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<!DOCTYPE html>
<html lang="en-us">
<head>
<link href="getusermedia.css" rel="stylesheet" type="text/css">
<title>Media Capture and Streams</title>
<meta content="text/html; charset=utf-8" http-equiv="Content-Type">
<script class="remove" src="respec-w3c.js" type="text/javascript"></script>
<script class="remove" src="getusermedia.js" type="text/javascript"></script>
</head>
<body>
<p class='copyright'>Initial Author of this Specification was Ian Hickson, Google Inc., with the following copyright statement:<br /> © Copyright 2004-2011 Apple Computer, Inc., Mozilla Foundation, and Opera Software ASA. You are granted a license to use, reproduce and create derivative works of this document.</p> <p class='copyright'>All subsequent changes since 26 July 2011 done by the W3C WebRTC Working Group (and previously the Device APIs Working Group) are under the following <a href="https://www.w3.org/policies/#copyright">Copyright</a> © 2011-2023 <a href="https://www.w3.org/">World Wide Web Consortium</a>. <abbr title="World Wide Web Consortium">W3C</abbr><sup>®</sup> <a href="https://www.w3.org/policies/#Legal_Disclaimer">liability</a>, <a href="https://www.w3.org/policies/#W3C_Trademarks">trademark</a> and <a href="https://www.w3.org/copyright/software-license/">permissive document license</a> rules apply.
<section id="abstract">
<p>This document defines a set of JavaScript APIs that allow local media,
including audio and video, to be requested from a platform.</p>
</section>
<section id="sotd">
<p>This document is not complete. The API is based on preliminary work done in the
WHATWG.</p>
<p>Before this document proceeds to Proposed Recommendation, the WebRTC Working Group intends to address <a href="https://www.w3.org/PM/horizontal/review.html?shortname=mediacapture-streams">issues that emerged from wide review</a>.</p>
</section>
<section class="informative" id="intro">
<h2>Introduction</h2>
<p>This document defines APIs for requesting access to local multimedia
devices, such as microphones or video cameras.</p>
<p>This document also defines the MediaStream API, which provides the means
to control where multimedia stream data is consumed, and provides some
control over the devices that produce the media. It also exposes
information about devices able to capture and render media.</p>
</section>
<section id="conformance">
<p>This specification defines conformance criteria that apply to a single
product: the <dfn>User Agent</dfn> that implements the interfaces that it
contains.</p>
<p>Conformance requirements phrased as algorithms or specific steps may be
implemented in any manner, so long as the end result is equivalent. (In
particular, the algorithms defined in this specification are intended to be
easy to follow, and not intended to be performant.)</p>
<p>Implementations that use ECMAScript [[ECMA-262]] to implement the APIs
defined in this specification must implement them in a manner consistent
with the ECMAScript Bindings defined in the Web IDL specification
[[WEBIDL]], as this specification uses that specification and
terminology.</p>
</section>
<section>
<h2>Terminology</h2>
<dl>
<dt><dfn data-export>source</dfn></dt>
<dd>
<p>A source is the "thing" providing the source of a media stream
track. The source is the broadcaster of the media itself. A source can
be a physical webcam, microphone, local video or audio file from the
user's hard drive, network resource, or static image. Note that this
document describes the use of microphone and camera type sources only,
the use of other source types is described in other documents.</p>
<p>An application that has no prior authorization regarding sources is
only given the number of available sources, their type and any
relationship to other devices. Additional information about sources can
become available when applications are authorized to use a source (see
<a href="#access-control-model"></a>).</p>
<p>Sources <strong>do not</strong> have constraints — tracks have
constraints. When a source is connected to a track, it must produce
media that conforms to the constraints present on that track, to that
track. Multiple tracks can be attached to the same source. [=User Agent=]
processing, such as downsampling, MAY be used to ensure that all tracks
have appropriate media.</p>
<p>Sources have constrainable properties which have
[=capabilities=] and [=settings=]
exposed on tracks. While the constrainable properties are "owned" by
the source, sources MAY be able to accommodate different demands at
once. For this reason, capabilities are common to any (multiple) tracks
that happen to be using the same source, whereas settings MAY differ
per track (e.g., if two different track objects bound to the same
source query capability and settings information, they will get back
the same capabilities, but may get different settings that are tailored
to satisfy their individual constraints).</p>
</dd>
<dt><dfn data-lt=settings>Setting</dfn> (Source Setting)</dt>
<dd>
<p>A setting refers to the immediate, current
value of the source's constrainable properties. Settings are always
read-only.</p>
<p>A source conditions may dynamically change, such as when a camera
switches to a lower frame rate due to low light conditions. In these
cases the tracks related to the affected source might not satisfy the
set constraints any longer. The platform SHOULD try to minimize such
excursions as far as possible, but will continue to deliver media even
when a temporary or permanent condition exists that prevents satisfying
the constraints.</p>
<p>Although settings are a property of the source, they are only
exposed to the application through the tracks attached to the source.
This is exposed via the <a>ConstrainablePattern</a> interface.</p>
</dd>
<dt><dfn data-lt=capabilities>Capability</dfn></dt>
<dd>
<p>For each constrainable property, there is a capability that
describes whether it is supported by the source and if so, the range of
supported values. As with settings, capabilities are exposed to the
application via the <a>ConstrainablePattern</a> interface.</p>
<p>The values of the supported capabilities must be normalized to the
ranges and enumerated types defined in this specification.</p>
<p>A {{MediaStreamTrack/getCapabilities()}} call on
a track returns the same underlying per-source capabilities for all
tracks connected to the source.</p>
<p>This API is intentionally simplified. Capabilities are not capable
of describing interactions between different values. For instance, it
is not possible to accurately describe the capabilities of a camera
that can produce a high resolution video stream at a low frame rate and
lower resolutions at a higher frame rate. Capabilities describe the
complete range of each value. Interactions between constraints are
exposed by attempting to apply constraints.</p>
</dd>
<dt><dfn>Constraint</dfn>s</dt>
<dd>
<p>Constraints provide a general control surface that allows
applications to both select an appropriate source for a track and, once
selected, to influence how a source operates.</p>
<p>Constraints limit the range of operating modes that a source can use
when providing media for a track. Without provided track constraints,
implementations are free to select a source's settings from the full
ranges of its supported capabilities. Implementations may also adjust
source settings at any time within the bounds imposed by all applied
constraints.</p>
<p>{{MediaDevices/getUserMedia()}} uses constraints
to help select an appropriate source for a track and configure it.
Additionally, the <a>ConstrainablePattern</a> interface on tracks
includes an API for dynamically changing the track's constraints at any
later time.</p>
<p>A track will not be connected to a source using
{{MediaDevices/getUserMedia()}} if its initial constraints cannot be
satisfied. However, the ability to meet the constraints on a track can
change over time, and constraints can be changed. If circumstances
change such that constraints cannot be met, the
<a>ConstrainablePattern</a> interface defines an appropriate error to
inform the application. [[[#the-model-sources-sinks-constraints-and-settings]]] explains how
constraints interact in more detail.</p>
<p>For each constrainable property, a constraint exists whose name
corresponds with the relevant source setting name and capability
name.</p>
<p>A constraint falls into one of three groups, depending on its place
in the constraints structure. The groups are:</p>
<ul>
<li><dfn>required constraints</dfn> are all <a data-lt=
"advanced constraints">non-advanced</a> constraints that are
<a>required</a>.</li>
<li><dfn>optional basic constraints</dfn> are the remaining
<a data-lt="advanced constraints">non-advanced</a> constraints.</li>
<li><dfn>advanced constraints</dfn> are all constraints specified
using the <code><a href="#dom-constraints-advanced">advanced</a></code>
keyword.</li>
</ul>
<p>In general, [=User Agents=] will have more flexibility to optimize the
media streaming experience the fewer constraints are applied, so
application authors are strongly encouraged to use <a>required
constraints</a> sparingly.</p>
</dd>
</dl>
</section>
<section id="stream-api">
<h2>MediaStream API</h2>
<section>
<h2>Introduction</h2>
<p>The two main components in the MediaStream API are the
{{MediaStreamTrack}} and {{MediaStream}} interfaces. The
{{MediaStreamTrack}} object represents media of a single
type that originates from one media source in the [=User Agent=], e.g. video
produced by a web camera. A {{MediaStream}} is used to
group several {{MediaStreamTrack}} objects into one unit
that can be recorded or rendered in a media element.</p>
<p>Each {{MediaStream}} can contain zero or more
{{MediaStreamTrack}} objects. All tracks in a
{{MediaStream}} are intended to be synchronized when
rendered. This is not a hard requirement, since it might not be possible
to synchronize tracks from sources that have different clocks. Different
{{MediaStream}} objects do not need to be
synchronized.</p>
<p class="note">While the intent is to synchronize tracks, it could be
better in some circumstances to permit tracks to lose synchronization. In
particular, when tracks are remotely sourced and real-time [[?WEBRTC]],
it can be better to allow loss of synchronization than to accumulate
delays or risk glitches and other artifacts. Implementations are expected
to understand the implications of choices regarding synchronization of
playback and the effect that these have on user perception.</p>
<p>A single {{MediaStreamTrack}} can represent
multi-channel content, such as stereo or 5.1 audio or stereoscopic video,
where the channels have a well defined relationship to each other.
Information about channels might be exposed through other APIs, such as
[[?WEBAUDIO]], but this specification provides no direct access to
channels.</p>
<p>A {{MediaStream}} object has an input and an output
that represent the combined input and output of all the object's tracks.
The output of the {{MediaStream}} controls how the object
is rendered, e.g., what is saved if the object is recorded to a file or
what is displayed if the object is used in a [^video^] element.
A single {{MediaStream}} object can be attached to
multiple different outputs at the same time.</p>
<p>A new {{MediaStream}} object can be created from
existing media streams or tracks using the
{{MediaStream/MediaStream()}} constructor. The constructor argument
can either be an existing {{MediaStream}} object, in
which case all the tracks of the given stream are added to the new
{{MediaStream}} object, or an array of
{{MediaStreamTrack}} objects. The latter form makes it
possible to compose a stream from different source streams.</p>
<p>Both {{MediaStream}} and
{{MediaStreamTrack}} objects can be cloned. A cloned
{{MediaStream}} contains clones of all member tracks from
the original stream. A cloned {{MediaStreamTrack}} has a
<a href="#constrainable-interface">set of constraints</a> that is
independent of the instance it is cloned from, which allows media from
the same source to have different constraints applied for different
<a>consumer</a>s. The {{MediaStream}} object is also used in
contexts outside {{MediaDevices/getUserMedia}}, such as [[?WEBRTC]].</p>
</section>
<section>
<h2>{{MediaStream}}</h2>
<p id=
"mediastream-constructor">The MediaStream <dfn data-idl data-dfn-for=MediaStream>constructor</dfn> composes a new
stream out of existing tracks. It takes an optional argument of type
{{MediaStream}} or an array of
{{MediaStreamTrack}} objects. When the constructor is invoked, the User
Agent must run the following steps:</p>
<ol class=algorithm>
<li>
<p>Let <var>stream</var> be a newly constructed
{{MediaStream}} object.</p>
</li>
<li>
<p>Initialize <var>stream</var>.{{MediaStream/id}} attribute to a newly generated
value.</p>
</li>
<li>
<p>If the constructor's argument is present, run the following
steps:</p>
<ol>
<li>
<p>Construct a set of tracks <var>tracks</var> based on the type
of argument:</p>
<ul>
<li>
<p>A {{MediaStream}} object:</p>
<p>Let <var>tracks</var> be a set containing all the
{{MediaStreamTrack}} objects in the
{{MediaStream}} <a>track
set</a>.</p>
</li>
<li>
<p>A sequence of {{MediaStreamTrack}}
objects:</p>
<p>Let <var>tracks</var> be a set containing all the
{{MediaStreamTrack}} objects in the provided
sequence.</p>
</li>
</ul>
</li>
<li>
<p>For each {{MediaStreamTrack}},
<var>track</var> , in <var>tracks</var>, run the following
steps:</p>
<ol>
<li>
<p>If <var>track</var> is already in <var>stream</var>'s
[=track set=], skip
<var>track</var>.</p>
</li>
<li>
<p>Otherwise, add <var>track</var> to <var>stream</var>'s
[=track set=].</p>
</li>
</ol>
</li>
</ol>
</li>
<li>
<p>Return <var>stream</var>.</p>
</li>
</ol>
<p>The tracks of a {{MediaStream}} are stored in a
<dfn class="export">track set</dfn>. The track set MUST contain the
{{MediaStreamTrack}} objects that correspond to the
tracks of the stream. The relative order of the tracks in the set is User
Agent defined and the API will never put any requirements on the order.
The proper way to find a specific {{MediaStreamTrack}}
object in the set is to look it up by its {{MediaStreamTrack/id}}.</p>
<p>An object that reads data from the output of a
{{MediaStream}} is referred to as a
{{MediaStream}} <dfn data-export data-for=MediaStream>consumer</dfn>. The list of
{{MediaStream}} consumers currently include media
elements (such as [^video^] and
[^audio^]) [[HTML]], Web Real-Time Communications
(WebRTC; {{RTCPeerConnection}}) [[?WEBRTC]], media recording
(<code class=fixme>MediaRecorder</code>) [[?mediastream-recording]], image capture
(<code class=fixme>ImageCapture</code>) [[?image-capture]], and web audio
({{MediaStreamAudioSourceNode}}) [[?WEBAUDIO]].</p>
<p class="note">{{MediaStream}} consumers must be able to
handle tracks being added and removed. This behavior is specified per
consumer.</p>
<p>A {{MediaStream}} object is said to be <dfn data-dfn-for="stream" id=
"stream-active">active</dfn> when it has at least one
{{MediaStreamTrack}} that has not [=MediaStreamTrack/ended=]. A {{MediaStream}} that does not
have any tracks or only has tracks that are [= MediaStreamTrack/ended =]
is <dfn data-dfn-for="stream" id="stream-inactive">inactive</dfn>.</p>
<p>A {{MediaStream}} object is said to be <dfn data-dfn-for=stream id=
"stream-audible" data-dfn-for=stream>audible</dfn> when it has at least one
{{MediaStreamTrack}} whose {{MediaStreamTrack/[[Kind]]}} is <a>"audio"</a>
that has not [=MediaStreamTrack/ended=]. A {{MediaStream}} that does not have any
audio tracks or only has audio tracks that are [=MediaStreamTrack/ended=] is
<dfn id="stream-inaudible" data-dfn-for=stream>inaudible</dfn>.</p>
<p>The [=User Agent=] may update a {{MediaStream}}'s [=track set=] in response to, for example, an external
event. This specification does not specify any such cases, but other
specifications using the MediaStream API may. One such example is the
WebRTC 1.0 [[?WEBRTC]] specification where the [=track set=] of a {{MediaStream}}, received
from another peer, can be updated as a result of changes to the media
session.</p>
<p>To <dfn class="abstract-op" data-dfn-for="MediaStream">add a track</dfn> <var>track</var> to a
{{MediaStream}} <var>stream</var>, the [=User Agent=] MUST
run the following steps:</p>
<ol class=algorithm>
<li>
<p>If <var>track</var> is already in <var>stream's</var> [=track set=], then abort these steps.</p>
</li>
<li>
<p>Add <var>track</var> to <var>stream</var>'s [=track set=].</p>
</li>
<li>
<p>[= Fire a track event=] named {{addtrack}} with
<var>track</var> at <var>stream</var>.</p>
</li>
</ol>
<p>To <dfn class="abstract-op" data-dfn-for="MediaStream">remove a track</dfn> <var>track</var> from a
{{MediaStream}} <var>stream</var>, the [=User Agent=] MUST
run the following steps:</p>
<ol class=algorithm>
<li>
<p>If <var>track</var> is not in <var>stream's</var> [=track set=], then abort these steps.</p>
</li>
<li>
<p>[=set/Remove=] <var>track</var> from <var>stream</var>'s [=track set=].</p>
</li>
<li>
<p>[= Fire a track event =] named {{removetrack}} with
<var>track</var> at <var>stream</var>.</p>
</li>
</ol>
<div>
<pre class="idl"
>[Exposed=Window]
interface MediaStream : EventTarget {
constructor();
constructor(MediaStream stream);
constructor(sequence<MediaStreamTrack> tracks);
readonly attribute DOMString id;
sequence<MediaStreamTrack> getAudioTracks();
sequence<MediaStreamTrack> getVideoTracks();
sequence<MediaStreamTrack> getTracks();
MediaStreamTrack? getTrackById(DOMString trackId);
undefined addTrack(MediaStreamTrack track);
undefined removeTrack(MediaStreamTrack track);
MediaStream clone();
readonly attribute boolean active;
attribute EventHandler onaddtrack;
attribute EventHandler onremovetrack;
};</pre>
<section>
<h2>Constructors</h2>
<dl data-link-for="MediaStream" data-dfn-for="MediaStream" class=
"constructors">
<dt>{{MediaStream}}</dt>
<dd>
<p>See the <a href="#mediastream-constructor">MediaStream
constructor algorithm</a></p>
<div>
<em>No parameters.</em>
</div>
</dd>
<dt>{{MediaStream}}</dt>
<dd>
<p>See the <a href="#mediastream-constructor">MediaStream
constructor algorithm</a></p>
</dd>
<dt>{{MediaStream}}</dt>
<dd>
<p>See the <a href="#mediastream-constructor">MediaStream
constructor algorithm</a></p>
</dd>
</dl>
</section>
<section>
<h2>Attributes</h2>
<dl data-link-for="MediaStream" data-dfn-for="MediaStream" class=
"attributes">
<dt>{{id}} of type {{DOMString}}, readonly</dt>
<dd>
<p>The <dfn data-idl>id</dfn> attribute MUST return the value to
which it was initialized when the object was created.</p>
<p>When a {{MediaStream}} is created, the User
Agent MUST generate an identifier string, and MUST initialize the
object's {{id}}
attribute to that string, unless the object is created as part of
a special purpose algorithm that specifies how the stream id must
be initialized. A good practice is to use a UUID [[rfc4122]],
which is 36 characters long in its canonical form. To avoid
fingerprinting, implementations SHOULD use the forms in section
4.4 or 4.5 of RFC 4122 when generating UUIDs.</p>
<p>An example of an algorithm that specifies how the stream id
must be initialized is the algorithm to associate an incoming
network component with a {{MediaStream}} object. [[?WEBRTC]]</p>
</dd>
<dt><dfn>active</dfn> of type {{boolean}}, readonly</dt>
<dd>
<p>The {{active}} attribute MUST return
<code>true</code> if this {{MediaStream}} is
[= stream/active =] and <code>false</code>
otherwise.</p>
</dd>
<dt><dfn>onaddtrack</dfn> of type {{EventHandler}}</dt>
<dd>
<p>The event type of this event handler is {{addtrack}}.</p>
</dd>
<dt><dfn>onremovetrack</dfn> of type {{EventHandler}}</dt>
<dd>
<p>The event type of this event handler is {{removetrack}}.</p>
</dd>
</dl>
</section>
<section>
<h2>Methods</h2>
<dl data-link-for="MediaStream" data-dfn-for="MediaStream" class=
"methods">
<dt><dfn>getAudioTracks()</dfn></dt>
<dd>
<p>Returns a sequence of {{MediaStreamTrack}}
objects representing the audio tracks in this stream.</p>
<p>The {{getAudioTracks}}
method MUST return a sequence that represents a snapshot of all
the {{MediaStreamTrack}} objects in this stream's
[=track set=] whose {{MediaStreamTrack/[[Kind]]}} is equal to
<a>"audio"</a>. The conversion from the [=track set=] to the sequence is [=User Agent=] defined
and the order does not have to be stable between calls.</p>
</dd>
<dt><dfn>getVideoTracks()</dfn></dt>
<dd>
<p>Returns a sequence of {{MediaStreamTrack}}
objects representing the video tracks in this stream.</p>
<p>The {{getVideoTracks}}
method MUST return a sequence that represents a snapshot of all
the {{MediaStreamTrack}} objects in this stream's
[=track set=] whose {{MediaStreamTrack/[[Kind]]}} is equal to
<a>"video"</a>. The conversion from the
[=track set=] to the sequence is [=User Agent=] defined
and the order does not have to be stable between calls.</p>
</dd>
<dt><dfn>getTracks()</dfn></dt>
<dd>
<p>Returns a sequence of {{MediaStreamTrack}}
objects representing all the tracks in this stream.</p>
<p>The {{getTracks}} method
MUST return a sequence that represents a snapshot of all the
{{MediaStreamTrack}} objects in this stream's
[=track set=], regardless of {{MediaStreamTrack/[[Kind]]}}. The
conversion from the [=track set=] to the sequence is User
Agent defined and the order does not have to be stable between
calls.</p>
</dd>
<dt><dfn>getTrackById()</dfn></dt>
<dd>
<p>The {{getTrackById}}
method MUST return either a {{MediaStreamTrack}}
object from this stream's [=track set=]
whose {{MediaStreamTrack/[[Id]]}} is
equal to <var>trackId</var>, or <code>null</code>, if no such track
exists.</p>
</dd>
<dt><dfn>addTrack()</dfn></dt>
<dd>
<p>Adds the given {{MediaStreamTrack}} to this
{{MediaStream}}.</p>
<p>When the {{addTrack}} method is
invoked, the [=User Agent=] MUST run the following steps:</p>
<ol class=algorithm>
<li>
<p>Let <var>track</var> be the methods argument and
<var>stream</var> the {{MediaStream}} object
on which the method was called.</p>
</li>
<li>
<p>If <var>track</var> is already in <var>stream</var>'s
[=track set=], then abort these
steps.</p>
</li>
<li>
<p>[=MediaStream/Add a track|Add=] <var>track</var> to <var>stream</var>'s [=track set=].</p>
</li>
</ol>
</dd>
<dt><dfn>removeTrack()</dfn></dt>
<dd>
<p>Removes the given {{MediaStreamTrack}} object
from this {{MediaStream}}.</p>
<p>When the {{removeTrack}}
method is invoked, the [=User Agent=] MUST run the following
steps:</p>
<ol class=algorithm>
<li>
<p>Let <var>track</var> be the methods argument and
<var>stream</var> the {{MediaStream}} object
on which the method was called.</p>
</li>
<li>
<p>If <var>track</var> is not in <var>stream's</var> [=track set=], then abort these steps.</p>
</li>
<li>
<p>[=MediaStream/Remove a track|Remove=] <var>track</var> from <var>stream</var>'s [=track set=].</p>
</li>
</ol>
</dd>
<dt><dfn>clone()</dfn></dt>
<dd>
<p>Clones the given {{MediaStream}} and all its
tracks.</p>
<p>When the {{clone()}} method is invoked, the User
Agent MUST run the following steps:</p>
<ol class=algorithm>
<li>
<p>Let <var>streamClone</var> be a newly constructed
{{MediaStream}} object.</p>
</li>
<li>
<p>Initialize <var>streamClone</var>.{{MediaStream.id}} to a newly
generated value.</p>
</li>
<li>
<p><a href="#track-clone">Clone each track</a> in this
{{MediaStream}} object and add the result to
<var>streamClone</var>'s <a>track
set</a>.</p>
</li>
<li>Return <var>streamClone</var>.</li>
</ol>
</dd>
</dl>
</section>
</div>
</section>
<section>
<h2>{{MediaStreamTrack}}</h2>
<p>A {{MediaStreamTrack}} object represents a media
source in the [=User Agent=]. An example source is a device connected to the
[=User Agent=]. Other specifications may define sources for
{{MediaStreamTrack}} that override the behavior specified
here. Several {{MediaStreamTrack}} objects can represent
the same media source, e.g., when the user chooses the same camera in the
UI shown by two consecutive calls to {{MediaDevices/getUserMedia()}}.</p>
<p>A {{MediaStreamTrack}} source defines the following properties:
<ol>
<li>A source has a <dfn data-export>MediaStreamTrack source type</dfn>.
It is set to either {{MediaStreamTrack}} or a subtype of {{MediaStreamTrack}}.
By default, it is set to {{MediaStreamTrack}}. </li>
<li>A source has <dfn data-export>MediaStreamTrack source-specific construction steps</dfn>
that are executed when creating a {{MediaStreamTrack}} from a source.
The steps take a newly created {{MediaStreamTrack}} as input. By default, the steps are empty.</li>
<li>A source has <dfn data-export>MediaStreamTrack source-specific clone steps</dfn>
that are executed when cloning a {{MediaStreamTrack}} of the given source.
The steps take the source and destination {{MediaStreamTrack}}s as input. By default, the steps are empty.</li>
</ol>
</p>
<p>The data from a {{MediaStreamTrack}} object does not
necessarily have a canonical binary form; for example, it could just be
"the video currently coming from the user's video camera". This allows
[=User Agents=] to manipulate media in whatever fashion is most suitable on
the user's platform.</p>
<p>A script can indicate that a {{MediaStreamTrack}}
object no longer needs its source with the {{MediaStreamTrack/stop()}}
method. When all tracks
using a source have been stopped or ended by some other means, the source
is <dfn id="source-stopped" data-lt="source stopped state">stopped</dfn>. If the source is a device
exposed by {{MediaDevices/getUserMedia()}}, then when
the source is stopped, the [=User Agent=] MUST run the following steps:</p>
<ol class=algorithm>
<li>
<p>Let <var>mediaDevices</var> be the {{MediaDevices}} object in question.</p>
<li>
<p>Let <var>deviceId</var> be the source device's {{MediaDeviceInfo/deviceId}}.</p>
</li>
<li>
<p>Set <var>mediaDevices</var>.{{MediaDevices/[[devicesLiveMap]]}}[<var>deviceId</var>] to
<code>false</code>.</p>
</li>
<li>
<p>If the [=permission state=]
of the permission associated with the device's kind and
<var>deviceId</var> for <var>mediaDevices</var>'s [=relevant settings object=],
is not {{PermissionState/"granted"}}, then set
<var>mediaDevices</var>.{{MediaDevices/[[devicesAccessibleMap]]}}[<var>deviceId</var>] to
<code>false</code>.</p>
</li>
</ol>
<p>To <dfn class="abstract-op">create a MediaStreamTrack</dfn> with an underlying
<var>source</var>, and a <var>mediaDevicesToTieSourceTo</var>, run the
following steps:</p>
<ol class=algorithm>
<li>
<p>Let <var>track</var> be a new object of type <var>source</var>'s [=MediaStreamTrack source type=].</p>
<p>Initialize track with the following internal slots:</p>
<ul data-dfn-for="MediaStreamTrack">
<li>
<p><dfn>[[\Source]]</dfn>,
initialized to <var>source</var>.</p>
</li>
<li>
<p><dfn>[[\Id]]</dfn>,
initialized to a newly generated unique identifier string. See
{{MediaStream.id}} attribute for guidelines on how to generate
such an identifier.</p>
</li>
<li>
<p><dfn>[[\Kind]]</dfn>,
initialized to <dfn><code>"audio"</code></dfn> if <var>source</var> is
an audio source, or <dfn><code>"video"</code></dfn> if
<var>source</var> is a video source.</p>
</li>
<li>
<p><dfn>[[\Label]]</dfn>,
initialized to <var>source</var>'s label, if provided by the User
Agent, or <code>""</code> otherwise. [=User Agents=] MAY label audio and
video sources (e.g., "Internal microphone" or "External USB Webcam").
</p>
</li>
<li>
<p><dfn>[[\ReadyState]]</dfn>,
initialized to {{MediaStreamTrackState/"live"}}.</p>
</li>
<li>
<p><dfn>[[\Enabled]]</dfn>,
initialized to <code>true</code>.</p>
</li>
<li>
<p><dfn>[[\Muted]]</dfn>,
initialized to <code>true</code> if <var>source</var> is
[= source/muted =], and <code>false</code> otherwise.</p>
</li>
<li>
<a data-link-for="constrainable object"
data-link-type="attribute">[[\Capabilities]]</a>,
<a data-link-for="constrainable object"
data-link-type="attribute">[[\Constraints]]</a>, and
<a data-link-for="constrainable object"
data-link-type="attribute">[[\Settings]]</a>, all initialized as
specified in the {{ConstrainablePattern}}.</p>
</li>
<li>
<p><dfn class="export">[[\Restrictable]]</dfn>, initialized to <code>false</code>.</p>
</li>
</ul>
</li>
<li>
<p>If <var>mediaDevicesToTieSourceTo</var> is not <code>null</code>,
[=tie track source to `MediaDevices`=] with <var>source</var> and <var>mediaDevicesToTieSourceTo</var>.</p>
</li>
<li><p>Run <var>source</var>'s [=MediaStreamTrack source-specific construction steps=]
with <var>track</var> as parameter.</p></li>
<li><p>Return <var>track</var>.</p></li>
</ol>
<p>To <dfn class="abstract-op">initialize the underlying source</dfn> of <var>track</var>
to <var>source</var>, run the following steps:</p>
<ol class="algorithm">
<li><p>Initialize <var>track</var>.{{MediaStreamTrack/[[Source]]}} to
<var>source</var>.</p></li>
<li><p>Initialize <var>track</var>'s <a data-link-for="constrainable object"
data-link-type="attribute">[[\Capabilities]]</a>,
<a data-link-for="constrainable object"
data-link-type="attribute">[[\Constraints]]</a>, and
<a data-link-for="constrainable object"
data-link-type="attribute">[[\Settings]]</a>, as
specified in the {{ConstrainablePattern}}.</p></li>
</ol>
<p>To <dfn class="abstract-op">tie track source to `MediaDevices`</dfn>, given <var>source</var> and
<var>mediaDevices</var>, run the following steps:
<ol class="algorithm">
<li>
<p>Add <var>source</var> to
<var>mediaDevices</var>.{{MediaDevices/[[mediaStreamTrackSources]]}}.</p>
</li>
</ol>
<p>To <dfn class="abstract-op">stop all sources</dfn> of a [=global object=], named <var>globalObject</var>,
the [=User Agent=] MUST run the following steps:</p>
<ol class="algorithm">
<li><p>For each {{MediaStreamTrack}} object <var>track</var> whose
<a data-cite="!HTML/#concept-relevant-global">relevant global object</a> is <var>globalObject</var>,
set <var>track</var>'s {{MediaStreamTrack/[[ReadyState]]}} to
{{MediaStreamTrackState/"ended"}}.</p></li>
<li><p>If <var>globalObject</var> is a {{Window}}, then for each <var>source</var> in
<var>globalObject</var>'s
[=associated `MediaDevices`=].{{MediaDevices/[[mediaStreamTrackSources]]}},
[= source/stopped | stop =] <var>source</var>.</p></li>
</ol>
<p>The [=User Agent=] MUST [=stop all sources=] of a <var>globalObject</var> in the following conditions:</p>
<ol>
<li><p>If <var>globalObject</var> is a {{Window}} object and the [=unloading document cleanup steps=]
are executed for its [=associated document=].</p></li>
<li><p>If <var>globalObject</var> is a {{WorkerGlobalScope}} object and its
<a data-cite="!HTML/workers.html#dom-workerglobalscope-closing">closing</a> flag is set to true.</p></li>
</ol>
<p>An implementation may use a per-source reference count to keep track
of source usage, but the specifics are out of scope for this
specification.</p>
<p>To <dfn class="abstract-op" id="track-clone">clone a track</dfn> the [=User Agent=] MUST run
the following steps:</p>
<ol class="algorithm">
<li>
<p>Let <var>track</var> be the {{MediaStreamTrack}}
object to be cloned.</p>
</li>
<li>
<p>Let <var>source</var> be <var>track</var>'s
{{MediaStreamTrack/[[Source]]}}.</p>
</li>
<li>
<p>Let <var>trackClone</var> be the result of
[=create a MediaStreamTrack | creating a MediaStreamTrack=] with
<var>source</var> and <code>null</code>.
</p>
</li>
<li>
<p>Set <var>trackClone</var>'s {{MediaStreamTrack/[[ReadyState]]}} to
<var>track</var>'s {{MediaStreamTrack/[[ReadyState]]}} value.</p>
</li>
<li>
<p>Set <var>trackClone</var>'s
<a data-link-for="constrainable object"
data-link-type="attribute">[[\Capabilities]]</a> to a clone of
<var>track</var>'s
<a data-link-for="constrainable object"
data-link-type="attribute">[[\Capabilities]]</a>.</p>
</li>
<li>
<p>Set <var>trackClone</var>'s
<a data-link-for="constrainable object"
data-link-type="attribute">[[\Constraints]]</a> to a clone of
<var>track</var>'s
<a data-link-for="constrainable object"
data-link-type="attribute">[[\Constraints]]</a>.</p>
</li>
<li>
<p>Set <var>trackClone</var>'s
<a data-link-for="constrainable object"
data-link-type="attribute">[[\Settings]]</a> to a clone of
<var>track</var>'s
<a data-link-for="constrainable object"
data-link-type="attribute">[[\Settings]]</a>.</p>
</li>
<li>
<p>Run <var>source</var> [=MediaStreamTrack source-specific clone steps=] with <var>track</var> and <var>trackClone</var> as parameters.</p>
</li>
<li>
<p>Return <var>trackClone</var>.</p>
</li>
</ol>
<section>
<h3>Media Flow and Life-cycle</h3>
<section>
<h4>Media Flow</h4>
<p>There are two dimensions related to the media flow for a
{{MediaStreamTrackState/"live"}} {{MediaStreamTrack}} : muted / not
muted, and enabled / disabled.</p>
<p><dfn class="export" data-dfn-for="MediaStreamTrack" data-dfn-type="dfn" id=
"track-muted">Muted</dfn> refers to the input to the
{{MediaStreamTrack}}. A {{MediaStreamTrack}} is [= MediaStreamTrack/muted =]
when its source is <dfn data-dfn-for="source">muted</dfn>,
i.e. temporarily unable to provide the track with data.
Live samples MUST NOT be made available to a
{{MediaStreamTrack}} while it is [=MediaStreamTrack/muted=].</p>
<p>The [=MediaStreamTrack/muted=] state is outside the control of web applications, but can be observed by
the application by reading the {{MediaStreamTrack/muted}} attribute and listening
to the associated events {{mute}} and {{unmute}}. The reasons for a
{{MediaStreamTrack}} to be muted are defined by its <a>source</a>.</p>
<p>For camera and microphone sources, the reasons to [=source/muted|mute=] are
[=implementation-defined=]. This allows user agents to implement privacy
mitigations in situations like:
the user pushing a physical mute button on the microphone, the user
closing a laptop lid with an embedded camera, the user toggling a
control in the operating system, the user clicking a mute button in the
[=User Agent=] chrome, the [=User Agent=] (on behalf of the user) mutes, etc.</p>
<p>On some operating systems, microphone access may
get stolen from the [=User Agent=] when another application with higher-audio priority gets access to it,
for instance in case of an incoming phone call on mobile OS. The [=User Agent=] SHOULD provide
this information to the web application through {{MediaStreamTrack/muted}} and
its associated events.</p>
<p>Whenever the [=User Agent=] initiates such an [= implementation-defined=]
change for camera or microphone sources, it MUST queue a
task, using the user interaction task source, to [=MediaStreamTrack/set a track's muted
state=] to the state desired by the user.</p>
<div class="note">This does not apply to [=source|sources=] defined in
other specifications. Other specifications need to define their own steps
to [=MediaStreamTrack/set a track's muted state=] if desired.</div>
<p>To <dfn class="export abstract-op" data-dfn-for="MediaStreamTrack"
id="set-track-muted">set a track's muted state</dfn> to
<var>newState</var>, the [=User Agent=] MUST run the following steps:</p>
<ol class="algorithm">
<li>
<p>Let <var>track</var> be the {{MediaStreamTrack}} in
question.</p>
</li>
<li>
<p>If <var>track</var>.{{MediaStreamTrack/[[Muted]]}} is already
<var>newState</var>, then abort these steps.</p>
</li>
<li>
<p>Set <var>track</var>.{{MediaStreamTrack/[[Muted]]}} to
<var>newState</var>.</p>
</li>
<li>
<p>If <var>newState</var> is <code>true</code> let
<var>eventName</var> be {{mute}}, otherwise
{{unmute}}.</p>
</li>
<li>
<p>[=Fire an event=] named <var>eventName</var> on
<var>track</var>.</p>
</li>
</ol>
<p><dfn data-export id="track-enabled" data-dfn-for="MediaStreamTrack" data-dfn-type="dfn" data-lt="track enabled state|enabled" data-lt-noDefault>Enabled/disabled</dfn> on the other hand is
available to the application to control (and observe) via the
{{MediaStreamTrack/enabled}}
attribute.</p>
<p>The result for the consumer is the same in the sense that whenever
{{MediaStreamTrack}} is muted or disabled (or both) the
consumer gets zero-information-content, which means silence for audio
and black frames for video. In other words, media from the source only
flows when a {{MediaStreamTrack}} object is both
unmuted and enabled. For example, a video element sourced by a
{{MediaStream}} containing only muted or disabled {{MediaStreamTrack}}s
for audio and video, is playing but rendering black video frames in
silence.</p>
<p>For a newly created {{MediaStreamTrack}} object, the
following applies: the track is always enabled unless stated otherwise
(for example when cloned) and the muted state reflects the state of the
source at the time the track is created.</p>
</section>
<section>
<h4>Life-cycle</h4>
<p>A {{MediaStreamTrack}} has two states in its
life-cycle: live and ended. A newly created
{{MediaStreamTrack}} can be in either state depending
on how it was created. For example, cloning an ended track results in a
new ended track. The current state is reflected by the object's
{{MediaStreamTrack/readyState}}
attribute.</p>
<p>In the live state, the track is active and media
(or zero-information-content if the {{MediaStreamTrack}} is
[= MediaStreamTrack/muted =] or [= MediaStreamTrack/enabled | disabled =])
is available for use by consumers.</p>
<p>If the source is a device exposed by `navigator.mediaDevices.`{{MediaDevices/getUserMedia()}},
then when a track becomes either
muted or disabled, and this brings all tracks connected to the device
to be either muted, disabled, or stopped, then the UA MAY, using the
device's {{MediaDeviceInfo/deviceId}}, <var>deviceId</var>, set
`navigator.mediaDevices.`{{MediaDevices/[[devicesLiveMap]]}}[<var>deviceId</var>] to <code>false</code>,
provided the UA sets it back to <code>true</code> as soon as any
unstopped track connected to this device becomes un-muted or enabled
again.</p>
<p>When a {{MediaStreamTrackState/"live"}}, [= MediaStreamTrack/muted | unmuted =], and
[= MediaStreamTrack/enabled =] track sourced by a device exposed
by {{MediaDevices/getUserMedia()}} becomes either
[= MediaStreamTrack/muted =] or [= MediaStreamTrack/enabled | disabled =],
and this brings <em>all</em> tracks connected to the device (across all
[=navigables=] the user agent operates) to be either
muted, disabled, or stopped, then the UA SHOULD relinquish the device
within 3 seconds while allowing time for a reasonably-observant user to
become aware of the transition. The UA SHOULD attempt to reacquire the
device as soon as any live track sourced by the device
becomes both [= MediaStreamTrack/muted | unmuted =] and
[= MediaStreamTrack/enabled =] again, provided that track's
[=relevant global object=]'s [=associated `Document`=]
[=Document/is in view=] at that time. If the
document is not [=Document/is in view|in view=] at that time,
the UA SHOULD instead queue a task to [=MediaStreamTrack/muted|mute=] the
track, and not queue a task to [=MediaStreamTrack/muted|unmute=] it until
the document comes [=Document/is in view|into view=].
If reacquiring the device fails, the UA MUST
[= track ended by the User agent | end the track =] (The UA MAY end it earlier
should it detect a device problem, like the device being physically
removed).</p>
<div class="note">
<p>The intent is to give users the assurance of privacy that having
physical camera (and microphone) hardware lights off brings, by
aligning physical and logical “privacy indicators”, at least while the
current document is the sole user of a device.</p>
<p>While other applications and documents using the device
simultaneously may interfere with this intent at times, they do not
interfere with the rules laid forth.</p>
</div>
<p>A {{MediaStreamTrack}} object is said to
<em>end</em> when the source of the track is disconnected or
exhausted.</p>
<p>If all {{MediaStreamTrack}}s that are using the same
source are [= MediaStreamTrack/ended =], the source will be
[= source/stopped =].</p>
<p>After the application has invoked the {{MediaStreamTrack/stop()}}
method on a {{MediaStreamTrack}} object, or once the [=source=] of a
{{MediaStreamTrack}} permanently ends production of live samples to its tracks,
whichever is sooner, a {{MediaStreamTrack}} is said to be
<dfn id="track-ended" data-dfn-for="MediaStreamTrack" data-dfn-type="dfn" data-export>ended</dfn>.</p>
<p>For camera and microphone sources, the reasons for a source to
[=MediaStreamTrack/ended|end=] besides {{MediaStreamTrack/stop()}} are
[=implementation-defined=]
(e.g., because the user rescinds the permission for the page to
use the local camera, or because the User
Agent has instructed the track to end for any reason).</p>
<p>When a {{MediaStreamTrack}} <var>track</var>
<dfn data-lt="track ended by the User agent" data-for=MediaStreamTrack data-export id="ends-nostop">ends for any reason other than the {{MediaStreamTrack/stop()}} method being
invoked</dfn>, the [=User Agent=] MUST queue a task that runs the following
steps:</p>
<ol class="algorithm">
<li>
<p>If <var>track</var>'s {{MediaStreamTrack/[[ReadyState]]}}
has the value {{MediaStreamTrackState/"ended"}} already, then abort these
steps.</p>
</li>
<li>
<p>Set <var>track</var>'s {{MediaStreamTrack/[[ReadyState]]}}
to {{MediaStreamTrackState/"ended"}}.</p>
</li>
<li>
<p>Notify <var>track</var>'s {{MediaStreamTrack/[[Source]]}} that <var>track</var> is
[= MediaStreamTrack/ended =] so that the source may be [= source/stopped =], unless other
{{MediaStreamTrack}} objects depend on it.</p>
</li>
<li>
<p>[=Fire an event=] named <a data-link-type=event>ended</a> at the object.</p>
</li>
</ol>
<p>If the end of the track was reached due to a user request, the event
source for this event is the user interaction event source.</p>
<p>To invoke the <dfn class="abstract-op">device permission revocation algorithm</dfn> with <var>permissionName</var>,
run the following steps:</p>
<ol class="algorithm">
<li>
<p>Let <var>tracks</var> be the set of all currently
{{MediaStreamTrackState/"live"}} <code>MediaStreamTrack</code>s
whose permission associated with this kind of track (<a>"camera"</a> or <a>"microphone"</a>)
matches <var>permissionName</var>.</p>
</li>
<li>
<p>For each <var>track</var> in <var>tracks</var>,
<a href="#ends-nostop">end</a> the track.</p>
</li>
</ol>
</section>
</section>
<section>
<h3>Tracks and Constraints</h3>
<p>{{MediaStreamTrack}} is a <a>constrainable
object</a> as defined in the <a href=
"#constrainable-interface">Constrainable Pattern</a> section.
Constraints are set on tracks and may affect sources.</p>
<p>Whether <code><a>Constraints</a></code> were provided at track
initialization time or need to be established later at runtime, the
APIs defined in the <a>ConstrainablePattern</a> Interface allow the
retrieval and manipulation of the constraints currently established on
a track.</p>
<p>Once ended, a track will continue exposing a
<dfn id="list-of-inherent-constrainable-track-properties">
list of inherent constrainable track properties</dfn>.
This list contains <code><a href="#def-constraint-deviceId">deviceId</a></code>,
<code><a href="#def-constraint-facingMode">facingMode</a></code> and
<code><a href="#def-constraint-groupId">groupId</a></code>.
</p>
</section>
<section id="media-stream-track-interface-definition">
<h3>Interface Definition</h3>
<div>
<pre class="idl"
>[Exposed=Window]
interface MediaStreamTrack : EventTarget {
readonly attribute DOMString kind;
readonly attribute DOMString id;
readonly attribute DOMString label;
attribute boolean enabled;
readonly attribute boolean muted;
attribute EventHandler onmute;
attribute EventHandler onunmute;
readonly attribute MediaStreamTrackState readyState;
attribute EventHandler onended;
MediaStreamTrack clone();
undefined stop();
MediaTrackCapabilities getCapabilities();
MediaTrackConstraints getConstraints();
MediaTrackSettings getSettings();
Promise<undefined> applyConstraints(optional MediaTrackConstraints constraints = {});
};</pre>
<section>
<h2>Attributes</h2>
<dl data-link-for="MediaStreamTrack" data-dfn-for=
"MediaStreamTrack" class="attributes">
<dt>{{kind}} of type {{DOMString}}, readonly</dt>
<dd>