This document defines a set of JavaScript APIs that allow local media, including audio and video, to be requested from a platform.

This document is not complete. It is subject to major changes and, while early experimentations are encouraged, it is therefore not intended for implementation. The API is based on preliminary work done in the WHATWG. The Media Capture Task Force expects this specification to evolve significantly based on:

Introduction

Access to multimedia streams (video, audio, or both) from local devices (video cameras, microphones, Web cams) can have a number of uses, such as real-time communication, recording, and surveillance.

This document defines the APIs used to get access to local devices that can generate multimedia stream data. This document also defines the stream API by which JavaScript is able to manipulate the stream data or otherwise process it.

This specification defines conformance criteria that apply to a single product: the user agent that implements the interfaces that it contains.

Implementations that use ECMAScript to implement the APIs defined in this specification must implement them in a manner consistent with the ECMAScript Bindings defined in the Web IDL specification [[!WEBIDL]], as this specification uses that specification and terminology.

Terminology

The EventHandler interface represents a callback used for event handlers as defined in [[!HTML5]].

The concepts queue a task and fires a simple event are defined in [[!HTML5]].

The terms event handlers and event handler event types are defined in [[!HTML5]].

Stream API

Introduction

The MediaStream interface is used to represent streams of media data, typically (but not necessarily) of audio and/or video content, e.g. from a local camera. The data from a MediaStream object does not necessarily have a canonical binary form; for example, it could just be "the video currently coming from the user’s video camera". This allows user agents to manipulate media streams in whatever fashion is most suitable on the user’s platform.

Each MediaStream object can contain zero or more tracks, in particular audio and video tracks. All tracks in a MediaStream are intended to be synchronized when rendered. Different MediaStreams do not need to be synchronized.

Each track in a MediaStream object has a corresponding MediaStreamTrack object.

A MediaStreamTrack represents content comprising one or more channels, where the channels have a defined well known relationship to each other (such as a stereo or 5.1 audio signal).

A channel is the smallest unit considered in this API specification.

A MediaStream object has an input and an output. The input depends on how the object was created: a LocalMediaStream object generated by a getUserMedia() call (which is described later in this document), for instance, might take its input from the user’s local camera. The output of the object controls how the object is used, e.g., what is saved if the object is written to a file or what is displayed if the object is used in a video element.

Each track in a MediaStream object can be disabled, meaning that it is muted in the object’s output. All tracks are initially enabled.

A MediaStream can be finished, indicating that its inputs have forever stopped providing data.

The output of a MediaStream object MUST correspond to the tracks in its input. Muted audio tracks MUST be replaced with silence. Muted video tracks MUST be replaced with blackness.

A new MediaStream object can be created from existing MediaStreamTrack objects using the MediaStream() constructor. The constructor takes two lists of MediaStreamTrack objects as arguments: one for audio tracks and one for video tracks. The lists can either be the track lists of another stream, subsets of such lists, or compositions of MediaStreamTrack objects from different MediaStream objects.

The ability to duplicate a MediaStream, i.e. create a new MediaStream object from the track lists of an existing stream, allows for greater control since separate MediaStream instances can be manipulated and consumed individually.

The LocalMediaStream interface is used when the user agent is generating the stream’s data (e.g. from a camera or streaming it from a local video file).

When a LocalMediaStream object is being generated from a local file (as opposed to a live audio/video source), the user agent SHOULD stream the data from the file in real time, not all at once. The MediaStream object is also used in contexts outside getUserMedia, such as [[!WEBRTC10]]. In both cases, ensuring a realtime stream reduces the ease with which pages can distinguish live video from pre-recorded video, which can help protect the user’s privacy.

MediaStream

The MediaStream() constructor takes zero or one argument. If the argument, trackContainers, is supplied, it specifies a list of MediaStream, MediaStreamTrackList and MediaStreamTrack objects. The list objects specifies existing tracks whose sources will be used to constuct the tracks in the new MediaStream object. A MediaStreamTrack object specifies a track directly, while MediaStream and MediaStreamTrackList objects specifiy all tracks contained within these objects. When the constructor is invoked, the UA must run the following steps:

  1. Let trackContainers be the constructor’s argument, if any, or null otherwise.

  2. Let stream be a newly constructed MediaStream object.

  3. Set stream’s label attribute to a newly generated value.

  4. If trackContainers is not null, then run the following sub steps for every element, trackContainer, in trackContainers:

    1. If trackContainer is null, then abort these steps and continue with the next element.

    2. If trackContainer is of type MediaStreamTrack, then run the following sub steps:

      1. Add track: Let track be the MediaStreamTrack about to be processed.

      2. If track’s kind attribute is not "audio" or "video", then throw a SyntaxError exception.

      3. If track has ended or if there is already a MediaStreamTrack contained within stream that has the same underlying source as track, then abort these steps.

      4. Create a new MediaStreamTrack object and let it inherit track’s underlying source, kind and label attributes. Append the new MediaStreamTrack to the corresponding track list (audioTracks or videoTracks) in stream according to kind.

    3. If trackContainer is of type MediaStreamTrackList, then run the sub steps labeled Add track (above) for every MediaStreamTrack in trackContainer.

    4. If trackContainer is of type MediaStream, then run the sub steps labeled Add track (above) for every MediaStreamTrack in trackContainer’s two track lists (audioTracks and videoTracks).

  5. Return stream.

A MediaStream can have multiple audio and video sources (e.g. because the user has multiple microphones, or because the real source of the stream is a media resource with many media tracks). The stream represented by a MediaStream thus has zero or more tracks.

The tracks of a MediaStream are stored in two track lists represented by MediaStreamTrackList objects: one for audio tracks and one for video tracks. The two track lists MUST contain the MediaStreamTrack objects that correspond to the tracks of the stream. The relative order of all tracks in a user agent MUST be stable. Tracks that come from a media resource whose format defines an order MUST be in the order defined by the format; tracks that come from a media resource whose format does not define an order MUST be in the relative order in which the tracks are declared in that media resource. Within these constraints, the order is user agent defined.

An object that reads data from the output of a MediaStream is referred to as a MediaStream consumer. The list of MediaStream consumers currently includes the media elements and the PeerConnection API specified in [[!WEBRTC10]].

MediaStream consumers must be able to handle tracks being added and removed. This behavior is specified per consumer.

A MediaStream object is said to be finished when all tracks belonging to the stream have ended. When this happens for any reason other than the stop() method being invoked, the user agent MUST queue a task that runs the following steps:

  1. If the object’s ended attribute has the value true already, then abort these steps. (The stop() method was probably called just before the stream stopped for other reasons, e.g. the user clicked an in-page stop button and then the user agent provided stop button.)

  2. Set the object’s ended attribute to true.

  3. Fire a simple event named ended at the object.

If the end of the stream was reached due to a user request, the task source for this task is the user interaction task source. Otherwise the task source for this task is the networking task source.

The union type we want to express here is not supported by ReSpec at the moment. Until it is supported, let TracksUnionType be defined as (MediaStream? or MediaStreamTrackList or MediaStreamTrack)[].

readonly attribute DOMString label

When a LocalMediaStream object is created, the user agent MUST generate a globally unique identifier string, and MUST initialize the object’s label attribute to that string. Such strings MUST only use characters in the ranges U+0021, U+0023 to U+0027, U+002A to U+002B, U+002D to U+002E, U+0030 to U+0039, U+0041 to U+005A, U+005E to U+007E, and MUST be 36 characters long.

When a MediaStream is created from another using the MediaStream() constructor, the label attribute is initialized to a newly generated value.

The label attribute MUST return the value to which it was initialized when the object was created.

readonly attribute MediaStreamTrackList audioTracks

Returns a MediaStreamTrackList object representing the audio tracks that can be enabled and disabled.

The audioTracks attribute MUST return an array host object for objects of type MediaStreamTrack that is fixed length and read only. The same object MUST be returned each time the attribute is accessed.

readonly attribute MediaStreamTrackList videoTracks

Returns a MediaStreamTrackList object representing the video tracks that can be enabled and disabled.

The videoTracks attribute MUST return an array host object for objects of type MediaStreamTrack that is fixed length and read only. The same object MUST be returned each time the attribute is accessed.

attribute boolean ended

The MediaStream.ended attribute MUST return true if the MediaStream has finished, and false otherwise.

When a MediaStream object is created, its ended attribute MUST be set to false, unless it is being created using the MediaStream() constructor whose arguments are lists of MediaStreamTrack objects that are all ended, in which case the MediaStream object MUST be created with its ended attribute set to true.

attribute EventHandler onended
This event handler, of type ended, MUST be supported by all objects implementing the MediaStream interface.

LocalMediaStream

Before the web application can access the user's media input devices it must let getUserMedia() create a LocalMediaStream . Once the application is done using, e.g., a webcam and a microphone, it may revoke its own access by calling stop() on the LocalMediaStream.

A web application may, once it has access to a LocalMediaStream , use the MediaStream() constructor to construct additional MediaStream objects. Since a derived MediaStream object is created from the tracks of an existing stream, it cannot use any media input devices that have not been approved by the user.

void stop()

When a LocalMediaStream object’s stop() method is invoked, the user agent MUST queue a task that runs the following steps on every track:

  1. Let track be the current MediaStreamTrack object.

  2. End track. The track starts outputting only silence and/or blackness, as appropriate.

  3. Dereference track’s underlying media source.

  4. If the reference count of track’s underlying media source is greater than zero, then abort these steps.

  5. Permanently stop the generation of data for track’s source. If the data is being generated from a live source (e.g., a microphone or camera), then the user agent SHOULD remove any active "on-air" indicator for that source. If the data is being generated from a prerecorded source (e.g. a video file), any remaining content in the file is ignored.

The task source for the tasks queued for the stop() method is the DOM manipulation task source.

MediaStreamTrack

A MediaStreamTrack object represents a media source in the user agent. Several MediaStreamTrack objects can represent the same media source, e.g., when the user chooses the same camera in the UI shown by two consecutive calls to getUserMedia() .

A MediaStreamTrack object can reference its media source in two ways, either with a strong or a weak reference, depending on how the track was created. For example, a track in a MediaStream, derived from a LocalMediaStream with the MediaStream() constructor, has a weak reference to a local media source, while a track in a LocalMediaStream has a strong reference. This means that a track in a MediaStream, derived from a LocalMediaStream, will end if there is no non-ended track in a LocalMediaStream which references the same local media source.

The concept with strong and weak references to media sources allows the web application to derive new MediaStream objects from LocalMediaStream objects (created via getUserMedia()) and still be able to revoke all given permissions with LocalMediaStream.stop().

A MediaStreamTrack object is said to end when the user agent learns that no more data will ever be forthcoming for this track.

When a MediaStreamTrack object ends for any reason (e.g., because the user rescinds the permission for the page to use the local camera, or because the data comes from a finite file and the file’s end has been reached and the user has not requested that it be looped, or because the UA has instructed the track to end for any reason, or because the reference count of the track’s underlying media source has reached zero, it is said to be ended. When track instance track ends for any reason other than the stop() method being invoked on the LocalMediaStream object that represents track, the user agent MUST queue a task that runs the following steps:

  1. If the track’s readyState attribute has the value ENDED (2) already, then abort these steps.

  2. Set track’s readyState attribute to ENDED (2).

  3. Fire a simple event named ended at the object.

If the end of the stream was reached due to a user request, the event source for this event is the user interaction event source.

readonly attribute DOMString kind

The MediaStreamTrack.kind attribute MUST return the string "audio" if the object’s corresponding track is or was an audio track, "video" if the corresponding track is or was a video track, and a user agent defined string otherwise.

readonly attribute DOMString label

User agents MAY label audio and video sources (e.g., "Internal microphone" or "External USB Webcam"). The MediaStreamTrack.label attribute MUST return the label of the object’s corresponding track, if any. If the corresponding track has or had no label, the attribute MUST instead return the empty string.

Thus the kind and label attributes do not change value, even if the MediaStreamTrack object is disassociated from its corresponding track.

attribute boolean enabled

The MediaStreamTrack.enabled attribute, on getting, MUST return the last value to which it was set. On setting, it MUST be set to the new value, and then, if the MediaStreamTrack object is still associated with a track, MUST enable the track if the new value is true, and disable it otherwise.

Thus, after a MediaStreamTrack is disassociated from its track, its enabled attribute still changes value when set; it just doesn’t do anything with that new value.

const unsigned short LIVE = 0

The track is active (the track’s underlying media source is making a best-effort attempt to provide data in real time).

The output of a track in the LIVE state can be switched on and off with the enabled attribute.

const unsigned short MUTED = 1

The track is muted (the track’s underlying media source is temporarily unable to provide data).

A MediaStreamTrack in a LocalMediaStream may be muted if the user temporarily revokes the web application’s permission to use a media input device.

const unsigned short ENDED = 2

The track has ended (the track’s underlying media source is no longer providing data, and will never provide more data for this track).

For example, a video track in a LocalMediaStream finishes if the user unplugs the USB web camera that acts as the track’s media source.

readonly attribute unsigned short readyState

The readyState attribute represents the state of the track. It MUST return the value to which the user agent last set it (as defined below). It can have the following values: LIVE, MUTED or ENDED.

When a MediaStreamTrack object is created, its readyState is either LIVE (0) or MUTED (1), depending on the state of the track’s underlying media source. For example, a track in a LocalMediaStream, created with getUserMedia(), MUST initially have its readyState attribute set to LIVE (1).

attribute EventHandler onmute
This event handler, of type muted, MUST be supported by all objects implementing the MediaStreamTrack interface.
attribute EventHandler onunmute
This event handler, of type unmuted, MUST be supported by all objects implementing the MediaStreamTrack interface.
attribute EventHandler onended
This event handler, of type ended, MUST be supported by all objects implementing the MediaStreamTrack interface.

URL

static DOMString createObjectURL (MediaStream stream)

Mints a Blob URL to refer to the given MediaStream.

When the createObjectURL() method is called with a MediaStream argument, the user agent MUST return a unique Blob URL for the given MediaStream. [[!FILE-API]]

For audio and video streams, the data exposed on that stream MUST be in a format supported by the user agent for use in audio and video elements.

A Blob URL is 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 MediaStream and LocalMediaStream objects.

MediaStreamTrackList

A MediaStreamTrackList object’s corresponding MediaStream refers to the MediaStream object which the current MediaStreamTrackList object is a property of.

readonly attribute unsigned long length
Returns the number of tracks in the list.
MediaStreamTrack item(unsigned long index)
Returns the MediaStreamTrack object at the specified index.
void add(MediaStreamTrack track)

Adds the given MediaStreamTrack to this MediaStreamTrackList according to the ordering rules for tracks.

When the add() method is invoked, the user agent MUST run the following steps:

  1. Let track be the MediaStreamTrack argument.

  2. Let stream be the MediaStreamTrackList object’s corresponding MediaStream object.

  3. If stream is finished, throw an INVALID_STATE_ERR exception.

  4. If track is already in the MediaStreamTrackList object’s internal list, then abort these steps.

  5. Add track to the end of the MediaStreamTrackList object’s internal list.

void remove(MediaStreamTrack track)

Removes the given MediaStreamTrack from this MediaStreamTrackList.

When the remove() method is invoked, the user agent MUST run the following steps:

  1. Let track be the MediaStreamTrack argument.

  2. Let stream be the MediaStreamTrackList object’s corresponding MediaStream object.

  3. If stream is finished, throw an INVALID_STATE_ERR exception.

  4. If track is not in the MediaStreamTrackList object’s internal list, then abort these steps.

  5. Remove track from the MediaStreamTrackList object’s internal list.

attribute EventHandler onaddtrack
This event handler, of type addtrack, MUST be supported by all objects implementing the MediaStreamTrackList interface.
attribute EventHandler onremovetrack
This event handler, of type removetrack, MUST be supported by all objects implementing the MediaStreamTrackList interface.

MediaStreams as Media Elements

A MediaStream may be assigned to media elements as defined in HTML5 [[!HTML5]] by calling createObjectURL to obtain a URL for the MediaStream and then setting the media elements src attribute to that URL. A MediaStream is not preloadable or seekable and represents a simple, potentially infinite, linear media timeline. The timeline starts at 0 and increments linearly in real time as long as the MediaStream is playing. The timeline does not increment when the MediaStream is paused.

Do we also need to support direct assignment and access of the underlying stream?

The nature of the MediaStream places certain restrictions on the behavior and attribute values of the associated media element and on the operations that can be performed on it, as shown below:

  • Whenever the user agent runs the media element load algorithm, reaches the resource fetch phase of this algorithm and determines that the media resource in question is a MediaStream, it MUST immediately abort the resource selection algorithm, setting the media.readystate to HAVE_ENOUGH_DATA.
  • The UA MUST NOT buffer data from a MediaStream. When playing, the UA MUST always play the current data from the stream.
Attribute Name Attribute Type Valid Values Additional considerations
src DOMString a local URI referencing a MediaStream N.B. Revocation of the URI does not count as a change to this field.
currentSrc DOMString a local URI referencing a MediaStream -
preload DOMString none A MediaStream cannot be preloaded.
buffered TimeRanges buffered.length MUST return 1.
buffered.start(0) MUST return 0.
buffered.end(0) MUST return 0.
A MediaStream cannot be preloaded. Therefore, the amount buffered is always an empty TimeRange.
currentTime double Any positive integer. The initial value is 0 and the values increments linearly in real time whenever the stream is playing. The value is the current stream position, in seconds. The UA MUST ignore attempts to set this attribute.
duration double Infinity A MediaStream does not have a pre-defined duration.

If the underlying MediaStream is destroyed, the UA MUST set this property to the value of the last known currentTime.

seeking boolean false A MediaStream is not seekable. Therefore, this attribute MUST always have the value false.
defaultPlaybackRate double 1.0 A MediaStream is not seekable. Therefore, this attribute MUST always have the value 1.0 and any attempt to alter it MUST fail.
playbackRate double 1.0 A MediaStream is not seekable. Therefore, this attribute MUST always have the value 1.0 and any attempt to alter it MUST fail.
played TimeRanges played.length MUST return 1.
played.start(0) MUST return 0.
played.end(0) MUST return the last known currentTime.
A MediaStream's timeline always consists of a single range, starting at 0 and extending up to the currentTime.
seekable TimeRanges seekable.length MUST return 0.
seekable.start() MUST return currentTime.
seekable.end() MUST return currentTime.
A MediaStream is not seekable.
startOffsetTime Date Not-a-Number (NaN) A MediaStream does not specify a timeline offset.
loop boolean false A MediaStream has no defined end and therefore cannot be looped.

Event summary

The following event fires on MediaStream objects:

Event name Interface Fired when...
ended Event The MediaStream finished as a result of all tracks in the MediaStream ending.

The following event fires on MediaStreamTrack objects:

Event name Interface Fired when...
muted Event The MediaStreamTrack object's source is temporarily unable to provide data.
unmuted Event The MediaStreamTrack object's source is live again after having been temporarily
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