Redirected Walking

It was found out that, when only visual input was supplied, people could successfully estimate the amount of change in the direction but not the path they followed [38]. This makes it possible to manipulate the visual cues to keep the users in the tracking area without being noticed. Various experiments have been performed to analyze the user’s perception on virtual and real worlds. It was found that the perceived egocentric distances were often underestimated in VR as compared to actual distances [39], unless the virtual environment started as a replica of the real environment [40]. Similarly, travelled distances in virtual worlds were underestimated [41]. Likewise, virtual walking speeds were often underestimated by the users as compared to the real walking speeds [42].

Redirection is a way of manipulating the user’s visual cues to keep them in the tracking area [19]. With this technique, larger virtual environments can be explored within a smaller tracking area. There are some variations of redirected walking techniques, and different taxonomies have been proposed in the literature. Steinicke et

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al. proposed a classification based on the types of gains applied; translation, rotation or curvature [43]. On the other hand, Suma et al. proposed a different classification based on the geometric flexibility, the detectability of the technique and the continuity [44]. In this taxonomy, the repositioning and reorientation techniques can either be overt or subtle according to the detectability, and either continuous or discrete according to the gain application time.

In their study, Steinicke et al. reported the positive and negative limits of the gains that could be applied without getting noticed by the users [43], [45], [46]. Reported limits were listed as -14% and 26% for translation, -20% and 49% for rotation, and 22 meters for curvature radius. For the rotational gains, limits were also examined in detail for the body rotations [47]. For small body rotations like 10 degrees, the limits became -14% and 102%. The sensitivity for scene motion was further examined for head yaws [48]. The scene could be rotated 5% against the head rotation and 11% with the head rotation. Using gain values outside these limit values were suggested by the researchers to increase the cognitive load and decrease the task performance [49].

Some sophisticated redirected walking techniques use dynamic curvature gains according to the speed of the user [50]. However, no significant difference was found between changing the rotational gain gradually and instantaneously during a full rotation [51]. Another technique called “Seven League Boots” predicted the aimed travel direction by combining eye direction and former displacement data, and applied translational gains on that direction [52]. The displacements in the perpendicular directions were not scaled so that unintentional vertical and sideways head movements

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were not exaggerated. For traveling in long and straight hallways, this method was preferred by the users. Another recent study suggested calculating the redirection parameters based on the architectural layout of the virtual environment [53]. If the users were asked to focus on a cognitive task, these limits could be expanded and no negative impact was noted in spatial memory [54]. Bruder et al. examined the limits of the gains for individuals using an electric wheelchair controlled by joystick [55]. The possible range for the gain values was found to be larger for such redirected driving.

Even if redirection techniques are used, in some cases, users may still reach the edge of the tracked area. For those situations, some additional methods were developed and used alongside the redirection technique. Williams at al. proposed “Freeze-Backup”, “Freeze-Turn”, and “2:1-Turn” methods as alternatives [56]. In the Freeze-Backup method, when the user approaches to the border of the actual area, the virtual environment positions stay frozen, and the user can take a few steps backwards. In the Freeze-Turn method, again the virtual environment stays frozen and the user can make a 180 degree turn to continue, walking back toward the center of the tracked area. Finally, in the 2:1 Turn method, the user makes a 180 degree turn and the virtual world makes a simultaneous 360 degree turn. The 360 degree turn better masks the unusual 180 degree turn the user is taking.

These methods can be effective but they also inject interruptions into the user experience that have a negative effect on the presence and the quality of the experience [57]. As an alternative, Kohli et al. [58] and Peck et al. [59] integrated virtual distracters into the experience to make the users rotate and face towards the center of the tracked

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area without explicitly interrupting the experience to prompt the user to turn. The aim was to avoid the explicit interruptions that caused breaks in the presence. Using distracters was found to be more natural as compared to mentioned explicit breaks. With the distracter based methods, the redirection was also less noticeable. Furthermore, addition of these distracters did not degrade wayfinding and navigation as compared to basic redirected walking [59]. A recent study suggested using a context-sensitive approach by spawning some events in the virtual world that are related to the narrative, so that the user’s orientation is changed with the help of redirection gains without being noticed [60].

Redirection algorithms can also be altered to involve passive haptic feedback objects as well [45], [61]. A proxy object in the real environment representing virtual objects with similar size, shape and surface structure can support passive haptic feedback to the users. Although more difficult to utilize, these passive haptic feedbacks was reported to improve the virtual reality experience significantly [62]. Another redirection technique for exploring architectural 3D models scales the virtual room to fit into the real room, so that users can feel the real walls when they reach to the virtual walls [63]. In this study, to let the users go through a virtual door in a virtual wall, an intense redirection was used so that they did not hit the real walls.