Virtual Environments and Environmental
Instruments
Stephen R. Ellis
NASA Ames Research Center
Moffett Field, California 94035
and
University of California, Berkeley
Berkeley, California 94720
silly@eos.arc.nasa.gov
Excerpts from Simulated and Virtual Realities,
K. Carr and R. England, eds. Taylor & Francis, London. pp. 11-51, reproduced
with permission [1]. Omissions are by request
of the publisher and are indicated with ellipses (...).
Table of Contents
1. Communication and Environments
1.1 Virtual environments are
media
Virtual environments created through computer graphics are communications
media (Licklider et al., 1978). They have both physical and abstract
components like other media. Paper, for example, is a communication medium
but the paper is itself only one possible physical embodiment of the abstraction
of a two-dimensional surface onto which marks may be made. Consequently,
there are alternative instantiations of the same abstraction. As an alternative
to paper, for example, the Apple Newton series of intelligent information
appliances resemble handwriting-recognizing magic slates on which users
write commands and data with a stylus (see Apple Computer Co., 1992). The
corresponding abstraction for head-coupled, virtual image, stereoscopic
displays that synthesize a coordinated sensory experience is an environment.
Recent advances and cost reductions in the underlying technology used to
create virtual environments have made possible the development of new interactive
systems that can subjectively displace their users to real or imaginary
remote locations.
Different expressions have been used to describe these synthetic experiences.
Terms like "virtual world" or "virtual environment"
seem preferable since they are linguistically conservative, less subject
to journalistic hyperbole and easily related to well-established usage as
in the term "virtual image" of geometric optics. These so called
"virtual reality" media several years ago caught the international
public imagination as a qualitatively new human-machine interface (Pollack,
1989; D'Arcy, 1990; Stewart, 1991; Brehde, 1991), but they, in fact, arise
from continuous development in several technical and nontechnical areas
during the past 25 years (Ellis, 1990, 1993; Brooks, 1988; Kalawsky, 1993).
Because of this history, it is important to ask why displays of this sort
have only recently captured public attention.
The reason for the recent attention stems mainly from a change in the
perception of the accessibility of the technology. Though its roots, as
discussed below, can be traced to the beginnings of flight simulation and
telerobotics displays, recent drops in the cost of interactive 3D graphics
systems and miniature video displays have made it realistic to consider
a wide variety of new applications for virtual environment displays. Furthermore,
many video demonstrations in the mid-1980's gave the impression that indeed
this interactive technology was ready to go. In fact, at that time, considerable
development was needed before it could be practicable and these design needs
still persist for many applications. Nevertheless, virtual environments
can indeed become Ivan Sutherland's "ultimate computer display";
but in order to insure that they provide effective communications channels
between their human users and their underlying environmental simulations,
they must be designed.
1.2 Optimal design
A well designed human-machine interface affords the user an efficient
and effortless flow of information between the device and its human operator.
When users are given sufficient control over the pattern of this interaction,
they themselves can evolve efficient interaction strategies that match the
coding of their communications to the machine to the characteristics of
their communication channel (Zipf, 1949; Mandelbrot, 1982; Ellis and Hitchcock,
1986; Grudin and Norman, 1993). Successful interface design should strive
to reduce this adaptation period by analysis of the users' task and their
performance limitations and strengths. This analysis requires understanding
of the operative design metaphor for the interface in question, i.e. the
abstract or formal description of the interface in question.
The dominant interaction metaphor for the human computer interface changed
in the 1980's. Modern graphical interfaces, like those first developed at
Xerox PARC (Smith et al., 1982) and used for the Apple Macintosh,
have transformed the "conversational" interaction from one in
which users "talked" to their computers to one in which they "acted
out" their commands within a "desktop" display. This so-called
desktop metaphor provides the users with an illusion of an environment in
which they enact system or application program commands by manipulating
graphical symbols on a computer screen.
1.3 Extensions of the desk-top metaphor
Virtual environment displays represent a three-dimensional generalization
of the two-dimensional desk-top metaphor [2]. The
central innovation in the concept, first stated and elaborated by Ivan Sutherland
(1965; 1970) and Myron Krueger (1977; 1983) with respect to interactive
graphics interfaces was that the pictorial interface generated by the computer
could became a palpable, concrete illusion of a synthetic but apparently
physical environment. In Sutherland's terms, this image would be the "ultimate
computer display." These synthetic environments may be experienced
either from egocentric or exocentric viewpoints. That is to say, the users
may appear to actually be immersed in the environment or see themselves
represented as a "You are here" symbol (Levine, 1984) which they
can control through an apparent window into an adjacent environment.
The objects in this synthetic space, as well as the space itself, may
be programmed to have arbitrary properties. However, the successful extension
of the desk-top metaphor to a full "environment" requires an understanding
of the necessary limits to programmer creativity in order to insure that
the environment is comprehensible and usable. These limits derive from human
experience in real environments and illustrate a major connection between
work in telerobotics and virtual environments. For reasons of simulation
fidelity, previous telerobotic and aircraft simulations, which have many
of the aspects of virtual environments, also have had to take explicitly
into account real-world kinematic and dynamic constraints in ways now usefully
studied by the designers of totally synthetic environments (Hashimoto et
al., 1986; Bussolari et al., 1988; Kim et al., 1988; Tachi
et al., 1989; Bejczy et al., 1990; Sheridan, 1992; Cardullo,
1993).
1.4 Environments
Successful synthesis of an environment requires some analysis of the
parts that make up the environment. The theater of human activity may be
used as a reference for defining an environment and may be thought of as
having three parts: a content, a geometry, and a dynamics
(Ellis, 1991).
 |
| Decomposition of an environment into its abstract functional
components. |
Content
The objects and actors in the environment are its content.
These objects may be described by vectors which identify their position,
orientation, velocity, and acceleration in the environmental space, as well
as other distinguishing characteristics such as their color, texture, and
energy. This vector is thus a description of the properties of the
objects. The subset of all the terms of the characteristic vector which
is common to every actor and object of the content may be called the position
vector. Though the actors in an environment may for some interactions
be considered objects, they are distinct from objects in that in addition
to characteristics they have capacities to initiate interactions
with other objects. The basis of these initiated interactions is the storage
of energy or information within the actors, and their ability to control
the release of this stored information or energy after a period of time.
The self is a distinct actor in the environment which provides a
point of view establishing the frame of reference from which the
environment may be constructed. All parts of the environment that are exterior
to the self may be considered the field of action. As an example, the balls
on a billiard table may be considered the content of the billiard table
environment and the cue ball controlled by the pool player maybe considered
the self. The additional energy and information that makes the cue
ball an actor is imparted to it by the cue controlled by the pool player
and his knowledge of game rules.
Geometry
The geometry is a description of the environmental field of action. It
has dimensionality, metrics, and extent. The dimensionality
refers to the number of independent descriptive terms needed to specify
the position vector for every element of the environment. The metrics are
systems of rules that may be applied to the position vector to establish
an ordering of the contents and to establish the concept of geodesic or
the loci of minimal distance paths between points in the environmental space.
The extent of the environment refers to the range of possible values for
the elements of the position vector. The environmental space or field of
action may be defined as the cartesian product of all the elements of the
position vector over their possible ranges. An environmental trajectory
is a time-history of an object through the environmental space. Since kinematic
constraints may preclude an object from traversing the space along some
paths, these constraints are also part of the environment's geometric description.
Dynamics
The dynamics of an environment are the rules of interaction among
its contents describing their behaviour as they exchange energy or information.
Typical examples of specific dynamical rules may be found in the differential
equations of newtonian dynamics describing the responses of billiard balls
to impacts of the cue ball. For other environments, these rules also may
take the form of grammatical rules or even of look-up tables for pattern-match-triggered
action rules. For example, a syntactically correct command typed at a computer
terminal can cause execution of a program with specific parameters. In this
case the meaning and information of the command plays the role of the energy,
and the resulting rate of change in the logical state of the affected device,
plays the role of acceleration.
This analogy suggests the possibility of developing a semantic
or informational mechanics in which some measure of motion through
the state space of an information processing device may be related to the
meaning or information content of the incoming messages. In such a mechanics,
the proportionality constant relating the change in motion to the message
content might be considered the semantic or informational
mass of the program. A principle difficulty in developing a useful definition
of "mass" from this analogy is that information processing devices
typically can react in radically different ways to slight variations in
the surface structure of the content of the input. Thus it is difficult
to find a technique to analyze the input to establish equivalence classes
analogous to alternate distributions of substance with equivalent centres
of mass. The centre-of-gravity rule for calculating the centre of mass is
an example of how various apparently variant mass distributions may be reduced
to a smaller number of equivalent objects in a way simplifying consistent
theoretical analysis as might be required for a physical simulation on a
computer.
The usefulness of analyzing environments into these abstract components,
content, geometry, and dynamics, primarily arises when designers search
for ways to enhance operator interaction with their simulations. For example,
this analysis has organized the search for graphical enhancements for pictorial
displays of aircraft and spacecraft traffic (McGreevy and Ellis, 1986; Ellis
et al., 1987; Grunwald and Ellis, 1988, 1991, 1993). However, it
also can help organize theoretical thinking about what it means to be in
an environment through reflection concerning the experience of physical
reality.
1.5 Sense of physical reality
Our sense of physical reality is a construction derived from the symbolic,
geometric, and dynamic information directly presented to our senses. But
it is noteworthy that many of the aspects of physical reality are only presented
in incomplete, noisy form. For example, though our eyes provide us only
with a fleeting series of snapshots of only parts of objects present in
our visual world, through a priori "knowledge" brought
to perceptual analysis of our sensory input, we accurately interpret these
objects to continue to exist in their entirety [3].
(Gregory, 1968, 1980, 1981; Hochberg, 1986). Similarly, our goal-seeking
behaviour appears to filter noise by benefiting from internal dynamical
models of the objects we may track or control (Kalman, 1960; Kleinman et
al., 1970). Accurate perception consequently involves considerable a
priori knowledge about the possible structure of the world. This knowledge
is under constant recalibration based on error feedback. The role of error
feedback has been classically mathematically modeled during tracking behaviour
(McRuer and Weir, 1969; Jex et al., 1966; Hess, 1987) and notably
demonstrated in the behavioural plasticity of visual-motor coordination
(Welch, 1978; Held et al., 1966; Held and Durlach, 1991) and in vestibular
and ocular reflexes (Jones et al., 1984; Zangemeister and Hansen,
1985; Zangemeister, 1991).
Thus, a large part of our sense of physical reality is a consequence
of internal processing rather than being something that is developed only
from the immediate sensory information we receive. Our sensory and cognitive
interpretive systems are predisposed to process incoming information in
ways that normally result in a correct interpretation of the external environment,
and in some cases they may be said to actually "resonate" with
specific patterns of input that are uniquely informative about our environment
(Gibson, 1950; Heeger, 1989; Koenderink and van Doorn, 1977; Regan and Beverley,
1979).
These same constructive processes are triggered by the displays used
to present virtual environments. However, since the incoming sensory information
is mediated by the display technology, these constructive processes will
be triggered only to the extent the displays provide high perceptual fidelity.
Accordingly, virtual environments can come in different stages of completeness,
which may be usefully distinguished by their extent of what may be called
"virtualization".
2. Virtualization
2.1 Definition of virtualization
Virtualization may be defined as the "process by which a viewer
interprets patterned sensory impressions to represent objects in an environment
other than that from which the impressions physically originate." A
classical example would be that of a virtual image as defined in geometrical
optics. A viewer of such an image sees the rays emanating from it as if
they originated from a point that could be computed by the basic lens law
rather than from their actual location.
Virtualization most clearly applies to the two sense modalities associated
with tremote stimuli, vision and audition. In audition as in vision, stimuli
can be synthesized so as to appear to be originating from sources other
than their physical origin (Wightman and Kistler, 1898a, 1898b). But carefully
designed haptic stimuli that provide illusory senses of contact, shape and
position clearly also show that virtualization can be applied to other sensory
dimensions (Lackner, 1988). In fact, one could consider the normal functioning
of the human sensory systems as the special case in which the interpretation
of patterned sensory impressions results in the perception of real objects
in the surrounding physical environment, which are in fact the physical
energy sources. In this respect perception of reality resolves to the case
in which, through a process of cartesian systematic doubt, it is impossible
for an observer to refute the hypothesis that the apparent source of the
sensory stimulus is indeed the physical source.
Virtualization, however, extends beyond the objects to the spaces in
which they themselves may move. Consequently, a more detailed discussion
of what it means to virtualize an environment is required.
2.2 Levels of virtualization
. . .
2.3 Environmental viewpoints and controlled elements
. . .
2.4 Breakdown by technological functions
. . .
2.5 Spatial and environmental instruments
Like the computer graphics pictures drawn on a display surface, the enveloping
synthetic environment created by a head-mounted display may be designed
to convey specific information. Thus, just as a spatial display generated
by computer graphics may be transformed into a spatial instrument by selection
and coupling of its display parameters to specific communicated variables,
so too may a synthetic environment be transformed into an environmental
instrument by design of its content, geometry, and dynamics (Ellis and Grunwald,
1989a,b). Transformations of virtual environments into useful environmental
instruments, however, are more constrained than those used to make spatial
instruments because the user must actually inhabit the environmental instrument.
Accordingly, the transformations and coupling of actions to effects within
an environmental instrument must not diverge too far from those transformations
and couplings actually experienced in the physical world, especially if
the instrument is to be used without disorientation, poor motor coordination,
and motion sickness. Thus, spatial instruments may be developed from a greater
variety of distortions in the viewing geometry and scene content than environmental
instruments. Environmental instruments, however, may be well-designed if
their creators have appropriate theoretical and practical understanding
of the constraints. Thus, the advent of virtual environment displays provides
a veritable cornucopia of opportunity for research in human perception,
motor-control, and interface technology.
3. Origins of Virtual Environments
3.1 Early visionaries
The obvious, intuitive appeal that virtual environment technology has
is probably rooted in the human fascination with vicarious experiences in
imagined environments. In this respect, virtual environments may be thought
of as originating with the earliest human cave art (Fagan, 1985), though
Lewis Carroll's Through the Looking-Glass (1883) certainly is a more
modern example of this fascination.
Fascination with alternative, synthetic realities has been continued
in more contemporary literature. Aldous Huxley's "feelies" in
Brave New World (1932) were also a kind of virtual environment, a
cinema with sensory experience extended beyond sight and sound. A similar
fascination must account for the popularity of microcomputer role-playing
adventure games such as Wizardry (Greenberg and Woodhead, 1980).
Motion pictures, and especially stereoscopic movies, of course, also provide
examples of noninteractive spaces (Lipton, 1982). Theatre provides an example
of corresponding performance environment which is more interactive and has
been discussed as a source of useful metaphors for human interface design
(Laural, 1991).
The contemporary interest in imagined environments has been particularly
stimulated by the advent of sophisticated, relatively inexpensive, interactive
techniques allowing the inhabitants of these environments to move about
and manually interact with computer graphics objects in three-dimensional
spaces. This kind of environment was envisioned in the science fiction plots
(Daley, 1982) of the movie TRON (1981) and in William Gibson's Neuromancer
(1984), yet the first actual synthesis of such a system using a head-mounted
stereo display was made possible much earlier in the middle 1960's by Ivan
Sutherland, who developed special-purpose fast graphics hardware specifically
for the purpose of experiencing computer synthesized environments through
head-mounted graphics displays (Sutherland, 1965, 1970).
Another early synthesis of a synthetic, interactive environment was implemented
by Myron Krueger using back-projection and video processing techniques (Krueger,
1977, 1983, 1985) in the 1970's. Unlike the device developed for Sutherland,
Krueger's environment was projected onto a wall-sized screen. In Krueger's
VIDEOPLACE, the users' images appears in a two-dimensional graphic
video world created by a computer. The VIDEOPLACE computer analyzed
video images to determine when an object was touched by an inhabitant, and
it could then generate a graphic or auditory response. One advantage of
this kind of environment is that the remote video-based position measurement
does not necessarily, encumber the user with position sensors. A more recent
and sophisticated version of this mode of experience of virtual environments
is the implementation from the University of Illinois called, with apologies
to Plato, the "Cave" (Cruz-Neira et al., 1992).
3.2 Vehicle simulation and three-dimensional cartography
Probably the most important source of virtual environment technology
comes from previous work in fields associated with the development of realistic
vehicle simulators, primarily for aircraft (Rolfe and Staples, 1986; CAE
Electronics, 1991; McKinnon and Kruk, 1991; Cardullo, 1993) but also automobiles
(Stritzke, 1991) and ships (Veldhuyzen and Stassen, 1977; Schuffel, 1987).
The inherent difficulties in controlling the actual vehicles often require
that operators be highly trained. Since acquiring this training on the vehicles
themselves could be dangerous or expensive, simulation systems synthesize
the content, geometry, and dynamics of the control environment for training
and for testing of new technology and procedures.
These systems usually cost millions of dollars and have recently involved
helmet-mounted displays to recreate part of the environment (Lypaczewski
et al., 1986; Barrette et al., 1990; Furness, 1986, 1987;
Kaiser Electronics, 1990). Declining costs have now brought the cost of
a virtual environment display down to that of an expensive workstation and
made possible "personal simulators" for everyday use (Foley, 1987;
Fisher et al., 1986; Kramer, 1992; Bassett, 1992).
The simulator's interactive visual displays are made by computer graphics
hardware and algorithms. Development of special-purpose hardware, such as
matrix multiplication devices, was an essential step that enabled generation
of real-time, that is, greater than 20 Hz, interactive three dimensional
graphics (Sutherland, 1965, 1970; Myers and Sutherland, 1968). More recent
examples are the "geometry engine" (Clark, 1980, 1982) and the
"reality engine" in Silicon Graphics IRIS workstations.
These "graphics engines" now can project literally millions of
shaded or textured polygons, or other graphics primitives, per second (Silicon
Graphics, 1993). Though this number may seem large, rendering of naturalistic
objects and surfaces can require rendering hundreds of thousands of polygons.
Efficient software techniques are also important for improved three-dimensional
graphics performance. "Oct-tree" data structures, for example,
have been shown to dramatically improve processing speed for inherently
volumetric structures (Jenkins and Tanimoto, 1980; Meagher, 1984). Additionally,
special variable resolution rendering techniques for head-mounted systems
also can be implemented to match the variable resolution of the human visual
system and thus not waste computer resources rendering polygons that the
user would be unable to see (Netrovali and Haskell, 1988; Cowdry, 1986;
Hitchner and McGreevy, 1993).
Since vehicle simulation may involve moving-base simulators, programming
the appropriate correlation between visual and vestibular simulation is
crucial for a complete simulation of an environment. Moreover, failure to
match these two stimuli correctly can lead to motion sickness (AGARD, 1988).
Paradoxically, however, since the effective travel of most moving-base simulators
is limited, designers must learn to introduce subthreshold visual-vestibular
mismatches to produce illusions of greater freedom of movement. These allowable
mismatches are built into so-called "washout" models (Bussolari
et al., 1988; Curry et al., 1976) and are key elements for
creating illusions of extended movement. For example, a slowly implemented
pitch-up of a simulator can be used as a dynamic distortion to create an
illusion of forward acceleration. Understanding the tolerable dynamic limits
of visual-vestibular miscorrelation will be an important design consideration
for wide field-of-view head-mounted displays.
The use of informative distortion is also well-established in cartography
(Monmonier, 1991) and is used to help create convincing three-dimensional
environments for simulated vehicles. Cartographic distortion is also obvious
in global maps which must warp a spherical surface into a plane (Cotter,
1966; Robinson et al., 1984) and three-dimensional maps, which often
use significant vertical scale exaggeration (6-20x) to clearly present topographic
features. Explicit informative geometric distortion is sometimes incorporated
into maps and cartograms presenting geographically indexed statistical data
(Tobler, 1963, 1976; Tufte, 1983, 1990; Bertin, 1967/1983), but the extent
to which such informative distortion may be incorporated into simulated
environments is constrained by the user's movement-related physiological
reflexes. If the viewer is constrained to actually be in the environment,
deviations from a natural environmental space can cause disorientation and
motion sickness (Crampton, 1990; Oman, 1991). For this reason, virtual space
or virtual image formats are more suitable when successful communication
of the spatial information may be achieved only through spatial distortions.
Even in these formats the content of the environment may have to be enhanced
by aids such as graticules to help the user discount unwanted aspects of
the geometric distortion (McGreevy and Ellis, 1986; Ellis et al.,
1987; Ellis and Hacisalihzade, 1990).
In some environmental simulations the environment itself is the object
of interest. Truly remarkable animations have been synthesized from image
sequences taken by NASA spacecraft which mapped various planetary surfaces.
When electronically combined with surface altitude data, the surface photography
can be used to synthesize flights over the surface through positions never
reached by the spacecraft's camera (Hussey, 1990). Recent developments have
made possible the use of these synthetic visualizations of planetary and
Earth surfaces for interactive exploration and they promise to provide planetary
scientists with the new capability of "virtual planetary exploration"
(NASA, 1990; Hitchner, 1992; McGreevy, 1993).
3.3 Physical and logical simulation
Visualization of planetary surfaces suggests the possibility that not
only the substance of the surface may be modeled but also its dynamic characteristics.
Dynamic simulations for virtual environments may be developed from ordinary
high-level programming languages like Pascal or C, but this usually requires
considerable time for development. Interesting alternatives for this kind
of simulation have been provided by simulation and modeling languages such
as SLAM II, with a graphical display interface, and TESS (Pritsker, 1986).
These very high-level languages provide tools for defining and implementing
continuous or discrete dynamic models. They can facilitate construction
of precise systems models (Cellier, 1991).
(Click on the image for an expanded view) |
 |
| The process of representing a graphic object in virtual
space allows a number of different opportunities to introduce informative
geometric distortions or enhancements. These either may be a modification
of the transforming matrix during the process of object definition or may
be modifications of an element of a model. These modifications may take
place (1) in an object relative coordinate system used to define the object's
shape, or (2) in an affine or even curvilinear object shape transformation,
or (3) during the placement transformation that positions the transformed
object in world coordinates, or (4) in the viewing transformation or (5)
in the final viewport transformation. The perceptual consequences of informative
distortions are different depending on where they are introduced. For example,
object transformations will not impair perceived positional stability of
objects displayed in a head-mounted format, whereas changes of the viewing
transformation, such as magnification, will. |
Another alternative made possible by graphical interfaces to computers is
a simulation development environment in which the simulation is created
through manipulation of icons representing its separate elements, such as
integrators, delays, or filters, so as to connect them into a functioning
virtual machine. A microcomputer program called Pinball Construction
Set published in 1982 by Bill Budge is a widely distributed early example
of this kind of simulation system. It allowed the user to create custom
simulated pinball machines on the computer screen simply by moving icons
from a toolkit into an "active region" of the display where they
would become animated. A more educational, and detailed example of this
kind of simulator was written as educational software by Warren Robinett.
This program, called Rocky's Boots (Robinett, 1982), allowed users to connect
icons representing logic circuit elements, that is, and-gates and or-gates,
into functioning logic circuits that were animated at a slow enough rate
to reveal their detailed functioning. More complete versions of this type
of simulation have now been incorporated into graphical interfaces to simulation
and modeling languages and are available through widely distributed object
oriented interfaces such as the interface builder distributed with NeXT®
computers.
The dynamical properties of virtual spaces and environments may also
be linked to physical simulations. Prominent, noninteractive examples of
this technique are James Blinn's physical animations in the video physics
courses, The Mechanical Universe and Beyond the Mechanical Universe
(Blinn, 1987, 1991). These physically correct animations are particularly
useful in providing students with subjective insights into dynamic three-dimensional
phenomena such as magnetic fields. Similar educational animated visualizations
have been used for courses on visual perception (Kaiser et al., 1990)
and computer-aided design (Open University and BBC, 1991). Physical simulation
is more instructive, however, if it is interactive, and if interactive virtual
spaces have been constructed which allow users to interact with nontrivial
physical simulations by manipulating synthetic objects whose behaviour is
governed by realistic dynamics (Witkin et al., 1987, 1990). Particularly
interesting are interactive simulations of anthropomorphic figures moving
according to realistic limb kinematics and following higher level behavioural
laws (Zeltzer and Johnson, 1991).
(Click on the image for an expanded view) |
 |
| Unusual environments sometimes have unusual dynamics. The
orbital motion of a satellite in a low earth orbit (upper panels) changes
when thrust v is made either in the direction of orbital motion,
V0, (left) or opposed to orbital motion (right) and and
indicated by the change of the original orbit (dashed lines) to the new
orbit (solid line). When the new trajectory is viewed in a frame of reference
relative to the initial thrust point on the original orbit (Earth is down,
orbital velocity is to the right, see lower panels), the consequences of
the burn appear unusual. Forward thrusts (left) cause nonuniform, backward,
trochoidal movement. Backward thrusts (right) cause the reverse. |
Some unusual natural environments are difficult to work in because their
inherent dynamics are unfamiliar and may be nonlinear. The immediate environment
around an orbiting spacecraft is an example. When expressed in a spacecraft-relative
frame of reference known as "local-vertical-local-horizontal",
the consequences of manoeuvring thrusts become markedly counter-intuitive
and nonlinear (NASA, 1985). Consequently, a visualization tool designed
to allow manual planning of manoeuvres in this environment has taken account
of these difficulties (Grunwald and Ellis, 1988, 1991, 1993; Ellis and Grunwald,
1989b). This display system most directly assists planning by providing
visual feedback of the consequences of the proposed plans. Its significant
features enabling interactive optimization of orbital manoeuvres include
an "inverse dynamics" algorithm that removes control nonlinearities.
Through a "geometric spreadsheet", the display creates a synthetic
environment that provides the user control of thruster burns which allows
independent solutions to otherwise coupled problems of orbital manoeuvring.
Although this display is designed for a particular space application, it
illustrates a technique that can be applied generally to interactive optimization
of constrained nonlinear functions.
3.4 Scientific and medical visualization
Visualizing physical phenomena may be accomplished not only by constructing
simulations of the phenomena but also by graphs and plots of the physical
parameters themselves (Blinn, 1987, 1991). For example, multiple time functions
of force and torque at the joints of a manipulator or limb while it is being
used for a test movement may be displayed (see, for example, Pedotti et
al., 1978) or a simulation of the test apparatus in question itself
may be interactively animated.
 |
| Successive CAT scan x-ray images may be digitized and used
to synthesize a volumetric data set which then may be electronically processed
to identify specific tissue. Here bone is isolated from the rest of the
data set and presents a striking image that even non-radiologists may be
tempted to interpret. Forthcoming hardware will give physicians access to
this type of volumetric imagery for the cost of a car. Different tissues
in volumetric data sets from CAT scan X-ray slices may be given arbitrary
visual properties by digital processing in order to aid visualization. In
this image tissue surrounding the bone is made partially transparent so
as to make the skin surface as well as the underlying bone of the skull
clearly visible. This processing is an example of enhancement of the content
of a synthetic environment. (Photograph courtesy of Octree Corporation,
Cupertino, CA) |
One application for which a virtual space display already has been demonstrated
some time ago in a commercial product has been in visualization of volumetric
medical data (Meagher, 1984). These images are typically constructed from
a series of two-dimensional slices of CAT, PET, or MRI images in order to
allow doctors to visualize normal or abnormal anatomical structures in three
dimensions. Because the different tissue types may be identified digitally,
the doctors may perform an "electronic dissection" and selectively
remove particular tissues. In this way truly remarkable skeletal images
may be created which currently aid orthopaedic and cranio-facial surgeons
to plan operations. These volumetric databases also are useful for shaping
custom-machined prosthetic bone implants and for directing precision robotic
boring devices for precise fit between implants and surrounding bone (Taylor
et al., 1990). Though these static databases have not yet been presented
to doctors as full virtual environments, existing technology is adequate
to develop improved virtual space techniques for interacting with them and
may be able to enhance the usability of the existing displays for teleoperated
surgery (Green et al., 1992; UCSD Medical School, 1994; Satava and
Ellis, 1994). Related scene-generation technology can already render detailed
images of this sort based on architectural drawings and can allow prospective
clients to visualize walkthroughs of buildings or furnished rooms that have
not yet been constructed (Greenberg, 1991; Airey et al., 1990; Nomura
et al., 1992).
3.5 Teleoperation and telerobotics and manipulative
simulation
 |
| A proximity operations planning display presents a virtual
space that enables operators to plan orbital manoeuvres despite counter-intuitive,
nonlinear dynamics and operational constraints, such as plume impingement
restrictions. The operator may use the display to visualize his proposed
trajectories. Violations of the constraints appear as graphics objects,
i.e. circles and arcs, which inform him of the nature and extent of each
violation. This display provides a working example of how informed design
of a planning environment's symbols, geometry, and dynamics can extend human
planning capacity into new realms. (Photograph courtesy of NASA) |
The second major technical influence on the development of virtual environment
technology is research on teleoperation and telerobotic simulation (Goertz,
1964; Vertut and Coiffet, 1986; Sheridan, 1992). Indeed, virtual environments
have existed before the name itself, as telerobotic and teleoperations simulations.
The display technology, however, in these cases was usually panel-mounted
rather than head-mounted. Two notable exceptions were the head-controlled/head-referenced
display developed for control of remote viewing systems by Raymond Goertz
at Argonne National Laboratory (Goertz et al., 1965) and a head-mounted
system developed by Charles Comeau and James Bryan of Philco (Comeau and
Brian, 1961). The development of these systems anticipated many of the applications
and design issues that confront the engineering of effective virtual environment
systems. Their discussions of the field-of-view/image resolution trade-off
is strikingly contemporary. A key difficulty, then and now, was lack of
a convenient and precise head tracker. The current popular, electromagnetic,
six-degree-of-freedom position tracker developed by Polhemus Navigation
(Raab et al., 1979; also see Ascension Technology Corp., 1990; Polhemus
Navigation Systems, 1990; Barnes, 1992) consequently was an important technological
advance. Interestingly, this was anticipated by similar work at Philco (Comeau
and Bryan, 1961) which was limited, however, to electromagnetic sensing
of orientation. In other techniques for tracking the head position, accelerometers,
optical tracking hardware (CAE Electronics, 1991; Wang et al., 1990),
or acoustic systems (Barnes, 1992) may be used. These more modern sensors
are much more convenient than those used by the pioneering work of Goertz
and Sutherland, who used mechanical position sensors, but the important,
dynamic characteristics of these sensors have only recently begun to be
fully described (Adelstein, Johnston and Ellis, 1992).
 |
| Virtual environment technology may assist visualization
of the results of aerodynamic simulations. Here a DataGlove is used to control
the position of a "virtual" source of smoke in a wind-tunnel simulation
so the operator can visualize the local pattern of air flow. In this application
the operator uses a viewing device incorporating TV monitors (McDowall et
al., 1990) to present a stereo view of the smoke trail around the test
model also shown in the desk-top display on the table (Levit and Bryson,
1991). (Photograph courtesy of NASA) |
A second key component of a teleoperation work station, or of a virtual
environment, is a sensor for coupling hand position to the position of the
end-effector at a remote work site. The earlier mechanical linkages used
for this coupling have been replaced by joysticks or by more complex sensors
that can determine hand shape, as well as position. Modern joysticks are
capable of measuring simultaneously all three rotational and three translational
components of motion. Some of the joysticks are isotonic (BASYS, 1990; CAE
Electronics, 1991; McKinnon and Kruk, 1991) and allow significant travel
or rotation along the sensed axes, whereas others are isometric and sense
the applied forces and torques without displacement (Spatial Systems, 1990).
Though the isometric sticks with no moving parts benefit from simpler construction,
the user's kinematic coupling in his hand make it difficult for him to use
them to apply signals in one axis without cross-coupled signals in other
axes. Consequently, these joysticks use switches for shutting down unwanted
axes during use. Careful design of the breakout forces and detents for the
different axes on the isotonic sticks allow a user to minimize cross-coupling
in control signals while separately controlling the different axes (CAE
Electronics, 1991; McKinnon and Kruk, 1991).
 |
| Visual virtual environment display systems have three basic
parts: a head-referenced visual display, head and/or body position sensors,
a technique for controlling the visual display based on head and/or body
movement. One of the earliest system of this sort, shown above, was developed
by Philco engineers (Comeau and Bryan, 1961) using a head-mounted, biocular,
virtual image viewing system, a Helmholtz coil electromagnetic head-orientation
sensor, and a remote TV camera slaved to head orientation to provide the
visual image. Today this would be called a telepresence viewing system.
The first system to replace the video signal with a totally synthetic image
produced through computer graphics, was demonstrated by Ivan Sutherland
for very simple geometric forms (Sutherland, 1965). |
Although the mechanical bandwidth might have been only of the order of 2-5
Hz, the early mechanical linkages used for telemanipulation provided force-feedback
conveniently and passively. In modern electronically coupled systems force-feedback
or "feel" must be actively provided, usually by electric motors.
Although systems providing six degrees of freedom with force-feedback on
all axes are mechanically complicated, they have been constructed and used
for a variety of manipulative tasks (Bejczy and Salisbury, 1980; Hannaford,
1989; Jacobson et al., 1986; Jacobus et al., 1992; Jacobus,
1992). Interestingly, force-feedback appears to be helpful in the molecular
docking work at the University of North Carolina in which chemists manipulate
molecular models of drugs in a computer graphics physical simulation in
order to find optimal orientations for binding sites on other molecules
(Ouh-young et al., 1989).
High-fidelity force-feedback requires electromechanical bandwidths over
30 Hz. Most manipulators do not have this high a mechanical response. A
force-reflecting joystick with these characteristics, however, has been
designed and built (Adelstein and Rosen, 1991, 1992). Because of the required
dynamic characteristics for high fidelity, it is not compact and is carefully
designed to protect its operators from the strong, high-frequency forces
it is capable of producing (see Fisher et al. (1990) for some descriptions
of typical manual interface specifications; also Brooks and Bejczy (1986)
for a review of control sticks).
Manipulative interfaces may provide varying degrees of manual dexterity.
Relatively crude interfaces for rate-controlled manipulators may allow experienced
operators to accomplish fine manipulation tasks. Access to this level of
proficiency, however, can be aided by coordinated displays of high visual
resolution, by use of position control derived from inverse kinematic analysis
of the manipulator, by more intuitive control of the interface, and by more
anthropomorphic linkages on the manipulator.
 |
| A high-fidelity, force-reflecting two-axis joystick designed
to study human tremor. (Photograph courtesy of B. Dov Adelstein) |
An early example of a dexterous, anthropomorphic robotic end-effector is
the hand by Tomovic and Boni (Tomovic and Boni, 1962). A more recent example
is the Utah/MIT hand (Jacobson et al., 1984). Such hand-like end
effectors with large numbers of degrees of freedom may be manually controlled
directly by hand-shape sensors; for example, the Exos, exoskeleton hand
(Exos, 1990).
Significantly, the users of the Exos hand often turn off a number of
the joints, raising the possibility that there may be a limit to the number
of degrees of freedom usefully incorporated into a dexterous master controller
(Marcus, 1991). Less bulky hand shape measurement devices have also been
developed using fiber optic or other sensors (Zimmerman et al., 1987;
W Industries, 1991). Use of these alternatives, however, involves significant
trade-offs of resolution, accuracy, force-reflection and calibration stability
as compared with the more bulky sensors. A more recent hand-shape measurement
device has been developed that combines high static and dynamic positional
fidelity with intuitive operation and convenient donning and doffing (Kramer,
1992).
 |
| Experienced operators of industrial manipulator arms (centre)
can develop great dexterity (see drawing on right) even with ordinary two-degree-of-freedom,
joystick interfaces (left) for the control of robot arms with adequate mechanical
bandwidth. Switches on the control box shift control to the various joints
on the arm. The source of the dexterity illustrated here is the high dynamic
fidelity of the control, a fidelity that needs to be reproduced if supposedly
more natural haptic virtual environment interfaces are to be useful (Photographs
courtesy of Deep Ocean Engineering, San Leandro, CA). |
As suggested by the informal comments of Exos hand-master users who shut
down apparently unneeded degrees of freedom on their hand-shape sensor,
the number of degrees of freedom that need to be monitored by sensors used
for virtual environment displays can become the subject of formal investigations.
For example, the head position sensors used on the Fakespace Boom constrain
natural head roll in the coronal plane. Furthermore, anecdotal observations
of architectural walk-throughs with closed, head-mounted displays have indicated
that it was better to disable head-roll tracking because, combined with
sensor lag, it seemed to make the use of the display unpleasant (J. Nomura,
Personal communication, 1993). Accordingly, one might reasonably ask what
benefits in position and orientation display head-roll tracking provides.
This question has been investigated for manipulative interaction with targets
placed within arm's length of a user. The results show that if large head
rotations with respect to the torso are not required (> ~ 50°), head
roll tracking provides only minor improvements in the users' ability to
rotate objects into alignment with targets presented in a virtual environment
(Adelstein and Ellis, 1993). Thus, roll-tracking and roll-compensation on
some telepresence camera platforms may be unnecessary if the user's interface
to the control of the remote device does not require large head-torso rotations.
3.6. Photography, cinematography, video technology
 |
| An exoskeleton hand-shape measurement system in a dexterous
hand master using accurate Hall-effect flexion sensors which is suitable
to drive a dexterous end-effector. (Photograph courtesy of Exos, Inc, Burlington,
MA) |
  |
Since photography, cinema, and television are formats for presenting synthetic
environments, it is not surprising that technology associated with special
effects for these media have been applied to virtual environments. The LEEP
optics, which are commonly used in many "virtual reality" stereo-viewers,
were originally developed for a stereoscopic camera system using matched
camera and viewing optics to cancel the aberrations of the wide angle lens.
The LEEP system field of view is approximately 110° x 55°, but it
depends on how the measurement is taken (Howlett, 1991). Though this viewer
does not allow adjustment for interpupilary distance, its large entrance
pupil (30 mm radius) removes the need for such adjustment. The stereoscopic
image pairs used with these optics, however, are presented 62 mm apart,
closer together than the average interpupilary distance. This choice is
a useful design feature which reduces the likelihood that average users
need to diverge their eyes to achieve binocular fusion.
  |
| Apparatus used to study the benefits of incorporating head-roll
tracking into a head-mounted telepresence display. The left panel shows
a stereo video camera mounted on a 3-degree-of-freedom platform that is
slaved in orientation to the head orientation of an operator wearing a head-mounted
video display at a remote site. The operator sees the video images from
the camera and uses them to reproduce the orientation and position of rectangular
test objects distributed on matching cylindrical work surfaces. |
An early development of a more complete environmental illusion through cinematic
virtual space was Morton Heilig's "Sensorama." It provided a stereo,
wide-field-of-view, egocentric display with coordinated binaural sound,
wind, and odour effects (Heilig, 1955). A more recent, interactive virtual
space display was implemented by the MIT Architecture Machine Group in the
form of a video-disk-based, interactive map of Aspen, Colorado (Lippman,
1980). The interactive map provided a video display of what the user would
have seen had he actually been there moving through the town. Similar interactive
uses of video-disk technology have been explored at the MIT Media Lab (Brand,
1987). One feature that probably distinguishes the multimedia work mentioned
here from the more scientific and engineering studies reported previously,
is that the media artists, as users of the enabling technologies, have more
interest in synthesizing highly integrated environments including sight,
sound, touch, and smell. A significant part of their goal is the integrated
experience of a "synthetic place". On the other hand, the simulator
designers are only interested in capturing the total experience insofar
as this experience helps specific training and testing. Realism is itself
not their goal, but effective communication and training are.
3.7 Role of engineering and physiological models
Since the integration of the equipment necessary to synthesize a virtual
environment represents such a technical challenge in itself, there is a
tendency for groups working in this area to focus their attention only on
collecting and integrating the individual technologies for conceptual demonstrations
in highly controlled settings. The videotaped character of many of these
demonstrations of early implementation often has suggested system performance
far beyond actually available technology. The visual resolution of the cheaper,
wide field displays using LCD technology has often been, for example, implicitly
exaggerated by presentation techniques using overlays of users wearing displays
and images taken directly from large-format graphics monitors. In fact,
the users of many of these displays are, for practical purposes, legally
blind.
 |
| A graphic model of a manipulator arm electronically superimposed
on a video signal from a remote worksite to assist users who must contend
with time delay in their control actions (Photograph courtesy of JPL, Pasadena,
CA). |
Accomplishment of specific tasks in real environments, however, places distinct
real performance requirements on the simulation of which visual resolution
is just an example. These requirements may be determined empirically for
each task, but a more general approach is to use human performance models
to help specify them. There are good general collections that can provide
this background design data (e.g. Borah et al., 1978; Boff et
al., 1986; Elkind et al., 1989) and there are specific examples
of how scientific and engineering knowledge and computer-graphics-based
visualization can be used to help designers conform to human performance
constraints (Monheit and Badler, 1990; Phillips et al., 1990; Larimer
et al., 1991). Useful sources on human sensory and motor capacities
relevant to virtual environments are also available (Brooks and Bejczy,
1986; Howard, 1982; Blauert, 1983; Goodale, 1990; Durlach et al.,
1991; Ellis et al., 1993).
Because widely available current technology limits the graphics and simulation
update rate in virtual environments to less than 20 Hz, understanding the
control characteristics of human movement, visual tracking, and vestibular
responses is important for determining the practical limits to useful work
in these environments. Theories of grasp, manual tracking (Jex et al.,
1966), spatial hearing (Blauert, 1983; Wenzel, 1991), vestibular response,
and visual-vestibular correlation (Oman, 1991; Oman et al., 1986)
all can help to determine performance guidelines.
Predictive knowledge of system performance is not only useful for matching
interfaces to human capabilities, but it is also useful in developing effective
displays for situations in which human operators must cope with significant
time lags, for example those > 250 ms, or other control difficulties.
In these circumstances, accurate dynamic or kinematic models of the controlled
element allow the designer to give the user control over a predictor which
he may move to a desired location and which will be followed by the actual
element (Hashimoto et al., 1986; Bejczy et al., 1990).
 |
| Though very expensive, the CAE Fiber Optic Helmet Mounted
display, FOHMD (left panel), is one of the highest-performance virtual environment
systems used as a head-mounted aircraft simulator display. It can present
an overall visual field 162° x 83.5° with 5-arcmin resolution with
a high resolution inset of 24° x 18° of 1.5 arcmin resolution. It
has a bright display, 30 foot-lambert, and a fast, optical head-tracker:
60-Hz sampling, with accelerometer augmentation. The Kaiser WideEye®
display (right panel) is a roughly comparable, monochrome device designed
for actual flight in aircraft as a head-mounted heads-up display. It has
a much narrower field of view (monocular: 40°, or binocular with 50%
overlap 40° x 60°; visual resolution is approximately 3 arcmin).
(Photographs courtesy of CAE Electronics, Montreal, Canada; Kaiser Electronics,
San José, CA) |
Another source of guidelines is the performance and design of existing high-fidelity
systems themselves. Of the virtual environment display systems, probably
the one with the best visual display is the CAE Fiber Optic Helmet Mounted
Display or the "FOHMD" (Lypaczewski et al., 1986; Barrette
et al., 1990) which is used in military aircraft simulators. It presents
two 83.5° monocular fields of view with adjustable binocular overlap,
typically of about 38° in early versions, giving a full horizontal field-of-view
of up to 162°. Similarly, the Wright-Patterson Air Force Base Visually
Coupled Airborne Systems Simulator or "VCASS" display, also presents
a very wide field of view, and has been used to study the consequences of
field-of-view restriction on several visual tasks (Wells and Venturino,
1990). Their results support other reports that indicate that visual performance
is influenced by increased field-of-view, but that this influence wanes
as fields of view greater than 60° are used (Hatada et al., 1980).
A significant feature of the FOHMD is that the 60-Hz sampling of head
position had to be augmented by signals from helmet-mounted accelerometers
to perceptually stabilize the graphics imagery during head movement. Without
the accelerometer signals, perceptual stability of the enveloping environment
requires head-position sampling over 100 Hz, as illustrated by well-calibrated
teleoperations viewing systems developed in Japan (Tachi et al.,
1984, 1989). In general, it is difficult to calibrate the head-mounted,
virtual image displays used in these integrated systems. One solution is
to use a see-through system and to compare the positions of real objects
and superimposed computer-generated objects (Hirose et al., 1990,
1992; Ellis and Bucher, 1994; Janin et al., 1993; Rolland, 1994).
Technical descriptions with performance data for fully integrated systems
have not been generally available or accurately detailed (Fisher et al.,
1986; Stone, 1991a,b), but this situation should change as reports are published
in a number of journals, i.e. IEEE Computer Graphics and Applications;
Computer Systems in Engineering; Presence: the Journal of Teleoperations
and Virtual Environments; Pixel: the Magazine of Scientific Visualization;
Ergonomics; and Human Factors. Compendiums of the human factors
design issues are available (e.g. Ellis et al., 1993), and there
are books collecting manufacturers' material which ostensibly describes
the performance of the component technology (e.g. Kalawsky, 1993). But due
to the absence of standards and the novelty of the equipment, developers
are likely to find these descriptions still incomplete and sometimes misleading.
Consequently, users of the technology must often measure the basic performance
measurements of components themselves (e.g. Adelstein et al., 1992).
4. Virtual Environments: Performance and Trade-offs
. . .
Notes
- Earlier versions of some parts of this paper
appeared as "Nature and origin of virtual environments: a bibliographical
essay," in Computer Systems in Engineering, 2 (4), 321-346,
1991 and as "What are virtual environments?" in IEEE Computer
Graphics and Applications, 14 (1), 17-22, 1994.
- Higher dimensional displays have also been
described. See Inselberg (1985) or Feiner and Beshers (1990) for alternative
approaches.
- This "knowledge" should not be thought
of as the conscious, abstract knowledge that is acquired in school. It
rather takes the form of tacit acceptance of specific constraints on the
possibilities of change such as that are reflected in Gestalt Laws, e.g.
common fate or good continuation. Its origin may be sought in the phylogenetic
history of a species, shaped by the process of natural selection and physical
law, and documented by the history of the earth's biosphere.
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[papers]||[top]
Advanced Displays and Spatial Perception Laboratory
Human Information Processing Research Branch
Moffett Field, CA 94035-1000
1.1 Virtual environments are media