Virtual Reality – NAS Home

Virtual RealityDefinition:Virtual reality has been notoriously difficult to define over theyears. Many people take "virtual" to mean fake or unreal, and "reality" to refer to the real world. This results in an oxymoron.The actual definition of virtual, however, is "to have the effect ofbeing such without actually being such". The definition of "reality" is "the property of being real", andone of the definitionsof "real" is "to have concrete existence". Using these definitions"virtual reality" means "to have the effect of concrete existencewithout actually having concrete existence", which is exactly theeffect achieved in a good virtual reality system. There is norequirement that the virtual environment match the real world.Inspired by these considerations, for the virtual windtunnel we adapt the following definition:Virtual reality is the use of computer technology to createthe effect of an interactive three-dimensional world in which theobjects have a sense of spatial presence.In this definition, "spatial presence" means that the objectsin the environment effectively have a location in three-dimensionalspace relative to and independent of your position. Note that this is an effect, not an illusion. The basic idea is to presentthe correct cues to your perceptual and cognitive system so that your brain interprets those cues as objects "out there" in thethree-dimensional world. These cues have been surprisingly simpleto provide using computer graphics: simply render a three-dimensional object (in stereo) from a pointof view which matches the positions of your eyes as you move about.If the objects in the environment interact with you then the effectof spatial presence is greatly heightened.Note also that we do not require that the virtual reality experiencebe "immersive". While for some applications the sense of immersionis highly desirable, we do not feel that it is required for virtual reality. The main point of virtual reality, and the primary difference between conventional three-dimensional computer graphicsand virtual reality is that in virtual reality you are working withThings as opposed to Pictures of Things.Requirements:The primary requirement of virtual reality is that the scenebe re-rendered from your current point of view as you move about.The frame rate at which the scene must be re-rendered depends on the application. For applications like the virtual windtunnel,it turns out that a minimum frame rate of 10 frames per second is enough to support the sense of spatial presence. While motionat this frame rate is clearly discontinuous, if properly done our cognitive systems will interpret the resulting imagesas three-dimensional objects "out there".

The other requirement is that interactive objects in the environmentcontinuously respond to your commands after only a small delay. Justhow long a delay can be tolerated depends on the application, but forapplications like the virtual windtunnel delays of up to about a tenth of a second can be allowed. Longer delays result in a significantlydegraded ability to control objects in the virtual environment.

We summarize the Virtual Reality Performance Requirements:

For more information on VR see the papers found on Steve Bryson's home page.

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Virtual Reality - NAS Home