4. Developments in VR Technology

4.2 Functional Characterisation of a VR System (cont)

4.2.2 Functional Decomposition

4.2.3 VR System Configuration

For the purposes of this report it was considered appropriate to partition a VR system into three categories; non-immersion, semi-immersion and full immersion systems dependant on the degree of immersion present. This partitioning facilitates consideration of the peripheral interface and ultimately cost.

The main features of a desk top VR system are its use of a computer generated virtual environment which is delivered by a conventional desk based high resolution monitor and fairly standard interaction devices such as keyboards and mice. Desk-top VR is essentially based on an enhanced version of the familiar personal computer, though not necessarily limited to IBM clones. If a 3D environment is being used then 3D interaction devices such as a Space Ball may be more appropriate. Desk-top VR applications do not generally demand the highest graphics performance, meaning that top of the range ‘PC clones’ can be used. However, for true 3D a means of viewing the display stereoscopically will be required.

Advantages

The significant advantage of a desk-top VR system is the cost since it is significantly lower than other forms of VR system. However, a desk-top VR system provides almost no sense of immersion in a virtual environment. For some applications this may be acceptable but where perception of scale is important then this can be a serious problem.

Appendix C shows the functional decomposition of a current generation desk-top VR system. It will be noted that no special hardware is required other than that required for a multi-media system.

Issues

There a number of issues that will require attention in the near future including:

1. Convergence of Desk-top VR software: There are several products available for desk-top VR applications such as Superscape and VRML. At the moment these products are quite distinct and aimed at different applications. However, the demand for 3D web based systems may lead to the situation where VRML or its successor will inherit the functionality of a desk-top VR system. The future strategy of Microsoft and its next generation of internet based operating systems could easily reduce the difference between Superscape type products and 3D web browsers. The popularity of network based operating systems may cause problems for current vendors of desk-top VR software.
2. Interaction devices: There are several reasonable interaction styles (based on screen pointing systems) available for desk-top use but these are essentially limited to 2D display presentations. A keyboard/mouse combination are not the ideal interaction devices to use for manipulation and navigation in a 3D virtual environment. As soon as a stereoscopic display is used then these interaction styles can be quite clumsy to use. A more effective 3D interaction device is urgently required.
3. Performance limitations: The low cost of desk-top VR systems make them extremely attractive for many applications in the future. However, this low cost comes at the expense of high performance. The immersive VR systems will be able to out perform non-immersive systems simply because of the computational and graphics performance available in the larger systems. Over time the performance of desk-top VR systems will migrate towards the higher performance of immersive systems. It should also be stressed that the immersive systems will also get better.

The term semi or partial immersive VR has only recently emerged as a description for an extremely interesting VR concept based on a fixed, wide angle display system. Many of the concepts for semi immersive VR spun off directly from the flight simulation field. Therefore, it is no suprise that the enabling technologies are highly developed and generally available off-the-shelf.

The main components of this type of VR system include a reasonsably high performance graphics computer and a wide angle display in excess of 60°. This can be provided by either :

• a large screen monitor,
• a large screen television projector or,
• multiple television projection systems.

Low cost LCD based projection can be used but care must be taken to ensure that they are used on flat screens otherwise considerable distortions will occur. Resolution should not be neglected as this will be a function of the display technology and field of view.

Advantages

Projection based systems provide a greater sense of presence than desk-top systems because of the wider field of view. For example, an IMAX theatre exploits the sense of presence one gets in a very wide field of view display system. The quality of the projected image is also a very important factor. For the highest resolution it may be necessary to employ a number of projection systems, each projector making up a part of the composite picture. Typical resolutions for multiple projection systems range from 1000 to 3000 lines which are considerably better than that provided by head mounted displays. The cost of providing and maintaining a semi immersion system can be very high compared to a desk-top system but the increased sense of immersion and presence can often be justified. Unlike VR systems where a head mounted display is used, a projection VR system allows a number of people to share and be involved in the same virtual environment. This aspect has important implications for use as an educational tool since it will allow a teacher and student to work in the same virtual environment.

The term Reality Centre was developed by Silicon Graphics Inc for semi immersive VR systems. The Reality Centre has proved itself to be a very powerful method of presenting virtual environments. Consequently, there is expected to be a growth of such facilities across the world. At the moment their cost is fairly prohibitive so these may develop initially as regional centres.

Unlike desk-top VR systems a projection based VR system can go a long way towards producing a visual image that allows a true sense of scale to be achieved. The use of multiple projection based systems can result in extremely high resolution images being produced - but this comes with a significant cost increase.

As will be discussed later Reality Centres (or for that matter any VR system) can be networked together. This will provide a very flexible teleconferencing facility bringing together key people from anywhere in the world.

Issues

1. Display media: Unfortunately, it is not just a matter of taking a television projection system and connecting this to a graphics systems. It is important to take careful account of aspects like projection geometry and screen curvature as these affect the final image displayed to the user. For instance LCD television projectors do not incorporate any form of screen curvature correction. This means that they are useless for use with curved screens.
2. Interaction devices: This is an important issue and relates to how someone interacts with a large immersive display. In a flight simulator a pilot puts demands into the system via an inceptor (joystick) and the aircraft model interprets this as a flight control input. However, in a Reality Centre the user will not only want to navigate (fly through) the enviroment, they are likely to want to interact with virtual objects.
3. Stereographic projection can be achieved with projection systems in a number of ways. The most popular is to employ some form of shuttered spectacle device that operates in synchronism with the graphics system. By generating alternating left and right eye views it is possible to create effective stereoscopic images. The important issue is where to apply this concept to take full advantage of the stereoscopic presentation. Special steps have to be taken when using shutter spectacles with television projection systems because of the long persistence time of green phosphor.
4. Multi-user interaction: One of the major advantages of a semi immersion system is the multi-user capability. Assuming that the system can be set up for several different view points the next issue becomes: How do you pass control of the environment from one user to another? At the moment the use of a 3D joystick seems to be the only viable option.

A fully immersive VR system is one that tends to be thought of first when most people think of a VR system. To achieve full immersion the user has to employ a head-coupled display which is either head mounted or arranged to move with the head. A sense of full immersion is achieved because the display provides a visual image wherever the user is looking. Consequently, a head coupled display provides a 360° field of regard. The field of view of a head coupled display is also very important and it is essential to note that the sense of immersion will be a function of the quality of the display provided in terms of resolution, field of view, update rate and image lags etc.

Advantages

All fully immersive VR systems give a sense of presence in the virtual environment that cannot be equalled by other VR approaches. This is a direct consequence of having a field of regard of 360° where images can be presented wherever the user is looking. The ability to exclude visible features of a real environment can lead to the sense of immersion taking place very quickly.

Issues

There are many issues relating to head coupled displays and it is not possible to list them here. Current issues surrounding the use of fully immersive VR systems revolve around limitations in current technology. However, it is expected over time that many of these issues will be resolved. The most critical issues include: human factors, required display resolution, weight of head mounted display, image lags, cost.

4.2.4 Comparision between Different VR Categories

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