RTCRtpSender.setParameters() - Web APIs 编辑

The setParameters() method of the RTCRtpSender interface applies changes the configuration of sender's track, which is the MediaStreamTrack for which the RTCRtpSender is responsible.

In other words, setParameters() updates the configuration of the RTP transmission as well as the encoding configuration for a specific outgoing media track on the WebRTC connection.

Syntax

var promise = rtcRtpSender.setParameters(parameters)

Parameters

parameters

A parameters object previously obtained by calling the same sender's getParameters() method, with the desired changes to the sender's configuration parameters. These parameters include potential codecs that could be use for encoding the sender's track. The available parameters are:

encodings

An array of RTCRtpEncodingParameters objects, each specifying the parameters for a single codec that could be used to encode the track's media.

transactionId

A string containing a unique ID for the last set of parameters applied; this value is used to ensure that setParameters() can only be called to alter changes made by a specific previous call to getParameters(). Once this parameter is initially set, it cannot be changed.

Obsolete properties

degradationPreference

Specifies the preferred way the WebRTC layer should handle optimizing bandwidth against quality in constrained-bandwidth situations; the value comes from the RTCDegradationPreference enumerated string type, and the default is balanced.

priority

A string from the RTCPriorityType enumerated type which indicates the encoding's priority. The default value is low.

Return value

A Promise that resolves when the RTCRtpSender.track property is updated with the given parameters.

Exceptions

If an error occurs, the returned promise is rejected with the appropriate exception from the list below.

InvalidModificationError

One of the following problems was detected:

• The number of encodings specified in the parameters object's encodings property does not match the number of encodings currently listed for the RTCRtpSender. You cannot change the number of encoding options once the sender has been created.
• The order of the specified encodings has changed from the current list's order.
• An attempt has been made to alter a property that cannot be changed after the sender is first created.

InvalidStateError

The transceiver of which the RTCRtpSender is a part is not running, or has no parameters to set.

OperationError

Any error condition which occurs that isn't covered by one of the other error codes results in an OperationError.

RangeError

The value specified for scaleResolutionDownBy is less than 1.0, which would result in scaling up rather than down, which is not allowed; or one or more of the specified encodings' maxFramerate values is less than 0.0.

In addition, if a WebRTC error occurs while configuring or accessing the media, an RTCError is thrown with its errorDetail set to hardware-encoder-error.

Description

It's important to keep in mind that you can't create an RTCRtpSendParameters object yourself and expect it to work. Instead, you must first call getParameters(), modify the received parameters object, then pass that object into setParameters(). WebRTC uses the parameters object's transactionId property to ensure that when you set parameters, your changes are based on the most recent parameters rather than an out of date configuration.

Examples

One use case for setParameters() is to try to reduce network bandwidth use in constrained environments by altering the resolution and/or bit rate of the media being transmitted by the RTCRtpSender.

Currently, some browsers have limitations on their implementations that may cause issues. For that reason, two examples are given here. The first shows how to use setParameters() when all browsers fully support the parameters being used, while the second example demonstrates workarounds to help solve limitations in browsers with incomplete support for the maxBitrate and scaleResolutionDownBy parameters.

By the specification

Once all browsers implement the spec fully, this implementation of setVideoParams() will do the job. This demonstrates how everything should work. You should probably use the second example, below, for now. But this is a clearer demonstration of the basic concept of first fetching the parameters, then altering them, then setting them.

async function setVideoParams(sender, height, bitrate) {
  const scaleRatio = sender.track.getSettings().height/height;
  const params = sender.getParameters();

  params.encodings[0].scaleResolutionDownBy = Math.max(ratio, 1);
  params.encodings[0].maxBitrate = bitrate;
  await sender.setParameters(params);
}

In calling this function, you specify a sender, as w.ell as the height you wish to scale the sender's video to, as well as a maximum bitrate to permit the sender to transmit. A scaling factor for the size of the video, scaleRatio, is computed. Then the sender's current parameters are fetched using getParameters().

The parameters are then altered by changing the first encodings object's scaleResolutionDownBy and maxBitrate to the calculated scaling factor and the specified maximum bitrate.

The changed parameters are then saved by calling the sender's setParameters() method.

Currently compatible implementation

As mentioned above, the previous example shows how things are meant to work. Unfortunately, there are implementation issues preventing this in many browsers right now. For that reason, if you want to be compatible with iPhone and other devices running Safari, and with Firefox, use code more like this:

async function setVideoParams(sender, height, bitrate) {
  const scaleRatio = sender.track.getSettings().height/height;
  const params = sender.getParameters();

  // If encodings is null, create it

  if (!params.encodings) {
    params.encodings = [{ }];
  }

  params.encodings[0].scaleResolutionDownBy = Math.max(ratio, 1);
  params.encodings[0].maxBitrate = bitrate;
  await sender.setParameters(params);

  // If the newly changed value of scaleResolutionDownBy is 1,
  // use applyConstraints() to be sure the height is constrained,
  // since scaleResolutionDownBy may not be implemented

  if (sender.getParameters().encodings[0].scaleResolutionDownBy == 1) {
    await sender.track.applyConstraints({ height });
  }
}

The differences here:

• If encodings is null, we create it, in order to ensure that we can then set the parameters successfully without crashing.
• If, after setting the parameters, the value of scaleResolutionDownBy is still 1, we call the sender's track's applyConstraints() method to constrain the track's height to height. This compensates for an unimplemented scaleResolutionDownBy (as is the case in Safari as of this writing).

This code will cleanly fall back and work the normal way if the browser fully implements the used features.

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• WebRTC API
• Codecs used by WebRTC
• Web media technologies