WebRTC视频码率控制（二）—— QP检测

WebRTC在视频编码过程中会进行QP检测，目的是让视频质量维持在可接受范围的前提下，调节整体视频表现，如分辨率、帧率。这里要注意的是，QP检测机制只是利用QP分析结果对分辨率、帧率进行调节，并不对编码QP做直接调整。说句题外话，通常编码器也不会对外提供QP设置接口，QP主要是由帧率、码率、复杂度等因素计算得到，这部分内容将会另写文章分享。

QP检测的主体代码在quality_scaler.cc的QualityScaler类中，后者作为observer注册到VideoStreamEncoder中，VideoStreamEncoder内完成了相关流程的控制。

void VideoStreamEncoder::ReconfigureEncoder() {
  // ...
  ConfigureQualityScaler(info);
}

void VideoStreamEncoder::ConfigureQualityScaler(
    const VideoEncoder::EncoderInfo& encoder_info) {
  RTC_DCHECK_RUN_ON(&encoder_queue_);
  const auto scaling_settings = encoder_info.scaling_settings;
  // 允许进行QP检测的主要条件是，degradation_preference允许调整分辨率
  // 换句话说，摄像头流允许，而共享屏幕不允许。
  const bool quality_scaling_allowed =
      IsResolutionScalingEnabled(degradation_preference_) &&
      scaling_settings.thresholds;

  if (quality_scaling_allowed) {
    if (quality_scaler_ == nullptr) {
      //...
      AdaptationObserverInterface* observer = this;
      quality_scaler_ = std::make_unique<QualityScaler>(
          &encoder_queue_, observer,
          experimental_thresholds ? *experimental_thresholds
                                  : *(scaling_settings.thresholds));
      has_seen_first_significant_bwe_change_ = false;
      initial_framedrop_ = 0;
    }
  } else {
    quality_scaler_.reset(nullptr);
    initial_framedrop_ = kMaxInitialFramedrop;
  }
  // ...
}

经过初始化后，QP阈值的下限、上限默认分别为：24、37。

QualityScaler::QualityScaler(rtc::TaskQueue* task_queue,
                             AdaptationObserverInterface* observer,
                             VideoEncoder::QpThresholds thresholds,
                             int64_t sampling_period_ms) {
  // ...
  check_qp_task_ = RepeatingTaskHandle::DelayedStart(
    task_queue->Get(), TimeDelta::ms(GetSamplingPeriodMs()), [this]() {
      CheckQp();
      return TimeDelta::ms(GetSamplingPeriodMs());
    });
  // ...
}

与此相应，在QualityScaler析构时终止检测：

QualityScaler::~QualityScaler() {
  RTC_DCHECK_RUN_ON(&task_checker_);
  check_qp_task_.Stop();
}

// 传入回调参数EncodedImage中携带的QP
VideoStreamEncoder::RunPostEncode(EncodedImage encoded_image,
                                  int64_t time_sent_us, ...) {
  // ...
    if (quality_scaler_ && encoded_image.qp_ >= 0)
      quality_scaler_->ReportQp(encoded_image.qp_, time_sent_us);
  // ...
}

void QualityScaler::ReportQp(int qp, int64_t time_sent_us) {
  // ...
  average_qp_.AddSample(qp);
  if (qp_smoother_high_)
    qp_smoother_high_->Add(qp, time_sent_us);
  if (qp_smoother_low_)
    qp_smoother_low_->Add(qp, time_sent_us);
}

void QualityScaler::CheckQp() {
  // ...
    // Check if we should scale up or down based on QP.
  const absl::optional<int> avg_qp_high =
      qp_smoother_high_ ? qp_smoother_high_->GetAvg()
                        : average_qp_.GetAverageRoundedDown();
  const absl::optional<int> avg_qp_low =
      qp_smoother_low_ ? qp_smoother_low_->GetAvg()
                       : average_qp_.GetAverageRoundedDown();
  if (avg_qp_high && avg_qp_low) {
    if (*avg_qp_high > thresholds_.high) {
      ReportQpHigh();
      return;
    }
    if (*avg_qp_low <= thresholds_.low) {
      // QP has been low. We want to try a higher resolution.
      ReportQpLow();
      return;
    }
  }
}

// 当QP过低时，通过AdaptUp对编码进行升级
void QualityScaler::ReportQpLow() {
  // ...
  observer_->AdaptUp(AdaptationObserverInterface::AdaptReason::kQuality);
  // ...
}

// 当QP过高时，通过AdaptDown对编码进行降级
void QualityScaler::ReportQpHigh() {
  // ...
  if (observer_->AdaptDown(
          AdaptationObserverInterface::AdaptReason::kQuality)) {
    ClearSamples();
  } else {
    adapt_failed_ = true;
  }
  // ...
}