modules/rtp_rtcp/source/rtp_packetizer_av1.cc

/*
 *  Copyright (c) 2019 The WebRTC project authors. All Rights Reserved.
 *
 *  Use of this source code is governed by a BSD-style license
 *  that can be found in the LICENSE file in the root of the source
 *  tree. An additional intellectual property rights grant can be found
 *  in the file PATENTS.  All contributing project authors may
 *  be found in the AUTHORS file in the root of the source tree.
 */
#include "modules/rtp_rtcp/source/rtp_packetizer_av1.h"

#include <stddef.h>
#include <stdint.h>

#include <algorithm>

#include "api/array_view.h"
#include "api/video/video_frame_type.h"
#include "modules/rtp_rtcp/source/leb128.h"
#include "modules/rtp_rtcp/source/rtp_packet_to_send.h"
#include "rtc_base/byte_buffer.h"
#include "rtc_base/checks.h"
#include "rtc_base/logging.h"

namespace webrtc {
namespace {
constexpr int kAggregationHeaderSize = 1;
// when there are 3 or less OBU (fragments) in a packet, size of the last one
// can be omited.
constexpr int kMaxNumObusToOmitSize = 3;
constexpr uint8_t kObuSizePresentBit = 0b0'0000'010;
constexpr int kObuTypeSequenceHeader = 1;
constexpr int kObuTypeTemporalDelimiter = 2;
constexpr int kObuTypeTileList = 8;
constexpr int kObuTypePadding = 15;

// Overhead introduced by "even distribution" of packet sizes.
constexpr size_t kBytesOverheadEvenDistribution = 1;
// Experimentally determined minimum amount of potential savings per packet to
// make "even distribution" of packet sizes worthwhile.
constexpr size_t kMinBytesSavedPerPacketWithEvenDistribution = 10;

bool ObuHasExtension(uint8_t obu_header) {
  return obu_header & 0b0'0000'100;
}

bool ObuHasSize(uint8_t obu_header) {
  return obu_header & kObuSizePresentBit;
}

int ObuType(uint8_t obu_header) {
  return (obu_header & 0b0'1111'000) >> 3;
}

// Given `remaining_bytes` free bytes left in a packet, returns max size of an
// OBU fragment that can fit into the packet.
// i.e. MaxFragmentSize + Leb128Size(MaxFragmentSize) <= remaining_bytes.
int MaxFragmentSize(int remaining_bytes) {
  if (remaining_bytes <= 1) {
    return 0;
  }
  for (int i = 1;; ++i) {
    if (remaining_bytes < (1 << 7 * i) + i) {
      return remaining_bytes - i;
    }
  }
}

}  // namespace

RtpPacketizerAv1::RtpPacketizerAv1(rtc::ArrayView<const uint8_t> payload,
                                   RtpPacketizer::PayloadSizeLimits limits,
                                   VideoFrameType frame_type,
                                   bool is_last_frame_in_picture,
                                   bool even_distribution)
    : frame_type_(frame_type),
      obus_(ParseObus(payload)),
      packets_(even_distribution ? PacketizeAboutEqually(obus_, limits)
                                 : Packetize(obus_, limits)),
      is_last_frame_in_picture_(is_last_frame_in_picture) {}

std::vector<RtpPacketizerAv1::Obu> RtpPacketizerAv1::ParseObus(
    rtc::ArrayView<const uint8_t> payload) {
  std::vector<Obu> result;
  rtc::ByteBufferReader payload_reader(payload);
  while (payload_reader.Length() > 0) {
    Obu obu;
    payload_reader.ReadUInt8(&obu.header);
    obu.size = 1;
    if (ObuHasExtension(obu.header)) {
      if (payload_reader.Length() == 0) {
        RTC_DLOG(LS_ERROR) << "Malformed AV1 input: expected extension_header, "
                              "no more bytes in the buffer. Offset: "
                           << (payload.size() - payload_reader.Length());
        return {};
      }
      payload_reader.ReadUInt8(&obu.extension_header);
      ++obu.size;
    }
    if (!ObuHasSize(obu.header)) {
      obu.payload = rtc::MakeArrayView(
          reinterpret_cast<const uint8_t*>(payload_reader.Data()),
          payload_reader.Length());
      payload_reader.Consume(payload_reader.Length());
    } else {
      uint64_t size = 0;
      if (!payload_reader.ReadUVarint(&size) ||
          size > payload_reader.Length()) {
        RTC_DLOG(LS_ERROR) << "Malformed AV1 input: declared size " << size
                           << " is larger than remaining buffer size "
                           << payload_reader.Length();
        return {};
      }
      obu.payload = rtc::MakeArrayView(
          reinterpret_cast<const uint8_t*>(payload_reader.Data()), size);
      payload_reader.Consume(size);
    }
    obu.size += obu.payload.size();
    // Skip obus that shouldn't be transfered over rtp.
    int obu_type = ObuType(obu.header);
    if (obu_type != kObuTypeTemporalDelimiter &&  //
        obu_type != kObuTypeTileList &&           //
        obu_type != kObuTypePadding) {
      result.push_back(obu);
    }
  }
  return result;
}

int RtpPacketizerAv1::AdditionalBytesForPreviousObuElement(
    const Packet& packet) {
  if (packet.packet_size == 0) {
    // Packet is still empty => no last OBU element, no need to reserve space
    // for it.
    return 0;
  }
  if (packet.num_obu_elements > kMaxNumObusToOmitSize) {
    // There is so many obu elements in the packet, all of them must be
    // prepended with the length field. That imply space for the length of the
    // last obu element is already reserved.
    return 0;
  }
  // No space was reserved for length field of the last OBU element, but that
  // element becoming non-last, so it now requires explicit length field.
  // Calculate how many bytes are needed to store the length in leb128 format.
  return Leb128Size(packet.last_obu_size);
}

std::vector<RtpPacketizerAv1::Packet> RtpPacketizerAv1::Packetize(
    rtc::ArrayView<const Obu> obus,
    PayloadSizeLimits limits) {
  std::vector<Packet> packets;
  if (obus.empty()) {
    return packets;
  }
  // Ignore certian edge cases where packets should be very small. They are
  // inpractical but adds complexity to handle.
  if (limits.max_payload_len - limits.last_packet_reduction_len < 3 ||
      limits.max_payload_len - limits.first_packet_reduction_len < 3) {
    RTC_DLOG(LS_ERROR) << "Failed to packetize AV1 frame: requested packet "
                          "size is unreasonable small.";
    return packets;
  }
  // Aggregation header is present in all packets.
  limits.max_payload_len -= kAggregationHeaderSize;

  // Assemble packets. Push to current packet as much as it can hold before
  // considering next one. That would normally cause uneven distribution across
  // packets, specifically last one would be generally smaller.
  packets.emplace_back(/*first_obu_index=*/0);
  int packet_remaining_bytes =
      limits.max_payload_len - limits.first_packet_reduction_len;
  for (size_t obu_index = 0; obu_index < obus.size(); ++obu_index) {
    const bool is_last_obu = obu_index == obus.size() - 1;
    const Obu& obu = obus[obu_index];

    // Putting `obu` into the last packet would make last obu element stored in
    // that packet not last. All not last OBU elements must be prepend with the
    // element length. AdditionalBytesForPreviousObuElement calculates how many
    // bytes are needed to store that length.
    int previous_obu_extra_size =
        AdditionalBytesForPreviousObuElement(packets.back());
    int min_required_size =
        packets.back().num_obu_elements >= kMaxNumObusToOmitSize ? 2 : 1;
    if (packet_remaining_bytes < previous_obu_extra_size + min_required_size) {
      // Start a new packet.
      packets.emplace_back(/*first_obu_index=*/obu_index);
      packet_remaining_bytes = limits.max_payload_len;
      previous_obu_extra_size = 0;
    }
    Packet& packet = packets.back();
    // Start inserting current obu into the packet.
    packet.packet_size += previous_obu_extra_size;
    packet_remaining_bytes -= previous_obu_extra_size;
    packet.num_obu_elements++;

    bool must_write_obu_element_size =
        packet.num_obu_elements > kMaxNumObusToOmitSize;
    // Can fit all of the obu into the packet?
    int required_bytes = obu.size;
    if (must_write_obu_element_size) {
      required_bytes += Leb128Size(obu.size);
    }
    int available_bytes = packet_remaining_bytes;
    if (is_last_obu) {
      // If this packet would be the last packet, available size is smaller.
      if (packets.size() == 1) {
        available_bytes += limits.first_packet_reduction_len;
        available_bytes -= limits.single_packet_reduction_len;
      } else {
        available_bytes -= limits.last_packet_reduction_len;
      }
    }
    if (required_bytes <= available_bytes) {
      // Insert the obu into the packet unfragmented.
      packet.last_obu_size = obu.size;
      packet.packet_size += required_bytes;
      packet_remaining_bytes -= required_bytes;
      continue;
    }

    // Fragment the obu.
    int max_first_fragment_size = must_write_obu_element_size
                                      ? MaxFragmentSize(packet_remaining_bytes)
                                      : packet_remaining_bytes;
    // Because available_bytes might be different than
    // packet_remaining_bytes it might happen that max_first_fragment_size >=
    // obu.size. Also, since checks above verified `obu` should not be put
    // completely into the `packet`, leave at least 1 byte for later packet.
    int first_fragment_size = std::min(obu.size - 1, max_first_fragment_size);
    if (first_fragment_size == 0) {
      // Rather than writing 0-size element at the tail of the packet,
      // 'uninsert' the `obu` from the `packet`.
      packet.num_obu_elements--;
      packet.packet_size -= previous_obu_extra_size;
    } else {
      packet.packet_size += first_fragment_size;
      if (must_write_obu_element_size) {
        packet.packet_size += Leb128Size(first_fragment_size);
      }
      packet.last_obu_size = first_fragment_size;
    }

    // Add middle fragments that occupy all of the packet.
    // These are easy because
    // - one obu per packet imply no need to store the size of the obu.
    // - this packets are nor the first nor the last packets of the frame, so
    // packet capacity is always limits.max_payload_len.
    int obu_offset;
    for (obu_offset = first_fragment_size;
         obu_offset + limits.max_payload_len < obu.size;
         obu_offset += limits.max_payload_len) {
      packets.emplace_back(/*first_obu_index=*/obu_index);
      Packet& packet = packets.back();
      packet.num_obu_elements = 1;
      packet.first_obu_offset = obu_offset;
      int middle_fragment_size = limits.max_payload_len;
      packet.last_obu_size = middle_fragment_size;
      packet.packet_size = middle_fragment_size;
    }

    // Add the last fragment of the obu.
    int last_fragment_size = obu.size - obu_offset;
    // Check for corner case where last fragment of the last obu is too large
    // to fit into last packet, but may fully fit into semi-last packet.
    if (is_last_obu &&
        last_fragment_size >
            limits.max_payload_len - limits.last_packet_reduction_len) {
      // Split last fragments into two.
      RTC_DCHECK_GE(last_fragment_size, 2);
      // Try to even packet sizes rather than payload sizes across the last
      // two packets.
      int semi_last_fragment_size =
          (last_fragment_size + limits.last_packet_reduction_len) / 2;
      // But leave at least one payload byte for the last packet to avoid
      // weird scenarios where size of the fragment is zero and rtp payload has
      // nothing except for an aggregation header.
      if (semi_last_fragment_size >= last_fragment_size) {
        semi_last_fragment_size = last_fragment_size - 1;
      }
      last_fragment_size -= semi_last_fragment_size;

      packets.emplace_back(/*first_obu_index=*/obu_index);
      Packet& packet = packets.back();
      packet.num_obu_elements = 1;
      packet.first_obu_offset = obu_offset;
      packet.last_obu_size = semi_last_fragment_size;
      packet.packet_size = semi_last_fragment_size;
      obu_offset += semi_last_fragment_size;
    }
    packets.emplace_back(/*first_obu_index=*/obu_index);
    Packet& last_packet = packets.back();
    last_packet.num_obu_elements = 1;
    last_packet.first_obu_offset = obu_offset;
    last_packet.last_obu_size = last_fragment_size;
    last_packet.packet_size = last_fragment_size;
    packet_remaining_bytes = limits.max_payload_len - last_fragment_size;
  }
  return packets;
}

std::vector<RtpPacketizerAv1::Packet> RtpPacketizerAv1::PacketizeAboutEqually(
    rtc::ArrayView<const Obu> obus,
    PayloadSizeLimits limits) {
  std::vector<Packet> packets = Packetize(obus, limits);
  if (packets.size() <= 1) {
    return packets;
  }
  size_t packet_index = 0;
  size_t packet_size_left_unused = 0;
  for (const auto& packet : packets) {
    // Every packet has to have an aggregation header of size
    // kAggregationHeaderSize.
    int available_bytes = limits.max_payload_len - kAggregationHeaderSize;

    if (packet_index == 0) {
      available_bytes -= limits.first_packet_reduction_len;
    } else if (packet_index == packets.size() - 1) {
      available_bytes -= limits.last_packet_reduction_len;
    }
    if (available_bytes >= packet.packet_size) {
      packet_size_left_unused += (available_bytes - packet.packet_size);
    }
    packet_index++;
  }
  if (packet_size_left_unused >
      packets.size() * kMinBytesSavedPerPacketWithEvenDistribution) {
    // Calculate new limits with a reduced max_payload_len.
    size_t size_reduction = packet_size_left_unused / packets.size();
    RTC_DCHECK_GT(limits.max_payload_len, size_reduction);
    RTC_DCHECK_GT(size_reduction, kBytesOverheadEvenDistribution);
    limits.max_payload_len -= (size_reduction - kBytesOverheadEvenDistribution);
    if (limits.max_payload_len - limits.last_packet_reduction_len < 3 ||
        limits.max_payload_len - limits.first_packet_reduction_len < 3) {
      return packets;
    }
    std::vector<Packet> packets_even = Packetize(obus, limits);
    // The number of packets should not change in the second pass. If it does,
    // conservatively return the original packets.
    if (packets_even.size() == packets.size()) {
      return packets_even;
    }
    RTC_LOG(LS_WARNING) << "AV1 even distribution caused a regression in "
                           "number of packets from "
                        << packets.size() << " to " << packets_even.size();
  }
  return packets;
}

uint8_t RtpPacketizerAv1::AggregationHeader() const {
  const Packet& packet = packets_[packet_index_];
  uint8_t aggregation_header = 0;

  // Set Z flag: first obu element is continuation of the previous OBU.
  bool first_obu_element_is_fragment = packet.first_obu_offset > 0;
  if (first_obu_element_is_fragment)
    aggregation_header |= (1 << 7);

  // Set Y flag: last obu element will be continuated in the next packet.
  int last_obu_offset =
      packet.num_obu_elements == 1 ? packet.first_obu_offset : 0;
  bool last_obu_is_fragment =
      last_obu_offset + packet.last_obu_size <
      obus_[packet.first_obu + packet.num_obu_elements - 1].size;
  if (last_obu_is_fragment)
    aggregation_header |= (1 << 6);

  // Set W field: number of obu elements in the packet (when not too large).
  if (packet.num_obu_elements <= kMaxNumObusToOmitSize)
    aggregation_header |= packet.num_obu_elements << 4;

  // Set N flag: beginning of a new coded video sequence.
  // Encoder may produce key frame without a sequence header, thus double check
  // incoming frame includes the sequence header. Since Temporal delimiter is
  // already filtered out, sequence header should be the first obu when present.
  if (frame_type_ == VideoFrameType::kVideoFrameKey && packet_index_ == 0 &&
      ObuType(obus_.front().header) == kObuTypeSequenceHeader) {
    aggregation_header |= (1 << 3);
  }
  return aggregation_header;
}

bool RtpPacketizerAv1::NextPacket(RtpPacketToSend* packet) {
  if (packet_index_ >= packets_.size()) {
    return false;
  }
  const Packet& next_packet = packets_[packet_index_];

  RTC_DCHECK_GT(next_packet.num_obu_elements, 0);
  RTC_DCHECK_LT(next_packet.first_obu_offset,
                obus_[next_packet.first_obu].size);
  RTC_DCHECK_LE(
      next_packet.last_obu_size,
      obus_[next_packet.first_obu + next_packet.num_obu_elements - 1].size);

  uint8_t* const rtp_payload =
      packet->AllocatePayload(kAggregationHeaderSize + next_packet.packet_size);
  uint8_t* write_at = rtp_payload;

  *write_at++ = AggregationHeader();

  int obu_offset = next_packet.first_obu_offset;
  // Store all OBU elements except the last one.
  for (int i = 0; i < next_packet.num_obu_elements - 1; ++i) {
    const Obu& obu = obus_[next_packet.first_obu + i];
    size_t fragment_size = obu.size - obu_offset;
    write_at += WriteLeb128(fragment_size, write_at);
    if (obu_offset == 0) {
      *write_at++ = obu.header & ~kObuSizePresentBit;
    }
    if (obu_offset <= 1 && ObuHasExtension(obu.header)) {
      *write_at++ = obu.extension_header;
    }
    int payload_offset =
        std::max(0, obu_offset - (ObuHasExtension(obu.header) ? 2 : 1));
    size_t payload_size = obu.payload.size() - payload_offset;
    if (!obu.payload.empty() && payload_size > 0) {
      memcpy(write_at, obu.payload.data() + payload_offset, payload_size);
    }
    write_at += payload_size;
    // All obus are stored from the beginning, except, may be, the first one.
    obu_offset = 0;
  }
  // Store the last OBU element.
  const Obu& last_obu =
      obus_[next_packet.first_obu + next_packet.num_obu_elements - 1];
  int fragment_size = next_packet.last_obu_size;
  RTC_DCHECK_GT(fragment_size, 0);
  if (next_packet.num_obu_elements > kMaxNumObusToOmitSize) {
    write_at += WriteLeb128(fragment_size, write_at);
  }
  if (obu_offset == 0 && fragment_size > 0) {
    *write_at++ = last_obu.header & ~kObuSizePresentBit;
    --fragment_size;
  }
  if (obu_offset <= 1 && ObuHasExtension(last_obu.header) &&
      fragment_size > 0) {
    *write_at++ = last_obu.extension_header;
    --fragment_size;
  }
  RTC_DCHECK_EQ(write_at - rtp_payload + fragment_size,
                kAggregationHeaderSize + next_packet.packet_size);
  int payload_offset =
      std::max(0, obu_offset - (ObuHasExtension(last_obu.header) ? 2 : 1));
  memcpy(write_at, last_obu.payload.data() + payload_offset, fragment_size);
  write_at += fragment_size;

  RTC_DCHECK_EQ(write_at - rtp_payload,
                kAggregationHeaderSize + next_packet.packet_size);

  ++packet_index_;
  bool is_last_packet_in_frame = packet_index_ == packets_.size();
  packet->SetMarker(is_last_packet_in_frame && is_last_frame_in_picture_);
  return true;
}

}  // namespace webrtc