The media part of WebRTC covers how to access hardware capable of capturing video and audio, such as cameras and microphones, as well as how media streams work. It also covers display media, which is how an application can do screen capturing.
媒体设备
All cameras and microphones that are supported by the browser are accessed and
managed through the navigator.mediaDevices object. Applications can retrieve
the current list of connected devices and also listen for changes, since many
cameras and microhpones connect through USB and can be connected and
disconnected during the lifecycle of the application. Since the state of a media
device can change at any time, it is recommended that applications register for
device changes in order to properly handle changes.
约束
When accessing media devices, it is a good practice to provide as detailed constraints as possible. While it is possible to open the default camera and microphone with a simple constraint, it might deliver a media stream that is far from the most optimal for the application.
The specific constraints are defined in a MediaTrackConstraint object, one for
audio and one for video. The attributes in this object are of type
ConstraintLong, ConstraintBoolean, ConstraintDouble or
ConstraintDOMString. These can either be a specific value (e.g., a number,
boolean or string), a range (LongRange or DoubleRange with a minimum and
maximum value) or an object with either an ideal or exact definition. For a
specific value, the browser will attempt to pick something as close as possible.
For a range, the best value in that range will be used. When exact is
specified, only media streams that exactly match that constraint will be
returned.
Near
// Camera with a resolution as close to 640x480 as possible
{
"video": {
"width": 640,
"height": 480
}
}
Range
// Camera with a resolution in the range 640x480 to 1024x768
{
"video": {
"width": {
"min": 640,
"max": 1024
},
"height": {
"min": 480,
"max": 768
}
}
}
Exact
// Camera with the exact resolution of 1024x768
{
"video": {
"width": {
"exact": 1024
},
"height": {
"exact": 768
}
}
}
To determine the actual configuration a certain track of a media stream has,
we can call MediaStreamTrack.getSettings() which returns the
MediaTrackSettings currently applied.
It is also possible to update the constraints of a track from a media device we
have opened, by calling applyConstraints() on the track. This lets an
application re-configure a media device without first having to close the
existing stream.
显示媒体
An application that wants to be able to perform screen capturing and recording
must use the Display Media API. The function getDisplayMedia() (which is part
of navigator.mediaDevices is similar to getUserMedia() and is used for the
purpose of opening the content of the display (or a portion of it, such as a
window). The returned MediaStream works the same as when using
getUserMedia().
The constraints for getDisplayMedia() differ from the ones used for regular
video or audio input.
{
video: {
cursor: 'always' | 'motion' | 'never',
displaySurface: 'application' | 'browser' | 'monitor' | 'window'
}
}
The code snipet above shows how the special constraints for screen recording works. Note that these might not be supported by all browsers that have display media support.
流和轨道
A MediaStream represents a stream of media content, which consists of tracks
(MediaStreamTrack) of audio and video. You can retrieve all the tracks from
MediaStream by calling MediaStream.getTracks(), which returns an array of
MediaStreamTrack objects.
媒体流跟踪
A MediaStreamTrack has a kind property that is either audio or video,
indicating the kind of media it represents. Each track can be muted by toggling
its enabled property. A track has a Boolean property remote that indicates
if it is sourced by a RTCPeerConnection and coming from a remote peer.