Body Tracking

The Body component allows you to stream the full 3D pose of the human body to the server in real time using the WebXR Body Tracking API.

To use this mixed reality (XR) feature, you must run vuer behind an SSL proxy (WebXR requires HTTPS). We recommend using either:

• ngrok (paid, stable performance) — ngrok documentation

• localtunnel (free, open source) — localtunnel documentation

Warning

Please read the relevant documentation of ngrok or localtunnel before proceeding.

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Body Tracking API

You can access the full body pose by listening to the BODY_TRACKING_MOVE event. The event will send all tracked joint positions and orientations as transformation matrices, indexed by XRBodyJoint.

Warning

You must set the stream option to true to start streaming body pose data, otherwise the event will not be triggered. This is to avoid unnecessary uplink congestion from the client.

Warning

The WebXR Body Tracking API is still experimental and is currently only supported in the Quest Browser. You need to enable the WebXR Experiments flag for body tracking:

• Open chrome://flags in the Quest Browser.

• Search for “webxr” in the search bar.

• Enable the relevant Experiments flags for body-tracking.

Here’s what Body Tracking looks like on devices that support it (e.g., Meta Quest3).

Example

from vuer import Vuer, VuerSession
from vuer.schemas import Body
from asyncio import sleep

app = Vuer()


@app.add_handler("BODY_TRACKING_MOVE")
async def on_body_move(event, session):
    """
    Handle incoming BODY_TRACKING_MOVE events from the client.
    event.value should be a BodiesData dictionary:
      { jointName: { matrix: Float32Array-like, ... }, ... }
    """
    print(f"BODY_TRACKING_MOVE: key={event.key} ts={getattr(event, 'ts', None)}")

    # Example: print only the first joint to avoid large output
    if event.value:
        first_joint, first_data = next(iter(event.value.items()))
        print(
            first_joint,
            "matrix_len=",
            len(first_data.get("matrix", [])) if first_data else None,
        )


@app.spawn(start=True)
async def main(session: VuerSession):
    """
    Add the Body element to the scene and start streaming body tracking data.
    """
    session.upsert(
        Body(
            key="body_tracking",  # Optional unique identifier (default: "body_tracking")
            stream=True,  # Must be True to start streaming data
            fps=30,  # Send data at 30 frames per second
            hideIndicate=False,  # Hide joint indicators in the scene but still stream data
            showFrame=False,  # Display coordinate frames at each joint
            frameScale=0.02,  # Scale of the coordinate frames or markers
        ),
        to="children",
    )

    # Keep the session alive
    while True:
        await sleep(1)

Returned Data

The BODY_TRACKING_MOVE event sends a BodiesData object containing all joints and their transform matrices:

export type BodiesData = {
  [jointName: string]: {
    matrix: Float32Array; // 4x4 matrix in column-major order
  };
};

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Matrix Format

All transforms are 4×4 matrices stored in a 16-element Float32Array, column-major order:

⌈  a0  a4  a8  a12 ⌉
|  a1  a5  a9  a13 |
|  a2  a6  a10 a14 |
⌊  a3  a7  a11 a15 ⌋

Refer to MDN’s XR Rigid Transform documentation for details.

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Body Joints Order

The BODY_JOINT_KEYS list follows the WebXR Body Tracking Joints Specification and includes head, torso, arms, hands, and legs.

Example:

Joint Name	Example Index
hips	0
spine-lower	1
head	6
left-hand-thumb-tip	23
right-foot-ball	79

Full list: see BODY_JOINT_KEYS in the client code.

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Notes

WebXR Specification Differences XRHandJoint (hand joints) and XRBodyJoint (body joints) have different origin points and axis orientations.

• For hand joints, the -Y axis is typically perpendicular to the palm, and the -Z axis runs along the bone toward the fingertip.

• For body joints (e.g., hips, spine, shoulder), the -Y axis is perpendicular to the skin surface, and the -Z axis runs along the bone away from the joint. In particular, the local coordinate systems of wrist and hips have explicit but different definitions in the WebXR specification.