Use bitstream-level QP for libvpx VP8 quality.

webrtc/modules/video_coding/codecs/vp8/vp8_impl.cc

 /*
  *  Copyright (c) 2012 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.
  */
 
@@ skipping 582 matching lines @@
       configurations_[i].rc_target_bitrate = stream_bitrates[stream_idx];
       temporal_layers_[stream_idx]->ConfigureBitrates(
           stream_bitrates[stream_idx], inst->maxBitrate, inst->maxFramerate,
           &configurations_[i]);
     }
   }
 
   rps_.Init();
   // Disable both high-QP limits and framedropping. Both are handled by libvpx
   // internally.
-  const int kLowQpThreshold = 18;
-  const int kDisabledBadQpThreshold = 64;
+  const int kLowQpThreshold = 23;

[stefan-webrtc, 2016/04/15 09:21:08]

Please clarify in the code why this was chosen to be 23.

+  const int kDisabledBadQpThreshold = 128;
   // TODO(glaznev/sprang): consider passing codec initial bitrate to quality
   // scaler to avoid starting with HD for low initial bitrates.
   quality_scaler_.Init(kLowQpThreshold, kDisabledBadQpThreshold, false, 0, 0, 0,
                        codec_.maxFramerate);
 
   // Only apply scaling to improve for single-layer streams. The scaling metrics
   // use frame drops as a signal and is only applicable when we drop frames.
   quality_scaler_enabled_ = encoders_.size() == 1 &&
                             configurations_[0].rc_dropframe_thresh > 0 &&
                             codec_.codecSpecific.VP8.automaticResizeOn;
@@ skipping 431 matching lines @@
         encoded_images_[encoder_idx].qp_ = qp_128;
         encoded_complete_callback_->Encoded(encoded_images_[encoder_idx],
                                             &codec_specific, &frag_info);
       } else if (codec_.mode == kScreensharing) {
         result = WEBRTC_VIDEO_CODEC_TARGET_BITRATE_OVERSHOOT;
       }
     }
   }
   if (encoders_.size() == 1 && send_stream_[0]) {
     if (encoded_images_[0]._length > 0) {
-      int qp;
-      vpx_codec_control(&encoders_[0], VP8E_GET_LAST_QUANTIZER_64, &qp);
-      quality_scaler_.ReportQP(qp);
+      int qp_128;
+      vpx_codec_control(&encoders_[0], VP8E_GET_LAST_QUANTIZER, &qp_128);
+      quality_scaler_.ReportQP(qp_128);
     } else {
       quality_scaler_.ReportDroppedFrame();
     }
   }
   return result;
 }
 
 int VP8EncoderImpl::SetChannelParameters(uint32_t packetLoss, int64_t rtt) {
   rps_.SetRtt(rtt);
   return WEBRTC_VIDEO_CODEC_OK;
@@ skipping 340 matching lines @@
     return -1;
   }
   if (vpx_codec_control(copy->decoder_, VP8_SET_REFERENCE, ref_frame_) !=
       VPX_CODEC_OK) {
     return -1;
   }
   return 0;
 }
 
 }  // namespace webrtc