forked from w3c/mediacapture-main
-
Notifications
You must be signed in to change notification settings - Fork 0
/
getusermedia.html
4967 lines (4050 loc) · 193 KB
/
getusermedia.html
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html>
<!--
To publish this document, see instructions in README
-->
<html lang="en-us" xml:lang="en-us" xmlns="http://www.w3.org/1999/xhtml"
xmlns:ns="http://www.w3.org/1999/xhtml">
<head>
<link href="getusermedia.css" rel="stylesheet" type="text/css" />
<title>Media Capture and Streams</title>
<meta content="text/html; charset=utf-8" http-equiv="Content-Type" />
<script class="remove"
src="http://www.w3.org/Tools/respec/respec-w3c-common"
type="text/javascript"> // <!-- keep this comment --> // </script>
<script class="remove" src="getusermedia.js" type="text/javascript"> //
<!-- keep this comment --> // </script>
</head>
<body>
<section id="abstract">
<p>This document defines a set of JavaScript APIs that allow local
media, including audio and video, to be requested from a platform.</p>
</section>
<section id="sotd">
<p>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.</p>
</section>
<section class="informative" id="intro">
<h2>Introduction</h2>
<p>Access to multimedia streams (video, audio, or both) from local
devices (video cameras, microphones, Web cams) can have a number of
uses, such as real-time communication, recording, and surveillance.</p>
<p>This document defines the APIs used to get access to local devices
that can generate multimedia stream data. This document also defines the
MediaStream API by which JavaScript is able to manipulate the stream
data or otherwise process it.</p>
</section>
<section id="conformance">
<p>This specification defines conformance criteria that apply to a
single product: the <dfn>user agent</dfn> that implements the interfaces
that it contains.</p>
<p>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.)</p>
<p>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.</p>
</section>
<section>
<h2>Terminology</h2>
<dl>
<dt>
<i>HTML Terms:</i>
</dt>
<dd>
<p>The <code>
<a
href="http://dev.w3.org/html5/spec/webappapis.html#eventhandler">EventHandler</a>
</code> interface represents a callback used for event handlers as
defined in [[!HTML5]].</p>
<p>The concepts <dfn>
<a
href="http://dev.w3.org/html5/spec/webappapis.html#queue-a-task">queue
a task</a>
</dfn> and <dfn>
<a
href="http://dev.w3.org/html5/spec/webappapis.html#fire-a-simple-event">fires
a simple event</a>
</dfn> are defined in [[!HTML5]].</p>
<p>The terms <dfn>
<a
href="http://dev.w3.org/html5/spec/webappapis.html#event-handlers">event
handlers</a>
</dfn> and <dfn>
<a
href="http://dev.w3.org/html5/spec/webappapis.html#event-handler-event-type">
event handler event types</a>
</dfn> are defined in [[!HTML5]].</p>
</dd>
<dt>
<dfn>source</dfn>
</dt>
<dd>
<p>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.</p>
<p>Other than the source identifier (defined in
<code><a>MediaDeviceInfo</a>.deviceId</code>),
other bits of source identity are
<strong>never</strong> directly available to the application until
the user agent connects a source to a track. Once a source has been
"released" to the application (either via a permissions UI,
pre-configured allow-list, or some other release mechanism) the
application will be able discover additional source-specific
capabilities.</p>
<p>Sources <strong>do not</strong> have constraints -- tracks have
constraints. When a source is connected to a track, it must,
possibly in combination with UA processing (e.g., downsampling),
conform to the constraints present on that track (or set of
tracks).</p>
<p>Sources are released (un-attached) from a track when the
track is ended for any reason.</p>
<p>On the <code>
<a>MediaStreamTrack</a>
</code> object, sources are represented by a <code>
<a>sourceType</a>
</code> attribute. The behavior of APIs associated with the
source's capabilities and settings change depending on the source
type.</p>
<p>Sources have <code>
<a>capabilities</a>
</code> and <code>
<a>settings</a>
</code>. The capabilities and settings are "owned" by the source
and are common to any (multiple) tracks that happen to be using the
same source (e.g., if two different track objects bound to the same
source ask for the same capability or setting information, they will
get back the same answer).</p>
</dd>
<dt> <a>Setting</a> (Source Setting) </dt>
<dd>
<p>A setting refers to the immediate, current value of the source's
(optionally constrained) capabilities. Settings are always
read-only.</p>
<p>A source's settings can change dynamically over time due to
environmental conditions, sink configurations, or constraint
changes. A source's settings must always conform to the current set
of mandatory constraints that all of the tracks it is bound to have
defined, and should do its best to conform to the set of optional
constraints specified.</p>
<p>Although settings are a property of the source, they are only
exposed to the application through the tracks attached to the
source. The <a>ConstrainablePattern</a> interface provides this
exposure.</p>
</dd>
<dt>
<a>Capabilities</a>
</dt>
<dd>
<p>Source capabilities are the intrinsic "features" of a source
object. For each source setting, there is a corresponding capability
that describes whether it is supported by the source and if so, what
the range of supported values are. As with settings, capabilities
are exposed to the application via the <a>ConstrainablePattern</a>
interface.</p>
<p>The values of the supported capabilities must be normalized to
the ranges and enumerated types defined in this specification.</p>
<p>A <a>getCapabilities()</a> call on a track returns the same
underlying per-source capabilities for all tracks connected to the
source.</p>
<p>Source capabilities are effectively constant. Applications should
be able to depend on a specific source having the same capabilities
for any session.</p>
</dd>
<dt>
<dfn>Constraints</dfn>
</dt>
<dd>
<p>Constraints are an optional track feature for restricting the
range of allowed variability on a source. Without provided track
constraints, implementations are free to select a source's settings
from the full ranges of its supported capabilities, and to adjust
those settings at any time for any reason.</p>
<p>Constraints are exposed on tracks via the
<a>ConstrainablePattern</a> interface, which includes an API for
dynamically changing constraints. Note that <a>getUserMedia()</a>
also permits an initial set of constraints to be applied when the
track is first obtained.</p>
<p>It is possible for two tracks that share a unique source to apply
contradictory constraints. The <a>ConstrainablePattern</a> interface
supports the calling of an error handler when the conflicting
constraint is requested.</p>
<p>A correspondingly-named constraint exists for each corresponding
source setting name and capability name. 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 mandatory constraints sparingly.</p>
</dd>
<dt>
<code>RTCPeerConnection</code>
</dt>
<dd><dfn>
<code>RTCPeerConnection</code>
</dfn> is defined in [[!WEBRTC10]].</dd>
</dl>
</section>
<section id="stream-api">
<h2>MediaStream API</h2>
<section>
<h2>Introduction</h2>
<p>The two main components in the MediaStream API are the <code>
<a>MediaStreamTrack</a>
</code> and the <code>
<a>MediaStream</a>
</code> interfaces. The <code>
<a>MediaStreamTrack</a>
</code> object represents media originating from a single media
source in the user agent, e.g. video from a web camera. A <code>
<a>MediaStream</a>
</code> is used group several <code>
<a>MediaStreamTrack</a>
</code> objects into one unit that can be rendered in a media
element or recorded.</p>
<p>Each <code>
<a>MediaStream</a>
</code> can contain zero or more <code>
<a>MediaStreamTrack</a>
</code> objects. All tracks in a <code>
<a>MediaStream</a>
</code> are intended to be synchronized when rendered. Different
<code>
<a>MediaStream</a>
</code> objects do not need to be synchronized.</p>
<p class="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 [[!WEBRTC10]], 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.</p>
<p>A <code>
<a>MediaStreamTrack</a>
</code> represents content comprising one or more channels, where
the channels have a defined well known relationship to each other
(such as a stereo or 5.1 audio signal). A channel is the smallest unit
considered in this API specification.</p>
<p>
<img alt="A MediaStream" src="images/media-stream.png" width="418" />
</p>
<p>A <code>
<a>MediaStream</a>
</code> object has an input and an output that represent the
combined input and output of all the object's tracks. The output of
the <code>
<a>MediaStream</a>
</code> 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 <code>video</code> element.</p>
<p>A new <code>
<a>MediaStream</a>
</code> object can be created from accessible media sources (that
does not require any additional permissions) using the <code>
<a href="#dom-mediastream">MediaStream()</a>
</code> constructor. The constructor argument can either be an
existing <code>
<a>MediaStream</a>
</code> object, in which case all the tracks of the given stream are
added to the new <code>
<a>MediaStream</a>
</code> object, or an array of <code>
<a>MediaStreamTrack</a>
</code> objects. The latter form makes it possible to compose a
stream from different source streams.</p>
<p>Both <code>
<a>MediaStream</a>
</code> and <code>
<a>MediaStreamTrack</a>
</code> objects can be cloned. This allows for greater control since
the separate instances can be manipulated and <a
title="consumer">consumed</a> individually. A cloned <code>
<a>MediaStream</a>
</code> contains clones of all member tracks from the original
stream.</p>
<p>When a <code>
<a>MediaStream</a>
</code> object is being generated from a local file (as opposed to a
live audio/video source), the user agent SHOULD stream the data from
the file in real time, not all at once. The <code>MediaStream</code>
object is also used in contexts outside <code>getUserMedia</code>,
such as [[!WEBRTC10]]. In both cases, ensuring a realtime stream
reduces the ease with which pages can distinguish live video from
pre-recorded video, which can help protect the user's privacy.</p>
</section>
<section>
<h2>MediaStream</h2>
<p>The <dfn id="dom-mediastream">
<code>MediaStream()</code>
</dfn> constructor composes a new stream out of existing tracks. It
takes an optional argument of type <code>
<a>MediaStream</a>
</code> or an array of <code>
<a>MediaStreamTrack</a>
</code> objects. <dfn id="mediastream-constructor">When the
constructor is invoked</dfn>, the UA must run the following steps:</p>
<ol>
<li>
<p>Let <var>stream</var> be a newly constructed <code>
<a>MediaStream</a>
</code> object.</p>
</li>
<li>
<p>Initialize <var>stream's</var> <code>
<a href="#dom-mediastream-id">id</a>
</code> attribute to a newly generated value.</p>
</li>
<li>
<p>If the constructor's argument is present, run the sub steps
that corresponds to the argument type.</p>
<ul>
<li>
<p><code>Array</code> of <code>
<a>MediaStreamTrack</a>
</code> objects:</p>
<p>Run the following sub steps for each <code>
<a>MediaStreamTrack</a>
</code> in the array:</p>
<ol>
<li>
<p><em>Add track</em>: Let <var>track</var> be the <code>
<a>MediaStreamTrack</a>
</code> about to be processed.</p>
</li>
<li>
<p>Add <var>track</var> to <var>stream</var>'s <a
href="#track-set">track set</a>.</p>
</li>
</ol>
</li>
<li>
<p><code>
<a>MediaStream</a>
</code>:</p>
<p>Run the sub steps labeled <em>Add track</em> (above) for
every <code>
<a>MediaStreamTrack</a>
</code> in the argument stream's <a href="#track-set">track
set</a>.</p>
</li>
</ul>
</li>
<li>
<p>If <var>stream</var>'s <a href="#track-set">track set</a> is
empty or only contains ended tracks, set <var>stream</var>'s <code>
<a href="#dom-mediastream-active">active</a>
</code> attribute to <code>false</code>, otherwise set it to
<code>true</code>.</p>
</li>
<li>
<p>Return <var>stream</var>.</p>
</li>
</ol>
<p>The tracks of a <code>
<a>MediaStream</a>
</code> are stored in a <dfn id="track-set">track set</dfn>. The
track set MUST contain the <code>
<a>MediaStreamTrack</a>
</code> 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 <code>
<a>MediaStreamTrack</a>
</code> object in the set is to look it up by its <code>
<a href="#dom-mediastreamtrack-id">id</a>
</code>.</p>
<p>An object that reads data from the output of a <code>
<a>MediaStream</a>
</code> is referred to as a <code>
<a>MediaStream</a>
</code> <dfn>consumer</dfn>. The list of <code>
<a>MediaStream</a>
</code> consumers currently include the media elements [[HTML5]],
<code>RTCPeerConnection</code> [[WEBRTC10]],
<code>MediaRecorder</code> [[mediastream-recording]] and
<code>ImageCapture</code> [[image-capture]].</p>
<p class="note"><code>
<a>MediaStream</a>
</code> consumers must be able to handle tracks being added and
removed. This behavior is specified per consumer.</p>
<p>A <code>
<a>MediaStream</a>
</code> object is said to be <dfn
id="stream-inactive">MediaStream.inactive</dfn> when it does not have
any tracks or all tracks belonging to the stream have <a
href="#track-ended">ended</a>. Otherwise the stream is active. A <code>
<a>MediaStream</a>
</code> can start its life as inactive if it is constructed without
any tracks.</p>
<p>When a <code>
<a>MediaStream</a>
</code> goes from being active to inactive, the user agent MUST
queue a task that sets the object's <code>
<a href="#dom-mediastream-active">active</a>
</code> attribute to <code>false</code> and fire a simple event
named <code>
<a href="#event-mediastream-inactive">inactive</a>
</code> at the object. When a <code>
<a>MediaStream</a>
</code> goes from being inactive to active, the user agent MUST
queue a task that sets the object's <code>
<a href="#dom-mediastream-active">active</a>
</code> attribute to <code>true</code> and fire a simple event named
<code>
<a href="#event-mediastream-active">active</a>
</code> at the object.</p>
<p>If the stream's activity status changed due to a user request, the
task source for this <span title="concept-task">task</span> is the
user interaction task source. Otherwise the task source for this <span
title="concept-task">task</span> is the networking task source.</p>
<dl class="idl" title="interface MediaStream : EventTarget">
<dt>Constructor()</dt>
<dd> See the <a href="#mediastream-constructor">MediaStream
constructor algorithm</a> </dd>
<dt>Constructor(MediaStream stream)</dt>
<dd> See the <a href="#mediastream-constructor">MediaStream
constructor algorithm</a> </dd>
<dt>Constructor(sequence<MediaStreamTrack> tracks)</dt>
<dd> See the <a href="#mediastream-constructor">MediaStream
constructor algorithm</a> </dd>
<dt>readonly attribute DOMString id</dt>
<dd>
<p>When a <code>
<a>MediaStream</a>
</code> object is created, the user agent MUST generate an
identifier string, and MUST initialize the object's <code>
<a href="#dom-mediastream-id">id</a>
</code> attribute to that string. A good practice is to use an
UUID, which is 36 characters long in its canonical form.</p>
<p>The <dfn id="dom-mediastream-id">
<code>id</code>
</dfn> attribute MUST return the value to which it was
initialized when the object was created.</p>
</dd>
<dt>sequence<MediaStreamTrack> getAudioTracks()</dt>
<dd>
<p>Returns a sequence of <code>
<a>MediaStreamTrack</a>
</code> objects representing the audio tracks in this
stream.</p>
<p>The <dfn id="dom-mediastream-getaudiotracks">
<code>getAudioTracks()</code>
</dfn> method MUST return a sequence that represents a snapshot
of all the <code>
<a>MediaStreamTrack</a>
</code> objects in this stream's <a href="#track-set">track
set</a> whose <code>
<a href="#dom-mediastreamtrack-kind">kind</a>
</code> is equal to "<code>audio</code>". The conversion from
the <a href="#track-set">track set</a> to the sequence is user
agent defined and the order does not have to stable between
calls.</p>
</dd>
<dt>sequence<MediaStreamTrack> getVideoTracks()</dt>
<dd>
<p>Returns a sequence of <code>
<a>MediaStreamTrack</a>
</code> objects representing the video tracks in this
stream.</p>
<p>The <dfn id="dom-mediastream-getvideotracks">
<code>getVideoTracks()</code>
</dfn> method MUST return a sequence that represents a snapshot
of all the <code>
<a>MediaStreamTrack</a>
</code> objects in this stream's <a href="#track-set">track
set</a> whose <code>
<a href="#dom-mediastreamtrack-kind">kind</a>
</code> is equal to "<code>video</code>". The conversion from
the <a href="#track-set">track set</a> to the sequence is user
agent defined and the order does not have to stable between
calls.</p>
</dd>
<dt>sequence<MediaStreamTrack> getTracks()</dt>
<dd>
<p>Returns a sequence of <code>
<a>MediaStreamTrack</a>
</code> objects representing all the tracks in this stream.</p>
<p>The <dfn id="dom-mediastream-gettracks">
<code>getTracks()</code>
</dfn> method MUST return a sequence that represents a snapshot
of all the <code>
<a>MediaStreamTrack</a>
</code> objects in this stream's <a href="#track-set">track
set</a>, regardless of kind. The conversion from the <a
href="#track-set">track set</a> to the sequence is user agent
defined and the order does not have to stable between calls.</p>
</dd>
<dt>MediaStreamTrack? getTrackById(DOMString trackId)</dt>
<dd>
<p>The <dfn id="dom-mediastream-gettrackbyid">
<code>getTrackById()</code>
</dfn> method MUST return the first <code>
<a>MediaStreamTrack</a>
</code> object in this stream's <a href="#track-set">track
set</a> whose <code>
<a href="#dom-mediastreamtrack-id">id</a>
</code> is equal to <var>trackId</var>. The method MUST return
null if no track matches the <var>trackId</var> argument.</p>
</dd>
<dt>void addTrack(MediaStreamTrack track)</dt>
<dd>
<p>Adds the given <code>
<a>MediaStreamTrack</a>
</code> to this <code>
<a>MediaStream</a>
</code>.</p>
<p>When the <dfn id="dom-mediastream-addtrack">
<code>addTrack()</code>
</dfn> method is invoked, the user agent MUST run the following
steps:</p>
<ol>
<li>
<p>Let <var>track</var> be the <code>
<a>MediaStreamTrack</a>
</code> argument and <var>stream</var> this <code>
<a>MediaStream</a>
</code> object.</p>
</li>
<li>
<p>If <var>track</var> is already in <var>stream's</var> <a
href="#track-set">track set</a>, then abort these steps.</p>
</li>
<li>
<p>Add <var>track</var> to <var>stream</var>'s <a
href="#track-set">track set</a>.</p>
</li>
</ol>
</dd>
<dt>void removeTrack(MediaStreamTrack track)</dt>
<dd>
<p>Removes the given <code>
<a>MediaStreamTrack</a>
</code> from this <code>
<a>MediaStream</a>
</code>.</p>
<p>When the <dfn id="dom-mediastream-removetrack">
<code>removeTrack()</code>
</dfn> method is invoked, the user agent MUST remove the track,
indicated by the method's argument, from the stream's <a
href="#track-set">track set</a>, if present.</p>
</dd>
<dt>MediaStream clone()</dt>
<dd>
<p>Clones the given <code>
<a>MediaStream</a>
</code> and all its tracks.</p>
<p>When the <dfn id="dom-mediastream-clone">
<code>MediaStream.clone()</code>
</dfn> method is invoked, the user agent MUST run the following
steps:</p>
<ol>
<li>
<p>Let <var>streamClone</var> be a newly constructed <code>
<a>MediaStream</a>
</code> object.</p>
</li>
<li>
<p>Initialize <var>streamClone</var>'s <code>
<a href="#dom-mediastream-id">id</a>
</code> attribute to a newly generated value.</p>
</li>
<li>
<p>Let <var>clonedTracks</var> be a list that contains the
result of running <code>
<a
href="#dom-mediastreamtrack-clone">MediaStreamTrack.clone()</a>
</code> on all the tracks in the stream on which this method
was called. </p>
</li>
<li>
<p>Let <var>clonedTracks</var> be <var>streamClone</var>'s <a
href="#track-set">track set</a>.</p>
</li>
<li>Return <var>streamClone</var>.</li>
</ol>
</dd>
<dt>readonly attribute boolean active</dt>
<dd>
<p>The <dfn id="dom-mediastream-active">
<code>MediaStream.active</code>
</dfn> attribute returns true if the <code>
<a>MediaStream</a>
</code> is active (see <a href="#stream-inactive">inactive</a>),
and false otherwise.</p>
<p>When a <code>
<a>MediaStream</a>
</code> object is created, its <code>
<a href="#dom-mediastream-active">active</a>
</code> attribute MUST be set to true, unless stated otherwise
(for example by the <code>
<a href="#dom-mediastream">MediaStream()</a>
</code> constructor algorithm).</p>
</dd>
<dt>attribute EventHandler onactive</dt>
<dd>This event handler, of type <code>
<a href="#event-mediastream-active">active</a>
</code>, MUST be supported by all objects implementing the <code>
<a>MediaStream</a>
</code> interface.</dd>
<dt>attribute EventHandler oninactive</dt>
<dd>This event handler, of type <code>
<a href="#event-mediastream-inactive">inactive</a>
</code>, MUST be supported by all objects implementing the <code>
<a>MediaStream</a>
</code> interface.</dd>
<dt>attribute EventHandler onaddtrack</dt>
<dd>This event handler, of type <code>
<a href="#event-mediastream-addtrack">addtrack</a>
</code>, MUST be supported by all objects implementing the <code>
<a>MediaStream</a>
</code> interface.</dd>
<dt>attribute EventHandler onremovetrack</dt>
<dd>This event handler, of type <code>
<a href="#event-mediastream-removetrack">removetrack</a>
</code>, MUST be supported by all objects implementing the <code>
<a>MediaStream</a>
</code> interface.</dd>
</dl>
</section>
<section>
<h2>MediaStreamTrack</h2>
<p>A <code>
<a>MediaStreamTrack</a>
</code> object represents a media source in the user agent. Several
<code>
<a>MediaStreamTrack</a>
</code> 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 <code>
<a href="#dom-mediadevices-getusermedia">getUserMedia()</a>
</code> .</p>
<p>The data from a <code>
<a>MediaStreamTrack</a>
</code> 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.</p>
<p>A script can indicate that a track no longer needs its source with
the <code>
<a href="#dom-mediastreamtrack-stop">MediaStreamTrack.stop()</a>
</code> method. When all tracks using a source have been stopped,
the given permission for that source is revoked and the source is <dfn
id="source-stopped">stopped</dfn>. If the data is being generated from
a live source (e.g., a microphone or camera), then the user agent
SHOULD remove any active "on-air" indicator for that source. If the
data is being generated from a prerecorded source (e.g. a video file),
any remaining content in the file is ignored. An implementation may
use a per source reference count to keep track of source usage, but
the specifics are out of scope for this specification.</p>
<p>If there is no stored permission to use that source, the UA SHOULD
also remove the "permission granted" indicator for the source.</p>
<section>
<h3>Life-cycle and Media Flow</h3>
<h4>Life-cycle</h4>
<p>A <code>
<a>MediaStreamTrack</a>
</code> has two stages in its life-cycle: <code>live</code> and
<code>ended</code>. A newly created <code>
<a>MediaStreamTrack</a>
</code> can be in any stage depending on how it was created. For
example, cloning an ended track results in a new ended track. The
current stage is reflected by the object's <code>
<a href="#dom-mediastreamtrack-readystate">readyState</a>
</code> attribute.</p>
<p>In the <code>live</code> state, the track is active and media is
available for use by consumers (but may be replaced by
zero-information-content if the <code>
<a>MediaStreamTrack</a>
</code> is <a href="#track-muted">muted</a> or <a
href="#track-enabled">enabled</a>, see below).</p>
<p>A muted or disabled <code>
<a>MediaStreamTrack</a>
</code> renders either silence (audio), black frames (video), or a
zero-information-content equivalent. For example, a video element
sourced by a muted or disabled <code>
<a>MediaStreamTrack</a>
</code> (contained within a <code>
<a>MediaStream</a>
</code>), is playing but the rendered content is the muted output.
When all tracks connected to a source are muted or disabled, the
"on-air" or "recording" indicator for that source can be turned off;
when the track is no longer muted or disabled, it MUST be turned
back on.</p>
<p>The muted/unmuted state of a track reflects if the source
provides any media at this moment. The enabled/disabled state is
under application control and determines if the track outputs media
(to its consumers). Hence, media from the source only flows when a
<code>
<a>MediaStreamTrack</a>
</code> object is both unmuted and enabled.</p>
<p>A <code>
<a>MediaStreamTrack</a>
</code> is <a href="#track-muted">muted</a> 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 or
browser chrome. A track can also be muted by the user agent.</p>
<p>Applications are able to <a href="#track-enabled">enable</a> or
disable a <code>
<a>MediaStreamTrack</a>
</code> 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.</p>
<p>For a newly created <code>
<a>MediaStreamTrack</a>
</code> 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.</p>
<p>A <code>
<a>MediaStreamTrack</a>
</code> object is said to <em>end</em> when the source of the
track is disconnected or exhausted.</p>
<p>A <code>
<a>MediaStreamTrack</a>
</code> can be <dfn id="track-detached">detached</dfn> from its
source. It means that the track is no longer dependent on the source
for media data. If no other <code>
<a>MediaStreamTrack</a>
</code> is using the same source, the source will be <a
href="#source-stopped">stopped</a>. <code>
<a>MediaStreamTrack</a>
</code> attributes such as <code>
<a href="#dom-mediastreamtrack-kind">kind</a>
</code> and <code>
<a href="#dom-mediastreamtrack-label">label</a>
</code> MUST not change values when the source is detached.</p>
<p>When a <code>
<a>MediaStreamTrack</a>
</code> object ends for any reason (e.g., because the user
rescinds the permission for the page to use the local camera, or
because the data comes from a finite file and the file's end has
been reached and the user has not requested that it be looped, or
because the application invoked the <code>
<a href="#dom-mediastreamtrack-stop">stop()</a>
</code> method on the <code>
<a>MediaStreamTrack</a>
</code> object, or because the UA has instructed the track to end
for any reason) it is said to be <dfn
id="track-ended">ended</dfn>.</p>
<p>When a <code>
<a>MediaStreamTrack</a>
</code> <var>track</var> ends for any reason other than the <code>
<a href="#dom-mediastreamtrack-stop">stop()</a>
</code> method being invoked, the user agent MUST queue a task
that runs the following steps:</p>
<ol>
<li>
<p>If the <var>track's</var> <code>
<a href="#dom-mediastreamtrack-readystate">readyState</a>
</code> attribute has the value <code>ended</code> already,
then abort these steps.</p>
</li>
<li>
<p>Set <var>track's</var> <code>
<a href="#dom-mediastreamtrack-readystate">readyState</a>
</code> attribute to <code>ended</code>.</p>
</li>
<li>
<p><a href="#track-detached">Detach</a> <var>track's</var>
source.</p>
</li>
<li>
<p>Fire a simple event named <code>
<a href="#event-mediastreamtrack-ended">ended</a>
</code> at the object.</p>
</li>
</ol>
<p>If the end of the stream was reached due to a user request, the
event source for this event is the user interaction event
source.</p>
<h4>Media Flow</h4>
<p>There are two concepts related to the media flow for a
<code>live</code> <code>
<a>MediaStreamTrack</a>
</code>: muted or not, and enabled or disabled.</p>
<p><dfn id="track-muted">Muted</dfn> refers to the input to the
<code>
<a>MediaStreamTrack</a>
</code>. If live samples are not made available to the <code>
<a>MediaStreamTrack</a>
</code> it is muted.</p>
<p>Muted is out of control for the application, but can be observed
by the application by reading the <code>
<a href="#dom-mediastreamtrack-muted">muted</a>
</code> attribute and listening to the associated events <code>
<a href="#event-mediastreamtrack-mute">mute</a>
</code> and <code>
<a href="#event-mediastreamtrack-unmute">unmute</a>
</code>.There can be several reasons for a <code>
<a>MediaStreamTrack</a>
</code> to be muted: the user pushing a physical mute button on
the microphone, the user toggling a control in the operating system,
the user clicking a mute button in the browser chrome, the UA (on
behalf of the user) mutes, etc.</p>
<p><dfn id="track-enabled">Enabled/disabled</dfn> on the other hand
is available to application to control (and observe) via the <code>
<a href="#dom-mediastreamtrack-enabled">enabled</a>
</code> attribute.</p>
<p>The result for the consumer is the same in the meaning that
whenever <code>
<a>MediaStreamTrack</a>
</code> 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 <code>
<a>MediaStreamTrack</a>
</code> object is both unmuted and enabled. For example, a video
element sourced by a muted or disabled <code>
<a>MediaStreamTrack</a>
</code> (contained in a <code>
<a>MediaStream</a>
</code>), is playing but rendering blackness.</p>
<p>For a newly created <code>
<a>MediaStreamTrack</a>
</code> 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.</p>
</section>
<section>
<h3>Tracks and Constraints</h3>
<p>Constraints are set on tracks and may affect sources.</p>
<p>Whether <code>
<a>Constraints</a>
</code> were provided at track initialization time or need to be
established later at runtime, the APIs defined in the
<a>ConstrainablePattern</a> Interface allow the retrieval and
manipulation of the constraints currently established on a
track.</p>
<p>Each track maintains an internal version of the <code>
<a>Constraints</a>
</code> structure, namely a mandatory set of constraints (no
duplicates), and an optional ordered list of individual constraint
objects (may contain duplicates). The internal stored constraint
structure is exposed to the application by the <code>
<a>constraints</a>
</code> attribute, and may be modified by the <code>
<a>applyConstraints()</a>
</code> method.</p>
<p>When <code>
<a>applyConstraints()</a>
</code> is called, a user agent MUST queue a task to evaluate
those changes when the task queue is next serviced. Similarly, if
the <a>
<code>sourceType</code>
</a> changes, then the user agent MUST perform the same actions to
re-evaluate the constraints of each track affected by that source
change.</p>
<p>If the <code>
<a>MediaStreamError</a>