Orientation Sensor

1. Introduction

The Orientation Sensor API extends the Generic Sensor API [GENERIC-SENSOR] to provide generic information describing the device’s physical orientation in relation to a three dimensional Cartesian coordinate system.

The AbsoluteOrientationSensor class inherits from the OrientationSensor interface and describes the device’s physical orientation in relation to the Earth’s reference coordinate system.

Other subclasses describe the orientation in relation to other stationary directions, such as true north, or non stationary directions, like in relation to a devices own z-position, drifting towards its latest most stable z-position.

The data provided by the OrientationSensor subclasses are similar to data from DeviceOrientationEvent, but the Orientation Sensor API has the following significant differences:

1. The Orientation Sensor API represents orientation data in WebGL-compatible formats (quaternion, rotation matrix).

2. The Orientation Sensor API satisfies stricter latency requirements.

3. Unlike DeviceOrientationEvent, the OrientationSensor subclasses explicitly define which low-level motion sensors are used to obtain the orientation data, thus obviating possible interoperability issues.

4. Instances of OrientationSensor subclasses are configurable via SensorOptions constructor parameter.

2. Use Cases and Requirements

The use cases and requirements are discussed in the Motion Sensors Explainer document.

3. Examples

const sensor = new AbsoluteOrientationSensor();
const mat4 = new Float32Array(16);
sensor.start();
sensor.onerror = event => console.log(event.error.name, event.error.message);
sensor.onchange = () => {
  sensor.populateMatrix(mat4);
};

const sensor = new AbsoluteOrientationSensor({ frequency: 60 });
const mat4 = new Float32Array(16);
sensor.start();
sensor.onerror = event => console.log(event.error.name, event.error.message);
function draw(timestamp) {
  window.requestAnimationFrame(draw);
  try {
    sensor.populateMatrix(mat4);
  } catch(e) {
    // mat4 has not been updated.
  }
  // Drawing...
}
window.requestAnimationFrame(draw);

4. Security and Privacy Considerations

There are no specific security and privacy considerations beyond those described in the Generic Sensor API [GENERIC-SENSOR].

5. Model

The OrientationSensor class extends the Sensor class and provides generic interface representing device orientation data.

To access the orientation sensor’s latest reading, the user agent must invoke request sensor access abstract operation for each of the low-level sensors used by the concrete orientation sensor. The table below describes mapping between concrete orientation sensors and permission tokens defined by low-level sensors.

A latest reading per sensor of orientation type includes an entry whose key is "quaternion" and whose value contains a four element list. The elements of the list are equal to components of a unit quaternion [QUATERNIONS] [V_x * sin(θ/2), V_y * sin(θ/2), V_z * sin(θ/2), cos(θ/2)] where V is the unit vector (whole elements are V_x, V_y, and V_z) representing the axis of rotation, and θ is the rotation angle about the axis defined by the unit vector V.

Note: The quaternion components are arranged in the list as [q₁, q₂, q₃, q₀] [QUATERNIONS], i.e. the components representing the vector part of the quaternion go first and the scalar part component which is equal to cos(θ/2) goes after. This order is used for better compatibility with the most of the existing WebGL frameworks, however other libraries could use a different order when exposing quaternion as an array, e.g. [q₀, q₁, q₂, q₃].

The AbsoluteOrientationSensor class is a subclass of OrientationSensor which represents the Absolute Orientation Sensor.

The absolute orientation sensor is a high-level sensor which is created through sensor-fusion of the low-level motion sensors:

• accelerometer that measures acceleration,

• gyroscope that measures angular velocity, and

• magnetometer that measures magnetic field.

Note: Corresponding low-level sensors are defined in [ACCELEROMETER], [GYROSCOPE], and [MAGNETOMETER]. Regardless, the fusion is platform specific and can happen in software or hardware, i.e. on a sensor hub.

For the absolute orientation sensor the value of latest reading["quaternion"] represents the rotation of a device hosting motion sensors in relation to the Earth’s reference coordinate system defined as a three dimensional Cartesian coordinate system (x, y, z), where:

• x-axis is a vector product of y.z that is tangential to the ground and points east,

• y-axis is tangential to the ground and points towards magnetic north, and

• z-axis points towards the sky and is perpendicular to the plane made up of x and y axes.

The device’s local coordinate system is the same as defined by low-level motion sensors.

Note: Figure below represents the case where device’s local coordinate system and the Earth’s reference coordinate system are aligned, therefore, orientation sensor’s latest reading would represent 0 (rad) [SI] rotation about each axis.

6. API

6.1. The OrientationSensor Interface

typedef (Float32Array or Float64Array or DOMMatrix) RotationMatrixType;
interface OrientationSensor : Sensor {
  readonly attribute FrozenArray<double>? quaternion;
  void populateMatrix(RotationMatrixType targetMatrix);
};

6.1.1. OrientationSensor.quaternion

Returns a four-element FrozenArray whose elements contain the components of the unit quaternion representing the device orientation. In other words, this attribute returns latest reading["quaternion"].

6.1.2. OrientationSensor.populateMatrix()

The populateMatrix() method populates the given object with rotation matrix which is converted from the value of latest reading["quaternion"] [QUATCONV], as shown below:

where:

• W = cos(θ/2)

• X = V_x * sin(θ/2)

• Y = V_y * sin(θ/2)

• Z = V_z * sin(θ/2)

The rotation matrix is flattened in targetMatrix object according to the column-major order, as described in populate rotation matrix algorighm.

6.2. The AbsoluteOrientationSensor Interface

[Constructor(optional SensorOptions sensorOptions)]
interface AbsoluteOrientationSensor : OrientationSensor {
};

To Construct an AbsoluteOrientationSensor Object the user agent must invoke the construct a Sensor object abstract operation.

7. Acknowledgements

Tobie Langel for the work on Generic Sensor API.

8. Conformance

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

A conformant user agent must implement all the requirements listed in this specification that are applicable to user agents.

The IDL fragments in this specification must be interpreted as required for conforming IDL fragments, as described in the Web IDL specification. [WEBIDL]