diff -r 000000000000 -r 6474c204b198 content/media/AudioChannelFormat.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/content/media/AudioChannelFormat.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,72 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+#ifndef MOZILLA_AUDIOCHANNELFORMAT_H_
+#define MOZILLA_AUDIOCHANNELFORMAT_H_
+
+#include <stdint.h>
+
+#include "nsTArrayForwardDeclare.h"
+
+namespace mozilla {
+
+/*
+ * This file provides utilities for upmixing and downmixing channels.
+ *
+ * The channel layouts, upmixing and downmixing are consistent with the
+ * Web Audio spec.
+ *
+ * Channel layouts for up to 6 channels:
+ *   mono   { M }
+ *   stereo { L, R }
+ *          { L, R, C }
+ *   quad   { L, R, SL, SR }
+ *          { L, R, C, SL, SR }
+ *   5.1    { L, R, C, LFE, SL, SR }
+ *
+ * Only 1, 2, 4 and 6 are currently defined in Web Audio.
+ */
+
+/**
+ * Return a channel count whose channel layout includes all the channels from
+ * aChannels1 and aChannels2.
+ */
+uint32_t
+GetAudioChannelsSuperset(uint32_t aChannels1, uint32_t aChannels2);
+
+/**
+ * Given an array of input channel data, and an output channel count,
+ * replaces the array with an array of upmixed channels.
+ * This shuffles the array and may set some channel buffers to aZeroChannel.
+ * Don't call this with input count >= output count.
+ * This may return *more* channels than requested. In that case, downmixing
+ * is required to to get to aOutputChannelCount. (This is how we handle
+ * odd cases like 3 -> 4 upmixing.)
+ * If aChannelArray.Length() was the input to one of a series of
+ * GetAudioChannelsSuperset calls resulting in aOutputChannelCount,
+ * no downmixing will be required.
+ */
+void
+AudioChannelsUpMix(nsTArray<const void*>* aChannelArray,
+                   uint32_t aOutputChannelCount,
+                   const void* aZeroChannel);
+
+/**
+ * Given an array of input channels (which must be float format!),
+ * downmix to aOutputChannelCount, and copy the results to the
+ * channel buffers in aOutputChannels.
+ * Don't call this with input count <= output count.
+ */
+void
+AudioChannelsDownMix(const nsTArray<const void*>& aChannelArray,
+                     float** aOutputChannels,
+                     uint32_t aOutputChannelCount,
+                     uint32_t aDuration);
+
+// A version of AudioChannelsDownMix that downmixes int16_ts may be required.
+
+} // namespace mozilla
+
+#endif /* MOZILLA_AUDIOCHANNELFORMAT_H_ */