Make QualityScaler more responsive to downgrades.

webrtc/modules/video_coding/utility/quality_scaler.cc

Index: webrtc/modules/video_coding/utility/quality_scaler.cc
diff --git a/webrtc/modules/video_coding/utility/quality_scaler.cc b/webrtc/modules/video_coding/utility/quality_scaler.cc
index 68ae9218ea2162d1c39c6a47005d5c3f779e8b95..c6e566973101e01f1ec868f8810e614d7a3480e7 100644
--- a/webrtc/modules/video_coding/utility/quality_scaler.cc
+++ b/webrtc/modules/video_coding/utility/quality_scaler.cc
@@ -11,8 +11,11 @@
 
 namespace webrtc {
 
-static const int kMinFps = 10;
-static const int kMeasureSeconds = 5;
+static const int kMinFps = 5;
+static const int kMeasureSecondsDownscale = 3;
+// Threshold constant used until first downscale (to permit fast rampup).
+static const int kMeasureSecondsFastUpscale = 2;
+static const int kMeasureSecondsUpscale = 5;
 static const int kFramedropPercentThreshold = 60;
 static const int kHdResolutionThreshold = 700 * 500;
 static const int kHdBitrateThresholdKbps = 500;
@@ -23,9 +26,7 @@ const int QualityScaler::kDefaultLowQpDenominator = 3;
 const int QualityScaler::kDefaultMinDownscaleDimension = 90;
 
 QualityScaler::QualityScaler()
-    : num_samples_(0),
-      low_qp_threshold_(-1),
-      downscale_shift_(0),
+    : low_qp_threshold_(-1),
       framerate_down_(false),
       min_width_(kDefaultMinDownscaleDimension),
       min_height_(kDefaultMinDownscaleDimension) {}
@@ -35,12 +36,17 @@ void QualityScaler::Init(int low_qp_threshold,
                          bool use_framerate_reduction,
                          int initial_bitrate_kbps,
                          int width,
-                         int height) {
+                         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;
   // TODO(glaznev): Investigate using thresholds for other resolutions
@@ -55,6 +61,7 @@ void QualityScaler::Init(int low_qp_threshold,
     }
   }
   UpdateTargetResolution(init_width, init_height);
+  ReportFramerate(fps);
   target_framerate_ = -1;
 }
 
@@ -65,14 +72,14 @@ void QualityScaler::SetMinResolution(int min_width, int min_height) {
 
 // Report framerate(fps) to estimate # of samples.
 void QualityScaler::ReportFramerate(int framerate) {
-  num_samples_ = static_cast<size_t>(
-      kMeasureSeconds * (framerate < kMinFps ? kMinFps : framerate));
   framerate_ = framerate;
+  UpdateSampleCounts();
 }
 
 void QualityScaler::ReportQP(int qp) {
   framedrop_percent_.AddSample(0);
-  average_qp_.AddSample(qp);
+  average_qp_downscale_.AddSample(qp);
+  average_qp_upscale_.AddSample(qp);
 }
 
 void QualityScaler::ReportDroppedFrame() {
@@ -82,7 +89,8 @@ void QualityScaler::ReportDroppedFrame() {
 void QualityScaler::OnEncodeFrame(const VideoFrame& frame) {
   // Should be set through InitEncode -> Should be set by now.
   assert(low_qp_threshold_ >= 0);
-  assert(num_samples_ > 0);
+  assert(num_samples_upscale_ > 0);
+  assert(num_samples_downscale_ > 0);
 
   // Update scale factor.
   int avg_drop = 0;
@@ -90,9 +98,9 @@ void QualityScaler::OnEncodeFrame(const VideoFrame& frame) {
 
   // When encoder consistently overshoots, framerate reduction and spatial
   // resizing will be triggered to get a smoother video.
-  if ((framedrop_percent_.GetAverage(num_samples_, &avg_drop) &&
+  if ((framedrop_percent_.GetAverage(num_samples_downscale_, &avg_drop) &&
        avg_drop >= kFramedropPercentThreshold) ||
-      (average_qp_.GetAverage(num_samples_, &avg_qp) &&
+      (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.
@@ -107,7 +115,7 @@ void QualityScaler::OnEncodeFrame(const VideoFrame& frame) {
     } else {
       AdjustScale(false);
     }
-  } else if (average_qp_.GetAverage(num_samples_, &avg_qp) &&
+  } 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;
@@ -160,13 +168,26 @@ void QualityScaler::UpdateTargetResolution(int frame_width, int frame_height) {
 
 void QualityScaler::ClearSamples() {
   framedrop_percent_.Reset();
-  average_qp_.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;

[stefan-webrtc, 2016/04/01 13:35:10]

Should there be a time limit at which time we also switch to the longer window?
E.g., after 20 seconds?

[pbos-webrtc, 2016/04/04 12:32:39]

Don't think it's really worth the code complexity for a corner case where we'd
go up wrong once (and it could still be right)? WDYT?

+    UpdateSampleCounts();
+  }
   ClearSamples();
 }