Copyright © 2024 World Wide Web Consortium. 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, substantive changes since publication as a W3C Recommendation in November 2016 are:
changeType
()
method to switch among codecs or
bytestreams
MediaSource
objects off the main thread in
dedicated workers
createObjectURL
()
extension to the URL
object following
its integration in the File API [FILEAPI]
ManagedMediaSource
, ManagedSourceBuffer
, and
BufferedChangeEvent
interfaces supporting power-efficient streaming and active buffered
media cleanup by the user agent
For a full list of changes made 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 03 November 2023 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 URL created by
createObjectURL
()
. It is used to attach a MediaSource
object to an
HTMLMediaElement.
These URLs are the same as a blob URLs, 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
's
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.
SourceBuffer
configuration
A specific set of tracks distributed across one or more SourceBuffer
objects
owned by a single MediaSource
instance.
Implementations MUST support at least 1 MediaSource
object with the following
configurations:
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
interface 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 10.
HTMLMediaElement Extensions
to modify HTMLMediaElement
's seekable
behavior.
Each MediaSource
object has a [[has ever been
attached]] internal slot that stores a boolean
. It is initialized to false when
the MediaSource
object is created, and is set true in the extended
HTMLMediaElement
's resource fetch algorithm as described in the attaching to a media element algorithm. The extended resource fetch algorithm uses this internal
slot to conditionally fail attachment of a MediaSource
using a MediaSourceHandle
set on a HTMLMediaElement
's srcObject
attribute.
WebIDLenum ReadyState
{
"closed
",
"open
",
"ended
",
};
closed
open
SourceBuffer
objects in MediaSource
's sourceBuffers
.
ended
MediaSource
's
endOfStream
()
has been called.
WebIDLenum EndOfStreamError
{
"network
",
"decode
",
};
network
Terminates playback and signals that a network error has occurred.
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.
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
();
[SameObject, Exposed=DedicatedWorker]
readonly attribute MediaSourceHandle
handle
;
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
();
static boolean isTypeSupported
(DOMString type);
};
Contains a handle useful for attachment of a dedicated worker MediaSource
object to an
HTMLMediaElement
via srcObject
. The handle remains the same object
for this MediaSource
object across accesses of this attribute, but it is distinct for
each MediaSource
object.
This specification may eventually enable visibility of this attribute on MediaSource
objects on the main Window context. If so, specification care will be necessary to prevent
potential backwards incompatible changes, such as could happen if exceptions were thrown on
accesses to this attribute.
On getting, run the following steps:
MediaSource
object has not yet been created, then run the
following steps:
MediaSourceHandle
object and associated resources, linked internally to this
MediaSource
.
MediaSourceHandle
object that is this attribute's value.
Contains the list of SourceBuffer
objects associated with this MediaSource
. When
MediaSource
's readyState
equals "closed
" this list
will be empty. Once readyState
transitions to "open
"
SourceBuffer objects can be added to this list by using addSourceBuffer
()
.
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].
Section 3.15.5 Changes to selected/enabled track state describes how this attribute gets updated.
Indicates the current state of the MediaSource
object. When the MediaSource
is created readyState
MUST be set to "closed
".
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.
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.
Adds a new SourceBuffer
to sourceBuffers
.
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.
ManagedSourceBuffer
if this is a
ManagedMediaSource
, or a SourceBuffer
otherwise, with their respective associated
resources.
[[generate timestamps flag]]
to the value in the
"Generate Timestamps Flag" column of the Media Source Extensions™ Byte Stream Format Registry entry that is associated with
type.
[[generate timestamps flag]]
is true, set buffer's
mode
to "sequence
". Otherwise, set buffer's
mode
to "segments
".
sourceBuffers
.
addsourcebuffer
at this's
sourceBuffers
.
Removes a SourceBuffer
from sourceBuffers
.
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
.
Signals the end of the stream.
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.
Updates [[live seekable range]]
that is used in section
10.
HTMLMediaElement Extensions to modify HTMLMediaElement
's
seekable
behavior.
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.
Updates [[live seekable range]]
that is used in section
10.
HTMLMediaElement Extensions to modify HTMLMediaElement
's
seekable
behavior.
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.
Check 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
's
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.
When this method is invoked, the user agent must run the following steps:
Event name | Interface | Dispatched when... |
---|---|---|
sourceopen |
Event
|
MediaSource 's readyState transitions from "closed "
to "open " or from "ended " to "open ".
|
sourceended |
Event
|
MediaSource 's readyState transitions from "open "
to "ended ".
|
sourceclose |
Event
|
MediaSource 's 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.
The rest of this section describes a model for bounding information latency for
attachments of a 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.
An 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 fetch algorithm, 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 only be done by obtaining a handle from it using
handle
, 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.
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 continue the execution
of the resource fetch algorithm.
The first step of the resource fetch algorithm 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.
At the beginning of the "Otherwise (mode is local)" section of the resource fetch algorithm, execute the additional steps, below.
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
object, a MediaSourceHandle
object or a URL record whose
object is a MediaSource
object, then:
MediaSource
that was constructed in a DedicatedWorkerGlobalScope
, such as would occur if
attempting to use a MediaSource object URL from a
DedicatedWorkerGlobalScope
MediaSource
MediaSource
's handle
from the
DedicatedWorker to the Window context and assigning it to the media element's
srcObject
attribute is the only way to attach such a
MediaSource.
MediaSourceHandle
whose
[[Detached]]
internal slot is true
MediaSourceHandle
whose underlying
MediaSource
's [[has ever been attached]]
internal slot is
true
MediaSource
more than once using a
MediaSourceHandle
, even if the MediaSource
was constructed on
Window
and had been loaded previously using a MediaSource object URL.
This doesn't preclude subsequent use of a MediaSource object URL for a
Window
MediaSource
from succeeding though.
readyState
is NOT set to "closed
"
MediaSource
's [[has ever been attached]]
internal slot to true.
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
".
ManagedMediaSource
, then set streaming
attribute to false
.
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
's buffered
attribute
has all necessary media segments buffered for that position.
TimeRanges
of HTMLMediaElement
's
buffered
HTMLMediaElement
's readyState
attribute is
greater than HAVE_METADATA
, then set the
HTMLMediaElement
's readyState
attribute to
HAVE_METADATA
.
Per HTMLMediaElement ready states
[HTML] logic, HTMLMediaElement
's
readyState
changes may trigger events on the
HTMLMediaElement.
appendBuffer
()
call
causes the coded frame processing algorithm to set the
HTMLMediaElement
's readyState
attribute to a value
greater than HAVE_METADATA
.
The web application can use buffered
and
HTMLMediaElement
's 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
's 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
's 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
's readyState
attribute equals
HAVE_NOTHING
:
HTMLMediaElement
's buffered
does not contain a
TimeRanges
for the current playback position:
HTMLMediaElement
's readyState
attribute to
HAVE_METADATA
.
Per HTMLMediaElement ready states
[HTML] logic, HTMLMediaElement
's
readyState
changes may trigger events on the
HTMLMediaElement.
HTMLMediaElement
's buffered
contains a TimeRanges
that includes the current playback position and enough data to ensure uninterrupted playback:
HTMLMediaElement
's readyState
attribute to
HAVE_ENOUGH_DATA
.
Per HTMLMediaElement ready states
[HTML] logic, HTMLMediaElement
's
readyState
changes may trigger events on the
HTMLMediaElement.
HAVE_CURRENT_DATA
.
HTMLMediaElement
's buffered
contains a TimeRanges
that includes the current playback position and some time beyond the current playback
position, then run the following steps:
HTMLMediaElement
's readyState
attribute to
HAVE_FUTURE_DATA
.
Per HTMLMediaElement ready states
[HTML] logic, HTMLMediaElement
's
readyState
changes may trigger events on the
HTMLMediaElement.
HAVE_CURRENT_DATA
.
HTMLMediaElement
's 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
's readyState
attribute to
HAVE_CURRENT_DATA
.
Per HTMLMediaElement ready states
[HTML] logic, HTMLMediaElement
's
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
's readyState
attribute
equals HAVE_NOTHING
HTMLMediaElement
's readyState
attribute is
greater than HAVE_NOTHING
decode
"
Window
:
HTMLMediaElement
's readyState
attribute
equals HAVE_NOTHING
HTMLMediaElement
's 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, and
DedicatedWorkerGlobalScope
.
The MediaSourceHandle
interface 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.
Each MediaSourceHandle
object has a [[has ever
been assigned as srcobject]] internal slot that stores a boolean
. It is
initialized to false when the MediaSourceHandle
object is created, is set true in the
extended HTMLMediaElement
's srcObject
setter as described in
section 10.
HTMLMediaElement Extensions, and if true, prevents successful transfer of
the MediaSourceHandle
as described in section 4.1
Transfer.
MediaSourceHandle
objects are Transferable
, each having a [[Detached]] internal slot that is used to ensure that once the
handle object instance has been transferred, that instance cannot be transferred again.
WebIDL[Transferable, Exposed=(Window,DedicatedWorker)]
interface MediaSourceHandle
{};
The 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.
Implementors should be aware that assumption of "move" semantics implied by
Transferable
is not always reality. For example, extensions or internal
implementations of postMessage using broadcast may cause unintended multiple recipients
of a transferred MediaSourceHandle
. For this reason, implementations are guided to
not resolve which potential clone of a transferred MediaSourceHandle
is still valid
for attachment until and unless any handle for the underlying MediaSource
object is
used in the asynchronous portion of the media element's resource selection algorithm.
This is similar to the existing behavior for attachment via MediaSource object URLs,
which can be cloned easily, where such a URL is valid for at most one attachment start
(across all of its potentially many clones).
Implementations MUST support at most one attachment (load) via
srcObject
ever for the MediaSource
object underlying a
MediaSourceHandle
, regardless of potential cloning of the MediaSourceHandle
due
to varying implementations of Transferable
.
See attaching to a media element for how this is enforced during the asynchronous portion of the media element's resource selection algorithm.
MediaSourceHandle
is only exposed on Window
and DedicatedWorkerGlobalScope
contexts, and cannot successfully transfer between different agent clusters [ECMASCRIPT]. Transfer of a MediaSourceHandle
object can only succeed
within the same agent cluster.
For example, transfer of a MediaSourceHandle
object from either a Window
or
DedicatedWorkerGlobalScope
to either a SharedWorker or a ServiceWorker will not
succeed. Developers should be aware of this difference versus MediaSource object URLs
which are DOMString
s that can be communicated many ways. Even so, attaching to a media element using a MediaSource object URL can only succeed for a MediaSource
that was constructed in a Window
context. See also the integration of the
agent and agent cluster formalisms for Web Application APIs
[HTML] where related concepts such as dedicated worker agents are defined.
Transfer steps for a MediaSourceHandle
object MUST include the following step:
MediaSourceHandle
's [[has ever been assigned as srcobject]]
internal slot is true, then the transfer steps must fail by throwing a
DataCloneError
exception.
WebIDLenum AppendMode
{
"segments
",
"sequence
",
};
segments
sequence
timestampOffset
attribute will be updated if a new offset is needed to make the new media segments
adjacent to the previous media segment. Setting the timestampOffset
attribute in "sequence
" mode allows a media segment to be placed at a
specific position in the timeline without any knowledge of the timestamps in the media
segment.
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 updatestart
event.
onupdate
of type EventHandler
The event handler for the update
event.
onupdateend
of type EventHandler
The event handler for the updateend
event.
onerror
of type EventHandler
The event handler for the error
event.
onabort
of type EventHandler
The event handler for the abort
event.
appendBuffer
Appends the segment data in an BufferSource
[WEBIDL] to
the SourceBuffer
.
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.
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.
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. The start of the removal range, in seconds measured from presentation start time The end of the removal range, in seconds measured from presentation start time.
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
's buffered
, and MUST therefore
support uninterrupted playback within each range of HTMLMediaElement
's
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
|
SourceBuffer 's updating transitions from false to true.
|
update |
Event
|
A SourceBuffer 's append or remove successfully completed. SourceBuffer 's
updating transitions from true to false.
|
updateend |
Event
|
The append or remove of a SourceBuffer ended.
|
error |
Event
|
An error occurred during the append to a SourceBuffer . updating
transitions from true to false.
|
abort |
Event
|
The SourceBuffer 's 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
's
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
's
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
's
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
's readyState
attribute is
greater than HAVE_CURRENT_DATA
, then set the
HTMLMediaElement
's readyState
attribute to
HAVE_METADATA
.
Per HTMLMediaElement ready states
[HTML] logic, HTMLMediaElement
's
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
's readyState
attribute is
HAVE_NOTHING
, then set the HTMLMediaElement
's
readyState
attribute to HAVE_METADATA
.
Per HTMLMediaElement ready states
[HTML] logic, HTMLMediaElement
's
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
's readyState
attribute is
HAVE_METADATA
and the new coded frames cause
HTMLMediaElement
's buffered
to have a TimeRanges
for
the current playback position, then set the HTMLMediaElement
's
readyState
attribute to
HAVE_CURRENT_DATA
.
Per HTMLMediaElement ready states
[HTML] logic, HTMLMediaElement
's
readyState
changes may trigger events on the HTMLMediaElement.
If the HTMLMediaElement
's readyState
attribute is
HAVE_CURRENT_DATA
and the new coded frames cause
HTMLMediaElement
's buffered
to have a TimeRanges
that
includes the current playback position and some time beyond the current playback
position, then set the HTMLMediaElement
's readyState
attribute to HAVE_FUTURE_DATA
.
Per HTMLMediaElement ready states
[HTML] logic, HTMLMediaElement
's
readyState
changes may trigger events on the HTMLMediaElement.
If the HTMLMediaElement
's readyState
attribute is
HAVE_FUTURE_DATA
and the new coded frames cause
HTMLMediaElement
's buffered
to have a TimeRanges
that
includes the current playback position and enough data to ensure uninterrupted playback, then set the HTMLMediaElement
's readyState
attribute to HAVE_ENOUGH_DATA
.
Per HTMLMediaElement ready states
[HTML] logic, HTMLMediaElement
's
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
's
readyState
is greater than
HAVE_METADATA
, then set the HTMLMediaElement
's
readyState
attribute to HAVE_METADATA
and stall playback.
Per HTMLMediaElement ready states
[HTML] logic, HTMLMediaElement
's
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 3.15.5 Changes to selected/enabled track state.
[[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 addsourcebuffer
event.
onremovesourcebuffer
of type EventHandler
The event handler for the removesourcebuffer
event.
Allows the SourceBuffer objects in the list to be accessed with an array operator (i.e., []).
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.
|
A ManagedMediaSource
is a MediaSource
that actively manages its memory content.
Unlike a MediaSource
, the user agent can evict content through the
memory cleanup algorithm from its sourceBuffers
(populated with ManagedSourceBuffer
) for any reason.
WebIDL[Exposed=(Window,DedicatedWorker)]
interface ManagedMediaSource
: MediaSource
{
constructor
();
readonly attribute boolean streaming
;
attribute EventHandler onstartstreaming
;
attribute EventHandler onendstreaming
;
};
streaming
On getting:
Event name | Interface | Dispatched when... |
---|---|---|
startstreaming |
Event
|
A ManagedMediaSource 's streaming attribute changed from
false to true .
|
endstreaming |
Event
|
A ManagedMediaSource 's streaming attribute changed from
true to false .
|
The following steps are run periodically, whenever the SourceBuffer Monitoring algorithm is scheduled to run.
Having enough managed data to ensure uninterrupted playback is an implementation
defined 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 value of
streaming
. These transitions indicate when the user agent believes
it has enough data buffered or it needs more data respectively.
Being able to retrieve and buffer data in an efficient way is an implementation defined condition where the user agent determines that it can fetch new data in an energy efficient manner while able to achieve the desired memory usage.
MediaSource
SourceBuffer Monitoring algorithm.
buffered
attribute contains a TimeRanges
that includes the current
playback position and enough managed data to ensure uninterrupted playback and is
able to retrieve and buffer data in an efficient way
streaming
, queue an element task on the media element
that runs the following steps:
streaming
attribute to can play
uninterrupted and efficiently.
startstreaming
at the ManagedMediaSource
.
endstreaming
at the
ManagedMediaSource
.
sourceBuffers
:
WebIDL[Exposed=(Window,DedicatedWorker)]
interface BufferedChangeEvent
: Event {
constructor
(DOMString type, optional BufferedChangeEventInit
eventInitDict = {});
[SameObject] readonly attribute TimeRanges addedRanges
;
[SameObject] readonly attribute TimeRanges removedRanges
;
};
dictionary BufferedChangeEventInit
: EventInit {
TimeRanges addedRanges
;
TimeRanges removedRanges
;
};
addedRanges
updatestart
and updateend
events (which
would have occurred during the last run of the coded frame processing algorithm).
removedRanges
updatestart
and updateend
events (which
would have occurred during the last run of the coded frame removal or coded frame eviction algorithm or if the user agent evicted content in response to a
memory cleanup).
WebIDL[Exposed=(Window,DedicatedWorker)]
interface ManagedSourceBuffer
: SourceBuffer
{
attribute EventHandler onbufferedchange
;
};
onbufferedchange
An event handler IDL attribute whose event handler event type is
bufferedchange
.
Event name | Interface | Dispatched when... |
---|---|---|
bufferedchange |
BufferedChangeEvent
|
The ManagedSourceBuffer 's buffered range changed following a call to
appendBuffer () , remove () ,
endOfStream () , or as a consequence of the user agent running the
memory cleanup algorithm.
|
The following steps are run at the completion of all operations to the
ManagedSourceBuffer
buffer that would cause a buffer's
buffered
to change. That is once appendBuffer
()
,
remove
()
or memory cleanup algorithm have
completed.
buffered
attribute before the changes occurred.
buffered
TimeRanges
.
BufferedChangeEventInit
dictionary initialized with
added as its addedRanges
and removed as its
removedRanges
bufferedchange
at buffer using the
BufferedChangeEvent
interface, initialized with eventInitDict.
ManagedMediaSource
parent
activeSourceBuffers
:
currentTime
until such presentation could be retrieved again.
Implementations can use different strategies for selecting removal ranges so web
applications shouldn't depend on a specific behavior. The web application would listen
to the bufferedchange
event to observe whether portions of the buffered data have
been evicted.
This section specifies what existing HTMLMediaElement
's seekable
and HTMLMediaElement
's buffered
attributes on the
HTMLMediaElement
MUST return when a MediaSource
is attached to the element, and
what the existing HTMLMediaElement
's srcObject
attribute MUST also
do when it is set to be a MediaSourceHandle
object.
HTMLMediaElement
's seekable
The HTMLMediaElement
's 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
's buffered
attribute.
HTMLMediaElement
's buffered
attribute returns
an empty TimeRanges
object, then return an empty TimeRanges
object and
abort these steps.
HTMLMediaElement
's
buffered
attribute.
HTMLMediaElement
's buffered
The HTMLMediaElement
's 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.
HTMLMediaElement
's srcObject
If a HTMLMediaElement
's srcObject
attribute is assigned a
MediaSourceHandle
, then set [[has ever been assigned as srcobject]]
for that MediaSourceHandle
to true as part of the synchronous steps of
the extended HTMLMediaElement
's srcObject
setter that occur
before invoking the element's load algorithm.
This prevents transferring that MediaSourceHandle
object ever again, enabling clear
synchronous exception if that is attempted.
MediaSourceHandle
needs to be added to HTMLMediaElement
's MediaProvider IDL
typedef and related text involving media provider objects.
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 , nullable
On 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 , nullable
On 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 , nullable
On 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.
<video id="v" autoplay></video>
<script>
const video = document.getElementById("v");
const mediaSource = new MediaSource();
mediaSource.addEventListener("sourceopen", onSourceOpen);
video.src = window.URL.createObjectURL(mediaSource);
async function onSourceOpen(e) {
const mediaSource = e.target;
if (mediaSource.sourceBuffers.length > 0) return;
const sourceBuffer = mediaSource.addSourceBuffer(
'video/webm; codecs="vorbis,vp8"',
);
video.addEventListener("seeking", (e) => onSeeking(mediaSource, e.target));
video.addEventListener("progress", () =>
appendNextMediaSegment(mediaSource),
);
try {
const initSegment = await getInitializationSegment();
if (initSegment == null) {
// Error fetching the initialization segment. Signal end of stream with an error.
mediaSource.endOfStream("network");
return;
}
// Append the initialization segment.
sourceBuffer.addEventListener("updateend", function firstAppendHandler() {
sourceBuffer.removeEventListener("updateend", firstAppendHandler);
// Append some initial media data.
appendNextMediaSegment(mediaSource);
});
sourceBuffer.appendBuffer(initSegment);
} catch (error) {
// Handle errors that might occur during initialization segment fetching.
console.error("Error fetching initialization segment:", error);
mediaSource.endOfStream("network");
}
}
async function appendNextMediaSegment(mediaSource) {
if (
mediaSource.readyState === "closed" ||
mediaSource.sourceBuffers[0].updating
)
return;
// If we have run out of stream data, then signal end of stream.
if (!haveMoreMediaSegments()) {
mediaSource.endOfStream();
return;
}
try {
const mediaSegment = await getNextMediaSegment();
// 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);
}
catch (error) {
// Handle errors that might occur during media segment fetching.
console.error("Error fetching media segment:", error);
mediaSource.endOfStream("network");
}
}
function onSeeking(mediaSource, video) {
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);
}
// Example of async function for getting initialization segment
async function getInitializationSegment() {
// Implement fetching of the initialization segment
// This is just a placeholder function
}
// Example function for checking if there are more media segments
function haveMoreMediaSegments() {
// Implement logic to determine if there are more media segments
// This is just a placeholder function
}
// Example function for getting the next media segment
async function getNextMediaSegment() {
// Implement fetching of the next media segment
// This is just a placeholder function
}
// Example function for seeking to a specific media segment
function seekToMediaSegmentAt(currentTime) {
// Implement seeking logic
// This is just a placeholder function
}
</script>
<script>
async function setUpVideoStream() {
// Specific video format and codec
const mediaType = 'video/mp4; codecs="mp4a.40.2,avc1.4d4015"';
// Check if the type of video format / codec is supported.
if (!window.ManagedMediaSource?.isTypeSupported(mediaType)) {
return; // Not supported, do something else.
}
// Set up video and its managed source.
const video = document.createElement("video");
const source = new ManagedMediaSource();
video.controls = true;
await new Promise((resolve) => {
video.src = URL.createObjectURL(source);
source.addEventListener("sourceopen", resolve, { once: true });
document.body.appendChild(video);
});
const sourceBuffer = source.addSourceBuffer(mediaType);
// Set up the event handlers
sourceBuffer.onbufferedchange = (e) => {
console.log("onbufferedchange event fired.");
console.log(`Added Ranges: ${timeRangesToString(e.addedRanges)}`);
console.log(`Removed Ranges: ${timeRangesToString(e.removedRanges)}`);
};
source.onstartstreaming = async () => {
const response = await fetch("./videos/bipbop.mp4");
const buffer = await response.arrayBuffer();
await new Promise((resolve) => {
sourceBuffer.addEventListener("updateend", resolve, { once: true });
sourceBuffer.appendBuffer(buffer);
});
};
source.onendstreaming = async () => {
// Stop fetching new segments here
};
}
// Helper function...
function timeRangesToString(timeRanges) {
const ranges = [];
for (let i = 0; i < timeRanges.length; i++) {
ranges.push([timeRanges.start(i), timeRanges.end(i)]);
}
return "[" + ranges.map(([start, end]) => `[${start}, ${end})` ) + "]";
}
</script>
<body onload="setUpVideoStream()"></body>
The editors would like to thank Alex Giladi, Bob Lund, Chris Needham, Chris Poole, Chris Wilson, Cyril Concolato, Dale Curtis, David Dorwin, David Singer, Duncan Rowden, François Daoust, Frank Galligan, Glenn Adams, Jer Noble, Joe Steele, John Simmons, Kagami Sascha Rosylight, Kevin Streeter, Marcos Cáceres, Mark Vickers, Matt Ward, Matthew Gregan, Michael(tm) Smith, Michael Thornburgh, Mounir Lamouri, Paul Adenot, Philip Jägenstedt, Philippe Le Hegaret, Pierre Lemieux, Ralph Giles, Steven Robertson, and Tatsuya Igarashi for their contributions to this specification.
This section is non-normative.
The video playback quality metrics described in previous revisions of this specification
(e.g., sections 5 and 10 of the Candidate Recommendation) are
now being developed as part of [MEDIA-PLAYBACK-QUALITY]. Some implementations may have
implemented the earlier draft VideoPlaybackQuality
object and the HTMLVideoElement
extension method getVideoPlaybackQuality
()
described in those previous
revisions.
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