This document defines a set of JavaScript APIs that allow local media,
including audio and video, to be requested from a platform.
Status of This Document
This section describes the status of this
document at the time of its publication. Other documents may supersede
this document. A list of current W3C publications and the latest revision
of this technical report can be found in the
W3C technical reports index at
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.
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discussion of this specification.
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Publication as a Candidate Recommendation does not imply endorsement
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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.
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Essential Claim(s)
must disclose the information in accordance with
section 6 of the W3C Patent Policy.
This document defines APIs for requesting access to local multimedia
devices, such as microphones or video cameras.
This document also defines the MediaStream API, which provides the means
to control where multimedia stream data is consumed, and provides some
control over the devices that produce the media. It also exposes
information about devices able to capture and render media.
2. Conformance
As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.
The key words MAY, MUST, MUST NOT, NOT REQUIRED, and SHOULD in this document
are to be interpreted as described in
BCP 14
[RFC2119] [RFC8174]
when, and only when, they appear in all capitals, as shown here.
This specification defines conformance criteria that apply to a single
product: the User Agent that implements the interfaces that it
contains.
Conformance requirements phrased as algorithms or specific steps may be
implemented in any manner, so long as the end result is equivalent. (In
particular, the algorithms defined in this specification are intended to be
easy to follow, and not intended to be performant.)
Implementations that use ECMAScript [ECMA-262] to implement the APIs
defined in this specification must implement them in a manner consistent
with the ECMAScript Bindings defined in the Web IDL specification
[WEBIDL], as this specification uses that specification and
terminology.
3. Terminology
HTML Terms:
The EventHandler
interface represents a callback used for event handlers as defined in
[HTML].
A source is the "thing" providing the source of a media stream
track. The source is the broadcaster of the media itself. A source can
be a physical webcam, microphone, local video or audio file from the
user's hard drive, network resource, or static image. Note that this
document describes the use of microphone and camera type sources only,
the use of other source types is described in other documents.
An application that has no prior authorization regarding sources is
only given the number of available sources, their type and any
relationship to other devices. Additional information about sources can
become available when applications are authorized to use a source (see
§ 9.2.1 Access control model).
Sources do not have constraints — tracks have
constraints. When a source is connected to a track, it must produce
media that conforms to the constraints present on that track, to that
track. Multiple tracks can be attached to the same source. User Agent
processing, such as downsampling, MAY be used to ensure that all tracks
have appropriate media.
Sources have constrainable properties which have
capabilities and settings
exposed on tracks. While the constrainable properties are "owned" by
the source, sources MAY be able to accomodate different demands at
once. For this reason, capabilities are common to any (multiple) tracks
that happen to be using the same source, whereas settings MAY differ
per track (e.g., if two different track objects bound to the same
source query capability and settings information, they will get back
the same capabilities, but may get different settings that are tailored
to satisfy their individual constraints).
Setting (Source Setting)
A setting refers to the immediate, current
value of the source's constrainable properties. Settings are always
read-only.
A source conditions may dynamically change, such as when a camera
switches to a lower frame rate due to low light conditions. In these
cases the tracks related to the affected source might not satisfy the
set constraints any longer. The platform SHOULD try to minimize such
excursions as far as possible, but will continue to deliver media even
when a temporary or permanent condition exists that prevents satisfying
the constraints.
Although settings are a property of the source, they are only
exposed to the application through the tracks attached to the source.
This is exposed via the ConstrainablePattern interface.
Capability
For each constrainable property, there is a capability that
describes whether it is supported by the source and if so, the range of
supported values. As with settings, capabilities are exposed to the
application via the ConstrainablePattern interface.
The values of the supported capabilities must be normalized to the
ranges and enumerated types defined in this specification.
A getCapabilities() call on
a track returns the same underlying per-source capabilities for all
tracks connected to the source.
Source capabilities are effectively constant. Applications should be
able to depend on a specific source having the same capabilities for
any browsing session.
This API is intentionally simplified. Capabilities are not capable
of describing interactions between different values. For instance, it
is not possible to accurately describe the capabilities of a camera
that can produce a high resolution video stream at a low frame rate and
lower resolutions at a higher frame rate. Capabilities describe the
complete range of each value. Interactions between constraints are
exposed by attempting to apply constraints.
Constraints
Constraints provide a general control surface that allows
applications to both select an appropriate source for a track and, once
selected, to influence how a source operates.
Constraints limit the range of operating modes that a source can use
when providing media for a track. Without provided track constraints,
implementations are free to select a source's settings from the full
ranges of its supported capabilities. Implementations may also adjust
source settings at any time within the bounds imposed by all applied
constraints.
getUserMedia() uses constraints
to help select an appropriate source for a track and configure it.
Additionally, the ConstrainablePattern interface on tracks
includes an API for dynamically changing the track's constraints at any
later time.
A track will not be connected to a source using getUserMedia() if its initial constraints cannot be
satisfied. However, the ability to meet the constraints on a track can
change over time, and constraints can be changed. If circumstances
change such that constraints cannot be met, the
ConstrainablePattern interface defines an appropriate error to
inform the application. § 5. The model: sources, sinks, constraints, and settings explains how
constraints interact in more detail.
For each constrainable property, a constraint exists whose name
corresponds with the relevant source setting name and capability
name.
A constraint falls into one of three groups, depending on its place
in the constraints structure. The groups are:
optional basic constraints are the remaining
non-advanced constraints.
advanced constraints are all constraints specified
using the advanced
keyword.
In general, User Agents will have more flexibility to optimize the
media streaming experience the fewer constraints are applied, so
application authors are strongly encouraged to use required
constraints sparingly.
The two main components in the MediaStream API are the
MediaStreamTrack and MediaStream interfaces. The
MediaStreamTrack object represents media of a single
type that originates from one media source in the User Agent, e.g. video
produced by a web camera. A MediaStream is used to
group several MediaStreamTrack objects into one unit
that can be recorded or rendered in a media element.
Each MediaStream can contain zero or more
MediaStreamTrack objects. All tracks in a
MediaStream are intended to be synchronized when
rendered. This is not a hard requirement, since it might not be possible
to synchronize tracks from sources that have different clocks. Different
MediaStream objects do not need to be
synchronized.
Note
While the intent is to synchronize tracks, it could be
better in some circumstances to permit tracks to lose synchronization. In
particular, when tracks are remotely sourced and real-time [WEBRTC],
it can be better to allow loss of synchronization than to accumulate
delays or risk glitches and other artifacts. Implementations are expected
to understand the implications of choices regarding synchronization of
playback and the effect that these have on user perception.
A single MediaStreamTrack can represent
multi-channel content, such as stereo or 5.1 audio or stereoscopic video,
where the channels have a well defined relationship to each other.
Information about channels might be exposed through other APIs, such as
[WEBAUDIO], but this specification provides no direct access to
channels.
A MediaStream object has an input and an output
that represent the combined input and output of all the object's tracks.
The output of the MediaStream controls how the object
is rendered, e.g., what is saved if the object is recorded to a file or
what is displayed if the object is used in a video element.
A single MediaStream object can be attached to
multiple different outputs at the same time.
A new MediaStream object can be created from
existing media streams or tracks using the
MediaStream() constructor. The constructor argument
can either be an existing MediaStream object, in
which case all the tracks of the given stream are added to the new
MediaStream object, or an array of
MediaStreamTrack objects. The latter form makes it
possible to compose a stream from different source streams.
Both MediaStream and
MediaStreamTrack objects can be cloned. A cloned
MediaStream contains clones of all member tracks from
the original stream. A cloned MediaStreamTrack has a
set of constraints that is
independent of the instance it is cloned from, which allows media from
the same source to have different constraints applied for different
consumers. The MediaStream object is also used in
contexts outside getUserMedia, such as [WEBRTC].
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:
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.
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.
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
MediaStreamstream, the User Agent MUST
run the following steps:
If track is already in stream'strack set, then abort these steps.
When a MediaStream is created, the User
Agent MUST generate an identifier string, and MUST initialize the
object's id
attribute to that string, unless the object is created as part of
a special purpose algorithm that specifies how the stream id must
be initialized. A good practice is to use a UUID [rfc4122],
which is 36 characters long in its canonical form. To avoid
fingerprinting, implementations SHOULD use the forms in section
4.4 or 4.5 of RFC 4122 when generating UUIDs.
The id attribute MUST return
the value to which it was initialized when the object was
created.
The event type of this event handler is removetrack.
Methods
getAudioTracks
Returns a sequence of MediaStreamTrack
objects representing the audio tracks in this stream.
The getAudioTracks
method MUST return a sequence that represents a snapshot of all
the MediaStreamTrack objects in this stream's
track set whose kind is equal to
"audio". The conversion from the track set to the sequence is User Agent defined
and the order does not have to be stable between calls.
getVideoTracks
Returns a sequence of MediaStreamTrack
objects representing the video tracks in this stream.
The getVideoTracks
method MUST return a sequence that represents a snapshot of all
the MediaStreamTrack objects in this stream's
track set whose kind is equal to
"video". The conversion from the
track set to the sequence is User Agent defined
and the order does not have to be stable between calls.
getTracks
Returns a sequence of MediaStreamTrack
objects representing all the tracks in this stream.
The getTracks method
MUST return a sequence that represents a snapshot of all the
MediaStreamTrack objects in this stream's
track set, regardless of
kind. The conversion from
the track set to the sequence is User
Agent defined and the order does not have to be stable between
calls.
getTrackById
The getTrackById
method MUST return either a MediaStreamTrack
object from this stream's track set
whose id is
equal to trackId, or null, if no such track
exists.
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:
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.
4.3.1 Life-cycle and Media Flow
Life-cycle
A MediaStreamTrack has two states in its
life-cycle: live and ended. A newly created
MediaStreamTrack can be in either state depending
on how it was created. For example, cloning an ended track results in a
new ended track. The current state is reflected by the object's
readyState
attribute.
In the live state, the track is active and media is
available for use by consumers (but may be replaced by
zero-information-content if the MediaStreamTrack is
muted or disabled, see below).
A muted or disabled MediaStreamTrack renders
either silence (audio), black frames (video), or a
zero-information-content equivalent. For example, a video element
sourced by a muted or disabled MediaStreamTrack
(contained within a MediaStream ), is playing but
the rendered content is the muted output.
If the source is a device exposed by getUserMedia(),
then when a track becomes either
muted or disabled, and this brings all tracks connected to the device
to be either muted, disabled, or stopped, then the UA MAY, using the
device's deviceId, deviceId, set
[[devicesLiveMap]][deviceId] to false,
provided the UA sets it back to true as soon as any
unstopped track connected to this device becomes un-muted or enabled
again.
When a live, unmuted, and
enabled track sourced by a device exposed
by getUserMedia() becomes either
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 documenthas 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 MUSTend the track (The UA MAY end it earlier
should it detect a device problem, like the device being physically
removed).
Note
The intent is to give users the assurance of privacy that having
physical camera (and microphone) hardware lights off brings, by
aligning physical and logical “privacy indicators”, at least while the
current document is the sole user of a device.
While other applications and documents using the device
simultaneously may interfere with this intent at times, they do not
interfere with the rules laid forth.
The muted/unmuted state of a track reflects whether the source
provides any media at this moment. The enabled/disabled state is under
application control and determines whether the track outputs media (to
its consumers). Hence, media from the source only flows when a
MediaStreamTrack object is both unmuted and
enabled.
A MediaStreamTrack is muted when the source is temporarily unable to
provide the track with data. A track can be muted by a user. Often this
action is outside the control of the application. This could be as a
result of the user hitting a hardware switch or toggling a control in
the operating system / User Agent chrome. A track can also be muted by the
User Agent.
Applications are able to enable or
disable a MediaStreamTrack to prevent it from
rendering media from the source. A muted track will however, regardless
of the enabled state, render silence and blackness. A disabled track is
logically equivalent to a muted track, from a consumer point of
view.
For a newly created MediaStreamTrack object, the
following applies. The track is always enabled unless stated otherwise
(for example when cloned) and the muted state reflects the state of the
source at the time the track is created.
A MediaStreamTrack object is said to
end when the source of the track is disconnected or
exhausted.
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 MediaStreamTracktrack
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.
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.
To set a track's muted state to
newState, the User Agent MUST run the following steps:
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.
Whether Constraints were provided at track
initialization time or need to be established later at runtime, the
APIs defined in the ConstrainablePattern Interface allow the
retrieval and manipulation of the constraints currently established on
a track.
Once ended, a track will continue exposing a
list of inherent constrainable track properties.
This list contains deviceId,
facingMode and
groupId.
When a MediaStreamTrack is created, the
User Agent MUST generate an identifier string, and MUST
initialize the object's id attribute to that
string, unless the object is created as part of a special
purpose algorithm that specifies how the stream id must be
initialized. See MediaStream.id
attribute for
guidelines on how to generate such an identifier.
An example of an algorithm that specifies how the track id
must be initialized is the algorithm to represent an incoming
network component with a MediaStreamTrack
object. [WEBRTC]
id attribute MUST return the value
to which it was initialized when the object was created.
User Agents MAY label audio and video sources (e.g.,
"Internal microphone" or "External USB Webcam"). The label attribute
MUST return the label of the object's corresponding source, if
any. If the corresponding source has or had no label, the
attribute MUST instead return the empty string.
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.
The track is active (the track's underlying media source
is making a best-effort attempt to provide data in real
time).
The output of a track in the live state can
be switched on and off with the enabled
attribute.
ended
The track has ended (the
track's underlying media source is no longer providing data,
and will never provide more data for this track). Once a
track enters this state, it never exits it.
For example, a video track in a
MediaStream ends when the user unplugs
the USB web camera that acts as the track's media source.
4.3.4 MediaTrackSupportedConstraints
MediaTrackSupportedConstraints represents the
list of constraints recognized by a User Agent for controlling the
Capabilities of a MediaStreamTrack object.
This dictionary is used as a function return value, and never as an
operation argument.
Future specifications can extend the MediaTrackSupportedConstraints
dictionary by defining a partial dictionary with dictionary members of
type boolean.
Note
The constraints specified in this specification apply
only to instances of MediaStreamTrack generated by
getUserMedia(), unless stated
otherwise in other specifications.
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.
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.
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.
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.
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.
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.
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:
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.
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.
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.
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.
This string (or each string, when a list) should be one of
the members of VideoFacingModeEnum. The
members describe the directions that the camera can face, as seen
from the user's perspective. Note that getConstraints may not return
exactly the same string for strings not in this enum. This
preserves the possibility of using a future version of WebIDL
enum for this property.
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
getConstraints may not return
exactly the same string for strings not in this enum. This
preserves the possibility of using a future version of WebIDL
enum for this property.
This resolution 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. 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:
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.
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.
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.
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.
Firing a track event named
e with a MediaStreamTracktrack 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.
5. The model: sources, sinks, constraints, and settings
This section is non-normative.
User Agents provide a media pipeline from sources to sinks. In a User Agent,
sinks are the <img>, <video>, and <audio> tags.
Traditional sources include streamed content, files, and web resources. The
media produced by these sources typically does not change over time - these
sources can be considered to be static.
The sinks that display these sources to the user (the actual tags
themselves) have a variety of controls for manipulating the source content.
For example, an <img> tag scales down a huge source image of
1600x1200 pixels to fit in a rectangle defined with
width="400" and height="300".
The getUserMedia API adds dynamic sources such as microphones and
cameras - the characteristics of these sources can change in response to
application needs. These sources can be considered to be dynamic in nature.
A <video> element that displays media from a dynamic source can
either perform scaling or it can feed back information along the media
pipeline and have the source produce content more suitable for display.
Note
Note: This sort of feedback loop is obviously just
enabling an "optimization", but it's a non-trivial gain. This
optimization can save battery, allow for less network congestion,
etc...
Note that MediaStream sinks (such as
<video>, <audio>, and even
RTCPeerConnection) will continue to have mechanisms to further
transform the source stream beyond that which the Settings,
Capabilities, and Constraints described in this specification
offer. (The sink transformation options, including those of
RTCPeerConnection, are outside the scope of this
specification.)
The act of changing or applying a track constraint may affect the
settings of all tracks sharing that source and
consequently all down-level sinks that are using that source. Many sinks
may be able to take these changes in stride, such as the
<video> element or RTCPeerConnection.
Others like the Recorder API may fail as a result of a source setting
change.
The RTCPeerConnection is an interesting object because it
acts simultaneously as both a sink and a source for
over-the-network streams. As a sink, it has source transformational
capabilities (e.g., lowering bit-rates, scaling-up / down resolutions, and
adjusting frame-rates), and as a source it could have its own settings
changed by a track source.
To illustrate how changes to a given source impact various sinks,
consider the following example. This example only uses width and height,
but the same principles apply to all of the Settings exposed in this
specification. In the first figure a home client has obtained a video
source from its local video camera. The source's width and height settings
are 800 pixels and 600 pixels, respectively. Three
MediaStream objects on the home client contain tracks
that use this same <deviceId. The three media streams
are connected to three different sinks: a <video>
element (A), another <video> element (B), and a peer
connection (C). The peer connection is streaming the source video to a
remote client. On the remote client there are two media streams with tracks
that use the peer connection as a source. These two media streams are
connected to two <video> element sinks (Y and
Z).
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 MediaStreams
are connected to two different <video> element sinks A
and B. Sink A has been sized to width="1920" and
height="1200" and is displaying the source's video content
without any transformations. Sink B has been sized smaller and, as a
result, is scaling the video down to fit its rectangle of 320 pixels across
by 200 pixels down.
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.
6. MediaStreams in Media Elements
A MediaStream may be assigned to media elements. A
MediaStream is not preloadable or seekable and represents a
simple, potentially infinite, linear
media timeline. The timeline
starts at 0 and increments linearly in real time as long as the
media element is
potentially playing. The timeline does not increment when
the playout of the MediaStream is paused.
User Agents that support this specification MUST support the
srcObject
attribute of the HTMLMediaElement interface defined in
[HTML], which includes support for playing MediaStream
objects.
The [HTML] document outlines how the HTMLMediaElement
works with a media provider object. The following applies when the
media provider object is a MediaStream:
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
The User Agent MUST always play the current data from the
MediaStream and MUST NOT buffer.
Once playback has ended, it won't resume if new
MediaStreamTracks 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:
Once playback has ended, it won't resume if new audio
MediaStreamTracks are added to the
MediaStream unless autoplay is
true or the element is restarted, e.g., by the web
application calling play().
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
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.
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.
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.
Some operations throw or fire OverconstrainedError. This is
an extension of DOMException that carries additional
information related to constraints failure.
The MediaStreamTrack object's source will no
longer provide any data, either because the user revoked the
permissions, or because the source device has been ejected, or
because the remote peer permanently stopped sending data.
The following events fire on MediaDevices
objects:
The set of media devices, available to the User Agent, has
changed. The current list devices can be retrieved with the
enumerateDevices()
method.
9. Enumerating Local Media Devices
This section describes an API that the script can use to query the User
Agent about connected media input and output devices (for example a web
camera or a headset).
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.
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.
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.
The event type of this event handler is devicechange.
Methods
enumerateDevices
Collects information about the User Agent's available media
input and output devices.
This method returns a promise. The promise will be
fulfilled with a sequence of
MediaDeviceInfo objects representing the
User Agent's available media input and output devices if
enumeration is successful.
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:
Probe the User Agent for available media devices, and let be deviceList
be the list of all discovered devices.
Let microphoneList and cameraList 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).
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).
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.
Once authorization has been granted to one
of the capture devices, 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.
9.2.1 Access control model
The algorithm described above means that the access to media device
information depends on whether or not the browsing context 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.
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
9.2.2 Device information exposure
To perform a device enumeration can proceed check,
run the following steps:
A User Agent MAY at any point set the device information exposure back to false,
for instance if the User Agent decides to revoke device access on a given browsing context.
9.2.4 Exposure decision algorithm for devices other than camera and microphone
The exposure decision algorithm for devices other than camera and microphone
takes a device, microphoneList and cameraList as input
and returns a boolean to decide whether exposing the device to the web page or not.
By default, it returns false.
Other specifications can define the algorithm for specific device types.
The identifier of the represented device. The device MUST be
uniquely identified by its identifier and its kind.
To to ensure stored identifiers are recognized, the identifier
MUST be the same in documents of the same origin in top-level browsing contexts.
In nested browsing contexts,
the decision of whether or not the identifier is the same across
documents, MUST follow the User Agent's partitioning rules for
storage (such as localStorage), if any,
to not interfere with mitigations for cross-site correlation.
If the identifier can uniquely
identify the user, then it MUST be un-guessable in documents from
other origins to prevent the identifier from being used to
correlate the same user across different origins. An identifier
can be reused across origins as long as it is not tied to the user
and can be guessed by other means, like the User-Agent string.
If any local devices have been attached to a live
MediaStreamTrack in a page from this origin, or stored permission to access local
devices has been granted to this origin, then this identifier
MUST be persisted, except as detailed below. Unique and stable
identifiers let the application save, identify the availability
of, and directly request specific sources, across multiple
visits.
However, as long as no local device has been attached to a
live MediaStreamTrack in a page from this origin, and no stored permission to access local
devices has been granted to this origin, then the User Agent MAY
clear this identifier once the last browsing session from this
origin has been closed. If the User Agent chooses not to clear
the identifier in this condition, then it MUST provide for the
user to visibly inspect and delete the identifier, like a
cookie.
Since deviceId may persist
across browsing sessions and to reduce its potential as a
fingerprinting mechanism, deviceId is to be treated
as other persistent storage mechanisms such as cookies
[COOKIES], in that User Agents MUST NOT persist device
identifiers for sites that are blocked from using cookies, and
User Agents MUST rotate per-origin device identifiers when other
persistent storage are cleared.
A label describing this device (for example "External USB
Webcam"). This label is intended to allow the end user to tell the
difference between devices. Applications can’t assume that the
label contains any specific information, such as the device type or model.
If the device has no associated label, then this
attribute MUST return the empty string.
The group identifier of the represented device. Two devices
have the same group identifier if they belong to the same
physical device. For example, the audio input and output devices
representing the speaker and microphone of the same headset
have the same groupId.
The group identifier MUST be uniquely generated for each
document.
Returns a MediaTrackCapabilities object
describing the primary audio or video track of a device's
MediaStream (according to its
kind value), in the absence of any user-supplied
constraints. These capabilities MUST be identical to those that
would have been obtained by calling
getCapabilities() on the first
MediaStreamTrack of this type in a
MediaStream returned by
getUserMedia({deviceId: id}) where id
is the value of the deviceId attribute of this
MediaDeviceInfo.
If no access has been granted to any local devices and this
InputDeviceInfo has been filtered with respect to
unique identifying information (see above description of
enumerateDevices() result), then this method returns
an empty dictionary.
10. Obtaining local multimedia content
This section extends Navigator and
MediaDevices with APIs to request permission to access
media input devices available to the User Agent.
Alternatively, a local MediaStream can be captured
from certain types of DOM elements, such as the video element
[mediacapture-fromelement]. This can be useful for automated testing.
10.1 Legacy Interface Extensions
Note
The definition of getUserMedia() in this section reflects two major
changes from the method definition that has existed here for many
months.
First, the official definition for the getUserMedia() method, and
the one which developers are encouraged to use, is now at MediaDevices. This decision
reflected consensus as long as the original API remained available here
under the Navigator object for backwards compatibility reasons, since
the working group acknowledges that early users of these APIs have been
encouraged to define getUserMedia as "var getUserMedia =
navigator.getUserMedia || navigator.webkitGetUserMedia ||
navigator.mozGetUserMedia;" in order for their code to be functional
both before and after official implementations of getUserMedia() in
popular User Agents. To ensure functional equivalence, the getUserMedia()
method here is defined in terms of the method under MediaDevices.
Second, the decision to change all other callback-based methods in
the specification to be based on Promises instead required that the
navigator.getUserMedia() definition reflect this in its use of
navigator.mediaDevices.getUserMedia(). Because navigator.getUserMedia()
is now the only callback-based method remaining in the specification,
there is ongoing discussion as to a) whether it still belongs in the
specification, and b) if it does, whether its syntax should remain
callback-based or change in some way to use Promises. Input on these
questions is encouraged, particularly from developers actively using
today's implementations of this functionality.
Note that the other methods that changed from a callback-based
syntax to a Promises-based syntax were not considered to have been
implemented widely enough in any form to have to consider legacy
usage.
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:
The definition of getUserMedia() in this section reflects two major
changes from the method definition that has existed under Navigator for
many months.
First, the official definition for the getUserMedia() method, and
the one which developers are encouraged to use, is now the one defined
here under MediaDevices. This decision reflected consensus as long as
the original API remained available at Navigator.getUserMedia
under the Navigator object for backwards compatibility reasons, since
the working group acknowledges that early users of these APIs have been
encouraged to define getUserMedia as "var getUserMedia =
navigator.getUserMedia || navigator.webkitGetUserMedia ||
navigator.mozGetUserMedia;" in order for their code to be functional
both before and after official implementations of getUserMedia() in
popular User Agents. To ensure functional equivalence, the getUserMedia()
method under Navigator is defined in terms of the method here.
Second, the method defined here is Promises-based, while the one
defined under Navigator is currently still callback-based. Developers
expecting to find getUserMedia() defined under Navigator are strongly
encouraged to read the detailed Note given there.
The getSupportedConstraints method is provided to allow
the application to determine which constraints the User Agent
recognizes.
Returns a dictionary whose members are the constrainable
properties known to the User Agent. A supported constrainable
property MUST be represented and any constrainable properties not
supported by the User Agent MUST NOT be present in the returned
dictionary. The values returned represent what the User Agent
implements and will not change during a browsing session.
getUserMedia
Prompts the user for permission to use their Web cam or other
video or audio input.
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.
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,
rejectp with a new
DOMException object whose
name attribute has the value
"NotFoundError" and abort these steps.
If the value of the kind entry of
constraints is true, set CS to
the empty constraint set (no constraint). Otherwise,
continue with CS set to the value of the
kind entry of constraints.
Remove any constrainable property inside of
CS that are not defined for
MediaStreamTrack objects of type
kind. This means that audio-only constraints
inside of "video" and video-only constraints inside of
"audio" are simply ignored rather than causing
OverconstrainedError.
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.
For each media type kind in
requestedMediaTypes, run the following sub steps,
preferably at the same time:
Note
User Agents are encouraged to bundle concurrent
requests for different kinds of media into a single
user-facing permission prompt.
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-permissionMediaStreamTrack 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
MediaStreamTrackMUST 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,
rejectp 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,
rejectp 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.
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 rejectp with a new OverconstrainedError
created by calling
OverconstrainedError(constraint,
message).
Return p.
Note
In the algorithm above, constraints are checked twice - once at
device selection, and once after access approval. Time may have passed
between those checks, so it is conceivable that the selected device is
no longer suitable. In this case, a NotReadableError will result.
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
MediaStreamMUST NOT contain a video Track.
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 MediaStreamMUST NOT contain an
audio Track.
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.
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.
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 MUSTreject 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 MUSTresolve the returned
promise.
Note
Any constraint provided via this API will only be considered if the given
constrainable property is supported by the User Agent. JavaScript
application code is expected to first check, via
getSupportedConstraints(), that all the named properties
that are used are supported by the User Agent. The reason for this is that
WebIDL drops any unsupported names from the dictionary holding the
constraints, so the User Agent does not see them and the unsupported names
end up being silently ignored. This will cause confusing programming
errors as the JavaScript code will be setting constraints but the User Agent
will be ignoring them. User Agents that support (recognize) the name of a
required constraint but cannot satisfy it will generate an error, while
User Agents that do not support the constrainable property will not generate
an error.
The following examples may help to understand how constraints work. The
first shows a basic Constraint structure. Three constraints are given, each
of which the User Agent will attempt to satisfy individually. Depending
upon the resolutions available for this camera, it is possible that not all
three constraints can be satisfied at the same time. If so, the User Agent
will satisfy two if it can, or only one if not even two constraints can be
satisfied together. Note that if not all three can be satisfied
simultaneously, it is possible that there is more than one combination of
two constraints that could be satisfied. If so, the User Agent will
choose.
const supports = navigator.mediaDevices.getSupportedConstraints();
if (!supports.aspectRatio) {
// Treat like an error.
}
const constraints = {
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.
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."
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. For
example, an application might specify three advanced ConstraintSets, the
first asking for a frame rate greater than 500, the second asking for a
frame rate greater than 400, and the third asking for one greater than 300.
If the User Agent is capable of setting a frame rate greater than 500, it
will (and the subsequent two ConstraintSets will be trivially satisfied).
However, if the User Agent cannot set the frame rate above 500, it will
skip that ConstraintSet and attempt to set the frame rate above 400. If
that fails, it will then try to set it above 300. If the User Agent cannot
satisfy any of the three ConstraintSets, it will set the frame rate to any
value it can get. If the developers wanted to insist on 300 as a lower
bound, they could provide that as a 'min' value in the basic ConstraintSet.
In that case, the User Agent would fail altogether if it couldn't get a
value over 300, but would choose a value over 500 if possible, then try for
a value over 400.
Note that, unlike basic constraints, the constraints within a
ConstraintSet in the advanced list must be satisfied together or skipped
together. Thus, {width: 1920, height: 1280} is a request for that specific
resolution, not a request for that width or that height. One can think of
the basic constraints as requesting an 'or' (non-exclusive) of the
individual constraints, while each advanced ConstraintSet is requesting an
'and' of the individual constraints in the ConstraintSet. An application
may inspect the full set of Constraints currently in effect via the
getConstraints() accessor.
The specific value that the User Agent chooses for a constrainable
property is referred to as a Setting. For example, if the application
applies a ConstraintSet specifying that the frameRate must be at least 30
frames per second, and no greater than 40, the Setting can be any
intermediate value, e.g., 32, 35, or 37 frames per second. The application
can query the current settings of the object's constrainable properties via
the getSettings()
accessor.
11.1 Interface Definition
Although this specification formally defines
ConstrainablePattern as a WebIDL interface, it is actually a
template or pattern for other interfaces and cannot be inherited directly
since the return values of the methods need to be extended, something
WebIDL cannot do. Thus, each interface that wishes to make use of the
functionality defined here will have to provide its own copy of the
WebIDL for the functions and interfaces given here. However it can refer
to the semantics defined here, which will not change. See MediaStreamTrack Interface
Definition for an example of this.
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.
The getCapabilities()
method returns the dictionary of the names of the constrainable
properties that the object supports. When invoked, the User Agent
MUST return the value of the [[Capabilities]] internal
slot.
Note
It is possible that the underlying hardware may not exactly
map to the range defined for the constrainable property. Where
this is possible, the entry SHOULD define how to translate and
scale the hardware's setting onto the values defined for the
property. For example, suppose that a hypothetical
fluxCapacitance property ranges from -10 (min) to 10 (max), but
there are common hardware devices that support only values of
"off" "medium" and "full". The constrainable property
definition might specify that for such hardware, the User Agent
should map the range value of -10 to "off", 10 to "full", and 0
to "medium". It might also indicate that given a ConstraintSet
imposing a strict value of 3, the User Agent should attempt to
set the value of "medium" on the hardware, and that
getSettings() should return a
fluxCapacitance of 0, since that is the value defined as
corresponding to "medium".
The getConstraints() method returns the Constraints
that were the argument to the most recent successful invocation
of the ApplyConstraints algorithm on the object,
maintaining the order in which they were specified. Note that
some of the advanced ConstraintSets returned may not be currently
satisfied. To check which ConstraintSets are currently in effect,
the application should use getSettings. Instead of
returning the exact constraints as described above, the UA MAY
return a constraint set that has the identical effect in all
situations as the applied constraints. When invoked, the User Agent
MUST return the value of the [[Constraints]] internal slot.
getSettings
The getSettings() method returns the current
settings of all the constrainable properties of the object,
whether they are platform defaults or have been set by the
ApplyConstraints algorithm. Note that a setting is a
target value that complies with constraints, and therefore may
differ from measured performance at times. When invoked, the User
Agent MUST return the value of the [[Settings]]
internal slot.
applyConstraints
When the applyConstraints template method is invoked, the
User Agent MUST run the following steps:
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, rejectp 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.
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)
present 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
value for the constraint does not satisfy the constraint, the
fitness distance is positive infinity.
If the constraint is
not required, and does not apply for
this type of device, the fitness distance is 0 (that is, the
constraint does not influence the fitness distance).
If no ideal value is specified (constraintValue
either contains no member named 'ideal', or, if bare values are
to be treated as 'ideal', isn't a bare value), the fitness
distance is 0.
For all positive numeric constraints (such as
height, width, frameRate, aspectRatio, sampleRate and
sampleSize), the fitness distance is the result of the formula
For all string, enum and boolean constraints (e.g.
deviceId, groupId, facingMode, resizeMode, echoCancellation),
the fitness distance is the result of the formula
(actual == ideal) ? 0 : 1
More definitions:
We refer to each element of a ConstraintSet (other than the
special term 'advanced') as a 'constraint' since it is intended
to constrain the acceptable settings for the given property
from the full list or range given in the corresponding
Capability of the ConstrainablePattern object to a value that
is within the range or list of values it specifies.
We refer to the "effective Capability" C of an object O as
the possibly proper subset of the possible values of C (as
returned by getCapabilities) taking into consideration
environmental limitations and/or restrictions placed by other
constraints. For example given a ConstraintSet that constrains
the aspectRatio, height, and width properties, the values
assigned to any two of the properties limit the effective
Capability of the third. The set of effective Capabilities may
be platform dependent. For example, on a resource-limited
device it may not be possible to set properties P1 and P2 both
to 'high', while on another less limited device, this may be
possible.
A settings dictionary, which is a set of values for the
constrainable properties of an object O, satisfies
ConstraintSet CS if the fitness distance between the set
and CS is less than infinity.
A set of ConstraintSets CS1...CSn (n >= 1) can be
satisfied by an object O if it is possible to find a settings
dictionary of O that satisfies CS1...CSn simultaneously.
To apply a set of ConstraintSets CS1...CSn to object O is
to choose such a sequence of values that satisfy CS1...CSn and
assign them as the settings for the properties of O.
We define the SelectSettings algorithm as
follows:
Each constraint specifies one
or more values (or a range of values) for its property. A
property MAY appear more than once in the list of 'advanced'
ConstraintSets. If an empty list has been given as the value
for a constraint, it MUST be interpreted as if the constraint
were not specified (in other words, an empty constraint == no
constraint).
Note that unknown properties are discarded by WebIDL,
which means that unknown/unsupported required constraints
will silently disappear. To avoid this being a surprise,
application authors are expected to first use the
getSupportedConstraints() method as shown in the
Examples below.
Let object be the
ConstrainablePattern object on which this
algorithm is applied. Let copy be an unconstrained
copy of object (i.e., copy should behave
as if it were object with all ConstraintSets
removed.)
If candidates is empty, return
undefined as the result of the
SelectSettings algorithm.
Iterate over the 'advanced' ConstraintSets in
newConstraints in the order in which they were
specified. For each ConstraintSet:
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 UA SHOULD use the one with the smallest
fitness distance, as calculated in step 3, but
MAY prefer ones with resizeMode set to
"none" over "crop-and-scale".
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.
In a single operation, remove the existing constraints from
object, apply newConstraints, and apply
successfulSettings as the current settings.
Return undefined.
Note
Any implementation that has the same result as the algorithm
above is an allowed implementation. For instance, the
implementation may choose to keep track of the maximum and
minimum values for a setting that are OK under the constraints
considered, rather than keeping track of all possible values
for the setting.
Note
When picking a settings dictionary, the UA can use any
information available to it. Examples of such information may
be whether the selection is done as part of device selection in
getUserMedia, whether the energy usage of the camera varies
between the settings dictionaries, or whether using a settings
dictionary will cause the device driver to apply
resampling.
The User Agent MAY choose new settings for the constrainable
properties of the object at any time. When it does so it MUST
attempt to satisfy all current Constraints, in the manner
described in the algorithm above, let
successfulSettings be the resulting new settings, and
queue a task to run the following steps:
Let object be the
ConstrainablePattern object on which new
settings for one or more constrainable properties have changed.
Set object's [[Settings]] internal
slot to successfulSettings.
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.
Throughout this specification, the identifier
ConstrainDOMString is used to refer to the (DOMString or sequence<DOMString> or
ConstrainDOMStringParameters) type.
11.3 Capabilities
Capabilities is a dictionary containing one or more
key-value pairs, where each key MUST be a constrainable property, and
each value MUST be a subset of the set of values allowed for that
property. The exact syntax of the value expression depends on the type of
the property. The Capabilities dictionary specifies which constrainable
properties that can be applied, as constraints, to the constrainable
object. Note that the Capabilities of a constrainable objectMAY 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.
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:
Note in the example above that the aspectRatio would make clear that
arbitrary combination of widths and heights are not possible, although it
would still suggest that more than two resolutions were available.
A
specification using the Constrainable Pattern should not subclass the
below dictionary, but instead provide its own definition. See
MediaTrackCapabilities for an example.
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.
A specification using the Constrainable Pattern should not subclass
the below dictionary, but instead provide its own definition. See
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.
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.
This is the list of ConstraintSets that the User Agent MUST
attempt to satisfy, in order, skipping only those that cannot be
satisfied. The order of these ConstraintSets is significant. In
particular, when they are passed as an argument to
applyConstraints, the User Agent MUST try to satisfy
them in the order that is specified. Thus if advanced
ConstraintSets C1 and C2 can be satisfied individually, but not
together, then whichever of C1 and C2 is first in this list will
be satisfied, and the other will not. The User Agent MUST attempt
to satisfy all ConstraintSets in the list, even if some cannot be
satisfied. Thus, in the preceding example, if constraint C3 is
specified after C1 and C2, the User Agent will attempt to satisfy
C3 even though C2 cannot be satisfied. Note that a given property
name may occur only once in each ConstraintSet but may occur in
more than one ConstraintSet.
12. Examples
This sample code exposes a button. When clicked, the button is
disabled and the user is prompted to offer a stream. The user can cause
the button to be re-enabled by providing a stream (e.g., giving the page
access to the local camera) and then disabling the stream (e.g., revoking
that access).
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.
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 usegetUserMedia to request the camera or microphone
respectively. This is enforced by the request permission to use
algorithm.
Define any<kind>Accessible (e.g.
anyAudioAccessible, anyVideoAccessible) as the
logical OR of the [[kindsAccessibleMap]][kind] value and all
the [[devicesAccessibleMap]][deviceId] values for devices of
that kind.
Define any<kind>Live (e.g. anyAudioLive,
anyVideoLive) to be the logical OR of the
[[kindsAccessibleMap]][kind] value and all the
[[devicesLiveMap]][deviceId] values for devices of that
kind.
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:
The User Agent MUST indicate to the user when the value of
anyAccessible changes.
The User Agent MUST indicate to the user when the value of
anyLive changes.
If the User Agent provides indication to the user per
kind, then for each any<kind>Accessible value
and any<kind>Live value, it MUST at minimum indicate
when the value changes.
If the User Agent provides indication to the user per
device, then for each
[[devicesAccessibleMap]][deviceId] value and
[[devicesLiveMap]][deviceId] value, it MUST at minimum
indicate when the value changes.
Any false-to-true transition indicated MUST remain observable for a
sufficient time that a reasonably-observant user could become aware of
it. This SHOULD be at least 3 seconds.
Any of the above transition indications MAY be combined as long as
the combined indication cannot transition to false if any of its
component indications are still true.
and the following are encouraged behaviors for the User Agent:
The User Agent is encouraged to provide ongoing indication of the
current state of anyAccessible.
The User Agent is encouraged to provide ongoing indication of the
current state of anyLive and to make any generic hardware
device indicator light match.
If the User Agent provides indication to the user per
kind, then for each any<kind>Accessible value
and any<kind>Live value, it is encouraged to provide
ongoing indication of the current state of the value. It is also
encouraged to make any device-type-specific hardware indicator light
match the corresponding any<kind>Live value.
If the User Agent provides indication to the user per
device, then for each
[[devicesAccessibleMap]][deviceId] value and
[[devicesLiveMap]][deviceId] value, it is encouraged to
provide ongoing indication of the current state of the value. It is also
encouraged to make any device-specific hardware indicator light match the
corresponding [[devicesLiveMap]][deviceId] value.
Any of the above ongoing indications MAY be used instead of the
corresponding required transition indication provided the false-to-true
transition requirement is met.
15. Privacy and Security Considerations
This section is non-normative; it specifies no new behavior, but instead
summarizes information already present in other parts of the
specification.
This document extends the Web platform with the ability to manage input
devices for media - in this iteration, microphones, and cameras. It also
allows the manipulation of audio output devices (speakers and headphones).
Capturing audio and video exposes personally-identifiable information to
applications, and this specification requires obtaining explicit user
consent before sharing it.
Without authorization (to the "drive-by web"), it offers the ability to
tell how many devices there are of each class, and how they are grouped
together (e.g. a microphone and camera belonging to a single Web cam). The
identifiers for the devices are designed to not be useful for a fingerprint
that can track the user between origins, but the number and grouping of
devices adds to the fingerprint surface. It recommends to treat the
per-origin persistent identifier deviceId as other persistent
storage (e.g. cookies) are treated.
When authorization is given, this document describes how to get access
to, and use, media data from the devices mentioned. This data may be
sensitive; advice is given that indicators should be supplied to indicate
that devices are in use, but both the nature of authorization and the
indicators of in-use devices are platform decisions.
Authorization may be given on a case-by-case basis, or be persistent. In
the case of a case-by-case authorization, it is important that the user be
able to say "no" in a way that prevents the UI from blocking user
interaction until permission is given - either by offering a way to say a
"persistent NO" or by not using a modal permissions dialog.
When authorization to any media device is given, application developers
gain access to the labels of all available media capture devices. In most
cases, the labels are persistent across browsing sessions and across
origins that have also been granted authorization, and thus potentially
provide a way to track a given device across time and origins.
For origins to which permission has been granted, the
devicechange event will be emitted across browsing
contexts and origins each time a new media device is added or removed; user
agents can mitigate the risk of correlation of browsing activity across
origins by fuzzing the timing of these events.
Once a developer gains access to a media stream from a capture device,
the developer also gains access to detailed information about the device,
including its range of operating capabilities (e.g. available resolutions
for a camera). These operating capabilities are for the most part
persistent across browsing sessions and origins, and thus provide a way to
track a given device across time and origins.
Once access to a video stream from a capture device is obtained, that
stream can most likely be used to fingerprint uniquely the said device
(e.g. via dead pixel detection). Similarly, once access to an audio stream
is obtained, that stream can most likely be used to fingerprint user
location down to the level of a room or even simultaneous occupation of a
room by disparate users (e.g. via analysis of ambient audio or of unique
audio purposely played out of the device speaker). User-level mitigation
for both audio and video consists of covering up the camera and/or
microphone or revoking permission via User Agent chrome controls.
It is possible to use constraints so that the failure of a getUserMedia
call will return information about devices on the system without prompting
the user, which increases the surface available for fingerprinting. The
User Agent should consider limiting the rate at which failed getUserMedia
calls are allowed in order to limit this additional surface.
In the case of persistent authorization via a stored permission, it is
important that it is easy to find the list of granted permissions and
revoke permissions that the user wishes to revoke.
Once permission has been granted, the User Agent should make two things
readily apparent to the user:
That the page has access to the devices for which permission is
given
Whether or not any of the devices are presently recording ("on air")
indicator
Note
Developers of sites with stored permissions should be careful that
these permissions not be abused. These permissions can be revoked using
the [permissions] API.
In particular, they should not make it possible to automatically send
audio or video streams from authorized media devices to an end point that
a third party can select.
Indeed, if a site offered URLs such as
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.
16. Extensibility
This section is non-normative.
Although new versions of this specification may be produced in the
future, it is also expected that other standards will need to define new
capabilities that build upon those in this specification. The purpose of
this section is to provide guidance to creators of such extensions.
Any WebIDL-defined interfaces, methods, or attributes in the
specification may be extended. Two likely extension points are defining a
new media type and defining a new constrainable property.
16.1 Defining a new media type (beyond the existing Audio and Video
types)
At a minimum, defining a new media type would require
adding a new getXXXXTracks() method for the type to the
MediaStream interface,
if the new type requires user authorization, defining new
permissions for it, including a new PermissionDescriptor name
associated with the new 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).
Creators of extension specifications are strongly encouraged to notify
the specification maintainers on the specification repository.
Future versions of this specification and others created by the WebRTC Working Group will take into consideration all extensions they are
aware of in an attempt to reduce potential usage conflicts.
It is also likely that new consumers of MediaStreams
or MediaStreamTracks will be defined in the future. The
following section provides guidance.
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.
