**2. Virtual reality technology**

VR technology enables users to immerse themselves in an artificial environment simulated by a computer, with the ability to navigate through the environment and interact with objects in it. Two keywords in the field of VR are presence and immersion. VR immerses the user in a simulation so that the user has a sense of being present in the virtual environment. The degree of immersion depends primarily on the computer hardware used whereas presence is achieved if the virtual environment causes the user to suspend disbelief and accept the computer-generated experience as real.

A computer system that generates an artificial world that tricks the user into feeling part of it would not be a complete definition of VR because it lacks the vital component of interactivity (Louka & Balducelli, 2001). VR is a highly interactive, computer-based multimedia environment in which the user becomes the participant in a computergenerated world (Kim et al., 2000; Onyesolu, 2009a; Onyesolu & Akpado, 2009). It is the simulation of a real or imagined environment that can be experienced visually in the three dimensions of width, height, and depth and that may additionally provide an interactive experience visually in full real-time motion with sound and possibly with tactile and other forms of feedback. VR is a way for humans to visualize, manipulate and interact with computers and extremely complex data (Isdale, 1998). It is an artificial environment created with computer hardware and software and presented to the user in such a way that it appears and feels like a real environment (Baieier, 1993). VR is a computer-synthesized, three-dimensional environment in which a plurality of human participants, appropriately interfaced, may engage and manipulate simulated physical elements in the environment and, in some forms, may engage and interact with representations of other humans, past, present or fictional, or with invented creatures. It is a computer-based technology for simulating visual auditory and other sensory aspects of complex environments (Onyesolu, 2009b). VR incorporates 3D technologies that give a real-life illusion. VR creates a simulation of real-life situation (Haag et al., 1998). VR is a three-dimensional user interface in which the user can perform actions and experience their consequences. It is a multidimensional realtime simulation rather than a linear animation with predefined camera movement.

This is what distinguishes VR from recorded, computer generated, images used in films and on television and from real-time computer animation where the user is a passive viewer (Louka & Balducelli, 2001). Unlike pre-rendered images of a 3D environment, the user of a VR system can usually move around freely in a virtual environment. The manner in which a user interacts with a virtual environment depends on the hardware and software used (Louka & Balducelli, 2001). In most cases, an off-the-shelf desktop computer with a 3D graphics acceleration and appropriate software are all that is necessary. This form of VR is often called desktop or fishtank VR (Louka & Balducelli, 2001, Onyeslu & Eze, 2011). In a desktop VR configuration, a 2D pointing device such as a mouse is typically used to select and manipulate objects, choose menu option, etc. Desktop VR is an effective, relatively cheap, option. The desktop VR experience can be taken a step further by using a projector and a large screen to display the virtual environment so that several users can work together in a meeting room or control centre, with one user controlling the computer. The 2D pointing device can also be replaced by a 3D input-device, which may improve usability.

Some 3D graphics cards and software support stereoscopic display, enabling users to view the virtual environment with an enhanced sense of depth using either a desktop display or a suitable projector. Stereoscopic displays typically require that users wear special glasses. Some users find that stereoscopic viewing enhances their spatial understanding, however such display systems should only be used for brief viewing sessions (e.g. up to half an hour) to avoid simulator sickness (Louka & Balducelli, 2001).

For a greater sense of immersion, multi-screen projection systems or head-mounted displays can be used to place the viewer inside the model. While the hardware required to do this efficiently, with motion tracking and 3D input devices such as gloves, is relatively costly, immersive VR is particularly useful if a realistic view of a virtual environment is required. The user experiences the environment within the model as opposed to looking into it from the outside. VR systems based on immersive technology are relatively expensive, but can be justified when the benefits outweigh the cost (Louka & Balducelli, 2001).

In order to create usable virtual reality systems at a reasonable cost and with an appropriate level of performance, the application designer's understanding of the advantages and limitations of combinations of VR hardware and software is crucial (Louka & Balducelli, 2001).
