[Chromecast] Use perceptually linear scaling for stream volumes

chromecast/media/cma/backend/alsa/stream_mixer_alsa_input_impl.cc

Index: chromecast/media/cma/backend/alsa/stream_mixer_alsa_input_impl.cc
diff --git a/chromecast/media/cma/backend/alsa/stream_mixer_alsa_input_impl.cc b/chromecast/media/cma/backend/alsa/stream_mixer_alsa_input_impl.cc
index b85d1962f2ad52e662ec82025fd9347c4a6220e6..a68aad4955b6c754e8bd6b3c07742f6b58deef64 100644
--- a/chromecast/media/cma/backend/alsa/stream_mixer_alsa_input_impl.cc
+++ b/chromecast/media/cma/backend/alsa/stream_mixer_alsa_input_impl.cc
@@ -479,14 +479,15 @@ void StreamMixerAlsaInputImpl::SetPaused(bool paused) {
   }
 }
 
-void StreamMixerAlsaInputImpl::SetVolumeMultiplier(float multiplier) {
-  RUN_ON_MIXER_THREAD(SetVolumeMultiplier, multiplier);
+static float VolumeToScaleFactor(float volume_level) {
+  volume_level = std::max(0.0f, std::min(volume_level, 1.0f));
+  return std::max(0.0f, std::min(powf(volume_level, 0.5f * log2(10.0f)), 1.0f));

[kmackay, 2016/06/17 21:59:54]

Is it necessary to clamp the result? If 0 <= a <= 1, then 0 <= a^n <= 1 anyway.

[jyw, 2016/06/20 16:53:02]

Done.

+}
+
+void StreamMixerAlsaInputImpl::SetVolumeMultiplier(float volume_level) {
+  RUN_ON_MIXER_THREAD(SetVolumeMultiplier, volume_level);
   DCHECK(!IsDeleting());
-  if (multiplier > 1.0f)
-    multiplier = 1.0f;
-  if (multiplier < 0.0f)
-    multiplier = 0.0f;
-  volume_multiplier_ = multiplier;
+  volume_multiplier_ = VolumeToScaleFactor(volume_level);
 }
 
 }  // namespace media