype
A prototype was realised to demonstrate the capabilities of the proposed technique.
The prototype consists of two MEMS based laser pico projectors from Microvision,
Inc. [20], a retro-reflective diffuser screen as a transfer screen [16], a pupil-tracker
unit [21], a rotating mechanism which rotates the transfer screen around its center,
24
Chapter 3: Proposed Display
and a personal computer. Figure 3.7 a shows a photograph of the realised prototype.
Table 3.2: Prototype Parameters
Parameter
Projection Distance
Projection Angle
Screen size
Diffusing angle in major axis (FWHM)
Diffusing angle in minor axis (FWHM)
Distance between two projectors
Diameter of projector MEMS scanner
Diameter of eye pupil
Interpupillary Distance
3.2.1
Symbol
d
β
s
φ
ψ
p
dp
deye
IP D
Value
1180 mm
28.4o
600 mm
40o
0.2o
63 mm
1 mm
2 mm − 8 mm
63 mm
Mems Based Laser scanning Pico Projectors
Projectors used in the prototype are MEMS based laser scanning pico projectors from
Microvision, Inc. [20]. The device contains red, green, and blue color lasers which are
all linear polarized with polarization of green is perpendicular to the polarizations of
red and blue. The light from all three lasers is combined and scanned by a MEMS
scanner in two axes using a raster scan pattern. By doing so, the projector forms an
image on the projected surface, as in fig 3.8a.
25
Chapter 3: Proposed Display
lasers
screen
picop
Mems scanner
coin
(a)
photonics module
(b)
Figure 3.8: a) Left picture shows the pico projector and a sketch of interior photonics
module. Right bottom picture is a photo of MEMS scanner. b) Photonics module of
a pico projector.
Since there is no lenses inside the projector and lasers are collimated, the projected
image is always on focus. Thus, a focus free projector gives freedom to scale the
display system. The photonics module consisting of lasers is shown in the fig 3.8b.
Pico-projectors have been used in many different applications and settings. One of
them is Portable 3D Laser Projector Using Mixed Polarization Technique which is
developed in our group, [O1]. Polarization difference between lasers of pico-projector,
and Mems technology in the pico-projector allow us to present full resolution 3D image
with only a single pico-projector.
26
Chapter 3: Proposed Display
major diffusing axis
viewing
window
light
retro-reflector
diffuser
light source
transfer screen
(a)
(b)
(c)
Figure 3.9: a) The sketch of retro-reflective diffusing screen b) the picture of retroreflective diffusing screen c) SEM picture of aperiodic single axis diffuser from Luminit,
LLC.
3.2.2
Retro-reflective Diffusing Screen
The transfer screen is a layered composition of a retro-reflective sheet [15] from Reflexite
[16], and a 1D diffuser (large diffusing angle in major axis and small diffusing angle in
the perpendicular minor axis), as seen in fig 3.9. Two different types of 1D diffuser
have been tested as the diffuser layer of the transfer screen. Diffuser 1 is a randomly
patterned aperiodic diffuser from Luminit, LLC [17] with FWHM diffusing angle, φ,
of 40o in major axis, and, ψ of 0.2o in minor axis. Diffuser 2 is a conventional periodic
27
Chapter 3: Proposed Display
diffuser with FWHM diffusing angle, φ, of 20o in major axis, and, ψ of ∼ 0o in minor
axis. The retro-reflector sheet is a high fill factor corner cube retro-reflector array with
0.254 mm pitch size and it is hexagonal with 600 mm diagonal length. As the screen
size is equal to the retro-reflector sheet size, it can be enlarged using an improved
mechanical design to support a larger screen. All prototype parameters are presented
in Table 3.2.
(a)
(b)
Figure 3.10: (a) Created viewing slits at different rotation angles: 9 shots are superimposed in order to create the photograph. The two bright spots in the photograph
are pico projectors. (b) A sample picture of viewer, showing viewing slits on his eyes’
position.
3.2.3
Created Viewing Field
The creation of viewing field by rotating the transfer screen is explained in chapter 3.
As mentioned in chapter 3, it is enough to rotate only single axis diffuser in order to
create dynamic viewing slits in the viewing field. However, in the prototype, rotating
mechanism rotates both retro-reflector sheet and the 1D diffuser which makes the
realization of the prototype easier. The created viewing field is illustrated in Fig.
3.10a by taking pictures of viewer space for different rotation angles of viewing slits.
Figure 3.10b shows a sample picture of viewer while viewing slits appear on his eyes’
position.
28
Chapter 3: Proposed Display
(a)
(b)
(c)
Figure 3.11: Screen shots taken from different viewing positions: (a) left eye, (b)
between the two eyes, and (c) right eye.
Specular reflections create two bright lines across the transfer screen, which result
in double image along the bright lines. The specular reflections can be eliminated
by anti-reflection (AR) coating surfaces, or by tilting the transfer screen around the
vertical axis, y-axis. In the prototype, display screen has been tilted by 5o around the
vertical axis, so specular reflections are not observed on the transfer screen. Fig. 3.11a
is a picture of the screen taken from the left eye viewing slit. Fig. 3.11b is a picture
of the screen taken between left and right eye viewing slits, thus both right and left
eye images are observed. Fig. 3.11c is a picture of the screen taken from the right eye
viewing slit.
29
Chapter 3: Proposed Display
Table 3.3: Mechanical Design Parameters
Parameter
screen mass
Design 1
2 kg
Design 2
1 kg
Formula
md
screen radius
0.3 m
0.3 m
rd
screen inertia
0.09 kgm2
0.045 kgm2
angular velocity
0.5π rad/sec
0.5π rad/sec
acceleration time
0.2 sec
0.2 sec
15.7 rad/sec2
15.7 rad/sec2
1.41 N m
0.7 N m
∆w
∆t
τs = I × α
1:1
1:4
gearm /gears
0.94 N m
0.9 N m
τm = F × d
angular acceleration
screen torque
gear ratio
motor torque
3.2.4
I=
1
× md × rd2
2
w
t
α=
Mechanical Analysis of Screen Rotation
In the proposed display, screen rotation is a key element of the system implementation.
The angular speed and acceleration of the screen must be enough to track viewer
movements without any latency. In order to do so, the system must produce enough
torque and has an efficient power transmission between the motor and the screen.
Minimum angular speed of the screen is determined such that viewing slits must travel
between two edges of the viewing field in one second. Maximum rotation angle of the
screen is approximately 45o in one direction and angular distance between two edges
of the viewing field is 90o as it will be explained in 3.3. Thus, the screen must be
rotated with an angular velocity of 0.5π rad/sec. Acceleration time of the screen is
determined as 0.1 sec. In order to rotate screen with these specifications, two different
mechanical design have been applied to the prototype and the overall screen weight
has been improved during the thesis study. Mechanical design parameters can be
found in table 3.3. In the next paragraphs, two design will be compared and discussed.
30
Chapter 3: Proposed Display
timing belt
Figure 3.12: A picture of constructed prototype by using timing-belt system for power
transmission.
3.2.4.1
Mechanical Design 1
Figure 3.12 is a picture of the prototype with the mechanical design 1. In the first
mechanical design, a servo motor is used to rotate the screen. A gear with radius of
7.5 mm has been assembled to the servo motor. Another gear with the same radius of
7.5 mm has been produced by using a 3D printer and assembled onto the center of
the back side of screen, fig. 3.13b. The servo motor has been connected to the screen
via a timing-belt system with one-to-one gear ratio as seen in Fig. 3.13a. The screen
is placed onto a roller bearing and able to rotate around its center.
31
Chapter 3: Proposed Display
screen
timing
belt
camera
3D printed
gear
motor
gear
Back side of
the screen
(a)
(b)
Figure 3.13: a) Closer look at timing-belt system. b) 3D sketch of home-made gear
In the first mechanical design, the screen mass is 2 kg, and screen radius is 0.3 m.
In order to rotate this screen with angular acceleration of 15.7 rad/sec2 , 1.41 N m
torque is needed as calculated in Table 3.3. The servo motor used in this design is a
TowerPro MG995 servo motor with 0.94 N.m holding torque. Since the gear ratio is
1:1 between the motor and the screen, the servo motor provides 0.94 N.m to the screen.
The provided torque is smaller than the needed screen torque with aforementioned
speed configurations. Thus, we have applied a new mechanical design and decreased
the weight of the screen with a new mounting.
Chapter 3: Proposed Display
32
Figure 3.14: A picture of constructed prototype by using two-gear system for power
transmission.
3.2.4.2
Mechanical Design 2
In mechanical design 2, instead of using a timing belt between the motor and the
screen, a two-gear system has been implemented for torque transmission as seen in
3.14. The screen is placed onto a ring shaped disc with 0.3 m radius and it is made of
3 mm thick plexi glass material. The overall weight of the screen has been decreased
to 1 kg. The circumference of the disc has been cut in toothed shaped as seen in 3.15.
Thus, in the two gear system, one gear is the gear with 75 mm radius mounted onto
the motor, the other gear is the screen itself with 300 mm radius and gear ratio is 1:4.
By this implementation, a more efficient torque transmission has been achieved due to
increase in gear ratio, Table 3.3, and lighter screen was obtained.
33
Chapter 3: Proposed Display
(a)
(b)
Figure 3.15: a) The diameters of screen and motor gear. b) The produced gear-shaped
back plate for the screen
In order to rotate new screen with angular acceleration of 15.7 rad/sec2 , 0.7 N m
torque is needed as calculated in Table 3.3. The step motor used in the prototype is
Wantai 57BYGH420 which has holding torque of 0.9 N.m. Since the gear ratio is 1:4
between the motor and the screen, the step motor can provide 3.6 N.m to the screen.
The provided torque, 3.6 N.m, is much larger than the needed screen torque, 0.7 N m.
Thus, mechanical design 2 fulfill the needs of the screen rotation.
34
Chapter 3: Proposed Display
Reflected light
from screen
Right Projector
Right Projector
Left Projector
(a)
(b)
Figure 3.16: a) Reference image taken from pupil-tracker camera. All projectors are
off. b)Sample image. Right projector is on and left projector is off.
3.2.5
Pupil-Tracker and Camera-Projectors Calibration
Pupil tracking is a key element of the display since the rotation of the screen is
synchronized with the position of the viewer. There are many different methods to find
the position of viewer’s pupils. In the prototype, an OpenCv script which utilizes the
Haar Feature-based Cascade Classifiers is written and been used. Microsoft Lifecam
720p webcam is placed to a position where the camera can easily see the viewer and
projectors. The image captured from the camera is fed into the CPU then CPU
calculates the position of viewer’s eyes by using the OpenCv script. In the proposed
display, the important position is the relative position of viewer’s eyes with respect to
each projectors since the screen is rotated accordingly.
35
Chapter 3: Proposed Display
Extra
Brightness
Right projector
position
(a)
(b)
Figure 3.17: a) Difference image between fig 3.16b and fig 3.16a with lower threshold
value. b) Difference image with larger threshold value.
Since the prototype uses the relative position of viewer’s eyes with respect to the
projectors, the position of projectors must be known. Thus, each time prototype turns
on, position of projectors must be calculated. In order to calculate the position of
projectors, an OpenCv script is written which utilizes the image differencing method.
The method uses the brightness difference between a reference image and sample
image. When the prototype turns on, the camera captures a reference image which
is captured when both projectors are off. The reference image is shown in fig 3.16a.
Afterwards, sample image is captured when right projector projects a full white image,
and left projector is off. The sample image is shown in fig 3.16b. The sample image is
substracted from the reference image and a threshold is applied to the substracted
image. The substracted image can be seen in fig 3.17a. The largest difference between
two images are on the exact position of projector, thus threshold value is increased
and applied to the substracted image. A new image is obtained and shown in fig 3.17b.
As seen from the fig. 3.17b, the position of the right projector is found in terms of
pixels value.
The same procedure that is applied for right projector is applied for the left
projector and the position of left projector in terms of pixel values is found, too. By
36
Chapter 3: Proposed Display
placing position of projectors, (Xp , Yp ), and position of viewer’s pupils, (Xe , Ye ), which
are detected by the pupil-tracker into equ 3.1, angular position of viewer’s pupils is
found. By doing so, angular position of the viewer is obtained in real time, and the
screen is rotated accordingly as explained in 3.1.2.
3.3
Measurements and Results
As a quality measure, a crosstalk analysis of the realized prototype for both diffusers
was conducted. The crosstalk has been quantized by Eq. 3.8, where leakage is the
maximum luminance of light that leaks from unintended channel to the intended
channel, and ’signal’ is maximum luminance of the intended channel, [22].
crosstalk(%) =
leakage
× 100
signal
(3.8)
Thus, two luminance measurements of the screen are taken from each eye’s position
to calculate the crosstalk value of the corresponding eye. In the conducted experiments, only crosstalk values for the left eye has been measured, since the system is
approximately symmetrical. For the first luminance measurement, which determines
the leakage, full black image is projected by left-eye projector, and full white image is
projected by right-eye projector. For the second measurement, which determines the
signal, images for the first measurement are swapped. Luminance values have been
measured by a calibrated camera from Radiant Imaging, which takes photometrically
weighted photographs, and inserted into Eq. 3.8. By repeating the explained procedure above for different positions of camera in viewer’s space, and interpolating the
measured values; crosstalk, and luminance maps of viewer’s space have been obtained
for both transfer screen with diffuser 1, and transfer screen with diffuser 2.
37
Chapter 3: Proposed Display
(a)
(c)
(b)
(d)
Figure 3.18: (a) and (b) Interpolated crosstalk maps of viewer’s space at the projector
plane for diffusers I and II. (c) and (d) Horizontal cross-sections of viewing slits for
different rotation angle, a for diffusers I and II.
Figure 3.18a is the obtained crosstalk map of the viewer’s space in projector plane
for screen with diffuser 1. According to [23], crosstalk should be less than 5 % in
order to prevent reduced viewing comfort in half of population. Thus the maximum
rotation angle, θ, of viewing slits is 46o , beyond which the crosstalk is more than
5 %, and results in inseparable stereo images. Figure 3.18c illustrates the horizontal
cross-section of viewing slits for different rotation angles. The actual width, w, of
viewing slits, which is the full width at 5 % of maximum intensity, for 0o of rotation,
is 75 mm. By placing the actual width, w, of viewing slit into Eq. 3.7, the maximum
rotation angle, θ, of viewing slits is calculated as 530 , which validates the experimental
result of 46o .
38
Chapter 3: Proposed Display
(a)
(b)
Figure 3.19: (a) and (b) Interpolated luminance map of viewer’s space at the projector
plane for diffuser 1 and 2.
(a)
(b)
Figure 3.20: Crosstalk and luminance variations along the projection axis, z, for
diffuser 1 (a) and 2 (b) at the position (x,y) = (0,9)cm.
Figure 3.19a is the luminance map of viewer’s space in projector plane. The
luminance of the display decreases to 50% of the maximum luminance at around
420 mm away from the center of viewing slits, which are located at (0,0) in coordinate
system. Thus, the FWHM length, L, of viewing slits is 840 mm. By placing the
prototype parameters, stated in Table 3.2, into Eq. 3.2, the FWHM length, L, of
viewing slits is calculated as 859 mm, which validates the experimental result of
Chapter 3: Proposed Display
39
840 mm.
Figure 3.20a shows the crosstalk, and luminance variations in projection axis. The
crosstalk of the display is below 5 % for ±7.5 cm away from the projector plane. Thus,
the viewer still perceives stereo images away from the projector plane. However, as
the viewer moves away from the projector plane, perceived luminance varies over the
transfer screen.
In order to increase the viewing field by increasing the maximum rotation angle, θ,
of viewing slits, another transfer screen has been constructed by replacing diffuser 1,
40o × 0.2o aperiodic 1D diffuser, with diffuser 2, 20o × 0o periodic diffuser. Crosstalk,
and luminance maps of the viewer’s space have been obtained for the screen with
diffuser 2, and presented in Figs. 3.18b, 3.19b, and 3.20b. The crosstalk of the screen
with diffuser 2 is less than the diffuser 1, for the same area of viewer’s space, as
presented in Fig. 3.18a. The maximum rotation angle, θ, of viewing slits is 58o for
the prototype 2, and it is more than the maximum rotation angle, θ, of viewing slits
for the diffuser 1, which is 46o . Figure 3.18d illustrates the horizontal cross-section of
viewing slits for different rotation angles. As seen in Fig. 3.18d, the actual width, w,
of viewing slits, which is the full width at 5 % of maximum intensity for 0o of rotation,
is around 60 mm. By placing the actual width, w, of viewing slit into Eq. 3.7, the
maximum rotation angle, θ, of viewing slits is calculated as 61o , which validates the
experimental result of 58o .
Figure 3.19b is the luminance map of viewer’s space in projector plane for the the
screen with diffuser 2. As seen in Fig. 3.19b, the luminance of the display decreases to
50% of the maximum luminance at around 210 mm away from the center of viewing
slits. Thus, the FWHM length, L, of viewing slits is 420 mm. By placing the prototype
2 parameters, which are the projection distance, d, of 1180 mm, and the FWHM
diffusing angle, φ, of 20o into Eq. 3.2, the FWHM length, L, of viewing slits is
calculated as 416 mm, which validates the experimental result of 420 mm.
As explained in previous paragraphs, theoretical analysis of the system characteristics and experimental results match each other. The length, L of viewing slits depends
Chapter 3: Proposed Display
40
on diffusing angle of 1D diffuser in transfer screen. Due to the system design structure,
projected images passes through 1D diffuser twice. Thus, diffusing angle of diffuser
must be multiplied with two and placed into Equ. 3.2. However, experimental results
show that although there is double pass of light from diffuser, the effect of diffuser
doesn’t change significantly. In order to demonstrate the effect of double pass, a
simple experiment has been conducted. A single beam of laser light has been sent onto
single diffuser, light passed through diffuser once, and length of diffused light has been
measured. For double pass, the same experiment has been repeated with two diffusers
which are placed back to back, and with single diffuser with a mirror behind it. Two
diffusers and single diffuser with mirror behind it gave the same length for diffused
light which had double pass. However, length of diffused light for single diffuser is
slightly smaller than the double pass experiment. The difference between single pass
and double pass is smaller than the double. Thus, with little measurement error, the
theoretical analysis and experimental analysis of system characteristics match each
other. The effect of double pass on diffusing angle must be investigated as a further
study.
Figure 3.20b shows the crosstalk, and luminance variations in projection axis for
the screen with diffuser 2. The crosstalk for the diffuser 2 is less than the crosstalk for
diffuser 1, for ±7.5 cm away from the projector plane, as presented in Fig. 3.20a.
41
Chapter 3: Proposed Display
(a)
(b)
Figure 3.21: Images captured from a screen with (a) periodic and (b) aperiodic diffuser,
respectively. Moire artifacts are visible in the periodic screen.
As the periodic diffuser is used instead of aperiodic diffuser, the crosstalk of the
display in viewer’s space has decreased significantly. However, the mismatch between
the periodicity of retro-reflector sheet, and the periodicity of 1D diffuser created Moiré
patterns on transfer screen, as seen in Fig. 3.21a. Thus, the 1D diffuser must be
aperiodic in order to have a Moiré-free transfer screen, as seen in Fig. 3.21b. Although
the periodic diffuser creates Moiré patterns, subjects have stated that the display
presents successful stereoscopic vision for both types of the 1D diffuser.
s = 2 × d × tan(β/2)
(3.9)
In projection-based displays, screen size, s, is determined by the projection distance,
d, and the projection angle, β, as in Eq. 3.9. For the displays which project images on
Lambertian scattering surfaces, the screen size, s, is limited by the projector’s power,
since as the projection distance, d, increases, the luminance of the display decreases
dramatically.
Chapter 3: Proposed Display
42
As the screen in prototype is a high gain retro-reflective diffusing surface, all
generated power is concentrated into the viewing slits, and luminance is always
reasonably high in the viewing slits regardless of projection distance. Retro-reflective
surfaces can have gains of between 1k − 10k [24]. Thus, projection distance, d, and
screen size, s, is not limited by the projector power. However, it should also be
noted that, as the retro-reflector screens work under a specific acceptance angle limit,
projection angle, β can not exceed the acceptance angle limit. In the prototype,
projection angle, β, is 28.4o , which is smaller than the acceptance angle, 30o , of
retro-reflector.
3.4
Further Applications: Super Stereoscopy Technique for More Realistic 3D Display
The display proposed in the thesis study provides a pair of parallax images for two
eyes which invoke 3D perception for the viewer without need of any glasses. Since the
viewer perceives 3D images, viewer’s eyes converges to the right position of objects in
the presented image. However, viewer’s eyes focus on the screen to see sharp images,
as illustrated in Figs. 3.23(a) and 3.23(b). This is a conflict called as accomodationvergence conflict. Almost all 3D displays suffer from this conflict which leads viewing
discomfort for 3D displays especially when the virtual objects are closer than 0.5 m.
For a possible new application and solving this conflict in the proposed display, a new
technique that is named as Super Stereoscopy (SS3D) and developed by our group
is tested with the proposed display, as reported in [3]. The detailed analysis of the
technique is explained in [2].
43
Chapter 3: Proposed Display
(a)
(b)
Figure 3.22: (a) The proposed display used for testing and (b) created pinhole glass
prototype with pinhole diameter of 0.6 mm and pinhole separation of 1.4 mm [2].
The proposed display is used as a test bench for this new SS3D technique, 3.22a.
A prototype SS3D eyeglass is built to perform experimental tests in our lab 3.22b. On
both sides of the eyeglass, there are two pinholes with embedded color filters. The
pinholes have a diameter of 0.6 mm and a separation of 1.4 mm, with a red color filter
on the left and a cyan color filter on the right (as seen from the viewers side). In
order to solve accomodation-vergence conflict in the proposed display, the viewer of
the proposed display wears this special SS3D glasses and a specially created stereo
images are projected from the projectors. The SS3D technique and content creation
are explained in the next paragraphs.
Chapter 3: Proposed Display
44
Figure 3.23: Retinal image formations in regular stereoscopic 3D and SS3D systems.
(a) Blurred retinal images occured by accomodation of eyes on the virtual object plane.
(b) Sharp retinal image by accomodation of eyes on the screen. (c) Sharp retinal
images due to pinholes on SS3D glasses (Accomodation of eyes are on the virtual
object plane). (d) Two seperated sharp parallax images results an approximate blur
effect, [2].
As explained in [2], created stereoscopic glasses have pinholes which are smaller
than the pupil of the eye. Through these pinholes, images are seen sharp wherever eyes
are accommodated. Since accommodation doesn’t affected by sharpness of images,
eyes can accommodate where they converge. Thus, accommodation and convergence
are in agreement and accommodation-vergence conflict is solved. In order to add
natural blur for defocused objects, two pinholes for each eye are used. Two pinholes
per eye have different light selective filters which allows to view two different parallax
images per eye. Totally, there are 4 different parallax images. Two different parallax
per eye overlaps and a single sharp retinal image is seen at the virtual object plane
for a focused object, Fig. 3.23(c). For defocused object, two parallax images are seen
sharp and slightly shifted from each other which gives an approximate blur effect, as
seen in Fig. 3.23(d). Thus, while solving accommodation-vergence conflict, by given
blur effect, 3D images appear more natural.
Chapter 3: Proposed Display
45
Figure 3.24: Content creation procedure. (a) 4 different parallax images (b) Images
with corresponding colour channel (c) Superimposition of two parallax images, [2].
Fig. 3.24 illustrates how contents of SS3D are created. Every frame of the proposed
display has 4 different parallax images, Fig. 3.24(a). Two of these images are projected
by left projector, and the other two are projected by the right projector. Two parallax
images projected by one projector have different colour channels from each other.
Image passing through left pinhole of each eye has only red channel. Image passing
through right pinhole of each eye has green and blue channels, Fig. 3.24(b). These
two images with different channels are superimposed into a single image, then this
image is projected by corresponding projector, Fig. 3.24(c).
Chapter 3: Proposed Display
46
Figure 3.25: (a) Estimated distance by testers with different contents. (b) Right
eye content of the object (pumpkin) at 36 cm (c) Right eye content of the object
(pumpkin) at 48 cm. The image of the cartoon character appears on the screen at 1
m from the user [3].
Subjective test was conducted to investigate the proposed technique.A photograph
of the test bench is showed in Figure 3.22a which the distance between the user and
the screen is fixed at 100 cm through the tests. The subjects where chosen randomly
among the university students and staffs. All the participants were young and were
not using correction for their vision. In each case the users were first shown same
standard images in order to see if the subject were experienced enough to distinguish
a 3D image, to calibrate the white balance, and to apply the required alignment due
to differences in their IPDs. All the images shown in the test were static 3D images.
Viewers were asked to estimate the distance of 3D objects in the shown content. All
images contained a reference point at infinite distance, which is used to register stereo
pair correctly. Additionally, each image contained three different objects at different
distances which two of them (the right pumpkin and the cartoon character) were fixed
and one of them (the big pumpkin) was shown in different distances. Note that all of
the images were created in-house using open-source 3D models in Blender rendering
Chapter 3: Proposed Display
47
software. Figure 3.25 shows the estimated distances by testers with different contents.
Only 13% of subjects could perceive the most extreme content as 3D before using
SS3D glasses, but with the help of our purposed 3D glasses this number increased
to 100%. Major observed drawback of SS3D glasses during the test was found to
be a decrease in brightness of the screen by testers. Another possible problem was
resolution due to pinhole diffraction nature. As the distance between the viewer and
the display increases, the pinhole or the alternative slit structure limits the smallest
resolvable spot size to about 0.3 mm (at 500 mm subject distance).
As a conclusion, by introducing the new technique Super Stereoscopy and applying
it to the proposed display, the accomodation-vergence conflict has been resolved in
the proposed display and leaded to a new application area of the proposed display.
48
Chapter 4: Conclusion
Chapter 4
CONCLUSION
A new type of multi-view autostereoscopic projection display, using a pair of laser
based pico projectors with MEMS scanner, a pupil-tracking camera, and a rotating
retro-reflective diffuser screen is proposed and demonstrated in the thesis. In order to
realize proposed technique a prototype has been constructed. Different mechanical
designs and different elements are tested with the prototype. Two different light
diffusers, one is periodic diffuser and the other is aperiodic diffusers are tested in the
transfer screen. Two different mechanical designs are applied in order to rotate the
transfer screen. Theoretical analysis and experimental results of these different designs
and elements are reported in the thesis and they are in good agreement.
A Python script is written in order to control the transfer screen in real time and to
calibrate diffusing axis of the screen with initial position of mechanical unit. Another
script is written to check the position of projectors with respect to pupil-tracker system
since the prototype uses viewer’s position with respect to projectors to rotate transfer
screen.
The final prototype has a hexagonal shaped screen with 600 mm diagonal length.
A viewing field of 700 mm in horizontal axis, and 500 mm in vertical axis with a
crosstalk value below 5 %. The crosstalk remained below 5 % within ±7.5 cm in the
z-axis, i.e., user can move back and forth about 15 cm and the crosstalk remain at
acceptable levels while the luminance dropped by 25 % and 10 %. Beyond that range,
the luminance at the edges of the transfer screen decreases and corners of images are
vignetted.
As a further application of the proposed display and a solution to the well-known
problem of accomodation-vergence conflict in 3D displays, a new technique called as
Chapter 4: Conclusion
49
Super Stereoscopy and developed by our group is applied to the proposed display.
Subjective tests are performed and reported in the thesis. By applying Super Stereoscopy technique to the proposed display, the accomodation-vergence conflict has been
resolved in the proposed display and leaded to a new application area of the proposed
display.
The autostereoscopic projection display technique introduced in this thesis presents
3D images to a single viewer without need of any glasses. The technique is cost efficient
since it uses only two projectors rather than an array of projectors to provide large
viewing field. The screen size is scalable due to retro-reflector used in the screen.
The technique provides high-gain, and sufficient brightness even with a pair of low
lumen mobile projectors. There is no loss of 3D vision and discrete transition between
different perspectives when the viewer changes his position. The transition between
different perspectives is smooth as viewing slits track the eyes real-time. In the present
method, since neither projectors nor screen make translational movement, projected
images are always overlapped on the screen, and no distortion correction is required.
Some but not all possible application scenarios for the proposed displays are personal
entertainment displays used in transportation, training and entertainment simulators,
interactive design tables, etc.
50
Publication record
PUBLICATION RECORD
[O1] O. Eldes, K. Akşit, and H. Urey, “Multi-view autostereoscopic projection display
using rotating screen,” Optics Express 21, 29043–29054 (2013).
[O2] O. Eldes, K. Akşit, and H. Urey, “Paper No 17.4: 3D Auto-stereoscopic display using pico-projectors and rotating screen,” in “EURODISPLAY2013: 33rd
International Display Research Conference, September,” (SID, 2013).
[O3] O. Eldes, K. Akşit, and H. Urey, “Auto-Stereoscopic Projection Display using
Head-Tracker and Rotating Screen,” in “SID-ME 2014: Mid-Europe Chapter
Sprint Meeting at Istanbul, April,”(SID, 2014).
[O4] O. ELDES, “Method for autostereoscopic projection Displays,” (2015). WO
Patent App. PCT/IB2013/056,842.
[O5] K. Aksit, A. Niaki, O. Eldes, and H. Urey, “Super stereoscopy 3d glasses for
more realistic 3d vision,” in “3DTV-Conference: The True Vision - Capture,
Transmission and Display of 3D Video (3DTV-CON), 2014,” (2014), pp. 1–3.
[O6] K. Akşit, , Osman. Eldes, S. Viswanathen, M. Freeman, and H. Urey, “Portable
3D Laser Projector using Mixed Polarization Technique,” Journal of Display
Technology 8, 582–589 (2012).
[O7] K. Akşit, O. Eldes, S. Viswanathen, M. Freeman, and H. Urey, “Mixed Polarization 3D Technique for Scanned Laser Pico Projector Displays,” in “IMID2012:
The 12th International Meeting on Information Display, August,” (SID, 2012).
Bibliography
51
[O8] K. Aksit, O. Eldes, M. K. Hedili, and H. Urey, “Paper no 15.1: Augmented
reality and 3d displays using pico-projectors,” SID Symposium Digest of Technical
Papers 44, 243–246 (2013).
[O9] H. Urey, S. Holmstrom, U. Baran, K. Aksit, M. Hedili, and O. Eldes, “Mems
scanners and emerging 3d and interactive augmented reality display applications,” (Invited talk in The 17th International Conference on Solid-State Sensors,
Actuators and Microsystems, 2013).
[O10] H. Urey, K. Aksit, and O. Eldes, “Novel 3d displays using micro-optics and
mems,” in “International Conference on Fibre Optics and Photonics,” (Optical
Society of America, 2012).
[O11] K. Akşit, O. Eldes, and H. Urey, “Multiple Body Tracking for Interactive Mobile
Projectors,” in “IMID2012: The 12th International Meeting on Information
Display, August,” (SID, 2012).
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