Surround Sound handling by folding 5 channels down to stereo.

media/audio/audio_util.cc

Index: media/audio/audio_util.cc
===================================================================
--- media/audio/audio_util.cc	(revision 21354)
+++ media/audio/audio_util.cc	(working copy)
@@ -59,4 +59,93 @@
   return false;
 }
 
+// Channel order for AAC
+// From http://www.hydrogenaudio.org/forums/lofiversion/index.php/t40046.html
+// And Quicktime Pro, Movie Inspector
+const int kChannel_C = 0;
+const int kChannel_L = 1;
+const int kChannel_R = 2;
+const int kChannel_SL = 3;
+const int kChannel_SR = 4;
+const int kChannel_LFE = 5;
+
+template<class Format>
+Format ChannelsClampInternal(float val,
+                             const Format min_value,
+                             const Format max_value) {
+  if (val > static_cast<float>(max_value)) {
+    return max_value;
+  }
+  if (val < static_cast<float>(min_value)) {
+    return min_value;
+  }
+  return static_cast<Format>(val);
+}
+
+template<class Format>
+void FoldChannelsInternal(Format* buf_out,
+                          int sample_count,
+                          float volume,
+                          int channels,
+                          const int min_value,
+                          const int max_value) {
+  Format* buf_in = buf_out;
+  // mid_value is to adjust excess 128 notation in unsigned 8 bit samples
+  // to signed before adding channels.
+  const int mid_value = (max_value + min_value + 1) / 2;
+  const float kHalfPerceived = 0.707f; // 1/sqrt(2)
+  for (int i = 0; i < sample_count; ++i) {
+    float center_half = static_cast<float>(buf_in[kChannel_C] - mid_value) * kHalfPerceived;
+    float left = static_cast<float>(buf_in[kChannel_L] - mid_value);
+    float right = static_cast<float>(buf_in[kChannel_R] - mid_value);
+    float surround_left = static_cast<float>(buf_in[kChannel_SL] - mid_value);
+    float surround_right = static_cast<float>(buf_in[kChannel_SR] - mid_value);
+    buf_out[0] = ChannelsClampInternal(
+      (left + surround_left + center_half) * volume + mid_value,
+      static_cast<Format>(min_value), static_cast<Format>(max_value));
+    buf_out[1] = ChannelsClampInternal(
+      (right + surround_right + center_half) * volume + mid_value,
+      static_cast<Format>(min_value), static_cast<Format>(max_value));
+    buf_out += 2;
+    buf_in += channels;
+  }
+}
+
+bool FoldChannels(void* buf,
+                  size_t buflen,
+                  int channels,
+                  int bytes_per_sample,
+                  float volume) {
+  DCHECK(buf);
+  DCHECK(volume >= 0.0f && volume <= 1.0f);
+  if (channels >= 5 && channels <= 6 && bytes_per_sample > 0) {
+    int sample_count = buflen / (channels * bytes_per_sample);
+    if (bytes_per_sample == 1) {
+      FoldChannelsInternal(reinterpret_cast<uint8*>(buf),
+                           sample_count,
+                           volume,
+                           channels,
+                           0, 255);
+      return true;
+    } else if (bytes_per_sample == 2) {
+      FoldChannelsInternal(reinterpret_cast<int16*>(buf),
+                           sample_count,
+                           volume,
+                           channels,
+                           -32768,
+                           32767);
+      return true;
+    } else if (bytes_per_sample == 4) {
+      FoldChannelsInternal(reinterpret_cast<int32*>(buf),
+                           sample_count,
+                           volume,
+                           channels,
+                           0x80000000,
+                           0x7fffffff);
+      return true;
+    }
+  }
+  return false;
+}
+
 }  // namespace media