webrtc-qos

1. 输入编码器的帧率，分辨率控制
  PeerConnectionClient.java::createVideoTrack方法里面 加入一行 changeCaptureFormat(videoWidth,videoHeight,videoFps);
  
2. 编码器目标码率设置
  PeerConnectionClient.java::onLocalDescription方法里面，调用setVideoMaxBitrate方法进行设置，该方法主体如下:
  RtpParameters parameters = localVideoSender.getParameters();
  if (parameters.encodings.size() == 0) {
    Log.w(TAG, "RtpParameters are not ready.");
    return;
  }

  for (RtpParameters.Encoding encoding : parameters.encodings) {
    // Null value means no limit.
    encoding.maxBitrateBps = maxBitrateKbps == null ? null : maxBitrateKbps * BPS_IN_KBPS;
  }
  if (!localVideoSender.setParameters(parameters)) {
    Log.e(TAG, "RtpSender.setParameters failed.");
  }
  Log.d(TAG, "Configured max video bitrate to: " + maxBitrateKbps);
  
  3.最低分辨率设置位置: video_encoder.h
  int min_pixels_per_frame = 320 * 180;
  
  4.帧率分辨率动态调整位置:video_stream_encoder.cc
  最低帧率控制： const int kMinFramerateFps = 2;
  帧率升降幅度： 2/3
  分辨率升降幅度: 3/5
  
  5.首帧丢弃原则，和编码器的启动码率有关
  bool VideoStreamEncoder::DropDueToSize(uint32_t pixel_count) const {
  if (initial_framedrop_ < kMaxInitialFramedrop &&
      encoder_start_bitrate_bps_ > 0) {
    if (encoder_start_bitrate_bps_ < 300000 /* qvga */) {
      return pixel_count > 320 * 240;
    } else if (encoder_start_bitrate_bps_ < 500000 /* vga */) {
      return pixel_count > 640 * 480;
    }
  }
  return false;
}

3.linux版本启动码率初始值相关配置
  peer_connection_factory.cc::CreateCall_w
  const int kMinBandwidthBps = 30000;
  const int kStartBandwidthBps = 300000;
  const int kMaxBandwidthBps = 2000000;
  
6.编码器启动码率设置
struct BitrateConstraints {
  int min_bitrate_bps = 0;
  int start_bitrate_bps = kDefaultStartBitrateBps;
  int max_bitrate_bps = -1;
 private:
  static constexpr int kDefaultStartBitrateBps = 600000; //默认是300kbps
};

7.编码器初始最大码率设置
static int GetMaxDefaultVideoBitrateKbps(int width,
                                         int height,
                                         bool is_screenshare) {
  int max_bitrate;
  if (width * height <= 320 * 240) {
    max_bitrate = 600;
  } else if (width * height <= 640 * 480) {
    max_bitrate = 1700;
  } else if (width * height <= 960 * 540) {
    max_bitrate = 2000;
  } else {
    max_bitrate = 2500;
  }
  if (is_screenshare)
    max_bitrate = std::max(max_bitrate, 1200);
  return max_bitrate;
}

webrtc抗丢包方法配置：NACK_ONLY 或者 FEC_ONLY 或者 NACK + FEC 或者 KNONE
配置位置： VCMLossProtectionLogic::SetMethod
  
  3. svc vs simulticast
  https://webrtchacks.com/sfu-simulcast/
  https://www.meetecho.com/blog/simulcast-janus-ssrc/
  
  4. 接收端如果没有收到关键帧，会立马请求:RtpVideoStreamReceiver::OnAssembledFrame
    // Request a key frame as soon as possible.
  if (!has_received_frame_) {
    has_received_frame_ = true;
    if (frame->FrameType() != VideoFrameType::kVideoFrameKey) {
      // TODO(bugs.webrtc.org/10336): Allow the sender to ignore these messages
      // if it is relying on LNTF alone.
      RequestKeyFrame();
    }
  }