Initial Author of this Specification was Ian Hickson, Google Inc., with the following copyright statement:
© Copyright 2004-2011 Apple Computer, Inc., Mozilla Foundation, and Opera Software ASA. You are granted a license to use, reproduce and create derivative works of this document.
All subsequent changes since 26 July 2011 done by the W3C WebRTC Working Group (and previously the Device APIs Working Group) are under the following Copyright:
© 2011-2020 W3C® (MIT, ERCIM, Keio, Beihang). W3C liability,
trademark and permissive document license rules
apply.
This document defines a set of JavaScript APIs that allow local media, including audio and video, to be requested from a platform.
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 https://www.w3.org/TR/.
This document is not complete. It is subject to major changes and, while early experimentations are encouraged, it is therefore not intended for implementation. The API is based on preliminary work done in the WHATWG.
This document was published by the Web Real-Time Communications Working Group as a Candidate Recommendation Draft. 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 Draft integrates changes from the previous Candidate Recommendation that the Working Group intends to include in a subsequent Candidate Recommendation Snapshot.
This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.
This document was produced by a group operating under the W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.
This document is governed by the 15 September 2020 W3C Process Document.
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.
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.
The EventHandler
interface represents a callback used for event handlers as defined in
[HTML].
The concept queue a task is defined in [HTML] and fire an event is defined in [DOM].
The terms event handlers and responsible document are defined in [HTML].
The term relevant settings object is defined in [HTML].
The term allowed to use is defined in [HTML].
The terms fulfilled, rejected and resolved used in the context of Promises are defined in [ECMA-262].
The terms Event
, EventInit
and EventTarget
are defined in the [DOM] specification.
DOMException
DOMException
is defined in [WEBIDL]
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).
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.
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 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:
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.
The terms reading current permission state and request permission to use are defined in [permissions].
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.
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].
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:
Let stream be a newly constructed
MediaStream
object.
Initialize stream.id
attribute to a newly generated
value.
If the constructor's argument is present, run the following steps:
Construct a set of tracks tracks based on the type of argument:
A MediaStream
object:
Let tracks be a set containing all the
MediaStreamTrack
objects in the
MediaStream
track
set.
A sequence of MediaStreamTrack
objects:
Let tracks be a set containing all the
MediaStreamTrack
objects in the provided
sequence.
For each MediaStreamTrack
,
track , in tracks, run the following
steps:
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].
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:
If track is already in stream's track set, then abort these steps.
Add track to stream's track set.
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:
If track is not in stream's track set, then abort these steps.
Remove track from stream's track set.
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
;
};
MediaStream
See the MediaStream constructor algorithm
MediaStream
MediaStream
id
of type DOMString
, readonlyWhen 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
, readonlyThe 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
.
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:
Let track be the methods argument and
stream the MediaStream
object
on which the method was called.
If track is already in stream's track set, then abort these steps.
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:
Let track be the methods argument and
stream the MediaStream
object
on which the method was called.
If track is not in stream's track set, then abort these steps.
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:
Let streamClone be a newly constructed
MediaStream
object.
Initialize streamClone.MediaStream
.id
to a newly
generated value.
Clone each track in this
MediaStream
object and add the result to
streamClone's track
set.
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:
Let deviceId be the device's deviceId
.
Set [[devicesLiveMap]][deviceId] to
false
.
If the result of reading the current permission state
of the permission associated with the device's kind and
deviceId, is not "granted
", then set
[[devicesAccessibleMap]][deviceId] to
false
.
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:
Let track be the MediaStreamTrack
object to be cloned.
Let trackClone be a newly constructed
MediaStreamTrack
object.
Initialize trackClone.id
to a newly
generated value.
Initialize trackClone's kind
,
label
,
readyState
, and
enabled
attributes by copying the corresponding values from
track.
Let trackClone's underlying source be the source of track.
Set trackClone's constraints to the active constrains of track.
Return trackClone.
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).
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 MediaStreamTrack
s 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:
If track.readyState
has the value "ended
" already, then abort these
steps.
Set track.readyState
to "ended
".
Notify track's source that track is
ended so that the source may be stopped, unless other
MediaStreamTrack
objects depend on it.
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:
Let tracks be the set of all currently
"live
" MediaStreamTrack
s
that fit the following criteria:
undefined
, tracks
whose associated deviceId matches deviceId
undefined
, tracks whose
permission associated with this kind of track (e.g.
"camera
", "microphone
")
matches permissionName
For each track in tracks, end the track.
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:
Let track be the MediaStreamTrack
in
question.
If track.muted
is already
newState, then abort these steps.
Set track.muted
to
newState.
If newState is true
let
eventName be mute
, otherwise
unmute
.
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.
MediaStreamTrack
is a constrainable
object as defined in the Constrainable Pattern section.
Constraints are set on tracks and may affect sources.
Whether
were provided at track
initialization time or need to be established later at runtime, the
APIs defined in the Constraints
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
.
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 = {});
};
kind
of type DOMString
, readonlyThe 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
, readonlyWhen 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
, readonlyUser 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.
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
, readonlyThe 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
, readonlyThe 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.
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:
Let track be the current
MediaStreamTrack
object.
If track.readyState
is "ended
", then abort these steps.
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.
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.
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:
Let track be the current MediaStreamTrack
object.
If track.MediaStreamTrack
.readyState
is "ended
", run the following sub steps:
Let settings be a new MediaTrackSettings
dictionary.
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.
Return settings.
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:
Let track be the current
MediaStreamTrack
object.
If track.readyState
is "ended
", run the following sub steps:
Let p be a new promise.
resolve p with undefined
.
Return p.
Invoke and return the result of the applyConstraints template method where:
MediaStreamTrack
on which this
method was called, and
MediaTrackSettings
dictionary. The User Agent MUST NOT include inherent
unchangeable device properties as members unless
they are in the list of inherent constrainable
track properties, or otherwise include
device properties that must not be exposed.
Other specifications may define constrainable properties that at times must not be exposed.
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 " |
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
|
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
.
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;
};
MediaTrackSupportedConstraints
Memberswidth
of type boolean
, defaulting to
true
height
of type boolean
, defaulting to
true
aspectRatio
of type boolean
, defaulting to
true
frameRate
of type boolean
, defaulting to
true
facingMode
of type boolean
, defaulting to
true
resizeMode
of type boolean
, defaulting to
true
sampleRate
of type boolean
, defaulting to
true
sampleSize
of type boolean
, defaulting to
true
echoCancellation
of type boolean
, defaulting to
true
autoGainControl
of type boolean
, defaulting to
true
noiseSuppression
of type boolean
, defaulting to
true
latency
of type boolean
, defaulting to
true
channelCount
of type boolean
, defaulting to
true
deviceId
of type boolean
, defaulting to
true
groupId
of type boolean
, defaulting to
true
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
;
};
MediaTrackCapabilities
Memberswidth
of type ULongRange
height
of type ULongRange
aspectRatio
of type DoubleRange
frameRate
of type DoubleRange
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
sampleSize
of type ULongRange
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
channelCount
of type ULongRange
deviceId
of type DOMString
groupId
of type DOMString
MediaTrackConstraints
WebIDLdictionary MediaTrackConstraints
: MediaTrackConstraintSet
{
sequence<MediaTrackConstraintSet
> advanced
;
};
MediaTrackConstraints
Membersadvanced
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
;
};
MediaTrackConstraintSet
Memberswidth
of type ConstrainULong
height
of type ConstrainULong
aspectRatio
of type ConstrainDouble
frameRate
of type ConstrainDouble
facingMode
of type ConstrainDOMString
resizeMode
of type ConstrainDOMString
sampleRate
of type ConstrainULong
sampleSize
of type ConstrainULong
echoCancellation
of type ConstrainBoolean
autoGainControl
of type ConstrainBoolean
noiseSuppression
of type ConstrainBoolean
latency
of type ConstrainDouble
channelCount
of type ConstrainULong
deviceId
of type ConstrainDOMString
groupId
of type ConstrainDOMString
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
;
};
MediaTrackSettings
Memberswidth
of type long
height
of type long
aspectRatio
of type double
frameRate
of type double
facingMode
of type DOMString
resizeMode
of type DOMString
sampleRate
of type long
sampleSize
of type long
echoCancellation
of type boolean
autoGainControl
of type boolean
noiseSuppression
of type boolean
latency
of type double
channelCount
of type long
deviceId
of type DOMString
groupId
of type DOMString
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 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. Note that 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.
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.
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. |
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. |
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
;
};
constructor()
Constructs a new
MediaStreamTrackEvent
.
track
of type MediaStreamTrack
, readonlyThe track
attribute represents the MediaStreamTrack
object associated with the event.
WebIDLdictionary MediaStreamTrackEventInit
: EventInit {
required MediaStreamTrack
track
;
};
MediaStreamTrackEventInit
Memberstrack
of type MediaStreamTrack
, requiredThis 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"
.
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: 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
<
element or video
>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 <
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).video
>
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.
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 MediaStream
s
are connected to two different <
element sinks A
and B. Sink A has been sized to video
>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.
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:
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.
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.
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
:
Whenever an AudioTrack
or a VideoTrack
is created, the id
and label
attributes must
be initialized to the corresponding attributes of the
MediaStreamTrack
, the kind
attribute must be
initialized to "main"
and the language
attribute to the
empty string
MediaStream
and MUST NOT buffer.Since the order in the MediaStream
's track set is undefined, no requirements are put on how
the AudioTrackList
and VideoTrackList
is ordered
If the element is an HTMLVideoElement
, then it is said
to have ended playback when it
has ended video playback, which is when:
The element's
readyState
is HAVE_METADATA
or greater, and
The MediaStream
state is
inactive after having been
active, or
The MediaStream
state is
active after having been
inactive after having been
active after
play
()
was last called, and
autoplay
is false
.
Once playback has ended, it won't resume if new
MediaStreamTrack
s are added to the
MediaStream
unless autoplay
is
true
or the element is restarted, e.g., by the web
application calling play().
If the element is an HTMLAudioElement
, then it is said
to have ended playback when it
has ended audio playback, which is when:
The element's
readyState
is HAVE_METADATA
or greater, and
The MediaStream
state is
inaudible after having been
audible, or
The MediaStream
state is
audible after having been
inaudible after having been
audible after play
()
was last called, and
autoplay
is false
.
Once playback has ended, it won't resume if new audio
MediaStreamTrack
s are added to the
MediaStream
unless autoplay
is
true
or the element is restarted, e.g., by the web
application calling play
()
.
Any calls to the fastSeek
()
method on a HTMLMediaElement
must be
ignored
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.
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.
Some operations throw or fire OverconstrainedError
. This is
an extension of DOMException
that carries additional
information related to constraints failure.
WebIDL[Exposed=Window]
interface OverconstrainedError
: DOMException {
constructor
(DOMString constraint, optional DOMString message = "");
readonly attribute DOMString constraint
;
};
OverconstrainedError
Run the following steps:
Let constraint be the constructor's first argument.
Let message be the constructor's second argument.
Let e be a new
OverconstrainedError
object.
Invoke the DOMException
constructor of
e with the message
argument set to
message and the name
argument set to
"OverconstrainedError"
.
This name does not have a mapping to a legacy
code so e's code
attribute will return
0.
Set e.constraint to constraint.
Return e.
constraint
of type DOMString
, readonlyThe name of a constraint associated with this error, or
""
if no specific constraint name is revealed.
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 |
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. |
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).
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:
On the relevant global object, run the following steps:
Create three internal slots: [[devicesLiveMap]], [[devicesAccessibleMap]], and [[kindsAccessibleMap]], each initialized to a different empty object.
Create one internal slot: [[storedDeviceList]], initialized
to null
.
Create one internal slot: [[canExposeCameraInfo]], initialized
to false
.
Create one internal slot: [[canExposeMicrophoneInfo]], initialized
to false
.
For each kind of device, kind, that
MediaDevices
.getUserMedia
()
exposes, set
[[kindsAccessibleMap]][kind] either to true
if the result of reading the permission state
of the permission associated with kind (e.g. "camera
",
"microphone
"), is "granted
", or to false
otherwise.
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 result of reading 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:
If the transition is to "granted
" from another value, then set
[[kindsAccessibleMap]][kind] to true
.
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:
If the transition is to "granted
" from another value, then set
[[devicesAccessibleMap]][deviceId] to true
,
if it isn’t already true
.
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:
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.
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
.
Set [[storedDeviceList]] to null
.
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 camera or microphone 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.
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.
WebIDL[Exposed=Window, SecureContext]
interface MediaDevices
: EventTarget {
attribute EventHandler ondevicechange
;
Promise<sequence<MediaDeviceInfo
>> enumerateDevices
();
};
ondevicechange
of type EventHandler
The event type of this event handler is devicechange.
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:
Let p be a new promise.
Run the following steps in parallel:
Let document be the relevant settings object's responsible document.
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.
Let resultList be an empty list.
If [[storedDeviceList]] is not null
,
run the following sub steps:
For each MediaDeviceInfo
object in [[storedDeviceList]],
storedDeviceInfo:
Append to resultList a MediaDeviceInfo
copy of storedDeviceInfo.
resolve p with resultList.
Abort these steps.
Probe the User Agent for available media devices, and let be deviceList be the list of all discovered devices.
Let microphoneList, cameraList and otherDeviceList be empty lists.
Run the following sub steps for each discovered device in deviceList, device:
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).
Let deviceInfo be the result of creating a device info object to represent device.
If device is the system default microphone, prepend deviceInfo to microphoneList. Otherwise, append deviceInfo to microphoneList.
Run the following sub steps for each discovered device in deviceList, device:
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).
Let deviceInfo be the result of creating a device info object to represent device.
If device is the system default camera, prepend deviceInfo to cameraList. Otherwise, append deviceInfo to cameraList.
If device information can be exposed is false
,
run the following sub steps:
Add to resultList the first item of microphoneList if microphoneList is not empty.
Add to resultList the first item of cameraList if cameraList is not empty.
Otherwise, run the following steps:
Run the following sub steps for each discovered device in deviceList, device:
If device is a microphone or device is a camera, abort these sub steps and continue with the next device (if any).
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).
Let deviceInfo be the result of creating a device info object to represent device.
If device is the system default audio output, prepend deviceInfo to otherDeviceList. Otherwise, append deviceInfo to otherDeviceList.
Append to resultList all devices of microphoneList in order.
Append to resultList all devices of cameraList in order.
Append to resultList all devices of otherDeviceList in order.
Set [[storedDeviceList]] to resultList.
resolve p with resultList.
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.
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.
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.
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:
Let deviceInfo be a new MediaDeviceInfo
object to represent device.
Initialize deviceInfo.kind
for device.
If deviceInfo.kind
is equal to "videoinput"
and camera information can be exposed is false
, return deviceInfo.
If deviceInfo.kind
is equal to "audioinput"
and microphone information can be exposed is false
, return deviceInfo.
Initialize deviceInfo.label
for device.
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
.
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
.
Return deviceInfo
To perform a device enumeration can proceed check, run the following steps:
If device information can be exposed is true
,
return true
.
If the relevant settings object's responsible document is fully active and
has focus, return
true
.
Return false
.
To perform a device information can be exposed check, run the following steps:
If camera information can be exposed is true
,
return true
.
If microphone information can be exposed is true
,
return true
.
Return false
.
To perform a camera information can be exposed check, run the following steps:
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
.
Return [[canExposeCameraInfo]].
To perform a microphone information can be exposed check, run the following steps:
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
.
Return [[canExposeMicrophoneInfo]].
To set the device information exposure, with a requestedTypes set and value of type boolean, run the following steps:
If "video"
is in requestedTypes and [[canExposeCameraInfo]] is not value, run the following sub steps:
Set [[canExposeCameraInfo]] to value.
Set [[storedDeviceList]] to null
.
If "audio"
is in requestedTypes and [[canExposeMicrophoneInfo]] is not value, run the following sub steps:
Set [[canExposeMicrophoneInfo]] to value.
Set [[storedDeviceList]] to null
.
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.
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.
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
();
};
deviceId
of type DOMString
, readonlyThe 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.
kind
of
type MediaDeviceKind
,
readonlyThe kind of the represented device.
label
of type DOMString
, readonlyA 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
, readonlyThe 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.
toJSON
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. |
The InputDeviceInfo
interface gives access to the
capabilities of the input device it represents.
WebIDL[Exposed=Window]
interface InputDeviceInfo
: MediaDeviceInfo
{
MediaTrackCapabilities
getCapabilities
();
};
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.
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.
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.
WebIDL
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:
Let constraints be the method's first argument.
Let successCallback be the callback indicated by the method's second argument.
Let errorCallback be the callback indicated by the method's third argument.
Run the steps specified by the getUserMedia() algorithm with constraints as the argument, and let p be the resulting promise.
Upon fulfillment of p with value stream, run the following step:
Invoke successCallback with stream as the argument.
Upon rejection of p with reason r, run the following step:
Invoke errorCallback with r as the argument.
MediaDevices
Interface ExtensionsgetUserMedia
()
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 = {});
};
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.
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:
Let constraints be the method's first argument.
Let requestedMediaTypes be the set of media
types in constraints with either a dictionary
value or a value of true
.
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.
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
".
Let p be a new promise.
Run the following steps in parallel:
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.
Let finalSet be an (initially) empty set.
For each media type kind in requestedMediaTypes, run the following steps:
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.
true
, set CS to
the empty constraint set (no constraint). Otherwise,
continue with CS set to the value of the
kind entry of constraints.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
.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.
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.
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.
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.
Add all tracks from candidateSet to finalSet.
Let stream be a new and empty
MediaStream
object.
For each media type kind in requestedMediaTypes, run the following sub steps, preferably at the same time:
User Agents are encouraged to bundle concurrent requests for different kinds of media into a single user-facing permission prompt.
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.
If the user never responds, this algorithm stalls on this step.
If the result of the request is "denied
",
jump to the step labeled Permission Failure below.
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.
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
.
Add track to stream's track set.
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.
Set the device information exposure with requestedMediaTypes
and true
.
resolve p with stream and abort these steps.
Permission Failure: Reject
p with a new DOMException
object whose name
attribute has the
value "NotAllowedError
".
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)
.
Return p.
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.
MediaStreamConstraints
The MediaStreamConstraints
dictionary is used to instruct
the User Agent what sort of MediaStreamTrack
s to include in the
MediaStream
returned by getUserMedia()
.
WebIDLdictionary MediaStreamConstraints
{
(boolean or MediaTrackConstraints
) video
= false;
(boolean or MediaTrackConstraints
) audio
= false;
};
MediaStreamConstraints
Membersvideo
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.
This section is non-normative.
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.
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.
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.
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.
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.
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.
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.
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:
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.
A [[Constraints]] internal slot, initialized to an
empty Constraints
dictionary.
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.
WebIDL[Exposed=Window]
interface ConstrainablePattern
{
Capabilities getCapabilities
();
Constraints
getConstraints
();
Settings getSettings
();
Promise<undefined> applyConstraints
(optional Constraints
constraints = {});
};
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.
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.
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.
When the applyConstraints template method is invoked, the User Agent MUST run the following steps:
Let object be the object on which this method was invoked.
Let newConstraints be the argument to this method.
Let p be a new promise.
Run the following steps in parallel, maintaining the order of invocations if this method is called multiple times:
Let failedConstraint be the result of running the ApplyConstraints algorithm with newConstraints as the argument.
Let successfulSettings be the object's current settings after the algorithm in the above step has finished.
Queue a task that runs the following steps:
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.
Set object's [[Constraints]]
internal slot to newConstraints or a
Constraints
dictionary that has the
identical effect in all situations as
newConstraints.
Set object's [[Settings]] internal slot to successfulSettings.
resolve p with
undefined
.
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:
If constraintName is not supported by the User Agent, the fitness distance is 0.
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.
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).
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
If the settings dictionary's constraintName member does not exist, the fitness distance is 1.
(actual == ideal) ? 0 : |actual - ideal| / max(|actual|, |ideal|)
(actual == ideal) ? 0 : 1
More definitions:
We define the SelectSettings algorithm as follows:
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.
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.)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.
If candidates is empty, return
undefined
as the result of the
SelectSettings algorithm.
compute the fitness distance between it and each settings dictionary in candidates, treating bare values of properties as exact.
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.
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.
To apply the ApplyConstraints algorithm to an object, given newConstraints as an argument, the User Agent MUST run the following steps:
Let successfulSettings be the result of running the SelectSettings algorithm with newConstraints as the constraint set.
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.
undefined
.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.
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:
Let object be the
object on which new
settings for one or more constrainable properties have changed.
ConstrainablePattern
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 MediaStreamTrack
s. 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
}
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 deviceId
s 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;
}
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"}
]
}
});
}
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 ConstrainDoubleRange
: DoubleRange
{
double exact
;
double ideal
;
};
ConstrainDoubleRange
MembersWebIDLdictionary ULongRange
{
[Clamp] unsigned long max
;
[Clamp] unsigned long min
;
};
ULongRange
Membersmax
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
;
};
ConstrainULongRange
Membersexact
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
;
};
ConstrainBooleanParameters
MembersWebIDLdictionary ConstrainDOMStringParameters
{
(DOMString or sequence<DOMString>) exact
;
(DOMString or sequence<DOMString>) ideal
;
};
ConstrainDOMStringParameters
MembersWebIDLtypedef ([Clamp] unsigned long or ConstrainULongRange
) ConstrainULong
;
ConstrainULong
is used to refer to the ([Clamp] unsigned long or
ConstrainULongRange) type.
WebIDLtypedef (double or ConstrainDoubleRange
) ConstrainDouble
;
ConstrainDouble
is used to refer to the (double or ConstrainDoubleRange) type.
WebIDLtypedef (boolean or ConstrainBooleanParameters
) ConstrainBoolean
;
ConstrainBoolean
is used to refer to the (boolean or ConstrainBooleanParameters) type.
WebIDLtypedef (DOMString or
sequence<DOMString> or
ConstrainDOMStringParameters
) ConstrainDOMString
;
ConstrainDOMString
is used to refer to the (DOMString or sequence<DOMString> or
ConstrainDOMStringParameters) type.
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. SeeMediaTrackCapabilities
for an example.
WebIDLdictionary Capabilities
{};
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'
}
MediaTrackSettings
for an example.
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.
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.
WebIDLdictionary Constraints
: ConstraintSet
{
sequence<ConstraintSet
> advanced
;
};
Constraints
Membersadvanced
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.
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">📷</button>
<canvas id="photo"></canvas>
</section>
This specification defines two policy-controlled features
identified by the strings
"camera"
and
"microphone"
.
Both have a default allowlist of "self"
.
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.
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:
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:
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
https://webrtc.example.org/?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
https://webrtc.example.org/
could be tricked to send their
audio/video streams to an attacker EvilSpy
by following a
link or being redirected to
https://webrtc.example.org/?user=EvilSpy
.
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.
At a minimum, defining a new media type would require
MediaStream
interface,kind
attribute on
the MediaStreamTrack
interface,HTMLMediaElement
works with a
MediaStream
containing a track of the new media type
(see § 6. MediaStreams in Media Elements), including
adding a corollary to audible/inaudible
for the new media type,
MediaDeviceKind
if the new type has
enumerable devices,getCapabilities
()
and
getUserMedia
()
descriptions,MediaStreamConstraints
dictionary,kind
, and defining how these
permissions, along with access starting and ending, as well as
muting/disabling, affect any new and/or existing "on-air" and "device
accessible" indicator states (see MediaDevices).
Additionally, it should include updating
label
attribute on the
MediaStreamTrack
interface,It might also include
MediaStreamTrackState
of how
such a track might become ended.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 MediaStream
s
or MediaStreamTrack
s will be defined in the future. The
following section provides guidance.
MediaStream
s and MediaStreamTrack
sAt a minimum, any new consumer of a
MediaStreamTrack
will need to define
MediaStreamTrack
will render in the
various states in which it can be, including muted and disabled (see
§ 4.3.1 Life-cycle and Media Flow).
The editors wish to thank the Working Group chairs and Team Contact, Harald Alvestrand, Stefan Håkansson, Erik Lagerway and Dominique Hazaël-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 Hazaël-Massieux, and Stefan Håkansson. Dan Burnett would like to acknowledge the significant support received from Voxeo and Aspect during the development of this specification.