Copyright © 2022 W3C® (MIT, ERCIM, Keio, Beihang). W3C liability, trademark and permissive document license rules apply.
HTMLMediaElement
[HTML] 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.
This section describes the status of this document at the time of its publication. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at https://www.w3.org/TR/.
On top of editorial updates, substantives changes since publication as a W3C Recommendation in November 2016 are the addition of a changeType
()
method to switch codecs, the possibility to create and use MediaSource
objects off the main thread in dedicated workers, and the removal of the createObjectURL()
extension to the URL
object following its integration in the File API [FILEAPI]. For a full list of changes done since the previous version, see the commits.
The working group maintains a list of all bug reports that the editors have not yet tried to address.
Implementors should be aware that this specification is not stable. Implementors who are not taking part in the discussions are likely to find the specification changing out from under them in incompatible ways. Vendors interested in implementing this specification before it eventually reaches the Candidate Recommendation stage should track the GitHub repository and take part in the discussions.
This document was published by the Media Working Group as a Working Draft using the Recommendation track.
Publication as a Working Draft does not imply endorsement by W3C and its Members.
This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.
This document was produced by a group operating under the W3C Patent Policy. W3C maintains a public 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 contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.
This document is governed by the 2 November 2021 W3C Process Document.
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 more SourceBuffer
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 the SourceBuffer
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].
This specification was designed with the following goals in mind:
This specification defines:
The track buffers that provide coded frames for the enabled
audioTracks
, the selected
videoTracks
, and the
"showing"
or "hidden"
textTracks
. All these tracks are associated with
SourceBuffer
objects in the activeSourceBuffers
list.
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 with presentation 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
the appendWindowStart
and appendWindowEnd
attributes respectively.
A unit of media data that has a presentation timestamp, a decode timestamp, and a coded frame duration.
The duration of a coded frame. For video and text, the duration indicates how long the video frame or text SHOULD be displayed. For audio, the duration represents the sum of all the samples contained within the coded frame. For example, if an audio frame contained 441 samples @44100Hz the frame duration would be 10 milliseconds.
The sum of a coded frame presentation timestamp and its coded frame duration. It represents the presentation timestamp that immediately follows the coded frame.
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.
The decode timestamp indicates the latest time at which the frame needs to be decoded assuming instantaneous decoding and rendering of this and any dependant frames (this is equal to the presentation timestamp of the earliest frame, in presentation order, that is dependant on this frame). If frames can be decoded out of presentation order, then the decode timestamp MUST be present in or derivable from the byte stream. The user agent MUST run the append error algorithm if this is not the case. If frames cannot be decoded out of presentation order and a decode timestamp is not present in the byte stream, then the decode timestamp is equal to the presentation timestamp.
A sequence of bytes that contain all of the initialization information required to decode a sequence of media segments. This includes codec initialization data, Track ID mappings for multiplexed segments, and timestamp offsets (e.g., edit lists).
The byte stream format specifications in the byte stream format registry [MSE-REGISTRY] contain format specific examples.
A sequence of bytes that contain packetized & timestamped media data for a portion of the media timeline. Media segments are always associated with the most recently appended initialization segment.
The byte stream format specifications in the byte stream format registry [MSE-REGISTRY] contain format specific examples.
A MediaSource object URL is a unique Blob URI [FILEAPI] created by createObjectURL
()
. It is used to attach a MediaSource
object to an HTMLMediaElement.
These URLs are the same as a Blob URI, except that anything in the definition of that feature that refers to File and Blob 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
()
.
For example, the origin of the MediaSource object URL affects the way that the media element is consumed by canvas.
The parent media source of a SourceBuffer
object is the MediaSource
object that created it.
The presentation start time is the earliest time point in the presentation and specifies the initial playback position and earliest possible position. All presentations created using this specification have a presentation start time of 0.
For the purposes of determining if HTMLMediaElement
.buffered
contains a TimeRanges
that includes the current playback position, implementations MAY choose to allow a current playback position at or after presentation start time and before the first TimeRanges
to play the first TimeRanges
if that TimeRanges
starts within a reasonably short time, like 1 second, after presentation start time. This allowance accommodates the reality that muxed streams commonly do not begin all tracks precisely at presentation start time. Implementations MUST report the actual buffered range, regardless of this allowance.
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 a coded 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.
The order that coded frames are rendered in the presentation. The presentation order is achieved by ordering coded frames in monotonically increasing order by their presentation timestamps.
A reference to a specific time in the presentation. The presentation timestamp in a coded frame indicates when the frame SHOULD be rendered.
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.
The specific byte stream format specification that describes the format of the byte stream accepted by a SourceBuffer
instance. The
byte stream format specification, for a SourceBuffer
object, is initially selected based on the type passed to the
addSourceBuffer
()
call that created the object, and can be updated by changeType
()
calls on the object.
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:
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.
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 single initialization segment has a unique Track ID. The user agent MUST run the append error algorithm if the Track ID is not unique within the initialization segment.
A Track ID is a byte stream format specific identifier that marks sections of the byte stream as being part of a specific track. The Track ID in a track description identifies which sections of a media segment belong to that track.
The MediaSource object represents a source of media data for an HTMLMediaElement. It keeps track of the readyState
for this source as well as a list of SourceBuffer
objects that can be used to add media data to the presentation. MediaSource objects are created by the web application and then attached to an HTMLMediaElement. The application uses the SourceBuffer
objects in sourceBuffers
to add media data to this source. The HTMLMediaElement fetches this media data from the MediaSource
object when it is needed during playback.
Each MediaSource
object has a [[live seekable range]] internal slot
that stores a normalized TimeRanges object. It is initialized to an empty TimeRanges
object
when the MediaSource
object is created, is maintained by setLiveSeekableRange
()
and
clearLiveSeekableRange
()
, and is used in
HTMLMediaElement Extensions to modify
HTMLMediaElement
.seekable
behavior.
WebIDLenum 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 the SourceBuffer objects in sourceBuffers .
|
ended |
The source is still attached to a media element, but endOfStream () has been called.
|
Consider adding a "closing
" ReadyState
to indicate the source is in the process of being concurrently detached from a media element. This would be useful for some implementations of MediaSource
and SourceBuffer
in DedicatedWorkerGlobalScope
.
WebIDLenum EndOfStreamError
{
"network
",
"decode
"
};
Enumeration description | |
---|---|
network |
Terminates playback and signals that a network error has occurred. 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 occurred. 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. |
WebIDL[Exposed=(Window,DedicatedWorker)]
interface MediaSource
: EventTarget {
constructor
();
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
;
static readonly attribute boolean canConstructInDedicatedWorker
;
SourceBuffer
addSourceBuffer
(DOMString type);
undefined removeSourceBuffer
(SourceBuffer
sourceBuffer);
undefined endOfStream
(optional EndOfStreamError
error);
undefined setLiveSeekableRange
(double start, double end);
undefined clearLiveSeekableRange
();
MediaSourceHandle
getHandle
();
static boolean isTypeSupported
(DOMString type);
};
sourceBuffers
of type SourceBufferList
, readonly 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 the selected
video track, the
enabled
audio track(s), and the
"showing"
or "hidden"
text track(s).
SourceBuffer
objects in this list MUST appear in the same order as they appear in
the sourceBuffers
attribute; e.g., if only sourceBuffers[0] and
sourceBuffers[3] are in activeSourceBuffers
, then activeSourceBuffers[0] MUST
equal sourceBuffers[0] and activeSourceBuffers[1] MUST equal sourceBuffers[3].
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 readyState
MUST 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 the MediaSource
object is created.
On getting, run the following steps:
readyState
attribute is "closed
" then return NaN and abort these steps.On setting, run the following steps:
TypeError
exception and abort these steps.readyState
attribute is not "open
" then throw an InvalidStateError
exception and abort these steps.updating
attribute equals true on any SourceBuffer
in sourceBuffers
, then throw an InvalidStateError
exception and abort these steps.The duration change algorithm will adjust new duration higher if there is any currently buffered coded frame with a higher end time.
appendBuffer
()
and endOfStream
()
can update the duration under certain circumstances.
onsourceopen
of type EventHandler
The event handler for the
event.sourceopen
onsourceended
of type EventHandler
The event handler for the
event.sourceended
onsourceclose
of type EventHandler
The event handler for the
event.sourceclose
canConstructInDedicatedWorker
of type boolean
Returns true.
This attribute enables main thread and dedicated worker feature detection of support for
creating and using a MediaSource
object in a dedicated worker, and mitigates the need for higher latency
detection polyfills like attempting creation of a MediaSource
object from a dedicated worker, especially
if the feature is not supported.
addSourceBuffer
Adds a new SourceBuffer
to sourceBuffers
.
Parameter | Type | Nullable | Optional | Description |
---|---|---|---|---|
type | DOMString | ✘ | ✘ |
SourceBuffer
When this method is invoked, the user agent must run the following steps:
TypeError
exception and abort these steps.SourceBuffer
objects in sourceBuffers
, then throw a NotSupportedError
exception and abort these steps.QuotaExceededError
exception and abort these steps.
For example, a user agent MAY throw a QuotaExceededError
exception if the media element has reached the
HAVE_METADATA
readyState. This can occur if the user agent's media engine does not support adding more tracks during
playback.
readyState
attribute is not in the "open
" state then throw an InvalidStateError
exception and abort these steps.SourceBuffer
object and associated resources.[[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 with
type.
[[generate timestamps flag]]
equals true:mode
attribute on the new object to
"sequence
".
mode
attribute on the new object to
"segments
".
sourceBuffers
and queue a task to fire an event named addsourcebuffer
at sourceBuffers
.removeSourceBuffer
Removes a SourceBuffer
from sourceBuffers
.
Parameter | Type | Nullable | Optional | Description |
---|---|---|---|---|
sourceBuffer | SourceBuffer | ✘ | ✘ |
undefined
When this method is invoked, the user agent must run the following steps:
sourceBuffers
then throw a NotFoundError
exception and abort these steps.updating
attribute equals true, then run the following steps:
updating
attribute to false.abort
at sourceBuffer.updateend
at sourceBuffer.AudioTrackList
object returned by
sourceBuffer.audioTracks
.AudioTrack
object in the SourceBuffer audioTracks list, run the following steps:
sourceBuffer
attribute on the AudioTrack
object to null.AudioTrack
object from the SourceBuffer audioTracks list.
This should trigger AudioTrackList
[HTML] logic to queue a task to fire an event
named removetrack using TrackEvent
with the track
attribute initialized to the AudioTrack
object, at the SourceBuffer audioTracks list.
If the enabled
attribute on the AudioTrack
object was true at the beginning
of this removal step, then this should also trigger AudioTrackList
[HTML] logic to
queue a task to fire an event named change at the
SourceBuffer audioTracks list.
Window
, to
remove the AudioTrack
object (or instead, the Window
mirror of it if the MediaSource
object was constructed in a DedicatedWorkerGlobalScope
) from the media element:
AudioTrackList
object
returned by the audioTracks
attribute on the HTMLMediaElement.AudioTrack
object from the HTMLMediaElement audioTracks list.
This should trigger AudioTrackList
[HTML] logic to queue a task to
fire an event named removetrack using TrackEvent
with the
track
attribute initialized to the AudioTrack
object, at the
HTMLMediaElement audioTracks list. If the enabled
attribute on the
AudioTrack
object was true at the beginning of this removal step, then this should
also trigger AudioTrackList
[HTML] logic to queue a task to fire an event
named change at the HTMLMediaElement audioTracks list.
VideoTrackList
object returned by
sourceBuffer.videoTracks
.VideoTrack
object in the SourceBuffer videoTracks list, run the following steps:
sourceBuffer
attribute on the VideoTrack
object to null.VideoTrack
object from the SourceBuffer videoTracks list.
This should trigger VideoTrackList
[HTML] logic to queue a task to fire an event
named removetrack using TrackEvent
with the track
attribute initialized to the VideoTrack
object, at the SourceBuffer videoTracks list.
If the selected
attribute on the
VideoTrack
object was true at the beginning of this removal step, then this should also
trigger VideoTrackList
[HTML] logic to queue a task to fire an event named
change at the SourceBuffer videoTracks list.
Window
, to
remove the VideoTrack
object (or instead, the Window
mirror of it if the MediaSource
object was constructed in a DedicatedWorkerGlobalScope
) from the media element:
VideoTrackList
object
returned by the videoTracks
attribute on the HTMLMediaElement.VideoTrack
object from the HTMLMediaElement videoTracks list.
This should trigger VideoTrackList
[HTML] logic to queue a task to fire an event named removetrack using TrackEvent
with the
track
attribute initialized to the VideoTrack
object, at the
HTMLMediaElement videoTracks list. If the selected
attribute on the
VideoTrack
object was true at the beginning of this removal step, then this should also
trigger VideoTrackList
[HTML] logic to queue a task to fire an event named
change at the HTMLMediaElement videoTracks list.
TextTrackList
object returned by
sourceBuffer.textTracks
.TextTrack
object in the SourceBuffer textTracks list, run the following steps:
sourceBuffer
attribute on the TextTrack
object to null.TextTrack
object from the SourceBuffer textTracks list.
This should trigger TextTrackList
[HTML] logic to queue a task to fire an event
named removetrack using TrackEvent
with the track
attribute initialized to the TextTrack
object, at the SourceBuffer textTracks list.
If the mode
attribute on the
TextTrack
object was "showing"
or
"hidden"
at the beginning of this removal step, then this should also
trigger TextTrackList
[HTML] logic to queue a task to fire an event named
change at the SourceBuffer textTracks list.
Window
, to
remove the TextTrack
object (or instead, the Window
mirror of it if the MediaSource
object was constructed in a DedicatedWorkerGlobalScope
) from the media element:
TextTrackList
object
returned by the textTracks
attribute on the HTMLMediaElement.TextTrack
object from the HTMLMediaElement textTracks list.
This should trigger TextTrackList
[HTML] logic to queue a task to fire an event named removetrack using TrackEvent
with the
track
attribute initialized to the TextTrack
object, at the
HTMLMediaElement textTracks list. If the mode
attribute on the
TextTrack
object was "showing"
or
"hidden"
at the beginning of this removal step, then this
should also trigger TextTrackList
[HTML] logic to queue a task to fire an event named change at the HTMLMediaElement textTracks list.
activeSourceBuffers
, then remove sourceBuffer from
activeSourceBuffers
and queue a task to fire an event named removesourcebuffer
at the SourceBufferList
returned by activeSourceBuffers
.sourceBuffers
and queue a task to fire an event named
removesourcebuffer
at the SourceBufferList
returned by sourceBuffers
.endOfStream
Signals the end of the stream.
Parameter | Type | Nullable | Optional | Description |
---|---|---|---|---|
error | EndOfStreamError | ✘ | ✔ |
undefined
When this method is invoked, the user agent must run the following steps:
readyState
attribute is not in the "open
" state then throw an InvalidStateError
exception and abort these steps.updating
attribute equals true on any SourceBuffer
in sourceBuffers
, then throw an InvalidStateError
exception and abort these steps.setLiveSeekableRange
Updates [[live seekable range]]
that is used in
HTMLMediaElement 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 than start. |
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 than end. |
undefined
When this method is invoked, the user agent must run the following steps:
readyState
attribute is not "open
" then throw an InvalidStateError
exception and abort these steps.TypeError
exception and abort these steps.[[live seekable range]]
to be a new normalized TimeRanges object
containing a single range whose start position is start and end position is end.
clearLiveSeekableRange
Updates [[live seekable range]]
that is used in
HTMLMediaElement Extensions to modify
HTMLMediaElement
.seekable
behavior.
undefined
When this method is invoked, the user agent must run the following steps:
readyState
attribute is not "open
" then throw an InvalidStateError
exception and abort these steps.[[live seekable range]]
contains a range, then set [[live seekable range]]
to be a new empty TimeRanges
object.
getHandle
Creates a handle useful for attachment to an HTMLMediaElement
via srcObject
.
MediaSourceHandle
When this method is invoked, the user agent must run the following steps:
MediaSource
, then throw a
NotSupportedError
exception and abort these steps.
Implementations MAY choose to only allow handle creation for MediaSource
objects in a
DedicatedWorkerGlobalScope
, as a minimum requirement for enabling attachment of such a MediaSource
object to an HTMLMediaElement
.
Consider requiring conforming implementations to let such handle be created on a Window
MediaSource
object, too?
MediaSourceHandle
object and associated resources.isTypeSupported
, staticCheck to see whether the MediaSource
is capable of creating SourceBuffer
objects for the specified MIME type.
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
()
call SHOULD still fail if sufficient resources are not available to support the addition of a new SourceBuffer
.
This method returning true implies that HTMLMediaElement.canPlayType() will return "maybe" or "probably" since it does not make sense for a MediaSource
to support a type the HTMLMediaElement knows it cannot play.
Parameter | Type | Nullable | Optional | Description |
---|---|---|---|---|
type | DOMString | ✘ | ✘ |
boolean
When this method is invoked, the user agent must run the following steps:
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 ". |
When a Window
HTMLMediaElement
is attached to a DedicatedWorkerGlobalScope
MediaSource
, each context
has algorithms that depend on information from the other.
HTMLMediaElement
is exposed only to Window
contexts, but MediaSource
and related objects defined
in this specification are exposed in Window
and DedicatedWorkerGlobalScope
contexts. This lets
applications construct a MediaSource
object in either of those types of context and attach it to an
HTMLMediaElement
object in a Window
context using a MediaSource object URL or a
MediaSourceHandle
as described in the attaching to a media element algorithm. A MediaSource
object
is not Transferable
; it is only visible in the context where it was created.
Window
media element to a DedicatedWorkerGlobalScope
MediaSource
. While the model describes communication using
message passing, implementations MAY choose to communicate in potentially faster ways, such as using shared
memory and locks. Attachments to a Window
MediaSource
synchronously have the information already without
communicating it across contexts.
A MediaSource
that is constructed in a DedicatedWorkerGlobalScope
has a
[[port to main]] internal slot that stores a MessagePort
setup during
attachment and nulled during detachment. A Window
[[port to main]]
is always null.
HTMLMediaElement
extended by this specification and attached to a DedicatedWorkerGlobalScope
MediaSource
similarly has a [[port to worker]] internal slot
that stores a MessagePort
and a [[channel with worker]] internal
slot that stores a MessageChannel
, both setup during attachment and nulled during detachment. Both
[[port to worker]]
and [[channel with worker]]
are null unless
attached to a DedicatedWorkerGlobalScope
MediaSource
.
Algorithms in this specification that need to communicate information from a Window
HTMLMediaElement
to an attached DedicatedWorkerGlobalScope
MediaSource
, or vice versa, will use these internal ports
implicitly to post a message to their counterpart, where the implicit handler of the message runs steps as
described in the algorithms.
There are distinct mechanisms for attaching a MediaSource
to a media element depending on where the MediaSource
object was constructed, in a Window
versus in a DedicatedWorkerGlobalScope
:
Attaching a MediaSource
that was constructed in a Window
can be done by assigning a
MediaSource object URL for that MediaSource
to the media element src
attribute or the src attribute of a <source> inside a media element. A MediaSource object URL
is created by passing a MediaSource object to createObjectURL
()
.
Though implementations MAY allow MediaSource object URL creation in a
DedicatedWorkerGlobalScope
for a MediaSource
constructed in that worker, attempting to use that
MediaSource object URL to attach to a media element using either the src
attribute or the src attribute of a <source> inside a media element MUST fail in the media
element's resource selection aglorithm, as extended below.
Extending the object URL attachment mechanism to worker MediaSource object URLs would further propagate this idiom that is less preferred versus using srcObject, and would unnecessarily increase user agent interoperability risk and implementation complexity.
MediaSource
that was constructed in a DedicatedWorkerGlobalScope
can be done by
obtaining a handle from it using getHandle
, transferring that MediaSourceHandle
to the
Window
context and assigning it to the media element srcObject
attribute. For the
purposes of aligning this specification with HTMLMediaElement
resource loading and fetching
algorithms, the underlying DedicatedWorkerGlobalScope
MediaSource
is the MediaSource
object mentioned there, and the MediaSourceHandle
object is the media provider object.
MediaSourceHandle needs to be added to HTMLMediaElement's MediaProvider IDL typedef and related text involving media provider objects.
If the resource fetch algorithm was invoked with a media provider object that is a
MediaSource
object, a MediaSourceHandle
object or a URL record whose object is a MediaSource
object, then let mode be local, skip the first step in the resource fetch algorithm
(which may otherwise set mode to remote) and add the steps and clarifications below to the "Otherwise (mode is local)" section of the resource fetch algorithm.
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's src
attribute or selected child <source>
's src
attribute is a blob:
URL matching a MediaSource 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 the resource 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 [HTML], 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.
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.
MediaSource
that was constructed in a DedicatedWorkerGlobalScope
,
such as would occur if attempting to use a MediaSource object URL from a
DedicatedWorkerGlobalScope
MediaSource
readyState
is NOT set to "closed
"
MediaSource
was constructed in a DedicatedWorkerGlobalScope
, then setup worker
attachment communication and open the MediaSource
:[[channel with worker]]
to be a new MessageChannel
.[[port to worker]]
to the port1
value of
[[channel with worker]]
.port2
of
[[channel with worker]]
as both the value and the sole member of the transferList, and let
the result be serialized port2.
MediaSource
's DedicatedWorkerGlobalScope
that will
DedicatedWorkerGlobalScope
's realm, and set
[[port to main]]
to be the resulting deserialized clone of the transferred
port2
value of [[channel with worker]]
.readyState
attribute to "open
".sourceopen
at the MediaSource
.MediaSource
was constructed in a Window
:[[channel with worker]]
null.[[port to worker]]
null.[[port to main]]
null.readyState
attribute to "open
".sourceopen
at the MediaSource
.appendBuffer
()
.MediaSource
is attached.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 [HTML] references changed after implementations of this specification stabilized.
The following steps are run in any case where the media element is going to transition to NETWORK_EMPTY
and queue a task to fire an event named emptied at the media element. These steps SHOULD be run right before the transition.
MediaSource
was constructed in a DedicatedWorkerGlobalScope
:MediaSource
using an internal detach
message posted to
[[port to worker]]
.[[port to worker]]
null.[[channel with worker]]
null.detach
notification runs the remainder of these
steps in the DedicatedWorkerGlobalScope
MediaSource
.MediaSource
was constructed in a Window
:Window
MediaSource
.[[port to main]]
null.readyState
attribute to "closed
".duration
to NaN.SourceBuffer
objects from activeSourceBuffers
.removesourcebuffer
at activeSourceBuffers
.SourceBuffer
objects from sourceBuffers
.removesourcebuffer
at sourceBuffers
.sourceclose
at the MediaSource
.Going forward, this algorithm is intended to be externally called and run in any case where the attached MediaSource
, if any, must be detached from the media element. It MAY be called on HTMLMediaElement [HTML] operations like load() and resource fetch algorithm failures in addition to, or in place of, when the media element transitions to NETWORK_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" [HTML] is not considered a failure that triggers detachment.
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 the seek algorithm:
The media element looks for media segments containing the new playback position in each SourceBuffer
object in activeSourceBuffers
.
Any position within a TimeRanges
in the current value of the HTMLMediaElement
.buffered
attribute has all necessary media segments buffered for that position.
TimeRanges
of HTMLMediaElement
.buffered
HTMLMediaElement
.readyState
attribute is greater than
HAVE_METADATA
, then set the HTMLMediaElement
.readyState
attribute
to HAVE_METADATA
.
Per HTMLMediaElement ready states
[HTML] logic, HTMLMediaElement
.readyState
changes may trigger events on the HTMLMediaElement.
appendBuffer
()
call causes the coded frame processing algorithm to set
the HTMLMediaElement
.readyState
attribute to a value greater than HAVE_METADATA
.
The web application can use buffered
and HTMLMediaElement
.buffered
to determine what the media element needs to resume playback.
If the readyState
attribute is "ended
" and the new playback position is within a TimeRanges
currently in HTMLMediaElement
.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 than new playback position. This condition should only occur due to logic in buffered
when readyState
is "ended
".
The following steps are periodically run during playback to make sure that all of the SourceBuffer
objects in activeSourceBuffers
have enough data to ensure uninterrupted playback. Changes to activeSourceBuffers
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 the HAVE_ENOUGH_DATA
ready state.
These transitions indicate when the user agent believes it has enough data buffered or it needs more data respectively.
An implementation MAY choose to use bytes buffered, time buffered, the append rate, or any other metric it sees fit to
determine when it has enough data. The metrics used MAY change during playback so web applications SHOULD only rely on the value of
HTMLMediaElement
.readyState
to determine whether more data is needed or not.
When the media element needs more data, the user agent SHOULD transition it from HAVE_ENOUGH_DATA
to
HAVE_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.
HTMLMediaElement
.readyState
attribute equals HAVE_NOTHING
:HTMLMediaElement
.buffered
does not contain a TimeRanges
for the current playback position:HTMLMediaElement
.readyState
attribute to HAVE_METADATA
.
Per HTMLMediaElement ready states
[HTML] logic, HTMLMediaElement
.readyState
changes may trigger events on the HTMLMediaElement.
HTMLMediaElement
.buffered
contains a TimeRanges
that includes the current playback position and enough data to ensure uninterrupted playback:HTMLMediaElement
.readyState
attribute to HAVE_ENOUGH_DATA
.
Per HTMLMediaElement ready states
[HTML] logic, HTMLMediaElement
.readyState
changes may trigger events on the HTMLMediaElement.
HAVE_CURRENT_DATA
.HTMLMediaElement
.buffered
contains a TimeRanges
that includes the current playback position and some time beyond the current playback position, then run the following steps:HTMLMediaElement
.readyState
attribute to HAVE_FUTURE_DATA
.
Per HTMLMediaElement ready states
[HTML] logic, HTMLMediaElement
.readyState
changes may trigger events on the HTMLMediaElement.
HAVE_CURRENT_DATA
.HTMLMediaElement
.buffered
contains a TimeRanges
that ends at the current playback position and does not have a range covering the time immediately after the current position:HTMLMediaElement
.readyState
attribute to HAVE_CURRENT_DATA
.
Per HTMLMediaElement ready states
[HTML] logic, HTMLMediaElement
.readyState
changes may trigger events on the HTMLMediaElement.
During playback activeSourceBuffers
needs to be updated if the selected
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.
Also, when MediaSource
was constructed in a DedicatedWorkerGlobalScope
, then each change that occurs
to a Window
mirror of a track created previously by the implicit handler for the internal create
track mirror
message MUST also be made to the corresponding DedicatedWorkerGlobalScope
track
using an internal update track state
message posted to [[port to worker]]
whose implicit handler makes the change and runs the following steps.
Likewise, each change that occurs to a DedicatedWorkerGlobalScope
track MUST also be made to the
corresponding Window
mirror of the track using an internal update track state
message
posted to [[port to main]]
whose implicit handler makes the change to the mirror.
SourceBuffer
associated with the previously selected video track is not associated with any other enabled tracks, run the following steps:
SourceBuffer
from activeSourceBuffers
.removesourcebuffer
at activeSourceBuffers
SourceBuffer
associated with the newly selected video track is not already in activeSourceBuffers
, run the following steps:
SourceBuffer
to activeSourceBuffers
.addsourcebuffer
at activeSourceBuffers
SourceBuffer
associated with this track is not associated with any other enabled or selected track, then run the following steps:SourceBuffer
associated with the audio track from activeSourceBuffers
removesourcebuffer
at activeSourceBuffers
SourceBuffer
associated with this track is not already in activeSourceBuffers
, then run the following steps:
SourceBuffer
associated with the audio track to activeSourceBuffers
addsourcebuffer
at activeSourceBuffers
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:SourceBuffer
associated with the text track from activeSourceBuffers
removesourcebuffer
at activeSourceBuffers
mode
becomes "showing"
or "hidden"
and the SourceBuffer
associated with this track is not already in activeSourceBuffers
, then run the following steps:
SourceBuffer
associated with the text track to activeSourceBuffers
addsourcebuffer
at activeSourceBuffers
Follow these steps when duration
needs to change to a new duration.
duration
is equal to new duration, then return.SourceBuffer
objects in sourceBuffers
, then throw an InvalidStateError
exception and abort these steps.
SourceBuffer
objects in sourceBuffers
.This condition can occur because the coded frame removal algorithm preserves coded frames that start before the start of the removal range.
duration
to new duration.Window
to update the media
element's duration:
duration
to new duration.This algorithm gets called when the application signals the end of stream via an endOfStream
()
call or an algorithm needs to
signal a decode error. This algorithm takes an error parameter that indicates whether an error will be signalled.
readyState
attribute value to "ended
".sourceended
at the MediaSource
.SourceBuffer
objects in sourceBuffers
.
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.
network
"Window
:
HTMLMediaElement
.readyState
attribute equals
HAVE_NOTHING
HTMLMediaElement
.readyState
attribute is greater than
HAVE_NOTHING
decode
"Window
:
HTMLMediaElement
.readyState
attribute equals
HAVE_NOTHING
HTMLMediaElement
.readyState
attribute is greater than
HAVE_NOTHING
This algorithm is used to run steps on Window
from a MediaSource
attached from either the same
Window
or from a DedicatedWorkerGlobalScope
, usually to update the state of the attached
HTMLMediaElement
. This algorithm takes a steps parameter that lists the steps to run on Window
.
MediaSource
was constructed in a DedicatedWorkerGlobalScope
:mirror on window
message to [[port to main]]
whose
implicit handler in Window
will run steps. Return control to the caller without awaiting that
handler's receipt of the message.
Window
rather than these steps somehow
happening in the middle of some other Window
task's execution, andDedicatedWorkerGlobalScope
.
The MediaSourceHandle object represents a proxy for a MediaSource
object that is useful for attaching a
DedicatedWorkerGlobalScope
MediaSource
to a Window
HTMLMediaElement
using
srcObject
as described in the attaching to a media element algorithm.
This distinct object is necessary to attach a cross-context MediaSource
to a media element
because MediaSource
objects themselves are not transferable since they are event targets.
WebIDL[Exposed=(Window,DedicatedWorker)]
interface MediaSourceHandle
{};
MediaSourceHandle
transfer steps and transfer-receiving steps require the implementation to
maintain an implicit internal slot referencing the underlying MediaSource
to enable attaching to a media element using srcObject
and consequent setup of an attachment's cross-context communication model.
WebIDLenum 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. The |
WebIDL[Exposed=(Window,DedicatedWorker)]
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
;
undefined appendBuffer
(BufferSource data);
undefined abort
();
undefined changeType
(DOMString type);
undefined remove
(double start, unrestricted double end);
};
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, and can be updated by changeType
()
or setting this attribute.
On getting, Return the initial value or the last value that was successfully set.
On setting, run the following steps:
sourceBuffers
attribute of the parent media source, then throw an InvalidStateError
exception and abort these steps.updating
attribute equals true, then throw an InvalidStateError
exception and abort these steps.[[generate timestamps flag]]
equals true and new mode equals
"segments
", then throw a TypeError
exception and abort these steps.If the readyState
attribute of the parent media source is in the "ended
" state then run the following steps:
readyState
attribute of the parent media source to "open
"sourceopen
at the parent media source.[[append state]]
equals PARSING_MEDIA_SEGMENT
, then throw an
InvalidStateError
and abort these steps.sequence
", then set the [[group start timestamp]]
to the [[group end timestamp]]
.updating
of type boolean
, readonly Indicates whether the asynchronous continuation of an appendBuffer
()
or remove
()
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 the SourceBuffer
. 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:
sourceBuffers
attribute of the parent media source then throw an InvalidStateError
exception and abort these steps.SourceBuffer
object.TimeRanges
object containing a single range from 0 to highest end time.SourceBuffer
, run the following steps:
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.
readyState
is "ended
", then set the end time on the last range in track ranges to highest end time.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:
sourceBuffers
attribute of the parent media source, then throw an InvalidStateError
exception and abort these steps.updating
attribute equals true, then throw an InvalidStateError
exception and abort these steps.If the readyState
attribute of the parent media source is in the "ended
" state then run the following steps:
readyState
attribute of the parent media source to "open
"sourceopen
at the parent media source.[[append state]]
equals PARSING_MEDIA_SEGMENT
, then throw an
InvalidStateError
and abort these steps.mode
attribute equals "sequence
", then set the
[[group start timestamp]]
to new timestamp offset.audioTracks
of type AudioTrackList
, readonly AudioTrack
objects created by this object.
videoTracks
of type VideoTrackList
, readonly VideoTrack
objects created by this object.
textTracks
of type TextTrackList
, readonly 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:
sourceBuffers
attribute of the parent media source, then throw an
InvalidStateError
exception and abort these steps.updating
attribute equals true, then throw an InvalidStateError
exception and abort these steps.appendWindowEnd
then throw a TypeError
exception
and abort these steps.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:
sourceBuffers
attribute of the parent media source, then throw an
InvalidStateError
exception and abort these steps.updating
attribute equals true, then throw an InvalidStateError
exception and abort these steps.TypeError
and abort these steps.appendWindowStart
then throw a TypeError
exception and abort these
steps.onupdatestart
of type EventHandler
The event handler for the
event.updatestart
onupdate
of type EventHandler
The event handler for the
event.update
onupdateend
of type EventHandler
The event handler for the
event.updateend
onerror
of type EventHandler
The event handler for the
event.error
onabort
of type EventHandler
The event handler for the
event.abort
appendBuffer
Appends the segment data in an BufferSource
[WEBIDL] to the SourceBuffer
.
Parameter | Type | Nullable | Optional | Description |
---|---|---|---|---|
data | BufferSource | ✘ | ✘ |
undefined
When this method is invoked, the user agent must run the following steps:
[[input buffer]]
.updating
attribute to true.updatestart
at this SourceBuffer
object.abort
Aborts the current segment and resets the segment parser.
undefined
When this method is invoked, the user agent must run the following steps:
sourceBuffers
attribute of the parent media source then throw an InvalidStateError
exception and abort these steps.readyState
attribute of the parent media source is not in the "open
" state then throw an InvalidStateError
exception and abort these steps.InvalidStateError
exception and abort these steps.updating
attribute equals true, then run the following steps:
updating
attribute to false.abort
at this SourceBuffer
object.updateend
at this SourceBuffer
object.appendWindowStart
to the presentation start time.appendWindowEnd
to positive Infinity.changeType
Changes the MIME type associated with this object. Subsequent appendBuffer
()
calls will expect the newly appended bytes to conform to the new type.
Parameter | Type | Nullable | Optional | Description |
---|---|---|---|---|
type | DOMString |
✘ | ✘ |
undefined
When this method is invoked, the user agent must run the following steps:
TypeError
exception and abort these steps.sourceBuffers
attribute of the parent media source, then throw an InvalidStateError
exception and abort these steps.updating
attribute equals true, then throw an InvalidStateError
exception and abort these steps.SourceBuffer
objects in the sourceBuffers
attribute of the parent media source, then throw a NotSupportedError
exception and abort these steps.If the readyState
attribute of the parent media source is in the "ended
" state then run the following steps:
readyState
attribute of the parent media source to "open
".sourceopen
at the parent media source.[[generate timestamps flag]]
on this SourceBuffer
object to the value in
the "Generate Timestamps Flag" column of the byte stream format registry [MSE-REGISTRY] entry that is
associated with type.[[generate timestamps flag]]
equals true:mode
attribute on this SourceBuffer
object to "sequence
", including running the associated steps for that attribute being set.
mode
attribute on this SourceBuffer
object, without running any associated steps for that attribute being set.
[[pending initialization segment for changeType flag]]
on this SourceBuffer
object to true.
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. |
undefined
When this method is invoked, the user agent must run the following steps:
sourceBuffers
attribute of the parent media source then throw an InvalidStateError
exception and abort these steps.updating
attribute equals true, then throw an InvalidStateError
exception and abort these steps.duration
equals NaN, then throw a TypeError
exception and abort these steps.duration
, then throw a TypeError
exception and abort these steps.TypeError
exception and abort these steps.If the readyState
attribute of the parent media source is in the "ended
" state then run
the following steps:
readyState
attribute of the parent media source to "open
"sourceopen
at the parent media source.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 last coded 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 last frame duration variable that stores the coded frame duration of the last coded 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 highest coded frame end timestamp across all coded frames in the current coded frame group that were appended to this track buffer. The variable is initially unset to indicate that no coded 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 a random 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 the coded frames currently stored in the track buffer.
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 muxed SourceBuffer
.
For specification purposes, this information is treated as if it were stored in a normalized TimeRanges object. Intersected track buffer ranges are used to report HTMLMediaElement
.buffered
, and MUST therefore support uninterrupted playback within each range of HTMLMediaElement
.buffered
.
These coded frame group start times differ slightly from those mentioned in the coded frame processing algorithm in that they are the earliest presentation timestamp across all track buffers following a discontinuity. Discontinuities can occur within the coded frame processing algorithm or result from the coded frame removal algorithm, regardless of mode
.
The threshold for determining disjointness of track buffer ranges is implementation-specific. For example, to reduce unexpected playback stalls, implementations MAY approximate the coded 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 this track buffer. Implementations MAY also use coded frame group start times as range start times across track buffers in a muxed SourceBuffer
to further reduce unexpected playback stalls.
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 was aborted by an abort () call. updating transitions from true to false. |
Each SourceBuffer
object has an [[append state]] internal slot
that keeps track of the high-level segment parsing state. It is initially set to WAITING_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. |
Each SourceBuffer
object has an [[input buffer]] internal slot
that is a byte buffer that holds unparsed bytes across appendBuffer
()
calls. The buffer is
empty when the SourceBuffer
object is created.
Each SourceBuffer
object has a [[buffer full flag]] internal
slot that keeps track of whether appendBuffer
()
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.
Each SourceBuffer
object has a [[group start timestamp]]
internal slot that keeps track of the starting timestamp for a new coded frame group in the
"sequence
" mode. It is unset when the SourceBuffer object is created and gets updated when
the mode
attribute equals "sequence
" and the
timestampOffset
attribute is set, or the coded frame processing algorithm runs.
Each SourceBuffer
object has a [[group end timestamp]] internal
slot that stores the highest coded frame end timestamp across all coded frames in the current
coded frame group. It is set to 0 when the SourceBuffer object is created and gets updated by the
coded frame processing algorithm.
The [[group end timestamp]]
stores the highest coded 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.
Each SourceBuffer
object has a
[[generate timestamps flag]] internal slot that is a
boolean that keeps track of whether timestamps need to be generated for the coded frames passed to the
coded frame processing algorithm.
This flag is set by addSourceBuffer
()
when the SourceBuffer
object is created and is
updated by changeType
()
.
When the segment parser loop algorithm is invoked, run the following steps:
[[input buffer]]
is empty, then jump to the need more
data step below.[[input buffer]]
contains bytes that violate the SourceBuffer byte stream format specification, then run the append error algorithm and abort this algorithm.[[input buffer]]
.If the [[append state]]
equals WAITING_FOR_SEGMENT
, then run the following
steps:
[[input buffer]]
indicates the start of an initialization segment, set the [[append state]]
to PARSING_INIT_SEGMENT
.[[input buffer]]
indicates the start of a media segment, set [[append state]]
to PARSING_MEDIA_SEGMENT
.If the [[append state]]
equals PARSING_INIT_SEGMENT
, then run the following
steps:
[[input buffer]]
does not contain a complete initialization segment yet,
then jump to the need more data step below.[[input buffer]]
.[[append state]]
to WAITING_FOR_SEGMENT
.If the [[append state]]
equals PARSING_MEDIA_SEGMENT
, then run the following
steps:
[[first initialization segment received flag]]
is false or the
[[pending initialization segment for changeType flag]]
is true, then run the append error algorithm and abort this algorithm.[[input buffer]]
contains one or more complete coded frames, then run
the coded frame processing algorithm.
The frequency at which the coded frame processing algorithm is run is implementation-specific. The coded frame processing algorithm MAY be called when the input buffer contains the complete media segment or it MAY be called multiple times as complete coded frames are added to the input buffer.
SourceBuffer
is full and cannot accept more media data, then set the
[[buffer full flag]]
to true.[[input buffer]]
does not contain a complete media segment, then jump to
the need more data step below.[[input buffer]]
.[[append state]]
to WAITING_FOR_SEGMENT
.When the parser state needs to be reset, run the following steps:
[[append state]]
equals PARSING_MEDIA_SEGMENT
and the [[input buffer]]
contains some complete coded frames, then run the coded frame processing algorithm
until all of these complete coded frames have been processed.mode
attribute equals "sequence
", then set the
[[group start timestamp]]
to the [[group end timestamp]]
[[input buffer]]
.[[append state]]
to WAITING_FOR_SEGMENT
.This algorithm is called when an error occurs during an append.
updating
attribute to false.error
at this SourceBuffer
object.updateend
at this SourceBuffer
object.decode
".When an append operation begins, the following steps are run to validate and prepare the SourceBuffer
.
SourceBuffer
has been removed from the sourceBuffers
attribute of the parent media source then throw an InvalidStateError
exception and abort these steps.updating
attribute equals true, then throw an InvalidStateError
exception and abort these steps.MediaSource
was constructed in a Window
HTMLMediaElement
.error
attribute
is not null. If that attribute is null, then let recent element error be false.Window
case, but run
on the Window
HTMLMediaElement
on any change to its error
attribute and
communicated by using [[port to worker]]
implicit messages. If such a message has
not yet been received, then let recent element error be false.InvalidStateError
exception and abort these steps.If the readyState
attribute of the parent media source is in the "ended
" state then run the following steps:
readyState
attribute of the parent media source to "open
"
sourceopen
at the parent media source.If the [[buffer full flag]]
equals true, then throw a QuotaExceededError
exception
and abort these steps.
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
application SHOULD use remove
()
to explicitly free up space and/or reduce the size of the append.
When appendBuffer
()
is called, the following steps are run to process the appended data.
updating
attribute to false.update
at this SourceBuffer
object.updateend
at this SourceBuffer
object.Follow these steps when a caller needs to initiate a JavaScript visible range removal operation that blocks other SourceBuffer updates:
updating
attribute to true.updatestart
at this SourceBuffer
object.updating
attribute to false.update
at this SourceBuffer
object.updateend
at this SourceBuffer
object.The following steps are run when the segment parser loop successfully parses a complete initialization segment:
Each SourceBuffer object has a [[first initialization segment received flag]] internal slot 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.
Each SourceBuffer object has a
[[pending initialization segment for changeType flag]] internal slot
that tracks whether an initialization segment is needed since the most recent
changeType
()
. This flag is set to false when the SourceBuffer is created, set to true by
changeType
()
and reset to false by the algorithm below.
duration
attribute if it currently equals NaN:
[[first initialization segment received flag]]
is true, then run the following
steps:
User agents MAY consider codecs, that would otherwise be supported, as "not supported" here if the codecs were not
specified in type parameter passed to
(a) the most recently successful changeType
()
on this SourceBuffer
object, or
(b) if no successful changeType
()
has yet occurred on this object, the addSourceBuffer
()
that created this SourceBuffer
object.
For example, if the most recently successful changeType
()
was called with 'video/webm'
or 'video/webm; codecs="vp8"'
, and a video track containing vp9 appears in the initialization segment,
then the user agent MAY use this step to trigger a decode error even if the other two properties'
checks, above, pass. Implementations are encouraged to trigger error in such cases only when the codec
is indeed not supported or the other two properties' checks fail.
Web authors are encouraged to use changeType
()
, addSourceBuffer
()
and
isTypeSupported
()
with precise codec parameters to more proactively detect user agent
support. changeType
()
is required if the SourceBuffer
object's bytestream format
is changing.
If the [[first initialization segment received flag]]
is false, then run the following
steps:
User agents MAY consider codecs, that would otherwise be supported, as "not supported" here if the codecs were not
specified in type parameter passed to
(a) the most recently successful changeType
()
on this SourceBuffer
object, or
(b) if no successful changeType
()
has yet occurred on this object, the addSourceBuffer
()
that created this SourceBuffer
object.
For example, MediaSource.isTypeSupported('video/webm;codecs="vp8,vorbis"')
may return true, but if
addSourceBuffer
()
was called with 'video/webm;codecs="vp8"'
and a Vorbis track appears in the
initialization segment, then the user agent MAY use this step to trigger a decode error.
Implementations are encouraged to trigger error in such cases only when the codec is indeed not supported.
Web authors are encouraged to use changeType
()
, addSourceBuffer
()
and
isTypeSupported
()
with precise codec parameters to more proactively detect user agent
support. changeType
()
is required if the SourceBuffer
object's bytestream format
is changing.
For each audio track in the initialization segment, run following steps:
AudioTrack
object.id
property on
new audio track.language
property on new audio track.label
property on new audio track.kind
property on new audio track.
If this SourceBuffer
object's audioTracks
.length
equals 0, then run
the following steps:
enabled
property on new audio track to true.audioTracks
attribute on this SourceBuffer
object.
This should trigger AudioTrackList
[HTML] logic to
queue a task to fire an event named addtrack
using TrackEvent
with the track
attribute initialized to
new audio track, at the AudioTrackList
object referenced by the
audioTracks
attribute on this SourceBuffer
object.
DedicatedWorkerGlobalScope
:create track mirror
message to [[port to main]]
whose implicit handler in Window
runs the following steps:AudioTrack
object.audioTracks
attribute
on the HTMLMediaElement.audioTracks
attribute on the
HTMLMediaElement.
This should trigger AudioTrackList
[HTML] logic to
queue a task to fire an event named addtrack
using TrackEvent
with the track
attribute initialized to
mirrored audio track or new audio track, at the AudioTrackList
object referenced by
the audioTracks
attribute on the HTMLMediaElement.
For each video track in the initialization segment, run following steps:
VideoTrack
object.id
property on
new video track.language
property on new video track.label
property on new video track.kind
property on new video track.
If this SourceBuffer
object's videoTracks
.length
equals 0, then run
the following steps:
selected
property on new video track to true.videoTracks
attribute on this SourceBuffer
object.
This should trigger VideoTrackList
[HTML] logic to
queue a task to fire an event named addtrack
using TrackEvent
with the track
attribute initialized to
new video track, at the VideoTrackList
object referenced by the
videoTracks
attribute on this SourceBuffer
object.
DedicatedWorkerGlobalScope
:create track mirror
message to [[port to main]]
whose implicit handler in Window
runs the following steps:VideoTrack
object.videoTracks
attribute
on the HTMLMediaElement.videoTracks
attribute on the
HTMLMediaElement.
This should trigger VideoTrackList
[HTML] logic to
queue a task to fire an event named addtrack
using TrackEvent
with the track
attribute initialized to
mirrored video track or new video track, at the VideoTrackList
object referenced by
the videoTracks
attribute on the HTMLMediaElement.
For each text track in the initialization segment, run following steps:
TextTrack
object.id
property on
new text track.language
property on new text track.label
property on new text track.kind
property on new text track.mode
property on new text track equals
"showing"
or "hidden"
, then set
active track flag to true.
textTracks
attribute on this SourceBuffer
object.
This should trigger TextTrackList
[HTML] logic to
queue a task to fire an event named addtrack
using TrackEvent
with the track
attribute initialized to
new text track, at the TextTrackList
object referenced by the
textTracks
attribute on this SourceBuffer
object.
DedicatedWorkerGlobalScope
:create track mirror
message to [[port to main]]
whose implicit handler in Window
runs the following steps:TextTrack
object.textTracks
attribute
on the HTMLMediaElement.textTracks
attribute on the
HTMLMediaElement.
This should trigger TextTrackList
[HTML] logic to
queue a task to fire an event named addtrack
using TrackEvent
with the track
attribute initialized to
mirrored text track or new text track, at the TextTrackList
object referenced by the
textTracks
attribute on the HTMLMediaElement.
SourceBuffer
to activeSourceBuffers
.addsourcebuffer
at activeSourceBuffers
[[first initialization segment received flag]]
to true.[[pending initialization segment for changeType flag]]
to false.Window
:
HTMLMediaElement
.readyState
attribute is greater than
HAVE_CURRENT_DATA
, then set the
HTMLMediaElement
.readyState
attribute to
HAVE_METADATA
.
Per HTMLMediaElement ready states
[HTML] logic, HTMLMediaElement
.readyState
changes may trigger events on the HTMLMediaElement.
sourceBuffers
of the parent media source has
[[first initialization segment received flag]]
equal to true, then
use the parent media source's mirror if necessary algorithm to run the following step in
Window
:
HTMLMediaElement
.readyState
attribute is
HAVE_NOTHING
, then set the HTMLMediaElement
.readyState
attribute to HAVE_METADATA
.
Per HTMLMediaElement ready states
[HTML] logic,
HTMLMediaElement
.readyState
changes may trigger events on the
HTMLMediaElement. If transition from HAVE_NOTHING
to
HAVE_METADATA
occurs, it should trigger HTMLMediaElement logic to queue a task to fire an event named loadedmetadata at the media element.
When complete coded frames have been parsed by the segment parser loop then the following steps are run:
For each coded frame in the media segment run the following steps:
[[generate timestamps flag]]
equals true: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 situations SHOULD be in the byte stream format specifications or in separate extension specifications.
Implementations don't have to internally store timestamps in a double precision floating point representation. This representation is used here because it is the representation 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.
mode
equals "sequence
" and [[group start timestamp]]
is set, then run the following steps:
timestampOffset
equal to [[group start timestamp]]
minus
presentation timestamp.[[group end timestamp]]
equal to [[group start timestamp]]
.[[group start timestamp]]
.If timestampOffset
is not 0, then run the following steps:
timestampOffset
to the presentation timestamp.timestampOffset
to the decode timestamp.mode
equals "segments
":[[group end timestamp]]
to presentation timestamp.mode
equals "sequence
":[[group start timestamp]]
equal to the [[group end timestamp]]
.appendWindowStart
, then set the need 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.
Some implementations MAY choose to collect some of these coded frames with presentation timestamp less than appendWindowStart
and use them
to generate a splice at the first coded frame that has a presentation timestamp greater than or equal to appendWindowStart
even if
that frame is not a random access point. Supporting this requires multiple decoders or faster than real-time decoding so for now
this behavior will not be a normative requirement.
appendWindowEnd
, then set the need 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.
Some implementations MAY choose to collect coded frames with presentation timestamp less than appendWindowEnd
and frame end timestamp greater than appendWindowEnd
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 span appendWindowStart
, implementations MAY thus support gapless audio splicing.
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.
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.
The greater than check is needed because bidirectional prediction between coded frames can cause presentation timestamp to not be monotonically increasing even though the decode timestamps are monotonically increasing.
[[group end timestamp]]
, then set
[[group end timestamp]]
equal to frame end timestamp.[[generate timestamps flag]]
equals true, then set
timestampOffset
equal to frame end timestamp.If the HTMLMediaElement
.readyState
attribute is HAVE_METADATA
and the new coded frames cause HTMLMediaElement
.buffered
to have a TimeRanges
for the current playback position, then set the HTMLMediaElement
.readyState
attribute to HAVE_CURRENT_DATA
.
Per HTMLMediaElement ready states
[HTML] logic, HTMLMediaElement
.readyState
changes may trigger events on the HTMLMediaElement.
If the HTMLMediaElement
.readyState
attribute is HAVE_CURRENT_DATA
and the new coded frames cause HTMLMediaElement
.buffered
to have a TimeRanges
that includes the current playback position and some time beyond the current playback position, then set the HTMLMediaElement
.readyState
attribute to HAVE_FUTURE_DATA
.
Per HTMLMediaElement ready states
[HTML] logic, HTMLMediaElement
.readyState
changes may trigger events on the HTMLMediaElement.
If the HTMLMediaElement
.readyState
attribute is HAVE_FUTURE_DATA
and the new coded frames cause HTMLMediaElement
.buffered
to have a TimeRanges
that includes the current playback position and enough data to ensure uninterrupted playback, then set the HTMLMediaElement
.readyState
attribute to HAVE_ENOUGH_DATA
.
Per HTMLMediaElement ready states
[HTML] logic, HTMLMediaElement
.readyState
changes may trigger events on the HTMLMediaElement.
duration
, then run the
duration change algorithm with new duration set to the maximum of the current
duration and the [[group end timestamp]]
.Follow these steps when coded frames for a specific time range need to be removed from the SourceBuffer:
For each track buffer in this SourceBuffer
, run the following steps:
duration
If this track buffer has a random access point timestamp that is greater than or equal to end, then update remove end timestamp to that random access point timestamp.
Random access point timestamps can be different across tracks because the dependencies between coded frames within a track are usually different than the dependencies in another track.
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:
mode
equals "segments
":[[group end timestamp]]
to presentation timestamp.mode
equals "sequence
":[[group start timestamp]]
equal to the [[group end timestamp]]
.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.
If this object is in activeSourceBuffers
, the current playback position is greater than or equal to
start and less than the remove end timestamp, and HTMLMediaElement
.readyState
is greater than
HAVE_METADATA
, then set the HTMLMediaElement
.readyState
attribute to HAVE_METADATA
and stall playback.
Per HTMLMediaElement ready states
[HTML] logic, HTMLMediaElement
.readyState
changes may trigger events on the HTMLMediaElement.
This transition occurs because media data for the current position has been removed. Playback cannot progress until media for the current playback position is appended or the selected/enabled tracks change.
[[buffer full flag]]
equals true and this object is ready to accept more bytes,
then set the [[buffer full flag]]
to false.This algorithm is run to free up space in this SourceBuffer
when new data is appended.
Need to recognize step here that implementations MAY decide to set [[buffer full flag]]
true here if it predicts that processing new data in addition to any existing bytes in
[[input buffer]]
would exceed the capacity of the SourceBuffer
. Such a step enables
more proactive push-back from implementations before accepting new data which would overflow
resources, for example. In practice, at least one implementation already does this.
[[buffer full flag]]
equals false, then abort these steps.Implementations MAY use different methods for selecting removal ranges so web applications SHOULD NOT depend on a
specific behavior. The web application can use the buffered
attribute to observe whether portions of the buffered data have been evicted.
Follow these steps when the coded frame processing algorithm needs to generate a splice frame for two overlapping audio coded frames:
floor(x * sample_rate + 0.5) / sample_rate
).
For example, given the following values:
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)
Some implementations MAY apply fades to/from silence to coded frames on either side of the inserted silence to make the transition less jarring.
This is intended to allow new coded frame to be added to the track buffer as if overlapped frame had not been in the track buffer to begin with.
If the new coded frame is less than 5 milliseconds in duration, then coded frames that are appended after the new coded frame will be needed to properly render the splice.
See the audio splice rendering algorithm for details on how this splice frame is rendered.
The following steps are run when a spliced frame, generated by the audio splice frame algorithm, needs to be rendered by the media element:
Here is a graphical representation of this algorithm.
Follow these steps when the coded frame processing algorithm needs to generate a splice frame for two overlapping timed text coded frames:
This is intended to allow new coded frame to be added to the track buffer as if it hadn't overlapped any frames in track buffer to begin with.
SourceBufferList is a simple container object for SourceBuffer
objects. It provides read-only array access and fires events when the list is modified.
WebIDL[Exposed=(Window,DedicatedWorker)]
interface SourceBufferList
: EventTarget {
readonly attribute unsigned long length
;
attribute EventHandler onaddsourcebuffer
;
attribute EventHandler onremovesourcebuffer
;
getter
SourceBuffer
(unsigned long index);
};
length
of type unsigned long
, readonly Indicates the number of SourceBuffer
objects in the list.
onaddsourcebuffer
of type EventHandler
The event handler for the
event.addsourcebuffer
onremovesourcebuffer
of type EventHandler
The event handler for the
event.removesourcebuffer
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 |
✘ | ✘ |
SourceBuffer
When this method is invoked, the user agent must run the following steps:
length
attribute then return undefined and abort these steps.SourceBuffer
object in the list.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. |
This section specifies what existing attributes on the HTMLMediaElement
MUST return when a MediaSource
is attached to the element.
The HTMLMediaElement
.seekable
attribute returns a new static normalized TimeRanges object created based on the following steps:
MediaSource
was constructed in a
DedicatedWorkerGlobalScope
that is terminated or is closing
then return an empty TimeRanges
object and abort these steps.
This case is intended to handle implementations that may no
longer maintain any previous information about buffered or
seekable media in a MediaSource that was constructed in a
DedicatedWorkerGlobalScope that has been terminated by
terminate
()
or user agent execution of terminate a worker for the MediaSource's DedicatedWorkerGlobalScope, for
instance as the eventual result of
close
()
execution.
Should there be some (eventual) media element error transition in the case of an attached worker MediaSource having its context destroyed? The experimental Chromium implementation of worker MSE just keeps the element readyState, networkState and error the same as prior to that context destruction, though the seekable and buffered attributes each report an empty TimeRange.
duration
and [[live seekable range]]
,
determined as follows:
MediaSource
was constructed in a Window
duration
and set recent live seekable range to be
[[live seekable range]]
.duration
and [[live seekable range]]
were recently, updated by handling
implicit messages posted by the MediaSource
to its [[port to main]]
on every change
to duration
or [[live seekable range]]
.TimeRanges
object.HTMLMediaElement
.buffered
attribute.HTMLMediaElement
.buffered
attribute returns an empty TimeRanges
object, then return an empty TimeRanges
object and abort these steps.HTMLMediaElement
.buffered
attribute.
The HTMLMediaElement
.buffered
attribute returns a static normalized TimeRanges object based on the following steps.
MediaSource
was constructed in a
DedicatedWorkerGlobalScope
that is terminated or is closing
then return an empty TimeRanges
object and abort these steps.
This case is intended to handle implementations that may no
longer maintain any previous information about buffered or
seekable media in a MediaSource that was constructed in a
DedicatedWorkerGlobalScope that has been terminated by
terminate
()
or user agent execution of terminate a worker for the MediaSource's DedicatedWorkerGlobalScope, for
instance as the eventual result of
close
()
execution.
Should there be some (eventual) media element error transition in the case of an attached worker MediaSource having its context destroyed? The experimental Chromium implementation of worker MSE just keeps the element readyState, networkState and error the same as prior to that context destruction, though the seekable and buffered attributes each report an empty TimeRange.
MediaSource
was constructed in a Window
TimeRanges
object.activeSourceBuffers
.length does not equal 0 then run the following steps:
buffered
for each SourceBuffer
object in
activeSourceBuffers
.TimeRanges
object containing
a single range from 0 to highest end time.SourceBuffer
object in activeSourceBuffers
run the following
steps:
buffered
attribute on the current SourceBuffer
.readyState
is "ended
", then set the end time on the last
range in source ranges to highest end time.TimeRanges
resulting from the steps for the
Window
case, but run with the MediaSource
and its SourceBuffer
objects in their
DedicatedWorkerGlobalScope
and communicated by using [[port to main]]
implicit
messages on every update to the activeSourceBuffers
, readyState
, or any of
the buffering state that would change any of the values of each of those buffered
attributes of the activeSourceBuffers
.
The overhead of recalculating and communicating recent intersection ranges so frequently is one reason for allowing implementation flexibility to query this information on-demand using other mechanisms such as shared memory and locks as mentioned in cross-context communication model.
This section specifies extensions to the [HTML] AudioTrack
definition.
WebIDL[Exposed=(Window,DedicatedWorker)]
partial interface AudioTrack {
readonly attribute SourceBuffer
? sourceBuffer
;
};
AudioTrack
needs Window+DedicatedWorker
exposure.sourceBuffer
of type SourceBuffer
, readonly , nullableOn getting, run the following step:
SourceBuffer
that was created on the same realm
as this track, and if that SourceBuffer
has not been removed from the sourceBuffers
attribute of its parent media source:SourceBuffer
that created this track.DedicatedWorkerGlobalScope
SourceBuffer
notified its internal create track
mirror
handler in Window
to create this track, then the Window
copy of the track would
return null for this attribute.
This section specifies extensions to the [HTML] VideoTrack
definition.
WebIDL[Exposed=(Window,DedicatedWorker)]
partial interface VideoTrack {
readonly attribute SourceBuffer
? sourceBuffer
;
};
VideoTrack
needs Window+DedicatedWorker
exposure.sourceBuffer
of type SourceBuffer
, readonly , nullableOn getting, run the following step:
SourceBuffer
that was created on the same realm
as this track, and if that SourceBuffer
has not been removed from the sourceBuffers
attribute of its parent media source:SourceBuffer
that created this track.DedicatedWorkerGlobalScope
SourceBuffer
notified its internal create track
mirror
handler in Window
to create this track, then the Window
copy of the track would
return null for this attribute.
This section specifies extensions to the [HTML] TextTrack
definition.
WebIDL[Exposed=(Window,DedicatedWorker)]
partial interface TextTrack {
readonly attribute SourceBuffer
? sourceBuffer
;
};
sourceBuffer
of type SourceBuffer
, readonly , nullableOn getting, run the following step:
SourceBuffer
that was created on the same realm
as this track, and if that SourceBuffer
has not been removed from the sourceBuffers
attribute of its parent media source:SourceBuffer
that created this track.DedicatedWorkerGlobalScope
SourceBuffer
notified its internal create track
mirror
handler in Window
to create this track, then the Window
copy of the track would
return null for this attribute.
The bytes provided through appendBuffer
()
for a SourceBuffer
form a logical byte stream. The format and
semantics of these byte streams are defined in byte stream format specifications.
The byte stream format registry [MSE-REGISTRY] provides mappings between a MIME type that may be passed to addSourceBuffer
()
,
isTypeSupported
()
or changeType
()
and the byte stream format expected by a SourceBuffer
using that
MIME type for parsing newly appended data. 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, its SourceBuffer
implementation MUST conform to the
byte stream format specification listed in the registry entry.
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.
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:
AudioTrack
, VideoTrack
, and TextTrack
attribute values
from data in initialization segments.
If the byte stream format covers a format similar to one covered in the in-band tracks spec [INBANDTRACKS], then it SHOULD try to use the same attribute mappings so that Media Source Extensions playback and non-Media Source Extensions playback provide the same track information.
The number and type of tracks are not consistent.
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.
Unsupported codec changes occur across initialization segments.
See the initialization segment received algorithm, addSourceBuffer
()
and changeType
()
for details and examples of codec changes.
Video frame size changes. The user agent MUST support seamless playback.
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.
Audio channel count changes. The user agent MAY support this seamlessly and could trigger downmixing.
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.
buffered
attribute.
This is intended to simplify switching between audio streams where the frame boundaries don't always line up across encodings (e.g., Vorbis).
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 constraints MAY be defined, for example to simplify implementation.
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 in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
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>
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 draftVideoPlaybackQuality
object and the HTMLVideoElement
extension method getVideoPlaybackQuality()
described in those previous revisions.
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