目录

前言

正文

前言

WebRTC的编码模式有很多种，其实是可以通过参数进行控制的，今天我们就来一探究竟，本文以M67为例。

《WebRTC工作原理精讲》系列-总览

正文

直接看关键函数：bool WebRtcVideoChannel::AddSendStream(const StreamParams& sp) 

调用AddSendStream（）方法时，WebRTC首先会检验StreamParams参数是否有效，然后判断SSRC是否可用。创建WebRtcVideoSendStream实例的时候，将参数传递给WebRtcVideoSendStream对象。参数sp经过CreateRtpParametersWithEncodings处理后，赋值给rtp_parameters_。 

webrtc::DegradationPreference
WebRtcVideoChannel::WebRtcVideoSendStream::GetDegradationPreference() const { // Do not adapt resolution for screen content as this will likely // result in blurry and unreadable text. // |this| acts like a VideoSource to make sure SinkWants are handled on the // correct thread. webrtc::DegradationPreference degradation_preference; if (rtp_parameters_.degradation_preference !=
      webrtc::DegradationPreference::BALANCED) { // If the degradationPreference is different from the default value, assume // it is what we want, regardless of trials or other internal settings. degradation_preference = rtp_parameters_.degradation_preference;
  } else if (!enable_cpu_overuse_detection_) {
    degradation_preference = webrtc::DegradationPreference::DISABLED;
  } else if (parameters_.options.is_screencast.value_or(false)) {
    degradation_preference = webrtc::DegradationPreference::MAINTAIN_RESOLUTION;
  } else if (webrtc::field_trial::IsEnabled( "WebRTC-Video-BalancedDegradation")) {
    degradation_preference = webrtc::DegradationPreference::BALANCED;
  } else { // TODO(orphis): The default should be BALANCED as the standard mandates. // Right now, there is no way to set it to BALANCED as it would change // the behavior for any project expecting MAINTAIN_FRAMERATE by default. degradation_preference = webrtc::DegradationPreference::MAINTAIN_FRAMERATE;
  } return degradation_preference;
}

GetDegradationPreference（）方法在获