HTML5之MSE标准为移动端的直播应用带来超低延时的播放体验

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原文地址：http://www.zhiboshequ.com/news/604.html

随着移动互联网应用的大规模普及，移动端的视频播放体验日益受到重视。

2016年11月17日，由Google与Microsoft等互联网巨头主导的HTML5 MSE扩展标准已经正式发布，这标志着以往由Apple公司发布的HLS协议标准将会很快退出历史舞台，就像PC端的 Adobe Flash Player 播放器一样，都将被新的通用性更强的技术标准所代替。这些变化最终都将大幅提升终端用户的视频播放体验，无疑将受到行业的广泛支持和拥抱。

下面是MSE的 推荐标准，国内目前已有公司按照该标准在产品中有了具体实现。

标准原文如下：

↑←

Media Source Extensions™

W3C Recommendation 17 November 2016

This version:

https://www.w3.org/TR/2016/REC-media-source-20161117/

Latest published version:

https://www.w3.org/TR/media-source/

Latest editor's draft:

https://w3c.github.io/media-source/

Previous version:

https://www.w3.org/TR/2016/PR-media-source-20161004/

Editors:

Matthew Wolenetz,Google Inc.

Jerry Smith,Microsoft Corporation

Mark Watson,Netflix Inc.

Aaron Colwell (until April 2015),Google Inc.

Adrian Bateman (until April 2015),Microsoft Corporation

Repository:

We are on GitHub

File a bug

Commit history

Mailing list:

[email protected]

Implementation:

Can I use Media Source Extensions?

Test Suite

Test Suite repository

Please check the errata for any errors or issues reported since publication.

The English version of this specification is the only normative version. Non-normativetranslations may also be available.

Copyright © 2016W3C^® (MIT,ERCIM,Keio,Beihang). W3C liability, trademark and permissive document license rules apply.

---

Abstract

This specification extends HTMLMediaElement [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.

Status of This Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of currentW3C publications and the latest revision of this technical report can be found in theW3C technical reports index at https://www.w3.org/TR/.

The working group maintains a list of all bug reports. New features for this specification are expected to be incubated in theWeb Platform Incubator Community Group.

One editorial issue (removing the exposure ofcreateObjectURL(mediaSource) in workers) was addressed since the previous publication. For the list of changes done since the previous version, see thecommits.

By publishing this Recommendation, W3C expects the functionality specified in this Recommendation will not be affected by changes to File API. The Working Group will continue to track these specifications.

This document was published by the HTML Media Extensions Working Group as a Recommendation. If you wish to make comments regarding this document, theGitHub repository is preferred for discussion of this specification. Historical discussion can also be found in themailing list archives).

In September 2016, the Working Group used an implementation report to move this document to Recommendation.

This document has been reviewed by W3C Members, by software developers, and by otherW3C groups and interested parties, and is endorsed by the Director as aW3C Recommendation. It is a stable document and may be used as reference material or cited from another document.W3C's role in making the Recommendation is to draw attention to the specification and to promote its widespread deployment. This enhances the functionality and interoperability of the Web.

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy.W3C maintains apublic list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes containsEssential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

This document is governed by the 1 September 2015 W3C Process Document.

Table of Contents

1. Introduction

This section is non-normative.

This specification allows JavaScript to dynamically construct media streams for <audio> and <video>. It defines a MediaSource object that can serve as a source of media data for an HTMLMediaElement. MediaSource objects have one or moreSourceBuffer objects. Applications append data segments to the SourceBuffer objects, and can adapt the quality of appended data based on system performance and other factors. Data from theSourceBuffer objects is managed as track buffers for audio, video and text data that is decoded and played. Byte stream specifications used with these extensions are available in the byte stream format registry [MSE-REGISTRY].

1.1 Goals

This specification was designed with the following goals in mind:

• Allow JavaScript to construct media streams independent of how the media is fetched.
• Define a splicing and buffering model that facilitates use cases like adaptive streaming, ad-insertion, time-shifting, and video editing.
• Minimize the need for media parsing in JavaScript.
• Leverage the browser cache as much as possible.
• Provide requirements for byte stream format specifications.
• Not require support for any particular media format or codec.

This specification defines:

• Normative behavior for user agents to enable interoperability between user agents and web applications when processing media data.
• Normative requirements to enable other specifications to define media formats to be used within this specification.

1.2 Definitions

Active Track Buffers

The track buffers that provide coded frames for the enabledaudioTracks, theselectedvideoTracks, and the"showing" or"hidden"textTracks. All these tracks are associated withSourceBuffer objects in theactiveSourceBuffers list.

Append Window

A presentation timestamp range used to filter out coded frames while appending. The append window represents a single continuous time range with a single start time and end time. Coded frames withpresentation timestamp within this range are allowed to be appended to the SourceBuffer while coded frames outside this range are filtered out. The append window start and end times are controlled by theappendWindowStart andappendWindowEnd attributes respectively.

Coded Frame

A unit of media data that has a presentation timestamp, a decode timestamp, and a coded frame duration.

Coded Frame Duration

The duration of a coded frame. For video and text, the duration indicates how long the video frame or textSHOULD 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.

Coded Frame End Timestamp

The sum of a coded frame presentation timestamp and its coded frame duration. It represents the presentation timestamp that immediately follows the coded frame.

Coded Frame Group

A group of coded frames that are adjacent and have monotonically increasing decode timestamps without any gaps. Discontinuities detected by the coded frame processing algorithm and abort() calls trigger the start of a new coded frame group.

Decode Timestamp

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 thepresentation timestamp of the earliest frame, in presentation order, that is dependant on this frame). If frames can be decoded out ofpresentation order, then the decode timestampMUST be present in or derivable from the byte stream. The user agentMUST run the append error algorithm if this is not the case. If frames cannot be decoded out ofpresentation order and a decode timestamp is not present in the byte stream, then the decode timestamp is equal to thepresentation timestamp.

Initialization Segment

A sequence of bytes that contain all of the initialization information required to decode a sequence ofmedia segments. This includes codec initialization data,Track ID mappings for multiplexed segments, and timestamp offsets (e.g., edit lists).

Note

The byte stream format specifications in the byte stream format registry [MSE-REGISTRY] contain format specific examples.

Media Segment

A sequence of bytes that contain packetized & timestamped media data for a portion of themedia timeline. Media segments are always associated with the most recently appendedinitialization segment.

Note

The byte stream format specifications in the byte stream format registry [MSE-REGISTRY] contain format specific examples.

MediaSource object URL

A MediaSource object URL is a unique Blob URI [FILE-API] created bycreateObjectURL(). It is used to attach aMediaSource object to an HTMLMediaElement.

These URLs are the same as a Blob URI, except that anything in the definition of that feature that refers toFile andBlob objects is hereby extended to also apply to MediaSource objects.

The origin of the MediaSource object URL is the relevant settings object of this during the call to createObjectURL().

Note

For example, the origin of the MediaSource object URL affects the way that the media element isconsumed by canvas.

Parent Media Source

The parent media source of a SourceBuffer object is the MediaSource object that created it.

Presentation Start Time

The presentation start time is the earliest time point in the presentation and specifies theinitial playback position andearliest possible position. All presentations created using this specification have a presentation start time of 0.

Note

For the purposes of determining if HTMLMediaElement.buffered contains aTimeRange that includes the current playback position, implementationsMAY choose to allow a current playback position at or afterpresentation start time and before the first TimeRange to play the firstTimeRange if thatTimeRange starts within a reasonably short time, like 1 second, afterpresentation start time. This allowance accommodates the reality that muxed streams commonly do not begin all tracks precisely atpresentation start time. ImplementationsMUST report the actual buffered range, regardless of this allowance.

Presentation Interval

The presentation interval of a coded frame is the time interval from its presentation timestamp to the presentation timestamp plus the coded frame's duration. For example, if a coded frame has a presentation timestamp of 10 seconds and acoded frame duration 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.

Presentation Order

The order that coded frames are rendered in the presentation. The presentation order is achieved by orderingcoded frames in monotonically increasing order by theirpresentation timestamps.

Presentation Timestamp

A reference to a specific time in the presentation. The presentation timestamp in acoded frame indicates when the frameSHOULD be rendered.

Random Access Point

A position in a media segment 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.

SourceBuffer byte stream format specification

The specific byte stream format specification that describes the format of the byte stream accepted by aSourceBuffer instance. Thebyte stream format specification, for a SourceBuffer object, is selected based on the type passed to theaddSourceBuffer() call that created the object.

SourceBuffer configuration

A specific set of tracks distributed across one or more SourceBuffer objects owned by a single MediaSource instance.

Implementations MUST support at least 1 MediaSource object with the following configurations:

• A single SourceBuffer with 1 audio track and/or 1 video track.
• Two SourceBuffers with one handling a single audio track and the other handling a single video track.

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.

Track Description

A byte stream format specific structure that provides the Track ID, codec configuration, and other metadata for a single track. Each track description inside a singleinitialization segment has a uniqueTrack ID. The user agentMUST run the append error algorithm if the Track ID is not unique within the initialization segment.

Track ID

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 atrack description identifies which sections of amedia segment belong to that track.

2. MediaSource Object

The MediaSource object represents a source of media data for an HTMLMediaElement. It keeps track of thereadyState for this source as well as a list ofSourceBuffer 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 theSourceBuffer objects insourceBuffers to add media data to this source. The HTMLMediaElement fetches this media data from theMediaSource object when it is needed during playback.

Each MediaSource object has a live seekable range variable that stores anormalized TimeRanges object. This variable is initialized to an empty TimeRanges object when theMediaSource object is created, is maintained by setLiveSeekableRange() andclearLiveSeekableRange(), and is used inHTMLMediaElement Extensions to modifyHTMLMediaElement.seekable behavior.

enum ReadyState {
    "closed",
    "open",
    "ended"
};

Enumeration description
closed	Indicates the source is not currently attached to a media element.
open	The source has been opened by a media element and is ready for data to be appended to theSourceBuffer objects insourceBuffers.
ended	The source is still attached to a media element, but endOfStream() has been called.

enum EndOfStreamError {
    "network",
    "decode"
};

Enumeration description
network	Terminates playback and signals that a network error has occured. 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.
decode	Terminates playback and signals that a decoding error has occured. 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.

[Constructor]
interface MediaSource : EventTarget {
    readonly attribute SourceBufferList    sourceBuffers;
    readonly attribute SourceBufferList    activeSourceBuffers;
    readonly attribute ReadyState          readyState;
             attribute unrestricted double duration;
             attribute EventHandler        onsourceopen;
             attribute EventHandler        onsourceended;
             attribute EventHandler        onsourceclose;
    SourceBuffer addSourceBuffer(DOMString type);
    void         removeSourceBuffer(SourceBuffer sourceBuffer);
    void         endOfStream(optional EndOfStreamError error);
    void         setLiveSeekableRange(double start, double end);
    void         clearLiveSeekableRange();
    static boolean isTypeSupported(DOMString type);
};

2.1 Attributes

sourceBuffers of type SourceBufferList, readonly

Contains the list of SourceBuffer objects associated with this MediaSource. When readyState equals "closed" this list will be empty. Once readyState transitions to "open" SourceBuffer objects can be added to this list by using addSourceBuffer().

activeSourceBuffers of type SourceBufferList, readonly

Contains the subset of sourceBuffers that are providing theselected video track, the enabled audio track(s), and the "showing" or"hidden" text track(s).

SourceBuffer objects in this listMUST appear in the same order as they appear in thesourceBuffers attribute; e.g., if only sourceBuffers[0] and sourceBuffers[3] are inactiveSourceBuffers, then activeSourceBuffers[0]MUST equal sourceBuffers[0] and activeSourceBuffers[1]MUST equal sourceBuffers[3].

Note

The Changes to selected/enabled track state section describes how this attribute gets updated.

readyState of type ReadyState, readonly

Indicates the current state of the MediaSource object. When the MediaSource is created readyStateMUST be set to"closed".

duration of type unrestricted double

Allows the web application to set the presentation duration. The duration is initially set to NaN when theMediaSource object is created.

On getting, run the following steps:

1. If the readyState attribute is"closed" then return NaN and abort these steps.
2. Return the current value of the attribute.

On setting, run the following steps:

1. If the value being set is negative or NaN then throw a TypeError exception and abort these steps.
2. If the readyState attribute is not"open" then throw anInvalidStateError exception and abort these steps.
3. If the updating attribute equals true on anySourceBuffer in sourceBuffers, then throw anInvalidStateError exception and abort these steps.
4. Run the duration change algorithm with new duration set to the value being assigned to this attribute.
   Note

   The duration change algorithm will adjust new duration higher if there is any currently buffered coded frame with a higher end time.

   Note

   appendBuffer() andendOfStream() can update the duration under certain circumstances.

onsourceopen of type EventHandler

The event handler for the sourceopen event.

onsourceended of type EventHandler

The event handler for the sourceended event.

onsourceclose of type EventHandler

The event handler for the sourceclose event.

2.2 Methods

addSourceBuffer

Adds a new SourceBuffer to sourceBuffers.

Parameter	Type	Nullable	Optional	Description
type	DOMString	✘	✘

Return type: SourceBuffer

When this method is invoked, the user agent must run the following steps:

1. If type is an empty string then throw a TypeError exception and abort these steps.
2. If type contains a MIME type that is not supported or contains a MIME type that is not supported with the types specified for the otherSourceBuffer objects in sourceBuffers, then throw aNotSupportedError exception and abort these steps.
3. If the user agent can't handle any more SourceBuffer objects or if creating a SourceBuffer based ontype would result in an unsupportedSourceBuffer configuration, then throw a QuotaExceededError exception and abort these steps.
   Note

   For example, a user agent MAY throw a QuotaExceededError exception if the media element has reached theHAVE_METADATA readyState. This can occur if the user agent's media engine does not support adding more tracks during playback.

4. If the readyState attribute is not in the"open" state then throw anInvalidStateError exception and abort these steps.
5. Create a new SourceBuffer object and associated resources.
6. Set the generate timestamps flag 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 withtype.

7. If the generate timestamps flag equals true:

   Set the mode attribute on the new object to "sequence".

   Otherwise:

   Set the mode attribute on the new object to "segments".

8. Add the new object to sourceBuffers andqueue a task tofire a simple event namedaddsourcebuffer atsourceBuffers.
9. Return the new object.

removeSourceBuffer

Removes a SourceBuffer from sourceBuffers.

Parameter	Type	Nullable	Optional	Description
sourceBuffer	SourceBuffer	✘	✘

Return type: void

When this method is invoked, the user agent must run the following steps:

1. If sourceBuffer specifies an object that is not in sourceBuffers then throw aNotFoundError exception and abort these steps.
2. If the sourceBuffer.updating attribute equals true, then run the following steps:
   1. Abort the buffer append algorithm if it is running.
   2. Set the sourceBuffer.updating attribute to false.
   3. Queue a task tofire a simple event namedabort atsourceBuffer.
   4. Queue a task tofire a simple event namedupdateend atsourceBuffer.
3. Let SourceBuffer audioTracks list equal the AudioTrackList object returned bysourceBuffer.audioTracks.
4. If the SourceBuffer audioTracks list is not empty, then run the following steps:
   1. Let HTMLMediaElement audioTracks list equal the AudioTrackList object returned by theaudioTracks attribute on the HTMLMediaElement.
   2. For each AudioTrack object in theSourceBuffer audioTracks list, run the following steps:
      1. Set the sourceBuffer attribute on theAudioTrack object to null.
      2. Remove the AudioTrack object from theHTMLMediaElement audioTracks list.
         Note

         This should trigger AudioTrackList [HTML51] logic to queue a task to fire a trusted event named removetrack, that does not bubble and is not cancelable, and that uses theTrackEvent interface, with thetrack attribute initialized to theAudioTrack object, at theHTMLMediaElement audioTracks list. If the enabled attribute on theAudioTrack object was true at the beginning of this removal step, then this should also triggerAudioTrackList [HTML51] logic to queue a task to fire a simple event named change at theHTMLMediaElement audioTracks list

      3. Remove the AudioTrack object from theSourceBuffer audioTracks list.
         Note

         This should trigger AudioTrackList [HTML51] logic to queue a task to fire a trusted event named removetrack, that does not bubble and is not cancelable, and that uses theTrackEvent interface, with thetrack attribute initialized to theAudioTrack object, at theSourceBuffer audioTracks list. If the enabled attribute on theAudioTrack object was true at the beginning of this removal step, then this should also triggerAudioTrackList [HTML51] logic to queue a task to fire a simple event named change at theSourceBuffer audioTracks list

5. Let SourceBuffer videoTracks list equal the VideoTrackList object returned bysourceBuffer.videoTracks.
6. If the SourceBuffer videoTracks list is not empty, then run the following steps:
   1. Let HTMLMediaElement videoTracks list equal the VideoTrackList object returned by thevideoTracks attribute on the HTMLMediaElement.
   2. For each VideoTrack object in theSourceBuffer videoTracks list, run the following steps:
      1. Set the sourceBuffer attribute on theVideoTrack object to null.
      2. Remove the VideoTrack object from theHTMLMediaElement videoTracks list.
         Note

         This should trigger VideoTrackList [HTML51] logic to queue a task to fire a trusted event named removetrack, that does not bubble and is not cancelable, and that uses theTrackEvent interface, with thetrack attribute initialized to theVideoTrack object, at theHTMLMediaElement videoTracks list. If the selected attribute on theVideoTrack object was true at the beginning of this removal step, then this should also triggerVideoTrackList [HTML51] logic to queue a task to fire a simple event named change at theHTMLMediaElement videoTracks list

      3. Remove the VideoTrack object from theSourceBuffer videoTracks list.
         Note

         This should trigger VideoTrackList [HTML51] logic to queue a task to fire a trusted event named removetrack, that does not bubble and is not cancelable, and that uses theTrackEvent interface, with thetrack attribute initialized to theVideoTrack object, at theSourceBuffer videoTracks list. If the selected attribute on theVideoTrack object was true at the beginning of this removal step, then this should also triggerVideoTrackList [HTML51] logic to queue a task to fire a simple event named change at theSourceBuffer videoTracks list

7. Let SourceBuffer textTracks list equal the TextTrackList object returned bysourceBuffer.textTracks.
8. If the SourceBuffer textTracks list is not empty, then run the following steps:
   1. Let HTMLMediaElement textTracks list equal the TextTrackList object returned by thetextTracks attribute on the HTMLMediaElement.
   2. For each TextTrack object in theSourceBuffer textTracks list, run the following steps:
      1. Set the sourceBuffer attribute on theTextTrack object to null.
      2. Remove the TextTrack object from theHTMLMediaElement textTracks list.
         Note

         This should trigger TextTrackList [HTML51] logic to queue a task to fire a trusted event named removetrack, that does not bubble and is not cancelable, and that uses theTrackEvent interface, with thetrack attribute initialized to theTextTrack object, at theHTMLMediaElement textTracks list. If the mode attribute on theTextTrack object was"showing" or"hidden" at the beginning of this removal step, then this should also triggerTextTrackList [HTML51] logic to queue a task to fire a simple event named change at theHTMLMediaElement textTracks list.

      3. Remove the TextTrack object from theSourceBuffer textTracks list.
         Note

         This should trigger TextTrackList [HTML51] logic to queue a task to fire a trusted event named removetrack, that does not bubble and is not cancelable, and that uses theTrackEvent interface, with thetrack attribute initialized to theTextTrack object, at theSourceBuffer textTracks list. If the mode attribute on theTextTrack object was"showing" or"hidden" at the beginning of this removal step, then this should also triggerTextTrackList [HTML51] logic to queue a task to fire a simple event named change at theSourceBuffer textTracks list.

9. If sourceBuffer is in activeSourceBuffers, then removesourceBuffer fromactiveSourceBuffers andqueue a task tofire a simple event namedremovesourcebuffer at theSourceBufferList returned byactiveSourceBuffers.
10. Remove sourceBuffer from sourceBuffers andqueue a task tofire a simple event namedremovesourcebuffer at theSourceBufferList returned bysourceBuffers.
11. Destroy all resources for sourceBuffer.

endOfStream

Signals the end of the stream.

Parameter	Type	Nullable	Optional	Description
error	EndOfStreamError	✘	✔

Return type: void

When this method is invoked, the user agent must run the following steps:

1. If the readyState attribute is not in the"open" state then throw anInvalidStateError exception and abort these steps.
2. If the updating attribute equals true on anySourceBuffer in sourceBuffers, then throw anInvalidStateError exception and abort these steps.
3. Run the end of stream algorithm with the error parameter set to error.

setLiveSeekableRange

Updates the live seekable range variable used inHTMLMediaElement Extensions to modify HTMLMediaElement.seekable behavior.

Parameter	Type	Nullable	Optional	Description
start	double	✘	✘	The start of the range, in seconds measured from presentation start time. While set, and if duration equals positive Infinity,HTMLMediaElement.seekable will return a non-empty TimeRanges object with a lowest range start timestamp no greater thanstart.
end	double	✘	✘	The end of range, in seconds measured from presentation start time. While set, and if duration equals positive Infinity,HTMLMediaElement.seekable will return a non-empty TimeRanges object with a highest range end timestamp no less thanend.

Return type: void

When this method is invoked, the user agent must run the following steps:

1. If the readyState attribute is not"open" then throw anInvalidStateError exception and abort these steps.
2. If start is negative or greater than end, then throw a TypeError exception and abort these steps.
3. Set live seekable range to be a newnormalized TimeRanges object containing a single range whose start position isstart and end position isend.

clearLiveSeekableRange

Updates the live seekable range variable used inHTMLMediaElement Extensions to modify HTMLMediaElement.seekable behavior.

No parameters.

Return type: void

When this method is invoked, the user agent must run the following steps:

1. If the readyState attribute is not"open" then throw anInvalidStateError exception and abort these steps.
2. If live seekable range contains a range, then setlive seekable range to be a new emptyTimeRanges object.

isTypeSupported, static

Check to see whether the MediaSource is capable of creating SourceBuffer objects for the specified MIME type.

Note

If true is returned from this method, it only indicates that the MediaSource implementation is capable of creating SourceBuffer objects for the specified MIME type. An addSourceBuffer() callSHOULD still fail if sufficient resources are not available to support the addition of a newSourceBuffer.

Note

This method returning true implies that HTMLMediaElement.canPlayType() will return "maybe" or "probably" since it does not make sense for aMediaSource to support a type the HTMLMediaElement knows it cannot play.

Parameter	Type	Nullable	Optional	Description
type	DOMString	✘	✘

Return type: boolean

When this method is invoked, the user agent must run the following steps:

1. If type is an empty string, then return false.
2. If type does not contain a valid MIME type string, then return false.
3. If type contains a media type or media subtype that the MediaSource does not support, then return false.
4. If type contains a codec that the MediaSource does not support, then return false.
5. If the MediaSource does not support the specified combination of media type, media subtype, and codecs then return false.
6. Return true.

2.3 Event Summary

Event name	Interface	Dispatched when...
sourceopen	Event	readyState transitions from"closed" to"open" or from"ended" to"open".
sourceended	Event	readyState transitions from"open" to"ended".
sourceclose	Event	readyState transitions from"open" to"closed" or"ended" to"closed".

2.4 Algorithms

2.4.1 Attaching to a media element

A MediaSource object can be attached to a media element by assigning aMediaSource object URL to the media elementsrc attribute or the src attribute of a <source> inside a media element. AMediaSource object URL is created by passing a MediaSource object to createObjectURL().

If the resource fetch algorithm was invoked with a media provider object that is a MediaSource object or a URL record whose object is a MediaSource object, then let mode be local, skip the first step in theresource fetch algorithm (which may otherwise set mode to remote) and add the steps and clarifications below to the"Otherwise (mode is local)" section of theresource fetch algorithm.

Note

The resource fetch algorithm's first step is expected to eventually align with selecting local mode for URL records whose objects are media provider objects. The intent is that if the HTMLMediaElement'ssrc attribute or selected child<source>'s src attribute is ablob: URL matching aMediaSource object URL when the respective src attribute was last changed, then that MediaSource object is used as the media provider object and current media resource in the local mode logic in theresource fetch algorithm. This also means that the remote mode logic that includes observance of any preload attribute is skipped when a MediaSource object is attached. Even with that eventual change to [HTML51], the execution of the following steps at the beginning of the local mode logic is still required when the current media resource is a MediaSource object.

Note

Relative to the action which triggered the media element's resource selection algorithm, these steps are asynchronous. The resource fetch algorithm is run after the task that invoked the resource selection algorithm is allowed to continue and a stable state is reached. Implementations may delay the steps in the "Otherwise" clause, below, until the MediaSource object is ready for use.

If readyState is NOT set to "closed"

Run the "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" steps of the resource fetch algorithm's media data processing steps list.

Otherwise

1. Set the media element's delaying-the-load-event-flag to false.
2. Set the readyState attribute to"open".
3. Queue a task tofire a simple event namedsourceopen at theMediaSource.
4. Continue the resource fetch algorithm by running the remaining "Otherwise (mode is local)" steps, with these clarifications:
   1. Text in the resource fetch algorithm or the media data processing steps list that refers to "the download", "bytes received", or "whenever new data for the current media resource becomes available" refers to data passed in viaappendBuffer().
   2. References to HTTP in the resource fetch algorithm and the media data processing steps list do not apply because the HTMLMediaElement does not fetch media data via HTTP when aMediaSource is attached.

Note

An attached MediaSource does not use the remote mode steps in the resource fetch algorithm, so the media element will not fire "suspend" events. Though future versions of this specification will likely remove "progress" and "stalled" events from a media element with an attached MediaSource, user agents conforming to this version of the specification may still fire these two events as these [HTML51] references changed after implementations of this specification stabilized.

2.4.2 Detaching from a media element

The following steps are run in any case where the media element is going to transition toNETWORK_EMPTY andqueue a task to fire a simple event named emptied at the media element. These steps SHOULD be run right before the transition.

1. Set the readyState attribute to"closed".
2. Update duration to NaN.
3. Remove all the SourceBuffer objects from activeSourceBuffers.
4. Queue a task tofire a simple event namedremovesourcebuffer atactiveSourceBuffers.
5. Remove all the SourceBuffer objects from sourceBuffers.
6. Queue a task tofire a simple event namedremovesourcebuffer atsourceBuffers.
7. Queue a task tofire a simple event namedsourceclose at theMediaSource.

Note

Going forward, this algorithm is intended to be externally called and run in any case where the attachedMediaSource, if any, must be detached from the media element. ItMAY 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 toNETWORK_EMPTY. 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.

2.4.3 Seeking

Run the following steps as part of the "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" step of theseek algorithm:

1. Note

   The media element looks for media segments containing the new playback position in each SourceBuffer object in activeSourceBuffers. Any position within aTimeRange in the current value of theHTMLMediaElement.buffered attribute has all necessary media segments buffered for that position.

   If new playback position is not in any TimeRange of HTMLMediaElement.buffered

   1. If the HTMLMediaElement.readyState attribute is greater thanHAVE_METADATA, then set theHTMLMediaElement.readyState attribute toHAVE_METADATA.
      Note

      Per HTMLMediaElement ready states [HTML51] logic, HTMLMediaElement.readyState changes may trigger events on the HTMLMediaElement.

   2. The media element waits until an appendBuffer() call causes thecoded frame processing algorithm to set the HTMLMediaElement.readyState attribute to a value greater thanHAVE_METADATA.
      Note

      The web application can use buffered andHTMLMediaElement.buffered to determine what the media element needs to resume playback.

   Otherwise

   Continue

   Note

   If the readyState attribute is"ended" and thenew playback position is within a TimeRange currently inHTMLMediaElement.buffered, then the seek operation must continue to completion here even if one or more currently selected or enabled track buffers' largest range end timestamp is less thannew playback position. This condition should only occur due to logic inbuffered whenreadyState is"ended".

2. The media element resets all decoders and initializes each one with data from the appropriateinitialization segment.
3. The media element feeds coded frames from the active track buffers into the decoders starting with the closest random access point before the new playback position.
4. Resume the seek algorithm at the "Await a stable state" step.

2.4.4 SourceBuffer Monitoring

The following steps are periodically run during playback to make sure that all of theSourceBuffer objects inactiveSourceBuffers haveenough data to ensure uninterrupted playback. Changes toactiveSourceBuffers also cause these steps to run because they affect the conditions that trigger state transitions.

Having enough data to ensure uninterrupted playback 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 theHAVE_ENOUGH_DATA ready state. These transitions indicate when the user agent believes it has enough data buffered or it needs more data respectively.

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 usedMAY change during playback so web applicationsSHOULD only rely on the value ofHTMLMediaElement.readyState to determine whether more data is needed or not.

Note

When the media element needs more data, the user agent SHOULD transition it from HAVE_ENOUGH_DATA toHAVE_FUTURE_DATA 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.

If the the HTMLMediaElement.readyState attribute equals HAVE_NOTHING:

1. Abort these steps.

If HTMLMediaElement.buffered does not contain a TimeRange for the current playback position:

1. Set the HTMLMediaElement.readyState attribute toHAVE_METADATA.
   Note

   Per HTMLMediaElement ready states [HTML51] logic, HTMLMediaElement.readyState changes may trigger events on the HTMLMediaElement.

2. Abort these steps.

If HTMLMediaElement.buffered contains a TimeRange that includes the current playback position and enough data to ensure uninterrupted playback:

1. Set the HTMLMediaElement.readyState attribute toHAVE_ENOUGH_DATA.
   Note

   Per HTMLMediaElement ready states [HTML51] logic, HTMLMediaElement.readyState changes may trigger events on the HTMLMediaElement.

2. Playback may resume at this point if it was previously suspended by a transition toHAVE_CURRENT_DATA.
3. Abort these steps.

If HTMLMediaElement.buffered contains a TimeRange that includes the current playback position and some time beyond the current playback position, then run the following steps:

1. Set the HTMLMediaElement.readyState attribute toHAVE_FUTURE_DATA.
   Note

   Per HTMLMediaElement ready states [HTML51] logic, HTMLMediaElement.readyState changes may trigger events on the HTMLMediaElement.

2. Playback may resume at this point if it was previously suspended by a transition toHAVE_CURRENT_DATA.
3. Abort these steps.

If HTMLMediaElement.buffered contains a TimeRange that ends at the current playback position and does not have a range covering the time immediately after the current position:

1. Set the HTMLMediaElement.readyState attribute toHAVE_CURRENT_DATA.
   Note

   Per HTMLMediaElement ready states [HTML51] logic, HTMLMediaElement.readyState changes may trigger events on the HTMLMediaElement.

2. Playback is suspended at this point since the media element doesn't have enough data to advance themedia timeline.
3. Abort these steps.

2.4.5 Changes to selected/enabled track state

During playback activeSourceBuffers needs to be updated if theselected video track, the enabled audio track(s), or a text track mode changes. When one or more of these changes occur the following steps need to be followed.

If the selected video track changes, then run the following steps:

1. If the SourceBuffer associated with the previously selected video track is not associated with any other enabled tracks, run the following steps:
   1. Remove the SourceBuffer from activeSourceBuffers.
   2. Queue a task tofire a simple event namedremovesourcebuffer atactiveSourceBuffers
2. If the SourceBuffer associated with the newly selected video track is not already inactiveSourceBuffers, run the following steps:
   1. Add the SourceBuffer to activeSourceBuffers.
   2. Queue a task tofire a simple event namedaddsourcebuffer atactiveSourceBuffers

If an audio track becomes disabled and the SourceBuffer associated with this track is not associated with any other enabled or selected track, then run the following steps:

1. Remove the SourceBuffer associated with the audio track from activeSourceBuffers
2. Queue a task tofire a simple event namedremovesourcebuffer atactiveSourceBuffers

If an audio track becomes enabled and the SourceBuffer associated with this track is not already in activeSourceBuffers, then run the following steps:

1. Add the SourceBuffer associated with the audio track to activeSourceBuffers
2. Queue a task tofire a simple event namedaddsourcebuffer atactiveSourceBuffers

If a text track mode becomes "disabled" and the SourceBuffer associated with this track is not associated with any other enabled or selected track, then run the following steps:

1. Remove the SourceBuffer associated with the text track from activeSourceBuffers
2. Queue a task tofire a simple event namedremovesourcebuffer atactiveSourceBuffers

If a text track mode becomes "showing" or "hidden" and the SourceBuffer associated with this track is not already in activeSourceBuffers, then run the following steps:

1. Add the SourceBuffer associated with the text track to activeSourceBuffers
2. Queue a task tofire a simple event namedaddsourcebuffer atactiveSourceBuffers

2.4.6 Duration change

Follow these steps when duration needs to change to anew duration.

1. If the current value of duration is equal tonew duration, then return.
2. If new duration is less than the highest presentation timestamp of any buffered coded frames for all SourceBuffer objects in sourceBuffers, then throw anInvalidStateError exception and abort these steps.
   Note

   Duration reductions that would truncate currently buffered media are disallowed. When truncation is necessary, useremove() to reduce the buffered range before updatingduration.

3. Let highest end time be the largest track buffer ranges end time across all the track buffers across all SourceBuffer objects in sourceBuffers.
4. If new duration is less than highest end time, then
   Note

   This condition can occur because the coded frame removal algorithm preserves coded frames that start before the start of the removal range.

   1. Update new duration to equal highest end time.
5. Update duration tonew duration.
6. Update the media duration tonew duration and run theHTMLMediaElement duration change algorithm.

2.4.7 End of stream algorithm

This algorithm gets called when the application signals the end of stream via anendOfStream() call or an algorithm needs to signal a decode error. This algorithm takes anerror parameter that indicates whether an error will be signalled.

1. Change the readyState attribute value to"ended".
2. Queue a task tofire a simple event namedsourceended at theMediaSource.

3. If error is not set

   1. Run the duration change algorithm with new duration set to the largest track buffer ranges end time across all the track buffers across all SourceBuffer objects in sourceBuffers.
      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.

   2. Notify the media element that it now has all of the media data.

   If error is set to "network"

   If the HTMLMediaElement.readyState attribute equals HAVE_NOTHING

   Run the "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" steps of the resource fetch algorithm's media data processing steps list.

   If the HTMLMediaElement.readyState attribute is greater than HAVE_NOTHING

   Run the " If the connection is interrupted after some media data has been received, causing the user agent to give up trying to fetch the resource" steps of the resource fetch algorithm's media data processing steps list.

   If error is set to "decode"

   If the HTMLMediaElement.readyState attribute equals HAVE_NOTHING

   Run the " 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" steps of the resource fetch algorithm's media data processing steps list.

   If the HTMLMediaElement.readyState attribute is greater than HAVE_NOTHING

   Run the media data is corrupted steps of the resource fetch algorithm's media data processing steps list.

3. SourceBuffer Object

enum AppendMode {
    "segments",
    "sequence"
};

Enumeration description
segments	The timestamps in the media segment determine where the coded frames are placed in the presentation. Media segments can be appended in any order.
sequence	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. ThetimestampOffset attribute will be updated if a new offset is needed to make the new media segments adjacent to the previous media segment. Setting thetimestampOffset attribute in"sequence" 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.

interface SourceBuffer : EventTarget {
             attribute AppendMode          mode;
    readonly attribute boolean             updating;
    readonly attribute TimeRanges          buffered;
             attribute double              timestampOffset;
    readonly attribute AudioTrackList      audioTracks;
    readonly attribute VideoTrackList      videoTracks;
    readonly attribute TextTrackList       textTracks;
             attribute double              appendWindowStart;
             attribute unrestricted double appendWindowEnd;
             attribute EventHandler        onupdatestart;
             attribute EventHandler        onupdate;
             attribute EventHandler        onupdateend;
             attribute EventHandler        onerror;
             attribute EventHandler        onabort;
    void appendBuffer(BufferSource data);
    void abort();
    void remove(double start, unrestricted double end);
};

3.1 Attributes

mode of type AppendMode

Controls how a sequence of media segments are handled. This attribute is initially set by addSourceBuffer() after the object is created.

On getting, Return the initial value or the last value that was successfully set.

On setting, run the following steps:

1. If this object has been removed from the sourceBuffers attribute of theparent media source, then throw an InvalidStateError exception and abort these steps.
2. If the updating attribute equals true, then throw anInvalidStateError exception and abort these steps.
3. Let new mode equal the new value being assigned to this attribute.
4. If generate timestamps flag equals true andnew mode equals"segments", then throw aTypeError exception and abort these steps.

5. If the readyState attribute of theparent media source is in the "ended" state then run the following steps:

   1. Set the readyState attribute of theparent media source to "open"
   2. Queue a task tofire a simple event namedsourceopen at theparent media source.
6. If the append state equalsPARSING_MEDIA_SEGMENT, then throw an InvalidStateError and abort these steps.
7. If the new mode equals "sequence", then set thegroup start timestamp to thegroup end timestamp.
8. Update the attribute to new mode.

updating of type boolean, readonly

Indicates whether the asynchronous continuation of an appendBuffer() orremove() operation is still being processed. This attribute is initially set to false when the object is created.

buffered of type TimeRanges, readonly

Indicates what TimeRanges are buffered in theSourceBuffer. This attribute is initially set to an empty TimeRanges object when the object is created.

When the attribute is read the following steps MUST occur:

1. If this object has been removed from the sourceBuffers attribute of theparent media source then throw an InvalidStateError exception and abort these steps.
2. Let highest end time be the largest track buffer ranges end time across all the track buffers managed by this SourceBuffer object.
3. Let intersection ranges equal a TimeRange object containing a single range from 0 tohighest end time.
4. For each audio and video track buffer managed by this SourceBuffer, run the following steps:
   Note

   Text track-buffers are included in the calculation of highest end time, above, but excluded from the buffered range calculation here. They are not necessarily continuous, nor should any discontinuity within them trigger playback stall when the other media tracks are continuous over the same time range.

   1. Let track ranges equal the track buffer ranges for the current track buffer.
   2. If readyState is"ended", then set the end time on the last range intrack ranges to highest end time.
   3. Let new intersection ranges equal the intersection between the intersection ranges and the track ranges.
   4. Replace the ranges in intersection ranges with the new intersection ranges.
5. If intersection ranges does not contain the exact same range information as the current value of this attribute, then update the current value of this attribute tointersection ranges.
6. Return the current value of this attribute.

timestampOffset of type double

Controls the offset applied to timestamps inside subsequent media segments that are appended to this SourceBuffer. The timestampOffset is initially set to 0 which indicates that no offset is being applied.

On getting, Return the initial value or the last value that was successfully set.

On setting, run the following steps:

1. Let new timestamp offset equal the new value being assigned to this attribute.
2. If this object has been removed from the sourceBuffers attribute of theparent media source, then throw an InvalidStateError exception and abort these steps.
3. If the updating attribute equals true, then throw anInvalidStateError exception and abort these steps.

4. If the readyState attribute of theparent media source is in the "ended" state then run the following steps:

   1. Set the readyState attribute of theparent media source to "open"
   2. Queue a task tofire a simple event namedsourceopen at theparent media source.
5. If the append state equalsPARSING_MEDIA_SEGMENT, then throw an InvalidStateError and abort these steps.
6. If the mode attribute equals"sequence", then set thegroup start timestamp tonew timestamp offset.
7. Update the attribute to new timestamp offset.

audioTracks of type AudioTrackList, readonly

The list of AudioTrack objects created by this object.

videoTracks of type VideoTrackList, readonly

The list of VideoTrack objects created by this object.

textTracks of type TextTrackList, readonly

The list of TextTrack objects created by this object.

appendWindowStart of type double

The presentation timestamp for the start of the append window. This attribute is initially set to the presentation start time.

On getting, Return the initial value or the last value that was successfully set.

On setting, run the following steps:

1. If this object has been removed from the sourceBuffers attribute of theparent media source, then throw an InvalidStateError exception and abort these steps.
2. If the updating attribute equals true, then throw anInvalidStateError exception and abort these steps.
3. If the new value is less than 0 or greater than or equal to appendWindowEnd then throw aTypeError exception and abort these steps.
4. Update the attribute to the new value.

appendWindowEnd of type unrestricted double

The presentation timestamp for the end of the append window. This attribute is initially set to positive Infinity.

On getting, Return the initial value or the last value that was successfully set.

On setting, run the following steps:

1. If this object has been removed from the sourceBuffers attribute of theparent media source, then throw an InvalidStateError exception and abort these steps.
2. If the updating attribute equals true, then throw anInvalidStateError exception and abort these steps.
3. If the new value equals NaN, then throw a TypeError and abort these steps.
4. If the new value is less than or equal to appendWindowStart then throw aTypeError exception and abort these steps.
5. Update the attribute to the new value.

onupdatestart of type EventHandler

The event handler for the updatestart event.

onupdate of type EventHandler

The event handler for the update event.

onupdateend of type EventHandler

The event handler for the updateend event.

onerror of type EventHandler

The event handler for the error event.

onabort of type EventHandler

The event handler for the abort event.

3.2 Methods

appendBuffer

Appends the segment data in an BufferSource[WEBIDL] to the source buffer.

Parameter	Type	Nullable	Optional	Description
data	BufferSource	✘	✘

Return type: void

When this method is invoked, the user agent must run the following steps:

1. Run the prepare append algorithm.
2. Add data to the end of the input buffer.
3. Set the updating attribute to true.
4. Queue a task tofire a simple event namedupdatestart at thisSourceBuffer object.
5. Asynchronously run the buffer append algorithm.

abort

Aborts the current segment and resets the segment parser.

No parameters.

Return type: void

When this method is invoked, the user agent must run the following steps:

1. If this object has been removed from the sourceBuffers attribute of theparent media source then throw an InvalidStateError exception and abort these steps.
2. If the readyState attribute of theparent media source is not in the "open" state then throw anInvalidStateError exception and abort these steps.
3. If the range removal algorithm is running, then throw an InvalidStateError exception and abort these steps.
4. If the updating attribute equals true, then run the following steps:
   1. Abort the buffer append algorithm if it is running.
   2. Set the updating attribute to false.
   3. Queue a task tofire a simple event namedabort at thisSourceBuffer object.
   4. Queue a task tofire a simple event namedupdateend at thisSourceBuffer object.
5. Run the reset parser state algorithm.
6. Set appendWindowStart to thepresentation start time.
7. Set appendWindowEnd to positive Infinity.

remove

Removes media for a specific time range.

Parameter	Type	Nullable	Optional	Description
start	double	✘	✘	The start of the removal range, in seconds measured from presentation start time.
end	unrestricted double	✘	✘	The end of the removal range, in seconds measured from presentation start time.

Return type: void

When this method is invoked, the user agent must run the following steps:

1. If this object has been removed from the sourceBuffers attribute of theparent media source then throw an InvalidStateError exception and abort these steps.
2. If the updating attribute equals true, then throw anInvalidStateError exception and abort these steps.
3. If duration equals NaN, then throw aTypeError exception and abort these steps.
4. If start is negative or greater than duration, then throw aTypeError exception and abort these steps.
5. If end is less than or equal to start or end equals NaN, then throw aTypeError exception and abort these steps.

6. If the readyState attribute of theparent media source is in the "ended" state then run the following steps:

   1. Set the readyState attribute of theparent media source to "open"
   2. Queue a task tofire a simple event namedsourceopen at theparent media source.
7. Run the range removal algorithm with start and end as the start and end of the removal range.

3.3 Track Buffers

A track buffer stores the track descriptions and coded frames for an individual track. The track buffer is updated as initialization segments and media segments are appended to the SourceBuffer.

Each track buffer has a last decode timestamp variable that stores the decode timestamp of the lastcoded frame appended in the currentcoded frame group. The variable is initially unset to indicate that nocoded frames have been appended yet.

Each track buffer has a last frame duration variable that stores thecoded frame duration of the lastcoded frame appended in the current coded frame group. The variable is initially unset to indicate that no coded frames have been appended yet.

Each track buffer has a highest end timestamp variable that stores the highestcoded frame end timestamp across allcoded frames in the current coded frame group that were appended to this track buffer. The variable is initially unset to indicate that nocoded frames have been appended yet.

Each track buffer has a need random access point flag variable that keeps track of whether the track buffer is waiting for arandom access point coded frame. The variable is initially set to true to indicate that random access point coded frame is needed before anything can be added to the track buffer.

Each track buffer has a track buffer ranges variable that represents the presentation time ranges occupied by thecoded frames currently stored in the track buffer.

Note

For track buffer ranges, these presentation time ranges are based on presentation timestamps, frame durations, and potentially coded frame group start times for coded frame groups across track buffers in a muxedSourceBuffer.

For specification purposes, this information is treated as if it were stored in anormalized TimeRanges object. Intersectedtrack buffer ranges are used to report HTMLMediaElement.buffered, andMUST therefore support uninterrupted playback within each range ofHTMLMediaElement.buffered.

Note

These coded frame group start times differ slightly from those mentioned in thecoded frame processing algorithm in that they are the earliestpresentation timestamp across all track buffers following a discontinuity. Discontinuities can occur within thecoded frame processing algorithm or result from the coded frame removal algorithm, regardless of mode. The threshold for determining disjointness oftrack buffer ranges is implementation-specific. For example, to reduce unexpected playback stalls, implementationsMAY approximate thecoded frame processing algorithm's discontinuity detection logic by coalescing adjacent ranges separated by a gap smaller than 2 times the maximum frame duration buffered so far in thistrack buffer. ImplementationsMAY also use coded frame group start times as range start times acrosstrack buffers in a muxedSourceBuffer to further reduce unexpected playback stalls.

3.4 Event Summary

Event name	Interface	Dispatched when...
updatestart	Event	updating transitions from false to true.
update	Event	The append or remove has successfully completed. updating transitions from true to false.
updateend	Event	The append or remove has ended.
error	Event	An error occurred during the append. updating transitions from true to false.
abort	Event	The append or remove was aborted by an abort() call.updating transitions from true to false.

3.5 Algorithms

3.5.1 Segment Parser Loop

All SourceBuffer objects have an internal append state variable that keeps track of the high-level segment parsing state. It is initially set toWAITING_FOR_SEGMENT and can transition to the following states as data is appended.

Append state name	Description
WAITING_FOR_SEGMENT	Waiting for the start of an initialization segment or media segment to be appended.
PARSING_INIT_SEGMENT	Currently parsing an initialization segment.
PARSING_MEDIA_SEGMENT	Currently parsing a media segment.

The input buffer is a byte buffer that is used to hold unparsed bytes acrossappendBuffer() calls. The buffer is empty when the SourceBuffer object is created.

The buffer full flag keeps track of whetherappendBuffer() is allowed to accept more bytes. It is set to false when the SourceBuffer object is created and gets updated as data is appended and removed.

The group start timestamp variable keeps track of the starting timestamp for a newcoded frame group in the"sequence" mode. It is unset when the SourceBuffer object is created and gets updated when themode attribute equals"sequence" and thetimestampOffset attribute is set, or thecoded frame processing algorithm runs.

The group end timestamp variable stores the highestcoded frame end timestamp across allcoded frames in the current coded frame group. It is set to 0 when the SourceBuffer object is created and gets updated by thecoded frame processing algorithm.

Note

The group end timestamp stores the highestcoded frame end timestamp across all track buffers in a SourceBuffer. Therefore, care should be taken in setting the mode attribute when appending multiplexed segments in which the timestamps are not aligned across tracks.

The generate timestamps flag is a boolean variable that keeps track of whether timestamps need to be generated for thecoded frames passed to thecoded frame processing algorithm. This flag is set byaddSourceBuffer() when the SourceBuffer object is created.

When the segment parser loop algorithm is invoked, run the following steps:

1. Loop Top: If the input buffer is empty, then jump to theneed more data step below.
2. If the input buffer contains bytes that violate theSourceBuffer byte stream format specification, then run the append error algorithm and abort this algorithm.
3. Remove any bytes that the byte stream format specifications say MUST be ignored from the start of theinput buffer.

4. If the append state equalsWAITING_FOR_SEGMENT, then run the following steps:

   1. If the beginning of the input buffer indicates the start of aninitialization segment, set theappend state toPARSING_INIT_SEGMENT.
   2. If the beginning of the input buffer indicates the start of amedia segment, setappend state toPARSING_MEDIA_SEGMENT.
   3. Jump to the loop top step above.

5. If the append state equalsPARSING_INIT_SEGMENT, then run the following steps:

   1. If the input buffer does not contain a completeinitialization segment yet, then jump to theneed more data step below.
   2. Run the initialization segment received algorithm.
   3. Remove the initialization segment bytes from the beginning of the input buffer.
   4. Set append state toWAITING_FOR_SEGMENT.
   5. Jump to the loop top step above.

6. If the append state equalsPARSING_MEDIA_SEGMENT, then run the following steps:

   1. If the first initialization segment received flag is false, then run theappend error algorithm and abort this algorithm.
   2. If the input buffer contains one or more completecoded frames, then run thecoded frame processing algorithm.
      Note

      The frequency at which the coded frame processing algorithm is run is implementation-specific. The coded frame processing algorithmMAY be called when the input buffer contains the complete media segment or itMAY be called multiple times as complete coded frames are added to the input buffer.

   3. If this SourceBuffer is full and cannot accept more media data, then set thebuffer full flag to true.
   4. If the input buffer does not contain a completemedia segment, then jump to theneed more data step below.
   5. Remove the media segment bytes from the beginning of the input buffer.
   6. Set append state toWAITING_FOR_SEGMENT.
   7. Jump to the loop top step above.
7. Need more data: Return control to the calling algorithm.

3.5.2 Reset Parser State

When the parser state needs to be reset, run the following steps:

1. If the append state equalsPARSING_MEDIA_SEGMENT and the input buffer contains some completecoded frames, then run thecoded frame processing algorithm until all of these completecoded frames have been processed.
2. Unset the last decode timestamp on alltrack buffers.
3. Unset the last frame duration on alltrack buffers.
4. Unset the highest end timestamp on alltrack buffers.
5. Set the need random access point flag on alltrack buffers to true.
6. If the mode attribute equals"sequence", then set thegroup start timestamp to thegroup end timestamp
7. Remove all bytes from the input buffer.
8. Set append state toWAITING_FOR_SEGMENT.

3.5.3 Append Error Algorithm

This algorithm is called when an error occurs during an append.

1. Run the reset parser state algorithm.
2. Set the updating attribute to false.
3. Queue a task tofire a simple event namederror at thisSourceBuffer object.
4. Queue a task tofire a simple event namedupdateend at thisSourceBuffer object.
5. Run the end of stream algorithm with the error parameter set to "decode".

3.5.4 Prepare Append Algorithm

When an append operation begins, the follow steps are run to validate and prepare theSourceBuffer.

1. If the SourceBuffer has been removed from the sourceBuffers attribute of theparent media source then throw an InvalidStateError exception and abort these steps.
2. If the updating attribute equals true, then throw anInvalidStateError exception and abort these steps.
3. If the HTMLMediaElement.error attribute is not null, then throw anInvalidStateError exception and abort these steps.

4. If the readyState attribute of theparent media source is in the "ended" state then run the following steps:

   1. Set the readyState attribute of theparent media source to "open"
   2. Queue a task tofire a simple event namedsourceopen at theparent media source.
5. Run the coded frame eviction algorithm.

6. If the buffer full flag equals true, then throw aQuotaExceededError exception and abort these step.

   Note

   This is the signal that the implementation was unable to evict enough data to accommodate the append or the append is too big. The web applicationSHOULD useremove() to explicitly free up space and/or reduce the size of the append.

3.5.5 Buffer Append Algorithm

When appendBuffer() is called, the following steps are run to process the appended data.

1. Run the segment parser loop algorithm.
2. If the segment parser loop algorithm in the previous step was aborted, then abort this algorithm.
3. Set the updating attribute to false.
4. Queue a task tofire a simple event namedupdate at thisSourceBuffer object.
5. Queue a task tofire a simple event namedupdateend at thisSourceBuffer object.

3.5.6 Range Removal

Follow these steps when a caller needs to initiate a JavaScript visible range removal operation that blocks other SourceBuffer updates:

1. Let start equal the starting presentation timestamp for the removal range, in seconds measured from presentation start time.
2. Let end equal the end presentation timestamp for the removal range, in seconds measured from presentation start time.
3. Set the updating attribute to true.
4. Queue a task tofire a simple event namedupdatestart at thisSourceBuffer object.
5. Return control to the caller and run the rest of the steps asynchronously.
6. Run the coded frame removal algorithm with start and end as the start and end of the removal range.
7. Set the updating attribute to false.
8. Queue a task tofire a simple event namedupdate at thisSourceBuffer object.
9. Queue a task tofire a simple event namedupdateend at thisSourceBuffer object.

3.5.7 Initialization Segment Received

The following steps are run when the segment parser loop successfully parses a complete initialization segment:

Each SourceBuffer object has an internal first initialization segment received flag that tracks whether the first initialization segment has been appended and received by this algorithm. This flag is set to false when the SourceBuffer is created and updated by the algorithm below.

1. Update the duration attribute if it currently equals NaN:

   If the initialization segment contains a duration:

   Run the duration change algorithm with new duration set to the duration in the initialization segment.

   Otherwise:

   Run the duration change algorithm with new duration set to positive Infinity.

2. If the initialization segment has no audio, video, or text tracks, then run the append error algorithm and abort these steps.
3. If the first initialization segment received flag is true, then run the following steps:
   1. Verify the following properties. If any of the checks fail then run the append error algorithm and abort these steps.
      • The number of audio, video, and text tracks match what was in the first initialization segment.
      • The codecs for each track, match what was specified in the first initialization segment.
      • If more than one track for a single type are present (e.g., 2 audio tracks), then theTrack IDs match the ones in the firstinitialization segment.
   2. Add the appropriate track descriptions from this initialization segment to each of the track buffers.
   3. Set the need random access point flag on all track buffers to true.
4. Let active track flag equal false.

5. If the first initialization segment received flag is false, then run the following steps:

   1. If the initialization segment contains tracks with codecs the user agent does not support, then run theappend error algorithm and abort these steps.
      Note

      User agents MAY consider codecs, that would otherwise be supported, as "not supported" here if the codecs were not specified in thetype parameter passed toaddSourceBuffer().
      For example, MediaSource.isTypeSupported('video/webm;codecs="vp8,vorbis"') may return true, but ifaddSourceBuffer() was called with 'video/webm;codecs="vp8"' and a Vorbis track appears in theinitialization segment, then the user agentMAY use this step to trigger a decode error.

   2. For each audio track in the initialization segment, run following steps:

      1. Let audio byte stream track ID be the Track ID for the current track being processed.
      2. Let audio language be a BCP 47 language tag for the language specified in theinitialization segment for this track or an empty string if no language info is present.
      3. If audio language equals the 'und' BCP 47 value, then assign an empty string toaudio language.
      4. Let audio label be a label specified in the initialization segment for this track or an empty string if no label info is present.
      5. Let audio kinds be a sequence of kind strings specified in the initialization segment for this track or a sequence with a single empty string element in it if no kind information is provided.
      6. For each value in audio kinds, run the following steps:
         1. Let current audio kind equal the value from audio kinds for this iteration of the loop.
         2. Let new audio track be a new AudioTrack object.
         3. Generate a unique ID and assign it to the id property onnew audio track.
         4. Assign audio language to the language property onnew audio track.
         5. Assign audio label to the label property onnew audio track.
         6. Assign current audio kind to the kind property onnew audio track.

         7. If audioTracks.length equals 0, then run the following steps:

            1. Set the enabled property onnew audio track to true.
            2. Set active track flag to true.
         8. Add new audio track to the audioTracks attribute on thisSourceBuffer object.
            Note

            This should trigger AudioTrackList [HTML51] logic to queue a task to fire a trusted event named addtrack, that does not bubble and is not cancelable, and that uses theTrackEvent interface, with thetrack attribute initialized tonew audio track, at theAudioTrackList object referenced by theaudioTracks attribute on thisSourceBuffer object.

         9. Add new audio track to the audioTracks attribute on the HTMLMediaElement.
            Note

            This should trigger AudioTrackList [HTML51] logic to queue a task to fire a trusted event named addtrack, that does not bubble and is not cancelable, and that uses theTrackEvent interface, with thetrack attribute initialized tonew audio track, at theAudioTrackList object referenced by theaudioTracks attribute on the HTMLMediaElement.

      7. Create a new track buffer to store coded frames for this track.
      8. Add the track description for this track to the track buffer.

   3. For each video track in the initialization segment, run following steps:

      1. Let video byte stream track ID be the Track ID for the current track being processed.
      2. Let video language be a BCP 47 language tag for the language specified in theinitialization segment for this track or an empty string if no language info is present.
      3. If video language equals the 'und' BCP 47 value, then assign an empty string tovideo language.
      4. Let video label be a label specified in the initialization segment for this track or an empty string if no label info is present.
      5. Let video kinds be a sequence of kind strings specified in the initialization segment for this track or a sequence with a single empty string element in it if no kind information is provided.
      6. For each value in video kinds, run the following steps:
         1. Let current video kind equal the value from video kinds for this iteration of the loop.
         2. Let new video track be a new VideoTrack object.
         3. Generate a unique ID and assign it to the id property onnew video track.
         4. Assign video language to the language property onnew video track.
         5. Assign video label to the label property onnew video track.
         6. Assign current video kind to the kind property onnew video track.

         7. If videoTracks.length equals 0, then run the following steps:

            1. Set the selected property onnew video track to true.
            2. Set active track flag to true.
         8. Add new video track to the videoTracks attribute on thisSourceBuffer object.
            Note

            This should trigger VideoTrackList [HTML51] logic to queue a task to fire a trusted event named addtrack, that does not bubble and is not cancelable, and that uses theTrackEvent interface, with thetrack attribute initialized tonew video track, at theVideoTrackList object referenced by thevideoTracks attribute on thisSourceBuffer object.

         9. Add new video track to the videoTracks attribute on the HTMLMediaElement.
            Note

            This should trigger VideoTrackList [HTML51] logic to queue a task to fire a trusted event named addtrack, that does not bubble and is not cancelable, and that uses theTrackEvent interface, with thetrack attribute initialized tonew video track, at theVideoTrackList object referenced by thevideoTracks attribute on the HTMLMediaElement.

      7. Create a new track buffer to store coded frames for this track.
      8. Add the track description for this track to the track buffer.

   4. For each text track in the initialization segment, run following steps:

      1. Let text byte stream track ID be the Track ID for the current track being processed.
      2. Let text language be a BCP 47 language tag for the language specified in theinitialization segment for this track or an empty string if no language info is present.
      3. If text language equals the 'und' BCP 47 value, then assign an empty string totext language.
      4. Let text label be a label specified in the initialization segment for this track or an empty string if no label info is present.
      5. Let text kinds be a sequence of kind strings specified in the initialization segment for this track or a sequence with a single empty string element in it if no kind information is provided.
      6. For each value in text kinds, run the following steps:
         1. Let current text kind equal the value from text kinds for this iteration of the loop.
         2. Let new text track be a new TextTrack object.
         3. Generate a unique ID and assign it to the id property onnew text track.
         4. Assign text language to the language property onnew text track.
         5. Assign text label to the label property onnew text track.
         6. Assign current text kind to the kind property onnew text track.
         7. Populate the remaining properties on new text track with the appropriate information from theinitialization segment.
         8. If the mode property onnew text track equals"showing" or"hidden", then setactive track flag to true.
         9. Add new text track to the textTracks attribute on thisSourceBuffer object.
            Note

            This should trigger TextTrackList [HTML51] logic to queue a task to fire a trusted event named addtrack, that does not bubble and is not cancelable, and that uses theTrackEvent interface, with thetrack attribute initialized tonew text track, at theTextTrackList object referenced by thetextTracks attribute on thisSourceBuffer object.

         10. Add new text track to the textTracks attribute on the HTMLMediaElement.
             Note

             This should trigger TextTrackList [HTML51] logic to queue a task to fire a trusted event named addtrack, that does not bubble and is not cancelable, and that uses theTrackEvent interface, with thetrack attribute initialized tonew text track, at theTextTrackList object referenced by thetextTracks attribute on the HTMLMediaElement.

      7. Create a new track buffer to store coded frames for this track.
      8. Add the track description for this track to the track buffer.
   5. If active track flag equals true, then run the following steps:
      1. Add this SourceBuffer to activeSourceBuffers.
      2. Queue a task tofire a simple event namedaddsourcebuffer atactiveSourceBuffers
   6. Set first initialization segment received flag to true.

6. If the HTMLMediaElement.readyState attribute isHAVE_NOTHING, then run the following steps:

   1. If one or more objects in sourceBuffers havefirst initialization segment received flag set to false, then abort these steps.
   2. Set the HTMLMediaElement.readyState attribute toHAVE_METADATA.
      Note

      Per HTMLMediaElement ready states [HTML51] logic, HTMLMediaElement.readyState changes may trigger events on the HTMLMediaElement. This particular transition should trigger HTMLMediaElement logic to queue a task to fire a simple event named loadedmetadata at the media element.

7. If the active track flag equals true and the HTMLMediaElement.readyState attribute is greater thanHAVE_CURRENT_DATA, then set theHTMLMediaElement.readyState attribute toHAVE_METADATA.
   Note

   Per HTMLMediaElement ready states [HTML51] logic, HTMLMediaElement.readyState changes may trigger events on the HTMLMediaElement.

3.5.8 Coded Frame Processing

When complete coded frames have been parsed by the segment parser loop then the following steps are run:

1. For each coded frame in the media segment run the following steps:

   1. Loop Top:

      If generate timestamps flag equals true:

      1. Let presentation timestamp equal 0.
      2. Let decode timestamp equal 0.

      Otherwise:

      1. Let presentation timestamp be a double precision floating point representation of the coded frame'spresentation timestamp in seconds.
         Note

         Special processing may be needed to determine the presentation and decode timestamps for timed text frames since this information may not be explicitly present in the underlying format or may be dependent on the order of the frames. Some metadata text tracks, like MPEG2-TS PSI data, may only have implied timestamps. Format specific rules for these situationsSHOULD be in thebyte stream format specifications or in separate extension specifications.

      2. Let decode timestamp be a double precision floating point representation of the coded frame's decode timestamp in seconds.
         Note

         Implementations don't have to internally store timestamps in a double precision floating point representation. This representation is used here because it is the represention for timestamps in the HTML spec. The intention here is to make the behavior clear without adding unnecessary complexity to the algorithm to deal with the fact that adding a timestampOffset may cause a timestamp rollover in the underlying timestamp representation used by the byte stream format. Implementations can use any internal timestamp representation they wish, but the addition of timestampOffset SHOULD behave in a similar manner to what would happen if a double precision floating point representation was used.

   2. Let frame duration be a double precision floating point representation of thecoded frame's duration in seconds.
   3. If mode equals"sequence" andgroup start timestamp is set, then run the following steps:
      1. Set timestampOffset equal togroup start timestamp -presentation timestamp.
      2. Set group end timestamp equal togroup start timestamp.
      3. Set the need random access point flag on alltrack buffers to true.
      4. Unset group start timestamp.

   4. If timestampOffset is not 0, then run the following steps:

      1. Add timestampOffset to thepresentation timestamp.
      2. Add timestampOffset to thedecode timestamp.
   5. Let track buffer equal the track buffer that the coded frame will be added to.

   6. If last decode timestamp for track buffer is set and decode timestamp is less than last decode timestamp:

      OR

      If last decode timestamp for track buffer is set and the difference between decode timestamp and last decode timestamp is greater than 2 times last frame duration:

      1. If mode equals "segments":

         Set group end timestamp to presentation timestamp.

         If mode equals "sequence":

         Set group start timestamp equal to the group end timestamp.

      2. Unset the last decode timestamp on alltrack buffers.
      3. Unset the last frame duration on alltrack buffers.
      4. Unset the highest end timestamp on alltrack buffers.
      5. Set the need random access point flag on alltrack buffers to true.
      6. Jump to the Loop Top step above to restart processing of the currentcoded frame.

      Otherwise:

      Continue.

   7. Let frame end timestamp equal the sum of presentation timestamp andframe duration.
   8. If presentation timestamp is less than appendWindowStart, then set theneed random access point flag to true, drop the coded frame, and jump to the top of the loop to start processing the next coded frame.
      Note

      Some implementations MAY choose to collect some of these coded frames withpresentation timestamp less thanappendWindowStart and use them to generate a splice at the first coded frame that has apresentation timestamp greater than or equal to appendWindowStart even if that frame is not arandom access point. Supporting this requires multiple decoders or faster than real-time decoding so for now this behavior will not be a normative requirement.

   9. If frame end timestamp is greater than appendWindowEnd, then set theneed random access point flag to true, drop the coded frame, and jump to the top of the loop to start processing the next coded frame.
      Note

      Some implementations MAY choose to collect coded frames withpresentation timestamp less thanappendWindowEnd andframe end timestamp greater thanappendWindowEnd and use them to generate a splice across the portion of the collected coded frames within the append window at time of collection, and the beginning portion of later processed frames which only partially overlap the end of the collected coded frames. Supporting this requires multiple decoders or faster than real-time decoding so for now this behavior will not be a normative requirement. In conjunction with collecting coded frames that spanappendWindowStart, implementationsMAY thus support gapless audio splicing.

   10. If the need random access point flag ontrack buffer equals true, then run the following steps:
       1. If the coded frame is not a random access point, then drop the coded frame and jump to the top of the loop to start processing the next coded frame.
       2. Set the need random access point flag ontrack buffer to false.
   11. Let spliced audio frame be an unset variable for holding audio splice information
   12. Let spliced timed text frame be an unset variable for holding timed text splice information
   13. If last decode timestamp fortrack buffer is unset andpresentation timestamp falls within thepresentation interval of acoded frame in track buffer, then run the following steps:
       1. Let overlapped frame be the coded frame in track buffer that matches the condition above.

       2. If track buffer contains audio coded frames:

          Run the audio splice frame algorithm and if a splice frame is returned, assign it to spliced audio frame.

          If track buffer contains video coded frames:

          1. Let remove window timestamp equal the overlapped frame presentation timestamp plus 1 microsecond.
          2. If the presentation timestamp is less than the remove window timestamp, then removeoverlapped frame fromtrack buffer.
             Note

             This is to compensate for minor errors in frame timestamp computations that can appear when converting back and forth between double precision floating point numbers and rationals. This tolerance allows a frame to replace an existing one as long as it is within 1 microsecond of the existing frame's start time. Frames that come slightly before an existing frame are handled by the removal step below.

          If track buffer contains timed text coded frames:

          Run the text splice frame algorithm and if a splice frame is returned, assign it to spliced timed text frame.

   14. Remove existing coded frames in track buffer:

       If highest end timestamp for track buffer is not set:

       Remove all coded frames from track buffer that have a presentation timestamp greater than or equal to presentation timestamp and less than frame end timestamp.

       If highest end timestamp for track buffer is set and less than or equal to presentation timestamp:

       Remove all coded frames from track buffer that have a presentation timestamp greater than or equal to highest end timestamp and less than frame end timestamp

   15. Remove all possible decoding dependencies on the coded frames removed in the previous two steps by removing all coded frames from track buffer between those frames removed in the previous two steps and the nextrandom access point after those removed frames.
       Note

       Removing all coded frames until the next random access point is a conservative estimate of the decoding dependencies since it assumes all frames between the removed frames and the next random access point depended on the frames that were removed.

   16. If spliced audio frame is set:

       Add spliced audio frame to the track buffer.

       If spliced timed text frame is set:

       Add spliced timed text frame to the track buffer.

       Otherwise:

       Add the coded frame with the presentation timestamp, decode timestamp, and frame duration to the track buffer.

   17. Set last decode timestamp fortrack buffer todecode timestamp.
   18. Set last frame duration fortrack buffer toframe duration.
   19. If highest end timestamp fortrack buffer is unset orframe end timestamp is greater thanhighest end timestamp, then sethighest end timestamp fortrack buffer to frame end timestamp.
       Note

       The greater than check is needed because bidirectional prediction between coded frames can causepresentation timestamp to not be monotonically increasing even though the decode timestamps are monotonically increasing.

   20. If frame end timestamp is greater than group end timestamp, then setgroup end timestamp equal toframe end timestamp.
   21. If generate timestamps flag equals true, then settimestampOffset equal toframe end timestamp.

2. If the HTMLMediaElement.readyState attribute isHAVE_METADATA and the newcoded frames causeHTMLMediaElement.buffered to have aTimeRange for the current playback position, then set theHTMLMediaElement.readyState attribute toHAVE_CURRENT_DATA.

   Note

   Per HTMLMediaElement ready states [HTML51] logic, HTMLMediaElement.readyState changes may trigger events on the HTMLMediaElement.

3. If the HTMLMediaElement.readyState attribute isHAVE_CURRENT_DATA and the newcoded frames causeHTMLMediaElement.buffered to have aTimeRange that includes the current playback position and some time beyond the current playback position, then set theHTMLMediaElement.readyState attribute toHAVE_FUTURE_DATA.

   Note

   Per HTMLMediaElement ready states [HTML51] logic, HTMLMediaElement.readyState changes may trigger events on the HTMLMediaElement.

4. If the HTMLMediaElement.readyState attribute isHAVE_FUTURE_DATA and the newcoded frames causeHTMLMediaElement.buffered to have aTimeRange that includes the current playback position andenough data to ensure uninterrupted playback, then set theHTMLMediaElement.readyState attribute toHAVE_ENOUGH_DATA.

   Note

   Per HTMLMediaElement ready states [HTML51] logic, HTMLMediaElement.readyState changes may trigger events on the HTMLMediaElement.

5. If the media segment contains data beyond the current duration, then run theduration change algorithm with new duration set to the maximum of the current duration and thegroup end timestamp.

3.5.9 Coded Frame Removal Algorithm

Follow these steps when coded frames for a specific time range need to be removed from the SourceBuffer:

1. Let start be the starting presentation timestamp for the removal range.
2. Let end be the end presentation timestamp for the removal range.

3. For each track buffer in this source buffer, run the following steps:

   1. Let remove end timestamp be the current value of duration

   2. If this track buffer has a random access point timestamp that is greater than or equal to end, then updateremove end timestamp to that random access point timestamp.

      Note

      Random access point timestamps can be different across tracks because the dependencies betweencoded frames within a track are usually different than the dependencies in another track.

   3. Remove all media data, from this track buffer, that contain starting timestamps greater than or equal to start and less than the remove end timestamp.

      1. For each removed frame, if the frame has a decode timestamp equal to the last decode timestamp for the frame's track, run the following steps:

         If mode equals "segments":

         Set group end timestamp to presentation timestamp.

         If mode equals "sequence":

         Set group start timestamp equal to the group end timestamp.

      2. Unset the last decode timestamp on alltrack buffers.
      3. Unset the last frame duration on alltrack buffers.
      4. Unset the highest end timestamp on alltrack buffers.
      5. Set the need random access point flag on alltrack buffers to true.
   4. Remove all possible decoding dependencies on the coded frames removed in the previous step by removing all coded frames from this track buffer between those frames removed in the previous step and the next random access point after those removed frames.
      Note

      Removing all coded frames until the next random access point is a conservative estimate of the decoding dependencies since it assumes all frames between the removed frames and the next random access point depended on the frames that were removed.

   5. If this object is in activeSourceBuffers, thecurrent playback position is greater than or equal to start and less than theremove end timestamp, andHTMLMediaElement.readyState is greater thanHAVE_METADATA, then set theHTMLMediaElement.readyState attribute toHAVE_METADATA and stall playback.

      Note

      Per HTMLMediaElement ready states [HTML51] logic, HTMLMediaElement.readyState changes may trigger events on the HTMLMediaElement.

      Note

      This transition occurs because media data for the current position has been removed. Playback cannot progress until media for thecurrent playback position is appended or the selected/enabled tracks change.

4. If buffer full flag equals true and this object is ready to accept more bytes, then set thebuffer full flag to false.

3.5.10 Coded Frame Eviction Algorithm

This algorithm is run to free up space in this source buffer when new data is appended.

1. Let new data equal the data that is about to be appended to this SourceBuffer.
2. If the buffer full flag equals false, then abort these steps.
3. Let removal ranges equal a list of presentation time ranges that can be evicted from the presentation to make room for thenew data.
   Note

   Implementations MAY use different methods for selectingremoval ranges so web applicationsSHOULD NOT depend on a specific behavior. The web application can use thebuffered attribute to observe whether portions of the buffered data have been evicted.

4. For each range in removal ranges, run the coded frame removal algorithm with start and end equal to the removal range start and end timestamp respectively.

3.5.11Audio Splice Frame Algorithm

Follow these steps when the coded frame processing algorithm needs to generate a splice frame for two overlapping audiocoded frames:

1. Let track buffer be the track buffer that will contain the splice.
2. Let new coded frame be the new coded frame, that is being added to track buffer, which triggered the need for a splice.
3. Let presentation timestamp be the presentation timestamp for new coded frame
4. Let decode timestamp be the decode timestamp for new coded frame.
5. Let frame duration be the coded frame duration of new coded frame.
6. Let overlapped frame be the coded frame in track buffer with a presentation interval that contains presentation timestamp.
7. Update presentation timestamp and decode timestamp to the nearest audio sample timestamp based on sample rate of the audio inoverlapped frame. If a timestamp is equidistant from both audio sample timestamps, then use the higher timestamp (e.g.,floor(x * sample_rate + 0.5) / sample_rate).
   Note

   For example, given the following values:

   • The presentation timestamp of overlapped frame equals 10.
   • The sample rate of overlapped frame equals 8000 Hz
   • presentation timestamp equals 10.01255
   • decode timestamp equals 10.01255

   presentation timestamp and decode timestamp are updated to 10.0125 since 10.01255 is closer to 10 + 100/8000 (10.0125) than 10 + 101/8000 (10.012625)

8. If the user agent does not support crossfading then run the following steps:
   1. Remove overlapped frame from track buffer.
   2. Add a silence frame to track buffer with the following properties:
      • The presentation timestamp set to the overlapped frame presentation timestamp.
      • The decode timestamp set to the overlapped frame decode timestamp.
      • The coded frame duration set to difference between presentation timestamp and theoverlapped framepresentation timestamp.
      Note

      Some implementations MAY apply fades to/from silence to coded frames on either side of the inserted silence to make the transition less jarring.

   3. Return to caller without providing a splice frame.
      Note

      This is intended to allow new coded frame to be added to the track buffer as ifoverlapped frame had not been in thetrack buffer to begin with.

9. Let frame end timestamp equal the sum of presentation timestamp andframe duration.
10. Let splice end timestamp equal the sum of presentation timestamp and the splice duration of 5 milliseconds.
11. Let fade out coded frames equal overlapped frame as well as any additional frames intrack buffer that have apresentation timestamp greater than presentation timestamp and less thansplice end timestamp.
12. Remove all the frames included in fade out coded frames from track buffer.
13. Return a splice frame with the following properties:
    • The presentation timestamp set to the overlapped frame presentation timestamp.
    • The decode timestamp set to the overlapped frame decode timestamp.
    • The coded frame duration set to difference between frame end timestamp and theoverlapped framepresentation timestamp.
    • The fade out coded frames equals fade-out coded frames.
    • The fade in coded frame equal new coded frame.
      Note

      If the new coded frame is less than 5 milliseconds in duration, then coded frames that are appended after thenew coded frame will be needed to properly render the splice.

    • The splice timestamp equals presentation timestamp.
    Note

    See the audio splice rendering algorithm for details on how this splice frame is rendered.

3.5.12Audio Splice Rendering Algorithm

The following steps are run when a spliced frame, generated by the audio splice frame algorithm, needs to be rendered by the media element:

1. Let fade out coded frames be the coded frames that are faded out during the splice.
2. Let fade in coded frames be the coded frames that are faded in during the splice.
3. Let presentation timestamp be the presentation timestamp of the first coded frame in fade out coded frames.
4. Let end timestamp be the sum of the presentation timestamp and the coded frame duration of the last frame in fade in coded frames.
5. Let splice timestamp be the presentation timestamp where the splice starts. This corresponds with the presentation timestamp of the first frame in fade in coded frames.
6. Let splice end timestamp equal splice timestamp plus five milliseconds.
7. Let fade out samples be the samples generated by decoding fade out coded frames.
8. Trim fade out samples so that it only contains samples between presentation timestamp and splice end timestamp.
9. Let fade in samples be the samples generated by decoding fade in coded frames.
10. If fade out samples and fade in samples do not have a common sample rate and channel layout, then convertfade out samples andfade in samples to a common sample rate and channel layout.
11. Let output samples be a buffer to hold the output samples.
12. Apply a linear gain fade out with a starting gain of 1 and an ending gain of 0 to the samples betweensplice timestamp andsplice end timestamp in fade out samples.
13. Apply a linear gain fade in with a starting gain of 0 and an ending gain of 1 to the samples betweensplice timestamp andsplice end timestamp in fade in samples.
14. Copy samples between presentation timestamp to splice timestamp fromfade out samples intooutput samples.
15. For each sample between splice timestamp and splice end timestamp, compute the sum of a sample fromfade out samples and the corresponding sample infade in samples and store the result inoutput samples.
16. Copy samples between splice end timestamp to end timestamp fromfade in samples intooutput samples.
17. Render output samples.

Note

Here is a graphical representation of this algorithm.

3.5.13Text Splice Frame Algorithm

Follow these steps when the coded frame processing algorithm needs to generate a splice frame for two overlapping timed textcoded frames:

1. Let track buffer be the track buffer that will contain the splice.
2. Let new coded frame be the new coded frame, that is being added to track buffer, which triggered the need for a splice.
3. Let presentation timestamp be the presentation timestamp for new coded frame
4. Let decode timestamp be the decode timestamp for new coded frame.
5. Let frame duration be the coded frame duration of new coded frame.
6. Let frame end timestamp equal the sum of presentation timestamp andframe duration.
7. Let first overlapped frame be the coded frame in track buffer with a presentation interval that contains presentation timestamp.
8. Let overlapped presentation timestamp be the presentation timestamp of the first overlapped frame.
9. Let overlapped frames equal first overlapped frame as well as any additional frames intrack buffer that have apresentation timestamp greater than presentation timestamp and less thanframe end timestamp.
10. Remove all the frames included in overlapped frames from track buffer.
11. Update the coded frame duration of the first overlapped frame to presentation timestamp -overlapped presentation timestamp.
12. Add first overlapped frame to the track buffer.
13. Return to caller without providing a splice frame.
    Note

    This is intended to allow new coded frame to be added to the track buffer as if it hadn't overlapped any frames intrack buffer to begin with.

4. SourceBufferList Object

SourceBufferList is a simple container object for SourceBuffer objects. It provides read-only array access and fires events when the list is modified.

interface SourceBufferList : EventTarget {
    readonly attribute unsigned long length;
             attribute EventHandler  onaddsourcebuffer;
             attribute EventHandler  onremovesourcebuffer;
    getter SourceBuffer (unsigned long index);
};

4.1 Attributes

length of type unsigned long, readonly

Indicates the number of SourceBuffer objects in the list.

onaddsourcebuffer of type EventHandler

The event handler for the addsourcebuffer event.

onremovesourcebuffer of type EventHandler

The event handler for the removesourcebuffer event.

4.2 Methods

getter

Allows the SourceBuffer objects in the list to be accessed with an array operator (i.e., []).

Parameter	Type	Nullable	Optional	Description
index	unsigned long	✘	✘

Return type: SourceBuffer

When this method is invoked, the user agent must run the following steps:

1. If index is greater than or equal to the length attribute then return undefined and abort these steps.
2. Return the index'th SourceBuffer object in the list.

4.3 Event Summary

Event name	Interface	Dispatched when...
addsourcebuffer	Event	When a SourceBuffer is added to the list.
removesourcebuffer	Event	When a SourceBuffer is removed from the list.

5. URL Object Extensions

This section specifies extensions to the URL[FILE-API] object definition.

[Exposed=Window]
partial interface URL {
    static DOMString createObjectURL(MediaSource mediaSource);
};

5.1 Methods

createObjectURL, static

Creates URLs for MediaSource objects.

Note

This algorithm is intended to mirror the behavior of the createObjectURL()[FILE-API] method, which does not auto-revoke the created URL. Web authors are encouraged to userevokeObjectURL()[FILE-API] for anyMediaSource object URL that is no longer needed for attachment to a media element.

Parameter	Type	Nullable	Optional	Description
mediaSource	MediaSource	✘	✘

Return type: DOMString

When this method is invoked, the user agent must run the following steps:

1. Return a unique MediaSource object URL that can be used to dereference the mediaSource argument.

6. HTMLMediaElement Extensions

This section specifies what existing attributes on the HTMLMediaElementMUST return when a MediaSource is attached to the element.

The HTMLMediaElement.seekable attribute returns a new staticnormalized TimeRanges object created based on the following steps:

If duration equals NaN:

Return an empty TimeRanges object.

If duration equals positive Infinity:

1. If live seekable range is not empty:
   1. Let union ranges be the union of live seekable range and theHTMLMediaElement.buffered attribute.
   2. Return a single range with a start time equal to the earliest start time in union ranges and an end time equal to the highest end time in union ranges and abort these steps.
2. If the HTMLMediaElement.buffered attribute returns an emptyTimeRanges object, then return an emptyTimeRanges object and abort these steps.
3. Return a single range with a start time of 0 and an end time equal to the highest end time reported by theHTMLMediaElement.buffered attribute.

Otherwise:

Return a single range with a start time of 0 and an end time equal to duration.

The HTMLMediaElement.buffered attribute returns a staticnormalized TimeRanges object based on the following steps.

1. Let intersection ranges equal an empty TimeRanges object.
2. If activeSourceBuffers.length does not equal 0 then run the following steps:
   1. Let active ranges be the ranges returned by buffered for eachSourceBuffer object in activeSourceBuffers.
   2. Let highest end time be the largest range end time in the active ranges.
   3. Let intersection ranges equal a TimeRange object containing a single range from 0 tohighest end time.
   4. For each SourceBuffer object in activeSourceBuffers run the following steps:
      1. Let source ranges equal the ranges returned by the buffered attribute on the currentSourceBuffer.
      2. If readyState is"ended", then set the end time on the last range insource ranges to highest end time.
      3. Let new intersection ranges equal the intersection between the intersection ranges and the source ranges.
      4. Replace the ranges in intersection ranges with the new intersection ranges.
3. If the current value of this attribute has not been set by this algorithm or intersection ranges does not contain the exact same range information as the current value of this attribute, then update the current value of this attribute tointersection ranges.
4. Return the current value of this attribute.

7. AudioTrack Extensions

This section specifies extensions to the HTML AudioTrack definition.

partial interface AudioTrack {
    readonly attribute SourceBuffer? sourceBuffer;
};

Attributes

sourceBuffer of type SourceBuffer, readonly , nullable

Returns the SourceBuffer that created this track. Returns null if this track was not created by aSourceBuffer or the SourceBuffer has been removed from the sourceBuffers attribute of itsparent media source.

8. VideoTrack Extensions

This section specifies extensions to the HTML VideoTrack definition.

partial interface VideoTrack {
    readonly attribute SourceBuffer? sourceBuffer;
};

Attributes

sourceBuffer of type SourceBuffer, readonly , nullable

Returns the SourceBuffer that created this track. Returns null if this track was not created by aSourceBuffer or the SourceBuffer has been removed from the sourceBuffers attribute of itsparent media source.

9. TextTrack Extensions

This section specifies extensions to the HTML TextTrack definition.

partial interface TextTrack {
    readonly attribute SourceBuffer? sourceBuffer;
};

Attributes

sourceBuffer of type SourceBuffer, readonly , nullable

Returns the SourceBuffer that created this track. Returns null if this track was not created by aSourceBuffer or the SourceBuffer has been removed from the sourceBuffers attribute of itsparent media source.

10. Byte Stream Formats

The bytes provided through appendBuffer() for aSourceBuffer form a logical byte stream. The format and semantics of these byte streams are defined inbyte stream format specifications. The byte stream format registry [MSE-REGISTRY] provides mappings between a MIME type that may be passed to addSourceBuffer() orisTypeSupported() and the byte stream format expected by aSourceBuffer created with that MIME type. Implementations are encouraged to register mappings for byte stream formats they support to facilitate interoperability. The byte stream format registry [MSE-REGISTRY] is the authoritative source for these mappings. If an implementation claims to support a MIME type listed in the registry, itsSourceBuffer implementation MUST conform to thebyte stream format specification listed in the registry entry.

Note

The byte stream format specifications in the registry are not intended to define new storage formats. They simply outline the subset of existing storage format structures that implementations of this specification will accept.

Note

Byte stream format parsing and validation is implemented in the segment parser loop algorithm.

This section provides general requirements for all byte stream format specifications:

• A byte stream format specification MUST defineinitialization segments andmedia segments.
• A byte stream format SHOULD provide references for sourcingAudioTrack,VideoTrack, and TextTrack attribute values from data in initialization segments.
  Note

  If the byte stream format covers a format similar to one covered in the in-band tracks spec [INBANDTRACKS], then itSHOULD try to use the same attribute mappings so that Media Source Extensions playback and non-Media Source Extensions playback provide the same track information.

• It MUST be possible to identify segment boundaries and segment type (initialization or media) by examining the byte stream alone.
• The user agent MUST run the append error algorithm when any of the following conditions are met:

  1. The number and type of tracks are not consistent.

     Note

     For example, if the first initialization segment has 2 audio tracks and 1 video track, then all initialization segments that follow it in the byte stream MUST describe 2 audio tracks and 1 video track.

  2. Track IDs are not the same acrossinitialization segments, for segments describing multiple tracks of a single type (e.g., 2 audio tracks).

  3. Codecs changes across initialization segments.

     Note

     For example, a byte stream that starts with an initialization segment that specifies a single AAC track and later contains aninitialization segment that specifies a single AMR-WB track is not allowed. Support for multiple codecs is handled with multipleSourceBuffer objects.

• The user agent MUST support the following:
  1. Track IDs changing acrossinitialization segments if the segments describes only one track of each type.

  2. Video frame size changes. The user agent MUST support seamless playback.

     Note

     This will cause the <video> display region to change size if the web application does not use CSS or HTML attributes (width/height) to constrain the element size.

  3. Audio channel count changes. The user agent MAY support this seamlessly and could trigger downmixing.

     Note

     This is a quality of implementation issue because changing the channel count may require reinitializing the audio device, resamplers, and channel mixers which tends to be audible.

• The following rules apply to all media segments within a byte stream. A user agent MUST:
  1. Map all timestamps to the same media timeline.
  2. Support seamless playback of media segments having a timestamp gap smaller than the audio frame size. User agentsMUST NOT reflect these gaps in thebuffered attribute.
     Note

     This is intended to simplify switching between audio streams where the frame boundaries don't always line up across encodings (e.g., Vorbis).

• The user agent MUST run the append error algorithm when any combination of an initialization segment and any contiguous sequence of media segments satisfies the following conditions:
  1. The number and type (audio, video, text, etc.) of all tracks in the media segments are not identified.
  2. The decoding capabilities needed to decode each track (i.e., codec and codec parameters) are not provided.
  3. Encryption parameters necessary to decrypt the content (except the encryption key itself) are not provided for all encrypted tracks.
  4. All information necessary to decode and render the earliest random access point in the sequence of media segments and all subsequence samples in the sequence (in presentation time) are not provided. This includes in particular,
     • Information that determines the intrinsic width and height of the video (specifically, this requires either the picture or pixel aspect ratio, together with the encoded resolution).
     • Information necessary to convert the video decoder output to a format suitable for display
  5. Information necessary to compute the global presentation timestamp of every sample in the sequence of media segments is not provided.

  For example, if I1 is associated with M1, M2, M3 then the above MUST hold for all the combinations I1+M1, I1+M2, I1+M1+M2, I1+M2+M3, etc.

Byte stream specifications MUST at a minimum define constraints which ensure that the above requirements hold. Additional constraintsMAY be defined, for example to simplify implementation.

11. Conformance

As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.

The key words MAY, MUST, MUST NOT, SHOULD, and SHOULD NOT are to be interpreted as described in [RFC2119].

12. Examples

Example use of the Media Source Extensions

<script>
  function onSourceOpen(videoTag, e) {
    var mediaSource = e.target;

    if (mediaSource.sourceBuffers.length > 0)
        return;

    var sourceBuffer = mediaSource.addSourceBuffer('video/webm; codecs="vorbis,vp8"');

    videoTag.addEventListener('seeking', onSeeking.bind(videoTag, mediaSource));
    videoTag.addEventListener('progress', onProgress.bind(videoTag, mediaSource));

    var initSegment = GetInitializationSegment();

    if (initSegment == null) {
      // Error fetching the initialization segment. Signal end of stream with an error.
      mediaSource.endOfStream("network");
      return;
    }

    // Append the initialization segment.
    var firstAppendHandler = function(e) {
      var sourceBuffer = e.target;
      sourceBuffer.removeEventListener('updateend', firstAppendHandler);

      // Append some initial media data.
      appendNextMediaSegment(mediaSource);
    };
    sourceBuffer.addEventListener('updateend', firstAppendHandler);
    sourceBuffer.appendBuffer(initSegment);
  }

  function appendNextMediaSegment(mediaSource) {
    if (mediaSource.readyState == "closed")
      return;

    // If we have run out of stream data, then signal end of stream.
    if (!HaveMoreMediaSegments()) {
      mediaSource.endOfStream();
      return;
    }

    // Make sure the previous append is not still pending.
    if (mediaSource.sourceBuffers[0].updating)
        return;

    var mediaSegment = GetNextMediaSegment();

    if (!mediaSegment) {
      // Error fetching the next media segment.
      mediaSource.endOfStream("network");
      return;
    }

    // NOTE: If mediaSource.readyState == “ended”, this appendBuffer() call will
    // cause mediaSource.readyState to transition to "open". The web application
    // should be prepared to handle multiple “sourceopen” events.
    mediaSource.sourceBuffers[0].appendBuffer(mediaSegment);
  }

  function onSeeking(mediaSource, e) {
    var video = e.target;

    if (mediaSource.readyState == "open") {
      // Abort current segment append.
      mediaSource.sourceBuffers[0].abort();
    }

    // Notify the media segment loading code to start fetching data at the
    // new playback position.
    SeekToMediaSegmentAt(video.currentTime);

    // Append a media segment from the new playback position.
    appendNextMediaSegment(mediaSource);
  }

  function onProgress(mediaSource, e) {
    appendNextMediaSegment(mediaSource);
  }
</script>

<video id="v" autoplay> </video>

<script>
  var video = document.getElementById('v');
  var mediaSource = new MediaSource();
  mediaSource.addEventListener('sourceopen', onSourceOpen.bind(this, video));
  video.src = window.URL.createObjectURL(mediaSource);
</script>
          

13. Acknowledgments

The editors would like to thank Alex Giladi, Bob Lund, Chris Poole, Cyril Concolato, David Dorwin, David Singer, Duncan Rowden, Frank Galligan, Glenn Adams, Jer Noble, Joe Steele, John Simmons, Kevin Streeter, Mark Vickers, Matt Ward, Matthew Gregan, Michael Thornburgh, Philip Jägenstedt, Pierre Lemieux, Ralph Giles, Steven Robertson, and Tatsuya Igarashi for their contributions to this specification.

A. VideoPlaybackQuality

This section is non-normative.

The video playback quality metrics described in previous revisions of this specification (e.g., sections 5 and 10 of the Candidate Recommendation) are now being developed as part of [ MEDIA-PLAYBACK-QUALITY]. Some implementations may have implemented the earlier draft VideoPlaybackQuality object and the HTMLVideoElement extension method getVideoPlaybackQuality() described in those previous revisions.

B. References

B.1 Normative references

[FILE-API]

Arun Ranganathan; Jonas Sicking. W3C. File API. 21 April 2015. W3C Working Draft. URL: https://www.w3.org/TR/FileAPI/

[HTML51]

Steve Faulkner; Arron Eicholz; Travis Leithead; Alex Danilo. W3C. HTML 5.1. 1 November 2016. W3C Recommendation. URL: https://www.w3.org/TR/html51/

[RFC2119]

S. Bradner. IETF. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119

[WEBIDL]

Cameron McCormack; Boris Zbarsky; Yves Lafon; Travis Leithead. W3C. Web IDL. 15 September 2016. W3C Proposed Recommendation. URL: https://www.w3.org/TR/WebIDL-1/

B.2 Informative references

[INBANDTRACKS]

Bob Lund; Silvia Pfeiffer. W3C. Sourcing In-band Media Resource Tracks from Media Containers into HTML. URL: https://dev.w3.org/html5/html-sourcing-inband-tracks/

[MEDIA-PLAYBACK-QUALITY]

Mounir Lamouri. W3C. Media Playback Quality. URL: https://wicg.github.io/media-playback-quality/

[MSE-REGISTRY]

Matthew Wolenetz; Jerry Smith; Aaron Colwell. W3C. Media Source Extensions™ Byte Stream Format Registry. URL: https://www.w3.org/TR/mse-byte-stream-format-registry/

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