useAfterPhysicsStep

Inject logic after mj_step each frame

Register a callback that runs after mj_step every physics frame. Use this for reading simulation results, computing rewards, or logging.

Signature

useAfterPhysicsStep(
  callback: (input: { model: MujocoModel; data: MujocoData }) => void
): void

Usage

Reading State

useAfterPhysicsStep(({ data }) => {
  const pos = data.xpos.subarray(3, 6); // Body 1 position
  console.log("Body position:", pos[0], pos[1], pos[2]);
});

Computing Rewards (RL)

useAfterPhysicsStep(({ data }) => {
  const targetPos = [0.5, 0, 0.3];
  const eePos = data.xpos.subarray(eeSiteId * 3, eeSiteId * 3 + 3);
  const dist = Math.sqrt(
    (eePos[0] - targetPos[0]) ** 2 +
    (eePos[1] - targetPos[1]) ** 2 +
    (eePos[2] - targetPos[2]) ** 2
  );
  rewardRef.current = -dist;
});

Logging Sensor Data

useAfterPhysicsStep(({ model, data }) => {
  if (data.time % 0.1 < model.opt.timestep) {
    logRef.current.push({
      time: data.time,
      qpos: Array.from(data.qpos),
      sensordata: Array.from(data.sensordata),
    });
  }
});

Execution Order

1. Provider zeros qfrc_applied
2. useBeforePhysicsStep callbacks
3. IK solving (if enabled)
4. mj_step
5. → Your useAfterPhysicsStep callbacks run here ←

Notes

• After-step runs at physics rate, not render rate
• Results are available immediately for rendering in the same frame
• Multiple after-step hooks compose — all run each frame
• Write results to refs (not React state) to avoid re-renders at 60fps