Three Degrees of Freedom (3DoF) vs Six (6DoF) in Extended Reality

 

We have talked about Extended Reality multiple times in this and other blogs. When we talk about the futuristic glasses or head mounted devices (HMD), we are talking about six degrees of freedom (6-DoF) as opposed to 3-DoF. So what exactly is the difference.

According to this blog post in Virtual Speech:

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:

  • 3 for rotational movement around the x, y, and z axes (also known as pitch, yaw, and roll)
  • 3 for translational movement along those axes, which can be thought of as moving forward or backward, left or right, and up or down

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.

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:

  • Looks left or right
  • Rotates their head up or down
  • Pivots left or right

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:

  • Google Cardboard
  • Oculus Go
  • Merge VR
  • Samsung Gear VR
  • Google Daydream

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:

  • Forward or backward
  • Laterally or vertically
  • Up or down

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:

  • Oculus Rift
  • Oculus Quest
  • HTC Vive
  • Windows Mixed Reality

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."

This video nicely explains the basics of 3DoF and 6DoF as well:

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