Terminology regarding various ``realities''

The term virtual reality dates back to German philosopher Immanuel Kant [341], although its use did not involve technology. Kant introduced the term to refer to the ``reality'' that exists in someone's mind, as differentiated from the external physical world, which is also a reality. The modern use the VR term was popularized by Jaron Lanier in the 1980s. Unfortunately, name virtual reality itself seems to be self contradictory, which is a philosophical problem rectified in [34] by proposing the alternative term virtuality. While acknowledging this issue, we will nevertheless continue onward with term virtual reality. The following distinction, however, will become important: The real world refers to the physical world that contains the user at the time of the experience, and the virtual world refers to the perceived world as part of the targeted VR experience.

Although the term VR is already quite encompassing, several competing terms related to VR are in common use at present. The term virtual environments predates widespread usage of VR and is preferred by most university researchers [109]. It is typically considered to be synonymous with VR; however, we emphasize in this book that the perceived environment could be a photographically captured ``real'' world just as well as a completely synthetic world. Thus, the perceived environment presented in VR need not seem ``virtual''. Augmented reality (AR) refers to systems in which most of the visual stimuli are propagated directly through glass or cameras to the eyes, and some additional structures, such as text and graphics, appear to be superimposed onto the user's world. The term mixed reality (MR) is sometimes used to refer to an entire spectrum that encompasses VR, AR, and ordinary reality [215]. People have realized that these decades-old terms and distinctions have eroded away in recent years, especially as unifying technologies have rapidly advanced. Therefore, attempts have been recently made to hastily unify them back together again under the headings XR, X Reality, VR/AR, AR/VR, VR/AR/MR and so on.

The related notion of Telepresence refers to systems that enable users to feel like they are somewhere else in the real world; if they are able to control anything, such as a flying drone, then teleoperation is an appropriate term. For our purposes, virtual environments, AR, mixed reality, telepresence, and teleoperation will all be considered as perfect examples of VR.

Figure 1.5: When considering a VR system, it is tempting to focus only on the traditional engineering parts: Hardware and software. However, it is equally important, if not more important, to understand and exploit the characteristics of human physiology and perception. Because we did not design ourselves, these fields can be considered as reverse engineering. All of these parts tightly fit together to form perception engineering.

The most important idea of VR is that the user's perception of reality has been altered through engineering, rather than whether the environment they believe they are in seems more ``real'' or ``virtual''. A perceptual illusion has been engineered. Thus, another reasonable term for this area, especially if considered as an academic discipline, could be perception engineering, engineering methods are being used to design, develop, and deliver perceptual illusions to the user. Figure 1.5 illustrates the ingredients of perception engineering, which also motivates the topics of book, which are a mixture of engineering and human psysiology and perception.