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This proposal extends HTMLMediaElement to allow JavaScript to generate media streams for playback. Allowing JavaScript to generate streams facilitates a variety of use cases like adaptive streaming and time shifting live streams.
This proposal allows JavaScript to dynamically construct media streams for <audio> and <video>.
It defines objects that allow JavaScript to pass media segments to an HTMLMediaElement.
A buffering model is also included to describe how the user agent should act when different media segments are
appended at different times. Byte stream specifications for WebM & ISO Base Media File Format are given to specify the
expected format of media segments used with these extensions.
This proposal was designed with the following goals in mind:
A sequence of bytes that contains 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).
Container specific examples of initialization segments:
A sequence of bytes that contain packetized & timestamped media data for a portion of the presentation timeline. Media segments are always associated with the most recently appended initialization segment.
Container specific examples of media segments:
A hypothetical buffer that contains a distinct sequence of initialization segments & media segments. When media segments are passed to append() they update the state of this buffer. The source buffer only allows a single media segment to cover a specific point in the presentation timeline of each track. If a media segment gets appended that contains media data overlapping (in presentation time) with media data from an existing segment, then the new media data will override the old media data. Since media segments depend on initialization segments the source buffer is also responsible for maintaining these associations. During playback, the media element pulls segment data out of the source buffers, demultiplexes it if necessary, and enqueues it into track buffers so it will get decoded and displayed. buffered describes the time ranges that are covered by media segments in the source buffer.
The set of source buffers that are providing the selected video track, the enabled audio tracks, and the "showing" or "hidden" text tracks. This is a subset of all the source buffers associated with a specific MediaSource object. See Changes to selected/enabled track state for details.
A hypothetical buffer that represents initialization and media data for a single AudioTrack, VideoTrack, or TextTrack that has been queued for playback. This buffer may not exist in actual implementations, but it is intended to represent media data that will be decoded no matter what media segments are appended to update the source buffer. This distinction is important when considering appends that happen close to the current playback position. See Source Buffer to Track Buffer transfer for details.
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 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. Appending media segments with negative timestamps will cause playback to terminate with a MediaError.MEDIA_ERR_DECODE error unless timestampOffset is used to make the timestamps greater than or equal to 0.
A MediaSource object URL is a unique Blob URI created by createObjectURL(). It is used to attach a MediaSource object to an HTMLMediaElement.
These URLs are the same as what the File API specification calls 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.
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.
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 must have a unique Track ID.
The subsections below outline the buffering model for this proposal. It describes how to add and remove source buffers from the presentation and describes the various rules and behaviors associated with appending data to an individual source buffer. At the highest level, the web application simply creates source buffers and appends a sequence of initialization segments and media segments to update the buffer's state. The media element pulls media data out of the source buffers, plays it, and fires events just like it would if a normal URL was passed to the src attribute. The web application is expected to monitor media element events to determine when it needs to append more media segments.
SourceBuffer objects can be created once a MediaSource object enters the "open" state. The application calls addSourceBuffer() with a type string that indicates the format of the data it intends to append to the new SourceBuffer. If the user agent supports the format and has sufficient resources, a new SourceBuffer object is created, added to sourceBuffers, and returned by the method. If the user agent doesn't support the specified format or can't support another SourceBuffer then it will throw an appropriate exception to signal why the request couldn't be satisfied.
Removing a SourceBuffer with removeSourceBuffer() releases all resources associated with the object. This includes destroying the all the segment data, track buffers, and decoders. The media element will also remove the appropriate tracks from audioTracks, videoTracks, & textTracks and fire the necessary change events. Playback may become degraded or stop if the currently selected VideoTrack or the only enabled AudioTracks are removed.
Updating the state of a source buffer requires appending at least one initialization segment and one or more media segments via append(). The following list outlines some of the basic rules for appending segments.
abort() is called.append()).To simplify the implementation and facilitate interoperability, a few constraints are placed on the initialization segments that are appended to a specific SourceBuffer:
SourceBuffer must describe 2 audio tracks and 1 video track.SourceBuffer objects.To simplify the implementation and facilitate interoperability, a few constraints are placed on the media segments that are appended to a specific SourceBuffer:
buffered.
Note: This is intended to simplify switching between audio streams where the frame boundaries don't always line up across encodings (e.g. Vorbis).
Once a new SourceBuffer has been created, it expects an initialization segment to be appended first. This first segment indicates the number and type of streams contained in the media segments that follow. This allows the media element to configure the necessary decoders and output devices. This first segment can also cause a HTMLMediaElement.readyState transition to HAVE_METADATA if this is the first SourceBuffer, or if it is the first track of a specific type (i.e. first audio, first video track, or first text track). If neither of the conditions hold then the tracks for this new SourceBuffer will just appear as disabled tracks and won't affect the current HTMLMediaElement.readyState until they are selected. The media element will also add the appropriate tracks to the audioTracks, videoTracks, & textTracks collections and fire the necessary change events. The description for append() contains all the details.
If a media segment is appended to a time range that is not covered by existing segments in the source buffer, then its data is copied directly into the source buffer. Addition of this data may trigger HTMLMediaElement.readyState transitions depending on what other data is buffered and whether the media element has determined if it can start playback. Calls to buffered will always reflect the current TimeRanges buffered in the SourceBuffer.
There are several ways that media segments can overlap segments in the source buffer. Behavior for the different overlap situations are described below. If more than one overlap applies, then the start overlap gets resolved first, followed by any complete overlaps, and finally the end overlap. If a segment contains multiple tracks then the overlap is resolved independently for each track.
The figure above shows how the source buffer gets updated when a new media segment completely overlaps a segment in the buffer. In this case, the new segment completely replaces the old segment.
The figure above shows how the source buffer gets updated when the beginning of a new media segment overlaps a segment in the buffer. In this case the new segment replaces all the old media data in the overlapping region. Since media segments are constrained to starting with random access points, this provides a seamless transition between segments.
When an audio frame in the source buffer overlaps with the start of the new media segment special behavior is required. At a minimum implementations must support dropping the old audio frame that overlaps the start of the new segment and insert silence for the small gap that is created. Higher quality implementations may support crossfading or crosslapping between the overlapping audio frames. No matter which strategy is implemented, no gaps should be created in the ranges reported by buffered and playback must never stall at the overlap.
The figure above shows how the source buffer gets updated when the end of a new media segment overlaps a segment in the buffer. In this case, the media element tries to keep as much of the old segment as possible. The amount saved depends on where the closest random access point, in the old segment, is to the end of the new segment. In the case of audio, if the gap is smaller than the size of an audio frame, then the media element should insert silence for this gap and not reflect it in buffered.
An implementation may keep old segment data before the end of the new segment to avoid creating a gap if it wishes. Doing this though can significantly increase implementation complexity and could cause delays at the splice point. The key property that must be preserved is the entirety of the new segment gets added to the source buffer and it is up to the implementation how much of the old segment data is retained. The web application can use buffered to determine how much of the old segment was preserved.
The figure above shows how the source buffer gets updated when the new media segment is in the middle of the old segment. This condition is handled by first resolving the start overlap and then resolving the end overlap.
The source buffer represents the media that the web application would like the media element to play. The track buffer contains the data that will actually get decoded and rendered. In most cases the track buffer will simply contain a subset of the source buffer near the current playback position. These two buffers start to diverge though when media segments that overlap or are very close to the current playback position are appended. Depending on the contents of the new media segment it may not be possible to switch to the new data immediately because there isn't a random access point close enough to the current playback position. The quality of the implementation determines how much data is considered "in the track buffer". It should transfer data to the track buffer as late as possible whilst maintaining seamless playback. Some implementations may be able to instantiate multiple decoders or decode the new data significantly faster than real-time to achieve a seamless splice immediately. Other implementations may delay until the next random access point before switching to the newly appended data. Notice that this difference in behavior is only observable when appending close to the current playback position. The track buffer represents a media subsegment, like a group of pictures or something with similar decode dependencies, that the media element commits to playing. This commitment may be influenced by a variety of things like limited decoding resources, hardware decode buffers, a jitter buffer, or the desire to limit implementation complexity.
Here is an example to help clarify the role of the track buffer. Say the current playback position has a timestamp of 8 and the media element pulled frames with timestamp 9 & 10 into the track buffer. The web application then appends a higher quality media segment that starts with a random access point at timestamp 9. The source buffer will get updated with the higher quality data, but the media element won't be able to switch to this higher quality data until the next random access point at timestamp 20. This is because a frame for timestamp 9 is already in the track buffer. As you can see the track buffer represents the "point of no return." for decoding. If a seek occurs the media element may choose to use the higher quality data since a seek might imply flushing the track buffer and the user expects a break in playback.
When a new media segment is appended, memory constraints may cause previously appended segments to get evicted from the source buffer. The eviction algorithm is implementation dependent, but segments that aren't likely to be needed soon are the most likely to get evicted. The buffered attribute allows the web application to monitor what time ranges are currently buffered in the source buffer.
For some use cases like ad-insertion or seamless playlists, the web application may want to insert a media segment in the presentation timeline at a location that is different than what the internal timestamps indicate. This can be accomplished by using the timestampOffset attribute on the SourceBuffer object. The value of timestampOffset is added to all timestamps inside a media segment before the contents of that segment are added to the source buffer. The timestampOffset applies to an entire media segment. An exception is thrown if the application tries to update the attribute when only part of a media segment has been appended. Both positive or negative offsets can be assigned to timestampOffset. If an offset causes a media segment timestamp to get converted to a time before the presentation start time, playback will terminate with a MediaError.MEDIA_ERR_DECODE error.
Here is a simple example to clarify how timestampOffset can be used. Say I have two sounds I want to play in sequence. The first sound is 5 seconds long and the second one is 10 seconds. Both sound files have timestamps that start at 0. First append the initialization segment and all media segments for the first sound. Now set timestampOffset to 5 seconds. Finally append the initialization segment and media segments for the second sound. This will result in a 15 second presentation that plays the two sounds in sequence.
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.
[Constructor]
interface MediaSource : EventTarget {
// All the source buffers created by this object.
readonly attribute SourceBufferList sourceBuffers;
// Subset of sourceBuffers that provide data for the selected/enabled tracks.
readonly attribute SourceBufferList activeSourceBuffers;
attribute unrestricted double duration;
SourceBuffer addSourceBuffer(DOMString type);
void removeSourceBuffer(SourceBuffer sourceBuffer);
enum State { "closed", "open", "ended" };
readonly attribute State readyState;
enum EndOfStreamError { "network", "decode" };
void endOfStream(optional EndOfStreamError error);
};
The sourceBuffers attribute contains the list of SourceBuffer objects associated with this MediaSource. When readyState equals "closed" this list will be empty. Once readyState transitions to "open" SourceBuffer objects can be added to this list by using addSourceBuffer().
The activeSourceBuffers attribute contains the subset of sourceBuffers that represents the active source buffers.
The duration attribute 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:
INVALID_ACCESS_ERR exception and abort these steps.readyState attribute is not "open" then throw an INVALID_STATE_ERR exception and abort these steps.Note: append() and endOfStream() can update the duration under certain circumstances.
The addSourceBuffer(type) method must run the following steps:
INVALID_ACCESS_ERR exception and abort these steps.SourceBuffer objects in sourceBuffers, then throw a NOT_SUPPORTED_ERR exception and abort these steps.QUOTA_EXCEEDED_ERR exception and abort these steps.readyState attribute is not in the "open" state then throw an INVALID_STATE_ERR exception and abort these steps.SourceBuffer object and associated resources.sourceBuffers and queue a task to fire a simple event named
addsourcebuffer at sourceBuffers.The removeSourceBuffer(sourceBuffer) method must run the following steps:
INVALID_ACCESS_ERR exception and abort these steps.sourceBuffers is empty then throw an INVALID_STATE_ERR exception and abort these steps.sourceBuffers then throw a NOT_FOUND_ERR exception and abort these steps.audioTracks, videoTracks, and textTracks for all tracks associated with sourceBuffer and queue a task to fire a simple event named
change at the modified lists.activeSourceBuffers, then remove it from activeSourceBuffers and queue a task to fire a simple event named
removesourcebuffer at activeSourceBuffers.sourceBuffers and queue a task to fire a simple event named
removesourcebuffer at sourceBuffers.The readyState attribute indicates the current state of the MediaSource object. It can have the following values:
"closed""open"SourceBuffer objects in sourceBuffers."ended"endOfStream() has been called. Appending data to SourceBuffer objects in this state is not allowed.When the MediaSource is created readyState must be set to "closed".
"network"Note: If the JavaScript fetching media data encounters a network error it should use this status code to terminate playback.
"decode"Note: If the JavaScript code fetching media data has problems parsing the data it should use this status code to terminate playback.
The endOfStream(error) method must run the following steps:
readyState attribute is not in the "open" state then throw an INVALID_STATE_ERR exception and abort these steps.readyState attribute value to "ended".sourceended at the MediaSource.SourceBuffer objects in sourceBuffers.Note: This allows the duration to properly reflect the end of the appended media segments. For example, if the duration was explicitly set to 10 seconds and only media segments for 0 to 5 seconds were appended before endOfStream() was called, then the duration will get updated to 5 seconds.
append() has been played."network"
HTMLMediaElement.readyState attribute equals HAVE_NOTHING
HTMLMediaElement.readyState attribute greater than HAVE_NOTHING
"decode"
HTMLMediaElement.readyState attribute equals HAVE_NOTHING
HTMLMediaElement.readyState attribute greater than HAVE_NOTHING
INVALID_ACCESS_ERR exception.| Event name | Interface | Dispatched when... |
|---|---|---|
sourceopen |
Event |
When readyState transitions from "closed" to "open" or from "ended" to "open". |
sourceended |
Event |
When readyState transitions from "open" to "ended". |
sourceclose |
Event |
When readyState transitions from "open" to "closed" or "ended" to "closed". |
A MediaSource object can be attached to a media element by assigning a MediaSource object URL 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().
If the resource fetch algorithm absolute URL matches the MediaSource object URL, run the following steps right before the "Perform a potentially CORS-enabled fetch" step in the resource fetch algorithm.
readyState is NOT set to "closed"
MEDIA_ERR_SRC_NOT_SUPPORTED error.readyState attribute to "open".sourceopen.append().The following steps are run in any case where the media element is going to transition to NETWORK_EMPTY and fire an emptied event. These steps should be run right before the transition.
readyState attribute to "closed".duration attribute to NaN.SourceBuffer objects from sourceBuffers and fire a removesourcebuffer event for each one.sourceclose at the MediaSource.seeking algorithm starts and has reached the stage where it is about to fire the seeking event.readyState attribute is set to "ended"
readyState attribute to "open"
sourceopen on the MediaSource object.seeking algorithm fires the seeking eventSourceBuffer object in activeSourceBuffers
activeSourceBuffers is missing media segments for the desired seek pointHTMLMediaElement.readyState attribute to HAVE_METADATA and fire the appropriate event for this transition.append(). The web application can use buffered to determine what the media element needs to resume playback.seeking algorithm and fires the seeked event indicating that the seek has completed.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. Appending new segments and changes to activeSourceBuffers also cause these steps to run because they affect the conditions that trigger state transitions. The web application can monitor changes in HTMLMediaElement.readyState to drive media segment appending.
buffered for all objects in activeSourceBuffers do not contain TimeRanges for the current playback position:HTMLMediaElement.readyState attribute to HAVE_METADATA and fire the appropriate event for this transition.buffered for all objects in activeSourceBuffers contain TimeRanges that include the current playback position and enough data to ensure uninterrupted playback:HTMLMediaElement.readyState attribute to HAVE_ENOUGH_DATA and fire the appropriate event for this transition.HAVE_CURRENT_DATA.buffered for at least one object in activeSourceBuffers contains a TimeRange that includes the current playback position but not enough data to ensure uninterrupted playback:HTMLMediaElement.readyState attribute to HAVE_FUTURE_DATA and fire the appropriate event for this transition.HAVE_CURRENT_DATA.buffered for at least one object in activeSourceBuffers contains a TimeRange that ends at the current playback position and does not have a range covering the time immediately after the current position:HTMLMediaElement.readyState attribute to HAVE_CURRENT_DATA and fire the appropriate event for this transition.During playback activeSourceBuffers needs to be updated if the selected video track, the enabled audio tracks, or a text track mode changes. When one or more of these changes occur the following steps need to be followed.
SourceBuffer associated with the previously selected video track is not associated with any other enabled tracks then remove it from activeSourceBuffers
SourceBuffer associated with the newly selected video track is not already in activeSourceBuffers then add it.SourceBuffer associated with this track is not associated with any other enabled or selected trackSourceBuffer associated with the audio track from activeSourceBuffers
SourceBuffer associated with this track is not already in activeSourceBuffers
SourceBuffer associated with the audio track to activeSourceBuffers
mode becomes "disabled" and the SourceBuffer associated with this track is not associated with any other enabled or selected trackSourceBuffer associated with the text track from activeSourceBuffers
mode becomes "showing" or "hidden" and the SourceBuffer associated with this track is not already in activeSourceBuffers
SourceBuffer associated with the text track to activeSourceBuffers
Follow these steps when duration needs to change to a new duration.
duration is equal to new duration, then abort these steps.duration to new duration.SourceBuffer objects in sourceBuffers.media controller duration to new duration and run the HTMLMediaElement duration change algorithm.
interface SourceBuffer : EventTarget {
// Returns the time ranges buffered.
readonly attribute TimeRanges buffered;
// Applies an offset to media segment timestamps.
attribute double timestampOffset;
// Append segment data.
void append(Uint8Array data);
// Abort the current segment append sequence.
void abort();
};
The buffered attribute indicates what TimeRanges are buffered in the SourceBuffer. When the attribute is read the following steps must occur:
sourceBuffers attribute of the MediaSource object that created it then throw an INVALID_STATE_ERR exception and abort these steps.The timestampOffset attribute 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, the initial value or the last value that was successfully set is returned. On setting, run following steps:
sourceBuffers attribute of the MediaSource object that created it, then throw an INVALID_STATE_ERR exception and abort these steps.readyState attribute of the MediaSource object that created this object is not in the "open" state, then throw an INVALID_STATE_ERR exception and abort these steps.INVALID_STATE_ERR and abort these steps.The append(data) method must run the following steps:
INVALID_ACCESS_ERR exception and abort these steps.sourceBuffers attribute of the MediaSource object that created it then throw an INVALID_STATE_ERR exception and abort these steps.readyState attribute of the MediaSource object that created this object is not in the "open" state then throw an INVALID_STATE_ERR exception and abort these steps.endOfStream("decode"), and abort these steps.timestampOffset is not 0:timestampOffset to them.endOfStream("decode"), and abort these steps.duration attribute if it currently equals NaN:HTMLMediaElement.readyState attribute is HAVE_NOTHING:HTMLMediaElement.readyState attribute to HAVE_METADATA and fire the appropriate event for this transition.HTMLMediaElement.readyState attribute is greater than HAVE_CURRENT_DATA and the initialization segment contains the first video or first audio track in the presentation:HTMLMediaElement.readyState attribute to HAVE_METADATA and fire the appropriate event for this transition.
audioTracks
AudioTrack and mark it as enabled.SourceBuffer to activeSourceBuffers.AudioTrack for each audio track in the initialization segment.videoTracks:VideoTrack and mark it as selected.SourceBuffer to activeSourceBuffers.VideoTrack for each video track in the initialization segment.textTracks
TextTrack for each text track in the initialization segment.mode is "showing" or "hidden" then add this SourceBuffer to activeSourceBuffers.HTMLMediaElement.readyState attribute is HAVE_METADATA and data causes all objects in activeSourceBuffers to have media data for the current playback position.HTMLMediaElement.readyState attribute to HAVE_CURRENT_DATA and fire the appropriate event for this transition.HTMLMediaElement.readyState attribute is HAVE_CURRENT_DATA and data causes all objects in activeSourceBuffers to have media data beyond the current playback position.HTMLMediaElement.readyState attribute to HAVE_FUTURE_DATA and fire the appropriate event for this transition.HTMLMediaElement.readyState attribute is HAVE_FUTURE_DATA and data causes all objects in activeSourceBuffers to have enough data to start playback.HTMLMediaElement.readyState attribute to HAVE_ENOUGH_DATA and fire the appropriate event for this transition.duration
HTMLMediaElement.buffered.The abort() method must run the following steps:
sourceBuffers attribute of the MediaSource object that created it then throw an INVALID_STATE_ERR exception and abort these steps.readyState attribute of the MediaSource object that created this object is not in the "open" state then throw an INVALID_STATE_ERR exception and abort these steps.buffered will reflect what data, if any, was kept.SourceBufferList is a simple container object for SourceBuffer objects. It provides read-only array access and fires events when the list is modified.
interface SourceBufferList : EventTarget {
readonly attribute unsigned long length;
getter SourceBuffer (unsigned long index);
};
The length attribute indicates the number of SourceBuffer objects in the list.
The getter SourceBuffer (unsigned long index) method allows the SourceBuffer objects in the list to be accessed with an array operator (i.e. []). This method 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. |
partial interface URL {
static DOMString createObjectURL(MediaSource mediaSource);
};
The createObjectURL(mediaSource) method must run the following steps.
Note: This algorithm is intended to mirror the behavior of the File API createObjectURL() method with autoRevoke set to true.
The bytes provided through append() for a SourceBuffer form a logical byte stream. The format of this byte stream depends on the media container format in use and is defined in a byte stream format specification. Byte stream format specifications based on WebM and the ISO Base Media File Format are provided below. If these formats are supported then the byte stream formats described below must be supported.
This section provides general requirements for all byte stream formats:
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.
Initialization segments are an optimization. They allow a byte stream format to avoid duplication of information in Media Segments that is the same for many Media Segments. Byte stream format specifications need not specify Initialization Segment formats, however. They may instead require that such information is duplicated in every Media Segment.
This section defines segment formats for implementations that choose to support WebM.
A WebM initialization segment must contain a subset of the elements at the start of a typical WebM file.
The following rules apply to WebM initialization segments:
A WebM media segment is a single Cluster element.
The following rules apply to WebM media segments:
A SimpleBlock element with its Keyframe flag set signals the location of a random access point for that track. Media segments containing multiple tracks are only considered a random access point if the first SimpleBlock for each track has its Keyframe flag set. The order of the multiplexed blocks should conform to the WebM Muxer Guidelines.
This section defines segment formats for implementations that choose to support the ISO Base Media File Format ISO/IEC 14496-12 (ISO BMFF).
An ISO BMFF initialization segment must contain a single Movie Header Box (moov). The tracks in the Movie Header Box must not contain any samples (i.e. the entry_count in the stts, stsc and stco boxes must be set to zero). A Movie Extends (mvex) box must be contained in the Movie Header Box to indicate that Movie Fragments are to be expected.
The initialization segment may contain Edit Boxes (edts) which provide a mapping of composition times for each track to the global presentation time.
An ISO BMFF media segment must contain a single Movie Fragment Box (moof) followed by one or more Media Data Boxes (mdat).
The following rules apply to ISO BMFF media segments:
A random access point as defined in this specification corresponds to a Stream Access Point of type 1 or 2 as defined in Annex I of ISO/IEC 14496-12.
Example use of the Media Source Extensions
<script>
function onSourceOpen(videoTag, e) {
var mediaSource = e.target;
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.
sourceBuffer.append(initSegment);
// Append some initial media data.
appendNextMediaSegment(mediaSource);
}
function appendNextMediaSegment(mediaSource) {
if (mediaSource.readyState == "ended")
return;
// If we have run out of stream data, then signal end of stream.
if (!HaveMoreMediaSegments()) {
mediaSource.endOfStream();
return;
}
var mediaSegment = GetNextMediaSegment();
if (!mediaSegment) {
// Error fetching the next media segment.
mediaSource.endOfStream("network");
return;
}
mediaSource.sourceBuffers[0].append(mediaSegment);
}
function onSeeking(mediaSource, e) {
var video = e.target;
// 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 media segments from the new playback position.
appendNextMediaSegment(mediaSource);
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>
| Version | Comment |
|---|---|
| 13 September 2012 | Added decode error & network error algorithms. |
| 24 August 2012 |
|
| 22 August 2012 |
|
| 17 August 2012 | Minor editorial fixes. |
| 09 August 2012 | Change presentation start time to always be 0 instead of using format specific rules about the first media segment appended. |
| 30 July 2012 | Added SourceBuffer.timestampOffset and MediaSource.duration. |
| 17 July 2012 | Replaced SourceBufferList.remove() with MediaSource.removeSourceBuffer(). |
| 02 July 2012 | Converted to the object-oriented API |
| 26 June 2012 | Converted to Editor's draft. |
| 0.5 | Minor updates before proposing to W3C HTML-WG. |
| 0.4 | Major revision. Adding source IDs, defining buffer model, and clarifying byte stream formats. |
| 0.3 | Minor text updates. |
| 0.2 | Updates to reflect initial WebKit implementation. |
| 0.1 | Initial Proposal |