> ## Documentation Index
> Fetch the complete documentation index at: https://docs.mentraglass.com/llms.txt
> Use this file to discover all available pages before exploring further.

# IMU

> Head-position events from the glasses inertial measurement unit.

`session.imu` reports which way the wearer is looking. The glasses' inertial
measurement unit resolves head pose into two states, `"up"` and `"down"`, and
fires an event each time that state changes. Use it to wake content when someone
glances up, or to dismiss it when they look back down.

This module needs the `IMU` hardware requirement in your
[manifest](/app-devs/core-concepts/miniapp-manifest). Mark it `REQUIRED` and
glasses without an IMU won't list your miniapp; mark it `OPTIONAL` and they still
run it without head-position events. There is no runtime permission to request.

## Head position

```typescript src/background/index.ts theme={null}
const off = session.imu.onHeadPosition(({ position }) => {
  if (position === "up") session.display.render([{type: "text", id: "msg", box: {x: 0, y: 0, w: 576, h: 288}, text: "Welcome back"}]);
  else session.display.render([]);
});
```

Each `HeadPositionData` has:

| Field      | Type             | Notes                                                          |
| ---------- | ---------------- | -------------------------------------------------------------- |
| `position` | `"up" \| "down"` | The new head pose. Fires on each transition, not continuously. |

## Raw accelerometer

`onAccel` streams raw accelerometer readings, `{x, y, z}` in g plus a timestamp.
This is G2-only today. On glasses without an exposed IMU stream the subscription
succeeds and no events arrive.

```typescript src/background/index.ts theme={null}
const off = session.imu.onAccel(({ x, y, z, timestamp }) => {
  // x, y, z in g; timestamp is Unix ms.
});
```

Each `AccelData` has:

| Field         | Type     | Notes                                         |
| ------------- | -------- | --------------------------------------------- |
| `x`, `y`, `z` | `number` | Accelerometer axes in g (gravity-normalized). |
| `timestamp`   | `number` | Unix ms timestamp of the reading.             |

Subscribing turns the sensor on and unsubscribing turns it off, so most callers
never touch it directly. `session.imu.setEnabled(true)` drives the sensor by hand
when you want to, for instance, a diagnostic toggle. It's a no-op on glasses
without an exposed IMU stream.

Gyroscope, magnetometer, and fused orientation aren't exposed yet.

## Cleaning up

Both methods return an unsubscribe function:

```typescript theme={null}
const off = session.imu.onHeadPosition(handler);
off(); // stop listening
```

Subscriptions are ref-counted, so the SDK only talks to the phone on the first
subscribe and last unsubscribe of a stream.