Earlier in this chapter, we briefly mentioned that HMDs sometimes monitor positional movement as well as rotational. There are a few different methods of tracking an HMD positionally, but so far, every solution includes an external component not connected to the headset. In the future as the hardware gets better, we can expect a solution to positional tracking as part of the headset itself (referred to as inside-out tracking).

The Oculus Rift's solution to positional tracking is called constellation tracking. It uses an infrared camera that faces the user to detect small infrared LED markers, invisible to the naked eye, and extrapolate positional movement values based on the number of pixels those markers move in a frame.

Here's what the tracking camera of the Oculus Rift looks like:

As long as the Rift is in view of this camera, the user can laterally move their head and the HMD's display will update to reflect it; this can be used for mechanics such as leaning into or understanding something or sticking your head out from behind a corner. The constellation tracker is capable of tracking the Rift in a seated or standing experience, which means you could even engage the player in limited full body movement. The Oculus Touch controllers, shipping in late 2016, will include an additional camera to improve the quality of tracking further.

This is an image of an early Oculus Rift prototype that shows the exposed IR trackers covering the outside of the device:

The consumer version of the Oculus Rift has these markers embedded in the strap on the back of the headset as well, so there are markers that can help the constellation system track you no matter which direction you're facing.

Generally, the more realistic a VR experience is, the more the user forgets about the world outside of it. Positional tracking adds a lot of realism to the feeling of looking around in VR, so it's a good idea to design your game in a way that will take full advantage of it.