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* 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 <stddef.h>
#include <stdint.h>
#include <memory>
#include <vector>
#include "api/audio/audio_frame.h"
#include "api/audio/channel_layout.h"
namespace webrtc {
// 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 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.
// This file is derived from Chromium's media/base/channel_mixer.h.
class ChannelMixer {
// To mix two channels into one and preserve loudness, we must apply
// (1 / sqrt(2)) gain to each.
static constexpr float kHalfPower = 0.707106781186547524401f;
ChannelMixer(ChannelLayout input_layout, ChannelLayout output_layout);
// Transforms all input channels corresponding to the selected |input_layout|
// to the number of channels in the selected |output_layout|.
// Example usage (downmix from stereo to mono):
// AudioFrame frame;
// frame.samples_per_channel_ = 160;
// frame.num_channels_ = 2;
// EXPECT_EQ(2u, frame.channels());
// mixer.Transform(&frame);
// EXPECT_EQ(1u, frame.channels());
void Transform(AudioFrame* frame);
bool IsUpMixing() const { return output_channels_ > input_channels_; }
// Selected channel layouts.
const ChannelLayout input_layout_;
const ChannelLayout output_layout_;
// Channel counts for input and output.
const size_t input_channels_;
const size_t output_channels_;
// 2D matrix of output channels to input channels.
std::vector<std::vector<float> > matrix_;
// 1D array used as temporary storage during the transformation.
std::unique_ptr<int16_t[]> audio_vector_;
// Number of elements allocated for |audio_vector_|.
size_t audio_vector_size_ = 0;
// Optimization case for when we can simply remap the input channels to output
// channels, i.e., when all scaling factors in |matrix_| equals 1.0.
bool remapping_;
// Delete the copy constructor and assignment operator.
ChannelMixer(const ChannelMixer& other) = delete;
ChannelMixer& operator=(const ChannelMixer& other) = delete;
} // namespace webrtc