Degrees of freedom (DoF) refer to the number of ways a rigid object can move through three dimensional space. There are six total degrees of freedom which describe every possible movement of an object:
VR headsets and input devices (e.g. hand controllers) are generally 3-DoF or 6-DoF.
Degrees of freedom is an essential concept in VR that allows human movement to be converted into movement within the VR environment.
Image showing the difference between 3-DoF (rotational movement) and 6-DoF (rotational and translational movement) with a VR headset.
3-DoF headsets allow us to track rotational motion but not translational. With a user wearing a VR headset, we can therefore track whether a user:
With 3-DoF, we cannot determine whether the user has moved (translational movement) about the scene by moving in real life.
Examples of 3-DoF VR headsets:
3-DoF headsets provide the simplest form of user tracking in VR. It relies mostly on inbuilt sensors (accelerometers, gyroscopes and magnetometers) that devices (such as a smartphone) use to measure movement.
6-DoF headsets allow us to track translational motion as well as rotational motion. We can determine whether a user has rotated their head and moved:
This type of tracking is important for VR experiences with translational motion and gives the user a lot more freedom to explore locations, inspect details and perform real life tasks in VR.
Examples of 6-DoF VR headsets:
There are various ways to achieve this. Early versions of 6-DoF headsets typically use positional tracking sensors (usually two physical devices positioned around the room which can track headset movement) - for example the Oculus Rift is embedded with infrared LEDs which are monitored by two 30cm tall sensor towers positioned approximately 1.5m apart.
Modern VR headsets may use inside-out tracking to achieve 6-DoF. From the Microsoft Mixed Reality documentation: "the tracking system uses two visible-light low-resolution cameras to observe features in your environment, and fuses this information with IMU data to determine a precise position of the device in your environment."
Deciding on which type of headset you need depends on your application and business need. It’s worth noting that 3-DoF headsets are typically cheaper than 6-DoF ones.
Here are some examples to highlight applications across the two: