Expose the internal MatrixBuilder class as ChannelMixingMatrix.

media/base/channel_mixing_matrix_unittest.cc

+// 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.
+
+// MSVC++ requires this to be set before any other includes to get M_SQRT1_2.
+#define _USE_MATH_DEFINES
+
+#include "media/base/channel_mixing_matrix.h"
+
+#include <cmath>
+
+#include "base/strings/stringprintf.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace media {
+
+// Test all possible layout conversions can be constructed and mixed.
+TEST(ChannelMixingMatrixTest, ConstructAllPossibleLayouts) {
+  for (ChannelLayout input_layout = CHANNEL_LAYOUT_MONO;
+       input_layout <= CHANNEL_LAYOUT_MAX;
+       input_layout = static_cast<ChannelLayout>(input_layout + 1)) {
+    for (ChannelLayout output_layout = CHANNEL_LAYOUT_MONO;
+         // TODO(wtc): why do we only test up to CHANNEL_LAYOUT_STEREO_DOWNMIX?
+         output_layout < CHANNEL_LAYOUT_STEREO_DOWNMIX;
+         output_layout = static_cast<ChannelLayout>(output_layout + 1)) {
+      // DISCRETE can't be tested here based on the current approach.
+      // CHANNEL_LAYOUT_STEREO_AND_KEYBOARD_MIC is not mixable.
+      if (input_layout == CHANNEL_LAYOUT_DISCRETE ||
+          input_layout == CHANNEL_LAYOUT_STEREO_AND_KEYBOARD_MIC ||
+          output_layout == CHANNEL_LAYOUT_DISCRETE ||
+          output_layout == CHANNEL_LAYOUT_STEREO_AND_KEYBOARD_MIC) {
+        continue;
+      }
+
+      SCOPED_TRACE(base::StringPrintf(
+          "Input Layout: %d, Output Layout: %d", input_layout, output_layout));
+      ChannelMixingMatrix matrix_builder(
+          input_layout,
+          ChannelLayoutToChannelCount(input_layout),
+          output_layout,
+          ChannelLayoutToChannelCount(output_layout));
+      std::vector<std::vector<float>> matrix;
+      matrix_builder.CreateTransformationMatrix(&matrix);
+    }
+  }
+}
+
+// Verify channels are mixed and scaled correctly.
+TEST(ChannelMixingMatrixTest, StereoToMono) {
+  ChannelLayout input_layout = CHANNEL_LAYOUT_STEREO;
+  ChannelLayout output_layout = CHANNEL_LAYOUT_MONO;
+  ChannelMixingMatrix matrix_builder(
+      input_layout,
+      ChannelLayoutToChannelCount(input_layout),
+      output_layout,
+      ChannelLayoutToChannelCount(output_layout));
+  std::vector<std::vector<float>> matrix;
+  bool remapping = matrix_builder.CreateTransformationMatrix(&matrix);
+
+  //                      Input: stereo
+  //                      LEFT  RIGHT
+  // Output: mono CENTER  0.5   0.5
+  //
+  EXPECT_FALSE(remapping);
+  EXPECT_EQ(1u, matrix.size());
+  EXPECT_EQ(2u, matrix[0].size());
+  EXPECT_EQ(0.5f, matrix[0][0]);
+  EXPECT_EQ(0.5f, matrix[0][1]);
+}
+
+TEST(ChannelMixingMatrixTest, MonoToStereo) {
+  ChannelLayout input_layout = CHANNEL_LAYOUT_MONO;
+  ChannelLayout output_layout = CHANNEL_LAYOUT_STEREO;
+  ChannelMixingMatrix matrix_builder(
+      input_layout,
+      ChannelLayoutToChannelCount(input_layout),
+      output_layout,
+      ChannelLayoutToChannelCount(output_layout));
+  std::vector<std::vector<float>> matrix;
+  bool remapping = matrix_builder.CreateTransformationMatrix(&matrix);
+
+  //                       Input: mono
+  //                       CENTER
+  // Output: stereo LEFT   1
+  //                RIGHT  1
+  //
+  EXPECT_TRUE(remapping);
+  EXPECT_EQ(2u, matrix.size());
+  EXPECT_EQ(1u, matrix[0].size());
+  EXPECT_EQ(1.0f, matrix[0][0]);
+  EXPECT_EQ(1u, matrix[1].size());
+  EXPECT_EQ(1.0f, matrix[1][0]);
+}
+
+TEST(ChannelMixingMatrixTest, FiveOneToMono) {
+  ChannelLayout input_layout = CHANNEL_LAYOUT_5_1;
+  ChannelLayout output_layout = CHANNEL_LAYOUT_MONO;
+  ChannelMixingMatrix matrix_builder(
+      input_layout,
+      ChannelLayoutToChannelCount(input_layout),
+      output_layout,
+      ChannelLayoutToChannelCount(output_layout));
+  std::vector<std::vector<float>> matrix;
+  bool remapping = matrix_builder.CreateTransformationMatrix(&matrix);
+
+  // Note: 1/sqrt(2) is shown as 0.707.
+  //
+  //                      Input: 5.1
+  //                      LEFT   RIGHT  CENTER  LFE    SIDE_LEFT  SIDE_RIGHT
+  // Output: mono CENTER  0.707  0.707  1       0.707  0.707      0.707

[wtc, 2014/10/23 16:55:13]

Dale: I added graphical representations of the matrices. I must admit I find the
sole 1 scaling factor for CENTER in this matrix very surprising, although it is
the expected result of our algorithm.

[DaleCurtis, 2014/10/23 17:15:27]

By all means, please analyze :) I intuitively chose that value because the
center channel in 5.1 is generally vocals, etc, other material which should be
featured more prominently than surrounds.

Based on listening to a few 5.1 -> mono down mixes it sounds good too, but I'd
very much welcome a new set of ears on it!

[wtc, 2014/10/23 18:39:45]

That's a good rationale and it's supported by your listening tests. I was
worried that the center channel uses the 1.0 scale simply because the output
layout also has a center channel.

+  //
+  EXPECT_FALSE(remapping);
+  EXPECT_EQ(1u, matrix.size());
+  EXPECT_EQ(6u, matrix[0].size());
+  EXPECT_EQ(static_cast<float>(M_SQRT1_2), matrix[0][0]);
+  EXPECT_EQ(static_cast<float>(M_SQRT1_2), matrix[0][1]);
+  // The center channel will be mixed at scale 1.
+  EXPECT_EQ(1.0f, matrix[0][2]);
+  EXPECT_EQ(static_cast<float>(M_SQRT1_2), matrix[0][3]);
+  EXPECT_EQ(static_cast<float>(M_SQRT1_2), matrix[0][4]);
+  EXPECT_EQ(static_cast<float>(M_SQRT1_2), matrix[0][5]);
+}
+
+TEST(ChannelMixingMatrixTest, DiscreteToDiscrete) {
+  const struct {
+    int input_channels;
+    int output_channels;
+  } test_case[] = {
+    {2, 2}, {2, 5}, {5, 2},
+  };
+
+  for (size_t n = 0; n < arraysize(test_case); n++) {
+    int input_channels = test_case[n].input_channels;
+    int output_channels = test_case[n].output_channels;
+    ChannelMixingMatrix matrix_builder(CHANNEL_LAYOUT_DISCRETE,
+                                       input_channels,
+                                       CHANNEL_LAYOUT_DISCRETE,
+                                       output_channels);
+    std::vector<std::vector<float>> matrix;
+    bool remapping = matrix_builder.CreateTransformationMatrix(&matrix);
+    EXPECT_TRUE(remapping);
+    EXPECT_EQ(static_cast<size_t>(output_channels), matrix.size());
+    for (int i = 0; i < output_channels; i++) {
+      EXPECT_EQ(static_cast<size_t>(input_channels), matrix[i].size());
+      for (int j = 0; j < input_channels; j++) {
+        if (i == j) {
+          EXPECT_EQ(1.0f, matrix[i][j]);
+        } else {
+          EXPECT_EQ(0.0f, matrix[i][j]);
+        }
+      }
+    }
+  }
+}
+
+}  // namespace media