Positional tracking detects the precise position of the head-mounted displays, controllers, other objects or body parts within Euclidean space. Positional tracking registers the exact position due to the recognition of the rotation (pitch, yaw, and roll) and recording of the translational movements. Since virtual reality is about emulating and altering reality it’s important that we can track accurately how objects (like the head or the hands) move in real life in order to represent them inside VR. Defining the position and orientation of a real object in space is determined with the help of special sensors or markers. Sensors record the signal from the real object when it moves or is moved and transmit the received information to the computer.
Optical methods represent a set of computer vision algorithms and tracking devices such as a camera of visible or infrared range, a stereo camera, and a depth camera. Optical tracking is based on the same principle as stereoscopic human vision. When a person looks at an object using binocular vision, he is able to define approximately at what distance the object is placed. Not enough just to install a pair of cameras to simulate the stereoscopic vision of a person. Cameras have to determine the distance to the object and its position in space, so it’s necessary to calibrate. Infants learn to calibrate their vision when they try to take something, correlating the location of the object with an outstretched hand. Optical systems are reliable and relatively non-expensive but it’s difficult to calibrate. Furthermore, the system requires a direct line of light without occlusions, otherwise, we receive wrong data. There are two approaches:
Inside-out tracking: the camera is based on the tracked device and the infrared markers are placed in stationary locations. This technology is used in Project Tango (SLAM), Vive and Microsoft HoloLens.
Outside-in tracking: the camera (or several cameras) is placed in a stationary location and the infrared markers are placed on the tracked device. The outside-in approach implies the presence of an external observer (the camera) that determines the position of a moving object by the characteristic points. Technology is usually used in high-end VR systems such as Oculus Rift, Playstation VR, and in many motion capture systems.
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