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Virtual Reality - History, Applications, Technology and Future

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Virtual Reality (VR), sometimes called Virtual Environments (VE) has drawn much attention in the last few years. Extensive media coverage causes this interest to grow rapidly. Very few people, however, really know what VR is, what its basic principles and its open problems are. In this paper a historical overview of virtual reality is presented, basic terminology and classes of VR systems are listed, followed by applications of this technology in science, work, and entertainment areas. An insightful study of typical VR systems is done. All components of VR application and interrelations between them are thoroughly examined: input devices, output devices and software. Additionally human factors and their implication on the design issues of VE are discussed . Finally, the future of VR is considered in two aspects: technological and social. New research directions, technological frontiers and potential applications are pointed out. The possible positive and negative influence of VR on li...
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Virtual Reality
History, Applications, Technology and Future
Tomasz Mazuryk and Michael Gervautz
Institute of Computer Graphics
Vienna University of Technology, Austria
[mazuryk|gervautz]@cg.tuwien.ac.at http://www.cg.tuwien.ac.at/
Abstract
Virtual Reality (VR), sometimes called Virtual Environments (VE) has drawn much attention in
the last few years. Extensive media coverage causes this interest to grow rapidly. Very few
people, however, really know what VR is, what its basic principles and its open problems are.
In this paper a historical overview of virtual reality is presented, basic terminology and classes
of VR systems are listed, followed by applications of this technology in science, work, and
entertainment areas. An insightful study of typical VR systems is done. All components of VR
application and interrelations between them are thoroughly examined: input devices, output
devices and software. Additionally human factors and their implication on the design issues of
VE are discussed. Finally, the future of VR is considered in two aspects: technological and
social. New research directions, technological frontiers and potential applications are pointed
out. The possible positive and negative influence of VR on life of average people is speculated.
1. Introduction
1.1. History
Nowadays computer graphics is used in many domains of our life. At the end of the 20
th
century it is difficult to imagine an architect, engineer, or interior designer working without a
graphics workstation. In the last years the stormy development of microprocessor technology
brings faster and faster computers to the market. These machines are equipped with better and
faster graphics boards and their prices fall down rapidly. It becomes possible even for an
average user, to move into the world of computer graphics. This fascination with a new
(ir)reality often starts with computer games and lasts forever. It allows to see the surrounding
world in other dimension and to experience things that are not accessible in real life or even not
yet created. Moreover, the world of three-dimensional graphics has neither borders nor
constraints and can be created and manipulated by ourselves as we wish – we can enhance it by
a fourth dimension: the dimension of our imagination...
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But not enough: people always want more. They want to step into this world and interact
with it – instead of just watching a picture on the monitor. This technology which becomes
overwhelmingly popular and fashionable in current decade is called Virtual Reality (VR). The
very first idea of it was presented by Ivan Sutherland in 1965: “make that (virtual) world in the
window look real, sound real, feel real, and respond realistically to the viewer’s
actions” [Suth65]. It has been a long time since then, a lot of research has been done and status
quo: “the Sutherland’s challenge of the Promised Land has not been reached yet but we are at
least in sight of it” [Broo95].
Let us have a short glimpse at the last three decades of research in virtual reality and its
highlights [Bala93a, Cruz93a, Giga93a, Holl95]:
Sensorama – in years 1960-1962 Morton Heilig created a multi-sensory simulator. A
prerecorded film in color and stereo, was augmented by binaural sound, scent, wind and
vibration experiences. This was the first approach to create a virtual reality system and it
had all the features of such an environment, but it was not interactive.
The Ultimate Display – in 1965 Ivan Sutherland proposed the ultimate solution of
virtual reality: an artificial world construction concept that included interactive graphics,
force-feedback, sound, smell and taste.
“The Sword of Damocles” – the first virtual reality system realized in hardware, not
in concept. Ivan Sutherland constructs a device considered as the first Head Mounted
Display (HMD), with appropriate head tracking. It supported a stereo view that was
updated correctly according to the user’s head position and orientation.
GROPE – the first prototype of a force-feedback system realized at the University of
North Carolina (UNC) in 1971.
VIDEOPLACE Artificial Reality created in 1975 by Myron Krueger – “a conceptual
environment, with no existence”. In this system the silhouettes of the users grabbed by
the cameras were projected on a large screen. The participants were able to interact one
with the other thanks to the image processing techniques that determined their positions in
2D screen’s space.
VCASS – Thomas Furness at the US Air Force’s Armstrong Medical Research
Laboratories developed in 1982 the Visually Coupled Airborne Systems Simulator – an
advanced flight simulator. The fighter pilot wore a HMD that augmented the out-the-
window view by the graphics describing targeting or optimal flight path information.
VIVED – VIrtual Visual Environment Display – constructed at the NASA Ames in 1984
with off-the-shelf technology a stereoscopic monochrome HMD.
VPL – the VPL company manufactures the popular DataGlove (1985) and the Eyephone
HMD (1988) – the first commercially available VR devices.
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BOOM commercialized in 1989 by the Fake Space Labs. BOOM is a small box
containing two CRT monitors that can be viewed through the eye holes. The user can
grab the box, keep it by the eyes and move through the virtual world, as the mechanical
arm measures the position and orientation of the box.
UNC Walkthrough projectin the second half of 1980s at the University of North
Carolina an architectural walkthrough application was developed. Several VR devices
were constructed to improve the quality of this system like: HMDs, optical trackers and
the Pixel-Plane graphics engine.
Virtual Wind Tunnel developed in early 1990s at the NASA Ames application that
allowed the observation and investigation of flow-fields with the help of BOOM and
DataGlove (see also section 1.3.2).
CAVE – presented in 1992 CAVE (CAVE Automatic Virtual Environment) is a virtual
reality and scientific visualization system. Instead of using a HMD it projects stereoscopic
images on the walls of room (user must wear LCD shutter glasses). This approach
assures superior quality and resolution of viewed images, and wider field of view in
comparison to HMD based systems (see also section 2.5.1).
Augmented Reality (AR) a technology that “presents a virtual world that enriches,
rather than replaces the real world” [Brys92c]. This is achieved by means of see-through
HMD that superimposes virtual three-dimensional objects on real ones. This technology
was previously used to enrich fighter pilot’s view with additional flight information
(VCASS). Thanks to its great potential – the enhancement of human vision – augmented
reality became a focus of many research projects in early 1990s (see also section 1.3.2).
1.2. What is VR? What is VR not?
At the beginning of 1990s the development in the field of virtual reality became much more
stormy and the term Virtual Reality itself became extremely popular. We can hear about Virtual
Reality nearly in all sort of media, people use this term very often and they misuse it in many
cases too. The reason is that this new, promising and fascinating technology captures greater
interest of people than e.g., computer graphics. The consequence of this state is that nowadays
the border between 3D computer graphics and Virtual Reality becomes fuzzy. Therefore in the
following sections some definitions of Virtual Reality and its basic principles are presented.
1.2.1. Some basic definitions and terminology
Virtual Reality (VR) and Virtual Environments (VE) are used in computer community
interchangeably. These terms are the most popular and most often used, but there are many
other. Just to mention a few most important ones: Synthetic Experience, Virtual Worlds,
Artificial Worlds or Artificial Reality. All these names mean the same:
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“Real-time interactive graphics with three-dimensional models, combined with a display
technology that gives the user the immersion in the model world and direct
manipulation.” [Fuch92]
“The illusion of participation in a synthetic environment rather than external observation
of such an environment. VR relies on a three-dimensional, stereoscopic head-tracker
displays, hand/body tracking and binaural sound. VR is an immersive, multi-sensory
experience.” [Giga93a]
“Computer simulations that use 3D graphics and devices such as the DataGlove to allow
the user to interact with the simulation.” [Jarg95]
“Virtual reality refers to immersive, interactive, multi-sensory, viewer-centered, three-
dimensional computer generated environments and the combination of technologies
required to build these environments.” [Cruz93a]
“Virtual reality lets you navigate and view a world of three dimensions in real time, with
six degrees of freedom. (...) In essence, virtual reality is clone of physical
reality.” [Schw95]
Although there are some differences between these definitions, they are essentially equivalent.
They all mean that VR is an interactive and immersive (with the feeling of presence) experience
in a simulated (autonomous) world [Zelt92] (see fig. 1.2.1.1) and this measure we will use
to determine the level of advance of VR systems.
(0,0,0)
(1,1,1)
Presence
Interaction
Autonomy
Virtual Reality
(0,0,1)
(0,1,1)
(1,0,1)
(1,0,0) (1,1,0)
(0,1,0)
Figure 1.2.1.1. Autonomy, interaction, presence in VR Zeltzers cube (adapted from [Zelt92]).
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Many people, mainly the researchers use the term Virtual Environments instead of Virtual
Reality because of the hype and the associated unrealistic expectations [Giga93a]. Moreover,
there are two important terms that must be mentioned when talking about VR: Telepresence and
Cyberspace. They are both tightly coupled with VR, but have a slightly different context:
Telepresence is a specific kind of virtual reality that simulates a real but remote (in
terms of distance or scale) environment. Another more precise definition says that
telepresence occurs when at the work site, the manipulators have the dexterity to allow
the operator to perform normal human functions; at the control station, the operator
receives sufficient quantity and quality of sensory feedback to provide a feeling of actual
presence at the worksite [Held92].
Cyberspace was invented and defined by William Gibson as a consensual
hallucination experienced daily by billions of legitimate operators (...) a graphics
representation of data abstracted from the banks of every computer in human
system [Gibs83]. Today the term Cyberspace is rather associated with entertainment
systems and World Wide Web (Internet).
1.2.2. Levels of immersion in VR systems
In a virtual environment system a computer generates sensory impressions that are delivered to
the human senses. The type and the quality of these impressions determine the level of
immersion and the feeling of presence in VR. Ideally the high-resolution, high-quality and
consistent over all the displays, information should be presented to all of the users
senses [Slat94]. Moreover, the environment itself should react realistically to the users
actions. The practice, however, is very different from this ideal case. Many applications
stimulate only one or a few of the senses, very often with low-quality and unsynchronized
information. We can group the VR systems accordingly to the level of immersion they offer to
the user (compare with [Isda93, Schw95]):
Desktop VR sometimes called Window on World (WoW) systems. This is the
simplest type of virtual reality applications. It uses a conventional monitor to display the
image (generally monoscopic) of the world. No other sensory output is supported.
Fish Tank VR improved version of Desktop VR. These systems support head
tracking and therefore improve the feeling of of being there thanks to the motion
parallax effect. They still use a conventional monitor (very often with LCD shutter glasses
for stereoscopic viewing) but generally do not support sensory output.
Immersive systems the ultimate version of VR systems. They let the user totally
immerse in computer generated world with the help of HMD that supports a stereoscopic
view of the scene accordingly to the users position and orientation. These systems may
be enhanced by audio, haptic and sensory interfaces.
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1.3. Applications of VR
1.3.1. Motivation to use VR
Undoubtedly VR has attracted a lot of interest of people in last few years. Being a new
paradigm of user interface it offers great benefits in many application areas. It provides an easy,
powerful, intuitive way of human-computer interaction. The user can watch and manipulate the
simulated environment in the same way we act in the real world, without any need to learn how
the complicated (and often clumsy) user interface works. Therefore many applications like flight
simulators, architectural walkthrough or data visualization systems were developed relatively
fast. Later on, VR has was applied as a teleoperating and collaborative medium, and of course
in the entertainment area.
1.3.2. Data and architectural visualization
For a long time people have been gathering a great amount of various data. The management of
megabytes or even gigabytes of information is no easy task. In order to make the full use of it,
special visualization techniques were developed. Their goal is to make the data perceptible and
easily accessible for humans. Desktop computers equipped with visualization packages and
simple interface devices are far from being an optimal solution for data presentation and
manipulation. Virtual reality promises a more intuitive way of interaction.
The first attempts to apply VR as a visualization tool were architectural walkthrough
systems. The pioneering works in this field were done at the University of North Carolina
beginning after year 1986 [Broo86], with the new system generations developed
constantly [Broo92b]. Many other research groups created impressive applications as well
just to mention the visualization of St. Peter Basilica at the Vatican presented at the Virtual
Reality World95 congress in Stuttgart or commercial Virtual Kitchen design tool. What is so
fantastic about VR to make it superior to a standard computer graphics? The feeling of presence
and the sense of space in a virtual building, which cannot be reached even by the most realistic
still pictures or animations. One can watch it and perceive it under different lighting conditions
just like real facilities. One can even walk through non-existent houses the destroyed ones
(see fig. 1.3.2.1) like e.g., the Frauenkirche in Dresden, or ones not even created yet.
Another discipline where VR is also very useful is scientific visualization. The navigation
through the huge amount of data visualized in three-dimensional space is almost as easy as
walking. An impressive example of such an application is the Virtual Wind Tunnel [Brys93f,
Brys93g] developed at the NASA Ames Research Center. Using this program the scientists
have the possibility to use a data glove to input and manipulate the streams of virtual smoke in
the airflow around a digital model of an airplane or space-shuttle. Moving around (using a
BOOM display technology) they can watch and analyze the dynamic behavior of airflow and
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easily find the areas of instability (see fig. 1.3.2.2). The advantages of such a visualization
system are convincing it is clear that using this technology, the design process of complicated
shapes of e.g., an aircraft, does not require the building of expensive wooden models any
more. It makes the design phase much shorter and cheaper. The success of NASA Ames
encouraged the other companies to build similar installations at Eurographics95 Volkswagen
in cooperation with the German Fraunhofer Institute presented a prototype of a virtual wind
tunnel for exploration of airflow around car bodies.
(a) (b)
Figure 1.3.2.1. VR in architecture: (a) Ephesos ruins (TU Vienna), (b) reconstruction of destroyed
Frauenkirche in Dresden (IBM).
(a) (b)
Figure 1.3.2.2. Exploration of airflow using Virtual Wind Tunnel developed at NASA Ames:
(a) outside view, (b) inside view (from [Brys93f]).
Other disciplines of scientific visualization that have also profited of virtual reality include
visualization of chemical molecules (see fig. 1.3.2.3), the digital terrain data of Mars
surface [Hitc93] etc.
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Figure 1.3.2.3. VR in chemistry: exploration of molecules.
Augmented reality (see fig. 1.3.2.4) offers the enhancement of human perception and was
applied as a virtual users guide to help completing some tasks: from the easy ones like laser
printer maintenance [Brys92c] to really complex ones like a technician guide in building a
wiring harness that forms part of an airplanes electrical system [Caud92]. An other example of
augmented reality application was developed at the UNC: its goal was to enhance a doctors
view with ultrasonic vision to enable him/her to gaze directly into the patients body [Baju92].
(a) (b)
Figure 1.3.2.4. Augmented Reality: (a) idea of AR (UNC),
(b) augmented reality ultrasound system (from [Stat95]).
1.3.3. Modeling, designing and planning
In modeling virtual reality offers the possibility of watching in real-time and in real-space what
the modeled object will look like. Just a few prominent examples: developed at the Fraunhofer
Institute Virtual Design (see fig. 1.3.3.1) or mentioned already before Virtual Kitchen tools
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for interior designers who can visualize their sketches. They can change colors, textures and
positions of objects, observing instantaneously how the whole surrounding would look like.
Figure 1.3.3.1. FhG Virtual Design (FhG IGD).
VR was also successfully applied to the modeling of surfaces [Brys92b, Butt92, Kame93].
The advantage of this technology is that the user can see and even feel the shaped surface under
his/her fingertips. Although these works are pure laboratory experiments, it is to believe that
great applications are possible in industry e.g., by constructing or improving car or aircraft
body shapes directly in the virtual wind tunnel!
1.3.4. Training and education
The use of flight simulators has a long history and we can consider them as the precursors of
todays VR. First such applications were reported in late 1950s [Holl95], and were constantly
improved in many research institutes mainly for the military purposes [Vinc93]. Nowadays
they are used by many civil companies as well, because they offer lower operating costs than
the real aircraft flight training and they are much safer (see fig. 1.3.4.1). In other disciplines
where training is necessary, simulations have also offered big benefits. Therefore they were
prosperously applied for determining the efficiency of virtual reality training of astronauts by
performing hazardous tasks in the space [Cate95]. Another applications that allow training of
medicine students in performing endosurgery [McGo94], operations of the eye [Hunt93,
Sinc94] and of the leg [Piep93] were proposed in recent years (see fig. 1.3.4.2). And finally a
virtual baseball coach [Ande93] has a big potential to be used in training and in entertainment as
well.
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(a) (b)
Figure 1.3.4.1. Advanced flight simulator of Boeing 777: (a) outside view, (b) inside view (from [Atla95]).
(a) (b)
Figure 1.3.4.2. VR in medicine: (a) eye surgery (from [Hunt93]), (b) leg surgery (FhG IGD).
One can say that virtual reality established itself in many disciplines of human activities, as a
medium that allows easier perception of data or natural phenomena appearance. Therefore the
education purposes seem to be the most natural ones. The intuitive presentation of construction
rules (virtual Lego-set), visiting a virtual museum, virtual painting studio or virtual music
playing [Loef95, Schr95] are just a few examples of possible applications. And finally thanks
to the enhanced user interface with broader input and output channels, VR allows people with
disabilities to use computers [Trev94, Schr95].
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1.3.5. Telepresence and teleoperating
Although the goal of telerobotics is autonomous operation, a supervising human operator is still
required in most of cases [Bola93]. Telepresence is a technology that allows people to operate
in remote environments by means of VR user interfaces (see fig. 1.3.5.1 and 1.3.5.2). In
many cases this form of remote control is the only possibility: the distant environment may be
hazardous to human health or life, and no other technology supports such a high level of
dexterity of operation. Figure 1.3.5.2 presents an example of master and slave parts of a
teleoperating system.
The nanomanipulator project [Tayl93] shows a different aspect of telepresence operating
in environment, remote in terms of scale. This system that uses a HMD and force-feedback
manipulation allows a scientist to see a microscope view, feel and manipulate the surface of the
sample. As the same category, the mentioned already before eye surgery system [Hunt93],
might be considered: beyond its training capabilities and remote operation, it offers the scaling
of movements (by factor 1 to 100) for precise surgery. In fact it may be also called a
centimanipulator.
HEAD MOUNTED
DISPLAY
3D SOUND CUEING
6DOF GESTURE
TRACKING
TACTILE INPUT
AND FEEDBACK
VOICE I/O
HEAD-SLAVED
STEREO CAMERAS
TELEOPERATOR
Figure 1.3.5.1. The idea of teleoperating (adapted from [Bola93]).
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Figure 1.3.5.2. The advanced teleoperation system developed at NOSC.
1.3.6. Cooperative working
Network based, shared virtual environments are likely to ease the collaboration between remote
users. The higher bandwidth of information passing may be used for cooperative working. The
big potential of applications in this field, has been noticed and multi-user VR becomes the focus
of many research programs like NPSNET [Mace94, Mace95b], AVIARY [Snow94a] and
others [Fahl93, Giga93b, Goss94]. Although these projects are very promising, their realistic
value will be determined in practice.
Some practical applications, however, already do exist just to mention a collaborative
CO-CAD desktop system [Gisi94] that enables a group of engineers to work together within a
shared virtual workspace. Other significant examples of distributed VR systems are training
applications: in inspection of hazardous area by multiple soldiers [Stan94] or in performing
complex tasks in open space by astronauts [Cate95, Loft95].
1.3.7. Entertainment
Constantly decreasing prices and constantly growing power of hardware has finally brought VR
to the masses it has found application in the entertainment. In last years W-Industry has
successfully brought to the market networked multi-player game systems (see fig. 1.3.7.1).
Beside these complicated installations, the market for home entertainment is rapidly expanding.
Video game vendors like SEGA and Nintendo sell simple VR games, and there is also an
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increasing variety of low-cost PC-based VR devices. Prominent examples include the Insidetrak
(a simplified PC version of the Polhemus Fastrak), i-glasses! (a low cost see-through HMD) or
Mattel PowerGlove.
Figure 1.3.7.1. VR in entertainment: Virtuality 1000DS from W-Industries (from [Atla95]).
Virtual reality recently went to Hollywood Facial Waldo and VActor systems developed by
SimGraphics allow to sample any emotion on an actors face and instantaneously transfer it
onto the face of any cartoon character [Dysa94]. The application field is enormous: VActor
system has been used to create commercial impressive videos with ultra low cost: USD10 a
second, where the todays industry standard is USD1,000 a second. Moreover, it may be used
in live presentations, and might be also extended to simulate body movements.
(a) (b)
Figure 1.3.7.2. Facial animation systems from SimGraphics:
(a) VActor Xpression, (b) Facial Waldo (from [Dysa94]).
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2. VR technology
2.1. A first look at VR applications: basic components
VR requires more resources than standard desktop systems do. Additional input and output
hardware devices and special drivers for them are needed for enhanced user interaction. But we
have to keep in mind that extra hardware will not create an immersive VR system. Special
considerations by making a project of such systems and special software [Zyda93b] are also
required. First, let us have a short look at the basic components of VR immersive applications:
HMD
Tracker
input data stream
output data stream
3DMouse
Figure 2.1.1. Basic components of VR immersive application.
Figure 2.1.1 depicts the most important parts of human-computer-human interaction loop
fundamental to every immersive system. The user is equipped with a head mounted display,
tracker and optionally a manipulation device (e.g., three-dimensional mouse, data glove etc.).
As the human performs actions like walking, head rotating (i.e. changing the point of view),
data describing his/her behavior is fed to the computer from the input devices. The computer
processes the information in real-time and generates appropriate feedback that is passed back to
the user by means of output displays.
In general: input devices are responsible for interaction, output devices for the feeling of
immersion and software for a proper control and synchronization of the whole environment.
2.1.1. Input devices
Input devices determine the way a user communicates with the computer. Ideally all these
devices together, should make users environment control as intuitive and natural as possible
they should be practically invisible [Brys93e]. Unfortunately, the current state of technology is
not advanced enough to support this, so naturalness may be reached in some very limited cases.
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In most of cases we still have to introduce some interaction metaphors that may become a
difficulty for an unskilled user.
2.1.2. Output devices
Output devices are responsible for the presentation of the virtual environment and its
phenomena to the user they contribute to the generation of an immersive feeling at most.
These include visual, auditory or haptic displays. As it is the case with input, the output devices
are also underdeveloped. The current state of technology does not allow to stimulate human
senses in a perfect manner, because VR output devices are far from ideal: they are heavy, low-
quality and low-resolution. In fact most systems support visual feedback, and only some of
them enhance it by audio or haptic information.
2.1.3. Software
Beyond input and output hardware, the underlying software plays a very important role. It is
responsible for the managing of I/O devices, analyzing incoming data and generating proper
feedback. The difference to conventional systems is that VR devices are much more complicated
than these used at the desktop they require extremely precise handling and send large
quantities of data to the system. Moreover, the whole application is time-critical and software
must manage it: input data must be handled timely and the system response that is sent to the
output displays must be prompt in order not to destroy the feeling of immersion.
2.2. Human factors
As virtual environments are supposed to simulate the real world, by constructing them we must
have knowledge how to fool the users senses [Holl95]. This problem is not a trivial task
and the sufficiently good solution has not yet been found: on the one hand we must give the
user a good feeling of being immersed, and on the other hand this solution must be feasible.
Which senses are most significant, what are the most important stimuli and of what quality do
they have to be in order to be accepted by the user?
Let us start by examining the contribution of each of the five human senses [Heil92]:
sight.................70 %
hearing..............20 %
smell ..................5 %
touch..................4 %
taste ...................1 %
This chart shows clearly that human vision provides the most of information passed to our brain
and captures most of our attention. Therefore the stimulation of the visual system plays a
principal role in fooling the senses and has become the focus of research. The second most
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important sense is hearing, which is also quite often taken into consideration (see section 2.5.3
for details). Touch in general, does not play a significant role, except for precise manipulation
tasks, when it becomes really essential (see section 2.3.3 and 2.5.2 for details). Smell and taste
are not yet considered in most VR systems, because of their marginal role and difficulty in
implementation.
The other aspects cannot be forgotten too: system synchronization (i.e. synchronization of
all stimuli with users actions), which contributes mainly to simulator sickness (see
section 2.2.2 for details) and finally the design issues (i.e. taking into account psychological
aspects) responsible for the depth of presence in virtual environments [Slat93, Slat94].
2.2.1. Visual perception characterization
As already mentioned before, visual information is the most important aspect in creating the
illusion of immersion in a virtual world. Ideally we should be able to generate feedback equal to
or exceeding the limits of the human visual system [Helm95]. Unfortunately todays
technology is not capable to do so, hence we will have to consider many compromises and their
implications on the quality of the resulting virtual environments.
Field of view
The human eye has both vertical and horizontal field of view (FOV) of approximately 180˚ by
180˚. The vertical range is limited by cheeks and eyebrows to about 150˚. The horizontal field
of view is also limited, and equals to 150˚: 60˚ towards the nose and 90˚ to the side [Heil92].
This gives 180˚ of total horizontal viewing range with a 120˚ binocular overlap, when focused
at infinity (see fig. 2.2.1.1).
(a) (b)
Figure 2.2.1.1. Human field of view: (a) vertical, (b) horizontal (from [Heil92]).
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For a comparison: a 21 monitor viewed from the distance of 50cm covers approximately 48˚ of
FOV, typical HMD supports 40˚ to 60˚ field of view. Some displays using wide field optics can
support up to 140˚ of FOV.
Visual acuity
Visual acuity is defined as the sharpness of viewing. It is measured as the fraction of a pixel
which spans one minute of arc horizontally [Cruz92]. Acuity changes for the different arc
distances from the line of sight. For the objects that are reasonably lighted and lie on-axis (and
therefore are projected onto the fovea the part of retina that can resolve finest details in the
image [Wysz82]) acuity is the best: the eye can resolve a separation of one minute of arc. The
area of highest acuity covers a region of about two degrees around line of sight. Sharpness of
viewing deteriorates rapidly beyond this central area (e.g., at 10˚ of the off-axis eccentricity it
drops to ten minutes of arc [Helm95]).
Even the best desktop visual displays are far from achieving this quality a 21 monitor
with the resolution of 1280x1024 viewed from the distance of 50cm supports a resolution of
2.8 minutes of arc. Typical HMD offer much worse arc resolutions they vary from four to six
arc minutes.
Temporal resolution
Temporal resolution of the eye refers to the flickering phenomena perceived by humans, when
watching a screen (e.g., CRT) that is updated by repeated impulses. Too low refresh rates,
especially for higher luminance and big displays, causes the perception of flickering. To avoid
this bad effect, a higher than the critical fusion frequency screen refresh rate (15Hz for small
screens and low illumination levels to 50Hz for big screens and high illumination levels) must
be used [Wysz82].
Todays technology supports this requirement fully currently available at the market CRT
monitors support 76Hz refresh-rates and more, and in case modern LCDs this problem does not
occur because the screen is updated constantly.
Luminance and color
The human eye has a dynamic range of ten orders of magnitude [Wysz82] which is far more
than any current available display can support. Moreover, none of the monitors can cover the
whole color gamut. Therefore special color mapping techniques [Fers94] must be used to
achieve possibly the best picture quality.
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Depth perception
To generate depth information and stereoscopic images the brain extracts information from the
pictures the eyes see and from the actual state of the eyes. This bits of information are called
depth cues. All of the depth cues may be divided into two groups: physiological (like
accommodation, convergence or stereopsis) and psychological (like overlap, object size, motion
parallax, linear perspective, texture gradient or height in visual field) [Sche94]. All of them
participate in generation of the depth information, but one must be careful not to provide
contradictory cues to the user.
2.2.2. Simulator sickness
There are potentially many sources of simulator sickness. Hardware imperfection may
contribute to the generation of sickness feeling, because it fails to provide perfect stimuli to
human senses. However, there are other crucial design issues: system latency and frame rate
variations.
A number of studies investigated this problem, which indicates its meaning and weight.
The studies of [Kenn92, Rega95] try to group and find out the intensity of all kind of maladies
occurring in use of flight simulators and VR systems. The most frequently observed symptoms
are: oculomotor dysfunctions (like eye strain, difficulty focusing, blurred vision), mental
dysfunctions (like fullness of head, difficulty concentrating, dizziness) or physiological
dysfunctions (like general discomfort, headache, sweating, increased salivation, nausea,
stomach awareness or even vomiting) [Kenn92]. However, while these indications sound very
frightening, it is important to mention that when 61% of the investigated subjects reported some
symptoms of sickness, only 5% experienced moderate and 2% severe malady [Rega95].
Latency and synchronization
The success of immersive applications depends not only on the quality of images but also on the
naturalness of the simulation. Desirable property of an intrinsic simulation is prompt, fluent and
synchronized response of the system. The main component of latency is produced by
rendering [Mine95b, Mazu95a], consequently frame update rates have the biggest effect on the
sense of presence and efficiency of performed tasks in VEs [Brys93d, Paus93a, Ware94,
Barf95]. Low latencies (below 100ms) have little effect on performance of flight
simulators [Card90] and frame rates of 15Hz seem to be sufficient to fulfill the sense of
presence in virtual environments [Barf95]. Nevertheless higher values (up to 60Hz) are
preferred [Deer93b], when performing fast movements or when perfect registration (e.g., in
augmented reality) is required [Azum94].
What are the physiological causes of the latency induced simulator sickness? One
hypothesis is that sickness arises from a mismatch between visual motion cues and the
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information that is sent to brain by the vestibular system [Helm95]. This might be the case for
both: motion based VR systems and static ones. This hypothesis seems to be correct because
the human individuals without functioning vestibular system are not subject to simulator
sickness [Eben92].
Frame rate variations
Non-constant frame rates may have a negative influence on the sense of presence and can also
cause simulator sickness. The humans are simply adapting to the slow system responses and
when the update does not come at the expected (even delayed) time-stamp our senses and brain
are disoriented. Therefore constant frame rate algorithms are developed [Funk93] (see also
section 2.4).
2.3. VR input devices
2.3.1. Position and orientation tracking devices
The absolute minimum of information that immersive VR requires, is the position and
orientation of the viewers head, needed for the proper rendering of images. Additionally other
parts of body may be tracked e.g., hands to allow interaction, chest or legs to allow the
graphical user representation etc. Three-dimensional objects have six degrees of freedom
(DOF): position coordinates (x, y and z offsets) and orientation (yaw, pitch and roll angles for
example). Each tracker must support this data or a subset of it [Holl95]. In general there are
two kinds of trackers: those that deliver absolute data (total position/orientation values) and
those that deliver relative data (i.e. a change of data from the last state).
The most important properties of 6DOF trackers, to be considered for choosing the right
device for the given application are [Meye92, Bhat93, Holl95]:
update rate defines how many measurements per second (measured in Hz) are made.
Higher update rate values support smoother tracking of movements, but require more
processing.
latency the amount of time (usually measured in ms) between the users real (physical)
action and the beginning of transmission of the report that represents this action. Lower
values contribute to better performance.
accuracy the measure of error in the reported position and orientation. Defined
generally in absolute values (e.g., in mm for position, or in degrees for orientation).
Smaller values mean better accuracy.
resolution smallest change in position and orientation that can be detected by the
tracker. Measured like accuracy in absolute values. Smaller values mean better
performance.
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range working volume, within which the tracker can measure position and orientation
with its specified accuracy and resolution, and the angular coverage of the tracker.
Beside these properties, some other aspects cannot be forgotten like the ease of use, size and
weight etc. of the device. These characteristics will be further used to determine the quality and
usefulness of different kinds of trackers.
Magnetic trackers
Magnetic trackers are the most often used tracking devices in immersive applications. They
typically consist of: a static part (emitter, sometimes called a source), a number of movable parts
(receivers, sometimes called sensors), and a control station unit. The assembly of emitter and
receiver is very similar: they both consist of three mutually perpendicular antennae. As the
antennae of the emitter are provided with current, they generate magnetic fields that are picked
up by the antennae of the receiver. The receiver sends its measurements (nine values) to the
control unit that calculates position and orientation of the given sensor. There are two kinds of
magnetic trackers that use either alternating current (AC) or direct current (DC) to generate
magnetic fields as the communication medium [Meye92].
The continuously changing magnetic field generated by AC magnetic trackers (e.g., 3Space
Isotrak, Fastrak or Insidetrak from Polhemus) induces currents in coils (i.e. antennae) of the
receiver (according to Maxwells law). The bad side-effect is the induction of eddy currents in
metal objects within this magnetic field. These currents generate their own magnetic fields that
interfere and distort the original one, which causes inaccurate measurements. The same effect
appears in vicinity of ferromagnetic objects.
DC trackers (e.g., Bird, Big Bird or Flock of Birds from Ascension) transmit a short series
of static magnetic fields in order to avoid the eddy current generation. Once the field reaches a
steady state (eddy currents are still generated but only at the beginning of measurement cycle)
the measurement is taken with the help of flux-gate magnetometers [Asce95b]. To eliminate the
influence of the Earths magnetic field, this constant component (measured when the transmitter
is shut off) is subtracted from the measured values. Although DC trackers eliminate the problem
of eddy current generation in metal objects, they are still sensitive to ferromagnetic
materials [Asce95b].
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Figure 2.3.1.1. Emitter and receiver units of Polhemus Fastrak.
Under optimal conditions (lack of any kind of magnetic interference) magnetic trackers have a
relatively good performance. For illustration we give a technical description of two commonly
used products Polhemus Fastrak [Polh93] and Ascension Flock of Birds [Asce95a]:
Tracker Max. #
of
sensors
Max.
range
(m)
Lag
(ms)
Max.
update
rate (Hz)
Accurac
y
(RMS)
Resolution
at distance
Polhemus
Fastrak
4 3.05 4 120 / # of
sensors
0.8 mm
0.15˚
5e-03mm per mm
0.025˚
Ascension
Flock of Birds
30 1 < 10 144 2.54 mm
0.5˚
0.5mm at 30cm
0.1˚ at 30cm
Table 2.3.1.1. Technical data of magnetic trackers.
Advantages:
sensors are small, light and handy
have no line-of-sight constraint
non-sensitive to acoustic interference
relatively high update rates and low latency
off-the-shelf availability
Disadvantages:
since magnitude of magnetic field strongly decreases with distance from the emitter, the
working volume of magnetic trackers is very limited and the resolution is getting worse as
the emitter-receiver distance is growing.
magnetic field is subject to distortion, caused by metal objects inside of it (AC trackers
only). Moreover, any external magnetic field generated e.g., by CRT displays or by
ferromagnetic objects in vicinity (both AC and DC trackers) may cause additional
distortion that leads to inaccurate measurements.
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Acoustic (ultrasonic) trackers
Acoustic trackers use ultrasonic waves (above 20kHz) for determining the position and
orientation of object in space. As the use of sound allows the determination of relative distance
between two points only, multiple emitters (typically three) and multiple receivers (typically
three) with known geometry are used to acquire a set of distances to calculate position and
orientation [Meye92]. There are two kinds of acoustic trackers they either use time-of-flight
(TOF) or phase-coherent (PC) measurements to determine the distance between a pair of points.
TOF trackers (e.g., Logitech 6DOF Ultrasonic Head Tracker, Mattel PowerGlove) measure the
flight time of short ultrasonic pulses from the source to the sensor. PC trackers (for example
used by I. Sutherland in 1968! [Suth68]) compare the phase of a reference signal with the
phase of the signal received by the sensors. The phase difference of 360˚ is equivalent to the
distance of one wavelength. The difference between two successive measurements of phases
allows to compute the distance change since the last measurement. As this method delivers
relative data (so the error tends to accumulate with time), development of PC trackers was
relinquished.
The typical working parameters of acoustic TOF trackers (taken from the Logitech 6DOF
specification see fig. 2.3.1.2) are:
range ................1.5m and 100˚ cone of angular coverage
update rate ..........50Hz
lag...................30ms
accuracy.............2% of distance from source and 0.1˚ of orientation
Figure 2.3.1.2. Logitech 6DOF Ultrasonic Tracker (from [Deer92]).
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Advantages (of TOF trackers):
light and small
relatively cheap (from USD1000)
do not suffer from magnetic interference
Disadvantages (of TOF trackers):
line-of-sight restriction
suffer from acoustic interference noise or echoes may lead to inaccurate measurements
low update rates
Optical trackers
There are many different kinds and configurations of optical trackers. Generally we can divide
them into three categories [Meye92]:
beacon trackers this approach uses a group of beacons (e.g., LEDs) and a set of
cameras capturing images of beacons pattern. Since the geometries of beacons and
detectors are known, position and orientation of the tracked body can be
derived [Wang90, Ward92]. There are two tracking paradigms: outside-in and inside-out
(see fig. 2.3.1.3).
pattern recognition these systems do not use any beacons they determine position
and orientation by comparing known patterns to the sensed ones [Meye92, Reki95]. No
fully functioning systems were developed up to now. A through-the-lens method of
tracking may become a challenge for the developers [Thom94].
laser ranging these systems transmit onto the object the laser light that is passed
through a diffraction grating. A sensor analyzes the diffraction pattern on the bodys
surface to calculate its position and orientation.
For all these systems the accuracy decreases significantly as the distance between sensors and
tracked objects grows [Meye92].
Advantages:
high update rates (up to 240Hz [Holl95]) in most of cases limited only by the speed of
the controlling computer
possibility of the extension to the large working volumes [Wang90, Ward92]
not sensible to the presence of metallic, ferromagnetic objects; not sensible to acoustic
interference
relatively good accuracy: magnitude orders of about 1mm and 0.1˚
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Disadvantages:
line-of-sight restriction
ambient light and infrared radiation may influence the performance
expensive and very often complicated construction
difficulties to track more than one object in one volume
(a) (b)
Figure 2.3.1.3. Beacon trackers: (a) outside-in and (b) inside-out tracking paradigms (UNC).
Mechanical trackers
A mechanical linkage of a few rigid arms with joints between them is used to measure position
and orientation of a free point (attached to the end of the structure) in relation to the base. The
angles at the joints are measured with the help of gears or potentiometers, which combined with
the knowledge of linkage construction allows to derive the required position and orientation
values (see fig 2.3.1.4). A prominent example of a mechanical tracking device is the BOOM
(Binocular Omni-Oriented Monitor) developed by Fake Space Labs (see fig. 2.3.1.5).
Figure 2.3.1.4. The idea of mechanical linkage (from [Brys93e]).
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Figure 2.3.1.5. Mechanical tracking device: BOOM from Fake Space Labs.
Advantages:
very accurate
immune to all kind of interferences (unless mechanical obstacles)
high update-rates (up to 300Hz)
may support force-feedback
Disadvantages:
not full freedom of movements due to the mechanical linkage
small working volume (about one cubic meter)
only one object can be tracked in one volume
2.3.2. Eye tracking
Head tracking allows proper rendering of images from the users point of view. The advantage
of the head tracking is that motion parallax cue can be provided, which improves the depth
perception. One more important aspect can be taken into account: the visual acuity of the eye
changes with the arc distance from the line-of-sight. It means that image does not need to have
equal resolution and quality over the whole display area. Objects that lie far the line-of-sight can
be represented coarsely, because the user will not notice it. Consequently, this may lead to the
dramatically decrease of rendering costs [Levo90, Funk93, Redd95]. Therefore eye-tracking
techniques may be incorporated to determine the gaze direction [Youn75, Stam93].
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In general, most important eye-tracking technologies can be grouped as follows:
limbus tracking the sharp boundary between iris and sclera (limbus) can be easily
identified. The infrared LEDs and photo-transistors are mounted on the users glasses to
monitor infrared spots reflections from the iris and sclera in order to determine the gaze
direction. This technique offers good accuracy (1˚ to 3˚), but limits vertical eye
movements (by extreme vertical eye movements limbus is partially obscured by eye-lids
what hinders exact measurement). It is used by e.g., the NAC Eye Mark eye tracker (see
fig. 2.3.2.1).
image tracking uses a video camera and image processing techniques to determine the
gaze direction. This technology offers good accuracy typically about 1˚ (used by e.g.,
ISCAN, Applied Science Labs 4000 SU-HMO [Holl95]).
electro-oculography (EOG) uses the electrodes placed beside the eyes to measure
the standing potential between cornea and retina. Typically, the recorded potentials are
very small: in the range of 15µV to 200µV. This approach has a questionable worth
because it is susceptible to external electric interference and muscle-action potentials.
corneal reflection uses photo-transistors to analyze a reflection of collimated beam of
light from the convex cornea surface. This approach offers relatively good accuracy (0.5˚
to 1˚), but it needs complex calibration, covers relatively small eye-movement area and is
sensitive to variations in cornea shape variations, tear fluids and corneal astigmatism.
Figure 2.3.2.1. NAC Eye Mark eye tracker (from [Levo90]).
2.3.3. 3D input devices
Beside trackers that capture users movements, many other input devices were developed to
make human-computer interaction easier and more intuitive. For full freedom of movements
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three-dimensional input devices seem the most natural. Attached to our body or hand-held, they
are generally used to select, move, modify etc. virtual objects. This chapter presents a broad
overview of most important of these devices.
3D Mice and Bats
This basic and simple user interaction tool is in general a joystick-like 6DOF device that can be
moved in space by hand. It is equipped with a tracker sensor to determine its
position/orientation and a few buttons that may trigger some actions [Ware90a]. Some 3D mice
may be equipped with a thumbball for additional movement control.
Gloves
Gloves are 3D input devices that can detect the joint angles of fingers. The measurement of
finger flexion is done with the help of fiber-optic sensors (e.g., VPL DataGlove), foil-strain
technology (e.g., Virtex CyberGlove) or resistive sensors (e.g., Mattel PowerGlove). The use
of gloves allows the user richer interaction than the 3D mouse, because hand gestures may be
recognized and translated into proper actions [Mine95a]. Additionally gloves are equipped with
a tracker that is attached to the users wrist to measure its position and orientation.
(a) (b)
Figure 2.3.3.1. Gloves: (a) VPL DataGlove, (b) Virtex CyberGlove (from [Stur94]).
An obvious extension of the data glove is a data suit that covers the whole body of the user. The
first step in this direction is capturing of the whole body movements with minimal number of
sensors [Badl93a]. In last few years more and more attention was paid to such devices, and
there are already commercial data suits on the market like e.g., the VPL DataSuit. An example
of application of the body tracking technology is the real-time animation of virtual actors in film
industry.
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Dexterous manipulators
Some applications (e.g., teleoperation, surgery) require extremely precise control. The data
gloves are very often not sufficient to fulfill these demands and therefore many dexterous
manipulators were developed, for example: the Master Manipulator [Iwat90], the Dexterous
Hand Master (DHM) from Utah University [Rohl93a, Rohl93b], further developed by EXOS
(see fig. 2.3.3.2a) or the DHM from NOSC (see fig. 2.3.3.2b). The Master Manipulator (see
fig. 2.5.2.1a) is a relatively simple device it supports only 9DOF control and force feedback
(see section 2.5.2 for details). It uses potentiometers to measure bending angles. Dexterous
Hand Masters are much more elaborated devices: they can trace three joints angles for each
finger (4DOF for each finger which makes total 20DOF for the whole hand). Moreover, they
guarantee high precision measurement of bending angles (error magnitude order of 1˚ in
contrast to 5˚-10˚ in case of gloves [Stur94]) thanks to Hall-effect [Tipl91] sensors.
(a) (b)
Figure 2.3.3.2. Dexterous manipulators: (a) EXOS Dexterous Hand Master (from [Stur94]),
(b) NOSC Dexterous Hand Master.
2.3.4. Desktop input devices
Beside sophisticated and expensive three-dimensional input devices, many special desktop tools
are very popular. The do not give so good and intuitive control like 3D devices and decrease the
immersion feeling, but are handy, simple in use and relatively cheap.
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SpaceBall
SpaceBall is a simple 6DOF input device (see fig. 2.3.4.1). The user can grab the ball with
his/her hand and manipulate it the device measures translation forces and rotation torques of
the ball and sends this data to the host computer. Additional buttons are built-in to enhance the
interaction possibilities.
Figure 2.3.4.1. SpaceBall desktop 6DOF input device (from [Vinc95]).
CyberMan
CyberMan is an extension of typical two-dimensional mouse (see fig. 2.3.4.2). It supports
6DOF input. With a help of small motor it can simulate quasi-haptic feedback: the part of these
device kept in hand can vibrate to indicate a collision, or force-feedback. CyberMan is most
often used in computer games.
Figure 2.3.4.2. CyberMan from Logitech desktop 6DOF input device.
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2D input devices
Many desktop systems are equipped only with standard 2D input mice. They do not support so
intuitive control of three-dimensional objects like any of previously described 6DOF
manipulators, but are very popular, wide-spread and cheap. Nevertheless, to allow the user a
relatively easy way of manipulation of 3D objects, software virtual controllers were
implemented. A virtual sphere controller a simulation of 3D trackball [Chen88] and other
tools [Niel86] support easy, interactive rotating and positioning of three-dimensional objects
with the use of simple 2D desktop mouse (for more advanced virtual controls 3D widgets see
section 2.4.2).
2.4. VR worlds: modeling, interaction and rendering
Every VR application must be effective by means of performance and interaction. This
requirement can be only fulfilled when all system parts input, interaction and output are
properly integrated one with the other. Nowadays, even the best hardware cannot support this
by itself it needs software assistance for precise control, resources management and
synchronization.
2.4.1. Construction of virtual worlds
Construction of virtual environments involves many different aspects that were not present in
standard computer graphics. The biggest challenge to trade is performance vs. natural look and
behavior. As already mentioned before, these requirements are contradictory: more convincing
models and better physical simulation demand more resources, thereby increasing
computational cost and affecting overall performance. Many different kinds of models
representing virtual worlds can be imagined: from simple models like a single unfurnished
room, to extremely complex ones like a the whole city with many buildings containing a lot of
chambers, each modeled with high amount of detail. While it is trivial to display a simple model
with adequate performance, but rendering millions of polygons would hinder interactive frame-
rates, even if we were able to load the whole scene into main memory. Hence it will never be
technically possible (the faster the hardware, the finer and more complex models will be), we
must develop dedicated data structures and algorithms allowing to produce the best image
quality with acceptable cost.
Data structures and modeling
For huge scenes containing millions of polygons, the challenge is to identify the relevant
(potentially visible) portion of the model, load data into memory and render it at interactive
frame-rates. In many cases it may still happen that the number of polygons of all visible objects
dramatically exceeds rendering capabilities. Therefore the other important aspect of the data
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structure construction is level-of-detail (LOD) definition (see fig. 2.4.1.1). Due to the
perspective projection distant objects appear smaller on the screen that the close ones (see
fig. 2.4.1.2). In the extreme case they may cover as little as one pixel! In this situation it does
not make sense to render them with the highest possible geometric resolution, because the user
will not notice it. Nevertheless, when the same objects are closer to the user they must be
rendered with a high resolution in order to let him/her see all the details.
(a) (b)
Figure 2.4.1.1. Multiple levels-of-detail of the same object: (a) low LOD, (b) high LOD (from [Funk93]).
Figure 2.4.1.2. Distant objects appear smaller on the screen than the close ones (from [Funk93]).
To achieve the best image quality at interactive frame rates, several approaches may be
used [Tell91, Funk92, Funk93, Falb93, Maci95, Scha96a]:
hierarchical scene database the scene is represented as a set of objects. Each object
of the scene is described with multiple LODs that represent different accuracy of object
representation (and contain different numbers of polygons). In extreme case objects can
be represented by one textured polygon [Maci95].
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visibility precomputation (analysis) the whole visual database is spatially
subdivided into cells connected by portals. The visibility analysis is performed on such a
prepared model in two phases: the preprocessing phase (determination of cell-to-cell and
cell-to-object visibility) and during the walkthrough phase (determination of eye-to-cell
and eye-to-object visibility). To improve the performance of this process the splitting
planes are chosen along the major obscuring elements e.g., walls, floors, ceilings or door
frames [Funk92] (see fig. 2.4.1.3).
memory management if the whole scene cannot be loaded into the main memory,
special algorithms for swapping in the relevant parts must be used. The loading of objects
from the disk can take relatively much time, so prediction of objects that might be
potentially visible in the near future has to be done and loading should start in advance
(prefetching), in order to avoid waiting in the rendering phase.
constant frame-rate rendering after all the potentially visible objects were
determined in the visibility preprocessing phase, it still may happen that not all of them
can be rendered with their highest resolution. To provide the best quality of the image
within a given time, the selection of LOD and rendering algorithm for each object must be
performed. Several properties of objects should be taken into account e.g., size on the
screen, importance for the user, focus (position on the screen, where he/she is looking) or
motion (for fast moving objects we cannot see many details) [Funk93]. As the graphics
pipeline in most graphical systems is used, the proper load balancing in each of the stages
must be taken into account [Funk93, Sowi95].
(a) (b)
Figure 2.4.1.3. Data pruning: (a) before and (b) after visibility computation (from [Funk92]).
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The demand for highly detail scenes has grown rapidly in the last years, so labor-intensive,
manual creation and processing became impractical. Automatic generation and processing of
models offers great possibilities: for example, creation of objects from multiple stereoscopic
images was proposed recently [Koch94]. Moreover, techniques that can generate automatically
multiple levels-of-detail from one high resolution polygonal representation are very helpful,
because they can accelerate the creation of hierarchical scene databases for interactive
walkthrough applications [Turk92, Ross93a, Heck94, Scha95a].
Physical simulation
Virtual reality may be a clone of physical (real) reality or a kind of closer not defined
(cyber)space that has it own rules. In both of these cases a simulation of the environment has to
be done. In case of newly defined cyberspace the task is relatively easy we can invent new
laws or use simplified physics. The real challenge is to simulate the rules of physics precisely,
because they are very complex phenomena: dynamics of objects, electromagnetic forces, atomic
forces etc. For the human-computer interaction purposes a subset of them has to be considered.
Newtons laws are the basis when simulating movements, collisions and force-interaction
between objects [Vinc95].
The simulation, collision detection [Zyda93c, Fang95] and animation of autonomous
objects, may be a very complex and time-consuming task, so other approaches must be applied
than in standard (i.e. non-real time) animation. The simulation process that manages the
behavior of the whole environment (including interaction between different users) should be run
in the background decoupled from the users interaction [Shaw92a, Shaw93b] in order to
support the full performance. The updates between these application parts are realized by means
of asynchronous operations.
The construction and maintaining of physically based, multi-user and therefore distributed
virtual environments is not an easy task. Beside usual expectations high efficiency support for
lag minimization it demands hardware independence, flexibility and high-level paradigms for
easy programming, maintaining and consistent user interface. A few prominent examples of VE
toolkits and systems (i.e. VE shells) are: MR (Minimal Reality) [Shaw93a, Shaw93b],
NPSNET [Zyda92b, Mace94, Mace95b], AVIARY [Snow93, Snow94a] or DIVE [Carl93a,
Carl93b].
2.4.2. Interacting with virtual worlds
The ultimate VR means that no user interface is needed at all every interaction task should be
as natural as in (real) reality. Unfortunately this is not possible today because of technical
problems. However, many techniques may be used to enhance the interaction model [Bala95],
but they still use some metaphors to make human-computer dialog easier.
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Interaction paradigms in 3D
The human hand is most dexterous part of our body in reality we use it (or both of them)
intuitively to perform a variety of actions: grabbing or moving objects, typing, opening doors,
precise manipulations etc. The most natural way of interacting with the computer is probably by
using the hand. Therefore the majority of already introduced in section 2.3 VR input devices
are coupled to our palm. They represent a broad variety of levels of advance, complexity and
price, so for different applications other devices and modes of interaction are used. Basic
interaction tasks in VEs are: camera control (for observation), navigation, object manipulation
and information access.
Camera control
Observation of the scene is essential to the user, because it provides information about his/her
location in virtual world. Intuitive camera control is fundamental it is responsible for the
immersion feeling. In the ideal case, where head-tracking is available the point of view is
directly set by rotating and moving the users head. This model is without doubt the most
convincing one, but on the other hand, not every system supports head-tracking capabilities.
Therefore other camera control models were developed [Ware90b] for non-immersive
applications. Camera movements can be steered with e.g., desktop spaceball devices. Two
control metaphors have been proven to be helpful in observing virtual worlds, and can be
changed during the interaction task according to the user needs:
eyeball in hand with this metaphor the user has to imagine that the spaceball
represents the eye he/she is watching the scene with. The user can intuitively translate and
rotate it (full 6DOF control) to change the viewing point and direction. This metaphor is
very useful when the user is immersed inside of the scene (i.e. the scene surrounds
him/her).
scene in hand with this metaphor the camera has the constant position and
orientation, and the whole scene can be manipulated (i.e. rotated and translated). This
metaphor is very useful, in the case when the user watches the whole scene (or some
specific objects of it) from outside. This is a natural way of observing from different
sides the objects that appear small and therefore can be kept in hand (it is easier to rotate
them, than to walk them around).
Navigation
In many cases, user may want to explore the whole (very often big) environment. Walking over
long distances cannot be realized so easily, because of the limited tracking range. Therefore an
appropriate transport medium is needed. In general, with the help of some input devices we can
define the motion of our body in virtual space. Depending on the type of application this may be
VIRTUAL REALITY HISTORY, APPLICATIONS, TECHNOLOGY AND FUTURE
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either driving (in 2D space) or flying (in 3D space). The principles of these navigation
paradigms are however the same [Robi93, Mine95a, Vince95]:
hand directed position and orientation of hand determines the direction of motion in
virtual world. In this approach different modes can be incorporated to specify the required
direction: pointing or crosshair mode. In the first case, moving is performed along a line
the pointing finger determines. In the second one, a cursor (crosshair) is attached to the
users hand and the line between the eye and cursor defines the moving direction.
gaze directed looking direction (head orientation) specifies the line of movement. It is
a relatively easy metaphor for an unskilled user but hinders looking around during the
motion because direction of motion is always attached to the gaze direction.
physical controls input devices like joysticks, 3D mice, spaceballs are used to
specify the motion direction. They allow precise control, but often the lack of
correspondence between device and motion may be confusing. However, construction of
special devices for certain applications may increase the feeling of immersion (e.g.,
steering wheel for driving simulation).
virtual controls instead of physical devices, virtual controls can be implemented.
This approach is hardware independent and therefore is much more flexible, but
interaction may be difficult because of lack of haptic feedback.
All these modes are based on the principle of steering a virtual vehicle through the space. The
user sitting inside of this vehicle can determine not only the direction but the speed and
acceleration of movements (e.g., pressing buttons, or by hand gestures). Moreover, he/she can
still rotate and move his/her head in his/her local coordinate system. The higher level navigation
models can be also incorporated:
teleporting the moving through the virtual world is realized with the help of
autonomous elevator-like devices or portals that once entered move the user to the
specified point of space. The obvious extension of this mode is goal driven navigation,
where the user can choose the target with a help of virtual menu [Jaco93] or a sensitive
map.
world scaling the distances in virtual world may be dynamically changed according to
the users needs. For example we can scale the world down and move to the desired
position (e.g., make one step one thousand kilometers long) and scale the world back to
the original size. The scaling of the model up can be also performed to allow the user
precise control (e.g., nanomanipulation [Tayl93] or eye surgery [Hunt93]).
Selection (object picking)
To perform any action that causes the change of virtual world state, the user must first select the
object that will be the subject of manipulation. There are two primary selection
VIRTUAL REALITY HISTORY, APPLICATIONS, TECHNOLOGY AND FUTURE
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techniques [Mine95a]: local and at-a-distance. In the local mode, selecting is done when the
collision between users hand represented with e.g., 3D cursor and object is detected. In the at-
the-distance mode, a ray is shot in to the scene to pick the object. The selection-ray can be
determined by hands orientation or gaze direction. An alternative selection-mode may be done
by choosing entries from the virtual menu [Jaco93].
Manipulation
Once the object is selected (which is signaled by e.g., highlighting it on the screen) the user
must be able to manipulate it: move, rotate, scale, change attributes etc. This can be achieved by
defining special button presses, hand gestures [Stur93] or menu entries that choose a proper
tool. These tools can be driven by physical input devices like mice, joysticks, sliders, gauges,
hand position tracking [Ware90a] or even by a nose-gesture interface [Henr92] :-).
A new paradigm of the 3D user interface and its use in the modeling process 3D widgets
(see fig. 2.4.2.1) were proposed lately [Broo92a]. Widgets encapsulate the geometry and
behavior, and therefore are flexible virtual controls that can be elaborated individually for the
application needs. Currently these widgets are used in a desktop system but porting them into
full immersive VR application seems to be straightforward.
(a) (b)
Figure 2.4.2.1. Manipulation of object with the help of 3D widgets:
(a) color-picker widgets, (b) rack widget (from [Broo92a]).
Information accessing
Nowadays, huge amounts of information are stored in computer memory and flow through
computer networks. These streams of data will be growing rapidly in the near future (data-
highways). The real problem will be rapid retrieval and comprehensive access to the relevant
information for a particular user. Standard computer interfaces are not capable to guarantee this
any more. Virtual reality with its broader input and output channels, autonomous guiding
VIRTUAL REALITY HISTORY, APPLICATIONS, TECHNOLOGY AND FUTURE
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agents [Ehma93] and space metaphors [Benf95] offers the enhancement of human perception
and makes information searching and understanding faster [Caud95, Mapl95].
To make the interaction and communication with virtual worlds successful we cannot think just
about one of previously listed interaction techniques. For each application area, other subset of
them will be needed to guarantee the optimal performance. Ideally, not only software but also
hardware should be transparent to the user and should provide maximum freedom and
naturalness. To achieve this, however, both refinement of hardware devices and software
paradigms for interaction are necessary.
2.4.3. Rendering of virtual worlds
The previous section described, the data structures and rendering approaches concerned with
them. This chapter will address other aspects of visualization of virtual worlds like camera
transformations, image generation and stereoscopy.
Visual display transformations
The visual display transformation for VR is much more complex than in standard computer
graphics. On one hand, there is a hierarchical scene database containing a number of objects
(like in standard computer graphics) and on the other hand is the user controlling the virtual
camera by moving his/her head, flying through the world, or manipulating it (e.g., scaling). To
provide a proper view of the scene, all these components are to be taken into consideration. The
determination of viewing parameters involves the calculation of a series of transformations
between coordinate systems (CS) that depend on hardware setup, users head position and state
of input devices [Mine95a, Robi95] (see fig. 2.4.3.1). This section describes how to calculate
display transformations for rendering of monoscopic images; for details concerning the
generation of stereoscopic images refer to the next section.
eye
world
room
head sensor
objects
measured by the tracker
fixed for a given
HMD geometry
fixed for a given
room geometry
modified when
objects are moved
modifed when user
moves or rotates
the room (vehicle)
tracker emitter
Figure 2.4.3.1. Coordinate system transformations for virtual reality (adapted from [Mine95a]).
VIRTUAL REALITY HISTORY, APPLICATIONS, TECHNOLOGY AND FUTURE
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To render the images we must know where the camera in the virtual world is. Therefore
following transformations must be calculated:
Eye-In-Sensor defines the position of the eye (virtual camera) in the trackers sensor
CS. These transformations (for left and right eye) are fixed for a given HMD geometry
(different HMDs can have trackers sensors mounted differently).
Sensor-In-Emitter defines position and orientation of the sensor in the trackers
emitter CS. This transformation changes dynamically as the user moves or rotates his/her
head and is measured by the tracking device.
Emitter-In-Room defines position and orientation of the tracking system in the
physical room it is placed in. This transformation is fixed for the given physical tracking
system setup in room.
Room-In-World defines position and orientation of the room (or user controlled
vehicle) in the world CS. This transformation changes dynamically according to the
users actions like flying, tilting or world scaling.
To resolve the final viewing transformation (from the object CS into the screen CS) we must
additionally take into account the viewing perspective projection and Object-In-World
transformations like in standard computer graphics [Fole90].
Stereoscopy
Our two eyes allow us to see three-dimensionally. Stereo vision relies on additional depth-cues
like eye convergence and stereopsis based on retinal disparity [McKe92b, Hodg93] (see
fig. 2.4.3.2) and therefore may greatly increase the feeling of immersion.
α
D
IPD
P1
P2
α
α
1
2
(a) (b)
Figure 2.4.3.2. Stereoscopic depth-cues: (a) eye convergence, (b) retinal disparity (from [Sche94]).
VIRTUAL REALITY HISTORY, APPLICATIONS, TECHNOLOGY AND FUTURE
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For stereo perception in computer graphics we must generate proper pairs of images (stereo
pairs). There are two kinds of VR systems that require different visual display transformations
in order to produce the proper stereoscopic images Fish Tank VR and immersive systems
(i.e. systems that use HMDs):
Fish Tank VR these systems use a standard desktop monitor to present the images.
The user is equipped with a head tracker and appropriate 3D glasses (see section 2.5.1).
Stereoscopic images are mainly created with the help of the off-axis projection [Hogd92].
This method uses two asymmetrical projections that are not centered on the main
projection axis (see fig. 2.4.3.3a). Alternatively a crossed-axis projection [Hodg92] can
be applied. In this approach the angle of convergence at the viewed point (see
fig. 2.4.3.4a) is used as the rotational angle for the scene (see fig. 2.4.3.4b). The
rotation is easy to implement, but it can produce divergent and vertical parallaxes (parallax
is the distance of homologous points on the screen). A divergent parallax can occur when
there are points that are far away from the center of rotation. Their parallax will get bigger
the further they are away, so the parallax is unbounded. Vertical parallax occurs when an
object is rotated under perspective projection [Sche94]. These artifacts can greatly affect
image quality.
immersive systems these systems use a HMD to present images to the user. They
use the on-axis projection [Hodg92]. For a generation of stereo images this method uses
parallel viewing rays for each of the eyes and the perspective projection (see
fig. 2.4.3.3b).
Screen plane
Screen border
Cameras
Cameras
- stereoscopic
regions
- monoscopic
regions
(a) (b)
Figure 2.4.3.3. Two centers of projection transformations:
(a) off-axis projection, (b) on-axis projection (from [Sche94]).
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ϕ
Plane of
Projection
ϕ
Left Right
(a) (b)
Figure 2.4.3.4. Crossed-axes projection: (a) eye convergence, (b) scene rotation (from [Sche94]).
Both of the two centers of projection methods (off-axis and on-axis) generate a proper
perspective but have the disadvantage that they create regions where only monoscopic
information is present (regions that are seen by only one of the eyes).
For the design of a stereoscopic system special considerations are to be taken into account,
because even very little distortion (caused by optics geometry or incorrect transformations) may
hinder the fusing of proper three-dimensional images [Robi92a, Jian93].
Image generation
Image generation is crucial in every VR system. Some aspects of it (like constant frame-rate
rendering and CS transformations) were already addressed in previous sections, but there is one
more important issue to mention: speed. To sustain an immersion feeling a high frame rate is
required, but visual scenes are getting larger and more detailed (the number of polygons in the
viewing frustum is growing). These two needs are addressed by high performance hardware
image generators (IG). Several vendors like SGI, Intergraph, Evans&Sutherland, Division or
SUN offer a variety of graphics boards that have different speeds, capabilities and prices. The
following properties must be considered [Last95]:
performance specifies how many Z-buffered polygons (generally triangles) per
second can be rendered. To achieve the peak performance triangle strips (meshes) are
used.
shading which shading algorithms are implemented in hardware: flat, Gouraud or
Phong.
lighting capability possibility of hardware assisted color calculation (based on e.g.,
defined light sources and material definition). This allows the use of models that do not
have precalculated vertex colors.
texturing capability possibility of texture mapping. This approach allows the
dramatic reduction of the scene geometry: instead of many small polygons, large textured
polygons are used. This also increases the quality of rendered images.
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The best graphic boards available on the market (e.g., RealityEngine
2
from SGI) provide up to
1.6 millions of 50 pixel, unlighted, flat-shaded and Z-buffered triangles (in mesh) per second.
Generally all graphic accelerators support lighting calculations and Gouraud-shading; very few
can perform Phong-shading. Use of these features, however, causes a performance penalty.
The use of textures in image generation can bring advantages: a smaller number of polygons is
required to achieve the expected image quality. The most prominent graphics board to date in
this field, RealityEngine
2
can render nearly one million of 50 pixel textured polygons per
second.
The use of hardware IG brings many performance advantages, but still cannot produce
images instantaneously. Therefore, alternative rendering techniques, hardware architectures and
configurations are developed constantly. Many of them base on the assumption that the biggest
error by the dynamic image viewing, is caused by head rotations. These approaches try to
resolve the dependency between orientation data coming from the tracker and the rendering
process. The first successfully performed attempt was the CAVE environment [Cruz92,
Cruz93b] (see also section 2.5.1) followed by the Virtual Portal [Deer93a]. In both of these
systems the user, instead of wearing a HMD, steps into a room whose walls are the projection
screens. This approach reduces the latency-based rotational error of head movement to zero,
because all the surrounding images are rendered based on the users position only. The
statistical data (only 2 from 9,000 people visiting the CAVE, experienced enough nausea to
complain about it) confirms this fully [Cruz93b]. A special hardware architecture the address
recalculation pipeline [Rega94] is built on the same principle, but allows the user to wear a
HMD. Six images are rendered in parallel on six walls of a fictive cube (six separate frame
buffers). This approach allows similarly how it was done in CAVE to detach the users
orientation from the rendering process. As soon as the rendering is completed, the users
orientation is measured and the appropriate pixels are copied into the final frame buffer.
An obvious extension of the CAVE rendering paradigm are cylindrical [Chen95] and
domed displays [Lant95]. In case of cylindrical displays (as used by QuickTime VR for
Macintosh) successive frames are rendered from a panoramic 360˚ high resolution photograph.
This approach can eliminate the distortions caused by the rendering on flat screens and requires
the pixel-copy operation only. Consequently this leads to a great reduction of the rendering cost:
a desktop computer without any hardware accelerated graphics can perform a real-time
walkthrough! A spherical projection (which is the ultimate one because of the human eye
structure) can additionally enhance the movement freedom in this case the user can not only
rotate his/her head left and right, he/she can also freely look up and down.
The combination of pixel-based techniques with high power image generators can bring
many advantages. The rendering of successive encapsulating surfaces can be performed
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detached from the users orientation, and the cost of the final image presentation is constant,
because it does not depend on the scenes complexity. This leads to a radical reduction of
motion-induced viewing errors.
2.5. VR output devices
2.5.1. Visual displays
Most research focused on presentation of visual information to the user. Beside fast view
updates, image quality plays an important role for generation of immersion feeling.
Section 2.2.1 (visual perception characterization) gave an overview of the most important
properties of the human visual system that must be taken into consideration for constructing
visual displays. The ideal display should have high resolution, high update-rate, wide field of
view, high brightness and contrast. On the other hand wearability cannot be forgotten: ease of
use, small weight etc. [McKen92b, Brys93e, Jian93, Holl95, Last95]. Unfortunately,
manufacturing a HMD that fulfills all these needs is beyond todays technology possibilities.
Technology
Two display technologies are currently available on the market:
CRT cathode ray tube displays are based on conventional television technology. They
offer relatively good image quality: high resolution (up to 1600x1280), sharp view and
big contrast. Their disadvantages are high weight and high power consumption. They
also generate high-frequency, strong magnetic fields that may be hazardous to the users
eyes [Holl95] and may have negative influence on the quality of measurements of
magnetic trackers.
LCD liquid crystal diode displays are a relatively new technology that is alternative to
standard CRT displays. LCD displays are flat, lightweight, have low power consumption
and lower emissions than CRTs. The biggest disadvantage is poor image quality: low
contrast, brightness and resolution (typically up to 720x480) [Holl95].
The small size of displays used in HMDs brings with it small FOV. To enhance the viewing
range special optics may be used such as LEEP or Fresnel lenses [Robi92a, Brys93e]. Both of
these approaches require a predistortion of the image that will be viewed through the special
optics (see fig. 2.5.1.1). Wide field optics is used for example by VPL Research for the
construction of their HMDs.
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(a) (b)
Figure 2.5.1.1. LEEP optics: (a) tracing of rays through the LEEP optics,
(b) predistortion grid for the LEEP optics (from [Robi92a]).
Beyond CRTs and LCDs, a virtual retinal display (VRD) was proposed [Koll93, Tidw95]. A
prototype of a VRD developed at the HITLab uses a modulated laser light that projects the
image directly onto the users retina. Although application of this system in practice is
questionable nowadays, it offers great potential quality improvement the goal of the project is
to achieve a resolution of 4000x3000 and a FOV bigger than 100˚.
VR visual output devices
Different type of VR systems from desktop to full immersion use different output visual
displays. They can vary from a standard computer monitor to a sophisticated HMDs. The
following section will present an overview of most often used displays in VR.
3D glasses
The simplest VR systems use only a monitor to present the scene to the user. However, the
window onto a world paradigm can be enhanced by adding a stereo view by use of LCD
shutter glasses [Deer92]. LCD shutter glasses support a three-dimensional view using
sequential stereo: with high frequency they close and open eye views in turn, when the proper
images are presented on the monitor (see fig. 2.5.1.2). An alternative solution uses a projection
screen instead of a CRT monitor. In this case polarization of light is possible and cheap
polarization glasses can be used to extract proper images for each of the eyes. A head movement
tracking can be added to support the user with motion parallax depth cue and increase the
realism of the presented images.
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Figure 2.5.1.2. Crystal Eyes LCD shutter glasses.
Surround displays
An alternative to standard desktop monitors are large projection screens. They offer not only
better image quality but also a wider field of view, which makes them very attractive for VR
applications. The total immersion demand may be fulfilled by a CAVE-like displays (see
fig. 2.5.1.3), where the user is surrounded by multiple flat screens [Cruz92, Cruz93b,
Deer93a] or one domed screen [Lant95]. Ideally it would support full 360˚ field of view (see
section 2.4.3 on image generation as well). The disadvantage of such projection systems is that
they are big, expensive, fragile and require precise hardware setup.
Figure 2.5.1.3. Surround display diagram: CAVE.
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Binocular Omni Oriented Monitors (BOOM)
Developed and commercialized by Fake Space Labs BOOMs are complex devices supporting
both mechanical tracking (see section 2.3.1) and stereoscopic displaying technology. Two
visual displays (for stereo view) are placed in a box mounted to a mechanical arm (see
fig. 2.3.1.5). The box can be grabbed by the user and the monitors can be watched through
two holes. As the mechanical construction supports usually counter-balance, the displays used
in the BOOMs need to be neither small nor lightweight. Therefore CRT technology can be used
for better resolution and image quality.
Head Mounted (Coupled) Displays (HMD)
HMDs are headsets incorporating two small CRT or LCD monitors placed in front of the users
eyes. The images are presented to the user based on his/her current position and orientation
measured by a tracker (see section 2.3.1). Since the HMD is mounted to the users head it must
fulfill strict ergonomic requirements: it should be relatively light, comfortable and easy to put on
and off. As any visual display it should also have possibly the best quality. These demands
force engineers to make hard trade-offs. Consequently, the prices and quality of HMDs vary
dramatically: from about 800 dollars for a low-cost, low-quality device to about one million (!)
dollars for hi-tech military HMDs [Holl95].
HMDs can be divided in two principle groups: opaque and see-through. Opaque HMDs
totally replace the users view with images of the virtual world and can be used in applications,
that create their own world like architectural walkthroughs, scientific visualization, games etc.
See-through HMDs superimpose computer generated images on real objects, augmenting the
real world with additional information (see also section 1.3.2). Most of the HMDs currently
available on the market support stereo viewing and can be driven either with PAL or NTSC
monitor signals [Vinc95].
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(a) (b)
(c) (d)
Figure 2.5.1.3. Different Head Mounted Displays: (a) Ivan Sutherlands HMD dated from 1968,
(b) low cost CyberMaxx HMD, (c) advanced military Sim Eye HMD,
(d) low cost see through HMD i-glasses! from Virtual I/O.
Name Type Technolog
y
Resolution
(H x V)
FOV
(H x V)
Est.
price
(USD)
Sim Eye see-through CRT 1280x1024 60˚x40˚ 200,000
CyberMaxx opaque LCD 180K pixels 60˚x53˚ 800
EyeGen3 opaque CRT 123,250 pixels 35˚x30˚ 10,000
i-glasses! opaque or
see-through
LCD 180K pixels 30˚ horizontal 800
Table 2.5.1.1. Technical data of example HMDs.
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2.5.2. Haptic displays
Haptic sensations perceived by humans can be divided into two main groups:
kinesthetic (force) feedback forces sensed by the muscles, joints and tendons.
tactile feedback includes feedback through the skin, like sense of touch, temperature,
texture or pressure on the skin surface.
These perception issues are extremely important when performing some precise manipulation
tasks. Manipulating every object in real world always causes a collision between the hand and
this object, which is perceived as haptic feedback. Therefore the many of dexterous
manipulators and some data gloves (see section 2.3.3) are equipped with devices simulating
these sensations [Giga93c, Vinc95]. A remote interaction with fragile objects (e.g., human eye
surgery or laboratory tasks) could not be completed accurately without proper haptic cues.
Force feedback
Object placement and manipulation requires a proper force, which is a quite natural
phenomenon for humans. To increase the naturalness of interaction in VE some devices are
equipped with force feedback. This includes a variety of manipulators from simple
gloves [Burd92] to sophisticated and mechanically complex exoskeletal hand masters (see
fig. 2.5.2.1).
Beside these hand mounted manipulators that are supposed to simulate the real interaction
with objects, several studies have proven that presence of force-feedback increases efficiency of
various placing and manipulation tasks. Therefore many devices were developed: from the
simplest ones like joysticks or desktop mice [Akam94] with kinesthetic feedback, through
force simulation with help of tense strings [Ishi94a, Ishi94b] to the UNC GROPE
project [Broo90]. All of them confirmed qualitative improvements of interaction it has been
found out that haptic display together with visual one, can enhance the perception and
understanding of natural phenomena like for example scalar or vector fields. The latest
prototype of GROPE consists of a ceiling-mounted arm (see fig. 2.5.2.2) coupled with a
computer and was used by the chemists for a drug-enzyme docking procedure. The application
in other areas like molecules or proteins properties investigation seems to bring big advantages.
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(a) (b)
Figure 2.5.2.1. Force feedback hand masters: (a) Master Manipulator (from [Iwat90]),
(b) force feedback structure for the data glove (from [Burd92]).
Figure 2.5.2.2. GROPE-III force feedback display (from [Broo90]).
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Tactile feedback
Tactile feedback is much more subtle than force feedback and therefore more difficult to
generate artificially. The simulation may be achieved with the help of vibrating nodules,
inflatable bubbles or electrorheological fluids (fluids whose viscosity increases with an applied
electric field) placed under the surface of a glove [Monk92, Giga93c, Holl95] (see
fig. 2.5.2.3). All these currently available technologies are unable to provide the whole
bandwidth of sensory data we obtain through our skin. They generate the indication of touch
with some surface but do not allow the user to recognize its structure.
Figure 2.5.2.3. CyberGlove equipped with tactile feedback units.
2.5.3. Audio displays
Sound can powerfully enhance the human perception ability. As an addition to the visual
information, auditory information can offer several benefits like [Wenz92, Barg93, Asth95,
Hahn95]:
additional channel of data passing
perception ability of information that is outside of visual display
alert or focus signals that attract the user or warn him/her
spatial orientation cues
possibility of parallel perception of many information streams
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In the simplest case, sound can indicate completion of some tasks or signal that some conditions
have been met (like collision with objects, placing of bodies etc.) without cluttering the screen.
This technique has been used for a long time in desktop systems and includes click-keyboards
or desktop sounds (including speech audio too) confirming certain actions or system events.
For these purposes a monaural sound is sufficient. In VR, however, more convincing three-
dimensional auditory displays can be used to simulate distance, direction, material and spatial
information about the environment. A successful area of application is architectural
visualization. Sound cues lead to better spatial impression and allow visual and acoustic
evaluation of buildings models, which is a big improvement for walkthrough applications.
To generate these spatial cues convincingly basic knowledge of the human sound
localization system is required. The duplex theory [Wenz92] accents the two primary hints that
play a leading role in this process: interaural time difference (ITD) and interaural intensity
difference (IID) (see fig. 2.5.3.1).
Figure 2.5.3.1. Duplex theory spatial cues: (a) interaural time difference,
(b) interaural intensity difference (from [Wenz92]).
Beside these two basic cues, other aspects should also be taken into account. Spatial
information reaching our ears strongly depends on the distance from the source, environment
geometry and material properties of objects in vicinity, which influences the creation of echoes.
At least the listeners pinnae (outer ear) has influence on spectral sound wave shaping by
reflecting and refracting it slightly. All these subtle effects altering the sound received by the ear
create the effect of spatial sound perception.
To synthesize artificially the sound that matches human perceptual abilities is a complex and
computationally expensive task. Moreover, the proper synchronization of sound and visual
events is extremely important in order not to disorient the user by contradictory cues. Three
basic steps are required for successful simulation of virtual acoustic environments [Asth95]:
sound generation every action in real world generates some sound (steps, object
collisions etc.). For acoustic simulation sound can be either generated (synthesized) or
sampled and played back.
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spatial propagation this is the most expensive part of the whole simulation: it
involves calculation of spatial propagation of sound waves through the environment.
Ideally all the reflections (echoes) from different objects and walls should be considered
according to the material properties and surface shapes of these objects.
mapping of parameters finally the calculated parameters should be mapped onto
proper sounds that will be delivered to the listener through headphones or speakers. To
do this, all rendered chunks (impulses) should be convoluted together, by use of special
filters, to produce one sound. The directional effect can be achieved by use of ITD and
IID cues and calculation of Head-Related Transfer-Functions (HRTF) [Wenz92].
3. The future of VR
The future of every new technology, including virtual reality, must be considered in two
different aspects: technological and social. Technological aspects include new research
directions and potential use of them for scientific aims. Social aspects include the influence of
new inventions on people: individuals and society as a whole.
3.1. Research directions in VR
The idea of the ultimate virtual environment as stated by Sutherland [Suth65] means that VR
should be indistinguishable from real reality (RR). Most of todays VR applications do not
conform to reality and have poor quality, but are still very useful and persuasive. Without doubt
VR has a big potential, but must be improved a lot to allow more comfortable and intuitive
interaction with virtual worlds. It does not have to simulate reality in every inch of existence: for
training, the simulation should closely match real operating conditions, while e.g., in the
UNCs nanomanipulator application we do not even have any reference to reality (since humans
cannot experience the interaction with molecules in real life).
Independently from the application and its purpose, human factors must be considered (see
section 2.2) or the system will fail to be sufficiently comfortable and intuitive. There is need for
mechanisms allowing people to easily adapt themselves and their behavior from VR to reality
and vice versa. To address these requirements better than current systems do, a lot of research
must be carried out and new technologies must be developed [Fuch92, Broo94]. Therefore
Andries van Dam called VR a forcing function [VanD93].
3.1.1. Ergonomics of visual displays
Up to now, the major interest was paid to visual feedback and visual display technologies.
Nevertheless the quality of nowadays shipped HMDs is far from ideal: resolution is
significantly below eyes resolving capability, luminance and color ranges do not cover the
whole eyes perception range (brightness range and gamut respectively), and finally the field of
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view is relatively narrow. All these disadvantages make virtual worlds appear artificial and
unreal, which severely contributes to the simulator sickness.
Despite continuous improvement of operational parameters, LCD- and CRT-based HMDs
(currently at 1280x1024 resolution on 1.2x1 area [Burd94]) will not offer the ultimate
quality. To overcome these miniaturization problems a competitive solution was presented
recently by HITLab a virtual retinal display [Koll93, Tidw95]. It uses a laser beam that
projects images directly on the users retina. Nowadays this technology offers approximately
the quality of an average LCD screen. Yet, it is very promising: theoretical limits of such
displays are essentially equal to the limits of the human eye.
An alternative approach for presenting images to VR user(s) are large projection screens.
Images can be seen with bare eyes, have better brightness and resolution than typical HMDs.
Stereo viewing is possible with light and comfortable LCD shutter- or polarization-glasses. For
the full immersion (360˚ look around) CAVE-type displays or recently introduced domed
projection screens can be used [Lant95]. Toshiba Corporation has lately developed a volume-
scanning display consisting of many slices of semi-transparent LCD screens. This new
technology allows three-dimensional viewing of stereoscopic images without any additional
equipment [Kame92, Kame93].
3.1.2. Tracking technologies
Todays tracking technologies have many limitations. First and foremost: in many cases the
tracked volume is very restricted. In practice the user is bound very closely to some point in
space (i.e. tracker reference point) and cannot walk around freely. Moreover, the quality of
tracking is often not sufficient most of currently used technologies are very sensitive to
environmental conditions (the quality of measurement decreases dramatically with the distance)
and introduce considerable latency.
An ideal tracker should be small and lightweight so that it can be comfortably worn by the
user. The working volume for the inside tracking should be big enough to allow free walking
for example in a big room (ten by ten meters?). And at least, the tracker should be immune to
any kind of interferences that would guarantee the equally high measurement precision in the
whole volume.
A partial solution for the inside tracking was developed at the UNC the optical ceiling
that allows tracking of the user inside an area of about three by four meters [Ward92]. This
approach gives the user unbound movement freedom and equal tracking precision in the whole
working volume. The inside-out tracking paradigm used in this system offers good quality of
orientation measurement but position measurements still lack the requested precision. The
whole installation is relatively expensive (needs ceiling LED panels and proper controlling of
VIRTUAL REALITY HISTORY, APPLICATIONS, TECHNOLOGY AND FUTURE
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them) and requires heavy optical equipment (cameras) to be attached to the HMD. Nevertheless
it is currently the best alternative for the ultimate inside tracking.
The idea of outside tracking is very promising it may open new possible applications of
VR, like navigation systems or place-sensitive information services. The currently existing
Global Positioning System (GPS) does not offer quality of position measurement that is
sufficient for virtual reality yet, but further development in this area may bring the required
improvements. Such a high precision GPS in combination with source-independent orientation
tracking devices (as used by the i-glasses! for example) may become then a solution for medium
quality but cheap and wide-spread global VR or AR systems.
3.1.3. Computing power and rendering architectures
Behind all virtual worlds a high computing power is hidden. It is the engine for the generation
of all kinds of feedback presented to the user. Though the most powerful computers are used in
VR, there is a continuing hunger for more MIPS and megabytes. More detailed and impressive
scenes require more storage capacity, more CPU performance and graphical power. Therefore
new processors and graphics boards are being developed in order to fulfill these needs. In
practice, however, standard UNIX workstations are utilized for VR applications that do not
guarantee real-time operation (UNIX is not a real time system!) and VR specific requirements
like constant frame-rate rendering (see section 2.4.1) or dynamic image registration are
compromised.
To overcome limitations of standard rendering architectures some novel VR systems correct
already rendered image before displaying it. Just to mention approaches that can be
implemented on conventional workstations like image deflection [So92, Mazu95a], Z-buffer
warping, image generation from the panoramic (360˚) photograph [Chen95] or dynamically
generated impostors [Maci95, Scha95b, Scha96b]. Other more complicated methods require
custom hardware architectures like address recalculation pipeline [Rega94], frameless
rendering [Bish94] or just in time pixels [Olan95]. The common aim of these techniques is
to minimize the dynamic viewing error (when the user moves through the scene). Combined
with prediction methods [Liang91, Azum94, Mazu95a], it is possible to achieve an essential
improvement of registration in virtual reality and augmented reality systems.
Finally, widely acknowledged and cross-platform standards in particular for device
drivers and APIs for these architectures are needed. Only this can assure rapid and (relatively)
easy development of new systems without a need for hand-crafted programming.
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3.1.4. User interfaces
VR means that no interface is needed: every kind of human-computer-human interaction
should be so natural and intuitive that neither learning nor adaptation should be necessary.
Though, we are far from this: todays interfaces are clumsy, often require heavy hardware
devices, complicated calibration steps and non-intuitive interaction paradigms. Hence they are
not easy to operate by the unskilled user.
Future interaction with virtual worlds should involve better input and output devices. Every
input device should be at the same time an output device that supports appropriate haptic
feedback. This is essential, because every action performed in the real world on some object
causes a reaction of this object. These cues allow humans to perform manipulation tasks
without seeing what happens our sense of touch informs us about it! Other senses must be
included into the interaction process like audio output and voice recognition for verbal
communication with computer and finally: taste and smell. Combination of all these sensations
would widen information passing channels between computer and human and make virtual
reality really realistic.
Gloves with feedback, dexterous and exoskeletal manipulators (for the hand and even
whole arm) are the first attempt to improve high quality haptic interfaces [Rohl93b, Stur94].
An extension of them might become a force feedback suit [Burd94] delivering haptic sensations
to the whole body. However, existing prototype devices are very complicated mechanical
constructions, heavy and uncomfortable in use.
Computer generated voice feedback (speech-audio) already does not seem to be a problem.
To fulfill the need of human-computer communication (e.g., with computer generated
autonomous actors or agents), speech recognition is also necessary. There is a couple of
commercial systems [Holl95] claiming good accuracy of recognition, with prices ranging from
150 to 30,000 US dollars. Very few of them, however, support big dictionaries and continuous
speech processing (whole sentences vs. single words). While it is easy to recognize and analyze
simple orders, the real problem is learning the computer to understand what the user intents (in
fact it is the Artificial Intelligence problem). Moreover, the high computing power for the
recognition and long training of the user are required.
3.1.5. Seamless virtual environments
Majority of VR research directions concentrated up to now, generally on the technical aspects.
The improvement of the performance, quality and responsiveness of virtual worlds was the
main problem. However, most of currently existing systems are in fact only test-beds that
cannot be used in any practical application. To construct seamless virtual environments proper
high-level software must be developed as the basis for real applications [Zyda93d]. To
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achieve this aim, following most important issues must be taken into consideration like
modeling interactive worlds, distributed multi-user architectures, effective user interaction
etc. [Fuch92, Zyda93d, Broo94].
Modeling is a crucial problem of virtual environments. Additionally the user must be able to
interact with the created model as with the real world. Therefore automatic world building tools
that allow easy, intuitive and inside-the-environment modeling are needed. Moreover, the
modeled objects should have natural behavior assigned to them (in some cases also autonomous
behavior), and very often they should obey the rules of physics. Such modeling tools have a
big potential to fuel the development of a broad variety of new VR applications.
Another important aspect are multi-user systems: in real world people share the same space
at the same time with other humans. To meet the same requirements in virtual worlds, the model
must be shared among multiple users (let each of them manipulate the model) and must be
properly updated in order to remain consistent. The todays networked information spaces are
still largely research prototypes. They are usually limited to a small number of users and run on
local networks for performance reasons. The example of large scale system is
NPSNET [Mace94] that works with a large number of concurrent users, but its underlying
networking protocol severely limits the variety of possible actions. Therefore further
development in this direction is necessary in order to support cooperative work, multi-user
training and collaboration with the help of VR.
3.1.6. Biomedical research
Sophisticated input and output devices are some of the most expensive parts of VR systems.
The development in the area of microelectronics gives a hope that new, high power silicon
architectures will be elaborated relatively fast. On the other hand, current standard output and
input devices are far below the satisfying quality. The improvement of them (better resolution,
precision etc.) is extremely expensive mainly because it is bound by technological frontiers.
To overcome these problems, biomedical signal processing could be used both for input
and output. Based on biosignals measured by electrodes, muscle activity could be
detected [Lust93]. By processing these signals, the positions of body parts could be tracked.
Moreover, this approach can be used for improvement of existing motion prediction techniques
(e.g., head movement). Knowing neural signal patterns that force muscle actions and knowing
the head transfer function (i.e. how the head reacts on muscle input), one could more
precisely predict the future position and orientation of the head.
The output of computers can be directly connected to the human nerves: instead of building
high (but still too low for the human eye) resolution displays, images could be fed directly into
the eye nerves, instead of providing force and tactile feedback, appropriate nerves in different
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parts of the body can be stimulated and so on. Ultimately one can imagine the direct stimulation
of brain cells in order to artificially generate sensations perceived by human senses. One could
just plug himself/herself into the computer as envisioned in William Gibsons science-fiction
novels [Gibs83]. The only question is: Is this what we really want?
3.2. Social aspects
Virtual reality is in its experimental stage today, but without doubt it has a great potential to alter
our life. People expect very much from this technology much more than it can offer yet. But
there is the other dark side: every new technology and every new invention brings a fear
and feeling of uncertainty with it (e.g., nuclear power). It can be used for good aim, but it can
be misused too. The more potential it has, the bigger the danger can be.
3.2.1. What are the expectations?
VR has found already an enormous number of applications in different areas of science (see
section 1.3). It became a perfect tool for architects, designers, physicists, chemists, doctors,
surgeons etc. All these disciplines, however, are closed for average people and therefore virtual
reality is becoming some kind of myth something extremely wonderful (a Promised Land?)
and at the same time something inaccessible.
Due to the high cost and fragility of equipment, up to the end of 1980 VR was hidden
behind laboratory walls. But in the beginning of the current decade a great interest of media
dragged it to the wide publicity. Moreover, the development of cheap and powerful hardware
allowed the spread of many installations opened to the public. The first were arcade
games [Atla95] computer games extended by an immersion feature using a HMD and a
tracking system. The great success of them forced the market appearance of further
entertainment systems: multi user car races, dungeon games, flight simulations and
others [Atla95].
Beside adventure games in cyberspace there are not many other applications that may have
a big influence on people or society yet. Nevertheless, there were already successful attempts of
use of the VR systems in medicine (e.g., curing of mental disorders, phobias [Whal93,
Vinc95], people with disabilities [Trev94]) and in education [Loef95, Schr95]. In the future,
VR technology will have a rapidly growing influence on almost every area of our life:
education school, a variety of training systems (e.g., driving license courses, sport
coaching [Ande93], flight simulation, military or astronauts training etc.), programs
explaining laws of nature (e.g., by placing the user between molecules, inside of
hurricane or letting him/her explore the galaxy), and even virtual universities without
lecture rooms will become usual in near future.
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information retrieval, processing and searching todays society is called an
information society and the need for new sources of easy accessible data will be
constantly growing. VR will offer the easiest access to information through virtual
libraries (not only books but films, music, stock-exchange data etc.), office electronic
data-files, guided sight-seeing tours (visiting virtual museums, buildings, cities, lands
etc.).
augmented reality with the help of see-through HMDs, additional information can be
displayed to the user pointing his/her attention to important objects of the real world,
showing the way to the specified aim (e.g., by highlighting the right way through the
city) or explaining the next step that must be performed to complete some tasks from the
complex ones like repairing complicated electronic devices or space shuttle elements in
open space to easy ones as operating faxes, laser-printers [Brys92c] or changing a car
tires etc.
new senses every information that cannot be acquired by human senses but can be
detected by technical sensors may be potentially seen by the user [Robi92b]. For example
a doctor may have a direct insight into patients body [Baju92], an electrician may see
wires in walls while fixing house installation or an engineer may see pipelines under the
ground when performing digging works etc.
passive entertainment as new information medium of 21
st
century, VR will replace
majority of passive entertainment activities like reading books, watching movies, TV,
listening to the music. In fact all of them will be unified in one big virtually multimedial
system.
active entertainment thanks to VR technology some computer games become more
realistic, and in consequence more interesting. It is to expect that other free-time activities
like e.g., playing music or sport exercises will be soon altered by VR technology.
communication and collaboration at work and at home people constantly
exchange huge amounts of data by communicating with other humans. Physical meetings
that are not always possible due to big distances and other obstacles, are replaced by
talking on the phone, or on-screen teleconferencing sessions. One can easily imagine
meetings in virtual space, virtual phone talking, virtual mailing and many more (in
practice every medium can be replaced by VR). These communication paradigms are not
bound by distance constraints and they are a promising alternative to the existing
collaboration media.
remote operation today, a remote operated TV-set is nothing spectacular. VR
technology allows to enhance basic idea of teleoperation. It can include really complex
tasks that require the dexterity of human hands. Teleoperated robots can in near future
replace people at workplaces that might be hazardous to their life of health. This includes
VIRTUAL REALITY HISTORY, APPLICATIONS, TECHNOLOGY AND FUTURE
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for example maintaining of nuclear power stations, works on height, works with
chemicals or viruses etc.
interactive design in the future every engineer will be able to design and test his/her
projects (engines, aerodynamics of bodies or even whole mechanical constructions) with
the help of VR. Testing a car, its road behavior, acceleration and other properties is a
fascinating and cheap alternative to todays design processes that last very often for years!
Eventually, average people will have the possibility to design their houses, hair styles or
clothes interactively and see immediately what the result will look like. Every visit at a
hair-dresser, tailor, or house designer would start with a VR session.
This list of expectations (or rather: wishes) can be extended infinitely and will never be
complete. The fact is that with the development of networks (i.e. data highways), everyone will
be able to rent a network line (like telephone or cable TV nowadays) and connect his/her
personal workstation (computer equipped with a HMD) to it. Then the use of virtual reality in
everyday life will become as common as the use of telephones, hoovers, TV-sets, videos, cars
or airplanes today.
3.2.2. What are the fears?
With the introduction of VR to society there is a need of finding ethical norms for it [Kall93,
Whal93]. People also should know about the potential dangers of the new technology: which
negative or even destructive influence it can bring along [Sher93, Whit93]. One may not
exaggerate because humans have a great ability of adaptation to new conditions: finally they will
always find their place in new (even virtual) realities. Nevertheless it is better to prevent than to
correct [Kall93].
Virtual reality systems of the future can be divided into four groups according to two
criteria: social vs. non-social and creative vs. non-creative [Ston93] (see fig. 3.2.2.1).
Non-social virtual realities allow a single user to interact with the environment. This can be
an interaction either: with a prefabricated (i.e. preprogrammed) environment (they are then
called: non-creative systems) or with an environment that can be modified according to the
users needs and wishes (they are then called: creative systems).
Social virtual realities on the other hand allow multiple users to interact with each other and
with the environment itself. Again, as with non-social systems, the environment can be
preprogrammed or it be created and altered by the user or a group of cooperating users.
VIRTUAL REALITY HISTORY, APPLICATIONS, TECHNOLOGY AND FUTURE
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single user
&
preprogrammed
environment
multiple users
&
preprogrammed
environment
single user
&
modifiable
environment
multiple users
&
modifiable
environment
non-creative:
creative:
social:
non-social:
Figure 3.2.2.1. Four types of VR systems (from [Ston93]).
Different types of VR systems can have different influences on peoples mentality. Non-social
virtual realities for example may lead to closing of people in their own worlds. This has
already partially happened some of the most fanatic computer-game players can hardly be
forced to come back to reality! And with more convincing and realistic systems, it can only
become worse... Non-creative applications (like games) may have an additional negative effect:
closing the user in the world that cannot be modified is against human nature and can lead to
degradation of our imagination.
Non-social and creative virtual worlds that potentially can be great tool for designers, are at
the same time even bigger temptation for complete escape from reality. They offer to the user
the possibility of modifying the surrounding according to ones wishes (which is very often not
possible in real world). Thanks to it, creating an artificial wonderland of dreams will be as easy
as building a house using a Lego-set. With these considerations several existential questions
arise: Is our everyday life so bad that so many people escape from it? Will VR make people at
least more happy? Which influence will it have on the ability of coexistence with other humans?
This last question, becomes even more important when considering social virtual worlds,
allowing people to communicate and collaborate. They can certainly be a great help in work and
in everyday life, but are they going to replace physical contacts totally? Even today a lot of
people spend hours on the telephone because they are too lazy to pay a visit to their friends. In
virtual reality, the user will be able to create an image of himself/herself often very idealized
and far from reality. Hidden behind our masks we will meet only equally perfect but cold
creatures. How long can one continue living without feelings and how destructive can it be?
How easy will it be to come back to reality and make contacts with real people? How easy will
it be to switch between real and virtual images, and can it eventually cause a virtual
schizophrenia?
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Beside the dangers of VR discussed previously, there are other more general hazards. TV
has in the 1960s increased the homicide rate in American society [Kall93]. VR can potentially
have the same influence on our society a few years from now. People playing brutal games may
identify themselves with the virtual heroes and adopt their violent behavior. With the
improvement of the simulation and visual quality of virtual worlds the differences between
reality and VR will be constantly disappearing and consequently people may become confused
what is real and what is virtual. In fact this process has already begun: military simulations are
becoming so close to reality that soldiers do not know any more whether they are remotely
steering a real death-machine or just making a training. This may eventually lead to the lack of
responsibility for our actions: one can kill cold-blooded thousands of innocent people not
knowing (or rather pretending not to know) if taking part in a simulation or a real
mission [Smit94].
All these questions are intentionally left open. The overwhelming evolution of virtual reality
technology indicates that there may be an all to real danger for society. VR may become the
ultimate drug for the masses. It is our responsibility to choose the right dose.
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