webrtc/modules/video_coding/utility/quality_scaler.cc

/*
 *  Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
 *
 *  Use of this source code is governed by a BSD-style license
 *  that can be found in the LICENSE file in the root of the source
 *  tree. An additional intellectual property rights grant can be found
 *  in the file PATENTS.  All contributing project authors may
 *  be found in the AUTHORS file in the root of the source tree.
 */
#include "webrtc/modules/video_coding/utility/quality_scaler.h"

namespace webrtc {

namespace {
static const int kMinFps = 5;
// Threshold constant used until first downscale (to permit fast rampup).
static const int kMeasureSecondsFastUpscale = 2;
static const int kMeasureSecondsUpscale = 5;
static const int kMeasureSecondsDownscale = 5;
static const int kFramedropPercentThreshold = 60;
// Min width/height to downscale to, set to not go below QVGA, but with some
// margin to permit "almost-QVGA" resolutions, such as QCIF.
static const int kMinDownscaleDimension = 140;
// Initial resolutions corresponding to a bitrate. Aa bit above their actual
// values to permit near-VGA and near-QVGA resolutions to use the same
// mechanism.
static const int kVgaBitrateThresholdKbps = 500;
static const int kVgaNumPixels = 700 * 500;  // 640x480
static const int kQvgaBitrateThresholdKbps = 250;
static const int kQvgaNumPixels = 400 * 300;  // 320x240
}  // namespace

QualityScaler::QualityScaler()
    : low_qp_threshold_(-1), framerate_down_(false) {}

void QualityScaler::Init(int low_qp_threshold,
                         int high_qp_threshold,
                         bool use_framerate_reduction,
                         int initial_bitrate_kbps,
                         int width,
                         int height,
                         int fps) {
  ClearSamples();
  low_qp_threshold_ = low_qp_threshold;
  high_qp_threshold_ = high_qp_threshold;
  use_framerate_reduction_ = use_framerate_reduction;
  downscale_shift_ = 0;
  // Use a faster window for upscaling initially (but be more graceful later).
  // This enables faster initial rampups without risking strong up-down
  // behavior later.
  measure_seconds_upscale_ = kMeasureSecondsFastUpscale;
  const int init_width = width;
  const int init_height = height;
  if (initial_bitrate_kbps > 0) {
    int init_num_pixels = width * height;
    if (initial_bitrate_kbps < kVgaBitrateThresholdKbps)
      init_num_pixels = kVgaNumPixels;
    if (initial_bitrate_kbps < kQvgaBitrateThresholdKbps)
      init_num_pixels = kQvgaNumPixels;
    while (width * height > init_num_pixels) {
      ++downscale_shift_;
      width /= 2;
      height /= 2;
    }
  }
  UpdateTargetResolution(init_width, init_height);
  ReportFramerate(fps);
  target_framerate_ = -1;
}

// Report framerate(fps) to estimate # of samples.
void QualityScaler::ReportFramerate(int framerate) {
  framerate_ = framerate;
  UpdateSampleCounts();
}

void QualityScaler::ReportQP(int qp) {
  framedrop_percent_.AddSample(0);
  average_qp_downscale_.AddSample(qp);
  average_qp_upscale_.AddSample(qp);
}

void QualityScaler::ReportDroppedFrame() {
  framedrop_percent_.AddSample(100);
}

void QualityScaler::OnEncodeFrame(const VideoFrame& frame) {
  // Should be set through InitEncode -> Should be set by now.
  assert(low_qp_threshold_ >= 0);
  assert(num_samples_upscale_ > 0);
  assert(num_samples_downscale_ > 0);

  // Update scale factor.
  int avg_drop = 0;
  int avg_qp = 0;

  // When encoder consistently overshoots, framerate reduction and spatial
  // resizing will be triggered to get a smoother video.
  if ((framedrop_percent_.GetAverage(num_samples_downscale_, &avg_drop) &&
       avg_drop >= kFramedropPercentThreshold) ||
      (average_qp_downscale_.GetAverage(num_samples_downscale_, &avg_qp) &&
       avg_qp > high_qp_threshold_)) {
    // Reducing frame rate before spatial resolution change.
    // Reduce frame rate only when it is above a certain number.
    // Only one reduction is allowed for now.
    // TODO(jackychen): Allow more than one framerate reduction.
    if (use_framerate_reduction_ && !framerate_down_ && framerate_ >= 20) {
      target_framerate_ = framerate_ / 2;
      framerate_down_ = true;
      // If frame rate has been updated, clear the buffer. We don't want
      // spatial resolution to change right after frame rate change.
      ClearSamples();
    } else {
      AdjustScale(false);
    }
  } else if (average_qp_upscale_.GetAverage(num_samples_upscale_, &avg_qp) &&
             avg_qp <= low_qp_threshold_) {
    if (use_framerate_reduction_ && framerate_down_) {
      target_framerate_ = -1;
      framerate_down_ = false;
      ClearSamples();
    } else {
      AdjustScale(true);
    }
  }
  UpdateTargetResolution(frame.width(), frame.height());
}

QualityScaler::Resolution QualityScaler::GetScaledResolution() const {
  return res_;
}

int QualityScaler::GetTargetFramerate() const {
  return target_framerate_;
}

const VideoFrame& QualityScaler::GetScaledFrame(const VideoFrame& frame) {
  Resolution res = GetScaledResolution();
  if (res.width == frame.width())
    return frame;

  scaler_.Set(frame.width(), frame.height(), res.width, res.height, kI420,
              kI420, kScaleBox);
  if (scaler_.Scale(frame, &scaled_frame_) != 0)
    return frame;

  scaled_frame_.set_ntp_time_ms(frame.ntp_time_ms());
  scaled_frame_.set_timestamp(frame.timestamp());
  scaled_frame_.set_render_time_ms(frame.render_time_ms());

  return scaled_frame_;
}

void QualityScaler::UpdateTargetResolution(int frame_width, int frame_height) {
  assert(downscale_shift_ >= 0);
  res_.width = frame_width;
  res_.height = frame_height;
  for (int shift = downscale_shift_;
       shift > 0 && (res_.width / 2 >= kMinDownscaleDimension) &&
       (res_.height / 2 >= kMinDownscaleDimension);
       --shift) {
    res_.width /= 2;
    res_.height /= 2;
  }
}

void QualityScaler::ClearSamples() {
  framedrop_percent_.Reset();
  average_qp_downscale_.Reset();
  average_qp_upscale_.Reset();
}

void QualityScaler::UpdateSampleCounts() {
  num_samples_downscale_ = static_cast<size_t>(
      kMeasureSecondsDownscale * (framerate_ < kMinFps ? kMinFps : framerate_));
  num_samples_upscale_ = static_cast<size_t>(
      measure_seconds_upscale_ * (framerate_ < kMinFps ? kMinFps : framerate_));
}

void QualityScaler::AdjustScale(bool up) {
  downscale_shift_ += up ? -1 : 1;
  if (downscale_shift_ < 0)
    downscale_shift_ = 0;
  if (!up) {
    // Hit first downscale, start using a slower threshold for going up.
    measure_seconds_upscale_ = kMeasureSecondsUpscale;
    UpdateSampleCounts();
  }
  ClearSamples();
}

}  // namespace webrtc