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<!DOCTYPE html>
<html lang="en" dir="ltr">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>Media Source Extensions</title>
<!-- script to register bugs -->
<script src="https://dvcs.w3.org/hg/webcomponents/raw-file/tip/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">
<style type="text/css">
.nonnormative { color: green; margin: 2em 0 2em 0em; padding: 0.5em 1em; border: none; background: #DDFFDD; }
.nonnormative h3 { color: inherit; background: inherit; }
.nonnormative:before { display: table; margin: -1em -0.5em -0.5em auto; width: auto; content: 'This section is non-normative.'; color: black; font-style: italic; border: solid 2px; background: white; padding: 0 0.25em; }
.iso-box { font-weight: bold; }
.iso-var { font-style: italic; }
table.old-table { border-collapse: collapse; border-style: hidden hidden none hidden; }
table.old-table thead, table tbody { border-bottom: solid; }
table.old-table tbody th:first-child { border-left: solid; }
table.old-table tbody th { text-align: left; }
table.old-table td, table th { border-left: solid; border-right: solid; border-bottom: solid thin; vertical-align: top; padding: 0.2em; }
dl.switch { padding-left: 2em; }
dl.switch > dt { text-indent: -1.5em; }
dl.switch > dt:before { content: '\21AA'; padding: 0 0.5em 0 0; display: inline-block; width: 1em; text-align: right; line-height: 0.5em; }
p + * > li, dd li { margin: 1em 0; }
@media screen { code :link, code :visited { color: inherit; } }
/* fix bug entry form styling */
body > form {
padding: 4px;
border: 1px solid red;
background-color: white;
</style>
<style>/*****************************************************************
* ReSpec 3 CSS
* Robin Berjon - http://berjon.com/
*****************************************************************/
/* --- INLINES --- */
em.rfc2119 {
text-transform: lowercase;
font-variant: small-caps;
font-style: normal;
color: #900;
}
h1 acronym, h2 acronym, h3 acronym, h4 acronym, h5 acronym, h6 acronym, a acronym,
h1 abbr, h2 abbr, h3 abbr, h4 abbr, h5 abbr, h6 abbr, a abbr {
border: none;
}
dfn {
font-weight: bold;
}
a.internalDFN {
color: inherit;
border-bottom: 1px solid #99c;
text-decoration: none;
}
a.externalDFN {
color: inherit;
border-bottom: 1px dotted #ccc;
text-decoration: none;
}
a.bibref {
text-decoration: none;
}
cite .bibref {
font-style: normal;
}
code {
color: #ff4500;
}
/* --- --- */
ol.algorithm { counter-reset:numsection; list-style-type: none; }
ol.algorithm li { margin: 0.5em 0; }
ol.algorithm li:before { font-weight: bold; counter-increment: numsection; content: counters(numsection, ".") ") "; }
/* --- TOC --- */
.toc a, .tof a {
text-decoration: none;
}
a .secno, a .figno {
color: #000;
}
ul.tof, ol.tof {
list-style: none outside none;
}
.caption {
margin-top: 0.5em;
font-style: italic;
}
/* --- TABLE --- */
table.simple {
border-spacing: 0;
border-collapse: collapse;
border-bottom: 3px solid #005a9c;
}
.simple th {
background: #005a9c;
color: #fff;
padding: 3px 5px;
text-align: left;
}
.simple th[scope="row"] {
background: inherit;
color: inherit;
border-top: 1px solid #ddd;
}
.simple td {
padding: 3px 10px;
border-top: 1px solid #ddd;
}
.simple tr:nth-child(even) {
background: #f0f6ff;
}
/* --- DL --- */
.section dd > p:first-child {
margin-top: 0;
}
.section dd > p:last-child {
margin-bottom: 0;
}
.section dd {
margin-bottom: 1em;
}
.section dl.attrs dd, .section dl.eldef dd {
margin-bottom: 0;
}
</style><style>/* --- ISSUES/NOTES --- */
div.issue-title, div.note-title {
padding-right: 1em;
min-width: 7.5em;
color: #b9ab2d;
}
div.issue-title { color: #e05252; }
div.note-title { color: #52e052; }
div.issue-title span, div.note-title span {
text-transform: uppercase;
}
div.note, div.issue {
margin-top: 1em;
margin-bottom: 1em;
}
.note > p:first-child, .issue > p:first-child { margin-top: 0 }
.issue, .note {
padding: .5em;
border-left-width: .5em;
border-left-style: solid;
}
div.issue, div.note {
padding: 0.5em;
margin: 1em 0;
position: relative;
clear: both;
}
span.note, span.issue { padding: .1em .5em .15em; }
.issue {
border-color: #e05252;
background: #fbe9e9;
}
.note {
border-color: #52e052;
background: #e9fbe9;
}
</style><style>/* --- WEB IDL --- */
pre.idl {
border-top: 1px solid #90b8de;
border-bottom: 1px solid #90b8de;
padding: 1em;
line-height: 120%;
}
pre.idl::before {
content: "WebIDL";
display: block;
width: 150px;
background: #90b8de;
color: #fff;
font-family: initial;
padding: 3px;
font-weight: bold;
margin: -1em 0 1em -1em;
}
.idlType {
color: #ff4500;
font-weight: bold;
text-decoration: none;
}
/*.idlModule*/
/*.idlModuleID*/
/*.idlInterface*/
.idlInterfaceID, .idlDictionaryID, .idlCallbackID, .idlEnumID {
font-weight: bold;
color: #005a9c;
}
a.idlEnumItem {
color: #000;
border-bottom: 1px dotted #ccc;
text-decoration: none;
}
.idlSuperclass {
font-style: italic;
color: #005a9c;
}
/*.idlAttribute*/
.idlAttrType, .idlFieldType, .idlMemberType {
color: #005a9c;
}
.idlAttrName, .idlFieldName, .idlMemberName {
color: #ff4500;
}
.idlAttrName a, .idlFieldName a, .idlMemberName a {
color: #ff4500;
border-bottom: 1px dotted #ff4500;
text-decoration: none;
}
/*.idlMethod*/
.idlMethType, .idlCallbackType {
color: #005a9c;
}
.idlMethName {
color: #ff4500;
}
.idlMethName a {
color: #ff4500;
border-bottom: 1px dotted #ff4500;
text-decoration: none;
}
/*.idlCtor*/
.idlCtorName {
color: #ff4500;
}
.idlCtorName a {
color: #ff4500;
border-bottom: 1px dotted #ff4500;
text-decoration: none;
}
/*.idlParam*/
.idlParamType {
color: #005a9c;
}
.idlParamName, .idlDefaultValue {
font-style: italic;
}
.extAttr {
color: #666;
}
/*.idlSectionComment*/
.idlSectionComment {
color: gray;
}
/*.idlConst*/
.idlConstType {
color: #005a9c;
}
.idlConstName {
color: #ff4500;
}
.idlConstName a {
color: #ff4500;
border-bottom: 1px dotted #ff4500;
text-decoration: none;
}
/*.idlException*/
.idlExceptionID {
font-weight: bold;
color: #c00;
}
.idlTypedefID, .idlTypedefType {
color: #005a9c;
}
.idlRaises, .idlRaises a.idlType, .idlRaises a.idlType code, .excName a, .excName a code {
color: #c00;
font-weight: normal;
}
.excName a {
font-family: monospace;
}
.idlRaises a.idlType, .excName a.idlType {
border-bottom: 1px dotted #c00;
}
.excGetSetTrue, .excGetSetFalse, .prmNullTrue, .prmNullFalse, .prmOptTrue, .prmOptFalse {
width: 45px;
text-align: center;
}
.excGetSetTrue, .prmNullTrue, .prmOptTrue { color: #0c0; }
.excGetSetFalse, .prmNullFalse, .prmOptFalse { color: #c00; }
.idlImplements a {
font-weight: bold;
}
dl.attributes, dl.methods, dl.constants, dl.constructors, dl.fields, dl.dictionary-members {
margin-left: 2em;
}
.attributes dt, .methods dt, .constants dt, .constructors dt, .fields dt, .dictionary-members dt {
font-weight: normal;
}
.attributes dt code, .methods dt code, .constants dt code, .constructors dt code, .fields dt code, .dictionary-members dt code {
font-weight: bold;
color: #000;
font-family: monospace;
}
.attributes dt code, .fields dt code, .dictionary-members dt code {
background: #ffffd2;
}
.attributes dt .idlAttrType code, .fields dt .idlFieldType code, .dictionary-members dt .idlMemberType code {
color: #005a9c;
background: transparent;
font-family: inherit;
font-weight: normal;
font-style: italic;
}
.methods dt code {
background: #d9e6f8;
}
.constants dt code {
background: #ddffd2;
}
.constructors dt code {
background: #cfc;
}
.attributes dd, .methods dd, .constants dd, .constructors dd, .fields dd, .dictionary-members dd {
margin-bottom: 1em;
}
table.parameters, table.exceptions {
border-spacing: 0;
border-collapse: collapse;
margin: 0.5em 0;
width: 100%;
}
table.parameters { border-bottom: 1px solid #90b8de; }
table.exceptions { border-bottom: 1px solid #deb890; }
.parameters th, .exceptions th {
color: #fff;
padding: 3px 5px;
text-align: left;
font-family: initial;
font-weight: normal;
text-shadow: #666 1px 1px 0;
}
.parameters th { background: #90b8de; }
.exceptions th { background: #deb890; }
.parameters td, .exceptions td {
padding: 3px 10px;
border-top: 1px solid #ddd;
vertical-align: top;
}
.parameters tr:first-child td, .exceptions tr:first-child td {
border-top: none;
}
.parameters td.prmName, .exceptions td.excName, .exceptions td.excCodeName {
width: 100px;
}
.parameters td.prmType {
width: 120px;
}
table.exceptions table {
border-spacing: 0;
border-collapse: collapse;
width: 100%;
}
</style><link rel="stylesheet" href="https://www.w3.org/StyleSheets/TR/W3C-WD"><!--[if lt IE 9]><script src='https://www.w3.org/2008/site/js/html5shiv.js'></script><![endif]--></head>
<body><div class="head">
<p>
<a href="http://www.w3.org/"><img width="72" height="48" src="https://www.w3.org/Icons/w3c_home" alt="W3C"></a>
</p>
<h1 class="title" id="title">Media Source Extensions</h1>
<h2 id="w3c-last-call-working-draft-05-september-2013"><abbr title="World Wide Web Consortium">W3C</abbr> Last Call Working Draft 05 September 2013</h2>
<dl>
<dt>This version:</dt>
<dd><a href="http://www.w3.org/TR/2013/WD-media-source-20130905/">http://www.w3.org/TR/2013/WD-media-source-20130905/</a></dd>
<dt>Latest published version:</dt>
<dd><a href="http://www.w3.org/TR/media-source/">http://www.w3.org/TR/media-source/</a></dd>
<dt>Latest editor's draft:</dt>
<dd><a href="http://dvcs.w3.org/hg/html-media/raw-file/default/media-source/media-source.html">http://dvcs.w3.org/hg/html-media/raw-file/default/media-source/media-source.html</a></dd>
<dt>Previous version:</dt>
<dd><a href="http://www.w3.org/TR/2013/WD-media-source-20130415/">http://www.w3.org/TR/2013/WD-media-source-20130415/</a></dd>
<dt>Editors:</dt>
<dd><span>Aaron Colwell</span>, <a href="http://www.google.com/">Google Inc.</a></dd>
<dd><span>Adrian Bateman</span>, <a href="http://www.microsoft.com/">Microsoft Corporation</a></dd>
<dd><span>Mark Watson</span>, <a href="http://www.netflix.com/">Netflix Inc.</a></dd>
</dl>
<p class="copyright">
<a href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a> ©
2013
<a href="http://www.w3.org/"><abbr title="World Wide Web Consortium">W3C</abbr></a><sup>®</sup>
(<a href="http://www.csail.mit.edu/"><abbr title="Massachusetts Institute of Technology">MIT</abbr></a>,
<a href="http://www.ercim.eu/"><abbr title="European Research Consortium for Informatics and Mathematics">ERCIM</abbr></a>,
<a href="http://www.keio.ac.jp/">Keio</a>, <a href="http://ev.buaa.edu.cn/">Beihang</a>), All Rights Reserved.
<abbr title="World Wide Web Consortium">W3C</abbr> <a href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>,
<a href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a> and
<a href="http://www.w3.org/Consortium/Legal/copyright-documents">document use</a> rules apply.
</p>
<hr>
</div>
<section id="abstract" class="introductory"><h2>Abstract</h2>
This specification extends HTMLMediaElement 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 <abbr title="World Wide Web Consortium">W3C</abbr>, please submit them via
<a href="https://www.w3.org/Bugs/Public/enter_bug.cgi?product=HTML%20WG&component=Media%20Source%20Extensions&short_desc=%5BMSE%5D%20">our public bug database</a>.
</p>
</section><section id="sotd" class="introductory"><h2>Status of This Document</h2>
<p>This document was developed through the <a href="http://lists.w3.org/Archives/Public/public-html-media/">HTML Media Task Force</a> and is published by the <a href="http://www.w3.org/html/wg/">HTML Working Group</a> as a
Last Call Working Draft. If you wish to make comments regarding this document in a manner that is tracked by the
W3C, please submit them via using our <a href="http://www.w3.org/Bugs/Public/enter_bug.cgi?product=HTML%20WG">public bug database</a> with the Component field set to “Media Source Extensions”.
If you cannot do this then you can also e-mail feedback to <a href="mailto:public-html-media@w3.org">public-html-media@w3.org</a> (<a href="mailto:public-html-media-request@w3.org">subscribe</a>, <a href="http://lists.w3.org/Archives/Public/public-html-media/">archives</a>), and
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<p>In order to be considered under the W3C Last Call Process, bugs must be filed or email sent to the mailing list
by 17 October 2013.</p>
<p>Significant changes between successive public drafts are tracked in the <a href="https://dvcs.w3.org/hg/html-media/raw-file/default/media-source/media-source-lc.html#revision-history">Revision History section</a>.</p>
<p>This document is intended to become a W3C Recommendation, either on its own or merged into an HTML
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</p><p>
Publication as a Last Call Working Draft does not imply endorsement by the <abbr title="World Wide Web Consortium">W3C</abbr> Membership.
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<p>
This document was produced by a group operating under the
<a href="http://www.w3.org/Consortium/Patent-Policy-20040205/">5 February 2004 <abbr title="World Wide Web Consortium">W3C</abbr> Patent Policy</a>.
<abbr title="World Wide Web Consortium">W3C</abbr> maintains a <a href="http://www.w3.org/2004/01/pp-impl/40318/status" rel="disclosure">public list of any patent disclosures</a>
made in connection with the deliverables of the group; that page also includes instructions for
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</p>
</section><section id="toc"><h2 class="introductory">Table of Contents</h2><ul class="toc"><li class="tocline"><a href="#introduction" class="tocxref"><span class="secno">1. </span>Introduction</a><ul class="toc"><li class="tocline"><a href="#goals" class="tocxref"><span class="secno">1.1 </span>Goals</a></li><li class="tocline"><a href="#definitions" class="tocxref"><span class="secno">1.2 </span>Definitions</a></li></ul></li><li class="tocline"><a href="#mediasource" class="tocxref"><span class="secno">2. </span>MediaSource Object</a><ul class="toc"><li class="tocline"><a href="#attributes" class="tocxref"><span class="secno">2.1 </span>Attributes</a></li><li class="tocline"><a href="#methods" class="tocxref"><span class="secno">2.2 </span>Methods</a></li><li class="tocline"><a href="#mediasource-events" class="tocxref"><span class="secno">2.3 </span>Event Summary</a></li><li class="tocline"><a href="#mediasource-algorithms" class="tocxref"><span class="secno">2.4 </span>Algorithms</a><ul class="toc"><li class="tocline"><a href="#mediasource-attach" class="tocxref"><span class="secno">2.4.1 </span>Attaching to a media element</a></li><li class="tocline"><a href="#mediasource-detach" class="tocxref"><span class="secno">2.4.2 </span>Detaching from a media element</a></li><li class="tocline"><a href="#mediasource-seeking" class="tocxref"><span class="secno">2.4.3 </span>Seeking</a></li><li class="tocline"><a href="#buffer-monitoring" class="tocxref"><span class="secno">2.4.4 </span>SourceBuffer Monitoring</a></li><li class="tocline"><a href="#active-source-buffer-changes" class="tocxref"><span class="secno">2.4.5 </span>Changes to selected/enabled track state</a></li><li class="tocline"><a href="#duration-change-algorithm" class="tocxref"><span class="secno">2.4.6 </span>Duration change</a></li><li class="tocline"><a href="#end-of-stream-algorithm" class="tocxref"><span class="secno">2.4.7 </span>End of stream algorithm</a></li></ul></li></ul></li><li class="tocline"><a href="#sourcebuffer" class="tocxref"><span class="secno">3. </span>SourceBuffer Object</a><ul class="toc"><li class="tocline"><a href="#attributes-1" class="tocxref"><span class="secno">3.1 </span>Attributes</a></li><li class="tocline"><a href="#methods-1" class="tocxref"><span class="secno">3.2 </span>Methods</a></li><li class="tocline"><a href="#track-buffers" class="tocxref"><span class="secno">3.3 </span>Track Buffers</a></li><li class="tocline"><a href="#sourcebuffer-events" class="tocxref"><span class="secno">3.4 </span>Event Summary</a></li><li class="tocline"><a href="#sourcebuffer-algorithms" class="tocxref"><span class="secno">3.5 </span>Algorithms</a><ul class="toc"><li class="tocline"><a href="#sourcebuffer-segment-parser-loop" class="tocxref"><span class="secno">3.5.1 </span>Segment Parser Loop</a></li><li class="tocline"><a href="#sourcebuffer-reset-parser-state" class="tocxref"><span class="secno">3.5.2 </span>Reset Parser State</a></li><li class="tocline"><a href="#sourcebuffer-append-error" class="tocxref"><span class="secno">3.5.3 </span>Append Error Algorithm</a></li><li class="tocline"><a href="#sourcebuffer-prepare-append" class="tocxref"><span class="secno">3.5.4 </span>Prepare Append Algorithm</a></li><li class="tocline"><a href="#sourcebuffer-buffer-append" class="tocxref"><span class="secno">3.5.5 </span>Buffer Append Algorithm</a></li><li class="tocline"><a href="#sourcebuffer-stream-append-loop" class="tocxref"><span class="secno">3.5.6 </span>Stream Append Loop</a></li><li class="tocline"><a href="#sourcebuffer-init-segment-received" class="tocxref"><span class="secno">3.5.7 </span>Initialization Segment Received</a></li><li class="tocline"><a href="#sourcebuffer-coded-frame-processing" class="tocxref"><span class="secno">3.5.8 </span>Coded Frame Processing</a></li><li class="tocline"><a href="#sourcebuffer-coded-frame-removal" class="tocxref"><span class="secno">3.5.9 </span>Coded Frame Removal Algorithm</a></li><li class="tocline"><a href="#sourcebuffer-coded-frame-eviction" class="tocxref"><span class="secno">3.5.10 </span>Coded Frame Eviction Algorithm</a></li><li class="tocline"><a href="#sourcebuffer-audio-splice-frame-algorithm" class="tocxref"><span class="secno">3.5.11 </span>Audio Splice Frame Algorithm</a></li><li class="tocline"><a href="#sourcebuffer-audio-splice-rendering-algorithm" class="tocxref"><span class="secno">3.5.12 </span>Audio Splice Rendering Algorithm</a></li><li class="tocline"><a href="#sourcebuffer-text-splice-frame-algorithm" class="tocxref"><span class="secno">3.5.13 </span>Text Splice Frame Algorithm</a></li></ul></li></ul></li><li class="tocline"><a href="#sourcebufferlist" class="tocxref"><span class="secno">4. </span>SourceBufferList Object</a><ul class="toc"><li class="tocline"><a href="#attributes-2" class="tocxref"><span class="secno">4.1 </span>Attributes</a></li><li class="tocline"><a href="#methods-2" class="tocxref"><span class="secno">4.2 </span>Methods</a></li><li class="tocline"><a href="#sourcebufferlist-events" class="tocxref"><span class="secno">4.3 </span>Event Summary</a></li></ul></li><li class="tocline"><a href="#videoplaybackquality" class="tocxref"><span class="secno">5. </span>VideoPlaybackQuality Object</a><ul class="toc"><li class="tocline"><a href="#attributes-3" class="tocxref"><span class="secno">5.1 </span>Attributes</a></li></ul></li><li class="tocline"><a href="#url" class="tocxref"><span class="secno">6. </span>URL Object Extensions</a><ul class="toc"><li class="tocline"><a href="#methods-3" class="tocxref"><span class="secno">6.1 </span>Methods</a></li></ul></li><li class="tocline"><a href="#htmlmediaelement-extensions" class="tocxref"><span class="secno">7. </span>HTMLMediaElement Extensions</a></li><li class="tocline"><a href="#htmlvideoelement-extensions" class="tocxref"><span class="secno">8. </span>HTMLVideoElement Extensions</a><ul class="toc"><li class="tocline"><a href="#methods-4" class="tocxref"><span class="secno">8.1 </span>Methods</a></li></ul></li><li class="tocline"><a href="#audio-track-extensions" class="tocxref"><span class="secno">9. </span>AudioTrack Extensions</a><ul class="toc"><li class="tocline"><a href="#attributes-4" class="tocxref"><span class="secno">9.1 </span>Attributes</a></li></ul></li><li class="tocline"><a href="#video-track-extensions" class="tocxref"><span class="secno">10. </span>VideoTrack Extensions</a><ul class="toc"><li class="tocline"><a href="#attributes-5" class="tocxref"><span class="secno">10.1 </span>Attributes</a></li></ul></li><li class="tocline"><a href="#text-track-extensions" class="tocxref"><span class="secno">11. </span>TextTrack Extensions</a><ul class="toc"><li class="tocline"><a href="#attributes-6" class="tocxref"><span class="secno">11.1 </span>Attributes</a></li></ul></li><li class="tocline"><a href="#byte-stream-formats" class="tocxref"><span class="secno">12. </span>Byte Stream Formats</a><ul class="toc"><li class="tocline"><a href="#webm" class="tocxref"><span class="secno">12.1 </span>WebM Byte Streams</a><ul class="toc"><li class="tocline"><a href="#webm-init-segments" class="tocxref"><span class="secno">12.1.1 </span>Initialization Segments</a></li><li class="tocline"><a href="#webm-media-segments" class="tocxref"><span class="secno">12.1.2 </span>Media Segments</a></li><li class="tocline"><a href="#webm-random-access-points" class="tocxref"><span class="secno">12.1.3 </span>Random Access Points</a></li></ul></li><li class="tocline"><a href="#iso" class="tocxref"><span class="secno">12.2 </span>ISO Base Media File Format Byte Streams</a><ul class="toc"><li class="tocline"><a href="#iso-init-segments" class="tocxref"><span class="secno">12.2.1 </span>Initialization Segments</a></li><li class="tocline"><a href="#iso-media-segments" class="tocxref"><span class="secno">12.2.2 </span>Media Segments</a></li><li class="tocline"><a href="#iso-random-access-points" class="tocxref"><span class="secno">12.2.3 </span>Random Access Points</a></li></ul></li><li class="tocline"><a href="#mpeg2ts" class="tocxref"><span class="secno">12.3 </span>MPEG-2 Transport Stream Byte Streams</a><ul class="toc"><li class="tocline"><a href="#mpeg2ts-general" class="tocxref"><span class="secno">12.3.1 </span>General</a></li><li class="tocline"><a href="#mpeg2ts-init-segments" class="tocxref"><span class="secno">12.3.2 </span>Initialization Segments</a></li><li class="tocline"><a href="#mpeg2ts-media-segments" class="tocxref"><span class="secno">12.3.3 </span>Media Segments</a></li><li class="tocline"><a href="#mpeg2ts-random-access-points" class="tocxref"><span class="secno">12.3.4 </span>Random Access Points</a></li><li class="tocline"><a href="#mpeg2ts-discontinuities" class="tocxref"><span class="secno">12.3.5 </span>Timestamp Rollover & Discontinuities</a></li></ul></li></ul></li><li class="tocline"><a href="#examples" class="tocxref"><span class="secno">13. </span>Examples</a></li><li class="tocline"><a href="#acknowledgements" class="tocxref"><span class="secno">14. </span>Acknowledgments</a></li><li class="tocline"><a href="#revision-history" class="tocxref"><span class="secno">15. </span>Revision History</a></li><li class="tocline"><a href="#references" class="tocxref"><span class="secno">A. </span>References</a><ul class="toc"><li class="tocline"><a href="#informative-references" class="tocxref"><span class="secno">A.1 </span>Informative references</a></li></ul></li></ul></section>
<section id="introduction">
<!--OddPage--><h2><span class="secno">1. </span>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 href="http://www.w3.org/TR/html5/embedded-content-0.html#htmlmediaelement">HTMLMediaElement</a> [<cite><a class="bibref" href="#bib-HTML5">HTML5</a></cite>].
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 for WebM, ISO Base Media File Format, and MPEG-2 Transport Streams are given to specify the
expected format of byte streams used with these extensions.</p>
<img src="pipeline_model.png" alt="Media Source Pipeline Model Diagram">
<section id="goals">
<h3><span class="secno">1.1 </span>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 byte stream definitions for WebM, the ISO Base Media File Format, and MPEG-2 Transport Streams.</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><span class="secno">1.2 </span>Definitions</h3>
<dl>
<dt id="active-track-buffers">Active Track Buffers</dt>
<dd><p>The <a href="#track-buffer">track buffers</a> that provide <a href="#coded-frame">coded frames</a> for the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-audiotrack-enabled">enabled</a></code>
<code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-media-audiotracks">audioTracks</a></code>, the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-videotrack-selected">selected</a></code> <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-media-videotracks">videoTracks</a></code>, and the
<code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-texttrack-showing">"showing"</a></code> or <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-texttrack-hidden">"hidden"</a></code> <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-media-texttracks">textTracks</a></code>. All these tracks are associated with
<a href="#idl-def-SourceBuffer" class="idlType"><code>SourceBuffer</code></a> objects in the <code><a href="#widl-MediaSource-activeSourceBuffers">activeSourceBuffers</a></code> list.</p>
</dd>
<dt id="append-window">Append Window</dt>
<dd><p>A presentation timestamp range used to filter out <a href="#coded-frame">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 presentation timestamps within this range are allowed to be appended
to the <a href="#idl-def-SourceBuffer" class="idlType"><code>SourceBuffer</code></a> while coded frames outside this range are filtered out. The append window start and end times are controlled by
the <code><a href="#widl-SourceBuffer-appendWindowStart">appendWindowStart</a></code> and <code><a href="#widl-SourceBuffer-appendWindowEnd">appendWindowEnd</a></code> attributes respectively.</p></dd>
<dt id="coded-frame">Coded Frame</dt>
<dd><p>A unit of media data that has a presentation timestamp and decode timestamp. The presentation timestamp indicates when the frame must be rendered. The decode timestamp indicates when the frame needs to be decoded. If frames can be decoded out of order, then the decode timestamp are present in the byte stream. The user agent must run the <a href="#end-of-stream-algorithm">end of stream algorithm</a> with the <var>error</var> parameter set to <code><a href="#idl-def-EndOfStreamError.decode">"decode"</a></code> if this is not the case. If frames cannot be decoded out of order and a decode timestamp is not present in the byte stream, then the decode timestamp is equal to the presentation timestamp.</p></dd>
<dt id="coded-frame-group">Coded Frame Group</dt>
<dd><p>A group of <a href="#coded-frame">coded frames</a> that are adjacent and monotonically increasing in decode time without any gaps. Discontinuities detected by the
<a href="#sourcebuffer-coded-frame-processing">coded frame processing algorithm</a> and <code><a href="#widl-SourceBuffer-abort-void">abort()</a></code> calls trigger the start of a new coded frame group.</p>
</dd>
<dt id="displayed-frame-delay">Displayed Frame Delay</dt>
<dd>
<p>The delay, to the nearest microsecond, between a frame's presentation time and the actual time it was displayed. 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 href="#media-segment">media segments</a>. This includes codec initialization data, <a href="#track-id">Track ID</a> mappings for multiplexed segments, and timestamp offsets (e.g. edit lists).</p>
<div class="note"><div class="note-title"><span>Note</span></div><p class="">The <a href="#byte-stream-formats">byte stream format specifications</a> contain format specific examples.</p></div>
</dd><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 href="http://www.w3.org/TR/html5/embedded-content-0.html#media-timeline">media timeline</a>. Media segments are always associated with the most recently appended <a href="#init-segment">initialization segment</a>.</p>
<div class="note"><div class="note-title"><span>Note</span></div><p class="">The <a href="#byte-stream-formats">byte stream format specifications</a> contain format specific examples.</p></div>
</dd>
<dt id="mediasource-object-url">MediaSource object URL</dt>
<dd>
<p>A MediaSource object URL is a unique <a href="http://www.w3.org/TR/FileAPI/#url">Blob URI</a> [<cite><a class="bibref" href="#bib-FILE-API">FILE-API</a></cite>] created by <code><a href="#widl-URL-createObjectURL-DOMString-MediaSource-mediaSource">createObjectURL()</a></code>. It is used to attach a <a href="#idl-def-MediaSource" class="idlType"><code>MediaSource</code></a> object to an HTMLMediaElement.</p>
<p>These URLs are the same as a <a href="http://www.w3.org/TR/FileAPI/#url">Blob URI</a>, except that anything in the definition of that feature that refers to <a href="http://www.w3.org/TR/FileAPI/#dfn-file">File</a> and <a href="http://www.w3.org/TR/FileAPI/#dfn-Blob">Blob</a> objects is hereby extended to also apply to <a href="#idl-def-MediaSource" class="idlType"><code>MediaSource</code></a> objects.</p>
<p>The <a href="http://www.w3.org/TR/html5/browsers.html#origin-0">origin</a> of the MediaSource object URL is specified by the <a href="http://www.w3.org/TR/FileAPI/#originOfBlob">Origin of Blob URIs</a> [<cite><a class="bibref" href="#bib-FILE-API">FILE-API</a></cite>].</p>
<div class="note"><div class="note-title"><span>Note</span></div><p class="">For example, the <a href="http://www.w3.org/TR/html5/browsers.html#origin-0">origin</a> of the MediaSource object URL affects the way that the media element is <a href="http://www.w3.org/TR/html5/embedded-content-0.html#security-with-canvas-elements">consumed by canvas</a>.</p></div>
</dd>
<dt id="parent-media-source">Parent Media Source</dt>
<dd><p>The parent media source of a <a href="#idl-def-SourceBuffer" class="idlType"><code>SourceBuffer</code></a> object is the <a href="#idl-def-MediaSource" class="idlType"><code>MediaSource</code></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 href="http://www.w3.org/TR/html5/embedded-content-0.html#initial-playback-position">initial playback position</a> and <a href="http://www.w3.org/TR/html5/embedded-content-0.html#earliest-possible-position">earliest possible position</a>. All presentations created using this specification have a presentation start time of 0.</p></dd>
<dt id="random-access-point">Random Access Point</dt>
<dd><p>A position in a <a href="#media-segment">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 href="#byte-stream-formats">byte stream format specification</a> that describes the format of the byte stream accepted by a <a href="#idl-def-SourceBuffer" class="idlType"><code>SourceBuffer</code></a> instance. The
<a href="#byte-stream-formats">byte stream format specification</a>, for a <a href="#idl-def-SourceBuffer" class="idlType"><code>SourceBuffer</code></a> object, is selected based on the <var>type</var> passed to the
<code><a href="#widl-MediaSource-addSourceBuffer-SourceBuffer-DOMString-type">addSourceBuffer()</a></code> call that created the object.</p></dd>
<dt id="track-description">Track Description</dt>
<dd><p>A byte stream format specific structure that provides the <a href="#track-id">Track ID</a>, codec configuration, and other metadata for a single track. Each track description inside a single <a href="#init-segment">initialization segment</a> has a unique <a href="#track-id">Track ID</a>. The user agent must run the <a href="#end-of-stream-algorithm">end of stream algorithm</a> with the <var>error</var> parameter set to <code><a href="#idl-def-EndOfStreamError.decode">"decode"</a></code> if the <a href="#track-id">Track ID</a> is not unique within the <a href="#init-segment">initialization 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 href="#track-description">track description</a> identifies which sections of a <a href="#media-segment">media segment</a> belong to that track.</p></dd>
</dl>
</section>
</section>
<section id="mediasource">
<!--OddPage--><h2><span class="secno">2. </span>MediaSource Object</h2>
<p>The MediaSource object represents a source of media data for an HTMLMediaElement. It keeps track of the <code><a href="#widl-MediaSource-readyState">readyState</a></code> for this source as well as a list of <a href="#idl-def-SourceBuffer" class="idlType"><code>SourceBuffer</code></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 href="#idl-def-SourceBuffer" class="idlType"><code>SourceBuffer</code></a> objects in <code><a href="#widl-MediaSource-sourceBuffers">sourceBuffers</a></code> to add media data to this source. The HTMLMediaElement fetches this media data from the <a href="#idl-def-MediaSource" class="idlType"><code>MediaSource</code></a> object when it is needed during playback.</p>
<pre class="idl"><span class="idlEnum" id="idl-def-ReadyState">enum <span class="idlEnumID">ReadyState</span> {
"<a href="#idl-def-ReadyState.closed" class="idlEnumItem">closed</a>",
"<a href="#idl-def-ReadyState.open" class="idlEnumItem">open</a>",
"<a href="#idl-def-ReadyState.ended" class="idlEnumItem">ended</a>"
};</span></pre><table class="simple"><tr><th colspan="2">Enumeration description</th></tr><tr><td><code id="idl-def-ReadyState.closed">closed</code></td><td>
Indicates the source is not currently attached to a media element.
</td></tr><tr><td><code id="idl-def-ReadyState.open">open</code></td><td>
The source has been opened by a media element and is ready for data to be appended to the <a href="#idl-def-SourceBuffer" class="idlType"><code>SourceBuffer</code></a> objects in <code><a href="#widl-MediaSource-sourceBuffers">sourceBuffers</a></code>.
</td></tr><tr><td><code id="idl-def-ReadyState.ended">ended</code></td><td>
The source is still attached to a media element, but <code><a href="#widl-MediaSource-endOfStream-void-EndOfStreamError-error">endOfStream()</a></code> has been called.
</td></tr></table>
<pre class="idl"><span class="idlEnum" id="idl-def-EndOfStreamError">enum <span class="idlEnumID">EndOfStreamError</span> {
"<a href="#idl-def-EndOfStreamError.network" class="idlEnumItem">network</a>",
"<a href="#idl-def-EndOfStreamError.decode" class="idlEnumItem">decode</a>"
};</span></pre><table class="simple"><tr><th colspan="2">Enumeration description</th></tr><tr><td><code id="idl-def-EndOfStreamError.network">network</code></td><td>
<p>Terminates playback and signals that a network error has occured.</p>
<div class="note"><div class="note-title"><span>Note</span></div><p class="">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></div>
</td></tr><tr><td><code id="idl-def-EndOfStreamError.decode">decode</code></td><td>
<p>Terminates playback and signals that a decoding error has occured.</p>
<div class="note"><div class="note-title"><span>Note</span></div><p class="">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></div>
</td></tr></table>
<pre class="idl"><span class="idlInterface" id="idl-def-MediaSource">[<span class="extAttr">Constructor</span>]
interface <span class="idlInterfaceID">MediaSource</span> : <span class="idlSuperclass"><code><a href="http://dom.spec.whatwg.org/#eventtarget" class="idlType">EventTarget</a></code></span> {
<span class="idlAttribute"> readonly attribute <span class="idlAttrType"><a href="#idl-def-SourceBufferList" class="idlType"><code>SourceBufferList</code></a></span> <span class="idlAttrName"><a href="#widl-MediaSource-sourceBuffers">sourceBuffers</a></span>;</span>
<span class="idlAttribute"> readonly attribute <span class="idlAttrType"><a href="#idl-def-SourceBufferList" class="idlType"><code>SourceBufferList</code></a></span> <span class="idlAttrName"><a href="#widl-MediaSource-activeSourceBuffers">activeSourceBuffers</a></span>;</span>
<span class="idlAttribute"> readonly attribute <span class="idlAttrType"><a href="#idl-def-ReadyState" class="idlType"><code>ReadyState</code></a></span> <span class="idlAttrName"><a href="#widl-MediaSource-readyState">readyState</a></span>;</span>
<span class="idlAttribute"> attribute <span class="idlAttrType"><code><a href="http://dev.w3.org/2006/webapi/WebIDL/#idl-unrestricted-double" class="idlType">unrestricted double</a></code></span> <span class="idlAttrName"><a href="#widl-MediaSource-duration">duration</a></span>;</span>
<span class="idlMethod"> <span class="idlMethType"><a href="#idl-def-SourceBuffer" class="idlType"><code>SourceBuffer</code></a></span> <span class="idlMethName"><a href="#widl-MediaSource-addSourceBuffer-SourceBuffer-DOMString-type">addSourceBuffer</a></span> (<span class="idlParam"><span class="idlParamType"><code><a href="http://dev.w3.org/2006/webapi/WebIDL/#idl-DOMString" class="idlType">DOMString</a></code></span> <span class="idlParamName">type</span></span>);</span>
<span class="idlMethod"> <span class="idlMethType"><code><a href="http://dev.w3.org/2006/webapi/WebIDL/#idl-void" class="idlType">void</a></code></span> <span class="idlMethName"><a href="#widl-MediaSource-removeSourceBuffer-void-SourceBuffer-sourceBuffer">removeSourceBuffer</a></span> (<span class="idlParam"><span class="idlParamType"><a href="#idl-def-SourceBuffer" class="idlType"><code>SourceBuffer</code></a></span> <span class="idlParamName">sourceBuffer</span></span>);</span>
<span class="idlMethod"> <span class="idlMethType"><code><a href="http://dev.w3.org/2006/webapi/WebIDL/#idl-void" class="idlType">void</a></code></span> <span class="idlMethName"><a href="#widl-MediaSource-endOfStream-void-EndOfStreamError-error">endOfStream</a></span> (<span class="idlParam">optional <span class="idlParamType"><a href="#idl-def-EndOfStreamError" class="idlType"><code>EndOfStreamError</code></a></span> <span class="idlParamName">error</span></span>);</span>
<span class="idlMethod"> static <span class="idlMethType"><code><a href="http://dev.w3.org/2006/webapi/WebIDL/#idl-boolean" class="idlType">boolean</a></code></span> <span class="idlMethName"><a href="#widl-MediaSource-isTypeSupported-boolean-DOMString-type">isTypeSupported</a></span> (<span class="idlParam"><span class="idlParamType"><code><a href="http://dev.w3.org/2006/webapi/WebIDL/#idl-DOMString" class="idlType">DOMString</a></code></span> <span class="idlParamName">type</span></span>);</span>
};</span></pre><section id="attributes"><h3><span class="secno">2.1 </span>Attributes</h3><dl class="attributes"><dt id="widl-MediaSource-activeSourceBuffers"><code>activeSourceBuffers</code> of type <span class="idlAttrType"><a href="#idl-def-SourceBufferList" class="idlType"><code>SourceBufferList</code></a></span>, readonly </dt><dd>
<p>Contains the subset of <code><a href="#widl-MediaSource-sourceBuffers">sourceBuffers</a></code> that are providing the
<a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-videotrack-selected">selected video track</a>, the
<a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-audiotrack-enabled">enabled audio tracks</a>, and the
<a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-texttrack-showing">"showing"</a> or <a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-texttrack-hidden">"hidden"</a> text tracks.
</p>
<div class="note"><div class="note-title"><span>Note</span></div><p class="">The <a href="#active-source-buffer-changes">Changes to selected/enabled track state</a> section describes how this attribute gets
updated.</p></div>
</dd><dt id="widl-MediaSource-duration"><code>duration</code> of type <span class="idlAttrType"><code><a href="http://dev.w3.org/2006/webapi/WebIDL/#idl-unrestricted-double" class="idlType">unrestricted double</a></code></span>, </dt><dd>
<p>Allows the web application to set the presentation duration. The duration is initially set to NaN when the <a href="#idl-def-MediaSource" class="idlType"><code>MediaSource</code></a> object is created.</p>
<p>On getting, run the following steps:</p>
<ol>
<li>If the <code><a href="#widl-MediaSource-readyState">readyState</a></code> attribute is <code><a href="#idl-def-ReadyState.closed">"closed"</a></code> 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 an <code><a href="http://dom.spec.whatwg.org/#dom-domexception-invalid_access_err">INVALID_ACCESS_ERR</a></code> exception and abort these steps.</li>
<li>If the <code><a href="#widl-MediaSource-readyState">readyState</a></code> attribute is not <code><a href="#idl-def-ReadyState.open">"open"</a></code> then throw an <code><a href="http://dom.spec.whatwg.org/#dom-domexception-invalid_state_err">INVALID_STATE_ERR</a></code> exception and abort these steps.</li>
<li>If the <code><a href="#widl-SourceBuffer-updating">updating</a></code> attribute equals true on any <a href="#idl-def-SourceBuffer" class="idlType"><code>SourceBuffer</code></a> in <code><a href="#widl-MediaSource-sourceBuffers">sourceBuffers</a></code>, then throw an <code><a href="http://dom.spec.whatwg.org/#dom-domexception-invalid_state_err">INVALID_STATE_ERR</a></code> exception and abort these steps.</li>
<li>Run the <a href="#duration-change-algorithm">duration change algorithm</a> with <var>new duration</var> set to the value being assigned to this attribute.
<div class="note"><div class="note-title"><span>Note</span></div><p class=""><code><a href="#widl-SourceBuffer-appendBuffer-void-ArrayBufferView-data">appendBuffer()</a></code>, <code><a href="#widl-SourceBuffer-appendStream-void-Stream-stream-unsigned-long-long-maxSize">appendStream()</a></code> and <code><a href="#widl-MediaSource-endOfStream-void-EndOfStreamError-error">endOfStream()</a></code> can update the duration under certain circumstances.</p></div>
</li>
</ol>
</dd><dt id="widl-MediaSource-readyState"><code>readyState</code> of type <span class="idlAttrType"><a href="#idl-def-ReadyState" class="idlType"><code>ReadyState</code></a></span>, readonly </dt><dd>
<p>Indicates the current state of the <a href="#idl-def-MediaSource" class="idlType"><code>MediaSource</code></a> object. When the <a href="#idl-def-MediaSource" class="idlType"><code>MediaSource</code></a> is created <code><a href="#widl-MediaSource-readyState">readyState</a></code> must be set to <code><a href="#idl-def-ReadyState.closed">"closed"</a></code>.
</p></dd><dt id="widl-MediaSource-sourceBuffers"><code>sourceBuffers</code> of type <span class="idlAttrType"><a href="#idl-def-SourceBufferList" class="idlType"><code>SourceBufferList</code></a></span>, readonly </dt><dd>
Contains the list of <a href="#idl-def-SourceBuffer" class="idlType"><code>SourceBuffer</code></a> objects associated with this <a href="#idl-def-MediaSource" class="idlType"><code>MediaSource</code></a>. When <code><a href="#widl-MediaSource-readyState">readyState</a></code> equals <code><a href="#idl-def-ReadyState.closed">"closed"</a></code> this list will be empty. Once <code><a href="#widl-MediaSource-readyState">readyState</a></code> transitions to <code><a href="#idl-def-ReadyState.open">"open"</a></code> SourceBuffer objects can be added to this list by using <code><a href="#widl-MediaSource-addSourceBuffer-SourceBuffer-DOMString-type">addSourceBuffer()</a></code>.
</dd></dl></section><section id="methods"><h3><span class="secno">2.2 </span>Methods</h3><dl class="methods"><dt id="widl-MediaSource-addSourceBuffer-SourceBuffer-DOMString-type"><code>addSourceBuffer</code></dt><dd>
<p>Adds a new <a href="#idl-def-SourceBuffer" class="idlType"><code>SourceBuffer</code></a> to <code><a href="#widl-MediaSource-sourceBuffers">sourceBuffers</a></code>.</p>
<p>Implementations must support at least 1 MediaSource object with the following <a href="#idl-def-SourceBuffer" class="idlType"><code>SourceBuffer</code></a> configurations. MediaSource objects must
support each of the configurations below, 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><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></p>
<table class="parameters"><tr><th>Parameter</th><th>Type</th><th>Nullable</th><th>Optional</th><th>Description</th></tr><tr><td class="prmName">type</td><td class="prmType"><code><code><a href="http://dev.w3.org/2006/webapi/WebIDL/#idl-DOMString" class="idlType">DOMString</a></code></code></td><td class="prmNullFalse">✘</td><td class="prmOptFalse">✘</td><td class="prmDesc"></td></tr></table><div><em>Return type: </em><code><a href="#idl-def-SourceBuffer" class="idlType"><code>SourceBuffer</code></a></code></div><p>When this method is invoked, the user agent must run the following steps:</p><ol class="method-algorithm">
<li>If <var>type</var> is null or an empty string then throw an <code><a href="http://dom.spec.whatwg.org/#dom-domexception-invalid_access_err">INVALID_ACCESS_ERR</a></code> 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 href="#idl-def-SourceBuffer" class="idlType"><code>SourceBuffer</code></a> objects in <code><a href="#widl-MediaSource-sourceBuffers">sourceBuffers</a></code>, then throw a <code><a href="http://dom.spec.whatwg.org/#dom-domexception-not_supported_err">NOT_SUPPORTED_ERR</a></code> exception and abort these steps.</li>
<li>If the user agent can't handle any more SourceBuffer objects then throw a <code><a href="http://dom.spec.whatwg.org/#dom-domexception-quota_exceeded_err">QUOTA_EXCEEDED_ERR</a></code> exception and abort these steps.
<div class="note"><div class="note-title"><span>Note</span></div><p class="">For example, a user agent may throw a <code><a href="http://dom.spec.whatwg.org/#dom-domexception-quota_exceeded_err">QUOTA_EXCEEDED_ERR</a></code> exception if the media element has reached the
<code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-media-have_metadata">HAVE_METADATA</a></code> readyState. This can occur if the user agent's media engine does not support adding more tracks during
playback.
</p></div>
</li>
<li>If the <code><a href="#widl-MediaSource-readyState">readyState</a></code> attribute is not in the <code><a href="#idl-def-ReadyState.open">"open"</a></code> state then throw an <code><a href="http://dom.spec.whatwg.org/#dom-domexception-invalid_state_err">INVALID_STATE_ERR</a></code> exception and abort these steps.</li>
<li>Create a new <a href="#idl-def-SourceBuffer" class="idlType"><code>SourceBuffer</code></a> object and associated resources.</li>
<li>Add the new object to <code><a href="#widl-MediaSource-sourceBuffers">sourceBuffers</a></code> and <a href="http://www.w3.org/TR/html5/webappapis.html#queue-a-task">queue a task</a> to <a href="http://www.w3.org/TR/html5/webappapis.html#fire-a-simple-event">fire a simple event</a> named <code><a href="#dom-evt-addsourcebuffer">addsourcebuffer</a></code> at <code><a href="#widl-MediaSource-sourceBuffers">sourceBuffers</a></code>.</li>
<li>Return the new object.</li>
</ol></dd><dt id="widl-MediaSource-endOfStream-void-EndOfStreamError-error"><code>endOfStream</code></dt><dd>
<p>Signals the end of the stream.</p>
<table class="parameters"><tr><th>Parameter</th><th>Type</th><th>Nullable</th><th>Optional</th><th>Description</th></tr><tr><td class="prmName">error</td><td class="prmType"><code><a href="#idl-def-EndOfStreamError" class="idlType"><code>EndOfStreamError</code></a></code></td><td class="prmNullFalse">✘</td><td class="prmOptTrue">✔</td><td class="prmDesc"></td></tr></table><div><em>Return type: </em><code><code><a href="http://dev.w3.org/2006/webapi/WebIDL/#idl-void" class="idlType">void</a></code></code></div><p>When this method is invoked, the user agent must run the following steps:</p><ol class="method-algorithm">
<li>If the <code><a href="#widl-MediaSource-readyState">readyState</a></code> attribute is not in the <code><a href="#idl-def-ReadyState.open">"open"</a></code> state then throw an <code><a href="http://dom.spec.whatwg.org/#dom-domexception-invalid_state_err">INVALID_STATE_ERR</a></code> exception and abort these steps.</li>
<li>If the <code><a href="#widl-SourceBuffer-updating">updating</a></code> attribute equals true on any <a href="#idl-def-SourceBuffer" class="idlType"><code>SourceBuffer</code></a> in <code><a href="#widl-MediaSource-sourceBuffers">sourceBuffers</a></code>, then throw an <code><a href="http://dom.spec.whatwg.org/#dom-domexception-invalid_state_err">INVALID_STATE_ERR</a></code> exception and abort these steps.</li>
<li>Run the <a href="#end-of-stream-algorithm">end of stream algorithm</a> with the <var>error</var> parameter set to <var>error</var>.</li>
</ol></dd><dt id="widl-MediaSource-isTypeSupported-boolean-DOMString-type"><code>isTypeSupported</code>, static</dt><dd>
<p>Check to see whether the <a href="#idl-def-MediaSource" class="idlType"><code>MediaSource</code></a> is capable of creating <a href="#idl-def-SourceBuffer" class="idlType"><code>SourceBuffer</code></a> objects for the the specified MIME type.</p>
<div class="note"><div class="note-title"><span>Note</span></div><p class="">
If true is returned from this method, it only indicates that the <a href="#idl-def-MediaSource" class="idlType"><code>MediaSource</code></a> implementation is capable of creating <a href="#idl-def-SourceBuffer" class="idlType"><code>SourceBuffer</code></a> objects for the specified MIME type. An <code><a href="#widl-MediaSource-addSourceBuffer-SourceBuffer-DOMString-type">addSourceBuffer()</a></code> call may still fail if sufficient resources are not available to support the addition of a new <a href="#idl-def-SourceBuffer" class="idlType"><code>SourceBuffer</code></a>.
</p></div>
<div class="note"><div class="note-title"><span>Note</span></div><p class="">
This method returning true implies that HTMLMediaElement.canPlayType() will return "maybe" or "probably" since it does not make sense for a <a href="#idl-def-MediaSource" class="idlType"><code>MediaSource</code></a> to support a type the HTMLMediaElement knows it cannot play.
</p></div>
<table class="parameters"><tr><th>Parameter</th><th>Type</th><th>Nullable</th><th>Optional</th><th>Description</th></tr><tr><td class="prmName">type</td><td class="prmType"><code><code><a href="http://dev.w3.org/2006/webapi/WebIDL/#idl-DOMString" class="idlType">DOMString</a></code></code></td><td class="prmNullFalse">✘</td><td class="prmOptFalse">✘</td><td class="prmDesc"></td></tr></table><div><em>Return type: </em><code><code><a href="http://dev.w3.org/2006/webapi/WebIDL/#idl-boolean" class="idlType">boolean</a></code></code></div><p>When this method is invoked, the user agent must run the following steps:</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></dd><dt id="widl-MediaSource-removeSourceBuffer-void-SourceBuffer-sourceBuffer"><code>removeSourceBuffer</code></dt><dd>
<p>Removes a <a href="#idl-def-SourceBuffer" class="idlType"><code>SourceBuffer</code></a> from <code><a href="#widl-MediaSource-sourceBuffers">sourceBuffers</a></code>.</p>
<table class="parameters"><tr><th>Parameter</th><th>Type</th><th>Nullable</th><th>Optional</th><th>Description</th></tr><tr><td class="prmName">sourceBuffer</td><td class="prmType"><code><a href="#idl-def-SourceBuffer" class="idlType"><code>SourceBuffer</code></a></code></td><td class="prmNullFalse">✘</td><td class="prmOptFalse">✘</td><td class="prmDesc"></td></tr></table><div><em>Return type: </em><code><code><a href="http://dev.w3.org/2006/webapi/WebIDL/#idl-void" class="idlType">void</a></code></code></div><p>When this method is invoked, the user agent must run the following steps:</p><ol class="method-algorithm">
<li>If <var>sourceBuffer</var> is null then throw an <code><a href="http://dom.spec.whatwg.org/#dom-domexception-invalid_access_err">INVALID_ACCESS_ERR</a></code> exception and abort these steps.</li>
<li>If <var>sourceBuffer</var> specifies an object that is not in <code><a href="#widl-MediaSource-sourceBuffers">sourceBuffers</a></code> then throw a <code><a href="http://dom.spec.whatwg.org/#dom-domexception-not_found_err">NOT_FOUND_ERR</a></code> exception and abort these steps.</li>
<li>If the <var>sourceBuffer</var>.<code><a href="#widl-SourceBuffer-updating">updating</a></code> attribute equals true, then run the following steps:
<ol>
<li>Abort the <a href="#sourcebuffer-buffer-append">buffer append </a> and <a href="#sourcebuffer-stream-append-loop">stream append loop</a> algorithms if they are running.</li>
<li>Set the <var>sourceBuffer</var>.<code><a href="#widl-SourceBuffer-updating">updating</a></code> attribute to false.</li>
<li><a href="http://www.w3.org/TR/html5/webappapis.html#queue-a-task">Queue a task</a> to <a href="http://www.w3.org/TR/html5/webappapis.html#fire-a-simple-event">fire a simple event</a> named <code><a href="#dom-evt-abort">abort</a></code> at <var>sourceBuffer</var>.</li>
<li><a href="http://www.w3.org/TR/html5/webappapis.html#queue-a-task">Queue a task</a> to <a href="http://www.w3.org/TR/html5/webappapis.html#fire-a-simple-event">fire a simple event</a> named <code><a href="#dom-evt-updateend">updateend</a></code> at <var>sourceBuffer</var>.</li>
</ol>
</li>
<li>Let <var>SourceBuffer audioTracks list</var> equal the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#audiotracklist">AudioTrackList</a></code> object returned by <var>sourceBuffer</var>.<code><a href="#widl-SourceBuffer-audioTracks">audioTracks</a></code>.</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 <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#audiotracklist">AudioTrackList</a></code> object returned by the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-media-audiotracks">audioTracks</a></code> attribute on the HTMLMediaElement.</li>
<li>Let the <var>removed enabled audio track flag</var> equal false.</li>
<li>For each <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#audiotrack">AudioTrack</a></code> object in the <var>SourceBuffer audioTracks list</var>, run the following steps:
<ol>
<li>Set the <code><a href="#widl-AudioTrack-sourceBuffer">sourceBuffer</a></code> attribute on the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#audiotrack">AudioTrack</a></code> object to null.</li>
<li>If the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-audiotrack-enabled">enabled</a></code> attribute on the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#audiotrack">AudioTrack</a></code> object is true, then
set the <var>removed enabled audio track flag</var> to true.</li>
<li>Remove the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#audiotrack">AudioTrack</a></code> object from the <var>HTMLMediaElement audioTracks list</var>.</li>
<li><a href="http://www.w3.org/TR/html5/webappapis.html#queue-a-task">Queue a task</a> to fire a <a href="http://www.w3.org/TR/html5/infrastructure.html#concept-events-trusted">trusted event</a> named <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#handler-tracklist-onremovetrack">removetrack</a></code>, that does not bubble and is not cancelable, and that uses the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#trackevent">TrackEvent</a></code> interface, at the <var>HTMLMediaElement audioTracks list</var>.</li>
<li>Remove the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#audiotrack">AudioTrack</a></code> object from the <var>SourceBuffer audioTracks list</var>.</li>
<li><a href="http://www.w3.org/TR/html5/webappapis.html#queue-a-task">Queue a task</a> to fire a <a href="http://www.w3.org/TR/html5/infrastructure.html#concept-events-trusted">trusted event</a> named <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#handler-tracklist-onremovetrack">removetrack</a></code>, that does not bubble and is not cancelable, and that uses the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#trackevent">TrackEvent</a></code> interface, at the <var>SourceBuffer audioTracks list</var>.</li>
</ol>
</li>
<li>If the <var>removed enabled audio track flag</var> equals true, then <a href="http://www.w3.org/TR/html5/webappapis.html#queue-a-task">queue a task</a> to <a href="http://www.w3.org/TR/html5/webappapis.html#fire-a-simple-event">fire a simple event</a> named <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#handler-tracklist-onchange">change</a></code> at the
<var>HTMLMediaElement audioTracks list</var>.</li>
</ol>
</li>
<li>Let <var>SourceBuffer videoTracks list</var> equal the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#videotracklist">VideoTrackList</a></code> object returned by <var>sourceBuffer</var>.<code><a href="#widl-SourceBuffer-videoTracks">videoTracks</a></code>.</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 <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#videotracklist">VideoTrackList</a></code> object returned by the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-media-videotracks">videoTracks</a></code> attribute on the HTMLMediaElement.</li>
<li>Let the <var>removed selected video track flag</var> equal false.</li>
<li>For each <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#videotrack">VideoTrack</a></code> object in the <var>SourceBuffer videoTracks list</var>, run the following steps:
<ol>
<li>Set the <code><a href="#widl-VideoTrack-sourceBuffer">sourceBuffer</a></code> attribute on the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#videotrack">VideoTrack</a></code> object to null.</li>
<li>If the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-videotrack-selected">selected</a></code> attribute on the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#videotrack">VideoTrack</a></code> object is true, then
set the <var>removed selected video track flag</var> to true.</li>
<li>Remove the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#videotrack">VideoTrack</a></code> object from the <var>HTMLMediaElement videoTracks list</var>.</li>
<li><a href="http://www.w3.org/TR/html5/webappapis.html#queue-a-task">Queue a task</a> to fire a <a href="http://www.w3.org/TR/html5/infrastructure.html#concept-events-trusted">trusted event</a> named <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#handler-tracklist-onremovetrack">removetrack</a></code>, that does not bubble and is not cancelable, and that uses the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#trackevent">TrackEvent</a></code> interface, at the <var>HTMLMediaElement videoTracks list</var>.</li>
<li>Remove the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#videotrack">VideoTrack</a></code> object from the <var>SourceBuffer videoTracks list</var>.</li>
<li><a href="http://www.w3.org/TR/html5/webappapis.html#queue-a-task">Queue a task</a> to fire a <a href="http://www.w3.org/TR/html5/infrastructure.html#concept-events-trusted">trusted event</a> named <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#handler-tracklist-onremovetrack">removetrack</a></code>, that does not bubble and is not cancelable, and that uses the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#trackevent">TrackEvent</a></code> interface, at the <var>SourceBuffer videoTracks list</var>.</li>
</ol>
</li>
<li>If the <var>removed selected video track flag</var> equals true, then <a href="http://www.w3.org/TR/html5/webappapis.html#queue-a-task">queue a task</a> to <a href="http://www.w3.org/TR/html5/webappapis.html#fire-a-simple-event">fire a simple event</a> named <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#handler-tracklist-onchange">change</a></code> at the
<var>HTMLMediaElement videoTracks list</var>.</li>
</ol>
</li>
<li>Let <var>SourceBuffer textTracks list</var> equal the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#texttracklist">TextTrackList</a></code> object returned by <var>sourceBuffer</var>.<code><a href="#widl-SourceBuffer-textTracks">textTracks</a></code>.</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 <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#texttracklist">TextTrackList</a></code> object returned by the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-media-texttracks">textTracks</a></code> attribute on the HTMLMediaElement.</li>
<li>Let the <var>removed enabled text track flag</var> equal false.</li>
<li>For each <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#texttrack">TextTrack</a></code> object in the <var>SourceBuffer textTracks list</var>, run the following steps:
<ol>
<li>Set the <code><a href="#widl-TextTrack-sourceBuffer">sourceBuffer</a></code> attribute on the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#texttrack">TextTrack</a></code> object to null.</li>
<li>If the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-texttrack-mode">mode</a></code> attribute on the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#texttrack">TextTrack</a></code> object is set to <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-texttrack-showing">"showing"</a></code> or
<code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-texttrack-hidden">"hidden"</a></code> , then set the <var>removed enabled text track flag</var> to true.</li>
<li>Remove the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#texttrack">TextTrack</a></code> object from the <var>HTMLMediaElement textTracks list</var>.</li>
<li><a href="http://www.w3.org/TR/html5/webappapis.html#queue-a-task">Queue a task</a> to fire a <a href="http://www.w3.org/TR/html5/infrastructure.html#concept-events-trusted">trusted event</a> named <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#handler-tracklist-onremovetrack">removetrack</a></code>, that does not bubble and is not cancelable, and that uses the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#trackevent">TrackEvent</a></code> interface, at the <var>HTMLMediaElement textTracks list</var>.</li>
<li>Remove the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#texttrack">TextTrack</a></code> object from the <var>SourceBuffer textTracks list</var>.</li>
<li><a href="http://www.w3.org/TR/html5/webappapis.html#queue-a-task">Queue a task</a> to fire a <a href="http://www.w3.org/TR/html5/infrastructure.html#concept-events-trusted">trusted event</a> named <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#handler-tracklist-onremovetrack">removetrack</a></code>, that does not bubble and is not cancelable, and that uses the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#trackevent">TrackEvent</a></code> interface, at the <var>SourceBuffer textTracks list</var>.</li>
</ol>
</li>
<li>If the <var>removed enabled text track flag</var> equals true, then <a href="http://www.w3.org/TR/html5/webappapis.html#queue-a-task">queue a task</a> to <a href="http://www.w3.org/TR/html5/webappapis.html#fire-a-simple-event">fire a simple event</a> named <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#handler-tracklist-onchange">change</a></code> at the
<var>HTMLMediaElement textTracks list</var>.</li>
</ol>
</li>
<li>If <var>sourceBuffer</var> is in <code><a href="#widl-MediaSource-activeSourceBuffers">activeSourceBuffers</a></code>, then remove <var>sourceBuffer</var> from <code><a href="#widl-MediaSource-activeSourceBuffers">activeSourceBuffers</a></code> and
<a href="http://www.w3.org/TR/html5/webappapis.html#queue-a-task">queue a task</a> to <a href="http://www.w3.org/TR/html5/webappapis.html#fire-a-simple-event">fire a simple event</a> named <code><a href="#dom-evt-removesourcebuffer">removesourcebuffer</a></code> at the <a href="#idl-def-SourceBufferList" class="idlType"><code>SourceBufferList</code></a> returned by <code><a href="#widl-MediaSource-activeSourceBuffers">activeSourceBuffers</a></code>.</li>
<li>Remove <var>sourceBuffer</var> from <code><a href="#widl-MediaSource-sourceBuffers">sourceBuffers</a></code> and <a href="http://www.w3.org/TR/html5/webappapis.html#queue-a-task">queue a task</a> to <a href="http://www.w3.org/TR/html5/webappapis.html#fire-a-simple-event">fire a simple event</a> named <code><a href="#dom-evt-removesourcebuffer">removesourcebuffer</a></code> at
the <a href="#idl-def-SourceBufferList" class="idlType"><code>SourceBufferList</code></a> returned by <code><a href="#widl-MediaSource-sourceBuffers">sourceBuffers</a></code>.</li>
<li>Destroy all resources for <var>sourceBuffer</var>.</li>
</ol></dd></dl></section>
<section id="mediasource-events">
<h3><span class="secno">2.3 </span>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><dfn id="dom-evt-sourceopen"><code>sourceopen</code></dfn></td>
<td><code>Event</code></td>
<td><code><a href="#widl-MediaSource-readyState">readyState</a></code> transitions from <code><a href="#idl-def-ReadyState.closed">"closed"</a></code> to <code><a href="#idl-def-ReadyState.open">"open"</a></code> or from <code><a href="#idl-def-ReadyState.ended">"ended"</a></code> to <code><a href="#idl-def-ReadyState.open">"open"</a></code>.</td>
</tr>
<tr>
<td><dfn id="dom-evt-sourceended"><code>sourceended</code></dfn></td>
<td><code>Event</code></td>
<td><code><a href="#widl-MediaSource-readyState">readyState</a></code> transitions from <code><a href="#idl-def-ReadyState.open">"open"</a></code> to <code><a href="#idl-def-ReadyState.ended">"ended"</a></code>.</td>
</tr>
<tr>
<td><dfn id="dom-evt-sourceclose"><code>sourceclose</code></dfn></td>
<td><code>Event</code></td>
<td><code><a href="#widl-MediaSource-readyState">readyState</a></code> transitions from <code><a href="#idl-def-ReadyState.open">"open"</a></code> to <code><a href="#idl-def-ReadyState.closed">"closed"</a></code> or <code><a href="#idl-def-ReadyState.ended">"ended"</a></code> to <code><a href="#idl-def-ReadyState.closed">"closed"</a></code>.</td>
</tr>
</tbody>
</table>
</section>
<section id="mediasource-algorithms">
<h3><span class="secno">2.4 </span>Algorithms</h3>
<section id="mediasource-attach">
<h4><span class="secno">2.4.1 </span>Attaching to a media element</h4>
<p> A <a href="#idl-def-MediaSource" class="idlType"><code>MediaSource</code></a> object can be attached to a media element by assigning a <a href="#mediasource-object-url">MediaSource object URL</a> to the media element <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#attr-media-src">src</a></code> attribute or the src attribute of a <source> inside a media element. A <a href="#mediasource-object-url">MediaSource object URL</a> is created by passing a MediaSource object to <code><a href="#widl-URL-createObjectURL-DOMString-MediaSource-mediaSource">createObjectURL()</a></code>.</p>
<p>If the <a href="http://www.w3.org/TR/html5/embedded-content-0.html#concept-media-load-resource">resource fetch algorithm</a> absolute URL matches the MediaSource object URL, run the following steps right before the <span>"<i>Perform a potentially CORS-enabled fetch</i>"</span>
step in the <a href="http://www.w3.org/TR/html5/embedded-content-0.html#concept-media-load-resource">resource fetch algorithm</a>.</p>
<dl class="switch">
<dt>If <code><a href="#widl-MediaSource-readyState">readyState</a></code> is NOT set to <code><a href="#idl-def-ReadyState.closed">"closed"</a></code></dt>
<dd>Run the <span>"<i>If the media data cannot be fetched at all, due to network errors, causing the user agent to give up trying to fetch the resource</i>"</span> steps of the <a href="http://www.w3.org/TR/html5/embedded-content-0.html#concept-media-load-resource">resource fetch algorithm</a>.</dd>
<dt>Otherwise</dt>
<dd>
<ol>
<li>Set the <code><a href="#widl-MediaSource-readyState">readyState</a></code> attribute to <code><a href="#idl-def-ReadyState.open">"open"</a></code>.</li>
<li>
<a href="http://www.w3.org/TR/html5/webappapis.html#queue-a-task">Queue a task</a> to <a href="http://www.w3.org/TR/html5/webappapis.html#fire-a-simple-event">fire a simple event</a> named <code><a href="#dom-evt-sourceopen">sourceopen</a></code> at the <a href="#idl-def-MediaSource" class="idlType"><code>MediaSource</code></a>.</li>
<li>Continue the <a href="http://www.w3.org/TR/html5/embedded-content-0.html#concept-media-load-resource">resource fetch algorithm</a> by running the <span>"<i>Perform a potentially CORS-enabled fetch</i>"</span> step. Text in the <a href="http://www.w3.org/TR/html5/embedded-content-0.html#concept-media-load-resource">resource fetch algorithm</a>
that refers to "the download" or "bytes received" refer to data passed in via <code><a href="#widl-SourceBuffer-appendBuffer-void-ArrayBufferView-data">appendBuffer()</a></code> and <code><a href="#widl-SourceBuffer-appendStream-void-Stream-stream-unsigned-long-long-maxSize">appendStream()</a></code>. References to
HTTP in the <a href="http://www.w3.org/TR/html5/embedded-content-0.html#concept-media-load-resource">resource fetch algorithm</a> do not apply because the HTMLMediaElement does not fetch media data via HTTP when a <a href="#idl-def-MediaSource" class="idlType"><code>MediaSource</code></a> is attached.</li>
</ol>
</dd>
</dl>
</section>
<section id="mediasource-detach">
<h4><span class="secno">2.4.2 </span>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 href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-media-network_empty">NETWORK_EMPTY</a> and <a href="http://www.w3.org/TR/html5/webappapis.html#queue-a-task">queue a task</a> to <a href="http://www.w3.org/TR/html5/webappapis.html#fire-a-simple-event">fire a simple event</a> named <a href="http://www.w3.org/TR/html5/embedded-content-0.html#event-mediacontroller-emptied">emptied</a> at the media element. These steps must be run right before the transition.</p>
<ol>
<li>Set the <code><a href="#widl-MediaSource-readyState">readyState</a></code> attribute to <code><a href="#idl-def-ReadyState.closed">"closed"</a></code>.</li>
<li>Set the <code><a href="#widl-MediaSource-duration">duration</a></code> attribute to NaN.</li>
<li>Remove all the <a href="#idl-def-SourceBuffer" class="idlType"><code>SourceBuffer</code></a> objects from <code><a href="#widl-MediaSource-activeSourceBuffers">activeSourceBuffers</a></code>.</li>
<li>
<a href="http://www.w3.org/TR/html5/webappapis.html#queue-a-task">Queue a task</a> to <a href="http://www.w3.org/TR/html5/webappapis.html#fire-a-simple-event">fire a simple event</a> named <code><a href="#dom-evt-removesourcebuffer">removesourcebuffer</a></code> at <code><a href="#widl-MediaSource-activeSourceBuffers">activeSourceBuffers</a></code>.</li>
<li>Remove all the <a href="#idl-def-SourceBuffer" class="idlType"><code>SourceBuffer</code></a> objects from <code><a href="#widl-MediaSource-sourceBuffers">sourceBuffers</a></code>.</li>
<li>
<a href="http://www.w3.org/TR/html5/webappapis.html#queue-a-task">Queue a task</a> to <a href="http://www.w3.org/TR/html5/webappapis.html#fire-a-simple-event">fire a simple event</a> named <code><a href="#dom-evt-removesourcebuffer">removesourcebuffer</a></code> at <code><a href="#widl-MediaSource-sourceBuffers">sourceBuffers</a></code>.</li>
<li>
<a href="http://www.w3.org/TR/html5/webappapis.html#queue-a-task">Queue a task</a> to <a href="http://www.w3.org/TR/html5/webappapis.html#fire-a-simple-event">fire a simple event</a> named <code><a href="#dom-evt-sourceclose">sourceclose</a></code> at the <a href="#idl-def-MediaSource" class="idlType"><code>MediaSource</code></a>.</li>
</ol>
</section>
<section id="mediasource-seeking">
<h4><span class="secno">2.4.3 </span>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 href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-media-seek">seek algorithm</a>:</p>
<ol>
<li>The media element looks for <a href="#media-segment">media segments</a> containing the <var>new playback position</var> in each <a href="#idl-def-SourceBuffer" class="idlType"><code>SourceBuffer</code></a> object in <code><a href="#widl-MediaSource-activeSourceBuffers">activeSourceBuffers</a></code>.
<dl class="switch">
<dt>If one or more of the objects in <code><a href="#widl-MediaSource-activeSourceBuffers">activeSourceBuffers</a></code> is missing <a href="#media-segment">media segments</a> for the <var>new playback position</var>
</dt>
<dd>
<ol>
<li>Set the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-media-readystate">HTMLMediaElement.readyState</a></code> attribute to <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-media-have_metadata">HAVE_METADATA</a></code>.</li>
<li>The media element waits for the necessary <a href="#media-segment">media segments</a> to be passed to <code><a href="#widl-SourceBuffer-appendBuffer-void-ArrayBufferView-data">appendBuffer()</a></code> or <code><a href="#widl-SourceBuffer-appendStream-void-Stream-stream-unsigned-long-long-maxSize">appendStream()</a></code>.
<div class="note"><div class="note-title"><span>Note</span></div><p class="">The web application can use <code><a href="#widl-SourceBuffer-buffered">buffered</a></code> to determine what the media element needs to resume playback.</p></div>
</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 href="#init-segment">initialization segment</a>.</li>
<li>The media element feeds <a href="#coded-frame">coded frames</a> from the <a href="#active-track-buffers">active track buffers</a> into the decoders starting with the
closest <a href="#random-access-point">random access point</a> before the the <var>new playback position</var>.</li>
<li>Resume the <a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-media-seek">seek algorithm</a> at the "<i>Await a stable state</i>" step.</li>
</ol>
</section>
<section id="buffer-monitoring">
<h4><span class="secno">2.4.4 </span>SourceBuffer Monitoring</h4>
<p>The following steps are periodically run during playback to make sure that all of the <a href="#idl-def-SourceBuffer" class="idlType"><code>SourceBuffer</code></a> objects in <code><a href="#widl-MediaSource-activeSourceBuffers">activeSourceBuffers</a></code> have <a href="#enough-data">enough data to ensure uninterrupted playback</a>. Appending new segments and changes to <code><a href="#widl-MediaSource-activeSourceBuffers">activeSourceBuffers</a></code> 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 <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-media-have_enough_data">HAVE_ENOUGH_DATA</a></code> ready state.
These transitions indicate when the user agent believes it has enough data buffered or it needs more data respectively.</p>
<div class="note"><div class="note-title"><span>Note</span></div><p class="">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
<code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-media-readystate">HTMLMediaElement.readyState</a></code> to determine whether more data is needed or not.</p></div>
<div class="note"><div class="note-title"><span>Note</span></div><p class="">When the media element needs more data, the user agent should transition it from <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-media-have_enough_data">HAVE_ENOUGH_DATA</a></code> to
<code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-media-have_future_data">HAVE_FUTURE_DATA</a></code> 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></div>
<dl class="switch">
<dt>If <code><a href="#widl-SourceBuffer-buffered">buffered</a></code> for all objects in <code><a href="#widl-MediaSource-activeSourceBuffers">activeSourceBuffers</a></code> do not contain <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#timeranges">TimeRanges</a></code> for the current playback position:</dt>
<dd>
<ol>
<li>Set the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-media-readystate">HTMLMediaElement.readyState</a></code> attribute to <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-media-have_metadata">HAVE_METADATA</a></code>.</li>
<li>If this is the first transition to <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-media-have_metadata">HAVE_METADATA</a></code>, then <a href="http://www.w3.org/TR/html5/webappapis.html#queue-a-task">queue a task</a> to <a href="http://www.w3.org/TR/html5/webappapis.html#fire-a-simple-event">fire a simple event</a> named <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#event-media-loadedmetadata">loadedmetadata</a></code> at the media element.</li>
<li>Abort these steps.</li>
</ol>
</dd>
<dt>If <code><a href="#widl-SourceBuffer-buffered">buffered</a></code> for all objects in <code><a href="#widl-MediaSource-activeSourceBuffers">activeSourceBuffers</a></code> contain <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#timeranges">TimeRanges</a></code> that include the current playback position and <a href="#enough-data">enough data to ensure uninterrupted playback</a>:</dt>
<dd>
<ol>
<li>Set the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-media-readystate">HTMLMediaElement.readyState</a></code> attribute to <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-media-have_enough_data">HAVE_ENOUGH_DATA</a></code>.</li>
<li>
<a href="http://www.w3.org/TR/html5/webappapis.html#queue-a-task">Queue a task</a> to <a href="http://www.w3.org/TR/html5/webappapis.html#fire-a-simple-event">fire a simple event</a> named <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#event-media-canplaythrough">canplaythrough</a></code> at the media element.</li>
<li>Playback may resume at this point if it was previously suspended by a transition to <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-media-have_current_data">HAVE_CURRENT_DATA</a></code>.</li>
<li>Abort these steps.</li>
</ol>
</dd>
<dt>If <code><a href="#widl-SourceBuffer-buffered">buffered</a></code> for at least one object in <code><a href="#widl-MediaSource-activeSourceBuffers">activeSourceBuffers</a></code> contains a <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#timeranges">TimeRange</a></code> that includes the current playback position but not <a href="#enough-data">enough data to ensure uninterrupted playback</a>:</dt>
<dd>
<ol>
<li>Set the <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-media-readystate">HTMLMediaElement.readyState</a></code> attribute to <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-media-have_future_data">HAVE_FUTURE_DATA</a></code>.</li>
<li>If the previous value of <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-media-readystate">HTMLMediaElement.readyState</a></code> was less than <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-media-have_future_data">HAVE_FUTURE_DATA</a></code>, then <a href="http://www.w3.org/TR/html5/webappapis.html#queue-a-task">queue a task</a> to <a href="http://www.w3.org/TR/html5/webappapis.html#fire-a-simple-event">fire a simple event</a> named <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#event-media-canplay">canplay</a></code> at the media element.</li>
<li>Playback may resume at this point if it was previously suspended by a transition to <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-media-have_current_data">HAVE_CURRENT_DATA</a></code>.</li>
<li>Abort these steps.</li>
</ol>
</dd>
<dt>If <code><a href="#widl-SourceBuffer-buffered">buffered</a></code> for at least one object in <code><a href="#widl-MediaSource-activeSourceBuffers">activeSourceBuffers</a></code> contains a <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#timeranges">TimeRange</a></code> 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 <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-media-readystate">HTMLMediaElement.readyState</a></code> attribute to <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-media-have_current_data">HAVE_CURRENT_DATA</a></code>.</li>
<li>If this is the first transition to <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-media-have_current_data">HAVE_CURRENT_DATA</a></code>, then <a href="http://www.w3.org/TR/html5/webappapis.html#queue-a-task">queue a task</a> to <a href="http://www.w3.org/TR/html5/webappapis.html#fire-a-simple-event">fire a simple event</a> named <code><a href="http://www.w3.org/TR/html5/embedded-content-0.html#event-media-loadeddata">loadeddata</a></code> at the media element.</li>
<li>Playback is suspended at this point since the media element doesn't have enough data to advance the <a href="http://www.w3.org/TR/html5/embedded-content-0.html#media-timeline">media timeline</a>.</li>
<li>Abort these steps.</li>
</ol>
</dd>
</dl>
</section>
<section id="active-source-buffer-changes">
<h4><span class="secno">2.4.5 </span>Changes to selected/enabled track state</h4>
<p>During playback <code><a href="#widl-MediaSource-activeSourceBuffers">activeSourceBuffers</a></code> needs to be updated if the <a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-videotrack-selected">selected video track</a>, the <a href="http://www.w3.org/TR/html5/embedded-content-0.html#dom-audiotrack-enabled">enabled audio tracks</a>, or a text track <a href="http://www.w3.org/TR/html5/embedded-content-0.html#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">