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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html>
<!--
To publish this document, see instructions in README
-->
<html lang="en-us" xml:lang="en-us" xmlns="http://www.w3.org/1999/xhtml"
xmlns:ns="http://www.w3.org/1999/xhtml">
<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="http://www.w3.org/Tools/respec/respec-w3c-common"
type="text/javascript"> // <!-- keep this comment --> // </script>
<script class="remove" src="getusermedia.js" type="text/javascript"> //
<!-- keep this comment --> // </script>
</head>
<body>
<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. It is subject to major changes and,
while early experimentations are encouraged, it is therefore not
intended for implementation. The API is based on preliminary work done
in the WHATWG.</p>
</section>
<section class="informative" id="intro">
<h2>Introduction</h2>
<p>Access to multimedia streams (video, audio, or both) from local
devices (video cameras, microphones, Web cams) can have a number of
uses, such as real-time communication, recording, and surveillance.</p>
<p>This document defines the APIs used to get access to local devices
that can generate multimedia stream data. This document also defines the
MediaStream API by which JavaScript is able to manipulate the stream
data or otherwise process it.</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 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>
<i>HTML Terms:</i>
</dt>
<dd>
<p>The <code>
<a
href="http://dev.w3.org/html5/spec/webappapis.html#eventhandler">EventHandler</a>
</code> interface represents a callback used for event handlers as
defined in [[!HTML5]].</p>
<p>The concepts <dfn>
<a
href="http://dev.w3.org/html5/spec/webappapis.html#queue-a-task">queue
a task</a>
</dfn> and <dfn>
<a
href="http://dev.w3.org/html5/spec/webappapis.html#fire-a-simple-event">fires
a simple event</a>
</dfn> are defined in [[!HTML5]].</p>
<p>The terms <dfn>
<a
href="http://dev.w3.org/html5/spec/webappapis.html#event-handlers">event
handlers</a>
</dfn> and <dfn>
<a
href="http://dev.w3.org/html5/spec/webappapis.html#event-handler-event-type">
event handler event types</a>
</dfn> are defined in [[!HTML5]].</p>
</dd>
<dt>
<dfn>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.</p>
<p>Some sources have an identifier which <em class="rfc2119"
title="must">must</em> be unique to the application (un-guessable by
another application) and persistent between application sessions
(e.g., the identifier for a given source device/application must
stay the same, but not be guessable by another application). Sources
that must have an identifier are camera and microphone sources;
local file sources are not required to have an identifier. Source
identifiers let the application save, identify the availability of,
and directly request specific sources.</p>
<p>Other than the identifier, other bits of source identity are
<strong>never</strong> directly available to the application until
the user agent connects a source to a track. Once a source has been
"released" to the application (either via a permissions UI,
pre-configured allow-list, or some other release mechanism) the
application will be able discover additional source-specific
capabilities.</p>
<p>Sources <strong>do not</strong> have constraints -- tracks have
constraints. When a source is connected to a track, it must,
possibly in combination with UA processing (e.g., downsampling),
conform to the constraints present on that track (or set of
tracks).</p>
<p>Sources will be released (un-attached) from a track when the
track is ended for any reason.</p>
<p>On the <code>
<a>MediaStreamTrack</a>
</code> object, sources are represented by a <code>
<a>sourceType</a>
</code> attribute. The behavior of APIs associated with the
source's capabilities and settings change depending on the source
type.</p>
<p>Sources have <code>
<a>capabilities</a>
</code> and <code>
<a>settings</a>
</code>. The capabilities and settings are "owned" by the source
and are common to any (multiple) tracks that happen to be using the
same source (e.g., if two different track objects bound to the same
source ask for the same capability or setting information, they will
get back the same answer).</p>
</dd>
<dt> <a>Setting</a> (Source Setting) </dt>
<dd>
<p>A setting refers to the immediate, current value of the source's
(optionally constrained) capabilities. Settings are always
read-only.</p>
<p>A source's settings can change dynamically over time due to
environmental conditions, sink configurations, or constraint
changes. A source's settings must always conform to the current set
of mandatory constraints that all of the tracks it is bound to have
defined, and should do its best to conform to the set of optional
constraints specified.</p>
<p>Although settings are a property of the source, they are only
exposed to the application through the tracks attached to the
source. The <a>ConstrainablePattern</a> interface provides this
exposure.</p>
<p>A conforming user-agent <em class="rfc2119"
title="must">must</em> support all the setting names defined in this
spec.</p>
</dd>
<dt>
<a>Capabilities</a>
</dt>
<dd>
<p>Source capabilities are the intrinsic "features" of a source
object. For each source setting, there is a corresponding capability
that describes whether it is supported by the source and if so, what
the range of supported values are. 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 <a>getCapabilities()</a> call on a track returns the same
underlying per-source capabilities for all tracks connected to the
source.</p>
<p>Source capabilities are effectively constant. Applications should
be able to depend on a specific source having the same capabilities
for any session.</p>
</dd>
<dt>
<a>Constraints</a>
</dt>
<dd>
<p>Constraints are an optional track feature for restricting the
range of allowed variability on a source. Without provided track
constraints, implementations are free to select a source's settings
from the full ranges of its supported capabilities, and to adjust
those settings at any time for any reason.</p>
<p>Constraints are exposed on tracks via the
<a>ConstrainablePattern</a> interface, which includes an API for
dynamically changing constraints. Note that <a>getUserMedia()</a>
also permits an initial set of constraints to be applied when the
track is first obtained.</p>
<p>It is possible for two tracks that share a unique source to apply
contradictory constraints. The <a>ConstrainablePattern</a> interface
supports the calling of an error handler when the conflicting
constraint is requested. After successful application of constraints
on a track (and its associated source), if at any later time the
track becomes <a>overconstrained</a>, the user agent MUST change the
track to the <a>muted</a> state.</p>
<p>A correspondingly-named constraint exists for each corresponding
source setting name and capability name. 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 mandatory constraints sparingly.</p>
</dd>
<dt>
<code>RTCPeerConnection</code>
</dt>
<dd><dfn>
<code>RTCPeerConnection</code>
</dfn> is defined in [[!WEBRTC10]].</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 <code>
<a>MediaStreamTrack</a>
</code> and the <code>
<a>MediaStream</a>
</code> interfaces. The <code>
<a>MediaStreamTrack</a>
</code> object represents media originating from a single media
source in the user agent, e.g. video from a web camera. A <code>
<a>MediaStream</a>
</code> is used group several <code>
<a>MediaStreamTrack</a>
</code> objects into one unit that can be rendered in a media
element or recorded.</p>
<p>Each <code>
<a>MediaStream</a>
</code> can contain zero or more <code>
<a>MediaStreamTrack</a>
</code> objects. All tracks in a <code>
<a>MediaStream</a>
</code> are intended to be synchronized when rendered. Different
<code>
<a>MediaStream</a>
</code> 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 [[!WEBRTC10]], 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 <code>
<a>MediaStreamTrack</a>
</code> represents content comprising one or more channels, where
the channels have a defined well known relationship to each other
(such as a stereo or 5.1 audio signal). A channel is the smallest unit
considered in this API specification.</p>
<p>
<img alt="A MediaStream" src="images/media-stream.png" width="418" />
</p>
<p>A <code>
<a>MediaStream</a>
</code> object has an input and an output that represent the
combined input and output of all the object's tracks. The output of
the <code>
<a>MediaStream</a>
</code> 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 <code>video</code> element.</p>
<p>A new <code>
<a>MediaStream</a>
</code> object can be created from accessible media sources (that
does not require any additional permissions) using the <code>
<a href="#dom-mediastream">MediaStream()</a>
</code> constructor. The constructor argument can either be an
existing <code>
<a>MediaStream</a>
</code> object, in which case all the tracks of the given stream are
added to the new <code>
<a>MediaStream</a>
</code> object, or an array of <code>
<a>MediaStreamTrack</a>
</code> objects. The latter form makes it possible to compose a
stream from different source streams.</p>
<p>Both <code>
<a>MediaStream</a>
</code> and <code>
<a>MediaStreamTrack</a>
</code> objects can be cloned. This allows for greater control since
the separate instances can be manipulated and <a
title="consumer">consumed</a> individually. A cloned <code>
<a>MediaStream</a>
</code> contains clones of all member tracks from the original
stream.</p>
<p>When a <code>
<a>MediaStream</a>
</code> object is being generated from a local file (as opposed to a
live audio/video source), the user agent SHOULD stream the data from
the file in real time, not all at once. The <code>MediaStream</code>
object is also used in contexts outside <code>getUserMedia</code>,
such as [[!WEBRTC10]]. In both cases, ensuring a realtime stream
reduces the ease with which pages can distinguish live video from
pre-recorded video, which can help protect the user's privacy.</p>
</section>
<section>
<h2>MediaStream</h2>
<p>The <dfn id="dom-mediastream">
<code>MediaStream()</code>
</dfn> constructor composes a new stream out of existing tracks. It
takes an optional argument of type <code>
<a>MediaStream</a>
</code> or an array of <code>
<a>MediaStreamTrack</a>
</code> objects. <dfn id="mediastream-constructor">When the
constructor is invoked</dfn>, the UA must run the following steps:</p>
<ol>
<li>
<p>Let <var>stream</var> be a newly constructed <code>
<a>MediaStream</a>
</code> object.</p>
</li>
<li>
<p>Initialize <var>stream's</var> <code>
<a href="#dom-mediastream-id">id</a>
</code> attribute to a newly generated value.</p>
</li>
<li>
<p>If the constructor's argument is present, run the sub steps
that corresponds to the argument type.</p>
<ul>
<li>
<p><code>Array</code> of <code>
<a>MediaStreamTrack</a>
</code> objects:</p>
<p>Run the following sub steps for each <code>
<a>MediaStreamTrack</a>
</code> in the array:</p>
<ol>
<li>
<p><em>Add track</em>: Let <var>track</var> be the <code>
<a>MediaStreamTrack</a>
</code> about to be processed.</p>
</li>
<li>
<p>Add <var>track</var> to <var>stream</var>'s <a
href="#track-set">track set</a>.</p>
</li>
</ol>
</li>
<li>
<p><code>
<a>MediaStream</a>
</code>:</p>
<p>Run the sub steps labeled <em>Add track</em> (above) for
every <code>
<a>MediaStreamTrack</a>
</code> in the argument stream's <a href="#track-set">track
set</a>.</p>
</li>
</ul>
</li>
<li>
<p>If <var>stream</var>'s <a href="#track-set">track set</a> is
empty or only contains ended tracks, set <var>stream</var>'s <code>
<a href="#dom-mediastream-active">active</a>
</code> attribute to <code>false</code>, otherwise set it to
<code>true</code>.</p>
</li>
<li>
<p>Return <var>stream</var>.</p>
</li>
</ol>
<p>The tracks of a <code>
<a>MediaStream</a>
</code> are stored in a <dfn id="track-set">track set</dfn>. The
track set MUST contain the <code>
<a>MediaStreamTrack</a>
</code> 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 <code>
<a>MediaStreamTrack</a>
</code> object in the set is to look it up by its <code>
<a href="#dom-mediastreamtrack-id">id</a>
</code>.</p>
<p>An object that reads data from the output of a <code>
<a>MediaStream</a>
</code> is referred to as a <code>
<a>MediaStream</a>
</code> <dfn>consumer</dfn>. The list of <code>
<a>MediaStream</a>
</code> consumers currently include the media elements [[HTML5]],
<code>RTCPeerConnection</code> [[WEBRTC10]],
<code>MediaRecorder</code> [[mediastream-rec]] and
<code>ImageCapture</code> [[mediastream-imagecap]].</p>
<p class="note"><code>
<a>MediaStream</a>
</code> consumers must be able to handle tracks being added and
removed. This behavior is specified per consumer.</p>
<p>A <code>
<a>MediaStream</a>
</code> object is said to be <dfn
id="stream-inactive">MediaStream.inactive</dfn> when it does not have
any tracks or all tracks belonging to the stream have <a
href="#track-ended">ended</a>. Otherwise the stream is active. A <code>
<a>MediaStream</a>
</code> can start its life as inactive if it is constructed without
any tracks.</p>
<p>When a <code>
<a>MediaStream</a>
</code> goes from being active to inactive, the user agent MUST
queue a task that sets the object's <code>
<a href="#dom-mediastream-active">active</a>
</code> attribute to <code>false</code> and fire a simple event
named <code>
<a href="#event-mediastream-inactive">inactive</a>
</code> at the object. When a <code>
<a>MediaStream</a>
</code> goes from being inactive to active, the user agent MUST
queue a task that sets the object's <code>
<a href="#dom-mediastream-active">active</a>
</code> attribute to <code>true</code> and fire a simple event named
<code>
<a href="#event-mediastream-active">active</a>
</code> at the object.</p>
<p>If the stream's activity status changed due to a user request, the
task source for this <span title="concept-task">task</span> is the
user interaction task source. Otherwise the task source for this <span
title="concept-task">task</span> is the networking task source.</p>
<dl class="idl" title="interface MediaStream : EventTarget">
<dt>Constructor()</dt>
<dd> See the <a href="#mediastream-constructor">MediaStream
constructor algorithm</a> </dd>
<dt>Constructor(MediaStream stream)</dt>
<dd> See the <a href="#mediastream-constructor">MediaStream
constructor algorithm</a> </dd>
<dt>Constructor(sequence<MediaStreamTrack> tracks)</dt>
<dd> See the <a href="#mediastream-constructor">MediaStream
constructor algorithm</a> </dd>
<dt>readonly attribute DOMString id</dt>
<dd>
<p>When a <code>
<a>MediaStream</a>
</code> object is created, the user agent MUST generate an
identifier string, and MUST initialize the object's <code>
<a href="#dom-mediastream-id">id</a>
</code> attribute to that string. A good practice is to use an
UUID, which is 36 characters long in its canonical form.</p>
<p>The <dfn id="dom-mediastream-id">
<code>id</code>
</dfn> attribute MUST return the value to which it was
initialized when the object was created.</p>
</dd>
<dt>sequence<MediaStreamTrack> getAudioTracks()</dt>
<dd>
<p>Returns a sequence of <code>
<a>MediaStreamTrack</a>
</code> objects representing the audio tracks in this
stream.</p>
<p>The <dfn id="dom-mediastream-getaudiotracks">
<code>getAudioTracks()</code>
</dfn> method MUST return a sequence that represents a snapshot
of all the <code>
<a>MediaStreamTrack</a>
</code> objects in this stream's <a href="#track-set">track
set</a> whose <code>
<a href="#dom-mediastreamtrack-kind">kind</a>
</code> is equal to "<code>audio</code>". The conversion from
the <a href="#track-set">track set</a> to the sequence is user
agent defined and the order does not have to stable between
calls.</p>
</dd>
<dt>sequence<MediaStreamTrack> getVideoTracks()</dt>
<dd>
<p>Returns a sequence of <code>
<a>MediaStreamTrack</a>
</code> objects representing the video tracks in this
stream.</p>
<p>The <dfn id="dom-mediastream-getvideotracks">
<code>getVideoTracks()</code>
</dfn> method MUST return a sequence that represents a snapshot
of all the <code>
<a>MediaStreamTrack</a>
</code> objects in this stream's <a href="#track-set">track
set</a> whose <code>
<a href="#dom-mediastreamtrack-kind">kind</a>
</code> is equal to "<code>video</code>". The conversion from
the <a href="#track-set">track set</a> to the sequence is user
agent defined and the order does not have to stable between
calls.</p>
</dd>
<dt>sequence<MediaStreamTrack> getTracks()</dt>
<dd>
<p>Returns a sequence of <code>
<a>MediaStreamTrack</a>
</code> objects representing all the tracks in this stream.</p>
<p>The <dfn id="dom-mediastream-gettracks">
<code>getTracks()</code>
</dfn> method MUST return a sequence that represents a snapshot
of all the <code>
<a>MediaStreamTrack</a>
</code> objects in this stream's <a href="#track-set">track
set</a>, regardless of kind. The conversion from the <a
href="#track-set">track set</a> to the sequence is user agent
defined and the order does not have to stable between calls.</p>
</dd>
<dt>MediaStreamTrack? getTrackById(DOMString trackId)</dt>
<dd>
<p>The <dfn id="dom-mediastream-gettrackbyid">
<code>getTrackById()</code>
</dfn> method MUST return the first <code>
<a>MediaStreamTrack</a>
</code> object in this stream's <a href="#track-set">track
set</a> whose <code>
<a href="#dom-mediastreamtrack-id">id</a>
</code> is equal to <var>trackId</var>. The method MUST return
null if no track matches the <var>trackId</var> argument.</p>
</dd>
<dt>void addTrack(MediaStreamTrack track)</dt>
<dd>
<p>Adds the given <code>
<a>MediaStreamTrack</a>
</code> to this <code>
<a>MediaStream</a>
</code>.</p>
<p>When the <dfn id="dom-mediastream-addtrack">
<code>addTrack()</code>
</dfn> method is invoked, the user agent MUST run the following
steps:</p>
<ol>
<li>
<p>Let <var>track</var> be the <code>
<a>MediaStreamTrack</a>
</code> argument and <var>stream</var> this <code>
<a>MediaStream</a>
</code> object.</p>
</li>
<li>
<p>If <var>track</var> is already in <var>stream's</var> <a
href="#track-set">track set</a>, then abort these steps.</p>
</li>
<li>
<p>Add <var>track</var> to <var>stream</var>'s <a
href="#track-set">track set</a>.</p>
</li>
</ol>
</dd>
<dt>void removeTrack(MediaStreamTrack track)</dt>
<dd>
<p>Removes the given <code>
<a>MediaStreamTrack</a>
</code> from this <code>
<a>MediaStream</a>
</code>.</p>
<p>When the <dfn id="dom-mediastream-removetrack">
<code>removeTrack()</code>
</dfn> method is invoked, the user agent MUST remove the track,
indicated by the method's argument, from the stream's <a
href="#track-set">track set</a>, if present.</p>
</dd>
<dt>MediaStream clone()</dt>
<dd>
<p>Clones the given <code>
<a>MediaStream</a>
</code> and all its tracks.</p>
<p>When the <dfn id="dom-mediastream-clone">
<code>MediaStream.clone()</code>
</dfn> method is invoked, the user agent MUST run the following
steps:</p>
<ol>
<li>
<p>Let <var>streamClone</var> be a newly constructed <code>
<a>MediaStream</a>
</code> object.</p>
</li>
<li>
<p>Initialize <var>streamClone</var>'s <code>
<a href="#dom-mediastream-id">id</a>
</code> attribute to a newly generated value.</p>
</li>
<li>
<p>Let <var>clonedTracks</var> be a list that contains the
result of running <code>
<a
href="#dom-mediastreamtrack-clone">MediaStreamTrack.clone()</a>
</code> on all the tracks in the stream on which this method
was called. </p>
</li>
<li>
<p>Let <var>clonedTracks</var> be <var>streamClone</var>'s <a
href="#track-set">track set</a>.</p>
</li>
<li>Return <var>streamClone</var>.</li>
</ol>
</dd>
<dt>readonly attribute boolean active</dt>
<dd>
<p>The <dfn id="dom-mediastream-active">
<code>MediaStream.active</code>
</dfn> attribute returns true if the <code>
<a>MediaStream</a>
</code> is active (see <a href="#stream-inactive">inactive</a>),
and false otherwise.</p>
<p>When a <code>
<a>MediaStream</a>
</code> object is created, its <code>
<a href="#dom-mediastream-active">active</a>
</code> attribute MUST be set to true, unless stated otherwise
(for example by the <code>
<a href="#dom-mediastream">MediaStream()</a>
</code> constructor algorithm).</p>
</dd>
<dt>attribute EventHandler onactive</dt>
<dd>This event handler, of type <code>
<a href="#event-mediastream-active">active</a>
</code>, MUST be supported by all objects implementing the <code>
<a>MediaStream</a>
</code> interface.</dd>
<dt>attribute EventHandler oninactive</dt>
<dd>This event handler, of type <code>
<a href="#event-mediastream-inactive">inactive</a>
</code>, MUST be supported by all objects implementing the <code>
<a>MediaStream</a>
</code> interface.</dd>
<dt>attribute EventHandler onaddtrack</dt>
<dd>This event handler, of type <code>
<a href="#event-mediastream-addtrack">addtrack</a>
</code>, MUST be supported by all objects implementing the <code>
<a>MediaStream</a>
</code> interface.</dd>
<dt>attribute EventHandler onremovetrack</dt>
<dd>This event handler, of type <code>
<a href="#event-mediastream-removetrack">removetrack</a>
</code>, MUST be supported by all objects implementing the <code>
<a>MediaStream</a>
</code> interface.</dd>
</dl>
</section>
<section>
<h2>MediaStreamTrack</h2>
<p>A <code>
<a>MediaStreamTrack</a>
</code> object represents a media source in the user agent. Several
<code>
<a>MediaStreamTrack</a>
</code> 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 <code>
<a href="#dom-mediadevices-getusermedia">getUserMedia()</a>
</code> .</p>
<p>The data from a <code>
<a>MediaStreamTrack</a>
</code> 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 track no longer needs its source with
the <code>
<a href="#dom-mediastreamtrack-stop">MediaStreamTrack.stop()</a>
</code> method. When all tracks using a source have been stopped,
the given permission for that source is revoked and the source is <dfn
id="source-stopped">stopped</dfn>. If the data is being generated from
a live source (e.g., a microphone or camera), then the user agent
SHOULD remove any active "on-air" indicator for that source. If the
data is being generated from a prerecorded source (e.g. a video file),
any remaining content in the file is ignored. 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>If there is no stored permission to use that source, the UA SHOULD
also remove the "permission granted" indicator for the source.</p>
<section>
<h3>Life-cycle and Media Flow</h3>
<h4>Life-cycle</h4>
<p>A <code>
<a>MediaStreamTrack</a>
</code> has two stages in its life-cycle: <code>live</code> and
<code>ended</code>. A newly created <code>
<a>MediaStreamTrack</a>
</code> can be in any stage depending on how it was created. For
example, cloning an ended track results in a new ended track. The
current stage is reflected by the object's <code>
<a href="#dom-mediastreamtrack-readystate">readyState</a>
</code> attribute.</p>
<p>In the <code>live</code> state, the track is active and media is
available for use by consumers (but may be replaced by
zero-information-content if the <code>
<a>MediaStreamTrack</a>
</code> is <a href="#track-muted">muted</a> or <a
href="#track-enabled">enabled</a>, see below).</p>
<p>A muted or disabled <code>
<a>MediaStreamTrack</a>
</code> renders either silence (audio), black frames (video), or a
zero-information-content equivalent. For example, a video element
sourced by a muted or disabled <code>
<a>MediaStreamTrack</a>
</code> (contained within a <code>
<a>MediaStream</a>
</code>), is playing but the rendered content is the muted output.
When all tracks connected to a source are muted or disabled, the
"on-air" or "recording" indicator for that source can be turned off;
when the track is no longer muted or disabled, it MUST be turned
back on.</p>
<p>The muted/unmuted state of a track reflects if the source
provides any media at this moment. The enabled/disabled state is
under application control and determines if the track outputs media
(to its consumers). Hence, media from the source only flows when a
<code>
<a>MediaStreamTrack</a>
</code> object is both unmuted and enabled.</p>
<p>A <code>
<a>MediaStreamTrack</a>
</code> is <dfn id="track-muted">muted</dfn> when the source is
temporarily unable to provide the track with data. A track can be
muted by a user. Often this action is outside the control of the
application. This could be as a result of the user hitting a
hardware switch, or toggling a control in the operating system or
browser chrome. A track can also be muted by the user agent.</p>
<p>Applications are able to <dfn id="track-enabled">enable</dfn> or
disable a <code>
<a>MediaStreamTrack</a>
</code> to prevent it from rendering media from the source. A
muted track will however, regardless of the enabled state, render
silence and blackness. A disabled track is logically equivalent to a
muted track, from a consumer point of view.</p>
<p>For a newly created <code>
<a>MediaStreamTrack</a>
</code> 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>
<p>A <code>
<a>MediaStreamTrack</a>
</code> object is said to <em>end</em> when the source of the
track is disconnected or exhausted.</p>
<p>A <code>
<a>MediaStreamTrack</a>
</code> can be <dfn id="track-detached">detached</dfn> from its
source. It means that the track is no longer dependent on the source
for media data. If no other <code>
<a>MediaStreamTrack</a>
</code> is using the same source, the source will be <a
href="#source-stopped">stopped</a>. <code>
<a>MediaStreamTrack</a>
</code> attributes such as <code>
<a href="#dom-mediastreamtrack-kind">kind</a>
</code> and <code>
<a href="#dom-mediastreamtrack-label">label</a>
</code> MUST not change values when the source is detached.</p>
<p>When a <code>
<a>MediaStreamTrack</a>
</code> object ends for any reason (e.g., because the user
rescinds the permission for the page to use the local camera, or
because the data comes from a finite file and the file's end has
been reached and the user has not requested that it be looped, or
because the application invoked the <code>
<a href="#dom-mediastreamtrack-stop">stop()</a>
</code> method on the <code>
<a>MediaStreamTrack</a>
</code> object, or because the UA has instructed the track to end
for any reason) it is said to be <dfn
id="track-ended">ended</dfn>.</p>
<p>When a <code>
<a>MediaStreamTrack</a>
</code> <var>track</var> ends for any reason other than the <code>
<a href="#dom-mediastreamtrack-stop">stop()</a>
</code> method being invoked, the user agent MUST queue a task
that runs the following steps:</p>
<ol>
<li>
<p>If the <var>track's</var> <code>
<a href="#dom-mediastreamtrack-readystate">readyState</a>
</code> attribute has the value <code>ended</code> already,
then abort these steps.</p>
</li>
<li>
<p>Set <var>track's</var> <code>
<a href="#dom-mediastreamtrack-readystate">readyState</a>
</code> attribute to <code>ended</code>.</p>
</li>
<li>
<p><a href="#track-detached">Detach</a> <var>track's</var>
source.</p>
</li>
<li>
<p>Fire a simple event named <code>
<a href="#event-mediastreamtrack-ended">ended</a>
</code> at the object.</p>
</li>
</ol>
<p>If the end of the stream was reached due to a user request, the
event source for this event is the user interaction event
source.</p>
<h4>Media Flow</h4>
<p>There are two concepts related to the media flow for a
<code>live</code> <code>
<a>MediaStreamTrack</a>
</code>: muted or not, and enabled or disabled.</p>
<p><dfn id="track-muted">Muted</dfn> refers to the input to the
<code>
<a>MediaStreamTrack</a>
</code>. If live samples are not made available to the <code>
<a>MediaStreamTrack</a>
</code> it is muted.</p>
<p>Muted is out of control for the application, but can be observed
by the application by reading the <code>
<a href="#dom-mediastreamtrack-muted">muted</a>
</code> attribute and listening to the associated events <code>
<a href="#event-mediastreamtrack-mute">mute</a>
</code> and <code>
<a href="#event-mediastreamtrack-unmute">unmute</a>
</code>.There can be several reasons for a <code>
<a>MediaStreamTrack</a>
</code> to be muted: the user pushing a physical mute button on
the microphone, the user toggling a control in the operating system,
the user clicking a mute button in the browser chrome, the UA (on
behalf of the user) mutes, etc.</p>
<p><dfn id="track-enabled">Enabled/disabled</dfn> on the other hand
is available to application to control (and observe) via the <code>
<a href="#dom-mediastreamtrack-enabled">enabled</a>
</code> attribute.</p>
<p>The result for the consumer is the same in the meaning that
whenever <code>
<a>MediaStreamTrack</a>
</code> 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 <code>
<a>MediaStreamTrack</a>
</code> object is both unmuted and enabled. For example, a video
element sourced by a muted or disabled <code>
<a>MediaStreamTrack</a>
</code> (contained in a <code>
<a>MediaStream</a>
</code>), is playing but rendering blackness.</p>
<p>For a newly created <code>
<a>MediaStreamTrack</a>
</code> 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>
<h3>Tracks and Constraints</h3>
<p>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>Each track maintains an internal version of the <code>
<a>Constraints</a>
</code> structure, namely a mandatory set of constraints (no
duplicates), and an optional ordered list of individual constraint
objects (may contain duplicates). The internal stored constraint
structure is exposed to the application by the <code>
<a>constraints</a>
</code> attribute, and may be modified by the <code>