Let Opus increase complexity for low bitrates

webrtc/modules/audio_coding/codecs/opus/audio_encoder_opus.cc

Index: webrtc/modules/audio_coding/codecs/opus/audio_encoder_opus.cc
diff --git a/webrtc/modules/audio_coding/codecs/opus/audio_encoder_opus.cc b/webrtc/modules/audio_coding/codecs/opus/audio_encoder_opus.cc
index f466926f195c8f386c685f6021b491b447714f8a..39d82a9d47035d9423a358961e51920eacaf152b 100644
--- a/webrtc/modules/audio_coding/codecs/opus/audio_encoder_opus.cc
+++ b/webrtc/modules/audio_coding/codecs/opus/audio_encoder_opus.cc
@@ -44,6 +44,9 @@ AudioEncoderOpus::Config CreateConfig(const CodecInst& codec_inst) {
   config.application = config.num_channels == 1 ? AudioEncoderOpus::kVoip
                                                 : AudioEncoderOpus::kAudio;
   config.supported_frame_lengths_ms.push_back(config.frame_size_ms);
+#if WEBRTC_OPUS_VARIABLE_COMPLEXITY
+  config.low_rate_complexity = 9;
+#endif
   return config;
 }
 
@@ -118,7 +121,11 @@ class AudioEncoderOpus::PacketLossFractionSmoother {
   rtc::ExpFilter smoother_;
 };
 
-AudioEncoderOpus::Config::Config() = default;
+AudioEncoderOpus::Config::Config() {
+#if WEBRTC_OPUS_VARIABLE_COMPLEXITY
+  low_rate_complexity = 9;
+#endif
+}
 AudioEncoderOpus::Config::Config(const Config&) = default;
 AudioEncoderOpus::Config::~Config() = default;
 auto AudioEncoderOpus::Config::operator=(const Config&) -> Config& = default;
@@ -133,6 +140,8 @@ bool AudioEncoderOpus::Config::IsOk() const {
     return false;
   if (complexity < 0 || complexity > 10)
     return false;
+  if (low_rate_complexity < 0 || low_rate_complexity > 10)
+    return false;
   return true;
 }
 
@@ -144,6 +153,12 @@ int AudioEncoderOpus::Config::GetBitrateBps() const {
     return num_channels == 1 ? 32000 : 64000;  // Default value.
 }
 
+int AudioEncoderOpus::Config::GetComplexity() const {
+  RTC_DCHECK(IsOk());
+  return GetBitrateBps() <= complexity_threshold_bps ? low_rate_complexity
+                                                     : complexity;
+}
+
 AudioEncoderOpus::AudioEncoderOpus(
     const Config& config,
     AudioNetworkAdaptorCreator&& audio_network_adaptor_creator)
@@ -250,6 +265,10 @@ void AudioEncoderOpus::SetTargetBitrate(int bits_per_second) {
       std::max(std::min(bits_per_second, kMaxBitrateBps), kMinBitrateBps));
   RTC_DCHECK(config_.IsOk());
   RTC_CHECK_EQ(0, WebRtcOpus_SetBitRate(inst_, config_.GetBitrateBps()));
+  if (complexity_ != config_.GetComplexity()) {
+    complexity_ = config_.GetComplexity();
+    RTC_CHECK_EQ(0, WebRtcOpus_SetComplexity(inst_, complexity_));
+  }
 }
 
 bool AudioEncoderOpus::EnableAudioNetworkAdaptor(
@@ -399,7 +418,8 @@ bool AudioEncoderOpus::RecreateEncoderInstance(const Config& config) {
   }
   RTC_CHECK_EQ(
       0, WebRtcOpus_SetMaxPlaybackRate(inst_, config.max_playback_rate_hz));
-  RTC_CHECK_EQ(0, WebRtcOpus_SetComplexity(inst_, config.complexity));
+  complexity_ = config.GetComplexity();
+  RTC_CHECK_EQ(0, WebRtcOpus_SetComplexity(inst_, complexity_));
   if (config.dtx_enabled) {
     RTC_CHECK_EQ(0, WebRtcOpus_EnableDtx(inst_));
   } else {