Outside-in tracking using markers is a well-developed and researched technology. Indeed, a group of researchers (Pustka et al., 2012) built a positional tracking system of this kind using only unmodified off-the-shelf mobile phones. Also, an early two-camera tracking system was described by Madritsch and Gervautz in 1996, and a system that used synchronized IR cameras, able to distinguish 6D targets, was described by Dorfmüller in 1999. <ref name=”5”> Pustka, D., Hülb, J.P., Willneff, J., Pankratz, F., Huber, M. and Klinker, G. (2012). Optical Outside-In Tracking using Unmodified Mobile Phones. IEEE International Symposium on Mixed and Augmented Reality</ref>
The main VR systems in the market - [[Oculus Rift]], [[HTC Vive]], and [[PlayStation VR]] - all use outside-in VR tracking. In each case, headsets and accessories are tracked by an external device. The Oculus Rift positional tracking ([[Constellation]]) is achieved by placing an optical sensor pointed at the headset, with the PlayStation VR having a similar setup. In the case of the HTC Vive, tracking is achieved with the [[Lighthouse]] sensors that are placed around the area to be tracked. While this kind of setup is mainly applied to higher-end VR systems, there have been some experiments with outside-in tracking applied to mobile VR. <ref name=”6”> Langley, H. (2017). Inside-out v Outside-in: How VR tracking works, and how it's going to change. Retrieved from https://www.wareable.com/vr/inside-out-vs-outside-in-vr-tracking-343</ref>
The outside-in tracking system needs room calibration after the cameras or sensors are placed, and the data acquired by the system is processed on a computer. <ref name=”3”></ref> <ref name=”5”></ref> Besides its application in VR, this type of tracking is used in motion capturing, as in the case of the film industry. <ref name=”2”></ref>
Outside-in tracking functions as the inverse of [[inside-out tracking]] (Figure 2). While the former places the sensors in a stationary location to track the VR goggle, in the latter the sensors are placed on the goggles and the markers in stationary locations. <ref name=”3”></ref> <ref name=”7”> Ishii, K. (2010). Augmented Reality: Fundamentals and nuclear related applications. Nuclear Safety and Simulation, 1(1)</ref>
==Pros and Cons of outside-in tracking==
At the moment, outside-in tracking has the best accuracy, with the possibility of adding more sensor trackers in the room to further increase it. The latency is also better with this type of tracking, reducing the probability of the user feeling nauseous. Nevertheless, outside-in systems do have limitations, mainly due to occlusion. This occurs when the cameras or sensors cannot track the object because it is out of their line of sight, which leads to tracking errors. <ref name=”6”></ref> <ref name=”8”> Welch,G. and Foxlin, E. (2002). Motion tracking: No silver bullet, but a respectable arsenal. IEEE Computer Graphics and Applications</ref>
==Future of outside-in tracking==
Outside-in tracking is expected to continue to be the main tracking technology in high-end VR systems as the best all-around solution. This market is likely to be the last to move towards the inside-out tracking technology. Other technologies like [[redirected walking]] could also have a great impact in VR positional tracking, further improving the characteristics of outside-in tracking. <ref name=”6”></ref>
==Devices using outside-in tracking==
