THIS CHAPTER REVIEWS THE STATE OF THE ART OF WALKING INTERFACES FOR VIRTUAL REALITY, FOCUSING FIRST ON ACADEMIC RESEARCH AND THEN ON COMMERCIAL POSSIBILITIES IT PRESENTS AND OVERVIEW OF AUDIO AND HAPTIC WALKING DISPLAYS, MODALITIES LESS INVESTIGATED WHEN CONSIDERING WALKING INTERACTIONS. One of the unresolved challenges in virtual reality applications is the locomotion problem. Navigation is a fundamental interaction task in virtual environments. Most VR applications give users the possibility to walk or move in the virtual world. One constraint that often comes along with VR setups is given by the limited workspace in which users are physically walking, also known as the problem of incompatible spaces [89]. Ideally a virtual reality world could be infinite, whereas the physical space where a person is interacting, is finite. The motion of a person is indeed bounded by either the walls of the simulation room or the range of the tracking system. Virtual navigation techniques must therefore cope with such physical restrictions. In this chapter we present several approaches to virtual navigation which cope with the problem of impossible spaces using different strategies, from walking in place to redirected walking to hardware based interfaces. The concepts behind afford progressively more effective physical walking, meanwhile gradually shifting the design focus from software to hardware issues.
5.1 WALKING IN PLACE There are several known VR locomotion metaphors in which the user is not required to walk [15], and therefore does not need to deal with workspace restrictions. Examples of these metaphors include teleportation, that is an instantaneous switch to a new location. Worlds In Miniature (WIM) [109] is a metaphor in which users hold a copy of the virtual world in their hands; from that copy, they can point to a location and be brought anywhere in the virtual world. Probably the most common navigation technique is the Flying Vehicle, where the environment is not manipulated; the illusion is that the user can move through the world, either by using a mock-up, a wand or other device. Walking in place (WIP) [141], simulates the physical act of walking without forward motion of the body; a virtual forward motion is introduced instead. When relying on WIP techniques for virtual locomotion the user performs stepping-like movements These steps in place serve as a proxy for real steps and enable the user to move through the virtual world while remaining stationary with respect to the physical environment.
The optical flow, that should match with the proprioceptive information coming during the physical walking act, is instead coupled to virtual proprioceptive cues. The sense of presence is greatly increased compared to static navigation techniques [167], though, other (mainly the vestibular) sensory cues of walking are missing. WIP techniques provide one possible solution to the problem emerging when an immersive virtual environment (IVE) offers a larger freedom of movement than the physical environment where the interaction is taking place. Such techniques are particularly useful when the spatial constraints are very prominent. WIP techniques also constitute an inexpensive, convenient and relatively natural alternative to these approaches. The advantages of WIP techniques include, but need not be limited to, convenience and cost-effectiveness [35], good performance on simple spatial orienting tasks [178], and generation of proprioceptive feedback similar, albeit not identical, to the one resulting from real walking [140]. Moreover, virtual locomotion accomplished via such stepping motions have been shown to elicit a more natural walking experience and a stronger sensation of presence compared to interaction via more traditional peripherals [167]. Combined, these potential advantages suggest the need for finding the best possible WIP technique. Arguably, the challenge of doing so is twofold. First, it includes the technical challenge of enabling users to control their virtual velocity in a manner that is both responsive and smooth [154]. Second, it is necessary to investigate how to increase the percieved naturalness of WIP locomotion; this means how to create an experience of walking in place through virtual environments as similar as possible to the experience of real walking. A part of the second challenge is to ensure that there is a natural correspondence between the gestures being performed and the resulting virtual velocity. The literature on human biomechanics tells us how to derive realistic walking speeds from gait properties, such as the step frequency [177]. However, realistic, virtual walking speeds need not always be perceived as natural. Wendt, Whitton and Brooks [177] proposed a WIP technique informed by human gait principles which is able to produce walking speeds that correspond better with those of real walking.
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