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[[Virtual reality]] technology (VR), allied with the development of immersive [[virtual environment]]s (VE), holds the promise of a myriad of uses such as exploring buildings, cities, [[tourism]] oriented [[virtual spaces]], [[training]], [[education]], or [[entertainment]] such as [[games|video games]], with [[HMD|head mounted displays]] (HMD) <ref name=”1”> Zhang, S. (2015). You can’t walk in a straight line – and that’s great for VR. Retrieved from www.wired.com/2015/08/cant-walk-straight-lineand-thats-great-vr</ref> <ref name=”2”> Steinicke, F., Bruder, G., Ropinski, T. and Hinrichs, K. (2008). Moving Towards Generally Applicable Redirected Walking. Proceedings of the Virtual Reality International Conference (VRIC), pages 15-24</ref> <ref name=”3”> Hodgson, E., Bachmannm, E. and Waller, D. (2011). Redirected Walking to Explore Virtual Environments: Assessing the Potential for Spatial Interference. ACM Transactions on Applied Perception, (8)4</ref>. Traditionally, the problem with exploring these VEs has been the fact that, in many existing VR systems, the user navigates the virtual world with hand-based input devices that control the direction, speed, acceleration and deceleration of movements, which decreases the sense of immersion. Other devices, such as [[omnidirectional treadmills|treadmills]], allow users to walk through VEs but even these do not allow for a great sense of immersion, since the user still has to change the direction manually. Various prototypes have been developed that try to improve walking as input to explore the virtual spaces such as [[omni-directional treadmills]], motion footpads, robot tiles, and motion carpets. These systems, despite being technological achievements, have the disadvantage of being costly and hardly scalable (they support only one user walking), and as such are not good candidates for advancement beyond the prototype stage <ref name=”2”></ref> <ref name=”4”> Steinicke, F., Bruder, G., Jerald, J., Frenz, H. and Lappe, M. (2010). Estimation of Detection Tresholds for Redirected Walking Techniques. IEEE Trans Vis Comput Graph., 16(1): 17-27</ref>.
The problem of how the user moves around when in VR is still unsolved in a total satisfactory manner, in order to maximize immersion <ref name=”1”></ref>. Real walking is more presence-enhancing when compared to the other techniques described above and, as such, presents itself as a possible solution <ref name =”5”> Steinicke, F., Bruder, G., Hinrichs, K. and Steed, A. (2009). Presence-Enhancing Real Walking User Interface for First-Person Video Games. Proceeding of the 2009 ACM SIGGRAPH Symposium on Video Games, pages 111-118</ref>. [[Presence ]] can be defined as the subjective feeling of being in the virtual environment, and is important for VE applications to further engage the user in a credible virtual place <ref name=”5”></ref> <ref name=”6”> Razzaque, S., Swapp, D., Slater, M., Whitton, M. C. and Steed, A. (2002). Redirected Walking in Place. EGVE '02 Proceedings of the workshop on Virtual environments, pages 123-130</ref>. Utilizing the user’s [[position tracking|position]] and [[orientation tracking ]] within a certain area, immersive virtual environments that use HMDs allows them to navigate through the virtual reality in a more natural manner. The position and orientation of the person are constantly updated, and the view in the HMD is correspondingly adjusted. However, it has been difficult to develop compelling large-scale VEs due to the limitations of the tracking technology (e.g. range) and access only to relatively small physical spaces in which the users can walk about <ref name=”3”></ref> <ref name=”7”> Hodgson, E. and Bachmannm, E. (2013). Comparing Four Approaches to Generalized Redirected Walking: Simulation and Live User Data. IEEE Trans Vis Comput Graph., 19(4):634-43</ref>. This leads to a need of a system that provides the user to walk over large distances in the virtual world while physically remaining constrained to a relatively small place <ref name=”2”></ref>. As an example, first-person video games in virtual reality would benefit of such technology by allowing gamers to experience the game immersively, not only because their [[field-of-view ]] is that of the virtual character but also because their movements would be tracked in-game, allowing for the players to cover long distances in virtual reality while staying in an small physical area <ref name=”5”></ref>.
Redirected walking is a possible solution to the problem of tracking physical distances in relation to virtual ones. This approach takes advantage of “people’s inability to detect small discrepancies between visual and proprioceptive sensory information during navigation <ref name=”3”></ref>”, and it allows the user to turn and walk in the VE using the body instead of a joystick while reducing the amount of physical space needed in relation to the virtual <ref name=”6”></ref>. According to Steinicke et al. (2009), when humans can use only vision to judge their motion through a virtual scene they can successfully estimate their momentary direction of self-motion but are not as well in perceiving their paths of travel. By creating the right mismatches between the physical movement of the user and the visual consequence in the VE, the user can be steered towards the center of the tracking space, away from the edges of the room <ref name=”3”></ref>. People don’t notice an increase or decrease in the virtual distance they have to walk, or if the virtual room is shifted so that they perceive their path as straight when in fact, the real path is curved. Even when the users turn their heads, if there is turn in the virtual space of 49 percent more or 20 percent less, this too will go unnoticed. As long as a movement is seen and sensed, the magnitude of that movement does not have to be precise <ref name=”1”></ref>. It is the limitation in the human perception for sensing position, orientation and movement that are exploited by the algorithms of redirected walking <ref name=”6”></ref>.
==How redirected walking is achieved==
The basic technique for redirected walking is to rotate the visual virtual scene around a vertical axis that is centered on the user’s head. When the user wants to walk in a straight line, he or she needs to turn physically to reach the goal. By inserting small rotations over time, the user is induced to return to the center of the physical area without realizing it. In this way, redirected walking allows for an increase in the amount of virtual space that can be simulated and traversed while the user is confined to a small physical area. This technique also prevents users from colliding with the walls of the room utilized <ref name=”3”></ref> <ref name=”6”></ref>. Also, the rotation of the visual scene must not increase the [[simulator sickness ]] of the users for it to be successful <ref name=”6”></ref>.
The achievement of redirected walking, with its correspondence between real and virtual movements that make the body function as an [[Input Devices|input device]], provides users with a rich spatial-sensory feedback that results in a greater sense of presence, and less of a chance of being disoriented in the VE. Indeed, according to Hodgson and Bachmann (2013), “virtual walking produces the same proprioceptive, inertial, and somatosensory cues that users experience while navigating in the real world.”
The biological basis for redirected walking can be seen in the phenomenon of when someone gets lost in the woods, for example, and walk in a circle without realizing it – even when trying to walk in a straight line <ref name=”3”></ref>. Souman et al. (2009) proved this phenomenon by showing that the tested participants walked in circles when they could not see the sun, and even when the sun was visible (to provide some sense of orientation) the participants sometimes went off from a straight course, even though they did not walk in circles in this case <ref> Souman, J. L., Frissen, I., Sreenivasa, M. N. and Ernst, M. O. (2009). Walking Straight into Circles. Current Biology, 19: 1538-1542</ref>.
