This specification extends 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.
Status of This Document
This section describes the status of this
document at the time of its publication. A list of current W3C
publications and the latest revision of this technical report can be found
in the W3C technical reports index at
https://www.w3.org/TR/.
On top of editorial updates, substantives changes since publication as a W3C Recommendation in November 2016 are the addition of a changeType() method to switch codecs, the possibility to create and use MediaSource objects off the main thread in dedicated workers, and the removal of the createObjectURL() extension to the URL object following its integration in the File API [FILEAPI]. For a full list of changes done since the previous version, see the commits.
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.
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
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Essential Claim(s)
must disclose the information in accordance with
section 6 of the W3C Patent Policy.
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].
1.1 Goals
This specification was designed with the following goals in mind:
Allow JavaScript to construct media streams independent of how the media is fetched.
Define a splicing and buffering model that facilitates use cases like adaptive streaming, ad-insertion, time-shifting, and video editing.
Minimize the need for media parsing in JavaScript.
Leverage the browser cache as much as possible.
Provide requirements for byte stream format specifications.
Not require support for any particular media format or codec.
This specification defines:
Normative behavior for user agents to enable interoperability between user agents and web applications when processing media data.
Normative requirements to enable other specifications to define media formats to be used within this specification.
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.
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.
A group of coded frames that are adjacent and have monotonically increasing decode timestamps without any gaps. Discontinuities detected by the
coded frame processing algorithm and abort() calls trigger the start of a new coded frame group.
Decode Timestamp
The decode timestamp indicates the latest time at which the frame needs to be decoded assuming instantaneous decoding and rendering of this and any dependant frames (this is equal to 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.
Initialization Segment
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).
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.
These URLs are the same as a Blob URI, except that anything in the definition of that feature that refers to File and Blob objects is hereby extended to also apply to MediaSource objects.
The 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.
Note
For the purposes of determining if HTMLMediaElement.buffered contains a TimeRanges that includes the current playback position, implementations MAY choose to allow a current playback position at or after presentation start time and before the first TimeRanges to play the first TimeRanges if that TimeRanges starts within a reasonably short time, like 1 second, after presentation start time. This allowance accommodates the reality that muxed streams commonly do not begin all tracks precisely at presentation start time. Implementations MUST report the actual buffered range, regardless of this allowance.
Presentation Interval
The presentation interval of a coded frame is the time interval from its presentation timestamp to the presentation timestamp plus the coded frame's duration. For example, if a coded frame has a presentation timestamp of 10 seconds and 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.
Presentation Order
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.
Presentation Timestamp
A reference to a specific time in the presentation. The presentation timestamp in a coded frame indicates when the frame SHOULD be rendered.
Random Access Point
A position in a media segment where decoding and continuous playback can begin without relying on any previous data in the segment. For video this tends to be the location of I-frames. In the case of audio, most audio frames can be treated as a random access point. Since video tracks tend to have a more sparse distribution of random access points, the location of these points are usually considered the random access points for multiplexed streams.
A specific set of tracks distributed across one or more SourceBuffer
objects owned by a single MediaSource instance.
Implementations MUST support at least 1 MediaSource object with the following
configurations:
A single SourceBuffer with 1 audio track and/or 1 video track.
Two SourceBuffers with one handling a single audio track and the other handling a single video track.
MediaSource objects MUST support each of the configurations above, but they are only
required to support one configuration at a time. Supporting multiple configurations at once
or additional configurations is a quality of implementation issue.
Track Description
A byte stream format specific structure that provides the Track ID, codec configuration, and other metadata for a single track. Each track description inside a 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.
Track ID
A Track ID is a byte stream format specific identifier that marks sections of the byte stream as being part of a specific track. The Track ID in a track description identifies which sections of a media segment belong to that track.
2. MediaSource Object
The MediaSource object represents a source of media data for an HTMLMediaElement. It keeps track of the readyState for this source as well as a list of SourceBuffer objects that can be used to add media data to the presentation. MediaSource objects are created by the web application and then attached to an HTMLMediaElement. The application uses the SourceBuffer objects in sourceBuffers to add media data to this source. The HTMLMediaElement fetches this media data from the MediaSource object when it is needed during playback.
Indicates the source is not currently attached to a media element.
open
The source has been opened by a media element and is ready for data to be appended to the SourceBuffer objects in sourceBuffers.
ended
The source is still attached to a media element, but endOfStream() has been called.
Issue 276: MSE-in-Workers: Consider adding a "closing" readyState to explain new `InvalidStateError` exception when closing underway
Consider adding a "closing" ReadyState to indicate the source is in the process of being concurrently detached from a media element. This would be useful for some implementations of MediaSource and SourceBuffer in DedicatedWorkerGlobalScope.
Terminates playback and signals that a network error has occurred.
Note
JavaScript applications SHOULD use this status code to terminate playback with a network error. For example, if a network error occurs while fetching media data.
decode
Terminates playback and signals that a decoding error has occurred.
Note
JavaScript applications SHOULD use this status code to terminate playback with a decode error. For example, if a parsing error occurs while processing out-of-band media data.
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.
Note
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:
If the handle for this MediaSource object has not yet been created, then run the following steps:
Let created handle be the result of creating a new MediaSourceHandle object
and associated resources, linked internally to this MediaSource.
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].
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.
When this method is invoked, the user agent must run the following steps:
If type is an empty string then throw a TypeError exception and abort these steps.
If type contains a MIME type that is not supported or contains a MIME type that is not supported with the types specified for the other SourceBuffer objects in sourceBuffers, then throw a NotSupportedError exception and abort these steps.
If the user agent can't handle any more SourceBuffer objects or if creating a SourceBuffer
based on type would result in an unsupported SourceBuffer configuration,
then throw a QuotaExceededError exception and abort these steps.
Note
For example, a user agent MAY throw a QuotaExceededError exception if the media element has reached the
HAVE_METADATA readyState. This can occur if the user agent's media engine does not support adding more tracks during
playback.
Create a new SourceBuffer object and associated resources.
Set the [[generate timestamps flag]] on the new object to the value in the "Generate
Timestamps Flag" column of the byte stream format registry [MSE-REGISTRY] entry that is associated with
type.
The start of the range, in seconds measured from presentation start time. While set, and if duration equals positive Infinity, HTMLMediaElement.seekable will return a non-empty TimeRanges object with a lowest range start timestamp no greater than start.
The end of range, in seconds measured from presentation start time. While set, and if duration equals positive Infinity, HTMLMediaElement.seekable will return a non-empty TimeRanges object with a highest range end timestamp no less than end.
Check to see whether the MediaSource is capable of creating SourceBuffer objects for the specified MIME type.
Note
If true is returned from this method, it only indicates that the MediaSource implementation is capable of creating SourceBuffer objects for the specified MIME type. An addSourceBuffer() call SHOULD still fail if sufficient resources are not available to support the addition of a new SourceBuffer.
Note
This method returning true implies that HTMLMediaElement.canPlayType() will return "maybe" or "probably" since it does not make sense for a MediaSource to support a type the HTMLMediaElement knows it cannot play.
The rest of this section describes a model for bounding information latency for attachments of a Window
media element to a DedicatedWorkerGlobalScopeMediaSource. 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 WindowMediaSource synchronously have the information already without
communicating it across contexts.
Algorithms in this specification that need to communicate information from a WindowHTMLMediaElement
to an attached DedicatedWorkerGlobalScopeMediaSource, 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.
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.
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.
Note
Relative to the action which triggered the media element's resource selection algorithm, these
steps are asynchronous. The resource fetch algorithm is run after the task that invoked the resource
selection algorithm is allowed to continue and a stable state is reached. Implementations may delay the steps
in the "Otherwise" clause, below, until the MediaSource object is ready for use.
This prevents using MediaSource object URLs for DedicatedWorker MediaSource
attachments. Transferring MediaSource.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.
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.
2.5.2 Detaching from a media element
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.
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.
2.5.3 Seeking
Run the following steps as part of the "Wait until the user agent has established whether or not the media data for the new playback position is available, and, if it is, until it has decoded enough data to play back that position" step of the seek algorithm:
The web application can use buffered and HTMLMediaElement.buffered to determine what the media element needs to resume playback.
Otherwise
Continue
Note
If the readyState attribute is "ended" and the new playback position is within a TimeRanges currently in HTMLMediaElement.buffered, then the seek operation must continue to completion here even if one or more currently selected or enabled track buffers' largest range end timestamp is less than new playback position. This condition should only occur due to logic in buffered when readyState is "ended".
The media element resets all decoders and initializes each one with data from the appropriate initialization segment.
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.
Note
An implementation MAY choose to use bytes buffered, time buffered, the append rate, or any other metric it sees fit to
determine when it has enough data. The metrics used MAY change during playback so web applications SHOULD only rely on the value of
HTMLMediaElement.readyState to determine whether more data is needed or not.
Note
When the media element needs more data, the user agent SHOULD transition it from HAVE_ENOUGH_DATA 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.
Playback may resume at this point if it was previously suspended by a transition to HAVE_CURRENT_DATA.
Abort these steps.
If HTMLMediaElement.buffered contains a TimeRanges that includes the current playback position and some time beyond the current playback position, then run the following steps:
Playback may resume at this point if it was previously suspended by a transition to HAVE_CURRENT_DATA.
Abort these steps.
If HTMLMediaElement.buffered contains a TimeRanges that ends at the current playback position and does not have a range covering the time immediately after the current position:
Playback is suspended at this point since the media element doesn't have enough data to advance the media timeline.
Abort these steps.
2.5.5 Changes to selected/enabled track state
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.
If the selected video track changes, then run the following steps:
If the SourceBuffer associated with the previously selected video track is not associated with any other enabled tracks, run the following steps:
If an audio track becomes disabled and the SourceBuffer associated with this track is not associated with any other enabled or selected track, then run the following steps:
If a text track mode becomes "disabled" and the SourceBuffer associated with this track is not associated with any other enabled or selected track, then run the following steps:
Duration reductions that would truncate currently buffered media are disallowed. When truncation is necessary, use remove() to reduce the buffered range before updating 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.
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.
Notify the media element that it now has all of the media data.
Post an internal 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.
Note
The purpose of the mirror message mechanism is to ensure that:
steps run asynchronously as their own task on Window rather than these steps somehow
happening in the middle of some other Window task's execution, and
steps are run without blocking the synchronous execution and return of this algorithm on
DedicatedWorkerGlobalScope.
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.
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.
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).
The timestamps in the media segment determine where the coded frames are placed in the presentation. Media segments can be appended in any order.
sequence
Media segments will be treated as adjacent in time independent of the timestamps in the media segment. Coded frames in a new media segment will be placed immediately after the coded
frames in the previous media segment. The 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.
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.
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.
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.
If readyState is "ended", then set the end time on the last range in track ranges to highest end time.
Let new intersection ranges equal the intersection between the intersection ranges and the track ranges.
Replace the ranges in intersection ranges with the new intersection ranges.
If intersection ranges does not contain the exact same range information as the
current value of this attribute, then update the current value of this attribute to
intersection ranges.
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:
Let new timestamp offset equal the new value being assigned to this attribute.
If the updating attribute equals true, then throw an InvalidStateError exception and abort these steps.
If type contains a MIME type that is not supported or contains a MIME type that is not supported with the types specified (currently or previously) of SourceBuffer objects in the sourceBuffers attribute of the parent media source, then throw a NotSupportedError exception and abort these steps.
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 track buffer ranges variable that represents the presentation time ranges occupied by the coded frames currently stored in the track buffer.
Note
For track buffer ranges, these presentation time ranges are based on presentation timestamps, frame durations, and potentially coded frame group start times for coded frame groups across track buffers in a muxed SourceBuffer.
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.
The append or remove has successfully completed. updating transitions from true to false.
updateend
Event
The append or remove has ended.
error
Event
An error occurred during the append. updating transitions from true to false.
abort
Event
The append was aborted by an abort() call. updating transitions from true to false.
4.5 Algorithms
4.5.1 Segment Parser Loop
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.
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.
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.
Let recent element error be true if the HTMLMediaElement.error attribute
is not null. If that attribute is null, then let recent element error be false.
Otherwise
Let recent element error be the value resulting from the steps for the Window case, but run
on the WindowHTMLMediaElement 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.
If recent element error is true, then throw an InvalidStateError 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.
4.5.5 Buffer Append
When appendBuffer() is called, the following steps are run to process the appended data.
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.
Update the duration attribute if it currently equals NaN:
If the initialization segment contains a duration:
Run the duration change algorithm with new duration set to the duration in
the initialization segment.
Otherwise:
Run the duration change algorithm with new duration set to positive Infinity.
Verify the following properties. If any of the checks fail then run the append error algorithm and abort these steps.
The number of audio, video, and text tracks match what was in the first initialization segment.
If more than one track for a single type are present (e.g., 2 audio tracks), then the Track IDs match the ones in the
first initialization segment.
The codecs for each track are supported by the user agent.
Note
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 initialization segment contains tracks with codecs the user agent does not support, then run the append error algorithm and abort these steps.
Note
User agents MAY consider codecs, that would otherwise be supported, as "not supported" here if the codecs were not
specified in 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.
Let audio byte stream track ID be the
Track ID for the current track being processed.
Let audio language be a BCP 47 language tag for the language
specified in the initialization segment for this track or an empty string if no
language info is present.
If audio language equals the 'und' BCP 47 value, then assign an empty string to audio language.
Let audio label be a label specified in the initialization segment for this track or an empty string if no
label info is present.
Let audio kinds be a sequence of kind strings specified in the
initialization segment for this track
or a sequence with a single empty string element in it
if no kind information is provided.
For each value in audio kinds, run the following steps:
Let current audio kind equal the value from audio kinds
for this iteration of the loop.
Let video byte stream track ID be the
Track ID for the current track being processed.
Let video language be a BCP 47 language tag for the language
specified in the initialization segment for this track or an empty string if no
language info is present.
If video language equals the 'und' BCP 47 value, then assign an empty string to video language.
Let video label be a label specified in the initialization segment for this track or an empty string if no
label info is present.
Let video kinds be a sequence of kind strings specified in the
initialization segment for this track
or a sequence with a single empty string element in it
if no kind information is provided.
For each value in video kinds, run the following steps:
Let current video kind equal the value from video kinds
for this iteration of the loop.
Let text byte stream track ID be the
Track ID for the current track being processed.
Let text language be a BCP 47 language tag for the language
specified in the initialization segment for this track or an empty string if no
language info is present.
If text language equals the 'und' BCP 47 value, then assign an empty string to text language.
Let text label be a label specified in the initialization segment for this track or an empty string if no
label info is present.
Let text kinds be a sequence of kind strings specified in the
initialization segment for this track
or a sequence with a single empty string element in it
if no kind information is provided.
For each value in text kinds, run the following steps:
Let current text kind equal the value from text kinds
for this iteration of the loop.
Let presentation timestamp be a double precision floating point representation of the coded frame's presentation timestamp in seconds.
Note
Special processing may be needed to determine the presentation and decode timestamps for timed text frames since this information may not be explicitly
present in the underlying format or may be dependent on the order of the frames. Some metadata text tracks, like MPEG2-TS PSI data, may only have implied timestamps.
Format specific rules for these situations SHOULD be in the byte stream format specifications or in separate extension specifications.
Let decode timestamp be a double precision floating point representation of the coded frame's decode timestamp in seconds.
Note
Implementations don't have to internally store timestamps in a double precision floating point representation. This
representation is used here because it is the 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.
Let frame duration be a double precision floating point representation of the coded frame's duration in seconds.
Jump to the Loop Top step above to restart processing of the current coded frame.
Otherwise:
Continue.
Let frame end timestamp equal the sum of presentation timestamp and frame duration.
If presentation timestamp is less than 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.
Note
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.
If frame end timestamp is greater than 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.
Note
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.
Let remove window timestamp equal the overlapped framepresentation timestamp plus 1 microsecond.
If the presentation timestamp is less than the remove window timestamp, then remove overlapped frame from track buffer.
Note
This is to compensate for minor errors in frame timestamp computations that can appear when converting back and forth between double precision
floating point numbers and rationals. This tolerance allows a frame to replace an existing one as long as it is within 1 microsecond of the existing
frame's start time. Frames that come slightly before an existing frame are handled by the removal step below.
Remove all coded frames from track buffer that have a presentation timestamp greater than or equal to
presentation timestamp and less than frame end timestamp.
If highest end timestamp for track buffer is set and less than or equal to presentation timestamp:
Remove all possible decoding dependencies on the coded frames removed in the previous two steps
by removing all coded frames from track buffer between those frames removed in the previous two steps and the next
random access point after those removed frames.
Note
Removing all coded frames until the next random access point is a conservative
estimate of the decoding dependencies since it assumes all frames between the removed frames and the next random access point
depended on the frames that were removed.
If spliced audio frame is set:
Add spliced audio frame to the track buffer.
If spliced timed text frame is set:
Add spliced timed text frame to the track buffer.
Otherwise:
Add the coded frame with the presentation timestamp, decode timestamp, and frame duration to the
track buffer.
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.
Let remove end timestamp be the current value of 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.
Note
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.
Remove all media data, from this track buffer, that contain starting timestamps greater than or equal to start and less than the remove end timestamp.
Remove all possible decoding dependencies on the coded frames removed in the previous step
by removing all coded frames from this track buffer between those frames removed in the previous step and the next
random access point after those removed frames.
Note
Removing all coded frames until the next random access point is a conservative
estimate of the decoding dependencies since it assumes all frames between the removed frames and the next random access point
depended on the frames that were removed.
This algorithm is run to free up space in this SourceBuffer when new data is appended.
Let new data equal the data that is about to be appended to this SourceBuffer.
Issue 289: Editorial? Coded Frame eviction algorithm needs to note that "buffer full flag" may be updated immediately based on |new data|
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.
Let removal ranges equal a list of presentation time ranges that can be evicted from the presentation to make room for the
new data.
Note
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.
For each range in removal ranges, run the coded frame removal algorithm with start and end equal to
the removal range start and end timestamp respectively.
Update presentation timestamp and decode timestamp to the nearest audio sample timestamp based on sample rate of the
audio in overlapped frame. If a timestamp is equidistant from both audio sample timestamps, then use the higher timestamp (e.g.,
floor(x * sample_rate + 0.5) / sample_rate).
Some implementations MAY apply fades to/from silence to coded frames on either side of the inserted silence to make the transition less
jarring.
Return to caller without providing a splice frame.
Note
This is intended to allow new coded frame to be added to the track buffer as if
overlapped frame had not been in the track buffer to begin with.
Let frame end timestamp equal the sum of presentation timestamp and frame duration.
Let splice end timestamp equal the sum of presentation timestamp and the splice duration of 5 milliseconds.
Let fade out coded frames equal overlapped frame as well as any additional frames in track buffer that
have a presentation timestamp greater than presentation timestamp and less than splice end timestamp.
Remove all the frames included in fade out coded frames from track buffer.
Return a splice frame with the following properties:
The fade out coded frames equals fade out coded frames.
The fade in coded frame equals new coded frame.
Note
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.
The splice timestamp equals presentation timestamp.
Note
See the audio splice rendering algorithm for details on how this splice frame is rendered.
4.5.12 Audio Splice Rendering
The following steps are run when a spliced frame, generated by the audio splice frame algorithm, needs to be rendered by the
media element:
Let fade out coded frames be the coded frames that are faded out during the splice.
Let fade in coded frames be the coded frames that are faded in during the splice.
Let presentation timestamp be the presentation timestamp of the first coded frame in fade out coded frames.
Let splice end timestamp equal splice timestamp plus five milliseconds.
Let fade out samples be the samples generated by decoding fade out coded frames.
Trim fade out samples so that it only contains samples between presentation timestamp and splice end timestamp.
Let fade in samples be the samples generated by decoding fade in coded frames.
If fade out samples and fade in samples do not have a common sample rate and channel layout, then convert
fade out samples and fade in samples to a common sample rate and channel layout.
Let output samples be a buffer to hold the output samples.
Apply a linear gain fade out with a starting gain of 1 and an ending gain of 0 to the samples between
splice timestamp and splice end timestamp in fade out samples.
Apply a linear gain fade in with a starting gain of 0 and an ending gain of 1 to the samples between splice timestamp and
splice end timestamp in fade in samples.
Copy samples between presentation timestamp to splice timestamp from fade out samples into output samples.
For each sample between splice timestamp and splice end timestamp, compute the sum of a sample from fade out samples and the
corresponding sample in fade in samples and store the result in output samples.
Copy samples between splice end timestamp to end timestamp from fade in samples into output samples.
Render output samples.
Note
Here is a graphical representation of this algorithm.
4.5.13 Text Splice Frame
Follow these steps when the coded frame processing algorithm needs to generate a splice frame for two overlapping timed text
coded frames:
Let track buffer be the track buffer that will contain the splice.
Let new coded frame be the new coded frame, that is being added to track buffer, which triggered the need for a splice.
Let frame end timestamp equal the sum of presentation timestamp and frame duration.
Let first overlapped frame be the coded frame in track buffer with a presentation interval that contains presentation timestamp.
Let overlapped presentation timestamp be the presentation timestamp of the first overlapped frame.
Let overlapped frames equal first overlapped frame as well as any additional frames in track buffer that
have a presentation timestamp greater than presentation timestamp and less than frame end timestamp.
Remove all the frames included in overlapped frames from track buffer.
Update the coded frame duration of the first overlapped frame to presentation timestamp minus overlapped presentation timestamp.
Add first overlapped frame to the track buffer.
Return to caller without providing a splice frame.
Note
This is intended to allow new coded frame to be added to the track buffer as if
it hadn't overlapped any frames in track buffer to begin with.
5. SourceBufferList Object
SourceBufferList is a simple container object for SourceBuffer objects. It provides read-only array access and fires events when the list is modified.
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.
Issue 277: MSE-in-Workers: Consider (eventually) transitioning attached element to error upon termination of MediaSource's worker/what should media element do?
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.
Let recent duration and recent live seekable range
respectively be the recent values of duration and [[live seekable range]],
determined as follows:
Let union ranges be the union of recent live seekable range and the
HTMLMediaElement.buffered attribute.
Return a single range with a start time equal to the earliest start time in union ranges and
an end time equal to the highest end time in union ranges and abort these steps.
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.
Issue 277: MSE-in-Workers: Consider (eventually) transitioning attached element to error upon termination of MediaSource's worker/what should media element do?
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.
Let recent intersection ranges be determined as follows:
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.
If the current value of this attribute has not been set by this algorithm or
recent intersection ranges does not contain the exact same range information as the
current value of this attribute, then update the current value of this attribute to
recent intersection ranges.
For example, if a DedicatedWorkerGlobalScopeSourceBuffer 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.
8. VideoTrack Extensions
This section specifies extensions to the [HTML] VideoTrack definition.
For example, if a DedicatedWorkerGlobalScopeSourceBuffer 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.
9. TextTrack Extensions
This section specifies extensions to the [HTML] TextTrack definition.
For example, if a DedicatedWorkerGlobalScopeSourceBuffer 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.
10. Byte Stream Formats
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.
Note
The byte stream format specifications in the registry are not intended to define new storage formats. They simply outline the subset of
existing storage format structures that implementations of this specification will accept.
Note
Byte stream format parsing and validation is implemented in the segment parser loop algorithm.
This section provides general requirements for all byte stream format specifications:
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.
It MUST be possible to identify segment boundaries and segment type (initialization or media) by examining the byte stream alone.
The user agent MUST run the append error algorithm when any of the following conditions are met:
The number and type of tracks are not consistent.
Note
For example, if the first initialization segment has 2 audio tracks and 1 video track, then all initialization segments that follow it in the byte stream MUST describe 2 audio tracks and 1 video track.
Track IDs are not the same across initialization segments, for segments describing multiple tracks of a single type (e.g., 2 audio tracks).
Video frame size changes. The user agent MUST support seamless playback.
Note
This will cause the <video> display region to change size if the web application does not use CSS or HTML attributes (width/height) to constrain the element size.
Audio channel count changes. The user agent MAY support this seamlessly and could trigger downmixing.
Note
This is a quality of implementation issue because changing the channel count may require reinitializing the audio device, resamplers, and channel mixers which tends to be audible.
The following rules apply to all media segments within a byte stream. A user agent MUST:
Support seamless playback of media segments having a timestamp gap smaller than the audio frame size. User agents MUST NOT reflect these gaps in the buffered attribute.
Note
This is intended to simplify switching between audio streams where the frame boundaries don't always line up across encodings (e.g., Vorbis).
The number and type (audio, video, text, etc.) of all tracks in the media segments are not identified.
The decoding capabilities needed to decode each track (i.e., codec and codec parameters) are not provided.
Encryption parameters necessary to decrypt the content (except the encryption key itself) are not provided for all encrypted tracks.
All information necessary to decode and render the earliest random access point in the sequence of media segments and all subsequence samples in the sequence
(in presentation time) are not provided. This includes in particular,
Information that determines the intrinsic width and height of the video (specifically, this requires either the picture or pixel aspect ratio, together with the encoded
resolution).
Information necessary to convert the video decoder output to a format suitable for display
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.
11. Conformance
As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.
The key words MAY, MUST, MUST NOT, SHOULD, and SHOULD NOT 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.
12. Examples
Example use of the Media Source Extensions
<script>
function onSourceOpen(videoTag, e) {
var mediaSource = e.target;
if (mediaSource.sourceBuffers.length > 0)
return;
var sourceBuffer = mediaSource.addSourceBuffer('video/webm; codecs="vorbis,vp8"');
videoTag.addEventListener('seeking', onSeeking.bind(videoTag, mediaSource));
videoTag.addEventListener('progress', onProgress.bind(videoTag, mediaSource));
var initSegment = GetInitializationSegment();
if (initSegment == null) {
// Error fetching the initialization segment. Signal end of stream with an error.
mediaSource.endOfStream("network");
return;
}
// Append the initialization segment.
var firstAppendHandler = function(e) {
var sourceBuffer = e.target;
sourceBuffer.removeEventListener('updateend', firstAppendHandler);
// Append some initial media data.
appendNextMediaSegment(mediaSource);
};
sourceBuffer.addEventListener('updateend', firstAppendHandler);
sourceBuffer.appendBuffer(initSegment);
}
function appendNextMediaSegment(mediaSource) {
if (mediaSource.readyState == "closed")
return;
// If we have run out of stream data, then signal end of stream.
if (!HaveMoreMediaSegments()) {
mediaSource.endOfStream();
return;
}
// Make sure the previous append is not still pending.
if (mediaSource.sourceBuffers[0].updating)
return;
var mediaSegment = GetNextMediaSegment();
if (!mediaSegment) {
// Error fetching the next media segment.
mediaSource.endOfStream("network");
return;
}
// NOTE: If mediaSource.readyState == “ended”, this appendBuffer() call will
// cause mediaSource.readyState to transition to "open". The web application
// should be prepared to handle multiple “sourceopen” events.
mediaSource.sourceBuffers[0].appendBuffer(mediaSegment);
}
function onSeeking(mediaSource, e) {
var video = e.target;
if (mediaSource.readyState == "open") {
// Abort current segment append.
mediaSource.sourceBuffers[0].abort();
}
// Notify the media segment loading code to start fetching data at the
// new playback position.
SeekToMediaSegmentAt(video.currentTime);
// Append a media segment from the new playback position.
appendNextMediaSegment(mediaSource);
}
function onProgress(mediaSource, e) {
appendNextMediaSegment(mediaSource);
}
</script>
<video id="v" autoplay> </video>
<script>
var video = document.getElementById('v');
var mediaSource = new MediaSource();
mediaSource.addEventListener('sourceopen', onSourceOpen.bind(this, video));
video.src = window.URL.createObjectURL(mediaSource);
</script>
13. Acknowledgments
The editors would like to thank Alex Giladi, Bob Lund, Chris Poole, Cyril Concolato, David Dorwin, David Singer, Duncan Rowden, Frank Galligan, Glenn Adams, Jer Noble, Joe Steele, John Simmons, Kevin Streeter, Mark Vickers, Matt Ward, Matthew Gregan, Michael Thornburgh, Philip Jägenstedt, Pierre Lemieux, Ralph Giles, Steven Robertson, and Tatsuya Igarashi for their contributions to this specification.
A. VideoPlaybackQuality
This section is non-normative.
The video playback quality metrics described in previous revisions of this specification (e.g., sections 5 and 10 of the Candidate Recommendation) are now being developed as part of [MEDIA-PLAYBACK-QUALITY]. Some implementations may have implemented the earlier draft VideoPlaybackQuality object and the HTMLVideoElement extension method getVideoPlaybackQuality() described in those previous revisions.
B. Issue summary
Issue 276: MSE-in-Workers: Consider adding a "closing" readyState to explain new `InvalidStateError` exception when closing underway
Issue 280: MSE-in-Workers: {Audio,Video,Text}Track{,List} IDL in HTML need additional DedicatedWorker in Exposed
Issue 289: Editorial? Coded Frame eviction algorithm needs to note that "buffer full flag" may be updated immediately based on |new data|
Issue 277: MSE-in-Workers: Consider (eventually) transitioning attached element to error upon termination of MediaSource's worker/what should media element do?
Issue 277: MSE-in-Workers: Consider (eventually) transitioning attached element to error upon termination of MediaSource's worker/what should media element do?
