Copyright © 2015-2017 W3C® (MIT, ERCIM, Keio, Beihang). W3C liability, trademark and document use rules apply.
This document defines how a stream of media can be captured from a DOM element, such as a
<video>
, <audio>
, or <canvas>
element, in the form of a MediaStream
[GETUSERMEDIA].
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.
This document was published by the Device and Sensors Working Group and the Web Real-Time Communications Working Group as a Working Draft. This document is intended to become a W3C Recommendation. If you wish to make comments regarding this document, please send them to public-media-capture@w3.org (subscribe, archives). All comments are welcome.
Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.
This document was produced by groups operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures (Device and Sensors Working Group) and a public list of any patent disclosures (Web Real-Time Communications Working Group) made in connection with the deliverables of each group; these pages also include instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.
This document is governed by the 1 September 2015 W3C Process Document.
This section is non-normative.
This document describes an extension to both HTML media elements and the HTML canvas element that enables the capture of the output of the element in the form of streaming media.
The captured media is formed into a MediaStream
[GETUSERMEDIA], which can then be consumed by the various APIs that process streams of media, such as WebRTC [
WEBRTC], or Web Audio [WEBAUDIO].
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 MUST and MUST NOT are to be interpreted as described in [RFC2119].
This specification defines conformance criteria that apply to a single product: the user agent that implements the interfaces that it contains.
Implementations that use ECMAScript to implement the APIs defined in this specification must implement them in a manner consistent with the ECMAScript Bindings defined in the Web IDL specification [WEBIDL], as this specification uses that specification and terminology.
Method
is defined on HTML [HTML5] media elements. Methods for capture are added to both captureStream
()HTMLMediaElement
and HTMLCanvasElement
.
Both MediaStream
and
expose the concept of a HTMLMediaElement
track
. Since there is no common type used for
, this document uses the term track to refer to either HTMLMediaElement
VideoTrack
or AudioTrack
.
MediaStreamTrack
is used to identify the media in a MediaStream
.
partial interface HTMLMediaElement
{
MediaStream
captureStream
();
};
captureStream
The captureStream()
method produces a real-time capture of the media that is rendered to the media element.
The captured MediaStream
comprises of MediaStreamTrack
s that render the content from the set of
selected (for
s, or other exclusively selected
track types) or enabled (for VideoTrack
s, or other track types that support multiple selections) tracks from the media element. If the media element does not have a selected or enabled tracks of a given type, then no
AudioTrack
MediaStreamTrack
of that type is present in the captured stream.
A <video>
element can therefore capture a video
MediaStreamTrack
and any number of audio
MediaStreamTrack
s. An <audio>
element can capture any number of audio MediaStreamTrack
s. In both cases, the set of captured MediaStreamTrack
s could be empty.
Unless and until there is a track of given type that is selected or enabled, no MediaStreamTrack
of that type is present in the captured stream. In particular, if the media element does not have a source assigned, then the captured
MediaStream
has no tracks. Consequently, a media element with a ready state of HAVE_NOTHING produces no captured MediaStreamTrack
instances. Once metadata is available and the selected or enabled tracks are determined, new captured
MediaStreamTrack
instances are created and added to the
MediaStream
.
A captured MediaStreamTrack
ends when playback
ends (and the ended
event fires) or when the track that it captures is no longer selected or enabled for playback. A track is no longer selected or enabled if the source is changed by setting the src
or
srcObject
attributes of the media element. The steps in MediaStreamTrack.stop()
are performed on the MediaStreamTrack
when it ends.
The set of captured MediaStreamTrack
s change if the source of the media element changes. If the source for the media element ends, a different source is selected.
If the selected VideoTrack
or enabled AudioTrack
s for the media element change, a addtrack
event with a new MediaStreamTrack
is generated for each track that was not previously selected or enabled; and a removetrack
events is generated for each track that ceases to be selected or enabled. A
MediaStreamTrack
MUST end prior to being removed from the
MediaStream
.
Since a MediaStreamTrack
can only end once, a track that is enabled, disabled and re-enabled will be captured as two separate tracks. Similarly, restarting playback after playback ends causes a new set of captured
MediaStreamTrack
instances to be created. Seeking during playback without changing track selection does not generate events or cause a captured
MediaStreamTrack
to end.
The MediaStreamTrack
s that comprise the captured
MediaStream
become muted or unmuted as the tracks they capture change state. At any time, a media element might not have active content available for capture on a given track for a variety of reasons:
MediaStreamTrack
that is acting as a source could be
muted or disabled.
Absence of content is reflected in captured tracks through the muted
attribute. A captured MediaStreamTrack
MUST have a
muted
attribute set to true
if its corresponding source track does not have available and accessible content. A
mute
event is raised on the MediaStreamTrack
when content availability changes.
What output a muted capture produces as a result will vary based on the type of media: a
ceases to capture new frames when muted, causing the captured stream to show the last captured frame; a muted
VideoTrack
produces silence.
AudioTrack
Whether a media element is actively rendering content (e.g., to a screen or audio device) has no effect on the content of captured streams. Muting the audio on a media element does not cause the capture to produce silence, nor does hiding a media element cause captured video to stop. Similarly, the audio level or volume of the media element does not affect the volume of captured audio.
Captured audio from an element with an effective playback rate other than 1.0 MUST be time-stretched. An unplayable playback rate causes the captured audio track to become muted.
The
method is added to the HTML [HTML5] canvas element. The resulting captureStream()
provides methods that allow for controlling when frames are sampled from the canvas.
CanvasCaptureMediaStreamTrack
partial interface HTMLCanvasElement
{
MediaStream
captureStream
(optional double frameRequestRate);
};
captureStream
The captureStream()
method produces a real-time video capture of the surface of the canvas. The resulting media stream has a single video
that matches the dimensions of the canvas element.
CanvasCaptureMediaStreamTrack
Content from a canvas that is not origin-clean MUST NOT be captured. This method throws a SecurityError exception if the canvas is not origin-clean.
A captured stream MUST immediately cease to capture content if the
origin-clean flag of the source canvas becomes false after the stream is created by captureStream()
. The captured MediaStreamTrack
MUST become muted, producing no new content while the canvas remains in this state.
Each track that captures a canvas has an internal
frameCaptureRequested
property that is set to true when a new frame is requested from the canvas.
The value of the
property on all new tracks is set to frameCaptureRequested
true
when the track is created. On creation of the captured track with a specific, non-zero frameRequestRate
, the user
agent starts a periodic timer at an interval of 1/frameRequestRate
seconds. At each activation of the timer, the
property is set to frameCaptureRequested
true
.
In order to support manual control of frame capture with the
requestFrame()
method, browsers MUST support a value of 0 for
frameRequestRate
. However, a captured stream MUST request capture of a frame when created, even if frameRequestRate
is zero.
This method throws a
NotSupportedError if frameRequestRate
is negative.
A new frame is requested from the canvas when
is true and the canvas is painted. Each time that the captured canvas is painted, the following steps are executed:
frameCaptureRequested
frameCaptureRequested
internal property of
track is set, add a new frame to track containing what was painted to the canvas.
frameRequestRate
value was specified, set the
frameCaptureRequested
internal property of track to false
.
When adding new frames to track containing what was painted to the canvas, the alpha channel content of the canvas must be captured and preserved if the canvas is not fully opaque. The consumers of this track
might not preserve the alpha channel.
This algorithm results in a captured track not starting until something changes in the canvas.
Parameter | Type | Nullable | Optional | Description |
---|---|---|---|---|
frameRequestRate |
double
|
✘ | ✔ |
MediaStream
CanvasCaptureMediaStreamTrack
The CanvasCaptureMediaStreamTrack
is an extension of
MediaStreamTrack
that provide a single requestFrame()
method. Applications that depend on tight control over the rendering of content to the media stream can use this method to control when frames from the canvas are captured.
interface CanvasCaptureMediaStreamTrack
: MediaStreamTrack
{
readonly attribute HTMLCanvasElement
canvas
;
void requestFrame
();
};
canvas
of type HTMLCanvasElement
, readonly
requestFrame
The requestFrame()
method allows applications to manually request that a frame from the canvas be captured and rendered into the track. In cases where applications progressively render to a canvas, this allows applications to avoid capturing a partially rendered frame.
As currently specified, this results in no SecurityError
or other error feedback if the canvas is not origin-clean. In part, this is because we don't track where requests for frames come from. Do we want to highlight that?
void
Media elements can render media resources from origins that differ from the origin of the media element. In those cases, the contents of the resulting MediaStreamTrack
MUST be protected from access by the document origin.
How this protection manifests will differ, depending on how the content is accessed. For instance, rendering inaccessible video to a canvas
element [2DCONTEXT] causes the origin-clean property of the canvas to become false
; attempting to create a Web Audio
MediaStreamAudioSourceNode
[WEBAUDIO] succeeds, but produces no information to the document origin (that is, only silence is transmitted into the audio context); attempting to transfer the media using WebRTC [WEBRTC] results in no information being transmitted.
The origin of the media that is rendered by a media element can change at any time. This is even the case for a single media resource. User agents MUST ensure that a change in the origin of media doesn't result in exposure of cross origin content.
This section will be removed before publication.
This document is based on the stream processing specification [streamproc] originally developed by Robert O'Callahan.