*This article was published in contribution to the new frontier of Intelligent Reality (IR). Click here to learn more about the 2022 IEEE 2nd International Conference on Intelligent Reality (ICIR).
If you engage with any of the types of Extended Reality (XR), from virtual reality to augmented reality and everything else in between, sooner or later you will encounter the terms 3DoF and 6DoF. DoF is an abbreviation for “degrees of freedom”. It refers to the number of ways an object can move. In order to create a virtual or augmented experience, it’s vital that you have good motion tracking so that the virtual components of their experience line up accurately with the real world components in 3D space. This is particularly important when tracking the movements of the person using the augmented reality or virtual reality headset.
How 3DoF and 6DoF are connected
There are two types of motion to consider: rotation and translation, and both have three degrees of freedom. There are three types of rotation: roll, pitch and yaw. If you imagine an airplane, roll occurs when one wing is higher than the other. Pitch occurs when either the nose or the tail of the airplane is higher. Yaw occurs when the plane rotates around its middle, such as when the plane turns left or right while taxiing on a runway. Combined, roll, pitch and yaw are three degrees of freedom. Typically, if a system can track roll, pitch and yaw, it is referred to as having three degrees of freedom, or 3DoF.
The second type of motion is translation. Translation simply means movement without rotation, such as moving forwards and backwards, left and right, or up and down. This type of motion is often described in terms of an x, y and z axis in 3D space. An object that can only move forwards and backwards along a single axis, like a train on tracks, has one degree of freedom, An object that can move forwards and backwards, and left and right has two degrees of motion. If the object can also move up and down, it has three degrees of freedom. Technically, a system that can track position in all three directions is also a 3DoF system. However, in this context, motion tracking in 3D without rotational tracking is quite rare, so 3DoF almost always refers to rotational tracking only.
The airplane we used to visualize roll, pitch and yaw can also travel forwards and backwards, left and right, and up and down. In other words, an airplane has three degrees of rotational freedom and three degrees of translational freedom. This gives the airplane a total of six degrees of freedom, or 6DoF. Any system that can track all six degrees of freedom is called a 6DoF system.
Until recently, it was much easier to achieve 3DoF rotational tracking than full 6DoF tracking. You can track roll, pitch and yaw using an inexpensive IMU sensor (inertial measurement unit sensor) such as the accelerometer or gyroscope found in a typical smartphone. It’s much harder to track translational motion. However, the more immersive you want an experience to feel, the more likely you are to need 6DoF. Creating a sense of immersion relies on good tracking, but also on a number of cognitive processes in the brain which are only just beginning to be understood.
Many of the early models of virtual reality headsets, such as the original Oculus Rift and even older headsets such as Virtual I/O glasses (from the 1990s) used 3DoF rotational tracking. Many input devices, such as the hand-held controller supplied with Microsoft’s Hololens, the controller that ships with Magic Leap’s headset, and many of the controllers that ship with VR headsets, also use 3DoF tracking.
Benefits of 3DoF versus 6DoF
The primary reason that all AR and VR systems don't use 6DoF is complexity and cost. The benefit of tracking 3DoF rotational motion is that it is relatively easy and inexpensive to implement. Probably the most well-known example of a low cost 3DoF device is Google Cardboard and other types of Mobile VR headsets. Cardboard started as a literal cardboard cutout that contained two lenses and could be assembled to hold a smartphone. The software used the IMU in the smartphone to provide basic rotational tracking with three degrees of freedom. The smartphone screen displayed images for the user’s left and right eye, which aligned with the pair of inexpensive plastic lenses. Millions of Google Cardboard devices were sold, helping drive the current growth of Virtual Reality. The Oculus Go, a now-discontinued VR headset from Meta, offered a more refined, but essentially identical experience. This type of VR headset offered a standalone VR experience, meaning the device was self-contained and did not require a wired connection to a PC or game console.
If the user stays in one location, rotational tracking with three degrees of freedom can be surprisingly effective. It also works well with 360 photos and 360 video (also called VR video), which are far cheaper and easier to produce than VR-ready 3D content. This led to the rise of many “seated” VR and AR experiences, from simulators and virtual tours to VR games, where the user would literally sit in a chair during the experience.
In the early days of the current iteration of VR headsets, there was a clear separation between low-end 3DoF headsets, such as Google Cardboard, and high end 6DoF headsets such as the HTC Vive. In a 6DoF headset, the user can walk around freely in a virtual space, providing a far more immersive experience. However, 6DoF systems were significantly more expensive, more complex to set up and use, and also required a wired connection to a powerful PC or game console. This made them unattractive for many potential users. For example, the HTC Vive PC VR system required an open, unobstructed area of at least 4m2, two base stations (both of which required power) and a very high end gaming PC costing thousands of dollars.
Challenges with 3DoF versus 6DoF
The human brain has evolved to have a refined set of processes and tools for identifying where we are in space, our orientation, the location of our limbs and how and if we are moving. While the brain can be fooled, it is quite sensitive to disparities between the visual data it receives and the sensory data it experiences. As you walk through the real world, the view you see is constantly changing because your position relative to your surroundings is changing. This is precisely what your brain expects because this is how the real world works. In a virtual world, or an augmented world, the virtual objects you see are all created by a computer, so it has to be told how your position has changed so that the view it shows you can be updated. Every time the user moves, the tracking system updates their position and the virtual scene is rendered to reflect their new position and orientation.
The challenge with 3DoF systems is that they can only detect rotation. They cannot detect translation. If a user takes three steps forwards, but does not rotate, a 3DoF system will not detect any movement. Since the system didn’t detect movement, the view of the VR environment being displayed in the headset will not be updated. However, the user’s brain is well aware that they have moved some distance. This conflict between what the user sees and what their body and brain experience causes discomfort for the user and can easily trigger motion sickness.
In a 6DoF tracking system, the user’s position and rotation are constantly being measured. As they move, the visuals being displayed are continually updated based on both rotation and position. Provided the system updates the visuals quickly enough, the user’s brain will not perceive any disparities between what the user sees and their actual movement. In other words, a 6DoF system allows the user to walk around in virtual reality, just as they do in the real world. This typically creates a far more immersive experience, and is far less likely to cause discomfort or nausea.
As noted earlier, early 6DoF virtual reality headsets were expensive and complex. However, developments in sensor technology and computer vision, and the steady increase in computational power, have led to the rise of “inside-out” tracking systems. This technique uses multiple small cameras embedded in a headset to track the user’s position quite accurately without the use of any external positional tracking sensors. One of the earliest headsets to use this type of 6DoF technology was Microsoft’s Hololens, an augmented reality headset. The technology was applied to Windows VR and is now ubiquitous. In addition to reducing the cost and complexity of 6DoF, virtual reality headsets such as the Quest from Meta also removed the need for a connected PC. This made 6DoF systems affordable and far easier to use, greatly reducing the barriers to the adoption of 6DoF.
A staggeringly wide range of 6DoF virtual reality experiences are now available, from dance and movement tutorials, to action and simulation games, to virtual reality tours and museums, and a wide range of serious applications such as industrial VR training. Augmented reality has fewer examples, partially because there are no broadly adopted AR headsets currently available.
But if 6DoF provides a more immersive experience, and it’s become simpler and less expensive for users, why not abandon 3DoF altogether? Why would anyone continue to use 3DoF? A significant challenge with 6DoF experiences is that they require fully realized virtual environments with 3D models. If you imagine a typical real world film set, the illusion it creates is broken as soon as you walk behind the set. 6DoF gives users the ability to “walk behind the set”, so the virtual environment has to be created with this in mind. This adds significant time, complexity and cost to the content authoring process. It is much cheaper to create content for a 3DoF system because, just like viewers watching a movie, the user is not free to walk around so various shortcuts can be used. However, recent developments in 360 video capture and improvements in 3D scanning suggest that creating robust 3D content will continue to get easier, further lowering the cost of production for 6DoF-friendly content.
Determining the best uses for 3DoF and 6DoF
As we’ve discussed here, determining whether to use 3DoF or 6DoF is not always an easy decision. While 6DoF has become a cheaper and less complex option over time, and it definitely provides a more immersive experience, it is still more expensive than 3DoF and the content creation requirements are significantly more demanding.
When users are reasonably likely to be seated, and the VR experience is intended to be quite short, 3DoF can still be an effective approach. For example, in virtual reality shopping applications delivered via the web, it can be assumed that most users will be sitting at a desk. Virtual concerts delivered as VR experiences via 360 video are also quite popular. If the viewer is sitting in a chair at home, the experience can be quite engaging.
Another interesting example is the use of 3DoF VR headsets with rollercoasters, which is being explored by several theme parks. A user riding a rollercoaster will experience both translational and rotational movement, but the amount and direction of the translation is known because the rollercoaster is on a track and the user is secured by a safety harness. This allows a near-seamless synchronization between the virtual and physical experience of the user. The result is an immersive experience almost as good as 6DoF but delivered at the much lower cost and complexity of 3DoF. Similarly, a 3DoF system used with a motion platform can deliver highly immersive results. The content developer simply has to synchronize the visuals with the predetermined movement of the motion platform.
Conversely, any experience where the user will move freely in 3D space should use 6DoF. Examples include exploring a virtual museum, chasing enemies down a virtual street, touring a famous landmark and getting hands-on training with a complex process.
Interest in augmented and virtual reality continues to grow. VR technology continues to improve, and various initiatives are working to improve interoperability and lower barriers to entry. One example is the work being done to bring responsive AR and VR experiences to the web.
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