explainer.md

WebXR Device API - Body Tracking

This document describes a design giving developers access to body-tracking XR systems, building on top of the WebXR device API

Use cases and scope

This API primarily exposes the joints of a person's body. It can be used to render an avatar in VR scenarios. It does not provide access to a full hbodynd mesh.

Accessing this API

This API will only be accessible if a body-tracking" XR feature is requested.

This API presents itself as an additional field on XRFrame, body. The body attribute will be non-null if the XR device supports hand tracking and the feature has been requested.

navigator.xr.requestSession({optionalFeatures: ["body-tracking"]}).then(...);

function renderFrame(session, frame) {
   // ...

      if (frame.body.hand) {
         // render a body
      }
   }
}

Joints

Each body is made up many bones, connected by joints. We name them with their connected bone, for example index-finger-phalanx-distal is the joint closer to the wrist connected to the distal phalanx bone of the index finger. The *-tip "joints" locate the tips of the fingers. The wrist joint is located at the composite joint between the wrist and forearm.

The joint spaces can be accessed via XRBody.get(), for example to access the leftmiddle knuckle joint one would use:

let joint = inputSource.hand.get("left-middle-finger-phalanx-distal");

All devices which support hand tracking will support or emulate all joints, so this method will always return a valid object as long as it is supplied with a valid joint name. If a joint is supported but not currently being tracked, the getter will still produce the XRBodySpace, but it will return null when run through getPose (etc).

Each joint space is an XRSpace, with its -Y direction pointing perpendicular to the skin, outwards from the palm, and -Z direction pointing along their associated bone, away from the wrist. This space will return null poses when the joint loses tracking.

For *-tip joints where there is no associated bone, the -Z direction is the same as that for the associated distal joint, i.e. the direction is along that of the previous bone.