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LPM

Overview

The LPM device receives live tracking data from an LPM system, so that a 3D object, a followspot, or any other value can follow a performer or a moving stage element in real time.

LPM stands for Local Position Measurement. It is a radar-based tracking system, originally developed by Inmotio and now part of the Riedel Communications portfolio, following an acquisition announced in December 2022. Each tracked person or object carries a transponder — in its original sports use, sewn into a vest — and a network of base stations installed around the venue measures the transponder's position, in two or three dimensions, in real time. It is used mostly for sports performance analysis, which is where the vocabulary of the device comes from.

An LPM system broadcasts one small UDP packet per tracked object — a tracker — and Onlyview simply listens for those packets. There is no connection to establish: as soon as packets arrive, the trackers they describe appear in the device.

The device is active on every machine of the show — Producer and Media Servers alike — and each machine reads the LPM broadcast directly rather than relaying the data internally. This keeps the tracking latency as low as possible, but it does mean that the broadcast has to reach all of them.

Each tracker is identified by a numeric ID, carried in the packet and encoded on 14 bits, so IDs run from 0 to 16383. Packets whose two error bits are set are discarded, so a tracker reporting an error is ignored rather than producing a wrong position.

Info

Onlyview reads only the position of each tracker, and its speed, which is used for extrapolation (see below). The other quantities carried by an LPM packet — quality, telemetry, acceleration, raw position, heading, roll and pitch — are not exposed.

Positions arrive in millimetres and are converted to metres, with the axes remapped to Onlyview's Y-up world. Depending on the packet format used by your LPM system, the tracker's speed is either read from the packet or, for the shorter format, computed by Onlyview from successive positions. Either way, the speed is used to extrapolate the position to the current frame, which smooths out the gap between two packets.

A tracker whose packets stop arriving is considered lost after one second, and its position becomes invalid.

Setup

  • LPM IP: The address Onlyview binds its listening socket to — in practice, the address of the network interface on which the LPM data arrives.
  • LPM port: The UDP port the LPM data arrives on.

Those are the only two settings, and they are read when the show starts.

Device Widget

The LPM device can be placed on a UserScreen. It shows the device name, the configured IP address, and one small square per tracker seen so far, labelled with its ID. The square tells you whether that tracker is currently being tracked or has been lost — a quick way to check, during a show, that the trackers are still reporting.

ActionGraph Nodes

LPM Tracker

Outputs the current position of one tracker. Available under Devices/LPM in the node palette.

Properties:

  • Device: The LPM device to read from.
  • Tracker ID: The ID of the tracker to follow.

Output Slots:

  • Position: Vec3 — the position of the tracker, in metres, in Onlyview's coordinate system.

The output is invalid when no device is selected, when no tracker with that ID has been received, or when the tracker has timed out. Downstream nodes therefore stop updating rather than jumping to a wrong position when tracking is lost.

Use one node per tracker you want to follow. The node has no input slots and no events.

Info

Tracking data usually arrives ahead of the video it has to match. To realign it with the rest of the show, insert a Delay node — the Delay (Vec3) variant delays a position.

See also

  • The Tracking study preview mode helps you design an optical camera rig, for tracking systems that need one.
  • OptiTrack — another tracking-data source, which also provides orientation.