useBodyState

Track body position, rotation, and velocities

Returns refs tracking a body’s position, quaternion, linear velocity, and angular velocity. Updated every physics frame without triggering re-renders.

Signature

useBodyState(name: string): {
  position: React.RefObject<THREE.Vector3>;
  quaternion: React.RefObject<THREE.Quaternion>;
  linearVelocity: React.RefObject<THREE.Vector3>;
  angularVelocity: React.RefObject<THREE.Vector3>;
}

Usage

import { useBodyState } from "mujoco-react";
import { useFrame } from "@react-three/fiber";

function BodyTracker() {
  const { position, quaternion } = useBodyState("block_1");

  const meshRef = useRef<THREE.Mesh>(null);

  useFrame(() => {
    // Mirror the body's position on a custom mesh
    meshRef.current?.position.copy(position.current);
    meshRef.current?.quaternion.copy(quaternion.current);
  });

  return <mesh ref={meshRef}><sphereGeometry args={[0.01]} /></mesh>;
}

Velocity Tracking

function SpeedDisplay() {
  const { linearVelocity } = useBodyState("ball");
  const textRef = useRef<any>(null);

  useFrame(() => {
    const speed = linearVelocity.current.length();
    if (textRef.current) textRef.current.text = `${speed.toFixed(2)} m/s`;
  });

  return <Text ref={textRef} position={[0, 0.5, 0]} />;
}

Return Value

Field	Type	Description
position	RefObject<THREE.Vector3>	World position from data.xpos
quaternion	RefObject<THREE.Quaternion>	World rotation from data.xquat (converted to Three.js order)
linearVelocity	RefObject<THREE.Vector3>	Linear velocity from data.cvel
angularVelocity	RefObject<THREE.Vector3>	Angular velocity from data.cvel

Notes

• Returns are refs, not state — read .current in useFrame or event handlers
• No re-renders on updates — this is intentional for 60fps performance
• The body is looked up by name once; if the name doesn’t exist, values stay at zero
• cvel layout is [angular(3), linear(3)] per body — the hook reorders for you