Accelerometer

Status of this document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at https://www.w3.org/TR/.

This document was published by the Devices and Sensors Working Group as a Candidate Recommendation. This document is intended to become a W3C Recommendation. This document will remain a Candidate Recommendation at least until 8 January 2020 in order to ensure the opportunity for wide review.

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The entrance criteria for this document to enter the Proposed Recommendation stage is to have a minimum of two independent and interoperable user agents that implementation all the features of this specification, which will be determined by passing the user agent tests defined in the test suite developed by the Working Group. The Working Group will prepare an implementation report to track progress.

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This document is governed by the 1 March 2019 W3C Process Document.

1. Introduction

The Accelerometer, LinearAccelerationSensor and GravitySensor APIs extends the Generic Sensor API [GENERIC-SENSOR] interface to provide information about acceleration applied to device’s X, Y and Z axis in local coordinate system defined by device.

2. Examples

let sensor = new Accelerometer();
sensor.start();

sensor.onreading = () => {
    console.log("Acceleration along X-axis: " + sensor.x);
    console.log("Acceleration along Y-axis: " + sensor.y);
    console.log("Acceleration along Z-axis: " + sensor.z);
}

sensor.onerror = event => console.log(event.error.name, event.error.message);

The following example shows how to use gravity sensor that provides readings in the screen coordinate system. The snippet will print message to the console when the dom screen is perpendicular to the ground and bottom of the rendered web page is pointing downwards.

let sensor = new GravitySensor({frequency: 5, referenceFrame: "screen"});

sensor.onreading = () => {
  if (sensor.y >= 9.8) {
    console.log("Web page is perpendicular to the ground.");
  }
}

sensor.start();

The following example detects shake gesture along x axis of the device, regardless of the orientation of the dom screen.

const shakeThreshold = 25;

let sensor = new LinearAccelerationSensor({frequency: 60});

sensor.addEventListener('reading', () => {
  if (sensor.x > shakeThreshold) {
    console.log("Shake detected.");
  }
});

sensor.start();

3. Use Cases and Requirements

The use cases and requirements are listed in the Motion Sensors Explainer and Sensor use cases documents.

4. Security and Privacy Considerations

Sensor readings provided by inertial sensors, such as accelerometer, could be used by adversaries to exploit various security threats, for example, keylogging, location tracking, fingerprinting and user identifying.

Research papers published by security community, for instance, [KEYSTROKEDEFENSE], indicate that by throttling the frequency, risks of successful attacks are not fully eliminated, while throttling may greatly affect usefulness of a web application with legitimate reasons to use the sensors.

The [TOUCHSIGNATURES] and [ACCESSORY] research papers propose that implementations can provide visual indication when inertial sensors are in use and/or require explicit user consent to access sensor readings. These mitigation strategies complement the generic mitigations defined in the Generic Sensor API [GENERIC-SENSOR].

5. Model

The Accelerometer sensor type’s associated Sensor subclass is the Accelerometer class.

The Accelerometer has a default sensor, which is the device’s main accelerometer sensor.

The Accelerometer has an associated sensor permission name which is "accelerometer".

A latest reading for a Sensor of Accelerometer sensor type includes three entries whose keys are "x", "y", "z" and whose values contain device’s acceleration about the corresponding axes. Values can contain also device’s linear acceleration or gravity depending on which object was instantiated.

The acceleration is the rate of change of velocity of a device with respect to time. Its unit is the metre per second squared (m/s²) [SI].

The frame of reference for the acceleration measurement must be inertial, such as, the device in free fall would provide 0 (m/s²) acceleration value for each axis.

The sign of the acceleration values must be according to the right-hand convention in a local coordinate system (see figure below).

Accelerometer coordinate system.

The LinearAccelerationSensor class is an Accelerometer's subclass. The LinearAccelerationSensor's latest reading contains device’s linear acceleration about the corresponding axes.

The linear acceleration is an acceleration that is applied to the device that hosts the sensor, without the contribution of a gravity force.

The GravitySensor class is an Accelerometer's subclass. The GravitySensor's latest reading contains device’s acceleration due to the effect of gravity force about the corresponding axes.

The gravity is a force that attracts an object to the center of the earth, or towards any other physical object having mass.

5.1. Reference Frame

The local coordinate system represents the reference frame for the Accelerometer, LinearAccelerationSensor, and the GravitySensor readings. It can be either the device coordinate system or the screen coordinate system.

The device coordinate system is defined as a three dimensional Cartesian coordinate system (x, y, z), which is bound to the physical device. For devices with a display, the origin of the device coordinate system is the center of the device display. If the device is held in its default position, the Y-axis points towards the top of the display, the X-axis points towards the right of the display and Z-axis is the vector product of X and Y axes and it points outwards from the display, and towards the viewer. The device coordinate system remains stationary regardless of the dom screen orientation (see figure below).

Device coordinate system.

The screen coordinate system is defined as a three dimensional Cartesian coordinate system (x, y, z), which is bound to the dom screen. The origin of the screen coordinate system in the center of the dom screen. The Y-axis always points towards the top of the dom screen, the X-axis points towards the right of the dom screen and Z-axis is the vector product of X and Y axes and it and it points outwards from the dom screen, and towards the viewer (see figure below).

Screen coordinate system.

The main difference between the device coordinate system and the screen coordinate system, is that the screen coordinate system always follows the dom screen orientation, i.e. it will swap X and Y axes in relation to the device if the current orientation type changes. In contrast, the device coordinate system will always remain stationary relative to the device.

6. API

6.1. The Accelerometer Interface

[SecureContext, Exposed=Window]
interface Accelerometer : Sensor {
  constructor(optional AccelerometerSensorOptions options = {});
  readonly attribute double? x;
  readonly attribute double? y;
  readonly attribute double? z;
};

enum AccelerometerLocalCoordinateSystem { "device", "screen" };

dictionary AccelerometerSensorOptions : SensorOptions {
  AccelerometerLocalCoordinateSystem referenceFrame = "device";
};

To construct an Accelerometer object the user agent must invoke the construct an accelerometer object abstract operation for the Accelerometer interface.

Supported sensor options for Accelerometer are "frequency" and "referenceFrame".

6.1.1. Accelerometer.x

The x attribute of the Accelerometer interface returns the result of invoking get value from latest reading with this and "x" as arguments. It represents the acceleration along x-axis.

6.1.2. Accelerometer.y

The y attribute of the Accelerometer interface returns the result of invoking get value from latest reading with this and "y" as arguments. It represents the acceleration along y-axis.

6.1.3. Accelerometer.z

The z attribute of the Accelerometer interface returns the result of invoking get value from latest reading with this and "z" as arguments. It represents the acceleration along z-axis.

6.2. The LinearAccelerationSensor Interface

[SecureContext, Exposed=Window]
interface LinearAccelerationSensor : Accelerometer {
  constructor(optional AccelerometerSensorOptions options = {});
};

To construct a LinearAccelerationSensor object the user agent must invoke the construct an accelerometer object abstract operation for the LinearAccelerationSensor interface.

Supported sensor options for LinearAccelerationSensor are "frequency" and "referenceFrame".

6.2.1. LinearAccelerationSensor.x

The x attribute of the LinearAccelerationSensor interface returns the result of invoking get value from latest reading with this and "x" as arguments. It represents the linear acceleration along x-axis.

6.2.2. LinearAccelerationSensor.y

The y attribute of the LinearAccelerationSensor interface returns the result of invoking get value from latest reading with this and "y" as arguments. It represents the linear acceleration along y-axis.

6.2.3. LinearAccelerationSensor.z

The z attribute of the LinearAccelerationSensor interface returns the result of invoking get value from latest reading with this and "z" as arguments. It represents the linear acceleration along z-axis.

6.3. The GravitySensor Interface

[SecureContext, Exposed=Window]
interface GravitySensor : Accelerometer {
  constructor(optional AccelerometerSensorOptions options = {});
};

To construct a GravitySensor object the user agent must invoke the construct an accelerometer object abstract operation for the GravitySensor interface.

Supported sensor options for GravitySensor are "frequency" and "referenceFrame".

6.3.1. GravitySensor.x

The x attribute of the GravitySensor interface returns the result of invoking get value from latest reading with this and "x" as arguments. It represents the effect of acceleration along x-axis due to gravity.

6.3.2. GravitySensor.y

The y attribute of the GravitySensor interface returns the result of invoking get value from latest reading with this and "y" as arguments. It represents the effect of acceleration along y-axis due to gravity.

6.3.3. GravitySensor.z

The z attribute of the GravitySensor interface returns the result of invoking get value from latest reading with this and "z" as arguments. It represents the effect of acceleration along z-axis due to gravity.

7. Abstract Operations

7.1. Construct an accelerometer object

input: accelerometer_interface, an Accelerometer interface identifier or an interface identifier whose inherited interfaces contains Accelerometer.; options, a AccelerometerSensorOptions object.
output: An Accelerometer object.

Let allowed be the result of invoking check sensor policy-controlled features with Accelerometer.
If allowed is false, then:
1. Throw a SecurityError DOMException.
Let accelerometer be a new instance of the interface identified by accelerometer_interface.
Invoke initialize a sensor object with accelerometer and options.
If options.referenceFrame is "screen", then:
1. Define local coordinate system for accelerometer as the screen coordinate system.
Otherwise, define local coordinate system for accelerometer as the device coordinate system.
Return accelerometer.

8. Automation

This section extends the automation section defined in the Generic Sensor API [GENERIC-SENSOR] to provide mocking information about the acceleration applied to the X, Y and Z axis of a device that hosts the sensor for the purposes of testing a user agent’s implementation of Accelerometer, LinearAccelerationSensor and GravitySensor APIs.

8.1. Mock Sensor Type

The Accelerometer class has an associated mock sensor type which is "accelerometer", its mock sensor reading values dictionary is defined as follows:

dictionary AccelerometerReadingValues {
  required double? x;
  required double? y;
  required double? z;
};

The LinearAccelerationSensor class has an associated mock sensor type which is "linear-acceleration", its mock sensor reading values dictionary is defined as follows:

dictionary LinearAccelerationReadingValues : AccelerometerReadingValues {
};

The GravitySensor class has an associated mock sensor type which is "gravity", its mock sensor reading values dictionary is defined as follows:

dictionary GravityReadingValues : AccelerometerReadingValues {
};

9. Acknowledgements

Tobie Langel for the work on Generic Sensor API.

10. 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]