Extract channel mixing matrix construction out of AudioMixer

base/cvd/cuttlefish/host/frontend/webrtc/BUILD.bazel

@@ -88,6 +88,15 @@ cf_cc_library(
     ],
 )
 
+cf_cc_library(
+    name = "libcuttlefish_webrtc_audio_channel_matrix",
+    srcs = ["audio_channel_matrix.cpp"],
+    hdrs = ["audio_channel_matrix.h"],
+    deps = [
+        "@abseil-cpp//absl/log:check",
+    ],
+)
+
 cf_cc_library(
     name = "libcuttlefish_webrtc_audio_mixer",
     srcs = ["audio_mixer.cpp"],
@@ -96,6 +105,7 @@ cf_cc_library(
     depend_on_what_you_use_enabled = False,
     include_cleaner_enabled = False,
     deps = [
+        ":libcuttlefish_webrtc_audio_channel_matrix",
         ":libcuttlefish_webrtc_audio_settings",
         "//cuttlefish/host/frontend/webrtc/libdevice:audio_sink",
         "//libbase",

base/cvd/cuttlefish/host/frontend/webrtc/audio_channel_matrix.cpp

@@ -0,0 +1,45 @@
+/*
+ * Copyright (C) 2026 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "cuttlefish/host/frontend/webrtc/audio_channel_matrix.h"
+
+#include <algorithm>
+#include <cstddef>
+#include <cstdint>
+#include <vector>
+
+#include "absl/log/check.h"
+
+namespace cuttlefish {
+
+std::vector<std::vector<float>> BuildChannelMixingMatrix(uint8_t dst_channels,
+                                                         uint8_t src_channels,
+                                                         float volume) {
+  CHECK(volume >= 0) << "volume can't be negative";
+
+  std::vector<std::vector<float>> matrix(
+      dst_channels, std::vector<float>(src_channels, 0.0f));
+
+  // As of now we only use direct channel mapping
+  const size_t mapped_channels = std::min(dst_channels, src_channels);
+  for (size_t i = 0; i < mapped_channels; ++i) {
+    matrix[i][i] = volume;
+  }
+
+  return matrix;
+}
+
+}  // namespace cuttlefish

base/cvd/cuttlefish/host/frontend/webrtc/audio_channel_matrix.h

@@ -0,0 +1,40 @@
+/*
+ * Copyright (C) 2026 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <cstdint>
+#include <vector>
+
+namespace cuttlefish {
+
+/**
+ * Builds the matrix used to mix a stream's source channels into a sink's
+ * destination channels.
+ *
+ * Each source channel is mapped to the destination channel of the same index
+ * and scaled by `volume`. Channels present on only one side are dropped.
+ *
+ * @param dst_channels Destination sink channels (e.g. 2 for a stereo sink)
+ * @param src_channels Source stream channels (e.g. 6 for 5.1 surround)
+ * @param volume Master stream volume [0.0 - 1.0]
+ * @return Mixing coefficients, indexed [destination channel][source channel]
+ */
+std::vector<std::vector<float>> BuildChannelMixingMatrix(uint8_t dst_channels,
+                                                         uint8_t src_channels,
+                                                         float volume);
+
+}  // namespace cuttlefish

base/cvd/cuttlefish/host/frontend/webrtc/audio_mixer.cpp

@@ -7,6 +7,8 @@
 
 #include "absl/log/check.h"
 
+#include "cuttlefish/host/frontend/webrtc/audio_channel_matrix.h"
+
 namespace cuttlefish {
 namespace {
 
@@ -170,12 +172,11 @@ void AudioMixer::OnPlayback(uint32_t stream_id, uint32_t stream_sample_rate,
   const auto frames_count = GetFrameCountAfterResampling(
       sample_rate_, stream_sample_rate, stream_frames_count);
 
-  std::unique_lock<std::mutex> lock(mutex_);
-
   // As of now we only use direct channel mapping
-  for(size_t i = 0; i < channles_map.size(); ++i) {
-    channles_map[i][i] = volume;
-  }
+  const std::vector<std::vector<float>> channels_map =
+      BuildChannelMixingMatrix(channels_count_, stream_channels_count, volume);
+
+  std::unique_lock<std::mutex> lock(mutex_);
 
   const bool need_notify = next_frame_.empty();  // no active streams
 
@@ -204,7 +205,7 @@ void AudioMixer::OnPlayback(uint32_t stream_id, uint32_t stream_sample_rate,
   const auto filled_frames_count =
       convert_fn(mixed_buffer_.data() + next_frame_id * frame_size_bytes_,
                  channels_count_, sample_rate_, buffer, stream_channels_count,
-                 stream_sample_rate, stream_frames_count, channles_map);
+                 stream_sample_rate, stream_frames_count, channels_map);
   CHECK(filled_frames_count <= frames_count);
 
   next_frame_[stream_id] = next_frame_id + filled_frames_count;

base/cvd/cuttlefish/host/frontend/webrtc/audio_mixer.h

@@ -58,16 +58,6 @@ class AudioMixer {
   // Frame index per stream to put next available data to
   std::unordered_map<uint32_t, size_t> next_frame_;
 
-  // Used to remap channels and apply volume levels
-  std::vector<std::vector<float>> channles_map = {{
-      {1, 0, 0, 0, 0, 0},
-      {0, 1, 0, 0, 0, 0},
-      {0, 0, 1, 0, 0, 0},
-      {0, 0, 0, 1, 0, 0},
-      {0, 0, 0, 0, 1, 0},
-      {0, 0, 0, 0, 0, 1},
-  }};
-
   ////////////////////////////////////////////////////
   ////////////////////////////////////////////////////