Media Capture and Streams

W3C Candidate Recommendation Snapshot

More details about this document
This version:
http://www-w3-org.eastshorepartners.com/TR/2021/CR-mediacapture-streams-20211013/
Latest published version:
http://www-w3-org.eastshorepartners.com/TR/mediacapture-streams/
Latest editor's draft:
http://w3c.github.io.eastshorepartners.com/mediacapture-main/
History:
Publication history
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Implementation report:
http://wpt.fyi.eastshorepartners.com/mediacapture-streams
Previous version:
http://www-w3-org.eastshorepartners.com/TR/2021/CRD-mediacapture-streams-20210819/
Editors:
Cullen Jennings (Cisco)
Bernard Aboba (Microsoft Corporation)
Jan-Ivar Bruaroey (Mozilla)
Henrik Bostr?m (Google)
Youenn Fablet (Apple)
Former editors:
Daniel C. Burnett (Invited Expert) - Until
Adam Bergkvist (Ericsson) - Until
Anant Narayanan (Mozilla) - Until
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GitHub w3c/mediacapture-main (pull requests, new issue, open issues)
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Abstract

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

Status of This Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www-w3-org.eastshorepartners.com/TR/.

This document is not complete. The API is based on preliminary work done in the WHATWG.

Before this document proceeds to Proposed Recommendation, the WebRTC Working Group intends to address issues that emerged from wide review.

This document was published by the Web Real-Time Communications Working Group as a Candidate Recommendation Snapshot. This document is intended to become a W3C Recommendation.

GitHub Issues are preferred for discussion of this specification. Alternatively, you can send comments to our mailing list. Please send them to public-webrtc@w3.org (subscribe, archives).

Publication as a Candidate Recommendation does not imply endorsement by the W3C Membership. A Candidate Recommendation Snapshot has received wide review and is intended to gather implementation experience.

This Candidate Recommendation is not expected to advance to Proposed Recommendation any earlier than 13 November 2021.

This document was produced by a group operating under the W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

This document is governed by the 15 September 2020 W3C Process Document.

1. Introduction

This section is non-normative.

This document defines APIs for requesting access to local multimedia devices, such as microphones or video cameras.

This document also defines the MediaStream API, which provides the means to control where multimedia stream data is consumed, and provides some control over the devices that produce the media. It also exposes information about devices able to capture and render media.

2. 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, NOT REQUIRED, and SHOULD 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.

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

Conformance requirements phrased as algorithms or specific steps may be implemented in any manner, so long as the end result is equivalent. (In particular, the algorithms defined in this specification are intended to be easy to follow, and not intended to be performant.)

Implementations that use ECMAScript [ECMA-262] 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.

3. Terminology

source

A source is the "thing" providing the source of a media stream track. The source is the broadcaster of the media itself. A source can be a physical webcam, microphone, local video or audio file from the user's hard drive, network resource, or static image. Note that this document describes the use of microphone and camera type sources only, the use of other source types is described in other documents.

An application that has no prior authorization regarding sources is only given the number of available sources, their type and any relationship to other devices. Additional information about sources can become available when applications are authorized to use a source (see § 9.2.1 Access control model).

Sources do not have constraints — tracks have constraints. When a source is connected to a track, it must produce media that conforms to the constraints present on that track, to that track. Multiple tracks can be attached to the same source. User Agent processing, such as downsampling, MAY be used to ensure that all tracks have appropriate media.

Sources have constrainable properties which have capabilities and settings exposed on tracks. While the constrainable properties are "owned" by the source, sources MAY be able to accomodate different demands at once. For this reason, capabilities are common to any (multiple) tracks that happen to be using the same source, whereas settings MAY differ per track (e.g., if two different track objects bound to the same source query capability and settings information, they will get back the same capabilities, but may get different settings that are tailored to satisfy their individual constraints).

Setting (Source Setting)

A setting refers to the immediate, current value of the source's constrainable properties. Settings are always read-only.

A source conditions may dynamically change, such as when a camera switches to a lower frame rate due to low light conditions. In these cases the tracks related to the affected source might not satisfy the set constraints any longer. The platform SHOULD try to minimize such excursions as far as possible, but will continue to deliver media even when a temporary or permanent condition exists that prevents satisfying the constraints.

Although settings are a property of the source, they are only exposed to the application through the tracks attached to the source. This is exposed via the ConstrainablePattern interface.

Capability

For each constrainable property, there is a capability that describes whether it is supported by the source and if so, the range of supported values. As with settings, capabilities are exposed to the application via the ConstrainablePattern interface.

The values of the supported capabilities must be normalized to the ranges and enumerated types defined in this specification.

A getCapabilities() call on a track returns the same underlying per-source capabilities for all tracks connected to the source.

Source capabilities are effectively constant. Applications should be able to depend on a specific source having the same capabilities for any browsing session.

This API is intentionally simplified. Capabilities are not capable of describing interactions between different values. For instance, it is not possible to accurately describe the capabilities of a camera that can produce a high resolution video stream at a low frame rate and lower resolutions at a higher frame rate. Capabilities describe the complete range of each value. Interactions between constraints are exposed by attempting to apply constraints.

Constraints

Constraints provide a general control surface that allows applications to both select an appropriate source for a track and, once selected, to influence how a source operates.

Constraints limit the range of operating modes that a source can use when providing media for a track. Without provided track constraints, implementations are free to select a source's settings from the full ranges of its supported capabilities. Implementations may also adjust source settings at any time within the bounds imposed by all applied constraints.

getUserMedia() uses constraints to help select an appropriate source for a track and configure it. Additionally, the ConstrainablePattern interface on tracks includes an API for dynamically changing the track's constraints at any later time.

A track will not be connected to a source using getUserMedia() if its initial constraints cannot be satisfied. However, the ability to meet the constraints on a track can change over time, and constraints can be changed. If circumstances change such that constraints cannot be met, the ConstrainablePattern interface defines an appropriate error to inform the application. § 5. The model: sources, sinks, constraints, and settings explains how constraints interact in more detail.

For each constrainable property, a constraint exists whose name corresponds with the relevant source setting name and capability name.

A constraint falls into one of three groups, depending on its place in the constraints structure. The groups are:

  • required constraints are all non-advanced constraints that are required.
  • optional basic constraints are the remaining non-advanced constraints.
  • advanced constraints are all constraints specified using the advanced keyword.

In general, User Agents will have more flexibility to optimize the media streaming experience the fewer constraints are applied, so application authors are strongly encouraged to use required constraints sparingly.

4. MediaStream API

4.1 Introduction

The two main components in the MediaStream API are the MediaStreamTrack and MediaStream interfaces. The MediaStreamTrack object represents media of a single type that originates from one media source in the User Agent, e.g. video produced by a web camera. A MediaStream is used to group several MediaStreamTrack objects into one unit that can be recorded or rendered in a media element.

Each MediaStream can contain zero or more MediaStreamTrack objects. All tracks in a MediaStream are intended to be synchronized when rendered. This is not a hard requirement, since it might not be possible to synchronize tracks from sources that have different clocks. Different MediaStream objects do not need to be synchronized.

Note

While the intent is to synchronize tracks, it could be better in some circumstances to permit tracks to lose synchronization. In particular, when tracks are remotely sourced and real-time [WEBRTC], it can be better to allow loss of synchronization than to accumulate delays or risk glitches and other artifacts. Implementations are expected to understand the implications of choices regarding synchronization of playback and the effect that these have on user perception.

A single MediaStreamTrack can represent multi-channel content, such as stereo or 5.1 audio or stereoscopic video, where the channels have a well defined relationship to each other. Information about channels might be exposed through other APIs, such as [WEBAUDIO], but this specification provides no direct access to channels.

A MediaStream object has an input and an output that represent the combined input and output of all the object's tracks. The output of the MediaStream controls how the object is rendered, e.g., what is saved if the object is recorded to a file or what is displayed if the object is used in a video element. A single MediaStream object can be attached to multiple different outputs at the same time.

A new MediaStream object can be created from existing media streams or tracks using the MediaStream() constructor. The constructor argument can either be an existing MediaStream object, in which case all the tracks of the given stream are added to the new MediaStream object, or an array of MediaStreamTrack objects. The latter form makes it possible to compose a stream from different source streams.

Both MediaStream and MediaStreamTrack objects can be cloned. A cloned MediaStream contains clones of all member tracks from the original stream. A cloned MediaStreamTrack has a set of constraints that is independent of the instance it is cloned from, which allows media from the same source to have different constraints applied for different consumers. The MediaStream object is also used in contexts outside getUserMedia, such as [WEBRTC].

4.2 MediaStream

The MediaStream constructor composes a new stream out of existing tracks. It takes an optional argument of type MediaStream or an array of MediaStreamTrack objects. When the constructor is invoked, the User Agent must run the following steps:

  1. Let stream be a newly constructed MediaStream object.

  2. Initialize stream.id attribute to a newly generated value.

  3. If the constructor's argument is present, run the following steps:

    1. Construct a set of tracks tracks based on the type of argument:

    2. For each MediaStreamTrack, track , in tracks, run the following steps:

      1. If track is already in stream's track set, skip track.

      2. Otherwise, add track to stream's track set.

  4. Return stream.

The tracks of a MediaStream are stored in a track set. The track set MUST contain the MediaStreamTrack objects that correspond to the tracks of the stream. The relative order of the tracks in the set is User Agent defined and the API will never put any requirements on the order. The proper way to find a specific MediaStreamTrack object in the set is to look it up by its id.

An object that reads data from the output of a MediaStream is referred to as a MediaStream consumer. The list of MediaStream consumers currently include media elements (such as video and audio) [HTML], Web Real-Time Communications (WebRTC; RTCPeerConnection) [WEBRTC], media recording (MediaRecorder) [mediastream-recording], image capture (ImageCapture) [image-capture], and web audio (MediaStreamAudioSourceNode) [WEBAUDIO].

Note

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 active when it has at least one MediaStreamTrack that has not ended. A MediaStream that does not have any tracks or only has tracks that are ended is inactive.

A MediaStream object is said to be audible when it has at least one MediaStreamTrack whose kind is "audio" that has not ended. A MediaStream that does not have any audio tracks or only has audio tracks that are ended is inaudible.

The User Agent may update a MediaStream's track set in response to, for example, an external event. This specification does not specify any such cases, but other specifications using the MediaStream API may. One such example is the WebRTC 1.0 [WEBRTC] specification where the track set of a MediaStream, received from another peer, can be updated as a result of changes to the media session.

To add a track track to a MediaStream stream, the User Agent MUST run the following steps:

  1. If track is already in stream's track set, then abort these steps.

  2. Add track to stream's track set.

  3. Fire a track event named addtrack with track at stream.

To remove a track track from a MediaStream stream, the User Agent MUST run the following steps:

  1. If track is not in stream's track set, then abort these steps.

  2. Remove track from stream's track set.

  3. Fire a track event named removetrack with track at stream.

WebIDL[Exposed=Window]
interface MediaStream : EventTarget {
  constructor();
  constructor(MediaStream stream);
  constructor(sequence<MediaStreamTrack> tracks);
  readonly attribute DOMString id;
  sequence<MediaStreamTrack> getAudioTracks();
  sequence<MediaStreamTrack> getVideoTracks();
  sequence<MediaStreamTrack> getTracks();
  MediaStreamTrack? getTrackById(DOMString trackId);
  undefined addTrack(MediaStreamTrack track);
  undefined removeTrack(MediaStreamTrack track);
  MediaStream clone();
  readonly attribute boolean active;
  attribute EventHandler onaddtrack;
  attribute EventHandler onremovetrack;
};

Constructors

MediaStream

See the MediaStream constructor algorithm

No parameters.
MediaStream

See the MediaStream constructor algorithm

MediaStream

See the MediaStream constructor algorithm

Attributes

id of type DOMString, readonly

When a MediaStream is created, the User Agent MUST generate an identifier string, and MUST initialize the object's id attribute to that string, unless the object is created as part of a special purpose algorithm that specifies how the stream id must be initialized. A good practice is to use a UUID [rfc4122], which is 36 characters long in its canonical form. To avoid fingerprinting, implementations SHOULD use the forms in section 4.4 or 4.5 of RFC 4122 when generating UUIDs.

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

active of type boolean, readonly

The active attribute MUST return true if this MediaStream is active and false otherwise.

onaddtrack of type EventHandler

The event type of this event handler is addtrack.

onremovetrack of type EventHandler

The event type of this event handler is removetrack.

Methods

getAudioTracks

Returns a sequence of MediaStreamTrack objects representing the audio tracks in this stream.

The getAudioTracks method MUST return a sequence that represents a snapshot of all the MediaStreamTrack objects in this stream's track set whose kind is equal to "audio". The conversion from the track set to the sequence is User Agent defined and the order does not have to be stable between calls.

getVideoTracks

Returns a sequence of MediaStreamTrack objects representing the video tracks in this stream.

The getVideoTracks method MUST return a sequence that represents a snapshot of all the MediaStreamTrack objects in this stream's track set whose kind is equal to "video". The conversion from the track set to the sequence is User Agent defined and the order does not have to be stable between calls.

getTracks

Returns a sequence of MediaStreamTrack objects representing all the tracks in this stream.

The getTracks method MUST return a sequence that represents a snapshot of all the MediaStreamTrack objects in this stream's track set, regardless of kind. The conversion from the track set to the sequence is User Agent defined and the order does not have to be stable between calls.

getTrackById

The getTrackById method MUST return either a MediaStreamTrack object from this stream's track set whose id is equal to trackId, or null, if no such track exists.

addTrack

Adds the given MediaStreamTrack to this MediaStream.

When the addTrack method is invoked, the User Agent MUST run the following steps:

  1. Let track be the methods argument and stream the MediaStream object on which the method was called.

  2. If track is already in stream's track set, then abort these steps.

  3. Add track to stream's track set.

removeTrack

Removes the given MediaStreamTrack object from this MediaStream.

When the removeTrack method is invoked, the User Agent MUST run the following steps:

  1. Let track be the methods argument and stream the MediaStream object on which the method was called.

  2. If track is not in stream's track set, then abort these steps.

  3. Remove track from stream's track set.

clone

Clones the given MediaStream and all its tracks.

When the clone() method is invoked, the User Agent MUST run the following steps:

  1. Let streamClone be a newly constructed MediaStream object.

  2. Initialize streamClone.MediaStream.id to a newly generated value.

  3. Clone each track in this MediaStream object and add the result to streamClone's track set.

  4. Return streamClone.

4.3 MediaStreamTrack

A MediaStreamTrack object represents a media source in the User Agent. An example source is a device connected to the User Agent. Other specifications may define sources for MediaStreamTrack that override the behavior specified here. 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().

The data from a MediaStreamTrack 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 in whatever fashion is most suitable on the user's platform.

A script can indicate that a MediaStreamTrack object no longer needs its source with the stop() method. When all tracks using a source have been stopped or ended by some other means, the source is stopped. If the source is a device exposed by getUserMedia(), then when the source is stopped, the UA MUST run the following steps:

  1. Let deviceId be the device's deviceId.

  2. Set [[devicesLiveMap]][deviceId] to false.

  3. If the permission state of the permission associated with the device's kind and deviceId, is not "granted", then set [[devicesAccessibleMap]][deviceId] to false.

At creation of any MediaStreamTrack, its underlying source is initialized. To initialize the underlying source of a MediaStreamTrack named track to a source named source, with an optional parameter tieSourceToContext, which value is true unless specified explicitely, the User Agent MUST run the following steps :

  1. Let source be track's underlying source.

  2. If tieSourceToContext is set to false, abort these steps.

  3. Let globalObject be track's relevant global object.

  4. Add source to globalObject's [[mediaStreamTrackSources]].

To stop all sources of a global object, named globalObject, the User Agent MUST run the following steps:

  1. For each MediaStreamTrack object track whose relevant global object is globalObject, set track.readyState to "ended".

  2. For each source in globalObject's [[mediaStreamTrackSources]], stop source.

The User Agent MUST stop all sources of a globalObject in the following conditions:

  1. If globalObject is a Window object and the unloading document cleanup steps are executed for its associated document.

  2. If globalObject is a WorkerGlobalScope object and its closing flag is set to true.

An implementation may use a per-source reference count to keep track of source usage, but the specifics are out of scope for this specification.

To clone a track the User Agent MUST run the following steps:

  1. Let track be the MediaStreamTrack object to be cloned.

  2. Let trackClone be a newly constructed MediaStreamTrack object.

  3. Initialize trackClone.id to a newly generated value.

  4. Initialize trackClone's kind, label, readyState, and enabled attributes by copying the corresponding values from track.

  5. Initialize the underlying source of trackClone to the source of track with tieSourceToContext equal to false.

  6. Set trackClone's constraints to the active constrains of track.

  7. Return trackClone.

4.3.1 Life-cycle and Media Flow

Life-cycle

A MediaStreamTrack has two states in its life-cycle: live and ended. A newly created MediaStreamTrack can be in either state depending on how it was created. For example, cloning an ended track results in a new ended track. The current state is reflected by the object's readyState attribute.

In the live state, the track is active and media is available for use by consumers (but may be replaced by zero-information-content if the MediaStreamTrack is muted or disabled, see below).

A muted or disabled MediaStreamTrack renders either silence (audio), black frames (video), or a zero-information-content equivalent. For example, a video element sourced by a muted or disabled MediaStreamTrack (contained within a MediaStream ), is playing but the rendered content is the muted output.

If the source is a device exposed by getUserMedia(), then when a track becomes either muted or disabled, and this brings all tracks connected to the device to be either muted, disabled, or stopped, then the UA MAY, using the device's deviceId, deviceId, set [[devicesLiveMap]][deviceId] to false, provided the UA sets it back to true as soon as any unstopped track connected to this device becomes un-muted or enabled again.

When a "live", unmuted, and enabled track sourced by a device exposed by getUserMedia() becomes either [= track/muted or disabled, and this brings all tracks connected to the device (regardless of browsing context) to be either muted, disabled, or stopped, then the UA SHOULD relinquish the device within 3 seconds while allowing time for a reasonably-observant user to become aware of the transition. The UA SHOULD attempt to reacquire the device as soon as any live track sourced by the device becomes both unmuted and enabled again, provided that track's relevant settings object's responsible document has focus at that time. If the document does not have focus at that time, the UA SHOULD instead queue a task to mute the track, and not queue a task to unmute it until the document regains focus. If reacquiring the device fails, the UA MUST end the track (The UA MAY end it earlier should it detect a device problem, like the device being physically removed).

Note

The intent is to give users the assurance of privacy that having physical camera (and microphone) hardware lights off brings, by aligning physical and logical “privacy indicators”, at least while the current document is the sole user of a device.

While other applications and documents using the device simultaneously may interfere with this intent at times, they do not interfere with the rules laid forth.

The muted/unmuted state of a track reflects whether the source provides any media at this moment. The enabled/disabled state is under application control and determines whether the track outputs media (to its consumers). Hence, media from the source only flows when a MediaStreamTrack object is both unmuted and enabled.

A MediaStreamTrack is muted when the source is temporarily unable to provide the track with data. A track can be muted by a user. Often this action is outside the control of the application. This could be as a result of the user hitting a hardware switch or toggling a control in the operating system / User Agent chrome. A track can also be muted by the User Agent.

Applications are able to enable or disable a MediaStreamTrack to prevent it from rendering media from the source. A muted track will however, regardless of the enabled state, render silence and blackness. A disabled track is logically equivalent to a muted track, from a consumer point of view.

For a newly created MediaStreamTrack object, the following applies. The track is always enabled unless stated otherwise (for example when cloned) and the muted state reflects the state of the source at the time the track is created.

A MediaStreamTrack object is said to end when the source of the track is disconnected or exhausted.

If all MediaStreamTracks that are using the same source are ended, the source will be stopped.

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 application invoked the stop() method on the MediaStreamTrack object, or because the User Agent has instructed the track to end for any reason) it is said to be ended.

When a MediaStreamTrack track ends for any reason other than the stop() method being invoked, the User Agent MUST queue a task that runs the following steps:

  1. If track.readyState has the value "ended" already, then abort these steps.

  2. Set track.readyState to "ended".

  3. Notify track's source that track is ended so that the source may be stopped, unless other MediaStreamTrack objects depend on it.

  4. Fire an event named ended at the object.

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

To invoke the device permission revocation algorithm with PermissionName permissionName and deviceId deviceId, run the following steps:

  1. Let tracks be the set of all currently "live" MediaStreamTracks that fit the following criteria:

    • If deviceId is not undefined, tracks whose associated deviceId matches deviceId
    • If deviceId is undefined, tracks whose permission associated with this kind of track (e.g. "camera", "microphone") matches permissionName
  2. For each track in tracks, end the track.

Media Flow

There are two dimensions related to the media flow for a "live" MediaStreamTrack : muted / not muted, and enabled / disabled.

Muted refers to the input to the MediaStreamTrack. If live samples are not made available to the MediaStreamTrack it is muted.

Muted is out of control for the application, but can be observed by the application by reading the muted attribute and listening to the associated events mute and unmute. There can be several reasons for a MediaStreamTrack to be muted: the user pushing a physical mute button on the microphone, the user closing a laptop lid with an embedded camera, the user toggling a control in the operating system, the user clicking a mute button in the User Agent chrome, the User Agent (on behalf of the user) mutes, etc.

Whenever the User Agent initiates such a change, it MUST queue a task, using the user interaction task source, to set a track's muted state to the state desired by the user.

task, using the user interaction task source, to set a track's muted state to the state desired by the user.

To set a track's muted state to newState, the User Agent MUST run the following steps:

  1. Let track be the MediaStreamTrack in question.

  2. If track.muted is already newState, then abort these steps.

  3. Set track.muted to newState.

  4. If newState is true let eventName be mute, otherwise unmute.

  5. Fire an event named eventName on track.

Enabled/disabled on the other hand is available to the application to control (and observe) via the enabled attribute.

The result for the consumer is the same in the sense that whenever MediaStreamTrack is muted or disabled (or both) the consumer gets zero-information-content, which means silence for audio and black frames for video. In other words, media from the source only flows when a MediaStreamTrack object is both unmuted and enabled. For example, a video element sourced by a muted or disabled MediaStreamTrack (contained in a MediaStream ), is playing but rendering blackness.

For a newly created MediaStreamTrack object, the following applies: the track is always enabled unless stated otherwise (for example when cloned) and the muted state reflects the state of the source at the time the track is created.

4.3.2 Tracks and Constraints

MediaStreamTrack is a constrainable object as defined in the Constrainable Pattern section. Constraints are set on tracks and may affect sources.

Whether Constraints were provided at track initialization time or need to be established later at runtime, the APIs defined in the ConstrainablePattern Interface allow the retrieval and manipulation of the constraints currently established on a track.

Once ended, a track will continue exposing a list of inherent constrainable track properties. This list contains deviceId, facingMode and groupId.

4.3.3 Interface Definition

WebIDL[Exposed=Window]
interface MediaStreamTrack : EventTarget {
  readonly attribute DOMString kind;
  readonly attribute DOMString id;
  readonly attribute DOMString label;
  attribute boolean enabled;
  readonly attribute boolean muted;
  attribute EventHandler onmute;
  attribute EventHandler onunmute;
  readonly attribute MediaStreamTrackState readyState;
  attribute EventHandler onended;
  MediaStreamTrack clone();
  undefined stop();
  MediaTrackCapabilities getCapabilities();
  MediaTrackConstraints getConstraints();
  MediaTrackSettings getSettings();
  Promise<undefined> applyConstraints(optional MediaTrackConstraints constraints = {});
};
Attributes
kind of type DOMString, readonly

The kind attribute MUST return the string "audio" if this object represents an audio track or "video" if this object represents a video track.

id of type DOMString, readonly

When a MediaStreamTrack is created, the User Agent MUST generate an identifier string, and MUST initialize the object's id attribute to that string, unless the object is created as part of a special purpose algorithm that specifies how the stream id must be initialized. See MediaStream.id attribute for guidelines on how to generate such an identifier.

An example of an algorithm that specifies how the track id must be initialized is the algorithm to represent an incoming network component with a MediaStreamTrack object. [WEBRTC]

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

label of type DOMString, readonly

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

enabled of type boolean

The enabled attribute controls the enabled state for the object.

On getting, the attribute MUST return the value to which it was last set. On setting, it MUST be set to the new value.

Note

Thus, after a MediaStreamTrack has ended, its enabled attribute still changes value when set; it just doesn't do anything with that new value.

muted of type boolean, readonly

The muted attribute MUST return true if the track is muted, and false otherwise.

onmute of type EventHandler

The event type of this event handler is mute.

onunmute of type EventHandler

The event type of this event handler is unmute.

readyState of type MediaStreamTrackState, readonly

The readyState attribute represents the state of the track. It MUST return the value as most recently set by the User Agent.

onended of type EventHandler

The event type of this event handler is ended.

Methods
clone

Clones this MediaStreamTrack.

When the clone() method is invoked, the User Agent MUST return the result of cloning this track.

stop

When a MediaStreamTrack object's stop() method is invoked, the User Agent MUST run following steps:

  1. Let track be the current MediaStreamTrack object.

  2. If track.readyState is "ended", then abort these steps.

  3. Notify track's source that track is ended.

    A source that is notified of a track ending will be stopped, unless other MediaStreamTrack objects depend on it.

  4. Set track.readyState to "ended".

getCapabilities

Returns the capabilites of the source that this MediaStreamTrack, the constrainable object, represents.

See ConstrainablePattern Interface for the definition of this method.

Since this method gives likely persistent, cross-origin information about the underlying device, it adds to the fingerprint surface of the device.(This is a fingerprinting vector.)

getConstraints

See ConstrainablePattern Interface for the definition of this method.

getSettings

When a MediaStreamTrack object's MediaStreamTrack.getSettings() method is invoked, the User Agent MUST run following steps:

  1. Let track be the current MediaStreamTrack object.

  2. If track.MediaStreamTrack.readyState is "ended", run the following sub steps:

    1. Let settings be a new MediaTrackSettings dictionary.

    2. For each property of the list of inherent constrainable track properties, add a corresponding property to settings if track had such property at the time it was ended, with the value at the time track was ended.

    3. Return settings.

  3. Return the current settings of the track as defined in ConstrainablePattern Interface.

applyConstraints

When a MediaStreamTrack object's applyConstraints() method is invoked, the User Agent MUST run following steps:

  1. Let track be the current MediaStreamTrack object.

  2. If track.readyState is "ended", run the following sub steps:

    1. Let p be a new promise.

    2. resolve p with undefined.

    3. Return p.

  3. Invoke and return the result of the applyConstraints template method where:

WebIDLenum MediaStreamTrackState {
  "live",
  "ended"
};
MediaStreamTrackState Enumeration description
live

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.

ended

The track has ended (the track's underlying media source is no longer providing data, and will never provide more data for this track). Once a track enters this state, it never exits it.

For example, a video track in a MediaStream ends when the user unplugs the USB web camera that acts as the track's media source.

4.3.4 MediaTrackSupportedConstraints

MediaTrackSupportedConstraints represents the list of constraints recognized by a User Agent for controlling the Capabilities of a MediaStreamTrack object. This dictionary is used as a function return value, and never as an operation argument.

Future specifications can extend the MediaTrackSupportedConstraints dictionary by defining a partial dictionary with dictionary members of type boolean.

Note

The constraints specified in this specification apply only to instances of MediaStreamTrack generated by MediaDevices.getUserMedia(), unless stated otherwise in other specifications.

WebIDLdictionary MediaTrackSupportedConstraints {
  boolean width = true;
  boolean height = true;
  boolean aspectRatio = true;
  boolean frameRate = true;
  boolean facingMode = true;
  boolean resizeMode = true;
  boolean sampleRate = true;
  boolean sampleSize = true;
  boolean echoCancellation = true;
  boolean autoGainControl = true;
  boolean noiseSuppression = true;
  boolean latency = true;
  boolean channelCount = true;
  boolean deviceId = true;
  boolean groupId = true;
};
Dictionary MediaTrackSupportedConstraints Members
width of type boolean, defaulting to true
See width for details.
height of type boolean, defaulting to true
See height for details.
aspectRatio of type boolean, defaulting to true
See aspectRatio for details.
frameRate of type boolean, defaulting to true
See frameRate for details.
facingMode of type boolean, defaulting to true
See facingMode for details.
resizeMode of type boolean, defaulting to true
See resizeMode for details.
sampleRate of type boolean, defaulting to true
See sampleRate for details.
sampleSize of type boolean, defaulting to true
See sampleSize for details.
echoCancellation of type boolean, defaulting to true
See echoCancellation for details.
autoGainControl of type boolean, defaulting to true
See autoGainControl for details.
noiseSuppression of type boolean, defaulting to true
See noiseSuppression for details.
latency of type boolean, defaulting to true
See latency for details.
channelCount of type boolean, defaulting to true
See channelCount for details.
deviceId of type boolean, defaulting to true
See deviceId for details.
groupId of type boolean, defaulting to true
See groupId for details.

4.3.5 MediaTrackCapabilities

MediaTrackCapabilities represents the Capabilities of a MediaStreamTrack object.

Future specifications can extend the MediaTrackCapabilities dictionary by defining a partial dictionary with dictionary members of appropriate type.

WebIDLdictionary MediaTrackCapabilities {
  ULongRange width;
  ULongRange height;
  DoubleRange aspectRatio;
  DoubleRange frameRate;
  sequence<DOMString> facingMode;
  sequence<DOMString> resizeMode;
  ULongRange sampleRate;
  ULongRange sampleSize;
  sequence<boolean> echoCancellation;
  sequence<boolean> autoGainControl;
  sequence<boolean> noiseSuppression;
  DoubleRange latency;
  ULongRange channelCount;
  DOMString deviceId;
  DOMString groupId;
};
Dictionary MediaTrackCapabilities Members
width of type ULongRange
See width for details.
height of type ULongRange
See height for details.
aspectRatio of type DoubleRange
See aspectRatio for details.
frameRate of type DoubleRange
See frameRate for details.
facingMode of type sequence<DOMString>

A camera can report multiple facing modes. For example, in a high-end telepresence solution with several cameras facing the user, a camera to the left of the user can report both "left" and "user". See facingMode for additional details.

resizeMode of type sequence<DOMString>

The User Agent MAY use cropping and downscaling to offer more resolution choices than this camera naturally produces. The reported sequence MUST list all the means the UA may employ to derive resolution choices for this camera. The value "none" MUST be present, indicating the ability to constrain the UA from cropping and downscaling. See resizeMode for additional details.

sampleRate of type ULongRange
See sampleRate for details.
sampleSize of type ULongRange
See sampleSize for details.
echoCancellation of type sequence<boolean>

If the source cannot do echo cancellation a single false is reported. If echo cancellation cannot be turned off, a single true is reported. If the script can control the feature, the source reports a list with both true and false as possible values. See echoCancellation for additional details.

autoGainControl of type sequence<boolean>

If the source cannot do auto gain control a single false is reported. If auto gain control cannot be turned off, a single true is reported. If the script can control the feature, the source reports a list with both true and false as possible values. See autoGainControl for additional details.

noiseSuppression of type sequence<boolean>

If the source cannot do noise suppression a single false is reported. If noise suppression cannot be turned off, a single true is reported. If the script can control the feature, the source reports a list with both true and false as possible values. See noiseSuppression for additional details.

latency of type DoubleRange
See latency for details.
channelCount of type ULongRange
See channelCount for details.
deviceId of type DOMString
See deviceId for details.
groupId of type DOMString
See groupId for details.

4.3.6 MediaTrackConstraints

WebIDLdictionary MediaTrackConstraints : MediaTrackConstraintSet {
  sequence<MediaTrackConstraintSet> advanced;
};
Dictionary MediaTrackConstraints Members
advanced of type sequence<MediaTrackConstraintSet>

See Constraints and ConstraintSet for the definition of this element.

Future specifications can extend the MediaTrackConstraintSet dictionary by defining a partial dictionary with dictionary members of appropriate type.

WebIDLdictionary MediaTrackConstraintSet {
  ConstrainULong width;
  ConstrainULong height;
  ConstrainDouble aspectRatio;
  ConstrainDouble frameRate;
  ConstrainDOMString facingMode;
  ConstrainDOMString resizeMode;
  ConstrainULong sampleRate;
  ConstrainULong sampleSize;
  ConstrainBoolean echoCancellation;
  ConstrainBoolean autoGainControl;
  ConstrainBoolean noiseSuppression;
  ConstrainDouble latency;
  ConstrainULong channelCount;
  ConstrainDOMString deviceId;
  ConstrainDOMString groupId;
};
Dictionary MediaTrackConstraintSet Members
width of type ConstrainULong
See width for details.
height of type ConstrainULong
See height for details.
aspectRatio of type ConstrainDouble
See aspectRatio for details.
frameRate of type ConstrainDouble
See frameRate for details.
facingMode of type ConstrainDOMString
See facingMode for details.
resizeMode of type ConstrainDOMString
See resizeMode for details.
sampleRate of type ConstrainULong
See sampleRate for details.
sampleSize of type ConstrainULong
See sampleSize for details.
echoCancellation of type ConstrainBoolean
See echoCancellation for details.
autoGainControl of type ConstrainBoolean
See autoGainControl for details.
noiseSuppression of type ConstrainBoolean
See noiseSuppression for details.
latency of type ConstrainDouble
See latency for details.
channelCount of type ConstrainULong
See channelCount for details.
deviceId of type ConstrainDOMString
See deviceId for details.
groupId of type ConstrainDOMString
See groupId for details.

4.3.7 MediaTrackSettings

MediaTrackSettings represents the Settings of a MediaStreamTrack object.

Future specifications can extend the MediaTrackSettings dictionary by defining a partial dictionary with dictionary members of appropriate type.

WebIDLdictionary MediaTrackSettings {
  long width;
  long height;
  double aspectRatio;
  double frameRate;
  DOMString facingMode;
  DOMString resizeMode;
  long sampleRate;
  long sampleSize;
  boolean echoCancellation;
  boolean autoGainControl;
  boolean noiseSuppression;
  double latency;
  long channelCount;
  DOMString deviceId;
  DOMString groupId;
};
Dictionary MediaTrackSettings Members
width of type long
See width for details.
height of type long
See height for details.
aspectRatio of type double
See aspectRatio for details.
frameRate of type double
See frameRate for details.
facingMode of type DOMString
See facingMode for details.
resizeMode of type DOMString
See resizeMode for details.
sampleRate of type long
See sampleRate for details.
sampleSize of type long
See sampleSize for details.
echoCancellation of type boolean
See echoCancellation for details.
autoGainControl of type boolean
See autoGainControl for details.
noiseSuppression of type boolean
See noiseSuppression for details.
latency of type double
See latency for details.
channelCount of type long
See channelCount for details.
deviceId of type DOMString
See deviceId for details.
groupId of type DOMString
See groupId for details.

4.3.8 Constrainable Properties

The names of the initial set of constrainable properties for MediaStreamTrack are defined below.

The following constrainable properties are defined to apply to both video and audio MediaStreamTrack objects:

Property Name Values Notes
deviceId DOMString The identifier of the device generating the content of the MediaStreamTrack. It conforms with the definition of MediaDeviceInfo.deviceId. Note that the setting of this property is uniquely determined by the source that is attached to the MediaStreamTrack. In particular, getCapabilities() will return only a single value for deviceId. This property can therefore be used for initial media selection with getUserMedia(). However, it is not useful for subsequent media control with applyConstraints(), since any attempt to set a different value will result in an unsatisfiable ConstraintSet. If a string of length 0 is used as a deviceId value constraint with getUserMedia(), it MAY be interpreted as if the constraint is not specified.
groupId DOMString The document-unique group identifier for the device generating the content of the MediaStreamTrack. It conforms with the definition of MediaDeviceInfo.groupId. Note that the setting of this property is uniquely determined by the source that is attached to the MediaStreamTrack. In particular, getCapabilities() will return only a single value for groupId. Since this property is not stable between browsing sessions, its usefulness for initial media selection with getUserMedia() is limited. It is not useful for subsequent media control with applyConstraints(), since any attempt to set a different value will result in an unsatisfiable ConstraintSet.

The following constrainable properties are defined to apply only to video MediaStreamTrack objects:

Property Name Values Notes
width ConstrainULong The width or width range, in pixels. As a capability, the range should span the video source's pre-set width values with min being equal to 1 and max being the largest width. The User Agent MUST support downsampling to any value between the min width range value and the native resolution width.
height ConstrainULong The height or height range, in pixels. As a capability, the range should span the video source's pre-set height values with min being equal to 1 and max being the largest height. The User Agent MUST support downsampling to any value between the min height range value and the native resolution height.
frameRate ConstrainDouble The exact frame rate (frames per second) or frame rate range. If video source's pre-set can determine frame rate values, the range, as a capacity, should span the video source's pre-set frame rate values with min being equal to 0 and max being the largest frame rate. The User Agent MUST support frame rates obtained from integral decimation of the native resolution frame rate. If this frame rate cannot be determined (e.g. the source does not natively provide a frame rate, or the frame rate cannot be determined from the source stream), then this value MUST refer to the User Agent's vsync display rate.
aspectRatio ConstrainDouble The exact aspect ratio (width in pixels divided by height in pixels, represented as a double rounded to the tenth decimal place) or aspect ratio range.
facingMode ConstrainDOMString This string (or each string, when a list) should be one of the members of VideoFacingModeEnum. The members describe the directions that the camera can face, as seen from the user's perspective. Note that getConstraints may not return exactly the same string for strings not in this enum. This preserves the possibility of using a future version of WebIDL enum for this property.
resizeMode ConstrainDOMString This string (or each string, when a list) should be one of the members of VideoResizeModeEnum. The members describe the means by which the resolution can be derived by the UA. In other words, whether the UA is allowed to use cropping and downscaling on the camera output.

The UA MAY disguise concurrent use of the camera, by cropping and/or downscaling to mimic native resolutions when "none" is used, but only when the camera is in use in another browsing context.(This is a fingerprinting vector.)

Note that getConstraints may not return exactly the same string for strings not in this enum. This preserves the possibility of using a future version of WebIDL enum for this property.

On systems where it's desirable to sometimes automatically flip the X and Y axis of the resulting captured video in response to ongoing environmental factors, the width, height and aspectRatio constraints and capabilities MUST remain unaffected in all algorithms and be considered in the primary orientation only, except for the getSettings() algorithm where settings for these constrainable properties MUST be flipped if necessary to match the returned dimensions of the captured video at any point in time.

The primary orientation of a system that supports flipping the X and Y axis of resulting captured video is defined by the User Agent for the particular system.

Note

On systems that support automatic switching between landscape and portrait mode, User Agents are encouraged to make landscape mode the primary orientation.

WebIDLenum VideoFacingModeEnum {
  "user",
  "environment",
  "left",
  "right"
};
VideoFacingModeEnum Enumeration description
user

The source is facing toward the user (a self-view camera).

environment

The source is facing away from the user (viewing the environment).

left

The source is facing to the left of the user.

right

The source is facing to the right of the user.

Below is an illustration of the video facing modes in relation to the user.
Illustration of video facing modes in relation to user

WebIDLenum VideoResizeModeEnum {
  "none",
  "crop-and-scale"
};
VideoResizeModeEnum Enumeration description
none

This resolution and frame rate is offered by the camera, its driver, or the OS.

Note: The UA MAY report this value to disguise concurrent use, but only when the camera is in use in another browsing context.(This is a fingerprinting vector.)

crop-and-scale

This resolution is downscaled and/or cropped from a higher camera resolution by the User Agent, or its frame rate is decimated by the User Agent. The media MUST NOT be upscaled, stretched or have fake data created that did not occur in the input source.

The following constrainable properties are defined to apply only to audio MediaStreamTrack objects:

Property Name Values Notes
sampleRate ConstrainULong The sample rate in samples per second for the audio data.
sampleSize ConstrainULong The linear sample size in bits. This constraint can only be satisfied for audio devices that produce linear samples.
echoCancellation ConstrainBoolean When one or more audio streams is being played in the processes of various microphones, it is often desirable to attempt to remove all the sound being played from the input signals recorded by the microphones. This is referred to as echo cancellation. There are cases where it is not needed and it is desirable to turn it off so that no audio artifacts are introduced. This allows applications to control this behavior.
autoGainControl ConstrainBoolean Automatic gain control is often desirable on the input signal recorded by the microphone. There are cases where it is not needed and it is desirable to turn it off so that the audio is not altered. This allows applications to control this behavior.
noiseSuppression ConstrainBoolean Noise suppression is often desirable on the input signal recorded by the microphone. There are cases where it is not needed and it is desirable to turn it off so that the audio is not altered. This allows applications to control this behavior.
latency ConstrainDouble The latency or latency range, in seconds. The latency is the time between start of processing (for instance, when sound occurs in the real world) to the data being available to the next step in the process. Low latency is critical for some applications; high latency may be acceptable for other applications because it helps with power constraints. The number is expected to be the target latency of the configuration; the actual latency may show some variation from that.
channelCount ConstrainULong The number of independent channels of sound that the audio data contains, i.e. the number of audio samples per sample frame.

4.4 MediaStreamTrackEvent

The addtrack and removetrack events use the MediaStreamTrackEvent interface.

The addtrack and removetrack events notify the script that the track set of a MediaStream has been updated by the User Agent.

Firing a track event named e with a MediaStreamTrack track means that an event with the name e, which does not bubble (except where otherwise stated) and is not cancelable (except where otherwise stated), and which uses the MediaStreamTrackEvent interface with the track attribute set to track, MUST be created and dispatched at the given target.

WebIDL[Exposed=Window]
interface MediaStreamTrackEvent : Event {
  constructor(DOMString type, MediaStreamTrackEventInit eventInitDict);
  [SameObject] readonly attribute MediaStreamTrack track;
};

Constructors

constructor()

Constructs a new MediaStreamTrackEvent.

Attributes

track of type MediaStreamTrack, readonly

The track attribute represents the MediaStreamTrack object associated with the event.

WebIDLdictionary MediaStreamTrackEventInit : EventInit {
  required MediaStreamTrack track;
};

Dictionary MediaStreamTrackEventInit Members

track of type MediaStreamTrack, required

5. The model: sources, sinks, constraints, and settings

This section is non-normative.

User Agents provide a media pipeline from sources to sinks. In a User Agent, sinks are the <img>, <video>, and <audio> tags. Traditional sources include streamed content, files, and web resources. The media produced by these sources typically does not change over time - these sources can be considered to be static.

The sinks that display these sources to the user (the actual tags themselves) have a variety of controls for manipulating the source content. For example, an <img> tag scales down a huge source image of 1600x1200 pixels to fit in a rectangle defined with width="400" and height="300".

Sources have a lifetime. By default, a source lifetime is tied to the context that created it. For instance, sources created by MediaDevices.getUserMedia() are considered as created by its navigator.mediaDevices context. Similarly, sources of RTCRtpReceiver objects are bound to the RTCPeerConnection itself, which is bound to its creation context. Except if stated explicitly in the definition of specific sources, a source is always stopped when its creation context goes away. It should be noted that two sources of different contexts may use the same capture device at the same time. One source can be stopped independently of the other one.

The getUserMedia API adds dynamic sources such as microphones and cameras - the characteristics of these sources can change in response to application needs. These sources can be considered to be dynamic in nature. A <video> element that displays media from a dynamic source can either perform scaling or it can feed back information along the media pipeline and have the source produce content more suitable for display.

Note

Note: This sort of feedback loop is obviously just enabling an "optimization", but it's a non-trivial gain. This optimization can save battery, allow for less network congestion, etc...

Note that MediaStream sinks (such as <video>, <audio>, and even RTCPeerConnection) will continue to have mechanisms to further transform the source stream beyond that which the Settings, Capabilities, and Constraints described in this specification offer. (The sink transformation options, including those of RTCPeerConnection, are outside the scope of this specification.)

The act of changing or applying a track constraint may affect the settings of all tracks sharing that source and consequently all down-level sinks that are using that source. Many sinks may be able to take these changes in stride, such as the <video> element or RTCPeerConnection. Others like the Recorder API may fail as a result of a source setting change.

The RTCPeerConnection is an interesting object because it acts simultaneously as both a sink and a source for over-the-network streams. As a sink, it has source transformational capabilities (e.g., lowering bit-rates, scaling-up / down resolutions, and adjusting frame-rates), and as a source it could have its own settings changed by a track source.

To illustrate how changes to a given source impact various sinks, consider the following example. This example only uses width and height, but the same principles apply to all of the Settings exposed in this specification. In the first figure a home client has obtained a video source from its local video camera. The source's width and height settings are 800 pixels and 600 pixels, respectively. Three MediaStream objects on the home client contain tracks that use this same <deviceId. The three media streams are connected to three different sinks: a <video> element (A), another <video> element (B), and a peer connection (C). The peer connection is streaming the source video to a remote client. On the remote client there are two media streams with tracks that use the peer connection as a source. These two media streams are connected to two <video> element sinks (Y and Z).

Changing media stream source effects: before the requested change

Note that at this moment, all of the sinks on the home client must apply a transformation to the original source's provided dimension settings. B is scaling the video down, A is scaling the video up (resulting in loss of quality), and C is also scaling the video up slightly for sending over the network. On the remote client, sink Y is scaling the video way down, while sink Z is not applying any scaling.

In response to applyConstraints() being called, one of the tracks wants a higher resolution (1920 by 1200 pixels) from the home client's video source.

Changing media stream source effects: after the requested change

Note that the source change immediately affects all of the tracks and sinks on the home client, but does not impact any of the sinks (or sources) on the remote client. With the increase in the home client source video's dimensions, sink A no longer has to perform any scaling, while sink B must scale down even further than before. Sink C (the peer connection) must now scale down the video in order to keep the transmission constant to the remote client.

While not shown, an equally valid settings change request could be made on the remote client's side. In addition to impacting sink Y and Z in the same manner as A, B and C were impacted earlier, it could lead to re-negotiation with the peer connection on the home client in order to alter the transformation that it is applying to the home client's video source. Such a change is NOT REQUIRED to change anything related to sink A or B or the home client's video source.

Note that this specification does not define a mechanism by which a change to the remote client's video source could automatically trigger a change to the home client's video source. Implementations may choose to make such source-to-sink optimizations as long as they only do so within the constraints established by the application, as the next example demonstrates.

It is fairly obvious that changes to a given source will impact sink consumers. However, in some situations changes to a given sink may also cause implementations to adjust a source's settings. This is illustrated in the following figures. In the first figure below, the home client's video source is sending a video stream sized at 1920 by 1200 pixels. The video source is also unconstrained, such that the exact source dimensions are flexible as far as the application is concerned. Two MediaStream objects contain tracks with the same deviceId, and those MediaStreams are connected to two different <video> element sinks A and B. Sink A has been sized to width="1920" and height="1200" and is displaying the source's video content without any transformations. Sink B has been sized smaller and, as a result, is scaling the video down to fit its rectangle of 320 pixels across by 200 pixels down.

Changing media stream sinks may affect sources: before the requested change

When the application changes sink A to a smaller dimension (from 1920 to 1024 pixels wide and from 1200 to 768 pixels tall), the User Agent's media pipeline may recognize that none of its sinks require the higher source resolution, and needless work is being done both on the part of the source and sink A. In such a case and without any other constraints forcing the source to continue producing the higher resolution video, the media pipeline MAY change the source resolution:

Changing media stream sinks may affect sources: after the requested change

In the above figure, the home client's video source resolution was changed to the greater of that from sink A and B in order to optimize playback. While not shown above, the same behavior could apply to peer connections and other sinks.

It is possible that constraints can be applied to a track which a source is unable to satisfy, either because the source itself cannot satisfy the constraint or because the source is already satisfying a conflicting constraint. When this happens, the promise returned from applyConstraints() will be rejected, without applying any of the new constraints. Since no change in constraints occurs in this case, there is also no required change to the source itself as a result of this condition. Here is an example of this behavior.

In this example, two media streams each have a video track that share the same source. The first track initially has no constraints applied. It is connected to sink N. Sink N has a resolution of 800 by 600 pixels and is scaling down the source's resolution of 1024 by 768 to fit. The other track has a required constraint forcing off the source's fill light; it is connected to sink P. Sink P has a width and height equal to that of the source.

Overconstrained application

Now, the first track adds a required constraint that the fill light should be forced on. At this point, both required constraints cannot be satisfied by the source (the fill light cannot be simultaneously on and off at the same time). Since this state was caused by the first track's attempt to apply a conflicting constraint, the constraint application fails and there is no change in the source's settings nor to the constraints on either track.

6. MediaStreams in Media Elements

A MediaStream may be assigned to media elements. 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 media element is potentially playing. The timeline does not increment when the playout of the MediaStream is paused.

User Agents that support this specification MUST support the srcObject attribute of the HTMLMediaElement interface defined in [HTML], which includes support for playing MediaStream objects.

The [HTML] document outlines how the HTMLMediaElement works with a media provider object. The following applies when the media provider object is a MediaStream:

The nature of the MediaStream places certain restrictions on the behavior of attributes of the associated HTMLMediaElement and on the operations that can be performed on it, as shown below:

Attribute Name Attribute Type Setter/Getter Behavior When Provider is a MediaStream Additional considerations
preload DOMString On getting: none. On setting: ignored. A MediaStream cannot be preloaded.
buffered TimeRanges buffered.length MUST return 0. A MediaStream cannot be preloaded. Therefore, the amount buffered is always an empty time range.
currentTime double Any non-negative integer. The initial value is 0 and the values increments linearly in real time whenever the stream is playing. The value is the official playback position, in seconds. Any attempt to alter it MUST be ignored.
seeking boolean false A MediaStream is not seekable. Therefore, this attribute MUST always return the value false.
defaultPlaybackRate double On getting: 1.0. On setting: ignored. A MediaStream is not seekable. Therefore, this attribute MUST always return the value 1.0 and any attempt to alter it MUST be ignored. Note that this also means that the ratechange event will not fire.
playbackRate double On getting: 1.0. On setting: ignored. A MediaStream is not seekable. Therefore, this attribute MUST always return the value 1.0 and any attempt to alter it MUST be ignored. Note that this also means that the ratechange event will not fire.
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. A MediaStream is not seekable.
loop boolean true, false Setting the loop attribute has no effect since a MediaStream has no defined end and therefore cannot be looped.

Since none of the setters listed above alter internal state of the HTMLMediaElement, once a MediaStream is no longer the element's assigned media provider object, the attributes listed will appear to resume the values they had before a stream was assigned to the element.

Note

A MediaStream stops being the element's assigned media provider object when srcObject is assigned null or a non-stream object, just ahead of the media element load algorithm. As a result, the ratechange event may fire (from step 7) if playbackRate and defaultPlaybackRate were different from before a MediaStream was assigned.

7. Error Handling

Some operations throw or fire OverconstrainedError. This is an extension of DOMException that carries additional information related to constraints failure.

7.1 OverconstrainedError Interface

WebIDL[Exposed=Window]
interface OverconstrainedError : DOMException {
  constructor(DOMString constraint, optional DOMString message = "");
  readonly attribute DOMString constraint;
};

7.1.1 Constructors

OverconstrainedError

Run the following steps:

  1. Let constraint be the constructor's first argument.

  2. Let message be the constructor's second argument.

  3. Let e be a new OverconstrainedError object.

  4. Invoke the DOMException constructor of e with the message argument set to message and the name argument set to "OverconstrainedError".

    Note

    This name does not have a mapping to a legacy code so e's code attribute will return 0.

  5. Set e.constraint to constraint.

  6. Return e.

7.1.2 Attributes

constraint of type DOMString, readonly

The name of a constraint associated with this error, or "" if no specific constraint name is revealed.

8. Event summary

This section is non-normative.

The following events fire on MediaStream objects:

Event name Interface Fired when...
addtrack MediaStreamTrackEvent A new MediaStreamTrack has been added to this stream. Note that this event is not fired when the script directly modifies the tracks of a MediaStream.
removetrack MediaStreamTrackEvent A MediaStreamTrack has been removed from this stream. Note that this event is not fired when the script directly modifies the tracks of a MediaStream.

The following events fire on MediaStreamTrack objects:

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

The MediaStreamTrack object's source will no longer provide any data, either because the user revoked the permissions, or because the source device has been ejected, or because the remote peer permanently stopped sending data.

The following events fire on MediaDevices objects:

Event name Interface Fired when...
devicechange Event The set of media devices, available to the User Agent, has changed. The current list devices can be retrieved with the enumerateDevices() method.

9. Enumerating Local Media Devices

This section describes an API that the script can use to query the User Agent about connected media input and output devices (for example a web camera or a headset).

9.2 MediaDevices

The MediaDevices object is the entry point to the API used to examine and get access to media devices available to the User Agent.

On page load, run the following steps:

  1. On the relevant global object, run the following steps:

    1. Create three internal slots: [[devicesLiveMap]], [[devicesAccessibleMap]], and [[kindsAccessibleMap]], each initialized to a different empty object.

    2. Create one internal slot: [[storedDeviceList]], initialized to null.

    3. Create one internal slot: [[canExposeCameraInfo]], initialized to false.

    4. Create one internal slot: [[canExposeMicrophoneInfo]], initialized to false.

    5. Create one internal slot: [[mediaStreamTrackSources]], initialized to an empty set.

  2. For each kind of device, kind, that MediaDevices.getUserMedia() exposes, set [[kindsAccessibleMap]][kind] either to true if the permission state of the permission associated with kind (e.g. "camera", "microphone"), is "granted", or to false otherwise.

  3. For each individual device that MediaDevices.getUserMedia() exposes, using the device's deviceId, deviceId, set [[devicesLiveMap]][deviceId] to false, and set [[devicesAccessibleMap]][deviceId] either to true if the permission state of the permission associated with the device’s kind and deviceId, is "granted", or to false otherwise.

For each kind of device, kind, that getUserMedia() exposes, whenever a transition occurs of the permission state of the permission associated with kind, run the following steps:

  1. If the transition is to "granted" from another value, then set [[kindsAccessibleMap]][kind] to true.

  2. If the transition is from "granted" to another value, then set [[kindsAccessibleMap]][kind] to false.

For each device that getUserMedia() exposes, whenever a transition occurs of the permission state of the permission associated with the device's kind and the device's deviceId, deviceId, run the following steps:

  1. If the transition is to "granted" from another value, then set [[devicesAccessibleMap]][deviceId] to true, if it isn’t already true.

  2. If the transition is from "granted" to another value, and the device is currently stopped, then set [[devicesAccessibleMap]][deviceId] to false.

When new media input and/or output devices of a MediaDeviceKind are made available where zero devices of that MediaDeviceKind were available before, or the lone input and/or output device of a MediaDeviceKind becomes unavailable, the User Agent MUST run the following device change notification steps in browsing contexts for which device enumeration can proceed is true and device information can be exposed is false, but in no other contexts:

  1. If [[storedDeviceList]] already lists the exact same set of devices in the same order as the list of devices that would be generated by a call to enumerateDevices now, then abort these steps.

    Note

    Due to the enumerateDevices algorithm, the above step limits firing the devicechange event to documents allowed to use enumerateDevices to enumerate devices of a particular MediaDeviceKind.

  2. Set [[storedDeviceList]] to null.

  3. Queue a task that fires a simple event named devicechange at the MediaDevices object.

    The User Agent MAY combine firing multiple events into firing one event when several events are due or when multiple devices are added or removed at the same time, e.g. a camera with a microphone.

Additionally, when new media input and/or output devices are made available, or any available input and/or output device becomes unavailable, or the system default for input and/or output devices changed, the User Agent MUST run the device change notification steps in browsing contexts for which device enumeration can proceed is true and device information can be exposed is true, but in no other contexts.

In both cases above, if a browsing context that was traversed comes to meet the device enumeration can proceed criteria later (e.g. gains focus), the User Agent MUST execute the device change notification steps on the browsing context at that time.

Note

These events are potentially triggered simultaneously on documents of different origins. User Agents MAY add fuzzing on the timing of events to avoid cross-origin activity correlation.(This is a fingerprinting vector.)

WebIDL[Exposed=Window, SecureContext]
interface MediaDevices : EventTarget {
  attribute EventHandler ondevicechange;
  Promise<sequence<MediaDeviceInfo>> enumerateDevices();
};

Attributes

ondevicechange of type EventHandler

The event type of this event handler is devicechange.

Methods

enumerateDevices

Collects information about the User Agent's available media input and output devices.

This method returns a promise. The promise will be fulfilled with a sequence of MediaDeviceInfo objects representing the User Agent's available media input and output devices if enumeration is successful.

Elements of this sequence that represent input devices will be of type InputDeviceInfo which extends MediaDeviceInfo.

Camera and microphone sources SHOULD be enumerable. Specifications that add additional types of source will provide recommendations about whether the source type should be enumerable.

When the enumerateDevices() method is called, the User Agent must run the following steps:

  1. Let p be a new promise.

  2. Run the following steps in parallel:

    1. Let document be the relevant settings object's responsible document.

    2. The User Agent MUST wait to proceed to the next step until device enumeration can proceed is true.

      The User Agent MAY wait to proceed to the next step until document is fully active and has focus.

    3. Let resultList be an empty list.

    4. If [[storedDeviceList]] is not null, run the following sub steps:

      1. For each MediaDeviceInfo object in [[storedDeviceList]], storedDeviceInfo:

      2. resolve p with resultList.

      3. Abort these steps.

    5. Probe the User Agent for available media devices, and let be deviceList be the list of all discovered devices.

    6. Let microphoneList, cameraList and otherDeviceList be empty lists.

    7. Run the following sub steps for each discovered device in deviceList, device:

      1. If device is not a microphone, or document is not allowed to use the feature identified by "microphone", abort these sub steps and continue with the next device (if any).

      2. Let deviceInfo be the result of creating a device info object to represent device.

      3. If device is the system default microphone, prepend deviceInfo to microphoneList. Otherwise, append deviceInfo to microphoneList.

    8. Run the following sub steps for each discovered device in deviceList, device:

      1. If device is not a camera, or document is not allowed to use the feature identified by "camera", abort these sub steps and continue with the next device (if any).

      2. Let deviceInfo be the result of creating a device info object to represent device.

      3. If device is the system default camera, prepend deviceInfo to cameraList. Otherwise, append deviceInfo to cameraList.

    9. If device information can be exposed is false, run the following sub steps:

      1. If microphoneList is not empty, truncate microphoneList to its first item.

      2. If cameraList is not empty, truncate cameraList to its first item.

    10. Run the following sub steps for each discovered device in deviceList, device:

      1. If device is a microphone or device is a camera, abort these sub steps and continue with the next device (if any).

      2. Run the exposure decision algorithm for devices other than camera and microphone, with device, microphoneList and cameraList as input. If the result of this algorithm is false, abort these sub steps and continue with the next device (if any).

      3. Let deviceInfo be the result of creating a device info object to represent device.

      4. If device is the system default audio output, prepend deviceInfo to otherDeviceList. Otherwise, append deviceInfo to otherDeviceList.

    11. Append to resultList all devices of microphoneList in order.

    12. Append to resultList all devices of cameraList in order.

    13. Append to resultList all devices of otherDeviceList in order.

    14. Set [[storedDeviceList]] to resultList.

    15. resolve p with resultList.

  3. Return p.

Since this method returns persistent information across browsing sessions and origins via the availability of media capture devices, it adds to the fingerprinting surface exposed by the User Agent.(This is a fingerprinting vector.)

As long as the responsible document did not capture, this method will limit exposure to two bits of information: whether there is a camera and whether there is a microphone. A User Agent may mitigate this by pretending the system has a camera and a microphone, for instance until the responsible document calls getUserMedia() with constraints deemed reasonable.(This is a fingerprinting vector.)

After the responsible document started capture, it provides additional persistent cross-origin information via the list of all media capture devices, including their grouping and human readable labels associated with the capture devices, which further adds to the fingerprinting surface. A User Agent may limit exposure by sanitizing device labels. This could for instance mean removing user names found in labels, but keeping device manufacturer or model information. It is important that the sanitized labels allow users to identify the corresponding devices. (This is a fingerprinting vector.)

9.2.1 Access control model

The algorithm described above means that the access to media device information depends on whether or not the responsible document did capture.

For camera and microphone devices, if the browsing context did not capture (i.e. getUserMedia() was not called or never resolved successfully), the MediaDeviceInfo object will contain a valid value for kind but empty strings for deviceId, label, and groupId. Additionally, at most one device of each kind will be listed in enumerateDevices() result.

Otherwise, the MediaDeviceInfo object will contain meaningful values for deviceId, kind, label, and groupId. All available devices are listed in enumerateDevices() result.

To perform creating a device info object to represent a discovered device, device, run the following steps:

  1. Let deviceInfo be a new MediaDeviceInfo object to represent device.

  2. Initialize deviceInfo.kind for device.

  3. If deviceInfo.kind is equal to "videoinput" and camera information can be exposed is false, return deviceInfo.

  4. If deviceInfo.kind is equal to "audioinput" and microphone information can be exposed is false, return deviceInfo.

  5. Initialize deviceInfo.label for device.

  6. If a stored deviceId exists for device, initialize deviceInfo.deviceId to that value. Otherwise, let deviceInfo.deviceId be a newly generated unique identifier as described under deviceId.

  7. If device belongs to the same physical device as a device already represented for document, initialize deviceInfo.groupId to the groupId value of the existing MediaDeviceInfo object. Otherwise, let deviceInfo.groupId be a newly generated unique identifier as described under groupId.

  8. Return deviceInfo

9.2.2 Device information exposure

To perform a device enumeration can proceed check, run the following steps:

  1. If device information can be exposed is true, return true.

  2. If the relevant settings object's responsible document is fully active and has focus, return true.

  3. Return false.

To perform a device information can be exposed check, run the following steps:

  1. If camera information can be exposed is true, return true.

  2. If microphone information can be exposed is true, return true.

  3. Return false.

To perform a camera information can be exposed check, run the following steps:

  1. If any of the local devices of kind "videoinput" are attached to a live MediaStreamTrack in the relevant settings object's responsible document, return true.

  2. Return [[canExposeCameraInfo]].

To perform a microphone information can be exposed check, run the following steps:

  1. If any of the local devices of kind "audioinput" are attached to a live MediaStreamTrack in the relevant settings object's responsible document, return true.

  2. Return [[canExposeMicrophoneInfo]].

9.2.3 Set device information exposure

To set the device information exposure, with a requestedTypes set and value of type boolean, run the following steps:

  1. If "video" is in requestedTypes and [[canExposeCameraInfo]] is not value, run the following sub steps:

    1. Set [[canExposeCameraInfo]] to value.

    2. Set [[storedDeviceList]] to null.

  2. If "audio" is in requestedTypes and [[canExposeMicrophoneInfo]] is not value, run the following sub steps:

    1. Set [[canExposeMicrophoneInfo]] to value.

    2. Set [[storedDeviceList]] to null.

Note

A User Agent MAY at any point set the device information exposure back to false, for instance if the User Agent decides to revoke device access on a given browsing context.

9.2.4 Exposure decision algorithm for devices other than camera and microphone

The exposure decision algorithm for devices other than camera and microphone takes a device, microphoneList and cameraList as input and returns a boolean to decide whether exposing the device to the web page or not.

By default, it returns false.

Other specifications can define the algorithm for specific device types.

9.3 Device Info

WebIDL[Exposed=Window, SecureContext]
interface MediaDeviceInfo {
  readonly attribute DOMString deviceId;
  readonly attribute MediaDeviceKind kind;
  readonly attribute DOMString label;
  readonly attribute DOMString groupId;
  [Default] object toJSON();
};

Attributes

deviceId of type DOMString, readonly

The identifier of the represented device. The device MUST be uniquely identified by its identifier and its kind.

To to ensure stored identifiers are recognized, the identifier MUST be the same in documents of the same origin in top-level browsing contexts. In nested browsing contexts, the decision of whether or not the identifier is the same across documents, MUST follow the User Agent's partitioning rules for storage (such as localStorage), if any, to not interfere with mitigations for cross-site correlation. If the identifier can uniquely identify the user, then it MUST be un-guessable in documents from other origins to prevent the identifier from being used to correlate the same user across different origins. An identifier can be reused across origins as long as it is not tied to the user and can be guessed by other means, like the User-Agent string.

If any local devices have been attached to a live MediaStreamTrack in a page from this origin, or stored permission to access local devices has been granted to this origin, then this identifier MUST be persisted, except as detailed below. Unique and stable identifiers let the application save, identify the availability of, and directly request specific sources, across multiple visits.

However, as long as no local device has been attached to a live MediaStreamTrack in a page from this origin, and no stored permission to access local devices has been granted to this origin, then the User Agent MAY clear this identifier once the last browsing session from this origin has been closed. If the User Agent chooses not to clear the identifier in this condition, then it MUST provide for the user to visibly inspect and delete the identifier, like a cookie.

Since deviceId may persist across browsing sessions and to reduce its potential as a fingerprinting mechanism, deviceId is to be treated as other persistent storage mechanisms such as cookies [COOKIES], in that User Agents MUST NOT persist device identifiers for sites that are blocked from using cookies, and User Agents MUST rotate per-origin device identifiers when other persistent storage are cleared.(This is a fingerprinting vector.)

kind of type MediaDeviceKind, readonly

The kind of the represented device.

label of type DOMString, readonly

A label describing this device (for example "External USB Webcam"). This label is intended to allow the end user to tell the difference between devices. Applications can’t assume that the label contains any specific information, such as the device type or model. If the device has no associated label, then this attribute MUST return the empty string.

groupId of type DOMString, readonly

The group identifier of the represented device. Two devices have the same group identifier if they belong to the same physical device. For example, the audio input and output devices representing the speaker and microphone of the same headset have the same groupId.

The group identifier MUST be uniquely generated for each document.

Methods

toJSON
When called, run [WEBIDL]'s default toJSON steps.
WebIDLenum MediaDeviceKind {
  "audioinput",
  "audiooutput",
  "videoinput"
};
MediaDeviceKind Enumeration description
audioinput

Represents an audio input device; for example a microphone.

audiooutput

Represents an audio output device; for example a pair of headphones.

videoinput

Represents a video input device; for example a webcam.

9.4 Input-specific Device Info

The InputDeviceInfo interface gives access to the capabilities of the input device it represents.

WebIDL[Exposed=Window]
interface InputDeviceInfo : MediaDeviceInfo {
  MediaTrackCapabilities getCapabilities();
};

Methods

getCapabilities

Returns a MediaTrackCapabilities object describing the primary audio or video track of a device's MediaStream (according to its kind value), in the absence of any user-supplied constraints. These capabilities MUST be identical to those that would have been obtained by calling getCapabilities() on the first MediaStreamTrack of this type in a MediaStream returned by getUserMedia({deviceId: id}) where id is the value of the deviceId attribute of this MediaDeviceInfo.

If no access has been granted to any local devices and this InputDeviceInfo has been filtered with respect to unique identifying information (see above description of enumerateDevices() result), then this method returns an empty dictionary.

10. Obtaining local multimedia content

This section extends Navigator and MediaDevices with APIs to request permission to access media input devices available to the User Agent.

Alternatively, a local MediaStream can be captured from certain types of DOM elements, such as the video element [mediacapture-fromelement]. This can be useful for automated testing.

10.1 Legacy Interface Extensions

Note
The definition of getUserMedia() in this section reflects two major changes from the method definition that has existed here for many months.

First, the official definition for the getUserMedia() method, and the one which developers are encouraged to use, is now at MediaDevices. This decision reflected consensus as long as the original API remained available here under the Navigator object for backwards compatibility reasons, since the working group acknowledges that early users of these APIs have been encouraged to define getUserMedia as "var getUserMedia = navigator.getUserMedia || navigator.webkitGetUserMedia || navigator.mozGetUserMedia;" in order for their code to be functional both before and after official implementations of getUserMedia() in popular User Agents. To ensure functional equivalence, the getUserMedia() method here is defined in terms of the method under MediaDevices.

Second, the decision to change all other callback-based methods in the specification to be based on Promises instead required that the navigator.getUserMedia() definition reflect this in its use of navigator.mediaDevices.getUserMedia(). Because navigator.getUserMedia() is now the only callback-based method remaining in the specification, there is ongoing discussion as to a) whether it still belongs in the specification, and b) if it does, whether its syntax should remain callback-based or change in some way to use Promises. Input on these questions is encouraged, particularly from developers actively using today's implementations of this functionality.

Note that the other methods that changed from a callback-based syntax to a Promises-based syntax were not considered to have been implemented widely enough in any form to have to consider legacy usage.

WebIDLpartial interface Navigator {
  [SecureContext] undefined getUserMedia(MediaStreamConstraints constraints,
                                    NavigatorUserMediaSuccessCallback successCallback,
                                    NavigatorUserMediaErrorCallback errorCallback);
};

Methods

getUserMedia

Prompts the user for permission to use their Web cam or other video or audio input.

The constraints argument is a dictionary of type MediaStreamConstraints.

The successCallback will be invoked with a suitable MediaStream object as its argument if the user accepts valid tracks as described in getUserMedia() on MediaDevices.

The errorCallback will be invoked if there is a failure in finding valid tracks or if the user denies permission, as described in getUserMedia() on MediaDevices.

When the getUserMedia() method is called, the User Agent MUST run the following steps:

  1. Let constraints be the method's first argument.

  2. Let successCallback be the callback indicated by the method's second argument.

  3. Let errorCallback be the callback indicated by the method's third argument.

  4. Run the steps specified by the getUserMedia() algorithm with constraints as the argument, and let p be the resulting promise.

  5. Upon fulfillment of p with value stream, run the following step:

    1. Invoke successCallback with stream as the argument.

  6. Upon rejection of p with reason r, run the following step:

    1. Invoke errorCallback with r as the argument.

10.2 MediaDevices Interface Extensions

Note
The definition of getUserMedia() in this section reflects two major changes from the method definition that has existed under Navigator for many months.

First, the official definition for the getUserMedia() method, and the one which developers are encouraged to use, is now the one defined here under MediaDevices. This decision reflected consensus as long as the original API remained available at Navigator.getUserMedia under the Navigator object for backwards compatibility reasons, since the working group acknowledges that early users of these APIs have been encouraged to define getUserMedia as "var getUserMedia = navigator.getUserMedia || navigator.webkitGetUserMedia || navigator.mozGetUserMedia;" in order for their code to be functional both before and after official implementations of getUserMedia() in popular User Agents. To ensure functional equivalence, the getUserMedia() method under Navigator is defined in terms of the method here.

Second, the method defined here is Promises-based, while the one defined under Navigator is currently still callback-based. Developers expecting to find getUserMedia() defined under Navigator are strongly encouraged to read the detailed Note given there.

The getSupportedConstraints method is provided to allow the application to determine which constraints the User Agent recognizes. Applications may need this information to use required constraints reliably or get predictable results from combinatory logic in advanced constraints.

WebIDLpartial interface MediaDevices {
  MediaTrackSupportedConstraints getSupportedConstraints();
  Promise<MediaStream> getUserMedia(optional MediaStreamConstraints constraints = {});
};

Methods

getSupportedConstraints

Returns a dictionary whose members are the constrainable properties known to the User Agent. A supported constrainable property MUST be represented and any constrainable properties not supported by the User Agent MUST NOT be present in the returned dictionary. The values returned represent what the User Agent implements and will not change during a browsing session.

getUserMedia

Prompts the user for permission to use their Web cam or other video or audio input.

The constraints argument is a dictionary of type MediaStreamConstraints.

This method returns a promise. The promise will be fulfilled with a suitable MediaStream object if the user accepts valid tracks as described below.

The promise will be rejected if there is a failure in finding valid tracks or if the user denies permission, as described below.

When the getUserMedia() method is called, the User Agent MUST run the following steps:

  1. Let constraints be the method's first argument.

  2. Let requestedMediaTypes be the set of media types in constraints with either a dictionary value or a value of true.

  3. If requestedMediaTypes is the empty set, return a promise rejected with a TypeError. The word "optional" occurs in the WebIDL due to WebIDL rules, but the argument MUST be supplied in order for the call to succeed.

  4. If the relevant settings object's responsible document is NOT fully active, return a promise rejected with a DOMException object whose name attribute has the value "InvalidStateError".

  5. Let p be a new promise.

  6. Run the following steps in parallel:

    1. The User Agent MUST wait to proceed to the next step until the relevant settings object's responsible document is fully active and has focus.

    2. Let finalSet be an (initially) empty set.

    3. For each media type kind in requestedMediaTypes, run the following steps:

      1. For each possible source for media of type kind, construct an unconstrained MediaStreamTrack with that source as its source.

        Call this set of tracks the candidateSet.

        If candidateSet is the empty set, reject p with a new DOMException object whose name attribute has the value "NotFoundError" and abort these steps.

      2. If the value of the kind entry of constraints is true, set CS to the empty constraint set (no constraint). Otherwise, continue with CS set to the value of the kind entry of constraints.
      3. Remove any constrainable property inside of CS that are not defined for MediaStreamTrack objects of type kind. This means that audio-only constraints inside of "video" and video-only constraints inside of "audio" are simply ignored rather than causing OverconstrainedError.
      4. If CS contains a member that is a required constraint and whose name is not in the list of allowed required constraints for device selection, then reject p with a TypeError, and abort these steps.

      5. Run the SelectSettings algorithm on each track in candidateSet with CS as the constraint set. If the algorithm returns undefined, remove the track from candidateSet. This eliminates devices unable to satisfy the constraints, by verifying that at least one settings dictionary exists that satisfies the constraints.

        If candidateSet is the empty set, let failedConstraint be any required constraint whose fitness distance was infinity for all settings dictionaries examined while executing the SelectSettings algorithm, or "" if there isn't one, and jump to the step labeled Constraint Failure below.

        This error gives information about what the underlying device is not capable of producing, before the user has given any authorization to any device, and can thus be used as a fingerprinting surface.(This is a fingerprinting vector.)

      6. Read the current permission state for all candidate devices in candidateSet that are not attached to a live MediaStreamTrack in the current browsing context. Remove from candidateSet any device for which the permission state is "denied".

        If candidateSet is now empty, indicating that all devices of this type are in state "denied", jump to the step labeled PermissionFailure below.

      7. Optionally, e.g., based on a previously-established user preference, for security reasons, or due to platform limitations, jump to the step labeled Permission Failure below.

      8. Add all tracks from candidateSet to finalSet.

    4. Let stream be a new and empty MediaStream object.

    5. For each media type kind in requestedMediaTypes, run the following sub steps, preferably at the same time:

      Note

      User Agents are encouraged to bundle concurrent requests for different kinds of media into a single user-facing permission prompt.

      1. Request permission to use a PermissionDescriptor with its name member set to the permission name associated with kind (e.g. "camera" for "video", "microphone" for "audio"), and, optionally, consider its deviceId member set to any appropriate device's deviceId, while considering all devices attached to a live and same-permission MediaStreamTrack in the current browsing context to have permission status "granted", resulting in a set of provided media. Same-permission in this context means a MediaStreamTrack that required the same level of permission to obtain as what is being requested (e.g. not isolated).

        When asking the user’s permission, the User Agent MUST disclose whether permission will be granted only to the device chosen, or to all devices of that kind.

        Let track be the provided media, which MUST be precisely one track of type kind from finalSet. The decision of which track to choose from the finalSet is completely up to the User Agent and may be determined by asking the user. Once selected, the source of the MediaStreamTrack MUST NOT change.

        The User Agent SHOULD use the value of the computed fitness distance from the SelectSettings algorithm as an input to the selection algorithm. However, it MAY also use other internally-available information about the devices, such as user preference.

        User Agents are encouraged to default to using the user's primary or system default device for kind (when possible). User Agents MAY allow users to use any media source, including pre-recorded media files.

      2. If the user never responds, this algorithm stalls on this step.

      3. If the result of the request is "denied", jump to the step labeled Permission Failure below.

      4. The result of the request is "granted". If a hardware error such as an OS/program/webpage lock prevents access, remove track from finalSet. If finalSet has no track of type kind, reject p with a new DOMException object whose name attribute has the value "NotReadableError" and abort these steps. Otherwise, restart these sub steps with the updated finalSet.

        If device access fails for any reason other than those listed above, remove track from finalSet. If finalSet has no track of type kind, reject p with a new DOMException object whose name attribute has the value "AbortError" and abort these steps. Otherwise, restart these sub steps with the updated finalSet.

      5. Using the granted device's deviceId, deviceId, set [[devicesLiveMap]][deviceId] to true, if it isn’t already true, and set the [[devicesAccessibleMap]][deviceId] to true, if it isn’t already true.

      6. Add track to stream's track set.

    6. Run the ApplyConstraints algorithm on all tracks in stream with the appropriate constraints. If any of them returns something other than undefined, let failedConstraint be that result and jump to the step labeled Constraint Failure below.

    7. Set the device information exposure with requestedMediaTypes and true.

    8. resolve p with stream and abort these steps.

    9. Permission Failure: Reject p with a new DOMException object whose name attribute has the value "NotAllowedError".

    10. Constraint Failure: Let message be either undefined or an informative human-readable message, let constraint be failedConstraint if device information can be exposed is true, or "" otherwise, and then reject p with a new OverconstrainedError created by calling OverconstrainedError(constraint, message).

  7. Return p.

Note

In the algorithm above, constraints are checked twice - once at device selection, and once after access approval. Time may have passed between those checks, so it is conceivable that the selected device is no longer suitable. In this case, a NotReadableError will result.

The allowed required constraints for device selection contains the following constraint names: width, height, aspectRatio, frameRate, facingMode, resizeMode, sampleRate, sampleSize, echoCancellation, autoGainControl, noiseSuppression, latency, channelCount, deviceId, groupId.

10.3 MediaStreamConstraints

The MediaStreamConstraints dictionary is used to instruct the User Agent what sort of MediaStreamTracks to include in the MediaStream returned by getUserMedia().

WebIDLdictionary MediaStreamConstraints {
  (boolean or MediaTrackConstraints) video = false;
  (boolean or MediaTrackConstraints) audio = false;
};

Dictionary MediaStreamConstraints Members

video of type (boolean or MediaTrackConstraints), defaulting to false

If true, it requests that the returned MediaStream contain a video track. If a Constraints structure is provided, it further specifies the nature and settings of the video Track. If false, the MediaStream MUST NOT contain a video Track.

audio of type (boolean or MediaTrackConstraints), defaulting to false

If true, it requests that the returned MediaStream contain an audio track. If a Constraints structure is provided, it further specifies the nature and settings of the audio Track. If false, the MediaStream MUST NOT contain an audio Track.

10.6 Implementation Suggestions

This section is non-normative.

Best Practice 1: Resource reservation

The User Agent is encouraged to reserve resources when it has determined that a given call to getUserMedia() will be successful. It is preferable to reserve the resource prior to resolving the returned promise. Subsequent calls to getUserMedia() (in this page or any other) should treat the resource that was previously allocated, as well as resources held by other applications, as busy. Resources marked as busy should not be provided as sources to the current web page, unless specified by the user. Optionally, the User Agent may choose to provide a stream sourced from a busy source but only to a page whose origin matches the owner of the original stream that is keeping the source busy.

This document recommends that in the permission grant dialog or device selection interface (if one is present), the user be allowed to select any available hardware as a source for the stream requested by the page (provided the resource is able to fulfill any specified required constraints). Although not specifically recommended as best practice, note that some User Agents may support the ability to substitute a video or audio source with local files and other media. A file picker may be used to provide this functionality to the user.

This document also recommends that the user be shown all resources that are currently busy as a result of prior calls to getUserMedia() (in this page or any other page that is still alive) and be allowed to terminate that stream and utilize the resource for the current page instead. If possible in the current operating environment, it is also suggested that resources currently held by other applications be presented and treated in the same manner. If the user chooses this option, the track corresponding to the resource that was provided to the page whose stream was affected must be removed.

Best Practice 2: Stored Permission s

When permission is requested for a device, the User Agent may choose to store this permission for later use by the same origin, so that the user does not need to grant permission again at a later time. It is a User Agent choice whether it offers functionality to store permission to each device separately, all devices of a given class, or all devices; the choice needs to be apparent to the user, and permission must have been granted for the entire set whose permission is being stored, e.g., to store permission to use all cameras the user must have given permission to use all cameras and not just one.

As described, this specification does not dictate whether or not granting permission results in a stored permission. When permission is not stored, permission will last only until such time as all MediaStreamTracks sourced from that device have been stopped.

Best Practice 3: Handling multiple devices

A MediaStream may contain more than one video and audio track. This makes it possible to include video from two or more webcams in a single stream object, for example. However, the current API does not allow a page to express a need for multiple video streams from independent sources.

It is recommended for multiple calls to getUserMedia() from the same page to be allowed as a way for pages to request multiple discrete video and/or audio streams.

Note also that if multiple getUserMedia() calls are done by a page, the order in which they request resources, and the order in which they complete, is not constrained by this specification.

A single call to getUserMedia() will always return a stream with either zero or one audio tracks, and either zero or one video tracks. If a script calls getUserMedia() multiple times before reaching a stable state, this document advises the UI designer that the permission dialogs should be merged, so that the user can give permission for the use of multiple cameras and/or media sources in one dialog interaction. The constraints on each getUserMedia call can be used to decide which stream gets which media sources.

Best Practice 4: Generating deviceIds

An efficient practice for generating a deviceId is to generate a cryptographic hash from a private key + (origin or origin + top-level origin, based on the user agents' partitioning rules) + salt + device's underlying (hardware) id in the driver, and present the resulting hash as an alphanumeric string. Using 32 bits or fewer for the hash is recommended, but not much lower, to avoid risk of collision.

A lower-entropy alternative, at the cost of storage, is to assign the numbers 0 through 255 randomly to each new device encountered for each origin or origin + top-level origin, based on the User Agent's partitioning rules, retiring the number that hasn't been seen the longest if numbers run out.

Best Practice 5: Device muting initiated by User Agent

A track sourced by a camera or microphone may be forcibly muted by a User Agent at any time, in order to manage a user's privacy. However, doing so may create web compatibility issues, as well as leak information about user activity, so caution is advised.

Best practice is to mute a camera or microphone track in the following instances:

  • An OS-level event for which the User Agent already suspends media playback globally, but JavaScript is not suspended. The rationale is users may otherwise be surprised if capture were to continue in this situation (unless they've intentionally configured it this way). If the OS-level event already causes frames to stop coming in on the track, then no new information of user activity is revealed by this. Even when this is not the case, revealing that capture is ending seems like a reasonable privacy tradeoff compared to continuing capture in situations that may surprise users.

  • A web page without focus re-enables a track when all tracks from that source are disabled, in order to delay resumption of capture until the page gains focus.

Best practice is to unmute a camera or microphone track it previously muted, in the following instances:

  • An OS-level event for which the User Agent already resumes media playback globally, and the page is visible to the user (e.g. not during a lock screen). User Agents may defer such action if it determines significant time has passed that may jeopardize a user's awareness of the earlier capture session.

  • A web page gains focus and has one or more enabled tracks that are also muted.

11. Constrainable Pattern

The Constrainable pattern allows applications to inspect and adjust the properties of objects implementing it (the constrainable object). It is broken out as a separate set of definitions so that it can be referred to by other specifications. The core concept is the Capability, which consists of a constrainable property of an object and the set of its possible values, which may be specified either as a range or as an enumeration. For example, a camera might be capable of framerates (a property) between 20 and 50 frames per second (a range) and may be able to be positioned (a property) facing towards the user, away from the user, or to the left or right of the user (an enumerated set). The application can examine a constrainable property's supported Capabilities via the getCapabilities() accessor.

The application can select the (range of) values it wants for an object's Capabilities by means of basic and/or advanced ConstraintSets and the applyConstraints() method. A ConstraintSet consists of the names of one or more properties of the object plus the desired value (or a range of desired values) for each property. Each of those property/value pairs can be considered to be an individual constraint. For example, the application may set a ConstraintSet containing two constraints, the first stating that the framerate of a camera be between 30 and 40 frames per second (a range) and the second that the camera should be facing the user (a specific value). How the individual constraints interact depends on whether and how they are given in the basic Constraint structure, which is a ConstraintSet with an additional 'advanced' property, or whether they are in a ConstraintSet in the advanced list. The behavior is as follows: all 'min', 'max', and 'exact' constraints in the basic Constraint structure are together treated as the required constraints, and if it is not possible to satisfy simultaneously all of those individual constraints for the indicated property names, the User Agent MUST reject the returned promise. Otherwise, it must apply the required constraints. Next, it will consider any ConstraintSets given in the advanced list, in the order in which they are specified, and will try to satisfy/apply each complete ConstraintSet (i.e., all constraints in the ConstraintSet together), but will skip a ConstraintSet if and only if it cannot satisfy/apply it in its entirety. Next, the User Agent MUST attempt to apply, individually, any 'ideal' constraints or a constraint given as a bare value for the property (referred to as optional basic constraints). Of these properties, it MUST satisfy the largest number that it can, in any order. Finally, the User Agent MUST resolve the returned promise.

Note
Any constraint provided via this API will only be considered if the given constrainable property is supported by the User Agent. JavaScript application code is expected to first check, via getSupportedConstraints(), that all the named properties that are used are supported by the User Agent. The reason for this is that WebIDL drops any unsupported names from the dictionary holding the constraints, so the User Agent does not see them and the unsupported names end up being silently ignored. This will cause confusing programming errors as the JavaScript code will be setting constraints but the User Agent will be ignoring them. User Agents that support (recognize) the name of a required constraint but cannot satisfy it will generate an error, while User Agents that do not support the constrainable property will not generate an error.

The following examples may help to understand how constraints work. The first shows a basic Constraint structure. Three constraints are given, each of which the User Agent will attempt to satisfy individually. Depending upon the resolutions available for this camera, it is possible that not all three constraints can be satisfied at the same time. If so, the User Agent will satisfy two if it can, or only one if not even two constraints can be satisfied together. Note that if not all three can be satisfied simultaneously, it is possible that there is more than one combination of two constraints that could be satisfied. If so, the User Agent will choose.

const stream = await navigator.mediaDevices.getUserMedia({
  video: {
    width: 1280,
    height: 720,
    aspectRatio: 3/2
  }
});

This next example adds a small bit of complexity. The ideal values are still given for width and height, but this time with minimum requirements on each as well as a minimum frameRate that must be satisfied. If it cannot satisfy the frameRate, width or height minimum it will reject the promise. Otherwise, it will try to satisfy the width, height, and aspectRatio target values as well and then resolve the promise.

try {
  const stream = await navigator.mediaDevices.getUserMedia({
    video: {
      width: {min: 640, ideal: 1280},
      height: {min: 480, ideal: 720},
      aspectRatio: 3/2,
      frameRate: {min: 20}
    }
  });
} catch (error) {
  if (error.name != "OverconstrainedError") {
    throw error;
  }
  // Overconstrained. Try again with a different combination (no prompt was shown)
}

This example illustrates the full control possible with the Constraints structure by adding the 'advanced' property. In this case, the User Agent behaves the same way with respect to the required constraints, but before attempting to satisfy the ideal values it will process the 'advanced' list. In this example the 'advanced' list contains two ConstraintSets. The first specifies width and height constraints, and the second specifies an aspectRatio constraint. Note that in the advanced list, these bare values are treated as 'exact' values. This example represents the following: "I need my video to be at least 640 pixels wide and at least 480 pixels high. My preference is for precisely 1920x1280, but if you can't give me that, give me an aspectRatio of 4x3 if at all possible. If even that is not possible, give me a resolution as close to 1280x720 as possible."

try {
  const stream = await navigator.mediaDevices.getUserMedia({
    video: {
      width: {min: 640, ideal: 1280},
      height: {min: 480, ideal: 720},
      frameRate: {min: 30},
      advanced: [
        {width: 1920, height: 1280},
        {aspectRatio: 4/3},
        {frameRate: {min: 50}},
        {frameRate: {min: 40}}
      ]
    }
  });
} catch (error) {
  if (error.name != "OverconstrainedError") {
    throw error;
  }
  // Overconstrained. Try again with a different combination (no prompt was shown)
}

The ordering of advanced ConstraintSets is significant. In the preceding example it is impossible to satisfy both the 1920x1280 ConstraintSet and the 4x3 aspect ratio ConstraintSet at the same time. Since the 1920x1280 occurs first in the list, the User Agent will attempt to satisfy it first. Application authors can therefore implement a backoff strategy by specifying multiple advanced ConstraintSets for the same property. The application also specifies two more advanced ConstraintSets, the first asking for a frame rate greater than 50, the second asking for a frame rate greater than 40. If the User Agent is capable of setting a frame rate greater than 50, it will (and the subsequent ConstraintSet will be trivially satisfied). However, if the User Agent cannot set the frame rate above 50, it will skip that ConstraintSet and attempt to set the frame rate above 40. In case the User Agent cannot satisfy either of the two ConstraintSets, the 'min' value in the basic ConstraintSet insists on 30 as a lower bound. In other words, the User Agent would fail altogether if it couldn't get a value over 30, but would choose a value over 50 if possible, then try for a value over 40.

Note that, unlike basic constraints, the constraints within a ConstraintSet in the advanced list must be satisfied together or skipped together. Thus, {width: 1920, height: 1280} is a request for that specific resolution, not a request for that width or that height. One can think of the basic constraints as requesting an 'or' (non-exclusive) of the individual constraints, while each advanced ConstraintSet is requesting an 'and' of the individual constraints in the ConstraintSet. An application may inspect the full set of Constraints currently in effect via the getConstraints() accessor.

The specific value that the User Agent chooses for a constrainable property is referred to as a Setting. For example, if the application applies a ConstraintSet specifying that the frameRate must be at least 30 frames per second, and no greater than 40, the Setting can be any intermediate value, e.g., 32, 35, or 37 frames per second. The application can query the current settings of the object's constrainable properties via the getSettings() accessor.

11.1 Interface Definition

Although this specification formally defines ConstrainablePattern as a WebIDL interface, it is actually a template or pattern for other interfaces and cannot be inherited directly since the return values of the methods need to be extended, something WebIDL cannot do. Thus, each interface that wishes to make use of the functionality defined here will have to provide its own copy of the WebIDL for the functions and interfaces given here. However it can refer to the semantics defined here, which will not change. See MediaStreamTrack Interface Definition for an example of this.

This pattern relies on the constrainable object defining three internal slots:

  1. A [[Capabilities]] internal slot, initialized to a Capabilities dictionary describing the aggregate allowable values for each constrainable property exposed, as explained under Capabilities, or an empty dictionary if it has none.

  2. A [[Constraints]] internal slot, initialized to an empty Constraints dictionary.

  3. A [[Settings]] internal slot, initialized to a Settings dictionary describing the currently active settings values for each constrainable property exposed, as explained under Settings, or an empty dictionary if it has none.

Template:
WebIDL[Exposed=Window]
interface ConstrainablePattern {
  Capabilities  getCapabilities();
  Constraints   getConstraints();
  Settings      getSettings();
  Promise<undefined> applyConstraints(optional Constraints constraints = {});
};

Methods

getCapabilities

The getCapabilities() method returns the dictionary of the names of the constrainable properties that the object supports. When invoked, the User Agent MUST return the value of the [[Capabilities]] internal slot.

Note

It is possible that the underlying hardware may not exactly map to the range defined for the constrainable property. Where this is possible, the entry SHOULD define how to translate and scale the hardware's setting onto the values defined for the property. For example, suppose that a hypothetical fluxCapacitance property ranges from -10 (min) to 10 (max), but there are common hardware devices that support only values of "off" "medium" and "full". The constrainable property definition might specify that for such hardware, the User Agent should map the range value of -10 to "off", 10 to "full", and 0 to "medium". It might also indicate that given a ConstraintSet imposing a strict value of 3, the User Agent should attempt to set the value of "medium" on the hardware, and that getSettings() should return a fluxCapacitance of 0, since that is the value defined as corresponding to "medium".

getConstraints

The getConstraints() method returns the Constraints that were the argument to the most recent successful invocation of the ApplyConstraints algorithm on the object, maintaining the order in which they were specified. Note that some of the advanced ConstraintSets returned may not be currently satisfied. To check which ConstraintSets are currently in effect, the application should use getSettings. Instead of returning the exact constraints as described above, the UA MAY return a constraint set that has the identical effect in all situations as the applied constraints. When invoked, the User Agent MUST return the value of the [[Constraints]] internal slot.

getSettings

The getSettings() method returns the current settings of all the constrainable properties of the object, whether they are platform defaults or have been set by the ApplyConstraints algorithm. Note that a setting is a target value that complies with constraints, and therefore may differ from measured performance at times. When invoked, the User Agent MUST return the value of the [[Settings]] internal slot.

applyConstraints

When the applyConstraints template method is invoked, the User Agent MUST run the following steps:

  1. Let object be the object on which this method was invoked.

  2. Let newConstraints be the argument to this method.

  3. Let p be a new promise.

  4. Run the following steps in parallel, maintaining the order of invocations if this method is called multiple times:

    1. Let failedConstraint be the result of running the ApplyConstraints algorithm with newConstraints as the argument.

    2. Let successfulSettings be the object's current settings after the algorithm in the above step has finished.

    3. Queue a task that runs the following steps:

      1. If failedConstraint is not undefined, let message be either undefined or an informative human-readable message, reject p with a new OverconstrainedError created by calling OverconstrainedError(failedConstraint, message), and abort these steps. The existing constraints remain in effect in this case.

      2. Set object's [[Constraints]] internal slot to newConstraints or a Constraints dictionary that has the identical effect in all situations as newConstraints.

      3. Set object's [[Settings]] internal slot to successfulSettings.

      4. resolve p with undefined.

  5. Return p.

The ApplyConstraints algorithm for applying constraints is stated below. Here are some preliminary definitions that are used in the statement of the algorithm:

We use the term settings dictionary for the set of values that might be applied as settings to the object.

For string valued constraints, we define "==" below to be true if one of the values in the sequence is exactly the same as the value being compared against.

We define the fitness distance between a settings dictionary and a constraint set CS as the sum, for each member (represented by a constraintName and constraintValue pair) which exists in CS, of the following values:

  1. If constraintName is not supported by the User Agent, the fitness distance is 0.

  2. If the constraint is required (constraintValue either contains one or more members named 'min', 'max', or 'exact', or is itself a bare value and bare values are to be treated as 'exact'), and the settings dictionary's constraintName member's value does not satisfy the constraint or doesn't exist, the fitness distance is positive infinity.

  3. If the constraint does not apply for this type of object, the fitness distance is 0 (that is, the constraint does not influence the fitness distance).

  4. If constraintValue is a boolean, but the constrainable property is not, then the fitness distance is based on whether the settings dictionary's constraintName member exists or not, from the formula

    (constraintValue == exists) ? 0 : 1
  5. If the settings dictionary's constraintName member does not exist, the fitness distance is 1.

  6. If no ideal value is specified (constraintValue either contains no member named 'ideal', or, if bare values are to be treated as 'ideal', isn't a bare value), the fitness distance is 0.
  7. For all positive numeric constraints (such as height, width, frameRate, aspectRatio, sampleRate and sampleSize), the fitness distance is the result of the formula
    (actual == ideal) ? 0 : |actual - ideal| / max(|actual|, |ideal|)
  8. For all string, enum and boolean constraints (e.g. deviceId, groupId, facingMode, resizeMode, echoCancellation), the fitness distance is the result of the formula
    (actual == ideal) ? 0 : 1

More definitions:

  • We refer to each element of a ConstraintSet (other than the special term 'advanced') as a 'constraint' since it is intended to constrain the acceptable settings for the given property from the full list or range given in the corresponding Capability of the ConstrainablePattern object to a value that is within the range or list of values it specifies.
  • We refer to the "effective Capability" C of an object O as the possibly proper subset of the possible values of C (as returned by getCapabilities) taking into consideration environmental limitations and/or restrictions placed by other constraints. For example given a ConstraintSet that constrains the aspectRatio, height, and width properties, the values assigned to any two of the properties limit the effective Capability of the third. The set of effective Capabilities may be platform dependent. For example, on a resource-limited device it may not be possible to set properties P1 and P2 both to 'high', while on another less limited device, this may be possible.
  • A settings dictionary, which is a set of values for the constrainable properties of an object O, satisfies ConstraintSet CS if the fitness distance between the set and CS is less than infinity.
  • A set of ConstraintSets CS1...CSn (n >= 1) can be satisfied by an object O if it is possible to find a settings dictionary of O that satisfies CS1...CSn simultaneously.
  • To apply a set of ConstraintSets CS1...CSn to object O is to choose such a sequence of values that satisfy CS1...CSn and assign them as the settings for the properties of O.

We define the SelectSettings algorithm as follows:

  1. Each constraint specifies one or more values (or a range of values) for its property. A property MAY appear more than once in the list of 'advanced' ConstraintSets. If an empty list has been given as the value for a constraint, it MUST be interpreted as if the constraint were not specified (in other words, an empty constraint == no constraint).

    Note that unknown properties are discarded by WebIDL, which means that unknown/unsupported required constraints will silently disappear. To avoid this being a surprise, application authors are expected to first use the getSupportedConstraints() method as shown in the Examples below.

  2. Let object be the ConstrainablePattern object on which this algorithm is applied. Let copy be an unconstrained copy of object (i.e., copy should behave as if it were object with all ConstraintSets removed.)
  3. For every possible settings dictionary of copy compute its fitness distance, treating bare values of properties as ideal values. Let candidates be the set of settings dictionaries for which the fitness distance is finite.

  4. If candidates is empty, return undefined as the result of the SelectSettings algorithm.

  5. Iterate over the 'advanced' ConstraintSets in newConstraints in the order in which they were specified. For each ConstraintSet:
    1. compute the fitness distance between it and each settings dictionary in candidates, treating bare values of properties as exact.

    2. If the fitness distance is finite for one or more settings dictionaries in candidates, keep those settings dictionaries in candidates, discarding others.

      If the fitness distance is infinite for all settings dictionaries in candidates, ignore this ConstraintSet.

  6. Select one settings dictionary from candidates, and return it as the result of the SelectSettings algorithm. The User Agent MUST use one with the smallest fitness distance, as calculated in step 3. If more than one settings dictionary have the smallest fitness distance, the User Agent chooses one of them based on system default property values and User Agent default property values.

For any property with a system default value for the selected device, the system default value SHOULD be used if compatible with the above algorithm. This is usually the case for properties like sampleRate or sampleSize. Other properties, like echoCancellation or resizeMode do not usually have system default values. The User Agent defines its own default values for these properties. Implementors need to be cautious to select good default values since they will often have an impact on how media content is generated.

Note

It is recommended to look at existing implementations to select meaningful default values. Note that default values may differ based on the system, for instance desktop vs. mobile. At time of writing, User Agent implementations tend to use the following default values, which were chosen for their suitability for using RTCPeerConnection as a sink:

  1. width set to 640.

  2. height set to 480.

  3. frameRate set to 30.

  4. echoCancellation set to true.

To apply the ApplyConstraints algorithm to an object, given newConstraints as an argument, the User Agent MUST run the following steps:

  1. Let successfulSettings be the result of running the SelectSettings algorithm with newConstraints as the constraint set.

  2. If successfulSettings is undefined, let failedConstraint be any required constraint whose fitness distance was infinity for all settings dictionaries examined while executing the SelectSettings algorithm, or "" if there isn't one, and then return failedConstraint and abort these steps.

  3. In a single operation, remove the existing constraints from object, apply newConstraints, and apply successfulSettings as the current settings.
  4. Return undefined.
Note

Any implementation that has the same result as the algorithm above is an allowed implementation. For instance, the implementation may choose to keep track of the maximum and minimum values for a setting that are OK under the constraints considered, rather than keeping track of all possible values for the setting.

Note

When picking a settings dictionary, the UA can use any information available to it. Examples of such information may be whether the selection is done as part of device selection in getUserMedia, whether the energy usage of the camera varies between the settings dictionaries, or whether using a settings dictionary will cause the device driver to apply resampling.

The User Agent MAY choose new settings for the constrainable properties of the object at any time. When it does so it MUST attempt to satisfy all current Constraints, in the manner described in the algorithm above, let successfulSettings be the resulting new settings, and queue a task to run the following steps:

  1. Let object be the ConstrainablePattern object on which new settings for one or more constrainable properties have changed.

  2. Set object's [[Settings]] internal slot to successfulSettings.

An example of Constraints that could be passed into applyConstraints() or returned as a value of constraints is below. It uses the constrainable properties defined for camera-sourced MediaStreamTracks. In this example, all constraints are ideal values, which means results are "best effort" based on the user's specific camera:

await track.applyConstraints({
  width: 1920,
  height: 1080,
  frameRate: 30,
});
const {width, height, frameRate} = track.getSettings();

console.log(`${width}x${height}x${frameRate}`); // 1920x1080x30, or it might be e.g.
                                                // 1280x720x30 as best effort

For finer control, an application can insist on an exact match, provided it's prepared to handle failure:

try {
  await track.applyConstraints({
    width: {exact: 1920},
    height: {exact: 1080},
    frameRate: {min: 25, ideal: 30, max: 30},
  });
  const {width, height, frameRate} = track.getSettings();

  console.log(`${width}x${height}x${frameRate}`); // 1920x1080x25-30!

} catch (error) {
  if (error.name != "OverconstrainedError") {
    throw error;
  }
  console.log(`This camera cannot produce the requested ${error.constraint}.`);
}

Constraints can also be passed into getUserMedia, not just as an initialization convenience, but to influence device selection. In this case, inherent constraints are also available.

Here's an example of using constraints to prefer a specific camera and microphone from a previous visit, with requirements on dimensions and a preference for stereo, to be applied once granted, and to help find suitable replacements in case the requested devices are no longer available (or in some user agents, overriden by the user).

try {
  const stream = await navigator.mediaDevices.getUserMedia({
    video: {
      deviceId: localStorage.camId,
      width: {min: 800, ideal: 1024, max: 1280},
      height: {min: 600}
    },
    audio: {
      deviceId: localStorage.micId,
      channelCount: 2
    }
  });

  // Granted. Store deviceIds for next time
  localStorage.camId = stream.getVideoTracks()[0].getSettings().deviceId;
  localStorage.micId = stream.getAudioTracks()[0].getSettings().deviceId;

} catch (error) {
  if (error.name != "OverconstrainedError") {
    throw error;
  }
  // Overconstrained. No suitable replacements found
}
Note

The above example avoids using {exact: deviceId}, so that browsers can immediately offer a choice between different cameras if your preferred device is not available.

The example also stores the deviceIds on every grant, in case they represent a new choice.

In contrast, here's an example of using constraints to implement an in-content camera picker. In this case, we use exact and rely solely on a deviceId that comes from the user picking from a list of choices:

async function switchCameraTrack(freshlyChosenDeviceId, oldTrack) {
  if (isMobile) {
    oldTrack.stop(); // Some platforms can only open one camera at a time.
  }
  const stream = await navigator.mediaDevices.getUserMedia({
    video: {
      deviceId: {exact: freshlyChosenDeviceId}
    }
  });
  const [track] = stream.getVideoTracks();
  localStorage.camId = track.getSettings().deviceId;
  return track;
}

Here's an example asking for a back camera on a phone, ideally in 720p, but accepting anything close to that. Note how constraints on dimensions are specified in landscape mode:

async function getBackCamera() {
  return await navigator.mediaDevices.getUserMedia({
    video: {
      facingMode: {exact: 'environment'},
      width: 1280,
      height: 720
    }
  });
}

Here's an example of "I want a native 16:9 resolution near 720p, but with an exact frame rate of 10 even if not natively available". This needs to be done in two steps: One to discover the native mode, and a second step to apply the custom frame rate. This also shows how to derive constraints from current settings, which may be rotated:

async function nativeResolutionButDecimatedFrameRate() {
  const stream = await navigator.mediaDevices.getUserMedia({
    video: {
      resizeMode: 'none', // means native resolution and frame rate
      width: 1280,
      height: 720,
      aspectRatio: 16 / 9 // aspect ratios may not be exactly accurate
    }
  });
  const [track] = stream.getVideoTracks();
  const {width, height, aspectRatio} = track.getSettings();

  // Constraints are in landscape, while settings may be rotated (portrait)
  if (width < height) {
    [width, height] = [height, width];
    aspectRatio = 1 / aspectRatio;
  }

  await track.applyConstraints({
    resizeMode: 'crop-and-scale',
    width: {exact: width},
    height: {exact: height},
    frameRate: {exact: 10},
    aspectRatio,
  });

  return stream;
}
Note
The above example assumes the primary orientation is landscape.

Here's an example showing how to use getSupportedConstraints, for cases where a constraint being ignored due to lack of support in a user agent is not tolerated by the application:

async function getFrontCameraRes() {
  const supports = navigator.mediaDevices.getSupportedConstraints();

  for (const constraint of ["facingMode", "aspectRatio", "resizeMode"]) {
    if (!(constraint in supports) {
      throw new OverconstrainedError(constraint, "Not supported");
    }
  }
  return await navigator.mediaDevices.getUserMedia({
    video: {
      facingMode: {exact: 'user'},
      advanced: [
        {aspectRatio: 16/9, height: 1080, resizeMode: "none"},
        {aspectRatio: 4/3, width: 1280, resizeMode: "none"}
      ]
    }
  });
}

11.2 Types for Constrainable Properties

The syntax for the specification of the set of legal values depends on the type of the values. In addition to the standard atomic types (boolean, long, double, DOMString), legal values include lists of any of the atomic types, plus min-max ranges, as defined below.

List values MUST be interpreted as disjunctions. For example, if a property 'facingMode' for a camera is defined as having legal values ["left", "right", "user", "environment"], this means that 'facingMode' can have the values "left", "right", "environment", and "user". Similarly Constraints restricting 'facingMode' to ["user", "left", "right"] would mean that the User Agent should select a camera (or point the camera, if that is possible) so that "facingMode" is either "user", "left", or "right". This Constraint would thus request that the camera not be facing away from the user, but would allow the User Agent to allow the user to choose other directions.

WebIDLdictionary DoubleRange {
  double max;
  double min;
};

Dictionary DoubleRange Members

max of type double

The maximum legal value of this property.

min of type double

The minimum value of this Property.

WebIDLdictionary ConstrainDoubleRange : DoubleRange {
  double exact;
  double ideal;
};

Dictionary ConstrainDoubleRange Members

exact of type double

The exact required value for this property.

ideal of type double

The ideal (target) value for this property.

WebIDLdictionary ULongRange {
  [Clamp] unsigned long max;
  [Clamp] unsigned long min;
};

Dictionary ULongRange Members

max of type unsigned long

The maximum legal value of this property.

min of type unsigned long

The minimum value of this property.

WebIDLdictionary ConstrainULongRange : ULongRange {
  [Clamp] unsigned long exact;
  [Clamp] unsigned long ideal;
};

Dictionary ConstrainULongRange Members

exact of type unsigned long

The exact required value for this property.

ideal of type unsigned long

The ideal (target) value for this property.

WebIDLdictionary ConstrainBooleanParameters {
  boolean exact;
  boolean ideal;
};

Dictionary ConstrainBooleanParameters Members

exact of type boolean

The exact required value for this property.

ideal of type boolean

The ideal (target) value for this property.

WebIDLdictionary ConstrainDOMStringParameters {
  (DOMString or sequence<DOMString>) exact;
  (DOMString or sequence<DOMString>) ideal;
};

Dictionary ConstrainDOMStringParameters Members

exact of type (DOMString or sequence<DOMString>)

The exact required value for this property.

ideal of type (DOMString or sequence<DOMString>)

The ideal (target) value for this property.

WebIDLtypedef ([Clamp] unsigned long or ConstrainULongRange) ConstrainULong;
Throughout this specification, the identifier ConstrainULong is used to refer to the ([Clamp] unsigned long or ConstrainULongRange) type.
WebIDLtypedef (double or ConstrainDoubleRange) ConstrainDouble;
Throughout this specification, the identifier ConstrainDouble is used to refer to the (double or ConstrainDoubleRange) type.
WebIDLtypedef (boolean or ConstrainBooleanParameters) ConstrainBoolean;
Throughout this specification, the identifier ConstrainBoolean is used to refer to the (boolean or ConstrainBooleanParameters) type.
WebIDLtypedef (DOMString or
         sequence<DOMString> or
         ConstrainDOMStringParameters) ConstrainDOMString;
Throughout this specification, the identifier ConstrainDOMString is used to refer to the (DOMString or sequence<DOMString> or ConstrainDOMStringParameters) type.

11.3 Capabilities

Capabilities is a dictionary containing one or more key-value pairs, where each key MUST be a constrainable property, and each value MUST be a subset of the set of values allowed for that property. The exact syntax of the value expression depends on the type of the property. The Capabilities dictionary specifies which constrainable properties that can be applied, as constraints, to the constrainable object. Note that the Capabilities of a constrainable object MAY be a subset of the properties defined in the Web platform, with a subset of the set values for those properties. Note that Capabilities are returned from the User Agent to the application, and cannot be specified by the application. However, the application can control the Settings that the User Agent chooses for constrainable properties by means of Constraints.

An example of a Capabilities dictionary is shown below. In this case, the constrainable object is a video source with a very limited set of Capabilities.

{
  frameRate: {min: 1.0, max: 60.0},
  facingMode: ['user', 'left']
}

The next example below points out that capabilities for range values provide ranges for individual constrainable properties, not combinations. This is particularly relevant for video width and height, since the ranges for width and height are reported separately. In the example, if the constrainable object can only provide 640x480 and 800x600 resolutions the relevant capabilities returned would be:

{
  width: {min: 640, max: 800},
  height: {min: 480, max: 600},
  aspectRatio: 4/3
}

Note in the example above that the aspectRatio would make clear that arbitrary combination of widths and heights are not possible, although it would still suggest that more than two resolutions were available.

A specification using the Constrainable Pattern should not subclass the below dictionary, but instead provide its own definition. See MediaTrackCapabilities for an example.
Template:
WebIDLdictionary Capabilities {};

11.4 Settings

Settings is a dictionary containing one or more key-value pairs. It MUST contain each key returned in getCapabilities() for which the property is defined on the object type it's returned on; for instance, an audio MediaStreamTrack has no "width" property. There MUST be a single value for each key and the value MUST be a member of the set defined for that property by getCapabilities(). The Settings dictionary contains the actual values that the User Agent has chosen for the object's constrainable properties. The exact syntax of the value depends on the type of the property.

A conforming User Agent MUST support all the constrainable properties defined in this specification.

An example of a Settings dictionary is shown below. This example is not very realistic in that a User Agent would actually be required to support more constrainable properties than just these.

{
  frameRate: 30.0,
  facingMode: 'user'
}
A specification using the Constrainable Pattern should not subclass the below dictionary, but instead provide its own definition. See MediaTrackSettings for an example.
Template:
WebIDLdictionary Settings {};

11.5 Constraints and ConstraintSet

Due to the limitations of WebIDL, interfaces implementing the Constrainable Pattern cannot simply subclass Constraints and ConstraintSet as they are defined here. Instead they must provide their own definitions that follow this pattern. See MediaTrackConstraints for an example of this.

Template:
WebIDLdictionary ConstraintSet {};

Each member of a ConstraintSet corresponds to a constrainable property and specifies a subset of the property's legal Capability values. Applying a ConstraintSet instructs the User Agent to restrict the settings of the corresponding constrainable properties to the specified values or ranges of values. A given property MAY occur both in the basic Constraints set and in the advanced ConstraintSets list, and MAY occur at most once in each ConstraintSet in the advanced list.

11.5.1 Dictionary Constraints Members

advanced of type sequence<ConstraintSet>

This is the list of ConstraintSets that the User Agent MUST attempt to satisfy, in order, skipping only those that cannot be satisfied. The order of these ConstraintSets is significant. In particular, when they are passed as an argument to applyConstraints, the User Agent MUST try to satisfy them in the order that is specified. Thus if advanced ConstraintSets C1 and C2 can be satisfied individually, but not together, then whichever of C1 and C2 is first in this list will be satisfied, and the other will not. The User Agent MUST attempt to satisfy all ConstraintSets in the list, even if some cannot be satisfied. Thus, in the preceding example, if constraint C3 is specified after C1 and C2, the User Agent will attempt to satisfy C3 even though C2 cannot be satisfied. Note that a given property name may occur only once in each ConstraintSet but may occur in more than one ConstraintSet.

12. Examples

This sample code exposes a button. When clicked, the button is disabled and the user is prompted to offer a stream. The user can cause the button to be re-enabled by providing a stream (e.g., giving the page access to the local camera) and then disabling the stream (e.g., revoking that access).

<button id="startBtn">Start</button>
<script>
const startBtn = document.getElementById('startBtn');

startBtn.onclick = async () => {
  try {
    startBtn.disabled = true;
    const constraints = {
      audio: true,
      video: true
    };

    const stream = await navigator.mediaDevices.getUserMedia(constraints);

    for (const track of stream.getTracks()) {
      track.onended = () => {
        startBtn.disabled = stream.getTracks().some((t) => t.readyState == 'live');
      };
    }
  } catch (err) {
    console.error(err);
  }
};
</script>

This example allows people to take photos of themselves from the local video camera. Note that the Image Capture specification [image-capture] provides a simpler way to accomplish this.

<script>
window.onload = async () => {
  const video = document.getElementById('monitor');
  const canvas = document.getElementById('photo');
  const shutter = document.getElementById('shutter');

  try {
    video.srcObject = await navigator.mediaDevices.getUserMedia({video: true});

    await new Promise(resolve => video.onloadedmetadata = resolve);
    canvas.width = video.videoWidth;
    canvas.height = video.videoHeight;
    document.getElementById('splash').hidden = true;
    document.getElementById('app').hidden = false;

    shutter.onclick = () => canvas.getContext('2d').drawImage(video, 0, 0);
  } catch (err) {
    console.error(err);
  }
};
</script>

<h1>Snapshot Kiosk</h1>

<section id="splash">
  <p id="errorMessage">Loading...</p>
</section>

<section id="app" hidden>
  <video id="monitor" autoplay></video>
  <button id="shutter">&#x1F4F7;</button>
  <canvas id="photo"></canvas>
</section>

13. Permissions Policy Integration

This specification defines two policy-controlled features identified by the strings "camera" and "microphone". Both have a default allowlist of "self".

Note

A document's permissions policy determines whether any content in that document is allowed to use getUserMedia to request camera or microphone respectively. If disabled in any document, no content in the document will be allowed to use getUserMedia to request the camera or microphone respectively. This is enforced by the request permission to use algorithm.

Additionally, enumerateDevices will only enumerate devices the document is allowed to use.

14. Privacy Indicator Requirements

For each kind of device that getUserMedia() exposes,

Define anyAccessible to be the logical OR of all any<kind>Accessible values.

Define anyLive to be the logical OR of all any<kind>Live values.

Then the following are requirements on the User Agent:

and the following are encouraged behaviors for the User Agent:

15. Privacy and Security Considerations

This section is non-normative; it specifies no new behavior, but instead summarizes information already present in other parts of the specification.

This document extends the Web platform with the ability to manage input devices for media - in this iteration, microphones, and cameras. It also allows the manipulation of audio output devices (speakers and headphones). Capturing audio and video exposes personally-identifiable information to applications, and this specification requires obtaining explicit user consent before sharing it.

Without authorization (to the "drive-by web"), it offers the ability to tell how many devices there are of each class, and how they are grouped together (e.g. a microphone and camera belonging to a single Web cam). The identifiers for the devices are designed to not be useful for a fingerprint that can track the user between origins, but the number and grouping of devices adds to the fingerprint surface. It recommends to treat the per-origin persistent identifier deviceId as other persistent storage (e.g. cookies) are treated.

When authorization is given, this document describes how to get access to, and use, media data from the devices mentioned. This data may be sensitive; advice is given that indicators should be supplied to indicate that devices are in use, but both the nature of authorization and the indicators of in-use devices are platform decisions.

Authorization may be given on a case-by-case basis, or be persistent. In the case of a case-by-case authorization, it is important that the user be able to say "no" in a way that prevents the UI from blocking user interaction until permission is given - either by offering a way to say a "persistent NO" or by not using a modal permissions dialog.

When authorization to any media device is given, application developers gain access to the labels of all available media capture devices. In most cases, the labels are persistent across browsing sessions and across origins that have also been granted authorization, and thus potentially provide a way to track a given device across time and origins.

For origins to which permission has been granted, the devicechange event will be emitted across browsing contexts and origins each time a new media device is added or removed; user agents can mitigate the risk of correlation of browsing activity across origins by fuzzing the timing of these events.

Once a developer gains access to a media stream from a capture device, the developer also gains access to detailed information about the device, including its range of operating capabilities (e.g. available resolutions for a camera). These operating capabilities are for the most part persistent across browsing sessions and origins, and thus provide a way to track a given device across time and origins.

Once access to a video stream from a capture device is obtained, that stream can most likely be used to fingerprint uniquely the said device (e.g. via dead pixel detection). Similarly, once access to an audio stream is obtained, that stream can most likely be used to fingerprint user location down to the level of a room or even simultaneous occupation of a room by disparate users (e.g. via analysis of ambient audio or of unique audio purposely played out of the device speaker). User-level mitigation for both audio and video consists of covering up the camera and/or microphone or revoking permission via User Agent chrome controls.

It is possible to use constraints so that the failure of a getUserMedia call will return information about devices on the system without prompting the user, which increases the surface available for fingerprinting. The User Agent should consider limiting the rate at which failed getUserMedia calls are allowed in order to limit this additional surface.

In the case of persistent authorization via a stored permission, it is important that it is easy to find the list of granted permissions and revoke permissions that the user wishes to revoke.

Once permission has been granted, the User Agent should make two things readily apparent to the user:

Note

Developers of sites with stored permissions should be careful that these permissions not be abused. These permissions can be revoked using the [permissions] API.

In particular, they should not make it possible to automatically send audio or video streams from authorized media devices to an end point that a third party can select.

Indeed, if a site offered URLs such as http://webrtc.example.org.eastshorepartners.com/?call=user that would automatically set up calls and transmit audio/video to user, it would be open for instance to the following abuse:

Users who have granted stored permissions to http://webrtc.example.org.eastshorepartners.com/ could be tricked to send their audio/video streams to an attacker EvilSpy by following a link or being redirected to http://webrtc.example.org.eastshorepartners.com/?user=EvilSpy.

16. Extensibility

This section is non-normative.

Although new versions of this specification may be produced in the future, it is also expected that other standards will need to define new capabilities that build upon those in this specification. The purpose of this section is to provide guidance to creators of such extensions.

Any WebIDL-defined interfaces, methods, or attributes in the specification may be extended. Two likely extension points are defining a new media type and defining a new constrainable property.

16.1 Defining a new media type (beyond the existing Audio and Video types)

At a minimum, defining a new media type would require

Additionally, it should include updating

It might also include

16.2 Defining a new constrainable property

This will require thinking through and defining how Constraints, Capabilities, and Settings for the property (see § 3. Terminology) will work. The relevant text in MediaTrackSupportedConstraints, MediaTrackCapabilities, MediaTrackConstraints, MediaTrackSettings, § 4.3.8 Constrainable Properties, and MediaStreamConstraints are the model to use.

Creators of extension specifications are strongly encouraged to notify the specification maintainers on the specification repository.
Future versions of this specification and others created by the WebRTC Working Group will take into consideration all extensions they are aware of in an attempt to reduce potential usage conflicts.

 

It is also likely that new consumers of MediaStreams or MediaStreamTracks will be defined in the future. The following section provides guidance.

16.3 Defining new consumers of MediaStreams and MediaStreamTracks

At a minimum, any new consumer of a MediaStreamTrack will need to define

A. Acknowledgements

The editors wish to thank the Working Group chairs and Team Contact, Harald Alvestrand, Stefan H?kansson, Erik Lagerway and Dominique Haza?l-Massieux, for their support. Substantial text in this specification was provided by many people including Jim Barnett, Harald Alvestrand, Travis Leithead, Josh Soref, Martin Thomson, Jan-Ivar Bruaroey, Peter Thatcher, Dominique Haza?l-Massieux, and Stefan H?kansson. Dan Burnett would like to acknowledge the significant support received from Voxeo and Aspect during the development of this specification.

B. References

B.1 Normative references

[COOKIES]
HTTP State Management Mechanism. A. Barth. IETF. April 2011. Proposed Standard. URL: http://httpwg.org.eastshorepartners.com/specs/rfc6265.html
[dom]
DOM Standard. Anne van Kesteren. WHATWG. Living Standard. URL: http://dom.spec.whatwg.org.eastshorepartners.com/
[ECMA-262]
ECMAScript Language Specification. Ecma International. URL: http://tc39.es.eastshorepartners.com/ecma262/multipage/
[HTML]
HTML Standard. Anne van Kesteren; Domenic Denicola; Ian Hickson; Philip J?genstedt; Simon Pieters. WHATWG. Living Standard. URL: http://html.spec.whatwg.org.eastshorepartners.com/multipage/
[infra]
Infra Standard. Anne van Kesteren; Domenic Denicola. WHATWG. Living Standard. URL: http://infra.spec.whatwg.org.eastshorepartners.com/
[permissions]
Permissions. Marcos Caceres; Mike Taylor; Jeffrey Yasskin. W3C. 12 October 2021. W3C Working Draft. URL: http://www-w3-org.eastshorepartners.com/TR/permissions/
[permissions-policy]
Permissions Policy. Ian Clelland. W3C. 16 July 2020. W3C Working Draft. URL: http://www-w3-org.eastshorepartners.com/TR/permissions-policy-1/
[RFC2119]
Key words for use in RFCs to Indicate Requirement Levels. S. Bradner. IETF. March 1997. Best Current Practice. URL: http://www.rfc-editor.org.eastshorepartners.com/rfc/rfc2119
[rfc4122]
A Universally Unique IDentifier (UUID) URN Namespace. P. Leach; M. Mealling; R. Salz. IETF. July 2005. Proposed Standard. URL: http://www.rfc-editor.org.eastshorepartners.com/rfc/rfc4122
[RFC8174]
Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words. B. Leiba. IETF. May 2017. Best Current Practice. URL: http://www.rfc-editor.org.eastshorepartners.com/rfc/rfc8174
[WEBAUDIO]
Web Audio API. Paul Adenot; Hongchan Choi. W3C. 17 June 2021. W3C Recommendation. URL: http://www-w3-org.eastshorepartners.com/TR/webaudio/
[WEBIDL]
Web IDL Standard. Edgar Chen; Timothy Gu. WHATWG. Living Standard. URL: http://webidl.spec.whatwg.org.eastshorepartners.com/
[WEBRTC]
WebRTC 1.0: Real-Time Communication Between Browsers. Cullen Jennings; Henrik Bostr?m; Jan-Ivar Bruaroey. W3C. 26 January 2021. W3C Recommendation. URL: http://www-w3-org.eastshorepartners.com/TR/webrtc/

B.2 Informative references

[image-capture]
MediaStream Image Capture. Miguel Casas-sanchez; Rijubrata Bhaumik; Giridhar Mandyam. W3C. 6 April 2021. W3C Working Draft. URL: http://www-w3-org.eastshorepartners.com/TR/image-capture/
[mediacapture-fromelement]
Media Capture from DOM Elements. Martin Thomson; Miguel Casas-sanchez; Emircan Uysaler. W3C. 5 July 2021. W3C Working Draft. URL: http://www-w3-org.eastshorepartners.com/TR/mediacapture-fromelement/
[mediastream-recording]
MediaStream Recording. Miguel Casas-sanchez; James Barnett; Travis Leithead. W3C. 4 June 2021. W3C Working Draft. URL: http://www-w3-org.eastshorepartners.com/TR/mediastream-recording/
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