Channel中的SetSendParameters

WebRtcVideoChannel::GetChangedSendParameters

negotiated_codecs：获取协商好的编码器，就是从本端获取跟远端匹配的视频编码器。

WebRtcVideoChannel::MapCodecs

std::vector<WebRtcVideoChannel::VideoCodecSettings>
WebRtcVideoChannel::MapCodecs(const std::vector<VideoCodec>& codecs) {
  if (codecs.empty()) {
    return {};
  }
  std::vector<VideoCodecSettings> video_codecs;
  std::map<int, VideoCodec::CodecType> payload_codec_type;
  // |rtx_mapping| maps video payload type to rtx payload type.
  std::map<int, int> rtx_mapping;
  webrtc::UlpfecConfig ulpfec_config;
  absl::optional<int> flexfec_payload_type;
  for (const VideoCodec& in_codec : codecs) {
    const int payload_type = in_codec.id;
    if (payload_codec_type.find(payload_type) != payload_codec_type.end()) {
      RTC_LOG(LS_ERROR) << "Payload type already registered: "
                        << in_codec.ToString();
      return {};
    }
    payload_codec_type[payload_type] = in_codec.GetCodecType();
    switch (in_codec.GetCodecType()) {
      ****
      case VideoCodec::CODEC_VIDEO: {
        video_codecs.emplace_back();
        video_codecs.back().codec = in_codec;
        break;
      }
    }
  }
  // One of these codecs should have been a video codec. Only having FEC
  // parameters into this code is a logic error.
  RTC_DCHECK(!video_codecs.empty());
 *****
  for (VideoCodecSettings& codec_settings : video_codecs) {
    const int payload_type = codec_settings.codec.id;
    codec_settings.ulpfec = ulpfec_config;
    codec_settings.flexfec_payload_type = flexfec_payload_type.value_or(-1);
    auto it = rtx_mapping.find(payload_type);
    if (it != rtx_mapping.end()) {
      const int rtx_payload_type = it->second;
      codec_settings.rtx_payload_type = rtx_payload_type;
    }
  }
  return video_codecs;
}

WebRtcVideoChannel::SelectSendVideoCodecs

std::vector<WebRtcVideoChannel::VideoCodecSettings>
WebRtcVideoChannel::SelectSendVideoCodecs(
    const std::vector<VideoCodecSettings>& remote_mapped_codecs) const {
    std::vector<webrtc::SdpVideoFormat> sdp_formats =
        encoder_factory_ ? encoder_factory_->GetImplementations()
        : std::vector<webrtc::SdpVideoFormat>();
    // The returned vector holds the VideoCodecSettings in term of preference.
    // They are orderd by receive codec preference first and local implementation
    // preference second.
    std::vector<VideoCodecSettings> encoders;
    for (const VideoCodecSettings& remote_codec : remote_mapped_codecs) {
        for (auto format_it = sdp_formats.begin();
             format_it != sdp_formats.end();) {
            // For H264, we will limit the encode level to the remote offered level
            // regardless if level asymmetry is allowed or not. This is strictly not
            // following the spec in https://tools.ietf.org/html/rfc6184#section-8.2.2
            // since we should limit the encode level to the lower of local and remote
            // level when level asymmetry is not allowed.
            if (IsSameCodec(format_it->name, format_it->parameters,
                            remote_codec.codec.name, remote_codec.codec.params)) {
                encoders.push_back(remote_codec);
                // To allow the VideoEncoderFactory to keep information about which
                // implementation to instantitate when CreateEncoder is called the two
                // parmeter sets are merged.
                encoders.back().codec.params.insert(format_it->parameters.begin(),
                                                    format_it->parameters.end());
                format_it = sdp_formats.erase(format_it);
            } else {
                ++format_it;
            }
        }
    }
    return encoders;
}

对比结果

发现都支持输入的4个编码器。后面会使用第一个作为视频编码器设置。

Stream中的SetSendParameters