Multi-user positioning technologies: Comparison and applications

The move from 3 to 6 Degrees of Freedom (DoF) was instrumental in revolutionising the way we interact in multi-user virtual environments.

While in a 3 DoF system users can turn their heads and look around, 6 Degrees of Freedom allows for both rotation and translation in physical space, resulting in much more natural and enriching collaborative experiences.

In this context, increasingly advanced multi-user positioning technologies are emerging to track the location and movements of users in the same physical space. But which one is the most suitable for which application?

Outside-In Tracking is based on placing sensors or cameras in the physical environment to track different devices worn by users, such as visors or controllers.

This technology stands out for its extremely high accuracy, as external sensors capture users’ movements by detecting the exact position of their hands and VR goggles. This makes Outside-In Tracking a perfect solution for environments that require rigorous tracking, such as advanced simulators or medical training.

It is also a very suitable solution when the environment is dynamic and changing, because once the room has been calibrated based on the position of the sensors, no further adjustments are necessary.

However, the initial setup of Outside-In Tracking can be complex, as it requires the strategic installation of sensors around the play area. On the other hand, it has limitations in case you want to cover large areas or a large number of simultaneous users.

Inside-Out Vision Based Tracking uses cameras and sensors embedded in Virtual Reality goggles to track the physical environment and users’ movements, eliminating the need for external sensors. This technology is common in stand-alone devices that do not require a PC connection, such as the PICO 4 Ultra or Quest 3.

Inside-Out Tracking stands out for its scalability and simplicity. Because it requires no additional hardware, it is easier to install and use, making it an ideal choice for rapid deployments.

However, its lower accuracy may be a limitation for applications that demand very detailed motion tracking. In addition, this type of technology does not allow for tracking additional elements to the VR goggles and/or controllers, such as tools or any other objects that users might need to manipulate during their immersive experience. It is also sensitive to changes in the environment, as its calibration depends on the visual characteristics of the space, so it may require readjustment if these change.

Marker-Based Tracking uses physical markers, such as QR codes or visual patterns, which are recognised by cameras or sensors installed in the VR goggles themselves. This technology makes it possible to estimate the position and orientation of users within the virtual environment based on information obtained from the detection of these markers in the real environment.

In short, it is a positioning system similar to the previous one in which visual elements are integrated to facilitate the work of the artificial vision system.

This method is effective for applications that require more accurate tracking than Inside-Out Tracking, although the level of accuracy is still lower than solutions using external sensors.

On the other hand, their reliance on physical markers can limit flexibility and freedom of movement, especially in dynamic environments or in full immersive experiences, where users or objects themselves may obstruct the markers, affecting the continuity of tracking.

Inertial Measurement Units (IMUs) are devices that combine accelerometers and gyroscopes to track the movements and orientation of users or items they carry. Ultimately, they provide valuable data on acceleration and rotation, enabling accurate representation of complex and fast movements.

This technology is especially useful when additional items or specific hardware, such as haptic gloves or motion capture suits, need to be tracked. For example, PICO offers the Motion Tracker, a wristband-like accessory with integrated super-low latency IMU sensors capable of recognising complete poses of users.

Used alone, IMUS can suffer from error accumulation over time (drift), so it is often complemented by other tracking technologies, such as cameras or optical sensors, aimed at correcting and adjusting tracking accuracy.

As we have already seen, the choice of the right technology depends on several factors such as the required accuracy, the size of the gaming area, the latency and the complexity of the environment. In addition, it should be noted that they can be used independently or together to create immersive and accurate experiences for multiple users.

For example, in a showroom of an industrial company with complex 3D models, it could be ideal to implement an Outside-In Tracking solution, while for a simulator that needs to support itinerant training actions, IMUs could be integrated with vision-based Inside-Out Tracking systems.

Selecting the right tracking technology is crucial to the success of any VR application. Understanding the strengths and limitations of each method is the first step in choosing the most appropriate solution for the specific context and requirements.