[Chromecast] Use perceptually linear scaling for stream volumes

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

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chromecast/media/cma/backend/alsa/stream_mixer_alsa_input_impl.h"
 
 #include <algorithm>
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
@@ skipping 461 matching lines @@
   }
   DCHECK_GE(fade_frames_remaining_, 0);
 
   if (state_ == kStateFadingOut) {
     // Tell the mixer that some more data might be available (since when fading
     // out, we can drain the queue completely).
     mixer_->OnFramesQueued();
   }
 }
 
-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
 }  // namespace chromecast