media/gpu/gpu_video_encode_accelerator_helpers.cc

// Copyright 2018 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.

#include "media/gpu/gpu_video_encode_accelerator_helpers.h"

#include <algorithm>

#include "base/logging.h"

namespace media {
namespace {
// The maximum size for output buffer, which is chosen empirically for
// 1080p video.
constexpr size_t kMaxBitstreamBufferSizeInBytes = 2 * 1024 * 1024;  // 2MB

// The mapping from resolution, bitrate, framerate to the bitstream buffer size.
struct BitstreamBufferSizeInfo {
  int coded_size_area;
  uint32_t bitrate_in_bps;
  uint32_t framerate;
  uint32_t buffer_size_in_bytes;
};

// The bitstream buffer size for each resolution. The table must be sorted in
// increasing order by the resolution. The value is decided by measuring the
// biggest buffer size, and then double the size as margin. (crbug.com/889739)
// TODO(akahuang): Measure the buffer size when we support 4K encoding.
constexpr BitstreamBufferSizeInfo kBitstreamBufferSizeTable[] = {
    {320 * 180, 100000, 30, 15000},
    {640 * 360, 500000, 30, 52000},
    {1280 * 720, 1200000, 30, 110000},
    {1920 * 1080, 4000000, 30, 380000},
};

}  // namespace

size_t GetEncodeBitstreamBufferSize(const gfx::Size& size,
                                    uint32_t bitrate,
                                    uint32_t framerate) {
  DCHECK_NE(framerate, 0u);
  for (auto& data : kBitstreamBufferSizeTable) {
    if (size.GetArea() <= data.coded_size_area) {
      // The buffer size is proportional to (bitrate / framerate), but linear
      // interpolation for smaller ratio is not enough. Therefore we only use
      // linear extrapolation for larger ratio.
      double ratio = std::max(
          1.0f * (bitrate / framerate) / (data.bitrate_in_bps / data.framerate),
          1.0f);
      return std::min(static_cast<size_t>(data.buffer_size_in_bytes * ratio),
                      kMaxBitstreamBufferSizeInBytes);
    }
  }
  return kMaxBitstreamBufferSizeInBytes;
}

// Get the maximum output bitstream buffer size. Because we don't change
// the buffer size when we update bitrate and framerate, we have to calculate
// the buffer size by the maximum bitrate.
// However, the maximum bitrate for intel chipset is 40Mbps. The buffer size
// calculated with this bitrate is always larger than 2MB. Therefore we just
// return the value.
// TODO(crbug.com/889739): Deprecate this function after we can update the
// buffer size while requesting new bitrate and framerate.
size_t GetEncodeBitstreamBufferSize() {
  // GetEncodeBitstreamBufferSize(size, 40000000, 30)
  return kMaxBitstreamBufferSizeInBytes;
}

}  // namespace media