Outside-in tracking

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See also: Markerless outside-in tracking and Positional tracking

Introduction

Figure 1. Outside-in tracking (Image: www.wareable.com)
Figure 2. Inside-out vs. outside-in tracking (Image: Ishii, 2010)

Outside-in tracking is a form of positional tracking and, generally, it is a method of optical tracking. When referring to virtual reality (VR), tracking is the process of tracing the scene coordinates of moving objects in real-time, such as head-mounted displays (HMDs) or motion controller peripherals. [1]

Outside-in VR tracking uses cameras or other sensors placed in a stationary location and oriented towards the tracked object (e.g. a headset) that moves freely around a designated area defined by the intersecting visual ranges of the cameras (Figure 1). The object is therefore observed from outside by the fixed tracking device. Usually, the tracked object has a known set of markers that are essential for the calculation of its position relative to the sensors. Also, while this type of positional tracking can be achieved using the visible light spectrum, it is common to use infra-red (IR) markers and cameras that can detect that type of light. [1] [2] [3]

The majority of commercially available optical trackers use outside-in tracking. Its accuracy and performance are dependent on various factors like optical sensors, tracking markers and targets, processing power, and tracking algorithms, all of which can vary greatly. [2] [4]

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. [5]

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. [6]

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. [3] [5] Besides its application in VR, this type of tracking is used in motion capturing, as in the case of the film industry. [2]

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. [3] [7]

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. [6] [8]

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. [6]

Devices using outside-in tracking

Rift (including Rift development kit)

PlayStation VR

Tracking systems using outside-in tracking

Constellation

PlayStation Camera

Neon Tracking System (used by HTC Link[9])

References

  1. 1.0 1.1 Ribo, M., Pinz, A. and Fuhrmann, A.L. (2001). A new optical tracking system for virtual and augmented reality applications. Instrumentation and Measurement Technology Conference Proceedings
  2. 2.0 2.1 2.2 Mehling, M. (2006). Implementation of a Low Cost Marker Based Infrared Optical Tracking System. PhD thesis, Fachhochschule Stuttgart
  3. 3.0 3.1 3.2 Boger, Y. (2014). Positional tracking: "Outside-in" vs. "Inside-out.” Retrieved from http://vrguy.blogspot.pt/2014/08/positional-tracking-outside-in-vs.html
  4. Wang, J.F., Azuma, R., Bishop, G., Chi, V., Eyles, J. and Fuchs, H. (1990). Tracking a head-mounted display in a room-sized environment with head-mounted cameras. Proceedings of SPIE 1990 Technical Symposium on Optical Engineering and Photonics in Aerospace Sensing, Orlando, Florida
  5. 5.0 5.1 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
  6. 6.0 6.1 6.2 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
  7. Ishii, K. (2010). Augmented Reality: Fundamentals and nuclear related applications. Nuclear Safety and Simulation, 1(1)
  8. Welch,G. and Foxlin, E. (2002). Motion tracking: No silver bullet, but a respectable arsenal. IEEE Computer Graphics and Applications
  9. [1]]