Being in a virtual reality simulation is like entering another world, but true immersion comes when you are able to move within this world. Here is where we need to understand basics of VR tracking, followed by the concept of Degrees of Freedom also known as DOF, which usually comes in 3DOF and 6DOf flavors.
What is the difference?
All objects around you that can move in 3D space move in 6 ways: 3 directional axes and 3 rotational axes. Each of these axes is a DoF. We live in a 3D world and interact it in all 6 DoF.
When it comes to VR, DoF is used to describe axes that are being tracked. Tracking comes from the ability to monitor a change of angle or distance on the axes by using hardware. When we refer to 3 DoF we mean orientation tracking.
This means the 3 axes which an object can be rotated about being tracked. This exists in mobile and standalone VR headsets like the new Oculus Go. If you turn your head while wearing a headset, it is able to track the angle change of this axes and allows you to look around in the environment.
The world within VR
Alongside having a wide field of view, virtual reality headsets are distinct from regular 3D displays in that they are tracked. PC VR, console VR and now even some standalone headsets even have positional tracking so you can lean, duck, and even walk around in VR.
Without VR motion tracking, you would find yourself somewhat restricted in the virtual world, unable to look around, move, and explore. The best case scenario would be to use a gamepad in order to control movement within the virtual environment – but that would only serve to shatter your feeling of immersion.
Being able to engage and interact with the virtual world the moment you pull on your VR headset, without any reminders of the real world, is crucial to the creation of an immersive experience.
Mobile VR headsets such as the Oculus Go, Samsung Gear VR, and Google Daydream View only have rotational tracking (3DoF). You are able to look up or down, to either side or tilt your head. But if you try to lean or actually move your head’s position, this is not tracked. The entire virtual world will move with you.
3DoF controllers are similar, rotation-only. They essentially act as laser selection pointers. This can be acceptable for seated content, but it doesn’t allow you to move around the virtual world physically, or to interact with it with your hands directly, 3DOF Tracking is always done with microscopic electromechanical gyroscopes.
But various vendors use varying technologies to enable positional tracking (6DoF). While there may be a common industry standard someday, none exists yet. Companies have different interpretation about which techniques are better used. The considerations for the various tracking systems each balance cost, ease of setup, tracking volume, controller tracking range, and modularity.
Given the above, you would think that all VR headsets would use 6DOF position tracking but that’s not the case, this is mainly due to cost. The hardware and software to have 6 DoF tracking are far more complex than 3 DoF tracking.
Most mobile VR headsets use the built it inertial measurement unit (IMU) sensor to read is orientation. This is the same sensor that turns your screen landscape when watching a video on your mobile. Whereas, 6 DoF systems need infrared or optical tracking systems to track specific points on their headset or controllers in 3D space.
The extra hardware increases the price but with greater functionality, and obviously flexibility.
VR Tracking with Optics
Optical methods of motion tracking usually use cameras of one sort or another. The person being tracked has optical markers. Usually dots of highly reflective material on certain known points of their body or on the equipment such as the HMD or handheld controllers.
In professional contexts where motion is captured for use in animation an actor’s body may be covered in such markers, but commercial systems designed for personal use in virtual reality may use only a few strategic markers or even no markers at all.
When a camera installation capable of calculating depth sees a marker it can map it to 3D space. For example, two cameras at known angles that both see the same dot allow for this mathematical calculation.
One issue that does arrive with this method is that of maker swap where two cameras see different dots but think they are the same one. Obviously, this provides incorrect tracking data and inaccurate motion tracking.
Reflective markers are known as passive markers as they reflect light, but there is another type of marker known as an active marker. These are computer controlled LEDs that allow for more accuracy and various workarounds for the weaknesses of passive marker methods. LEDs may be different colors or flash rapidly in sync with the capture system.
Although active marker systems perform better than passive ones, for the subject there are drawbacks. The user has to wear a power supply or be tethered to the system somehow, which is clearly an encumbrance.
The Leap Motion is another example of a marker-free tracking system, but rather than full body tracking the Leap Motion creates high-resolution real-time scans of objects in close proximity to it. In virtual reality contexts, the device can be attached to the front of an HMD and provide precisely digitized versions of the user’s hands, which then allows for natural interaction.
Full body tracking that rivals professional systems in terms of movement freedom and accuracy for the consumer are not yet with us. At the moment most virtual reality for consumers is a “sit-down experience”.
Products such as the Oculus Rift are designed for this sort of experience at the moment. Oculus itself recommends that you don’t stand up and wander around while wearing their HMD since you’ll likely trip over something and hurt yourself.
Non-Optical VR Tracking
The virtual reality equipment that you wear on your bodies such as an HMD or the controllers you hold such as the PlayStation Move, Oculus Touch or SteamVR controller contains micro-electromechanical sensors such as accelerometers, gyroscopes, and magnetometers.
The gyroscopes measure 360-degree rotation, accelerometers measure movement along the XYZ axes and magnetometers can determine orientation towards a magnetic field. Which means they can tell which way magnetic North is for example.
Thanks to developments in the automobile, aeronautical and computer industries these devices have become cheap, accurate and positively tiny compared to the full sized devices that came before them.
It’s especially the success and growth of the mobile device market that has driven the modern development of these sensors and it isn’t an exaggeration to say that the new generation of consumer virtual reality systems owes their existence to technologies developed for smartphone and tablets.
Combining these three sensors can provide a device with low-latency, precision motion data. This can be combined with optical methods such as infrared tracking or passive reflector tracking for truly robust motion tracking, but for untethered, self-contained systems these sensors can do an adequate job by themselves. As evidenced by various mobile virtual reality products.
Other non-optical tracking technologies are more unique. Direct electro-mechanical sensing of body movement is one such technology. Some modern version of the virtual reality glove and other haptic products also double as direct electro-mechanical motion trackers (see our article on haptics).
When you flex your fingers sensors in the glove are activated and convert that movement into electrical signals for motion tracking. Examples include the GloveOne and the Salto.
Another interesting approach, which is not virtual reality-specific, but has potential in this area is the Myo armband. The Myo interprets electrical impulses from the muscles in the forearm allowing for gesture-based controls of computer software.
What makes the device special is how unobtrusive it is since it is designed to be worn all day. Perhaps in the future, such direct sensing could be precise enough to replace other tracking methods and it is a technology worth keeping an eye on.
As technology advances we will see better means on tracking within VR and maybe someday we can have a realistic full immersion that rivals the real world.