Lighthouse

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Lighthouse is a laser-based inside-out positional tracking system developed by Valve for SteamVR and HTC Vive. It accurately tracks the position and orientation of the user's head-mounted Display and controllers in real time. Lighthouse enables the users to move anywhere and re-orient themselves in any position within the range of the SteamVR Base Stations. It is a key technology that enables SteamVR to create the first holodeck or full-room experience in Virtual Reality.

Valve plans to make Lighthouse freely available to all hardware manufactures.

How Does It Work?

Central to the Lighthouse technology are the Base Stations. These Base Stations are small rectangular objects placed in the tracking area. They serve as reference points for any positionally tracked devices such as the HMDs and controllers. Base Stations perform this function by constantly flooding the room with a non-visible light. The receptors on the tracked devices would intercept the light and figure out where they are in relation to the Base Stations. Multiple Base Stations (2 for SteamVR) allow the tracked devices to figure out where they are in the 3D space.

Each Base Station contains an IR beacon called Sync Blinker and 2 laser emitters that spin rapidly. 60 times per second, the Sync Blinker would emit a synchronization pulse and 1 of the 2 spinning lasers would sweep a beam across the room. The receptors, HMDs and controllers, are covered with photosensors that recognizes the synchronization pulse and the laser beams. When it detects a synchronization pulse, the receptor starts to count til one of its photosensors is hit by the laser beam. Lighthouse calculates When the photosensor is hit by the laser and Where that photosensor is located to find the exact position of the receptor in relation to the Base Station. When there are 2 Base Stations, the position and the orientation of the receptors in the 3D space of the room is established.[1]

Base Stations are vulnerable to occlusion. They require line of sight to the tracked objects. Base Stations are designed to be scalable. 2 Base Stations are placed in opposite sides of the room to minimize this problem. More Base Station can be placed to increase the tracking range.

Detailed Explanation

HTC Vive uses the IMUs as the primary position tracking system. It responds extremely quickly and updates at several hundred Hz (1000Hz sampling, 500Hz reporting). However, IMUs drift due to double-integration of error. The drift is on the order of meters per second. So what Lighthouse do is squelch that error 60 times per second (both have a 60Hz global position update rate) using their laserl sensors to provide an absolute position reference.

For Lighthouse high-speed position tracking performance is down ENTIRELY to IMU performance. It wouldn't be possible at all without another absolute reference system (optical, magnetic or otherwise) but it's the IMU that's doing the grunt-work.

However, the IMU is even more important for the Lighthouse than the Constellation. Lighthouse is a scanning system: not only do you not know the positions of markers at the same point in time, you don't even get the X and Y positions at the same point in time: there is a 4ms delay (4 scans per 16ms) between each laser strike for each sensor. If a controller is moving at a modest 1ms-1 , then between laser strikes it's moved 4mm! While throwing a controller like a cricket ball is extremely ill advised, a 150mph throw (~150mph hand speed) is 45ms-1, or 180mm between scans. Using the IMU data allows you to update parts of the position (X or Y coords, or polar spherical coords relative to the basestation, depending on how Valve are doing their math) independently of each other.

Tracking volume

See also: Tracking volume

HTC Vive

See also: HTC Vive

Htc vive tracking volume1.png

120°H x 120°V (>21 feet range)
Room size: 15 feet by 15 feet

Non-VR Applications of Lighthouse

Lighthouse could be used as a general purpose indoor positional tracking system. It be used in robotics, drones and smartphones. A receiver can be placed in these machines, allowing them to be accurately tracked with 6DOF.

History and Miscellaneous

The chief architect of the Lighthouse tracking system is Alan Yates.

One of the predecessors to Lighthouse is to plaster the room with QR codes.

References

  1. https://www.youtube.com/watch?v=xrsUMEbLtOs