audio/utility/channel_mixing_matrix.cc - src - Git at Google

 /*
  *  Copyright (c) 2019 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.
  */

 #include "audio/utility/channel_mixing_matrix.h"

 #include <stddef.h>

 #include <algorithm>

 #include "audio/utility/channel_mixer.h"
 #include "rtc_base/checks.h"
 #include "rtc_base/logging.h"

 namespace webrtc {

 static void ValidateLayout(ChannelLayout layout) {
   RTC_CHECK_NE(layout, CHANNEL_LAYOUT_NONE);
   RTC_CHECK_LE(layout, CHANNEL_LAYOUT_MAX);
   RTC_CHECK_NE(layout, CHANNEL_LAYOUT_UNSUPPORTED);
   RTC_CHECK_NE(layout, CHANNEL_LAYOUT_DISCRETE);
   RTC_CHECK_NE(layout, CHANNEL_LAYOUT_STEREO_AND_KEYBOARD_MIC);

   // Verify there's at least one channel.  Should always be true here by virtue
   // of not being one of the invalid layouts, but lets double check to be sure.
   int channel_count = ChannelLayoutToChannelCount(layout);
   RTC_DCHECK_GT(channel_count, 0);

   // If we have more than one channel, verify a symmetric layout for sanity.
   // The unit test will verify all possible layouts, so this can be a DCHECK.
   // Symmetry allows simplifying the matrix building code by allowing us to
   // assume that if one channel of a pair exists, the other will too.
   if (channel_count > 1) {
     // Assert that LEFT exists if and only if RIGHT exists, and so on.
     RTC_DCHECK_EQ(ChannelOrder(layout, LEFT) >= 0,
                   ChannelOrder(layout, RIGHT) >= 0);
     RTC_DCHECK_EQ(ChannelOrder(layout, SIDE_LEFT) >= 0,
                   ChannelOrder(layout, SIDE_RIGHT) >= 0);
     RTC_DCHECK_EQ(ChannelOrder(layout, BACK_LEFT) >= 0,
                   ChannelOrder(layout, BACK_RIGHT) >= 0);
     RTC_DCHECK_EQ(ChannelOrder(layout, LEFT_OF_CENTER) >= 0,
                   ChannelOrder(layout, RIGHT_OF_CENTER) >= 0);
   } else {
     RTC_DCHECK_EQ(layout, CHANNEL_LAYOUT_MONO);
   }
 }

 ChannelMixingMatrix::ChannelMixingMatrix(ChannelLayout input_layout,
                                          int input_channels,
                                          ChannelLayout output_layout,
                                          int output_channels)
     : input_layout_(input_layout),
       input_channels_(input_channels),
       output_layout_(output_layout),
       output_channels_(output_channels) {
   // Stereo down mix should never be the output layout.
   RTC_CHECK_NE(output_layout, CHANNEL_LAYOUT_STEREO_DOWNMIX);

   // Verify that the layouts are supported
   if (input_layout != CHANNEL_LAYOUT_DISCRETE)
     ValidateLayout(input_layout);
   if (output_layout != CHANNEL_LAYOUT_DISCRETE)
     ValidateLayout(output_layout);

   // Special case for 5.0, 5.1 with back channels when upmixed to 7.0, 7.1,
   // which should map the back LR to side LR.
   if (input_layout_ == CHANNEL_LAYOUT_5_0_BACK &&
       output_layout_ == CHANNEL_LAYOUT_7_0) {
     input_layout_ = CHANNEL_LAYOUT_5_0;
   } else if (input_layout_ == CHANNEL_LAYOUT_5_1_BACK &&
              output_layout_ == CHANNEL_LAYOUT_7_1) {
     input_layout_ = CHANNEL_LAYOUT_5_1;
   }
 }

 ChannelMixingMatrix::~ChannelMixingMatrix() = default;

 bool ChannelMixingMatrix::CreateTransformationMatrix(
     std::vector<std::vector<float>>* matrix) {
   matrix_ = matrix;

   // Size out the initial matrix.
   matrix_->reserve(output_channels_);
   for (int output_ch = 0; output_ch < output_channels_; ++output_ch)
     matrix_->push_back(std::vector<float>(input_channels_, 0));

   // First check for discrete case.
   if (input_layout_ == CHANNEL_LAYOUT_DISCRETE ||
       output_layout_ == CHANNEL_LAYOUT_DISCRETE) {
     // If the number of input channels is more than output channels, then
     // copy as many as we can then drop the remaining input channels.
     // If the number of input channels is less than output channels, then
     // copy them all, then zero out the remaining output channels.
     int passthrough_channels = std::min(input_channels_, output_channels_);
     for (int i = 0; i < passthrough_channels; ++i)
       (*matrix_)[i][i] = 1;

     return true;
   }

   // Route matching channels and figure out which ones aren't accounted for.
   for (Channels ch = LEFT; ch < CHANNELS_MAX + 1;
        ch = static_cast<Channels>(ch + 1)) {
     int input_ch_index = ChannelOrder(input_layout_, ch);
     if (input_ch_index < 0)
       continue;

     int output_ch_index = ChannelOrder(output_layout_, ch);
     if (output_ch_index < 0) {
       unaccounted_inputs_.push_back(ch);
       continue;
     }

     RTC_DCHECK_LT(static_cast<size_t>(output_ch_index), matrix_->size());
     RTC_DCHECK_LT(static_cast<size_t>(input_ch_index),
                   (*matrix_)[output_ch_index].size());
     (*matrix_)[output_ch_index][input_ch_index] = 1;
   }

   // If all input channels are accounted for, there's nothing left to do.
   if (unaccounted_inputs_.empty()) {
     // Since all output channels map directly to inputs we can optimize.
     return true;
   }

   // Mix front LR into center.
   if (IsUnaccounted(LEFT)) {
     // When down mixing to mono from stereo, we need to be careful of full scale
     // stereo mixes.  Scaling by 1 / sqrt(2) here will likely lead to clipping
     // so we use 1 / 2 instead.
     float scale =
         (output_layout_ == CHANNEL_LAYOUT_MONO && input_channels_ == 2)
             ? 0.5
             : ChannelMixer::kHalfPower;
     Mix(LEFT, CENTER, scale);
     Mix(RIGHT, CENTER, scale);
   }

   // Mix center into front LR.
   if (IsUnaccounted(CENTER)) {
     // When up mixing from mono, just do a copy to front LR.
     float scale =
         (input_layout_ == CHANNEL_LAYOUT_MONO) ? 1 : ChannelMixer::kHalfPower;
     MixWithoutAccounting(CENTER, LEFT, scale);
     Mix(CENTER, RIGHT, scale);
   }

   // Mix back LR into: side LR || back center || front LR || front center.
   if (IsUnaccounted(BACK_LEFT)) {
     if (HasOutputChannel(SIDE_LEFT)) {
       // If the input has side LR, mix back LR into side LR, but instead if the
       // input doesn't have side LR (but output does) copy back LR to side LR.
       float scale = HasInputChannel(SIDE_LEFT) ? ChannelMixer::kHalfPower : 1;
       Mix(BACK_LEFT, SIDE_LEFT, scale);
       Mix(BACK_RIGHT, SIDE_RIGHT, scale);
     } else if (HasOutputChannel(BACK_CENTER)) {
       // Mix back LR into back center.
       Mix(BACK_LEFT, BACK_CENTER, ChannelMixer::kHalfPower);
       Mix(BACK_RIGHT, BACK_CENTER, ChannelMixer::kHalfPower);
     } else if (output_layout_ > CHANNEL_LAYOUT_MONO) {
       // Mix back LR into front LR.
       Mix(BACK_LEFT, LEFT, ChannelMixer::kHalfPower);
       Mix(BACK_RIGHT, RIGHT, ChannelMixer::kHalfPower);
     } else {
       // Mix back LR into front center.
       Mix(BACK_LEFT, CENTER, ChannelMixer::kHalfPower);
       Mix(BACK_RIGHT, CENTER, ChannelMixer::kHalfPower);
     }
   }

   // Mix side LR into: back LR || back center || front LR || front center.
   if (IsUnaccounted(SIDE_LEFT)) {
     if (HasOutputChannel(BACK_LEFT)) {
       // If the input has back LR, mix side LR into back LR, but instead if the
       // input doesn't have back LR (but output does) copy side LR to back LR.
       float scale = HasInputChannel(BACK_LEFT) ? ChannelMixer::kHalfPower : 1;
       Mix(SIDE_LEFT, BACK_LEFT, scale);
       Mix(SIDE_RIGHT, BACK_RIGHT, scale);
     } else if (HasOutputChannel(BACK_CENTER)) {
       // Mix side LR into back center.
       Mix(SIDE_LEFT, BACK_CENTER, ChannelMixer::kHalfPower);
       Mix(SIDE_RIGHT, BACK_CENTER, ChannelMixer::kHalfPower);
     } else if (output_layout_ > CHANNEL_LAYOUT_MONO) {
       // Mix side LR into front LR.
       Mix(SIDE_LEFT, LEFT, ChannelMixer::kHalfPower);
       Mix(SIDE_RIGHT, RIGHT, ChannelMixer::kHalfPower);
     } else {
       // Mix side LR into front center.
       Mix(SIDE_LEFT, CENTER, ChannelMixer::kHalfPower);
       Mix(SIDE_RIGHT, CENTER, ChannelMixer::kHalfPower);
     }
   }

   // Mix back center into: back LR || side LR || front LR || front center.
   if (IsUnaccounted(BACK_CENTER)) {
     if (HasOutputChannel(BACK_LEFT)) {
       // Mix back center into back LR.
       MixWithoutAccounting(BACK_CENTER, BACK_LEFT, ChannelMixer::kHalfPower);
       Mix(BACK_CENTER, BACK_RIGHT, ChannelMixer::kHalfPower);
     } else if (HasOutputChannel(SIDE_LEFT)) {
       // Mix back center into side LR.
       MixWithoutAccounting(BACK_CENTER, SIDE_LEFT, ChannelMixer::kHalfPower);
       Mix(BACK_CENTER, SIDE_RIGHT, ChannelMixer::kHalfPower);
     } else if (output_layout_ > CHANNEL_LAYOUT_MONO) {
       // Mix back center into front LR.
       // TODO(dalecurtis): Not sure about these values?
       MixWithoutAccounting(BACK_CENTER, LEFT, ChannelMixer::kHalfPower);
       Mix(BACK_CENTER, RIGHT, ChannelMixer::kHalfPower);
     } else {
       // Mix back center into front center.
       // TODO(dalecurtis): Not sure about these values?
       Mix(BACK_CENTER, CENTER, ChannelMixer::kHalfPower);
     }
   }

   // Mix LR of center into: front LR || front center.
   if (IsUnaccounted(LEFT_OF_CENTER)) {
     if (HasOutputChannel(LEFT)) {
       // Mix LR of center into front LR.
       Mix(LEFT_OF_CENTER, LEFT, ChannelMixer::kHalfPower);
       Mix(RIGHT_OF_CENTER, RIGHT, ChannelMixer::kHalfPower);
     } else {
       // Mix LR of center into front center.
       Mix(LEFT_OF_CENTER, CENTER, ChannelMixer::kHalfPower);
       Mix(RIGHT_OF_CENTER, CENTER, ChannelMixer::kHalfPower);
     }
   }

   // Mix LFE into: front center || front LR.
   if (IsUnaccounted(LFE)) {
     if (!HasOutputChannel(CENTER)) {
       // Mix LFE into front LR.
       MixWithoutAccounting(LFE, LEFT, ChannelMixer::kHalfPower);
       Mix(LFE, RIGHT, ChannelMixer::kHalfPower);
     } else {
       // Mix LFE into front center.
       Mix(LFE, CENTER, ChannelMixer::kHalfPower);
     }
   }

   // All channels should now be accounted for.
   RTC_DCHECK(unaccounted_inputs_.empty());

   // See if the output |matrix_| is simply a remapping matrix.  If each input
   // channel maps to a single output channel we can simply remap.  Doing this
   // programmatically is less fragile than logic checks on channel mappings.
   for (int output_ch = 0; output_ch < output_channels_; ++output_ch) {
     int input_mappings = 0;
     for (int input_ch = 0; input_ch < input_channels_; ++input_ch) {
       // We can only remap if each row contains a single scale of 1.  I.e., each
       // output channel is mapped from a single unscaled input channel.
       if ((*matrix_)[output_ch][input_ch] != 1 || ++input_mappings > 1)
         return false;
     }
   }

   // If we've gotten here, |matrix_| is simply a remapping.
   return true;
 }

 void ChannelMixingMatrix::AccountFor(Channels ch) {
   unaccounted_inputs_.erase(
       std::find(unaccounted_inputs_.begin(), unaccounted_inputs_.end(), ch));
 }

 bool ChannelMixingMatrix::IsUnaccounted(Channels ch) const {
   return std::find(unaccounted_inputs_.begin(), unaccounted_inputs_.end(),
                    ch) != unaccounted_inputs_.end();
 }

 bool ChannelMixingMatrix::HasInputChannel(Channels ch) const {
   return ChannelOrder(input_layout_, ch) >= 0;
 }

 bool ChannelMixingMatrix::HasOutputChannel(Channels ch) const {
   return ChannelOrder(output_layout_, ch) >= 0;
 }

 void ChannelMixingMatrix::Mix(Channels input_ch,
                               Channels output_ch,
                               float scale) {
   MixWithoutAccounting(input_ch, output_ch, scale);
   AccountFor(input_ch);
 }

 void ChannelMixingMatrix::MixWithoutAccounting(Channels input_ch,
                                                Channels output_ch,
                                                float scale) {
   int input_ch_index = ChannelOrder(input_layout_, input_ch);
   int output_ch_index = ChannelOrder(output_layout_, output_ch);

   RTC_DCHECK(IsUnaccounted(input_ch));
   RTC_DCHECK_GE(input_ch_index, 0);
   RTC_DCHECK_GE(output_ch_index, 0);

   RTC_DCHECK_EQ((*matrix_)[output_ch_index][input_ch_index], 0);
   (*matrix_)[output_ch_index][input_ch_index] = scale;
 }

 }  // namespace webrtc
	/*
	* Copyright (c) 2019 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.
	*/

	#include "audio/utility/channel_mixing_matrix.h"

	#include <stddef.h>

	#include <algorithm>

	#include "audio/utility/channel_mixer.h"
	#include "rtc_base/checks.h"
	#include "rtc_base/logging.h"

	namespace webrtc {

	static void ValidateLayout(ChannelLayout layout) {
	RTC_CHECK_NE(layout, CHANNEL_LAYOUT_NONE);
	RTC_CHECK_LE(layout, CHANNEL_LAYOUT_MAX);
	RTC_CHECK_NE(layout, CHANNEL_LAYOUT_UNSUPPORTED);
	RTC_CHECK_NE(layout, CHANNEL_LAYOUT_DISCRETE);
	RTC_CHECK_NE(layout, CHANNEL_LAYOUT_STEREO_AND_KEYBOARD_MIC);

	// Verify there's at least one channel. Should always be true here by virtue
	// of not being one of the invalid layouts, but lets double check to be sure.
	int channel_count = ChannelLayoutToChannelCount(layout);
	RTC_DCHECK_GT(channel_count, 0);

	// If we have more than one channel, verify a symmetric layout for sanity.
	// The unit test will verify all possible layouts, so this can be a DCHECK.
	// Symmetry allows simplifying the matrix building code by allowing us to
	// assume that if one channel of a pair exists, the other will too.
	if (channel_count > 1) {
	// Assert that LEFT exists if and only if RIGHT exists, and so on.
	RTC_DCHECK_EQ(ChannelOrder(layout, LEFT) >= 0,
	ChannelOrder(layout, RIGHT) >= 0);
	RTC_DCHECK_EQ(ChannelOrder(layout, SIDE_LEFT) >= 0,
	ChannelOrder(layout, SIDE_RIGHT) >= 0);
	RTC_DCHECK_EQ(ChannelOrder(layout, BACK_LEFT) >= 0,
	ChannelOrder(layout, BACK_RIGHT) >= 0);
	RTC_DCHECK_EQ(ChannelOrder(layout, LEFT_OF_CENTER) >= 0,
	ChannelOrder(layout, RIGHT_OF_CENTER) >= 0);
	} else {
	RTC_DCHECK_EQ(layout, CHANNEL_LAYOUT_MONO);
	}
	}

	ChannelMixingMatrix::ChannelMixingMatrix(ChannelLayout input_layout,
	int input_channels,
	ChannelLayout output_layout,
	int output_channels)
	: input_layout_(input_layout),
	input_channels_(input_channels),
	output_layout_(output_layout),
	output_channels_(output_channels) {
	// Stereo down mix should never be the output layout.
	RTC_CHECK_NE(output_layout, CHANNEL_LAYOUT_STEREO_DOWNMIX);

	// Verify that the layouts are supported
	if (input_layout != CHANNEL_LAYOUT_DISCRETE)
	ValidateLayout(input_layout);
	if (output_layout != CHANNEL_LAYOUT_DISCRETE)
	ValidateLayout(output_layout);

	// Special case for 5.0, 5.1 with back channels when upmixed to 7.0, 7.1,
	// which should map the back LR to side LR.
	if (input_layout_ == CHANNEL_LAYOUT_5_0_BACK &&
	output_layout_ == CHANNEL_LAYOUT_7_0) {
	input_layout_ = CHANNEL_LAYOUT_5_0;
	} else if (input_layout_ == CHANNEL_LAYOUT_5_1_BACK &&
	output_layout_ == CHANNEL_LAYOUT_7_1) {
	input_layout_ = CHANNEL_LAYOUT_5_1;
	}
	}

	ChannelMixingMatrix::~ChannelMixingMatrix() = default;

	bool ChannelMixingMatrix::CreateTransformationMatrix(
	std::vector<std::vector<float>>* matrix) {
	matrix_ = matrix;

	// Size out the initial matrix.
	matrix_->reserve(output_channels_);
	for (int output_ch = 0; output_ch < output_channels_; ++output_ch)
	matrix_->push_back(std::vector<float>(input_channels_, 0));

	// First check for discrete case.
	if (input_layout_ == CHANNEL_LAYOUT_DISCRETE \|\|
	output_layout_ == CHANNEL_LAYOUT_DISCRETE) {
	// If the number of input channels is more than output channels, then
	// copy as many as we can then drop the remaining input channels.
	// If the number of input channels is less than output channels, then
	// copy them all, then zero out the remaining output channels.
	int passthrough_channels = std::min(input_channels_, output_channels_);
	for (int i = 0; i < passthrough_channels; ++i)
	(*matrix_)[i][i] = 1;

	return true;
	}

	// Route matching channels and figure out which ones aren't accounted for.
	for (Channels ch = LEFT; ch < CHANNELS_MAX + 1;
	ch = static_cast<Channels>(ch + 1)) {
	int input_ch_index = ChannelOrder(input_layout_, ch);
	if (input_ch_index < 0)
	continue;

	int output_ch_index = ChannelOrder(output_layout_, ch);
	if (output_ch_index < 0) {
	unaccounted_inputs_.push_back(ch);
	continue;
	}

	RTC_DCHECK_LT(static_cast<size_t>(output_ch_index), matrix_->size());
	RTC_DCHECK_LT(static_cast<size_t>(input_ch_index),
	(*matrix_)[output_ch_index].size());
	(*matrix_)[output_ch_index][input_ch_index] = 1;
	}

	// If all input channels are accounted for, there's nothing left to do.
	if (unaccounted_inputs_.empty()) {
	// Since all output channels map directly to inputs we can optimize.
	return true;
	}

	// Mix front LR into center.
	if (IsUnaccounted(LEFT)) {
	// When down mixing to mono from stereo, we need to be careful of full scale
	// stereo mixes. Scaling by 1 / sqrt(2) here will likely lead to clipping
	// so we use 1 / 2 instead.
	float scale =
	(output_layout_ == CHANNEL_LAYOUT_MONO && input_channels_ == 2)
	? 0.5
	: ChannelMixer::kHalfPower;
	Mix(LEFT, CENTER, scale);
	Mix(RIGHT, CENTER, scale);
	}

	// Mix center into front LR.
	if (IsUnaccounted(CENTER)) {
	// When up mixing from mono, just do a copy to front LR.
	float scale =
	(input_layout_ == CHANNEL_LAYOUT_MONO) ? 1 : ChannelMixer::kHalfPower;
	MixWithoutAccounting(CENTER, LEFT, scale);
	Mix(CENTER, RIGHT, scale);
	}

	// Mix back LR into: side LR \|\| back center \|\| front LR \|\| front center.
	if (IsUnaccounted(BACK_LEFT)) {
	if (HasOutputChannel(SIDE_LEFT)) {
	// If the input has side LR, mix back LR into side LR, but instead if the
	// input doesn't have side LR (but output does) copy back LR to side LR.
	float scale = HasInputChannel(SIDE_LEFT) ? ChannelMixer::kHalfPower : 1;
	Mix(BACK_LEFT, SIDE_LEFT, scale);
	Mix(BACK_RIGHT, SIDE_RIGHT, scale);
	} else if (HasOutputChannel(BACK_CENTER)) {
	// Mix back LR into back center.
	Mix(BACK_LEFT, BACK_CENTER, ChannelMixer::kHalfPower);
	Mix(BACK_RIGHT, BACK_CENTER, ChannelMixer::kHalfPower);
	} else if (output_layout_ > CHANNEL_LAYOUT_MONO) {
	// Mix back LR into front LR.
	Mix(BACK_LEFT, LEFT, ChannelMixer::kHalfPower);
	Mix(BACK_RIGHT, RIGHT, ChannelMixer::kHalfPower);
	} else {
	// Mix back LR into front center.
	Mix(BACK_LEFT, CENTER, ChannelMixer::kHalfPower);
	Mix(BACK_RIGHT, CENTER, ChannelMixer::kHalfPower);
	}
	}

	// Mix side LR into: back LR \|\| back center \|\| front LR \|\| front center.
	if (IsUnaccounted(SIDE_LEFT)) {
	if (HasOutputChannel(BACK_LEFT)) {
	// If the input has back LR, mix side LR into back LR, but instead if the
	// input doesn't have back LR (but output does) copy side LR to back LR.
	float scale = HasInputChannel(BACK_LEFT) ? ChannelMixer::kHalfPower : 1;
	Mix(SIDE_LEFT, BACK_LEFT, scale);
	Mix(SIDE_RIGHT, BACK_RIGHT, scale);
	} else if (HasOutputChannel(BACK_CENTER)) {
	// Mix side LR into back center.
	Mix(SIDE_LEFT, BACK_CENTER, ChannelMixer::kHalfPower);
	Mix(SIDE_RIGHT, BACK_CENTER, ChannelMixer::kHalfPower);
	} else if (output_layout_ > CHANNEL_LAYOUT_MONO) {
	// Mix side LR into front LR.
	Mix(SIDE_LEFT, LEFT, ChannelMixer::kHalfPower);
	Mix(SIDE_RIGHT, RIGHT, ChannelMixer::kHalfPower);
	} else {
	// Mix side LR into front center.
	Mix(SIDE_LEFT, CENTER, ChannelMixer::kHalfPower);
	Mix(SIDE_RIGHT, CENTER, ChannelMixer::kHalfPower);
	}
	}

	// Mix back center into: back LR \|\| side LR \|\| front LR \|\| front center.
	if (IsUnaccounted(BACK_CENTER)) {
	if (HasOutputChannel(BACK_LEFT)) {
	// Mix back center into back LR.
	MixWithoutAccounting(BACK_CENTER, BACK_LEFT, ChannelMixer::kHalfPower);
	Mix(BACK_CENTER, BACK_RIGHT, ChannelMixer::kHalfPower);
	} else if (HasOutputChannel(SIDE_LEFT)) {
	// Mix back center into side LR.
	MixWithoutAccounting(BACK_CENTER, SIDE_LEFT, ChannelMixer::kHalfPower);
	Mix(BACK_CENTER, SIDE_RIGHT, ChannelMixer::kHalfPower);
	} else if (output_layout_ > CHANNEL_LAYOUT_MONO) {
	// Mix back center into front LR.
	// TODO(dalecurtis): Not sure about these values?
	MixWithoutAccounting(BACK_CENTER, LEFT, ChannelMixer::kHalfPower);
	Mix(BACK_CENTER, RIGHT, ChannelMixer::kHalfPower);
	} else {
	// Mix back center into front center.
	// TODO(dalecurtis): Not sure about these values?
	Mix(BACK_CENTER, CENTER, ChannelMixer::kHalfPower);
	}
	}

	// Mix LR of center into: front LR \|\| front center.
	if (IsUnaccounted(LEFT_OF_CENTER)) {
	if (HasOutputChannel(LEFT)) {
	// Mix LR of center into front LR.
	Mix(LEFT_OF_CENTER, LEFT, ChannelMixer::kHalfPower);
	Mix(RIGHT_OF_CENTER, RIGHT, ChannelMixer::kHalfPower);
	} else {
	// Mix LR of center into front center.
	Mix(LEFT_OF_CENTER, CENTER, ChannelMixer::kHalfPower);
	Mix(RIGHT_OF_CENTER, CENTER, ChannelMixer::kHalfPower);
	}
	}

	// Mix LFE into: front center \|\| front LR.
	if (IsUnaccounted(LFE)) {
	if (!HasOutputChannel(CENTER)) {
	// Mix LFE into front LR.
	MixWithoutAccounting(LFE, LEFT, ChannelMixer::kHalfPower);
	Mix(LFE, RIGHT, ChannelMixer::kHalfPower);
	} else {
	// Mix LFE into front center.
	Mix(LFE, CENTER, ChannelMixer::kHalfPower);
	}
	}

	// All channels should now be accounted for.
	RTC_DCHECK(unaccounted_inputs_.empty());

	// See if the output \|matrix_\| is simply a remapping matrix. If each input
	// channel maps to a single output channel we can simply remap. Doing this
	// programmatically is less fragile than logic checks on channel mappings.
	for (int output_ch = 0; output_ch < output_channels_; ++output_ch) {
	int input_mappings = 0;
	for (int input_ch = 0; input_ch < input_channels_; ++input_ch) {
	// We can only remap if each row contains a single scale of 1. I.e., each
	// output channel is mapped from a single unscaled input channel.
	if ((*matrix_)[output_ch][input_ch] != 1 \|\| ++input_mappings > 1)
	return false;
	}
	}

	// If we've gotten here, \|matrix_\| is simply a remapping.
	return true;
	}

	void ChannelMixingMatrix::AccountFor(Channels ch) {
	unaccounted_inputs_.erase(
	std::find(unaccounted_inputs_.begin(), unaccounted_inputs_.end(), ch));
	}

	bool ChannelMixingMatrix::IsUnaccounted(Channels ch) const {
	return std::find(unaccounted_inputs_.begin(), unaccounted_inputs_.end(),
	ch) != unaccounted_inputs_.end();
	}

	bool ChannelMixingMatrix::HasInputChannel(Channels ch) const {
	return ChannelOrder(input_layout_, ch) >= 0;
	}

	bool ChannelMixingMatrix::HasOutputChannel(Channels ch) const {
	return ChannelOrder(output_layout_, ch) >= 0;
	}

	void ChannelMixingMatrix::Mix(Channels input_ch,
	Channels output_ch,
	float scale) {
	MixWithoutAccounting(input_ch, output_ch, scale);
	AccountFor(input_ch);
	}

	void ChannelMixingMatrix::MixWithoutAccounting(Channels input_ch,
	Channels output_ch,
	float scale) {
	int input_ch_index = ChannelOrder(input_layout_, input_ch);
	int output_ch_index = ChannelOrder(output_layout_, output_ch);

	RTC_DCHECK(IsUnaccounted(input_ch));
	RTC_DCHECK_GE(input_ch_index, 0);
	RTC_DCHECK_GE(output_ch_index, 0);

	RTC_DCHECK_EQ((*matrix_)[output_ch_index][input_ch_index], 0);
	(*matrix_)[output_ch_index][input_ch_index] = scale;
	}

	} // namespace webrtc