Mix to an intermediate buffer.

services/media/audio/platform/generic/output_formatter.h

+// Copyright 2015 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.
+
+#ifndef SERVICES_MEDIA_AUDIO_PLATFORM_GENERIC_OUTPUT_FORMATTER_H_
+#define SERVICES_MEDIA_AUDIO_PLATFORM_GENERIC_OUTPUT_FORMATTER_H_
+
+#include <memory>
+
+#include "mojo/services/media/common/interfaces/media_types.mojom.h"
+
+namespace mojo {
+namespace media {
+namespace audio {
+
+class OutputFormatter;
+using OutputFormatterPtr = std::unique_ptr<OutputFormatter>;
+
+class OutputFormatter {
+ public:
+  static OutputFormatterPtr Select(
+      const LpcmMediaTypeDetailsPtr& output_format);
+
+  ~OutputFormatter();
+
+  /**
+   * Take frames of audio from the source intermediate buffer and convert them
+   * to the proper sample format for the output buffer, clipping the audio as
+   * needed in the process.
+   *
+   * @note It is assumed that the source intermediate mixing buffer has the same
+   * number of channels and channel ordering as the output buffer.
+   *
+   * @param source A pointer to the normalized frames of audio to use as the
+   * source.
+   *
+   * @param dest A pointer to the destination buffer whose frames match the
+   * format described by output_format during the call to Select.
+   *
+   * @param frames The number of frames to produce.
+   */
+  virtual void ProduceOutput(const int32_t* source,
+                             void*          dest,
+                             uint32_t       frames) const = 0;
+
+  /**
+   * Fill a destination buffer with silence.
+   *
+   * @param dest A pointer to the destination buffer whose frames match the
+   * format described by output_format during the call to Select.
+   *
+   * @param frames The number of frames to produce.
+   */
+  virtual void FillWithSilence(void* dest, uint32_t frames) const = 0;
+
+  const LpcmMediaTypeDetailsPtr& format() const { return format_; }
+  uint32_t channels() const { return channels_; }
+  uint32_t bytes_per_sample() const { return bytes_per_sample_; }
+  uint32_t bytes_per_frame() const { return bytes_per_frame_; }
+
+ protected:
+  OutputFormatter(const LpcmMediaTypeDetailsPtr& output_format,
+                  uint32_t bytes_per_sample,
+                  uint32_t channels);
+
+  LpcmMediaTypeDetailsPtr format_;
+  uint32_t channels_ = 0;
+  uint32_t bytes_per_sample_  = 0;
+  uint32_t bytes_per_frame_ = 0;
+};
+
+}  // namespace audio
+}  // namespace media
+}  // namespace mojo
+
+#endif  // SERVICES_MEDIA_AUDIO_PLATFORM_GENERIC_OUTPUT_FORMATTER_H_