Add support for channel transforms.

media/base/channel_mixer.h

+// Copyright (c) 2012 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.
+
+#ifndef MEDIA_BASE_CHANNEL_MIXER_H_
+#define MEDIA_BASE_CHANNEL_MIXER_H_
+
+#include <vector>
+
+#include "base/basictypes.h"
+#include "media/base/channel_layout.h"
+#include "media/base/media_export.h"
+
+namespace media {
+
+class AudioBus;
+
+// ChannelMixer is for converting audio between channel layouts.  The conversion
+// matrix is built upon construction and used during each Transform() call.  The
+// algorithm works by generating a conversion matrix mapping each output channel
+// to list of input channels.  The transform is then done by scaling each input
+// channel according to the matrix and summing it into a single output channel.

[Chris Rogers, 2012/10/19 20:48:17]

This last sentence is a little confusing.  I think what you mean is something
like:

The transform renders all of the output channels, with each output channel
rendered according to a weighted sum of the relevant input channels as defined
in the matrix.

[DaleCurtis, 2012/10/19 22:33:20]

Done.

+class MEDIA_EXPORT ChannelMixer {
+ public:
+  ChannelMixer(ChannelLayout input, ChannelLayout output);
+  ~ChannelMixer();
+
+  // Transforms all channels from |input| into |output| channels.
+  void Transform(const AudioBus* input, AudioBus* output);
+
+ private:
+  // Constructor helper methods for managing unaccounted input channels.
+  void AccountFor(Channels ch);
+  bool IsUnaccounted(Channels ch);
+
+  // Helper methods for checking if |ch| exists in either |input_layout_| or
+  // |output_layout_| respectively.
+  bool HasInputChannel(Channels ch);
+  bool HasOutputChannel(Channels ch);
+
+  // Constructor helper methods for updating |matrix_| with the proper value for
+  // mixing |input_ch| into |output_ch|.  MixWithoutAccounting() does not remove
+  // the channel from |unaccounted_inputs_|.
+  void Mix(Channels input_ch, Channels output_ch, float scale);
+  void MixWithoutAccounting(Channels input_ch, Channels output_ch, float scale);
+
+  // Input and output channel layout provided during construction.
+  ChannelLayout input_layout_;
+  ChannelLayout output_layout_;
+
+  // Helper variable for tracking which inputs are currently unaccounted, should
+  // be empty after construction completes.
+  std::vector<Channels> unaccounted_inputs_;
+
+  // 2D matrix of output channels to input channels.
+  std::vector< std::vector<float> > matrix_;
+
+  // Optimization case for when we can simply remap the input channels to output
+  // channels and don't need to do a multiply-accumulate loop over |matrix_|.
+  bool remapping_;
+
+  DISALLOW_COPY_AND_ASSIGN(ChannelMixer);
+};
+
+}  // namespace media
+
+#endif  // MEDIA_BASE_CHANNEL_MIXER_H_