mersive virtual reality
G. Robertson, Stuart K. Card, and Jock D. Mackinlay, Xerox PARC
The goal of virtual reality systems is to place the user in a three-dimension- al environment that can be directly manipulated. Ideally, users cease to think of themselves as interacting with a computer and interact instead with the 3D environment. The usual defini- tion of VR involves full immersion. That is, users wear head-mounted ste- reo displays to provide full visual im- mersion and special gloves that allow six-degree-of-freedom input for di- rectly manipulating the environment.
An alternative form of VR is being explored in a number of research labs. Nonimmersive VR also places the user in a 3D environment that can be directly manipulated, but it does so with a conventional graphics worksta- tion using a monitor, a keyboard, and a mouse. The scene is displayed with the same 3D depth cues used in im- mersive VR: perspective view, hid- den-surface elimination, color, tex- ture, lighting, shading, and shadows. As in immersive VR, animation and simulation are interactively controlled in response to the users direct manip- ulation. Much of the technology used to support immersive and nonimmer- sive VR is the same. They use the
same 3D modeling and rendering and many of the same interaction tech- niques.
Immersive vs. nonimmersive VR. Full immersion is often seen as a major advantage. But our experience and the experiences of others suggest that, for many applications, the same effect is possible with proper 3D cues and inter- active animation. As the user controls the animation and focuses on it, he or she is drawn into the 3D world. Mental and emotional immersion takes place, in spite of the lack of visual or percep- tual immersion. Anyone who has re- cently played a good video arcade game, many of which are examples of nonimmersive VR, knows the truth of this. In addition, current immersive VR techniques cause display jitter and have a lag between the users head or hand movement and the resulting envi- ronmental change. These effects, which tend to inhibit the illusion of immer- sion, are not a problem in nonimmer- sive VR systems.
Similarly, it is often assumed that six-degree-of-freedom input devices give the user more direct control. At Xerox PARC, we have developed
environments in the US military
Moshell, University of Central Florida
A virtual environment is a generic, relatively low-cost real-time interac- tive simulation. The US military has been a primary supporter of real-time simulation for more than 60 years - since the development of the Link Flight Instrument Trainer in the late 1930s.
Flight simulation. Real-time visual simulation was invented to support carrier landing training, in-flight emergency training, and air-to-air combat.
cost, simulation for training is also driven by the lack of suitable large training areas. Except in the US west- ern desert and far at sea, NATO forc- es have almost no field training op- portunities because of noise, damage
Originally motivated by safety and
to roads and vegetation, and civilian opposition. In addition to training, simulation is being increasingly used during concept formation for new weapons and tactics and for mission rehearsal.
Early visual simulators used mov- able cameras and immense model boards. These systems were very ex- pensive to build and maintain and could not support multiple moving ve- hicles, except in limited circumstanc- es. In the 1970s, electronic image gen- erators began to replace model boards. Today, only a few model- board simulators remain in operation.
Some of the most difficult motion cues to simulate are the nonvisual ones involving the sustained accelera- tion of high-performance aircraft flight. The motion bases developed to
simple mouse/keyboard-controlled in- teraction techniques for viewpoint and object movement. They are easy to learn and use and are often faster than Dataglove interaction tech- niques, although our technique could be adapted to a glove. Andy van Dams group at Brown University has developed a series of direct-manipula- tion interaction techniques for creat- ing and manipulating objects using conventional input devices. Also, SGI Inventor,2 a recently introduced high- level 3D toolkit, provides a set of di- rect-manipulation interaction tech- niques for creating and manipulating objects using conventional input de- vices.
Spatialized 3D audio is possible in both types of VR, but immersive VR does have an advantage. Since the head orientation is known in immer- sive systems, the system can allow the user to localize sound - that is, home in on it by turning the head, as one does in the real world.
Advantages of nonimmersive VR systems. Nonimmersive VR systems have three advantages over immersive VR systems: evolutionary advantages caused by the current state of the computer industry, advantages in overcoming current technical limits or problems with immersive techniques,
(Continued on p . 83)
meet this need sometimes degraded rather than improved the training val- ue of the simulator for high-perfor- mance aircraft. Inflatable pressure suits and seats are also used to simulate the g-forces experienced by pilots.
Extensive research has been done concerning the side effects of simula- tion-based training, such as simulator sickness.2 Transport delay (the lag be- tween changing a control and the sim- ulators response) is usually the larg- est contributing factor to sim-sickness, which has symptoms similar to those of motion and space sickness. Many of these results are relevant to current efforts to build virtual environments with head-mounted displays, where sim-sickness is common.
Head-mounted displays. A good deal of military work has been done on techniques for displaying informa- tion and controlling weapons via dis- plays mounted on helmets. The Cobra helicopters chin-mounted machine gun, for instance, automatically points where the pilot looks; a mechanical
February 1993 81
Nonimmersive VR (contd from p . 81)
and advantages in the way the tech- niques are used.
Evolutionary. The evolutionary ad- vantages of nonimmersive VR are its use of familiar tools - display, key- board, and mouse - and lower start- up costs. Many office workers already have the basic tools on their desks, and graphics workstations are com- monplace and inexpensive compared to head-mounted displays and six-de- gree-of-freedom input devices. Until recently, supporting 3D smooth inter- active animation required advanced graphics accelerator hardware. Be- cause of rapid progress in processor technology, we are at the threshold of a period when fast processors can ac- complish the same task. These factors suggest that there will be a large ini- tial installed base into which nonim- mersive VR technology can be intro- duced.
Technical limits. The second advan- tage of nonimmersive VR has to do with limits or problems in current im- mersive VR technology. Immersive VR researchers and users spend much time worrying about the lag in six-de- gree-of-freedom input devices, display jitter, and synchronizing machines for stereo generation without loss of per- formance. Researchers often become slaves to the devices rather than fo- cusing on applications or interaction techniques.
The lag from head and hand track- ing to scene change is caused by scene rendering time and the handling of immersive input devices. There is no- ticeably less lag in nonimmersive VR because it eliminates tracking devices. Display jitter in immersive VR is caused by noise in the data coming from the head tracker and by the neck muscles moving the tracker. Both causes are eliminated in nonimmer- sive VR. Much of current immersive VR uses lower resolution display technology to get the display onto the head; thus, by default, nonimmersive VR has higher resolution displays. Likewise, producing and synchroniz- ing stereo images is not an issue in nonimmersive VR, which does not re- quire stereo vision. There is some evidence that, for some applications, animation is a more effective commu- nicator than stereo; hence, the ability to run without stereo can be a real ad- vantage.
The two types of VR systems have many interaction techniques and is- sues in common. Because of the tech- nology limits, it seems clear that it will
be easier to explore these common in- teraction techniques with nonimmer- sive VR.
Use. Typical office workers will probably be unwilling to put on spe- cial equipment to do their work, espe- cially if it obscures their surroundings. Nonimmersive VR does not prevent users from seeing what is around them and does not require wearing any spe- cial equipment. The use of immersive VR for extended time periods is likely to cause psychological and physical stress that most users will not tolerate. Nonimmersive VR stress factors are the same as those for general comput- er use and are likely to be much less than for immersive techniques.
Hybrid possibilities. Of course, there are a number of hybrid possibili- ties between these two kinds of VR. Six-degree-of-freedom input devices, like the VPL Dataglove, can be intro- duced into nonimmersive VR. Stereo video can be added without using head-mounted displays - for exam- ple, by using shuttered glasses syn- chronized with the display. Also, head tracking that produces parallax effects can be added without using head- mounted displays or stereo vision. Parallax effects - side-to-side head movements to see slight side-to-side scene shifts -would let the user look around an object.
Applications of nonimmersive VR. A number of places are researching and developing nonimmersive VR techniques and applications. The In- formation Visualizer3 is a nonimmer-