Degrees of freedom

Degrees of freedom or DOF are the ways an object can move within a space. There are a total of 6 degrees of freedom in a 3 dimensional space. The 6 DOF can be divided into 2 categories, rotational movements and translational movements. Each category has 3 DOF. The original Oculus Rift DK1 only had 3DOF. Oculus Rift headsets DK2 and later, and SteamVR HMDs all have 6 DOF. Both orientation tracking (rotation) and positional tracking (translation) are required to have a truly immersive VR experience.

Rotational movements

The 3 rotational movements are pitch, yaw, and roll. These movements are tracked by most HMDs' on-board sensors. As you tilt and turn your head, the HMD senses the movements and alters its display accordingly.

Rotational movements are tracked by IMUs (inertial measurement units), which consist of an accelerometer, gyroscope, and optionally a magnetometer. These IMUs measure the HMD's velocity, orientation, and gravitational forces to infer rotational orientation and movement. An IMU that has a magnetometer is often marketed as having "9 DOF". The 9 DOF are calculated by adding up the 3 DOF detected by each subsensor.

Translational movements

The 3 translational movements are left/right, forward/backward and up/down. Three dimensions are also referred to as X,Y,Z, but some people and programs use XYZ to mean lateral,vertical,depth, in that order, whereas others use it to mean lateral,depth,vertical, or others. Translational movements are usually tracked by an external camera or other sensors. Few HMDs use on-board sensors to track translational movements.

Tracking absolute translational position based on movement measurements is called dead reckoning.

The ability to track translational movements is required for positional tracking. (positional tracking is the ability to determine the absolute position of an object in a 3D environment)