Unify hardware and software QP thresholds.

Uses current libvpx (slightly older) thresholds to maintain a larger
window of stable QP, but maintains the newer H264 thresholds.

BUG=
R=glaznev@webrtc.org, mflodman@webrtc.org

Review-Url: https://codereview.webrtc.org/1966213002
Cr-Commit-Position: refs/heads/master@{#12734}

webrtc/modules/video_coding/utility/quality_scaler.cc

@@ -30,6 +30,16 @@ static const int kQvgaBitrateThresholdKbps = 250;
 static const int kQvgaNumPixels = 400 * 300;  // 320x240
 }  // namespace
 
+// QP thresholds are chosen to be high enough to be hit in practice when quality
+// is good, but also low enough to not cause a flip-flop behavior (e.g. going up
+// in resolution shouldn't give so bad quality that we should go back down).
+
+const int QualityScaler::kLowVp8QpThreshold = 29;
+const int QualityScaler::kBadVp8QpThreshold = 95;
+
+const int QualityScaler::kLowH264QpThreshold = 22;
+const int QualityScaler::kBadH264QpThreshold = 35;
+
 QualityScaler::QualityScaler() : low_qp_threshold_(-1) {}
 
 void QualityScaler::Init(int low_qp_threshold,

webrtc/modules/video_coding/utility/quality_scaler.h

@@ -37,6 +37,15 @@ class QualityScaler {
   const VideoFrame& GetScaledFrame(const VideoFrame& frame);
   int downscale_shift() const { return downscale_shift_; }
 
+  // QP is obtained from VP8-bitstream for HW, so the QP corresponds to the
+  // bitstream range of [0, 127] and not the user-level range of [0,63].
+  static const int kLowVp8QpThreshold;
+  static const int kBadVp8QpThreshold;
+
+  // H264 QP is in the range [0, 51].
+  static const int kLowH264QpThreshold;
+  static const int kBadH264QpThreshold;
+
  private:
   void AdjustScale(bool up);
   void UpdateTargetResolution(int frame_width, int frame_height);