Accelerometer

W3C Candidate Recommendation,

This version:
https://www.w3.org/TR/2018/CR-accelerometer-20180320/
Latest published version:
https://www.w3.org/TR/accelerometer/
Editor's Draft:
https://w3c.github.io/accelerometer/
Previous Versions:
Version History:
https://github.com/w3c/accelerometer/commits/gh-pages/index.bs
Feedback:
public-device-apis@w3.org with subject line “[accelerometer] … message topic …” (archives)
Issue Tracking:
GitHub
Editors:
Anssi Kostiainen (Intel Corporation)
Alexander Shalamov (Intel Corporation)
Bug Reports:
via the w3c/accelerometer repository on GitHub
Test Suite:
web-platform-tests on GitHub

Copyright © 2018 W3C® (MIT, ERCIM, Keio, Beihang). W3C liability, trademark and permissive document license rules apply.

Abstract

This specification defines Accelerometer, LinearAccelerationSensor and GravitySensor interfaces for obtaining information about acceleration applied to the X, Y and Z axis of a device that hosts the sensor.

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 Device 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 in order to ensure the opportunity for wide review.

If you wish to make comments regarding this document, please send them to public-device-apis@w3.org (subscribe, archives). When sending e-mail, please put the text “accelerometer” in the subject, preferably like this: “[accelerometer] …summary of comment…”. All comments are welcome.

Publication as a Candidate Recommendation does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

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.

This document was produced by a group operating under the W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

This document is governed by the 1 February 2018 W3C Process Document.

The CR exit criterion is two interoperable deployed implementations of each feature.

For changes since the previous version, see the online HTML diff tool.

The following features are at-risk, and may be dropped during the CR period:

“At-risk” is a W3C Process term-of-art, and does not necessarily imply that the feature is in danger of being dropped or delayed. It means that the WG believes the feature may have difficulty being interoperably implemented in a timely manner, and marking it as such allows the WG to drop the feature if necessary when transitioning to the Proposed Rec stage, without having to publish a new Candidate Rec without the feature first.

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

[Constructor(optional AccelerometerSensorOptions options), SecureContext,
  Exposed=Window]
interface Accelerometer : Sensor {
  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

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

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

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

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.

  1. Let allowed be the result of invoking check sensor policy-controlled features with Accelerometer.

  2. If allowed is false, then:

    1. Throw a SecurityError DOMException.

  3. Let accelerometer be a new instance of the interface identified by accelerometer_interface.

  4. Invoke initialize a sensor object with accelerometer and options.

  5. If options.referenceFrame is "screen", then:

    1. Define local coordinate system for accelerometer as the screen coordinate system.

  6. Otherwise, define local coordinate system for accelerometer as the device coordinate system.

  7. Return accelerometer.

8. Acknowledgements

Tobie Langel for the work on Generic Sensor API.

9. 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]

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[GENERIC-SENSOR]
Rick Waldron; et al. Generic Sensor API. URL: https://w3c.github.io/sensors/
[INFRA]
Anne van Kesteren; Domenic Denicola. Infra Standard. Living Standard. URL: https://infra.spec.whatwg.org/
[PERMISSIONS]
Mounir Lamouri; Marcos Caceres; Jeffrey Yasskin. Permissions. URL: https://w3c.github.io/permissions/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119
[SCREEN-ORIENTATION]
Mounir Lamouri; Marcos Caceres. The Screen Orientation API. URL: https://w3c.github.io/screen-orientation/
[WEBIDL]
Cameron McCormack; Boris Zbarsky; Tobie Langel. Web IDL. URL: https://heycam.github.io/webidl/

Informative References

[ACCESSORY]
Owusu, Emmanuel, et al. ACCessory: password inference using accelerometers on smartphones. 2012. Informational. URL: https://dl.acm.org/citation.cfm?id=2162095
[KEYSTROKEDEFENSE]
Song, Yihang, et al. Two novel defenses against motion-based keystroke inference attacks. 2014. Informational. URL: https://arxiv.org/abs/1410.7746
[SI]
SI Brochure: The International System of Units (SI), 8th edition. 2014. URL: http://www.bipm.org/en/publications/si-brochure/
[TOUCHSIGNATURES]
Mehrnezhad, Maryam, et al. Touchsignatures: identification of user touch actions and pins based on mobile sensor data via javascript. 2016. Informational. URL: https://arxiv.org/abs/1602.04115

IDL Index

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

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

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

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

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