Pose Tracking Methods: Outside-in VS Inside-out Tracking in VR

Daisy Dai Updated on May 11, 2024

In the context of virtual reality (VR), pose tracking refers to the process of accurately capturing the movement and position of the user's head, hands, and body to create a realistic and immersive experience. There are two primary methods of tracking used in VR: outside-in tracking and inside-out tracking. Let's explore each of them.

What’s the difference between outside-in and inside-out tracking?

The main difference between outside-in and inside-out tracking lies in the method used to capture and interpret the user's movements in virtual reality (VR).

In short, from their title and the image above, it is easy to understand their meaning. Outside-in tracking is the external sensors (like base stations) place around the VR headset to track the movement of the users. Inside-out tracking is the embedded sensors (like cameras) place on the headset or controllers to track the movement of the users.

And here are the key distinctions:

Outside-in Tracking	Inside-out Tracking
External sensors Sensor reference points Accuracy and precision Potential for reduced occlusion Larger tracking areas	Embedded sensors Environment analysis Portability and ease of setup Potential tracking limitations Cost-effective solution

Outside-in Tracking

In terms of outside-in tracking, it is obvious that outside-in tracking relies on external sensors or cameras placed in the environment. These sensors track the movement and position of markers or sensors attached to the user or their VR devices. Second, the external sensors provide a reference point for calculating the user's position and movement accurately. By triangulating the data from multiple sensors, the system can determine the user's position and orientation in 3D space. Third, outside-in tracking systems often provide high accuracy and precision due to the use of dedicated sensors and specialized tracking algorithms. Fourth, since the sensors are external to the user, they are less susceptible to occlusion issues where the user's body or objects block the line of sight between the sensors and the tracked devices. Last, with multiple sensors placed around the environment, outside-in tracking can cover larger physical spaces, enabling room scale VR.

Inside-out Tracking

For the inside-out tracking, it is known that inside-out tracking uses sensors embedded within the VR headset or controllers. These sensors, such as cameras or infrared sensors, capture the environment around the user. Second, inside-out tracking algorithms analyze the captured data to determine the user's position and movement relative to their surroundings. Computer vision techniques, simultaneous localization, and mapping (SLAM), or other methods are employed to interpret the sensor data. Third, inside-out tracking systems are typically more portable and easier to set up since they eliminate the need for external sensors. Users can start using inside-out tracking systems by simply putting on the VR headset. Fourth, inside-out tracking may have limitations in tracking the user's movements outside the field of view of the embedded sensors. Occlusion of the hands or controllers can also temporarily disrupt tracking accuracy. Last, inside-out tracking tends to be more cost-effective compared to outside-in tracking since it doesn't require additional sensor infrastructure.

It's important to note that the specific capabilities and limitations of outside-in and inside-out tracking can vary across different VR systems and generations. Technology advancements have led to continuous improvements in both tracking methods, offering users more immersive and accurate VR experiences.

How does VR tracking work?

Outside-in Tracking
Sensor Setup	In outside-in tracking, the VR system employs external sensors, such as cameras or infrared sensors, strategically positioned in the environment. These sensors serve as reference points and capture the movements and positions of markers or sensors attached to the user or their VR devices. For example, you can check the Pimax 8KX Setup to find out how to get the right position of the base stations and the whole setup issue of an outside-tracking VR headset.
Marker Detection	The external sensors detect and track the markers or sensors attached to the user's body or VR devices. These markers can be reflective markers, LED markers, or other identifiable features that the sensors can recognize.
Data Processing	The tracking data captured by the external sensors is sent to a computer or dedicated tracking hardware for processing. Sophisticated algorithms and triangulation techniques are applied to analyze the data and determine the precise position and orientation of the tracked markers in 3D space.
Pose Estimation	Based on the tracked markers' positions and the known geometry of the VR setup, the system can calculate the user's position and movement relative to the reference points provided by the external sensors. This information is used to update the virtual camera's position and orientation, allowing for realistic rendering of the virtual environment.
Feedback and Interaction	The accurately tracked user movements enable realistic interactions within the VR environment. The system can detect gestures, hand movements, or controller positions, allowing the user to interact with virtual objects or manipulate the virtual space.

Inside-out Tracking
Embedded Sensors	Inside-out tracking relies on sensors embedded within the VR headset or controllers. These sensors can include cameras, infrared sensors, or other specialized tracking technologies. They capture the user's environment and movements directly from the VR devices.
Environmental Analysis	The captured sensor data is processed using computer vision algorithms, simultaneous localization and mapping (SLAM) techniques, or other methods. These algorithms analyze the visual or positional information from the embedded sensors to interpret the user's movements and environment.
Position Estimation	By analyzing the visual input or using SLAM algorithms, the system determines the user's position and movement within the virtual environment. The tracked data is used to update the virtual camera's position and orientation, providing an accurate representation of the user's movements.
Tracking Limitations	Inside-out tracking may have limitations in tracking movements outside the field of view of the embedded sensors. Occlusion of the hands or controllers can temporarily disrupt tracking accuracy since the sensors rely on visual input.
Portability and Ease of Setup	Inside-out tracking offers the advantage of portability and ease of setup. Users can simply put on the VR headset or pick up the controllers, and the tracking system starts working without the need for external sensors or calibration.

Examples for Reference

Headset	Tracking Method	Resolution	FOV	Price ($)
Pimax Crystal	Inside-out & Outside-in	2880 x2880 pixels per eye	140°	$1599
Pimax 8KX	Outside-in	3840 X 2160 pixels per eye	200°	$1,387
Oculus Rift S	Inside-out	1280 x 1440 per eye	115°	$399
HTC Vive Pro	Outside-in	1440 x 1600 per eye	110°	$1,399
Valve Index	Outside-in	1440 x 1600 per eye	130°	$999
PlayStation VR	Outside-in	960 x 1080 per eye	100°	Start from $399
Oculus Quest 2	Inside-out	1832 x 1920 per eye	100°	Start from $299
HP Reverb G2	Inside-out	2160 x 2160 per eye	114°	$599