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media-source-respec.html
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<!DOCTYPE html SYSTEM "about:legacy-compat">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>Media Source Extensions</title>
<script src="respec-w3c-common.js" class="remove"></script>
<script src="media-source.js" class="remove"></script>
<script class="remove">
var respecConfig = {
// specification status (e.g., WD, LCWD, NOTE, etc.). If in doubt use ED.
specStatus: "ED",
useExperimentalStyles: true,
// the specification's short name, as in http://www.w3.org/TR/short-name/
shortName: "media-source",
// if there a publicly available Editor's Draft, this is the link
edDraftURI: "http://w3c.github.io/media-source/",
// if this is a LCWD, uncomment and set the end of its review period
// lcEnd: "2009-08-05",
/* Commented out to enable auto-updating of publishDate during respec execution
publishDate: "2016-07-26",
*/
previousMaturity: "CR",
previousPublishDate: "2015-07-05",
// editors, add as many as you like
// only "name" is required
editors: [
{ name: "Matthew Wolenetz", url: "",
company: "Google Inc.", companyURL: "http://www.google.com/" },
{ name: "Jerry Smith", url: "",
company: "Microsoft Corporation", companyURL: "http://www.microsoft.com/" },
{ name: "Mark Watson", url: "",
company: "Netflix Inc.", companyURL: "http://www.netflix.com/" },
{ name: "Aaron Colwell (until April 2015)", url: "",
company: "Google Inc.", companyURL: "http://www.google.com/" },
{ name: "Adrian Bateman (until April 2015)", url: "",
company: "Microsoft Corporation", companyURL: "http://www.microsoft.com/" },
],
mseDefGroupName: "media-source",
mseContributors: [
"Bob Lund",
"Alex Giladi",
"Duncan Rowden",
"Mark Vickers",
"Glenn Adams",
"Frank Galligan",
"Steven Robertson",
"Matt Ward",
"David Dorwin",
"Kevin Streeter",
"Joe Steele",
"Michael Thornburgh",
"Philip Jägenstedt",
"John Simmons",
"Pierre Lemieux",
"Cyril Concolato",
"Ralph Giles",
"David Singer",
"Tatsuya Igarashi",
"Chris Poole",
"Jer Noble",
"Matthew Gregan"
],
// name of the WG
wg: "HTML Media Extensions Working Group",
// URI of the public WG page
wgURI: "http://www.w3.org/html/wg/",
// name (without the @w3c.org) of the public mailing to which comments are due
wgPublicList: "public-html-media",
// URI of the patent status for this WG, for Rec-track documents
// !!!! IMPORTANT !!!!
// This is important for Rec-track documents, do not copy a patent URI from a random
// document unless you know what you're doing. If in doubt ask your friendly neighbourhood
// Team Contact.
wgPatentURI: "http://www.w3.org/2004/01/pp-impl/40318/status",
crEnd: "2016-06-7",
noIDLIn: true,
scheme: "https",
otherLinks: [{
key: 'Repository',
data: [{
value: 'We are on GitHub',
href: 'https://github.com/w3c/media-source/'
}, {
value: 'File a bug',
href: 'https://github.com/w3c/media-source/issues'
}, {
value: 'Commit history',
href: 'https://github.com/w3c/media-source/commits/gh-pages/media-source-respec.html'
}]
},{
key: 'Mailing list',
data: [{
value: 'public-html-media@w3.org',
href: 'https://lists.w3.org/Archives/Public/public-html-media/'
}]
},{
key: 'Implementation',
data: [{
value: 'Can I use Media Source Extensions?',
href: 'http://caniuse.com/#feat=mediasource'
}, {
value: 'Test Suite',
href: 'http://w3c-test.org/media-source/'
}, {
value: 'Test Suite repository',
href: 'https://github.com/w3c/web-platform-tests/tree/master/media-source'
}]
}],
preProcess: [ mediaSourcePreProcessor ],
// Empty definitions for objects declared in the document are here to
// prevent error messages from being displayed for references to these objects.
definitionMap: {
MediaSource: function() {},
SourceBuffer: function() {},
SourceBufferList: function() {},
AppendMode: function() {},
VideoPlaybackQuality: function() {},
ReadableStream: function() {},
ArrayBuffer: function() {},
ArrayBufferView: function() {},
},
postProcess: [ mediaSourcePostProcessor ],
localBiblio: {
"W3C-STREAMS-API": {
title: "W3C Streams API",
href: "http://www.w3.org/TR/streams-api/",
authors: ["Feras Moussa", "Takeshi Yoshino"],
status: "WD",
publisher: "W3C",
},
"WHATWG-STREAMS-API": {
title: "WHATWG Streams API",
href: "https://streams.spec.whatwg.org/",
authors: ["Domenic Denicola", "Takeshi Yoshino"],
publisher: "WHATWG",
},
"MSE-REGISTRY": {
title: "Media Source Extensions Byte Stream Format Registry",
href: "byte-stream-format-registry.html",
//href: "http://www.w3.org/2013/12/byte-stream-format-registry/",
authors: ["Matthew Wolenetz", "Jerry Smith", "Aaron Colwell"],
publisher: "W3C",
},
"INBANDTRACKS": {
title: "Sourcing In-band Media Resource Tracks from Media Containers into HTML",
href: "http://dev.w3.org/html5/html-sourcing-inband-tracks/",
authors: ["Bob Lund", "Silvia Pfeiffer"],
publisher: "W3C",
}
}
};
</script>
<!-- script to register bugs -->
<!-- Disabled unless/until it supports GitHub issues.
<script src="https://w3c.github.io/webcomponents/assets/scripts/bug-assist.js"></script>
<meta name="bug.short_desc" content="[MSE] ">
<meta name="bug.product" content="HTML WG">
<meta name="bug.component" content="Media Source Extensions">
-->
<link rel="stylesheet" href="mse.css">
</head>
<body>
<section id="sotd">
<p>The working group maintains <a href="https://github.com/w3c/media-source/issues">a list of all bug reports that the editors have not yet tried to address</a>; there may also be open bugs in the <a href="http://w3.org/brief/Mjcw">previous bug tracker</a>. This draft highlights some of the pending issues that are still to be discussed in the working group. No decision has been taken on the outcome of these issues including whether they are valid.</p>
<p>Implementors should be aware that this specification is not stable. <strong>Implementors who are not taking part in the discussions are likely to find the specification changing out from under them in incompatible ways.</strong> Vendors interested in implementing this specification before it eventually reaches the Candidate Recommendation stage should join the mailing list mentioned below and take part in the discussions.</p>
<p>The following features are <strong>at risk</strong> and may be removed:</p>
<ul>
<li><p>The <a def-id="appendStream">appendStream</a> method on the <a href="#sourcebuffer">sourceBuffer</a> object and algorithm steps that are used only by it.</p></li>
<li><p>The <a href="#videoplaybackquality">VideoPlaybackQuality</a> object and the <a href="#htmlvideoelement-extensions">HTMLVideoElement Extensions</a> that use it.</p></li>
<li><p>The <a def-id="totalFrameDelay">totalFrameDelay</a> attribute on the <a href="#videoplaybackquality">VideoPlaybackQuality</a> object.</p></li>
<li><p>The <a href=#idl-def-TrackDefault>TrackDefault</a> object and its related objects, attributes, methods and algorithms, including: <a href=#idl-def-TrackDefaultList>TrackDefaultList</a>, <a href=#widl-SourceBuffer-trackDefaults>SourceBuffer.trackDefaults</a>, <a href=#sourcebuffer-default-track-language>Default track language</a>, <a href=#sourcebuffer-default-track-label>Default track label</a>, and <a href=#sourcebuffer-default-track-kinds>Default track kinds</a>. Implementations still MUST at least include trivial versions of these default track language/label/kinds algorithms that are used by the <a href=#sourcebuffer-init-segment-received>initialization segment received algorithm</a>. These trivial versions would behave as follows: Trivial default track language and Trivial default track label algorithms MUST return an empty string, and Trivial default kinds algorithm MUST return a sequence with a single empty string element in it.</p></li>
</ul>
<p>
For this specification to exit the Candidate Recommendation stage,
two independent implementations as detailed in the <a href="http://dev.w3.org/html5/decision-policy/public-permissive-exit-criteria.html">CR Exit Criteria (Public Permissive version 3)</a> document
will be required to pass each test in the MSE test suite to be
developed by the HTML Media Extensions WG.
</p>
</section>
<section id="abstract">
This specification extends <a def-id="htmlmediaelement">HTMLMediaElement</a> [[HTML51]] to allow
JavaScript to generate media streams for playback.
Allowing JavaScript to generate streams facilitates a variety of use
cases like adaptive streaming and time shifting live streams.
<p>If you wish to make comments or file bugs regarding this document in a manner that is tracked by the editors, please submit them via
<a href="https://github.com/w3c/media-source/issues/new">our public bug database</a>.
</p>
</section>
<section id="introduction">
<h2>Introduction</h2>
<p>This specification allows JavaScript to dynamically construct media streams for <audio> and <video>.
It defines objects that allow JavaScript to pass media segments to an <a def-id="htmlmediaelement">HTMLMediaElement</a> [[!HTML51]].
A buffering model is also included to describe how the user agent acts when different media segments are
appended at different times. Byte stream specifications used with these extensions are available in the byte stream format registry [[MSE-REGISTRY]].</p>
<img src="pipeline_model.svg" alt="Media Source Pipeline Model Diagram" longdesc='pipeline_model_description.html#pipelinedesc'>
<section id="goals">
<h3>Goals</h3>
<p>This specification was designed with the following goals in mind:</p>
<ul>
<li>Allow JavaScript to construct media streams independent of how the media is fetched.</li>
<li>Define a splicing and buffering model that facilitates use cases like adaptive streaming, ad-insertion, time-shifting, and video editing.</li>
<li>Minimize the need for media parsing in JavaScript.</li>
<li>Leverage the browser cache as much as possible.</li>
<li>Provide requirements for byte stream format specifications.</li>
<li>Not require support for any particular media format or codec.</li>
</ul>
<p>This specification defines:</p>
<ul>
<li>Normative behavior for user agents to enable interoperability between user agents and web applications when processing media data.</li>
<li>Normative requirements to enable other specifications to define media formats to be used within this specification.</li>
</ul>
</section>
<section id="definitions">
<h3>Definitions</h3>
<dl>
<dt id="active-track-buffers">Active Track Buffers</dt>
<dd><p>The <a def-id="track-buffers"></a> that provide <a def-id="coded-frames"></a> for the <a def-id="audiotrack-enabled"></a>
<a def-id="audiotracks"></a>, the <a def-id="videotrack-selected"></a> <a def-id="videotracks"></a>, and the
<a def-id="texttrackmode-showing"></a> or <a def-id="texttrackmode-hidden"></a> <a def-id="texttracks"></a>. All these tracks are associated with
<a>SourceBuffer</a> objects in the <a def-id="activeSourceBuffers"></a> list.</p>
</dd>
<dt id="append-window">Append Window</dt>
<dd><p>A <a def-id="presentation-timestamp"></a> range used to filter out <a def-id="coded-frames"></a> while appending. The append window represents a single
continuous time range with a single start time and end time. Coded frames with <a def-id="presentation-timestamp"></a> within this range are allowed to be appended
to the <a>SourceBuffer</a> while coded frames outside this range are filtered out. The append window start and end times are controlled by
the <a def-id="appendWindowStart"></a> and <a def-id="appendWindowEnd"></a> attributes respectively.</p></dd>
<dt id="coded-frame">Coded Frame</dt>
<dd><p>A unit of media data that has a <a def-id="presentation-timestamp"></a>, a <a def-id="decode-timestamp"></a>, and a <a def-id="coded-frame-duration"></a>.</p></dd>
<dt id="coded-frame-duration">Coded Frame Duration</dt>
<dd>
<p>The duration of a <a def-id="coded-frame"></a>. For video and text, the duration indicates how long the video frame or text SHOULD be displayed. For audio, the duration represents the sum of all the samples contained within the coded frame. For example, if an audio frame contained 441 samples @44100Hz the frame duration would be 10 milliseconds.</p>
</dd>
<dt id="coded-frame-end-timestamp">Coded Frame End Timestamp</dt>
<dd>
<p>The sum of a <a def-id="coded-frame"></a> <a def-id="presentation-timestamp"></a> and its
<a def-id="coded-frame-duration"></a>. It represents the <a def-id="presentation-timestamp"></a> that immediately follows the coded frame.</p>
</dd>
<dt id="coded-frame-group">Coded Frame Group</dt>
<dd><p>A group of <a def-id="coded-frames"></a> that are adjacent and have monotonically increasing <a def-id="decode-timestamps"></a> without any gaps. Discontinuities detected by the
<a def-id="coded-frame-processing-algorithm"></a> and <a def-id="abort"></a> calls trigger the start of a new coded frame group.</p>
</dd>
<dt id="decode-timestamp">Decode Timestamp</dt>
<dd>
<p> The decode timestamp indicates the latest time at which the frame needs to be decoded assuming instantaneous decoding and rendering of this and any dependant frames (this is equal to the <a def-id="presentation-timestamp"></a> of the earliest frame, in <a def-id="presentation-order"></a>, that is dependant on this frame). If frames can be decoded out of <a def-id="presentation-order"></a>, then the decode timestamp MUST be present in or derivable from the byte stream. The user agent MUST run the <a def-id="append-decode-error-algorithm"></a> if this is not the case. If frames cannot be decoded out of <a def-id="presentation-order"></a> and a decode timestamp is not present in the byte stream, then the decode timestamp is equal to the <a def-id="presentation-timestamp"></a>.</p>
</dd>
<dt id="displayed-frame-delay">Displayed Frame Delay</dt>
<dd>
<p>The delay between a frame's presentation time and the actual time it was displayed, in a double-precision value in seconds and rounded to the nearest display refresh interval. This
delay is always greater than or equal to zero since frames MUST never be displayed before their presentation time. Non-zero delays are a sign of playback jitter
and possible loss of A/V sync.</p>
</dd>
<dt id="init-segment">Initialization Segment</dt>
<dd>
<p>A sequence of bytes that contain all of the initialization information required to decode a sequence of <a def-id="media-segments"></a>. This includes codec initialization data, <a def-id="track-id"></a> mappings for multiplexed segments, and timestamp offsets (e.g., edit lists).</p>
<p class="note">The <a def-id="byte-stream-format-specs"></a> in the byte stream format registry [[MSE-REGISTRY]] contain format specific examples.</p>
<dt id="media-segment">Media Segment</dt>
<dd>
<p>A sequence of bytes that contain packetized & timestamped media data for a portion of the <a def-id="media-timeline"></a>. Media segments are always associated with the most recently appended <a def-id="init-segment"></a>.</p>
<p class="note">The <a def-id="byte-stream-format-specs"></a> in the byte stream format registry [[MSE-REGISTRY]] contain format specific examples.</p>
</dd>
<dt id="mediasource-object-url">MediaSource object URL</dt>
<dd>
<p>A MediaSource object URL is a unique <a def-id="blob-uri"></a> [[!FILE-API]] created by <a def-id="createObjectURL"></a>. It is used to attach a <a>MediaSource</a> object to an HTMLMediaElement.</p>
<p>These URLs are the same as a <a def-id="blob-uri"></a>, except that anything in the definition of that feature that refers to <a def-id="File"></a> and <a def-id="Blob"></a> objects is hereby extended to also apply to <a>MediaSource</a> objects.</p>
<p>The <a def-id="origin">origin</a> of the MediaSource object URL is the <a def-id="relevant-settings-object"></a> of <code>this</code> during the call to <a def-id="createObjectURL"></a>.</p>
<p class="note">For example, the <a def-id="origin"></a> of the MediaSource object URL affects the way that the media element is <a href="https://www.w3.org/TR/html51/semantics-scripting.html#security-with-canvas-elements">consumed by canvas</a>.</p>
</dd>
<dt id="parent-media-source">Parent Media Source</dt>
<dd><p>The parent media source of a <a>SourceBuffer</a> object is the <a>MediaSource</a> object that created it.</p></dd>
<dt id="presentation-start-time">Presentation Start Time</dt>
<dd><p>The presentation start time is the earliest time point in the presentation and specifies the <a def-id="videoref" name="initial-playback-position">initial playback position</a> and <a def-id="videoref" name="earliest-possible-position">earliest possible position</a>. All presentations created using this specification have a presentation start time of 0.</dd>
<p class="note">For the purposes of determining if <a def-id="hme-buffered"></a> contains a <a def-id="timerange"></a> that includes the current playback position, implementations MAY choose to allow a current playback position at or after <a def-id="presentation-start-time"></a> and before the first <a def-id="timerange"></a> to play the first <a def-id="timerange"></a> if that <a def-id="timerange"></a> starts within a reasonably short time, like 1 second, after <a def-id="presentation-start-time"></a>. This allowance accommodates the reality that muxed streams commonly do not begin all tracks precisely at <a def-id="presentation-start-time"></a>. Implementations MUST report the actual buffered range, regardless of this allowance.</p>
<dt id="presentation-interval">Presentation Interval</dt>
<dd>
<p>The presentation interval of a <a def-id="coded-frame"></a> is the time interval from its <a def-id="presentation-timestamp"></a> to the <a def-id="presentation-timestamp"></a> plus the <a def-id="coded-frames-duration"></a>. For example, if a coded frame has a presentation timestamp of 10 seconds and a <a def-id="coded-frame-duration"></a> of 100 milliseconds, then the presentation interval would be [10-10.1). Note that the start of the range is inclusive, but the end of the range is exclusive.</p>
</dd>
<dt id="presentation-order">Presentation Order</dt>
<dd>
<p>The order that <a def-id="coded-frames"></a> are rendered in the presentation. The presentation order is achieved by ordering <a def-id="coded-frames"></a> in monotonically increasing order by their <a def-id="presentation-timestamps"></a>.</p>
</dd>
<dt id="presentation-timestamp">Presentation Timestamp</dt>
<dd>
<p>A reference to a specific time in the presentation. The presentation timestamp in a <a def-id="coded-frame"></a> indicates when the frame SHOULD be rendered.</p>
</dd>
<dt id="random-access-point">Random Access Point</dt>
<dd><p>A position in a <a def-id="media-segment"></a> where decoding and continuous playback can begin without relying on any previous data in the segment. For video this tends to be the location of I-frames. In the case of audio, most audio frames can be treated as a random access point. Since video tracks tend to have a more sparse distribution of random access points, the location of these points are usually considered the random access points for multiplexed streams.</p></dd>
<dt id="sourcebuffer-byte-stream-format-spec">SourceBuffer byte stream format specification</dt>
<dd><p>The specific <a def-id="byte-stream-format-spec"></a> that describes the format of the byte stream accepted by a <a>SourceBuffer</a> instance. The
<a def-id="byte-stream-format-spec"></a>, for a <a>SourceBuffer</a> object, is selected based on the <var>type</var> passed to the
<a def-id="addSourceBuffer"></a> call that created the object.</p></dd>
<dt id="sourcebuffer-configuration">SourceBuffer configuration</dt>
<dd><p>A specific set of tracks distributed across one or more <a>SourceBuffer</a>
objects owned by a single <a>MediaSource</a> instance.</p>
<p>Implementations MUST support at least 1 <a>MediaSource</a> object with the following
configurations:</p>
<ul>
<li>A single SourceBuffer with 1 audio track and/or 1 video track.</li>
<li>Two SourceBuffers with one handling a single audio track and the other handling a single video track.</li>
</ul>
<p>MediaSource objects MUST support each of the configurations above, but they are only
required to support one configuration at a time. Supporting multiple configurations at once
or additional configurations is a quality of implementation issue.</p>
</dd>
<dt id="track-description">Track Description</dt>
<dd><p>A byte stream format specific structure that provides the <a def-id="track-id"></a>, codec configuration, and other metadata for a single track. Each track description inside a single <a def-id="init-segment"></a> has a unique <a def-id="track-id"></a>. The user agent MUST run the <a def-id="append-decode-error-algorithm"></a> if the <a def-id="track-id"></a> is not unique within the <a def-id="init-segment"></a>.</p></dd>
<dt id="track-id">Track ID</dt>
<dd><p>A Track ID is a byte stream format specific identifier that marks sections of the byte stream as being part of a specific track. The Track ID in a <a def-id="track-description"></a> identifies which sections of a <a def-id="media-segment"></a> belong to that track.</p></dd>
</dl>
</section>
</section>
<section id="mediasource">
<h2>MediaSource Object</h2>
<p>The MediaSource object represents a source of media data for an HTMLMediaElement. It keeps track of the <a def-id="readyState"></a> for this source as well as a list of <a>SourceBuffer</a> objects that can be used to add media data to the presentation. MediaSource objects are created by the web application and then attached to an HTMLMediaElement. The application uses the <a>SourceBuffer</a> objects in <a def-id="sourceBuffers"></a> to add media data to this source. The HTMLMediaElement fetches this media data from the <a>MediaSource</a> object when it is needed during playback.</p>
<p>Each <a>MediaSource</a> object has a <dfn id="live-seekable-range">live seekable range</dfn> variable that stores a <a def-id="normalized-timeranges-object"></a>. This variable is initialized to an empty <a def-id="timeranges"></a> object when the <a>MediaSource</a> object is created, is maintained by <a def-id="setLiveSeekableRange"></a> and <a def-id="clearLiveSeekableRange"></a>, and is used in <a href="#htmlmediaelement-extensions">HTMLMediaElement Extensions</a> to modify <a def-id="hme-seekable"></a> behavior.</p>
<dl title="enum ReadyState" class="idl">
<dt>closed</dt>
<dd>
Indicates the source is not currently attached to a media element.
</dd>
<dt>open</dt>
<dd>
The source has been opened by a media element and is ready for data to be appended to the <a>SourceBuffer</a> objects in <a def-id="sourceBuffers"></a>.
</dd>
<dt>ended</dt>
<dd>
The source is still attached to a media element, but <a def-id="endOfStream"></a> has been called.
</dd>
</dl>
<dl title="enum EndOfStreamError" class="idl">
<dt>network</dt>
<dd>
<p>Terminates playback and signals that a network error has occured.</p>
<p class="note">JavaScript applications SHOULD use this status code to terminate playback with a network error. For example, if a network error occurs while fetching media data.</p>
</dd>
<dt>decode</dt>
<dd>
<p>Terminates playback and signals that a decoding error has occured.</p>
<p class="note">JavaScript applications SHOULD use this status code to terminate playback with a decode error. For example, if a parsing error occurs while processing out-of-band media data.</p>
</dd>
</dl>
<dl title="[Constructor] interface MediaSource : EventTarget" class='idl'>
<dt>readonly attribute SourceBufferList sourceBuffers</dt>
<dd>
Contains the list of <a>SourceBuffer</a> objects associated with this <a>MediaSource</a>. When <a def-id="readyState"></a> equals <a def-id="closed"></a> this list will be empty. Once <a def-id="readyState"></a> transitions to <a def-id="open"></a> SourceBuffer objects can be added to this list by using <a def-id="addSourceBuffer"></a>.
</dd>
<dt>readonly attribute SourceBufferList activeSourceBuffers</dt>
<dd>
<p>Contains the subset of <a def-id="sourceBuffers"></a> that are providing the
<a def-id="videoref" name="dom-videotrack-selected">selected video track</a>, the
<a def-id="videoref" name="dom-audiotrack-enabled">enabled audio track(s)</a>, and the
<a def-id="texttrackmode-showing"></a> or <a def-id="texttrackmode-hidden"></a> text track(s).
</p>
<p><a>SourceBuffer</a> objects in this list MUST appear in the same order as they appear in
the <a def-id="sourceBuffers"></a> attribute; e.g., if only sourceBuffers[0] and
sourceBuffers[3] are in <a def-id="activeSourceBuffers"></a>, then activeSourceBuffers[0] MUST
equal sourceBuffers[0] and activeSourceBuffers[1] MUST equal sourceBuffers[3].
</p>
<p class="note">The <a href="#active-source-buffer-changes">Changes to selected/enabled track state</a> section describes how this attribute gets
updated.</p>
</dd>
<dt>readonly attribute ReadyState readyState</dt>
<dd>
<p>Indicates the current state of the <a>MediaSource</a> object. When the <a>MediaSource</a> is created <a def-id="readyState"></a> MUST be set to <a def-id="closed"></a>.
</dd>
<dt>attribute unrestricted double duration</dt>
<dd>
<p>Allows the web application to set the presentation duration. The duration is initially set to NaN when the <a>MediaSource</a> object is created.</p>
<p>On getting, run the following steps:</p>
<ol>
<li>If the <a def-id="readyState"></a> attribute is <a def-id="closed"></a> then return NaN and abort these steps.</li>
<li>Return the current value of the attribute.</li>
</ol>
<p>On setting, run the following steps:</p>
<ol>
<li>If the value being set is negative or NaN then throw a <a def-id="type-error"></a> exception and abort these steps.</li>
<li>If the <a def-id="readyState"></a> attribute is not <a def-id="open"></a> then throw an <a def-id="invalid-state-error"></a> exception and abort these steps.</li>
<li>If the <a def-id="updating"></a> attribute equals true on any <a>SourceBuffer</a> in <a def-id="sourceBuffers"></a>, then throw an <a def-id="invalid-state-error"></a> exception and abort these steps.</li>
<li>Run the <a def-id="duration-change-algorithm"></a> with <var>new duration</var> set to the value being assigned to this attribute.
<p class="note"><a def-id="appendBuffer"></a>, <a def-id="appendStream"></a> and <a def-id="endOfStream"></a> can update the duration under certain circumstances.</p>
</li>
</ol>
</dd>
<dt>attribute EventHandler onsourceopen</dt>
<dd>
<p>The event handler for the <a def-id="sourceopen"></a> event.</p>
</dd>
<dt>attribute EventHandler onsourceended</dt>
<dd>
<p>The event handler for the <a def-id="sourceended"></a> event.</p>
</dd>
<dt>attribute EventHandler onsourceclose</dt>
<dd>
<p>The event handler for the <a def-id="sourceclose"></a> event.</p>
</dd>
<dt>SourceBuffer addSourceBuffer(DOMString type)</dt>
<dd>
<p>Adds a new <a>SourceBuffer</a> to <a def-id="sourceBuffers"></a>.</p>
<ol class="method-algorithm">
<li>If <var>type</var> is an empty string then throw a <a def-id="type-error"></a> exception and abort these steps.</li>
<li>If <var>type</var> contains a MIME type that is not supported or contains a MIME type that is not supported with the types specified for the other <a>SourceBuffer</a> objects in <a def-id="sourceBuffers"></a>, then throw a <a def-id="not-supported-error"></a> exception and abort these steps.</li>
<li>If the user agent can't handle any more SourceBuffer objects or if creating a SourceBuffer
based on <var>type</var> would result in an unsupported <a def-id="sourcebuffer-configuration"></a>,
then throw a <a def-id="quota-exceeded-error"></a> exception and abort these steps.
<p class="note">For example, a user agent MAY throw a <a def-id="quota-exceeded-error"></a> exception if the media element has reached the
<a def-id="have-metadata"></a> readyState. This can occur if the user agent's media engine does not support adding more tracks during
playback.
</p>
</li>
<li>If the <a def-id="readyState"></a> attribute is not in the <a def-id="open"></a> state then throw an <a def-id="invalid-state-error"></a> exception and abort these steps.</li>
<li>Create a new <a>SourceBuffer</a> object and associated resources.</li>
<li>Set the <a def-id="generate-timestamps-flag"></a> on the new object to the value in the
"Generate Timestamps Flag" column of the byte stream format registry [[MSE-REGISTRY]] entry
that is associated with <var>type</var>.
<li>
<dl class="switch">
<dt>If the <a def-id="generate-timestamps-flag"></a> equals true:</dt>
<dd>
Set the <a def-id="mode"></a> attribute on the new object to
<a def-id="AppendMode-sequence"></a>.
</dd>
<dt>Otherwise:</dt>
<dd>
Set the <a def-id="mode"></a> attribute on the new object to
<a def-id="AppendMode-segments"></a>.
</dd>
</dl>
</li>
<li>Add the new object to <a def-id="sourceBuffers"></a> and <a def-id="queue-a-task-to-fire-an-event-named"></a> <a def-id="addsourcebuffer"></a> at <a def-id="sourceBuffers"></a>.</li>
<li>Return the new object.</li>
</ol>
</dd>
<dt>void removeSourceBuffer(SourceBuffer sourceBuffer)</dt>
<dd>
<p>Removes a <a>SourceBuffer</a> from <a def-id="sourceBuffers"></a>.</p>
<ol class="method-algorithm">
<li>If <var>sourceBuffer</var> specifies an object that is not in <a def-id="sourceBuffers"></a> then throw a <a def-id="not-found-error"></a> exception and abort these steps.</li>
<li>If the <var>sourceBuffer</var>.<a def-id="updating"></a> attribute equals true, then run the following steps:
<ol>
<li>Abort the <a def-id="buffer-append"></a> and <a def-id="stream-append-loop"></a> algorithms if they are running.</li>
<li>Set the <var>sourceBuffer</var>.<a def-id="updating"></a> attribute to false.</li>
<li><a def-id="Queue-a-task-to-fire-an-event-named"></a> <a def-id="updateabort"></a> at <var>sourceBuffer</var>.</li>
<li><a def-id="Queue-a-task-to-fire-an-event-named"></a> <a def-id="updateend"></a> at <var>sourceBuffer</var>.</li>
</ol>
</li>
<li>Let <var>SourceBuffer audioTracks list</var> equal the <a def-id="audio-track-list"></a> object returned by <var>sourceBuffer</var>.<a def-id="sourcebuffer-audioTracks"></a>.</li>
<li>If the <var>SourceBuffer audioTracks list</var> is not empty, then run the following steps:
<ol>
<li>Let <var>HTMLMediaElement audioTracks list</var> equal the <a def-id="audio-track-list"></a> object returned by the <a def-id="audiotracks"></a> attribute on the HTMLMediaElement.</li>
<li>For each <a def-id="audio-track"></a> object in the <var>SourceBuffer audioTracks list</var>, run the following steps:
<ol>
<li>Set the <a def-id="audiotrack-sourceBuffer"></a> attribute on the <a def-id="audio-track"></a> object to null.</li>
<li>Remove the <a def-id="audio-track"></a> object from the <var>HTMLMediaElement audioTracks list</var>.</li>
<p class="note">
This should trigger <a def-id="audio-track-list"></a> [[HTML51]] logic to <a def-id="queue-and-fire-media-removetrack-with-track-attr-initialized-to"></a> the <a def-id="audio-track"></a> object, at the <var>HTMLMediaElement audioTracks list</var>.
If the <a def-id="audiotrack-enabled"></a> attribute on the <a def-id="audio-track"></a> object was true at the beginning of this removal step, then this should also trigger <a def-id="audio-track-list"></a> [[HTML51]] logic to <a def-id="queue-a-task-to-fire-an-event-named"></a> <a def-id="mediatracklist-change"></a> at the <var>HTMLMediaElement audioTracks list</var>
</p>
<li>Remove the <a def-id="audio-track"></a> object from the <var>SourceBuffer audioTracks list</var>.</li>
<p class="note">
This should trigger <a def-id="audio-track-list"></a> [[HTML51]] logic to <a def-id="queue-and-fire-media-removetrack-with-track-attr-initialized-to"></a> the <a def-id="audio-track"></a> object, at the <var>SourceBuffer audioTracks list</var>.
If the <a def-id="audiotrack-enabled"></a> attribute on the <a def-id="audio-track"></a> object was true at the beginning of this removal step, then this should also trigger <a def-id="audio-track-list"></a> [[HTML51]] logic to <a def-id="queue-a-task-to-fire-an-event-named"></a> <a def-id="mediatracklist-change"></a> at the <var>SourceBuffer audioTracks list</var>
</p>
</ol>
</li>
</ol>
</li>
<li>Let <var>SourceBuffer videoTracks list</var> equal the <a def-id="video-track-list"></a> object returned by <var>sourceBuffer</var>.<a def-id="sourcebuffer-videoTracks"></a>.</li>
<li>If the <var>SourceBuffer videoTracks list</var> is not empty, then run the following steps:
<ol>
<li>Let <var>HTMLMediaElement videoTracks list</var> equal the <a def-id="video-track-list"></a> object returned by the <a def-id="videotracks"></a> attribute on the HTMLMediaElement.</li>
<li>For each <a def-id="video-track"></a> object in the <var>SourceBuffer videoTracks list</var>, run the following steps:
<ol>
<li>Set the <a def-id="videotrack-sourceBuffer"></a> attribute on the <a def-id="video-track"></a> object to null.</li>
<li>Remove the <a def-id="video-track"></a> object from the <var>HTMLMediaElement videoTracks list</var>.</li>
<p class="note">
This should trigger <a def-id="video-track-list"></a> [[HTML51]] logic to <a def-id="queue-and-fire-media-removetrack-with-track-attr-initialized-to"></a> the <a def-id="video-track"></a> object, at the <var>HTMLMediaElement videoTracks list</var>.
If the <a def-id="videotrack-selected"></a> attribute on the <a def-id="video-track"></a> object was true at the beginning of this removal step, then this should also trigger <a def-id="video-track-list"></a> [[HTML51]] logic to <a def-id="queue-a-task-to-fire-an-event-named"></a> <a def-id="mediatracklist-change"></a> at the <var>HTMLMediaElement videoTracks list</var>
</p>
<li>Remove the <a def-id="video-track"></a> object from the <var>SourceBuffer videoTracks list</var>.</li>
<p class="note">
This should trigger <a def-id="video-track-list"></a> [[HTML51]] logic to <a def-id="queue-and-fire-media-removetrack-with-track-attr-initialized-to"></a> the <a def-id="video-track"></a> object, at the <var>SourceBuffer videoTracks list</var>.
If the <a def-id="videotrack-selected"></a> attribute on the <a def-id="video-track"></a> object was true at the beginning of this removal step, then this should also trigger <a def-id="video-track-list"></a> [[HTML51]] logic to <a def-id="queue-a-task-to-fire-an-event-named"></a> <a def-id="mediatracklist-change"></a> at the <var>SourceBuffer videoTracks list</var>
</p>
</ol>
</li>
</ol>
</li>
<li>Let <var>SourceBuffer textTracks list</var> equal the <a def-id="text-track-list"></a> object returned by <var>sourceBuffer</var>.<a def-id="sourcebuffer-textTracks"></a>.</li>
<li>If the <var>SourceBuffer textTracks list</var> is not empty, then run the following steps:
<ol>
<li>Let <var>HTMLMediaElement textTracks list</var> equal the <a def-id="text-track-list"></a> object returned by the <a def-id="texttracks"></a> attribute on the HTMLMediaElement.</li>
<li>For each <a def-id="text-track"></a> object in the <var>SourceBuffer textTracks list</var>, run the following steps:
<ol>
<li>Set the <a def-id="texttrack-sourceBuffer"></a> attribute on the <a def-id="text-track"></a> object to null.</li>
<li>Remove the <a def-id="text-track"></a> object from the <var>HTMLMediaElement textTracks list</var>.</li>
<p class="note">
This should trigger <a def-id="text-track-list"></a> [[HTML51]] logic to <a def-id="queue-and-fire-text-removetrack-with-track-attr-initialized-to"></a> the <a def-id="text-track"></a> object, at the <var>HTMLMediaElement textTracks list</var>.
If the <a def-id="texttrack-mode"></a> attribute on the <a def-id="text-track"></a> object was <a def-id="texttrackmode-showing"></a> or <a def-id="texttrackmode-hidden"></a> at the beginning of this removal step, then this should also trigger <a def-id="text-track-list"></a> [[HTML51]] logic to <a def-id="queue-a-task-to-fire-an-event-named"></a> <a def-id="texttracklist-change"></a> at the <var>HTMLMediaElement textTracks list</var>.
</p>
<li>Remove the <a def-id="text-track"></a> object from the <var>SourceBuffer textTracks list</var>.</li>
<p class="note">
This should trigger <a def-id="text-track-list"></a> [[HTML51]] logic to <a def-id="queue-and-fire-text-removetrack-with-track-attr-initialized-to"></a> the <a def-id="text-track"></a> object, at the <var>SourceBuffer textTracks list</var>.
If the <a def-id="texttrack-mode"></a> attribute on the <a def-id="text-track"></a> object was <a def-id="texttrackmode-showing"></a> or <a def-id="texttrackmode-hidden"></a> at the beginning of this removal step, then this should also trigger <a def-id="text-track-list"></a> [[HTML51]] logic to <a def-id="queue-a-task-to-fire-an-event-named"></a> <a def-id="texttracklist-change"></a> at the <var>SourceBuffer textTracks list</var>.
</p>
</ol>
</li>
</ol>
</li>
<li>If <var>sourceBuffer</var> is in <a def-id="activeSourceBuffers"></a>, then remove <var>sourceBuffer</var> from <a def-id="activeSourceBuffers"></a> and
<a def-id="queue-a-task-to-fire-an-event-named"></a> <a def-id="removesourcebuffer"></a> at the <a>SourceBufferList</a> returned by <a def-id="activeSourceBuffers"></a>.</li>
<li>Remove <var>sourceBuffer</var> from <a def-id="sourceBuffers"></a> and <a def-id="queue-a-task-to-fire-an-event-named"></a> <a def-id="removesourcebuffer"></a> at
the <a>SourceBufferList</a> returned by <a def-id="sourceBuffers"></a>.</li>
<li>Destroy all resources for <var>sourceBuffer</var>.</li>
</ol>
</dd>
<dt>void endOfStream(optional EndOfStreamError error)</dt>
<dd>
<p>Signals the end of the stream.</p>
<ol class="method-algorithm">
<li>If the <a def-id="readyState"></a> attribute is not in the <a def-id="open"></a> state then throw an <a def-id="invalid-state-error"></a> exception and abort these steps.</li>
<li>If the <a def-id="updating"></a> attribute equals true on any <a>SourceBuffer</a> in <a def-id="sourceBuffers"></a>, then throw an <a def-id="invalid-state-error"></a> exception and abort these steps.</li>
<li>Run the <a def-id="end-of-stream-algorithm"></a> with the <var>error</var> parameter set to <var>error</var>.</li>
</ol>
</dd>
<dt>void setLiveSeekableRange()</dt>
<dd>
<p>Updates the <a def-id="live-seekable-range"></a> variable used in <a href="#htmlmediaelement-extensions">HTMLMediaElement Extensions</a> to modify <a def-id="hme-seekable"></a> behavior.</p>
<dl class='parameters'>
<dt>double start</dt>
<dd>
The start of the range, in seconds measured from <a def-id="presentation-start-time"></a>. While set, and if <a def-id="duration"></a> equals positive Infinity, <a def-id="hme-seekable"></a> will return a non-empty TimeRanges object with a lowest range start timestamp no greater than <var>start</var>.
</dd>
<dt>double end</dt>
<dd>
The end of range, in seconds measured from <a def-id="presentation-start-time"></a>. While set, and if <a def-id="duration"></a> equals positive Infinity, <a def-id="hme-seekable"></a> will return a non-empty TimeRanges object with a highest range end timestamp no less than <var>end</var>.
</dd>
</dl>
<ol class="method-algorithm">
<li>If the <a def-id="readyState"></a> attribute is not <a def-id="open"></a> then throw an <a def-id="invalid-state-error"></a> exception and abort these steps.</li>
<li>If <var>start</var> is negative or greater than <var>end</var>, then throw a <a def-id="type-error"></a> exception and abort these steps.</li>
<li>Set <a def-id="live-seekable-range"></a> to be a new <a def-id="normalized-timeranges-object"></a> containing a single range whose start position is <var>start</var> and end position is <var>end</var>.
</ol>
</dd>
<dt>void clearLiveSeekableRange()</dt>
<dd>
<p>Updates the <a def-id="live-seekable-range"></a> variable used in <a href="#htmlmediaelement-extensions">HTMLMediaElement Extensions</a> to modify <a def-id="hme-seekable"></a> behavior.</p>
<ol class="method-algorithm">
<li>If the <a def-id="readyState"></a> attribute is not <a def-id="open"></a> then throw an <a def-id="invalid-state-error"></a> exception and abort these steps.</li>
<li>If <a def-id="live-seekable-range"></a> contains a range, then set <a def-id="live-seekable-range"></a> to be a new empty <a def-id="timeranges"></a> object.
</ol>
</dd>
<dt>static boolean isTypeSupported(DOMString type)</dt>
<dd>
<p>Check to see whether the <a>MediaSource</a> is capable of creating <a>SourceBuffer</a> objects for the specified MIME type.</p>
<ol class="method-algorithm">
<li>If <var>type</var> is an empty string, then return false.</li>
<li>If <var>type</var> does not contain a valid MIME type string, then return false.</li>
<li>If <var>type</var> contains a media type or media subtype that the MediaSource does not support, then return false.</li>
<li>If <var>type</var> contains a codec that the MediaSource does not support, then return false.</li>
<li>If the MediaSource does not support the specified combination of media type, media subtype, and codecs then return false.</li>
<li>Return true.</li>
</ol>
<p class="note">
If true is returned from this method, it only indicates that the <a>MediaSource</a> implementation is capable of creating <a>SourceBuffer</a> objects for the specified MIME type. An <a def-id="addSourceBuffer"></a> call SHOULD still fail if sufficient resources are not available to support the addition of a new <a>SourceBuffer</a>.
</p>
<p class="note">
This method returning true implies that HTMLMediaElement.canPlayType() will return "maybe" or "probably" since it does not make sense for a <a>MediaSource</a> to support a type the HTMLMediaElement knows it cannot play.
</p>
</dd>
</dl>
<section id="mediasource-events">
<h3>Event Summary</h3>
<table class="old-table">
<thead>
<tr>
<th>Event name</th>
<th>Interface</th>
<th>Dispatched when...</th>
</tr>
</thead>
<tbody>
<tr>
<td><a def-id="eventdfn">sourceopen</a></td>
<td><code>Event</code></td>
<td><a def-id="readyState"></a> transitions from <a def-id="closed"></a> to <a def-id="open"></a> or from <a def-id="ended"></a> to <a def-id="open"></a>.</td>
</tr>
<tr>
<td><a def-id="eventdfn">sourceended</a></td>
<td><code>Event</code></td>
<td><a def-id="readyState"></a> transitions from <a def-id="open"></a> to <a def-id="ended"></a>.</td>
</tr>
<tr>
<td><a def-id="eventdfn">sourceclose</a></td>
<td><code>Event</code></td>
<td><a def-id="readyState"></a> transitions from <a def-id="open"></a> to <a def-id="closed"></a> or <a def-id="ended"></a> to <a def-id="closed"></a>.</td>
</tr>
</tbody>
</table>
</section>
<section id="mediasource-algorithms">
<h3>Algorithms</h3>
<section id="mediasource-attach">
<h4>Attaching to a media element</h4>
<p> A <a>MediaSource</a> object can be attached to a media element by assigning a <a def-id="MediaSource-object-URL"></a> to the media element <a def-id="media-src"></a> attribute or the src attribute of a <source> inside a media element. A <a def-id="MediaSource-object-URL"></a> is created by passing a MediaSource object to <a def-id="createObjectURL"></a>.</p>
<p>If the <a def-id="resource-fetch-algorithm"></a> absolute URL matches the MediaSource object URL, ignore any <a def-id="preload"></a> attribute of the media element, skip any optional steps for when <a def-id="preload"></a> equals <a def-id="preload-none"></a>, and run the following steps right before the <a def-id="perform-potentially-cors-enabled-fetch"></a>
step in the <a def-id="resource-fetch-algorithm"></a>.</p>
<dl class="switch">
<dt>If <a def-id="readyState"></a> is NOT set to <a def-id="closed"></a></dt>
<dd>Run the <a def-id="media-data-cannot-be-fetched"></a> steps of the <a def-id="resource-fetch-algorithm"></a>.</dd>
<dt>Otherwise</dt>
<dd>
<ol>
<li>Set the media element's <a def-id="delaying-the-load-event-flag"></a> to false.</li>
<li>Set the <a def-id="readyState"></a> attribute to <a def-id="open"></a>.</li>
<li>
<a def-id="Queue-a-task-to-fire-an-event-named"></a> <a def-id="sourceopen"></a> at the <a>MediaSource</a>.</li>
<li>Continue the <a def-id="resource-fetch-algorithm"></a> by running the <a def-id="perform-potentially-cors-enabled-fetch"></a> step, with these clarifications:
<ol>
<li>Text in the <a def-id="resource-fetch-algorithm"></a> that refers to "the download" or "bytes received" refers to data passed in via <a def-id="appendBuffer"></a> and <a def-id="appendStream"></a>.</li>
<li>References to HTTP in the <a def-id="resource-fetch-algorithm"></a> do not apply because the HTMLMediaElement does not fetch media data via HTTP when a <a>MediaSource</a> is attached.</li>
<li>Text in the <a def-id="resource-fetch-algorithm"></a> that refers to transitions to "NETWORK_IDLE" and firing "suspend" events, and the paragraph that begins with "User agents may decide to not download more content at any time", do not apply to a media element that has a <a>MediaSource</a> object attached by this algorithm.</li>
</ol>
</li>
</ol>
</dd>
</dl>
</section>
<section id="mediasource-detach">
<h4>Detaching from a media element</h4>
<p>The following steps are run in any case where the media element is going to transition to <a def-id="videoref" name="dom-htmlmediaelement-network_empty">NETWORK_EMPTY</a> and <a def-id="queue-a-task-to-fire-an-event-named"></a> <a def-id="videoref" name="eventdef-media-emptied">emptied</a> at the media element. These steps SHOULD be run right before the transition.</p>
<ol>
<li>Set the <a def-id="readyState"></a> attribute to <a def-id="closed"></a>.</li>
<li>Update <a def-id="duration"></a> to NaN.</li>
<li>Remove all the <a>SourceBuffer</a> objects from <a def-id="activeSourceBuffers"></a>.</li>
<li>
<a def-id="Queue-a-task-to-fire-an-event-named"></a> <a def-id="removesourcebuffer"></a> at <a def-id="activeSourceBuffers"></a>.</li>
<li>Remove all the <a>SourceBuffer</a> objects from <a def-id="sourceBuffers"></a>.</li>
<li>
<a def-id="Queue-a-task-to-fire-an-event-named"></a> <a def-id="removesourcebuffer"></a> at <a def-id="sourceBuffers"></a>.</li>
<li>
<a def-id="Queue-a-task-to-fire-an-event-named"></a> <a def-id="sourceclose"></a> at the <a>MediaSource</a>.</li>
</ol>
<p class="note">Going forward, this algorithm is intended to be externally called and run in any case where the attached <a>MediaSource</a>, if any, must be detached from the media element. It MAY be called on HTMLMediaElement [[HTML51]] operations like load() and resource fetch algorithm failures in addition to, or in place of, when the media element transitions to <a def-id="videoref" name="dom-htmlmediaelement-network_empty">NETWORK_EMPTY</a>. Resource fetch algorithm failures are those which abort either the resource fetch algorithm or the resource selection algorithm, with the exception that the "Final step" [[HTML51]] is not considered a failure that triggers detachment.</p>
</section>
<section id="mediasource-seeking">
<h4>Seeking</h4>
<p>Run the following steps as part of the "<i>Wait until the user agent has established whether or not the media data for the new playback position is available, and, if it is, until it has decoded enough data to play back that position"</i> step of the <a def-id="hme-seek-algorithm"></a>:</p>
<ol>
<li>The media element looks for <a def-id="media-segments"></a> containing the <var>new playback position</var> in each <a>SourceBuffer</a> object in <a def-id="activeSourceBuffers"></a>.
<dl class="switch">
<dt>If one or more of the objects in <a def-id="activeSourceBuffers"></a> is missing <a def-id="media-segments"></a> for the <var>new playback position</var>
</dt>
<dd>
<ol>
<li>If the <a def-id="ready-state"></a> attribute is greater than
<a def-id="have-metadata"></a>, then set the <a def-id="ready-state"></a> attribute
to <a def-id="have-metadata"></a>.</li>
<p class="note">Per <a def-id="ready-states"></a> [[HTML51]] logic, <a def-id="ready-state"></a> changes may trigger events on the HTMLMediaElement.</p>
<li>The media element waits until an <a def-id="appendBuffer"></a> or an <a def-id="appendStream"></a> call causes the <a def-id="coded-frame-processing-algorithm"></a> to set
the <a def-id="ready-state"></a> attribute to a value greater than <a def-id="have-metadata"></a>.
<p class="note">The web application can use <a def-id="buffered"></a> to determine what the media element needs to resume playback.</p>
</li>
</ol>
</dd>
<dt>Otherwise</dt>
<dd>Continue</dd>
</dl>
</li>
<li>The media element resets all decoders and initializes each one with data from the appropriate <a def-id="init-segment"></a>.</li>
<li>The media element feeds <a def-id="coded-frames"></a> from the <a def-id="active-track-buffers"></a> into the decoders starting with the
closest <a def-id="random-access-point"></a> before the <var>new playback position</var>.</li>
<li>Resume the <a def-id="hme-seek-algorithm"></a> at the "<i>Await a stable state</i>" step.</li>
</ol>
</section>
<section id="buffer-monitoring">
<h4>SourceBuffer Monitoring</h4>
<p>The following steps are periodically run during playback to make sure that all of the <a>SourceBuffer</a> objects in <a def-id="activeSourceBuffers"></a> have <a def-id="enough-data"></a>. Changes to <a def-id="activeSourceBuffers"></a> also cause these steps to run because they affect the conditions that trigger state transitions.</p>
<p>Having <dfn id="enough-data">enough data to ensure uninterrupted playback</dfn> is an implementation specific condition where the user agent
determines that it currently has enough data to play the presentation without stalling for a meaningful period of time. This condition is
constantly evaluated to determine when to transition the media element into and out of the <a def-id="have-enough-data"></a> ready state.
These transitions indicate when the user agent believes it has enough data buffered or it needs more data respectively.</p>
<p class="note">An implementation MAY choose to use bytes buffered, time buffered, the append rate, or any other metric it sees fit to
determine when it has enough data. The metrics used MAY change during playback so web applications SHOULD only rely on the value of
<a def-id="ready-state"></a> to determine whether more data is needed or not.</p>
<p class="note">When the media element needs more data, the user agent SHOULD transition it from <a def-id="have-enough-data"></a> to
<a def-id="have-future-data"></a> early enough for a web application to be able to respond without causing an interruption in playback.
For example, transitioning when the current playback position is 500ms before the end of the buffered data gives the application roughly
500ms to append more data before playback stalls.</p>
<dl class="switch">
<dt>If the the <a def-id="ready-state"></a> attribute equals <a def-id="have-nothing"></a>:</dt>
<dd>
<ol>
<li>Abort these steps.</li>
</ol>
</dd>
<dt>If <a def-id="hme-buffered"></a> does not contain a <a def-id="timerange"></a> for the current playback position:</dt>
<dd>
<ol>
<li>Set the <a def-id="ready-state"></a> attribute to <a def-id="have-metadata"></a>.</li>
<p class="note">Per <a def-id="ready-states"></a> [[HTML51]] logic, <a def-id="ready-state"></a> changes may trigger events on the HTMLMediaElement.</p>
<li>Abort these steps.</li>
</ol>
</dd>
<dt>If <a def-id="hme-buffered"></a> contains a <a def-id="timerange"></a> that includes the current playback position and <a def-id="enough-data"></a>:</dt>
<dd>
<ol>
<li>Set the <a def-id="ready-state"></a> attribute to <a def-id="have-enough-data"></a>.</li>
<p class="note">Per <a def-id="ready-states"></a> [[HTML51]] logic, <a def-id="ready-state"></a> changes may trigger events on the HTMLMediaElement.</p>
<li>Playback may resume at this point if it was previously suspended by a transition to <a def-id="have-current-data"></a>.</li>
<li>Abort these steps.</li>
</ol>
</dd>
<dt>If <a def-id="hme-buffered"></a> contains a <a def-id="timerange"></a> that includes the current playback position and some time beyond the current playback position, then run the following steps:</dt>
<dd>
<ol>
<li>Set the <a def-id="ready-state"></a> attribute to <a def-id="have-future-data"></a>.</li>
<p class="note">Per <a def-id="ready-states"></a> [[HTML51]] logic, <a def-id="ready-state"></a> changes may trigger events on the HTMLMediaElement.</p>
<li>Playback may resume at this point if it was previously suspended by a transition to <a def-id="have-current-data"></a>.</li>
<li>Abort these steps.</li>
</ol>
</dd>
<dt>If <a def-id="hme-buffered"></a> contains a <a def-id="timerange"></a> that ends at the current playback position and does not have a range covering the time immediately after the current position:</dt>
<dd>
<ol>
<li>Set the <a def-id="ready-state"></a> attribute to <a def-id="have-current-data"></a>.</li>
<p class="note">Per <a def-id="ready-states"></a> [[HTML51]] logic, <a def-id="ready-state"></a> changes may trigger events on the HTMLMediaElement.</p>
<li>Playback is suspended at this point since the media element doesn't have enough data to advance the <a def-id="media-timeline"></a>.</li>
<li>Abort these steps.</li>
</ol>
</dd>
</dl>
</section>
<section id="active-source-buffer-changes">
<h4>Changes to selected/enabled track state</h4>
<p>During playback <a def-id="activeSourceBuffers"></a> needs to be updated if the <a def-id="videoref" name="dom-videotrack-selected">selected video track</a>, the <a def-id="videoref" name="dom-audiotrack-enabled">enabled audio track(s)</a>, or a text track <a def-id="videoref" name="dom-texttrack-mode">mode</a> changes. When one or more of these changes occur the following steps need to be followed.</p>
<dl class="switch">
<dt>If the selected video track changes, then run the following steps:</dt>
<dd>
<ol>
<li>If the <a>SourceBuffer</a> associated with the previously selected video track is not associated with any other enabled tracks, run the following steps:
<ol>
<li>Remove the <a>SourceBuffer</a> from <a def-id="activeSourceBuffers"></a>.</li>
<li>
<a def-id="Queue-a-task-to-fire-an-event-named"></a> <a def-id="removesourcebuffer"></a> at <a def-id="activeSourceBuffers"></a>
</li>
</ol>
</li>
<li>If the <a>SourceBuffer</a> associated with the newly selected video track is not already in <a def-id="activeSourceBuffers"></a>, run the following steps:
<ol>
<li>Add the <a>SourceBuffer</a> to <a def-id="activeSourceBuffers"></a>.</li>
<li>
<a def-id="Queue-a-task-to-fire-an-event-named"></a> <a def-id="addsourcebuffer"></a> at <a def-id="activeSourceBuffers"></a>
</li>
</ol>
</li>
</ol>
</dd>
<dt>If an audio track becomes disabled and the <a>SourceBuffer</a> associated with this track is not associated with any other enabled or selected track, then run the following steps:</dt>
<dd>
<ol>
<li>Remove the <a>SourceBuffer</a> associated with the audio track from <a def-id="activeSourceBuffers"></a>
</li>
<li>
<a def-id="Queue-a-task-to-fire-an-event-named"></a> <a def-id="removesourcebuffer"></a> at <a def-id="activeSourceBuffers"></a>
</li>
</ol>
</dd>
<dt>If an audio track becomes enabled and the <a>SourceBuffer</a> associated with this track is not already in <a def-id="activeSourceBuffers"></a>, then run the following steps:
</dt>
<dd>
<ol>
<li>Add the <a>SourceBuffer</a> associated with the audio track to <a def-id="activeSourceBuffers"></a>
</li>
<li>
<a def-id="Queue-a-task-to-fire-an-event-named"></a> <a def-id="addsourcebuffer"></a> at <a def-id="activeSourceBuffers"></a>
</li>
</ol>
</dd>
<dt>If a text track <a def-id="videoref" name="dom-texttrack-mode">mode</a> becomes <a def-id="texttrackmode-disabled"></a> and the <a>SourceBuffer</a> associated with this track is not associated with any other enabled or selected track, then run the following steps:</dt>
<dd>
<ol>
<li>Remove the <a>SourceBuffer</a> associated with the text track from <a def-id="activeSourceBuffers"></a>
</li>
<li>
<a def-id="Queue-a-task-to-fire-an-event-named"></a> <a def-id="removesourcebuffer"></a> at <a def-id="activeSourceBuffers"></a>
</li>
</ol>
</dd>
<dt>If a text track <a def-id="videoref" name="dom-texttrack-mode">mode</a> becomes <a def-id="texttrackmode-showing"></a> or <a def-id="texttrackmode-hidden"></a> and the <a>SourceBuffer</a> associated with this track is not already in <a def-id="activeSourceBuffers"></a>, then run the following steps:
</dt>
<dd>
<ol>
<li>Add the <a>SourceBuffer</a> associated with the text track to <a def-id="activeSourceBuffers"></a>
</li>
<li>
<a def-id="Queue-a-task-to-fire-an-event-named"></a> <a def-id="addsourcebuffer"></a> at <a def-id="activeSourceBuffers"></a>
</li>
</ol>
</dd>
</dl>
</section>
<section id="duration-change-algorithm">
<h4>Duration change</h4>
<p>Follow these steps when <a def-id="duration"></a> needs to change to a <var>new duration</var>.</p>
<ol>
<li>If the current value of <a def-id="duration"></a> is equal to <var>new duration</var>, then return.</li>
<li>If <var>new duration</var> is less than the highest <a def-id="presentation-timestamp"></a> of any buffered <a def-id="coded-frames"></a> for all <a>SourceBuffer</a> objects in <a def-id="sourceBuffers"></a>, then throw an <a def-id="invalid-state-error"></a> exception and abort these steps.</li>
<p class="note">Duration reductions that would truncate currently buffered media are disallowed. When truncation is necessary, use <a def-id="remove"></a> to reduce the buffered range before updating <a def-id="duration"></a>.</p>
<li>Update <a def-id="duration"></a> to <var>new duration</var>.</li>
<li>If a user agent is unable to partially render audio frames or text cues that start before and end after the <a def-id="duration"></a>, then run the following steps:</li>
<p class="note">This condition can occur because the <a def-id="coded-frame-removal-algorithm"></a> preserves audio frames and text cues that start before and end after the <a def-id="duration"></a>.</p>
<ol>
<li>Update <var>new duration</var> to the highest end time reported by the <a def-id="buffered"></a> attribute across all <a>SourceBuffer</a> objects in <a def-id="sourceBuffers"></a>.</li>
<li>Update <a def-id="duration"></a> to <var>new duration</var>.</li>
</ol>
<li>Update the <a def-id="hme-duration"></a> to <var>new duration</var> and run the <a def-id="hme-duration-change-algorithm"></a>.</li>
</ol>
</section>
<section id="end-of-stream-algorithm">
<h4>End of stream algorithm</h4>
<p>This algorithm gets called when the application signals the end of stream via an <a def-id="endOfStream"></a> call or an algorithm needs to
signal a decode error. This algorithm takes an <var>error</var> parameter that indicates whether an error will be signalled.</p>
<ol>
<li>Change the <a def-id="readyState"></a> attribute value to <a def-id="ended"></a>.</li>
<li>
<a def-id="Queue-a-task-to-fire-an-event-named"></a> <a def-id="sourceended"></a> at the <a>MediaSource</a>.</li>
<li><dl class="switch">
<dt>If <var>error</var> is not set</dt>
<dd>
<ol>
<li>Run the <a def-id="duration-change-algorithm"></a> with <var>new duration</var> set to the highest end time reported by the <a def-id="buffered"></a> attribute across all <a>SourceBuffer</a> objects in <a def-id="sourceBuffers"></a>.<br>
<p class="note">This allows the duration to properly reflect the end of the appended media segments. For example, if the duration was explicitly set to 10 seconds and only media segments for 0 to 5 seconds were appended before endOfStream() was called, then the duration will get updated to 5 seconds.</p>
</li>
<li>Notify the media element that it now has all of the media data.</li>
</ol>
</dd>
<dt>If <var>error</var> is set to <a def-id="network"></a>
</dt>
<dd>
<dl class="switch">
<dt>If the <a def-id="ready-state"></a> attribute equals <a def-id="have-nothing"></a>
</dt>
<dd>Run the <a def-id="media-data-cannot-be-fetched"></a> steps of the <a def-id="resource-fetch-algorithm"></a>.</dd>
<dt>If the <a def-id="ready-state"></a> attribute is greater than <a def-id="have-nothing"></a>
</dt>
<dd>Run the "<i>If the connection is interrupted after some media data has been received, causing the user agent to give up trying to fetch the resource</i>" steps of the <a def-id="resource-fetch-algorithm"></a>.</dd>
</dl>
</dd>
<dt>If <var>error</var> is set to <a def-id="decode"></a>
</dt>
<dd>
<dl class="switch">
<dt>If the <a def-id="ready-state"></a> attribute equals <a def-id="have-nothing"></a>
</dt>
<dd>Run the "<i>If the media data can be fetched but is found by inspection to be in an unsupported format, or can otherwise not be rendered at all</i>" steps of the <a def-id="resource-fetch-algorithm"></a>.</dd>
<dt>If the <a def-id="ready-state"></a> attribute is greater than <a def-id="have-nothing"></a>
</dt>
<dd>Run the <a def-id="media-data-is-corrupted"></a> steps of the <a def-id="resource-fetch-algorithm"></a>.</dd>
</dl>
</dd>
</dl>
</li>
</ol>
</section>
</section>
</section>
<section id="sourcebuffer">
<h2>SourceBuffer Object</h2>
<dl title="enum AppendMode" class="idl">
<dt>segments</dt>
<dd>
<p>The timestamps in the media segment determine where the <a def-id="coded-frames"></a> are placed in the presentation. Media segments can be appended in any order.</p>
</dd>
<dt>sequence</dt>
<dd>
<p>Media segments will be treated as adjacent in time independent of the timestamps in the media segment. Coded frames in a new media segment will be placed immediately after the coded
frames in the previous media segment. The <a def-id="timestampOffset"></a> attribute will be updated if a new offset is needed to make the new media segments adjacent to the previous media segment.
Setting the <a def-id="timestampOffset"></a> attribute in <a def-id="AppendMode-sequence"></a> mode allows a media segment to be placed at a specific position in the timeline without any knowledge
of the timestamps in the media segment.
</p>
</dd>
</dl>
<dl title="interface SourceBuffer : EventTarget" class="idl">
<dt>attribute AppendMode mode</dt>
<dd>
<p>Controls how a sequence of <a def-id="media-segments"></a> are handled. This attribute is initially set by <a def-id="addSourceBuffer"></a> after the object is created.</p>
<p>On getting, Return the initial value or the last value that was successfully set.</p>
<p>On setting, run the following steps:</p>
<ol>
<li>If this object has been removed from the <a def-id="sourceBuffers"></a> attribute of the <a def-id="parent-media-source"></a>, then throw an <a def-id="invalid-state-error"></a> exception and abort these steps.</li>
<li>If the <a def-id="updating"></a> attribute equals true, then throw an <a def-id="invalid-state-error"></a> exception and abort these steps.</li>
<li>Let <var>new mode</var> equal the new value being assigned to this attribute.</li>
<li>If <a def-id="generate-timestamps-flag"></a> equals true and <var>new mode</var> equals
<a def-id="AppendMode-segments"></a>, then throw a <a def-id="type-error"></a>
exception and abort these steps.</li>
<li>
<p>If the <a def-id="readyState"></a> attribute of the <a def-id="parent-media-source"></a> is in the <a def-id="ended"></a> state then run the following steps:</p>
<ol>
<li>Set the <a def-id="readyState"></a> attribute of the <a def-id="parent-media-source"></a> to <a def-id="open"></a></li>
<li><a def-id="Queue-a-task-to-fire-an-event-named"></a> <a def-id="sourceopen"></a> at the <a def-id="parent-media-source"></a>.</li>
</ol>
</li>
<li>If the <a def-id="append-state"></a> equals <a def-id="parsing-media-segment"></a>, then throw an <a def-id="invalid-state-error"></a> and abort these steps.</li>
<li>If the <var>new mode</var> equals <a def-id="AppendMode-sequence"></a>, then set the <a def-id="group-start-timestamp"></a> to the <a def-id="group-end-timestamp"></a>.</li>
<li>Update the attribute to <var>new mode</var>.</li>