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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

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