4.VR
Hardware
There are a number of specialized types of hardware devices that have been
developed or used for Virtual Reality applications.
4.1. Image
Generators
One of the most time consuming tasks in a VR system is the generation of the
images. Fast computer graphics opens a very large range of applications aside
from VR, so there has been a market demand for hardware acceleration for a long
while. There are currently a number of vendors selling image generator cards
for PC level machines, many of these are based on the Intel i860 processor.
These cards range in price from about $2000 up to $6 or $10,000. Silicon
Graphics Inc. has made a very profitable business of producing graphics
workstations. SGI boxes are some of the most common processors found in VR
laboratories and high end systems. SGI boxes range in price from under $10,000
to over $100,000. The simulator market has produced several companies that
build special purpose computers designed expressly for real time image
generation. These computers often cost several hundreds of thousands of dollars.
4.2. Manipulation
and Control Devices
One key element for interaction with a virtual world, is a means of tracking
the position of a real world object, such as a head or hand. There are numerous
methods for position tracking and control. Ideally a technology should provide
3 measures for position(X, Y, Z) and 3 measures of orientation (roll, pitch,
yaw). One of the biggest problem for position tracking is latency, or the time
required to make the measurements and preprocess them before input to the
simulation engine.
The simplest control hardware is a conventional mouse, trackball or joystick.
While these are two dimensional devices, creative programming can use them for
6D controls. There are a number of 3 and 6 dimensional mice/trackball/joystick
devices being introduced to the market at this time. These add some extra
buttons and wheels that are used to control not just the XY translation of a
cursor, but its Z dimension and rotations in all three directions. The Global
Devices 6D Controller is one such 6D joystick It looks like a racket ball
mounted on a short stick. You can pull and twist the ball in addition to the
left/right & forward/back of a normal joystick. Other 3D and 6D mice,
joystick and force balls are available from Logitech, Mouse System Corp. among
others.
One common VR device is the instrumented glove. The use of a glove to
manipulate objects in a computer is covered by a basic patent in the USA. Such
a glove is outfitted with sensors on the fingers as well as an overall
position/orientation tracker. There are a number of different types of sensors
that can be used. VPL (holders of the patent) made several DataGloves, mostly
using fiber optic sensors for finger bends and magnetic trackers for overall
position. Mattel manufactured the PowerGlove for use with the Nintendo game
system, for a short time. This device is easily adapted to interface to a
personal computer. It provides some limited hand location and finger position
data using strain gauges for finger bends and ultrasonic position sensors. The
gloves are getting rare, but some can still be found at Toys R' Us and other
discount stores. Anthony Clifton recently posted this suggestion for a" very
good resource for PowerGloves etc.: small children. A friend's son had gotten
a glove a couple years ago and almost NEVER used it, so I bought it off the
kid. Remember children like money more than toys they never use."
The concept of an instrumented glove has been extended to other body parts.
Full body suits with position and bend sensors have been used for capturing
motion for character animation, control of music synthesizers, etc. in addition
to VR applications.
Mechanical armatures can be used to provide fast and very accurate tracking.
Such armatures may look like a desk lamp (for basic position/orientation) or
they may be highly complex exoskeletons (for more detailed positions). The
drawbacks of mechanical sensors are the encumbrance of the device and its
restrictions on motion. Exos Systems builds one such exoskeleton for hand
control. It also provides force feedback. Shooting Star system makes a low cost
armature system for head tracking. Fake Space Labs and LEEP Systems make much
more expensive and elaborate armature systems for use with their display systems.
Ultrasonic sensors can be used to track position and orientation. A set of
emitters and receivers are used with a known relationship between the emitters
and between the receivers. The emitters are pulsed in sequence and the time lag
to each receiver is measured. Triangulation gives the position. Drawbacks to
ultrasonics are low resolution, long lag times and interference from echoes and
other noises in the environment. Logitech and Transition State are two
companies that provide ultrasonic tracking systems.
Magnetic trackers use sets of coils that are pulsed to produce magnetic
fields. The magnetic sensors determine the strength and angles of the fields.
Limitations of these trackers are a high latency for the measurement and
processing, range limitations, and interference from ferrous materials within
the fields. However, magnetic trackers seem to be one of the preferred methods.
The two primary companies selling magnetic trackers are Polhemus and Ascension.
Optical position tracking systems have been developed. One method uses a
ceiling grid LEDs and a head mounted camera. The LEDs are pulsed in sequence
and the cameras image is processed to detect the flashes. Two problems with
this method are limited space (grid size) and lack of full motion (rotations).
Another optical method uses a number of video cameras to capture simultaneous
images that are correlated by high speed computers to track objects. Processing
time (and cost of fast computers) is a major limiting factor here. One company
selling an optical tracker is Origin Instruments.
Inertial trackers have been developed that are small and accurate enough for
VR use. However, these devices generally only provide rotational measurements.
They are also not accurate for slow position changes.
4.3. Stereo
Vision
Stereo vision is often included in a VR system. This is accomplished by
creating two different images of the world, one for each eye. The images are
computed with the viewpoints offset by the equivalent distance between the
eyes. There are a large number of technologies for presenting these two images.
The images can be placed side-by-side and the viewer asked (or assisted) to
cross their eyes. The images can be projected through differently polarized
filters, with corresponding filters placed in front of the eyes. Anaglyph
images user red/blue glasses to provide a crude (no color) stereovision.
The two images can be displayed sequentially on a conventional monitor or
projection display. Liquid Crystal shutter glasses are then used to shut off
alternate eyes in synchronization with the display. When the brain receives the
images in rapid enough succession, it fuses the images into a single scene and
perceives depth. A fairly high display swapping rate (min. 60hz) is required to
avoid perceived flicker. A number of companies made low cost LC shutter glasses
for use with TVs (Sega, Nintendo, Toshiba, etc.). There are circuits and code
for hooking these up to a computer available on many of the On-line systems,
BBSs and Internet FTP sites mentioned later. However, locating the glasses
themselves is getting difficult as none are still being made or sold for their
original use. Stereographics sells a very nice commercial LC shutter system
called CrystalEyes.
Another alternative method for creating stereo imagery on a computer is to use
one of several split screen methods. These divide the monitor into two parts
and display left and right images at the same time. One method places the
images side by side and conventionally oriented. It may not use the full
screen or may otherwise alter the normal display aspect ratio. A special hood
viewer is placed against the monitor which helps the position the eyes
correctly and may contain a divider so each eye e sees only its own image. Most
of these hoods, such as the one for the V5 of Rend386, use fresnel lenses to
enhance the viewing. An alternative split screen method orients the images so
the top of each points out the side of the monitor. A special hood containing
mirrors is used to correctly orient the images. A very nice low cost (under
$200) unit of this type is the Cyberscope available from Simsalabim.
1.1.1. Health
Hazards from Stereoscopic Displays
There was an article supplement with CyberEdge Journal issue #17 entitled
"What's Wrong with your Head Mounted Display". It is a summary report on the
findings of a study done by the Edinburgh Virtual Environment Lab, Dept. of
Psychology, Univ. of Edinburgh on the eye strain effects of stereoscopic Head
Mounted Displays. There have been a number of anecdotal reports of stress with
HMDs and other stereoscopic displays, but few, if any, good clinical studies.
This study was done very carefully and the results are a cause for some concern.
The basic test was to put 20 young adults on a stationary bicycle and let them
cycle around a virtual rural road setting using a HMD (VPL LX EyePhone and a
second HMD LEEP optic equipped system). After 10 minutes of light exercise, the
subjects were tested...
"The results were alarming: measures of distance vision , binocular fusion and
convergence displayed clear signs of binocular stress in a significant number
of the subjects. Over half the subjects also reported symptoms of such stress,
such as blurred vision."
The article goes on to describe the primary reason for the stress - the
difference between the image focal depth and the disparity. Normally, the when
your eyes look at a close object they focus (accommodate) close and also rotate
inward (converge). When they accommodate on a far object, the eyes also
diverge. However, a stereoscopic display does not change the either the
effective focal plane (set by the optics) and the disparity depth. The eyes
strain to decouple the signals.
The article discusses some potential solutions, but notes that most of them
(dynamic focal/disparity) are difficult to implement. It mentions monoscopic
HMDs only to say that while they would seem to avoid the problems, they were
not tested. The article does not discuss non-HMD stereoscopic devices at all,
but I would extrapolate that they should show some similar problems. The full
article is available from CyberEdge Journal for a small fee.
There has been a fair bit of discussion ongoing in the sci.virtual-worlds
newsgroup (check the Sept./Oct. 93 archives) about this and some other studies.
One contributor, Dipl.-Ing. Olaf H. Kelle, University of Wuppertal, Germany,
reported only 10% of his users showing eye strain. His system is setup with a
focal depth of 3m which seems to be a better, more comfortable viewing
distance. Others have noted that long duration monitor use often leads to the
user staring or not blinking. It is common for VDT users to be cautioned to
look away from the screen occasionally to adjust their focal depth and to
blink. Another contributor, John Nagle provided the following list of other
potential problems with HMDs: electrical safety, Falling/tripping over real
world objects, simulator sickness (disorientation due to conflicting motion
signals from eyes and inner ear), Eye Strain, Induced post-HMD accidents ("some
flight simulators some flight simulators, usually those for military fighter
aircraft, it's been found necessary to forbid simulator users to fly or drive
for a period of time after flying the simulator".).
4.4. Head
Mounted Display (HMD)
One hardware device closely associated with VR is the Head Mounted Device
(HMD).
These
use some sort of helmet or goggles to place small video displays in front of
each eye, with special optics to focus and stretch the perceived field of
view. Most HMDs use two displays and can provide stereoscopic imaging. Others
use a single larger display to provide higher resolution, but without the
stereoscopic vision.
Most
lower cost HMDs ($3000-10,000 range ) use LCD displays, while others use small
CRTs, such as those found in camcorders. The more expensive HMDs use special
CRTs mounted along side the head or optical fibers to pipe the images from
non-head mounted displays. ($60,000 and up). A HMD requires a position tracker
in addition to the helmet. Alternatively, the display can be mounted on an
armature for support and tracking (a Boom display).
4.5. Force
and Touch (Haptic) Rendering
4.6. Motion
Rendering
