调用栈

WebrtcVideoSendStream::SetSendParameters

WebrtcVideoSendStream::SetCodec

void WebRtcVideoChannel::WebRtcVideoSendStream::SetCodec(
    const VideoCodecSettings& codec_settings) {
  RTC_DCHECK_RUN_ON(&thread_checker_);
  parameters_.encoder_config = CreateVideoEncoderConfig(codec_settings.codec);
  RTC_DCHECK_GT(parameters_.encoder_config.number_of_streams, 0);
  parameters_.config.rtp.payload_name = codec_settings.codec.name;
  parameters_.config.rtp.payload_type = codec_settings.codec.id;
  parameters_.config.rtp.raw_payload =
      codec_settings.codec.packetization == kPacketizationParamRaw;
  parameters_.config.rtp.ulpfec = codec_settings.ulpfec;
  parameters_.config.rtp.flexfec.payload_type =
      codec_settings.flexfec_payload_type;
  // Set RTX payload type if RTX is enabled.
  if (!parameters_.config.rtp.rtx.ssrcs.empty()) {
    if (codec_settings.rtx_payload_type == -1) {
      RTC_LOG(LS_WARNING)
          << "RTX SSRCs configured but there's no configured RTX "
             "payload type. Ignoring.";
      parameters_.config.rtp.rtx.ssrcs.clear();
    } else {
      parameters_.config.rtp.rtx.payload_type = codec_settings.rtx_payload_type;
    }
  }
  const bool has_lntf = HasLntf(codec_settings.codec);
  parameters_.config.rtp.lntf.enabled = has_lntf;
  parameters_.config.encoder_settings.capabilities.loss_notification = has_lntf;
  parameters_.config.rtp.nack.rtp_history_ms =
      HasNack(codec_settings.codec) ? kNackHistoryMs : 0;
  parameters_.codec_settings = codec_settings;
  // TODO(nisse): Avoid recreation, it should be enough to call
  // ReconfigureEncoder.
  RTC_LOG(LS_INFO) << "RecreateWebRtcStream (send) because of SetCodec.";
  RecreateWebRtcStream();
}

WebRtcVideoSendStream::RecreateWebRtcStream

void WebRtcVideoChannel::WebRtcVideoSendStream::RecreateWebRtcStream() {
  RTC_DCHECK_RUN_ON(&thread_checker_);
  if (stream_ != NULL) {
    call_->DestroyVideoSendStream(stream_);
  }
  RTC_CHECK(parameters_.codec_settings);
  RTC_DCHECK_EQ((parameters_.encoder_config.content_type ==
                 webrtc::VideoEncoderConfig::ContentType::kScreen),
                parameters_.options.is_screencast.value_or(false))
      << "encoder content type inconsistent with screencast option";
  parameters_.encoder_config.encoder_specific_settings =
      ConfigureVideoEncoderSettings(parameters_.codec_settings->codec);
  webrtc::VideoSendStream::Config config = parameters_.config.Copy();
  if (!config.rtp.rtx.ssrcs.empty() && config.rtp.rtx.payload_type == -1) {
    RTC_LOG(LS_WARNING) << "RTX SSRCs configured but there's no configured RTX "
                           "payload type the set codec. Ignoring RTX.";
    config.rtp.rtx.ssrcs.clear();
  }
  if (parameters_.encoder_config.number_of_streams == 1) {
    // SVC is used instead of simulcast. Remove unnecessary SSRCs.
    if (config.rtp.ssrcs.size() > 1) {
      config.rtp.ssrcs.resize(1);
      if (config.rtp.rtx.ssrcs.size() > 1) {
        config.rtp.rtx.ssrcs.resize(1);
      }
    }
  }
  stream_ = call_->CreateVideoSendStream(std::move(config),
                                         parameters_.encoder_config.Copy());
  parameters_.encoder_config.encoder_specific_settings = NULL;
  if (source_) {
    stream_->SetSource(source_, GetDegradationPreference());
  }
  // Call stream_->Start() if necessary conditions are met.
  UpdateSendState();
}

Call::CreateVideoSendSendStream

会有两个ssrc，各自的用处是，一个是视频流，一个是重传流。
simucast又有很多ssrc。

webrtc::VideoSendStream* Call::CreateVideoSendStream(
    webrtc::VideoSendStream::Config config,
    VideoEncoderConfig encoder_config) {
  if (config_.fec_controller_factory) {
    RTC_LOG(LS_INFO) << "External FEC Controller will be used.";
  }
  std::unique_ptr<FecController> fec_controller =
      config_.fec_controller_factory
          ? config_.fec_controller_factory->CreateFecController()
          : std::make_unique<FecControllerDefault>(clock_);
  return CreateVideoSendStream(std::move(config), std::move(encoder_config),
                               std::move(fec_controller));
}

小结