探索C++与Live555实现RTSP服务器的艺术（三）

// File: liveMedia/H264VideoRTPSink.cpp
// Repository: https://github.com/rgaufert/live555
void H264VideoRTPSink::doSpecialFrameHandling(unsigned fragmentationOffset,
                unsigned char* frameStart,
                unsigned numBytesInFrame,
                struct timeval framePresentationTime,
                unsigned numRemainingBytes) {
  // Set the RTP 'M' (marker) bit iff
  // 1/ The most recently delivered fragment was the end of (or the only fragment of) an NAL unit, and
  // 2/ This NAL unit was the last NAL unit of an 'access unit' (i.e. video frame).
  // (RTP 'M' bit is set for video frames only; not for audio frames)
  // Begin by checking condition 1/ above:
  if (numRemainingBytes == 0) { // This fragment ends the current NAL unit
    // Check for condition 2/ above:
    // This requires parsing the NAL unit's header byte, and the first byte of the NAL unit payload:
    if (numBytesInFrame > 0) {
      unsigned char nal_unit_type = (frameStart[0]&0x1F);
      if (nal_unit_type == 24 || nal_unit_type == 25 || nal_unit_type == 26 || nal_unit_type == 27) {
  // This is a STAP-A, STAP-B, MTAP16, or MTAP24 NAL unit.  Check the first NAL unit within it:
  if (numBytesInFrame > 1) {
    nal_unit_type = (frameStart[1]&0x1F);
  } else {
    // This NAL unit is unusable; we shouldn't have sent it
    nal_unit_type = 0;
  }
      }
      if (nal_unit_type == 6 || nal_unit_type == 7 || nal_unit_type == 8) {
  // This NAL unit is an SPS or PPS or a prefix NAL unit; next is a IDR picture
  fCurrentNALUnitEndsAccessUnit = True;
      } else if (nal_unit_type <= 5 && nal_unit_type > 0) {
  // This NAL unit is a VCL NAL unit (slice_layer_without_partitioning_rbsp)
  // We need to examine the first byte of the NAL unit's RBSP payload - the "slice header":
  unsigned char* sliceHeader = &frameStart[1]; // if the NAL unit is not fragmented
  unsigned numBytesInNALUnitPayload = numBytesInFrame - 1;
  // Begin by making sure we have at least one byte of the NAL unit payload data:
  if (numBytesInNALUnitPayload > 0) {
    if (nal_unit_type == 28 || nal_unit_type == 29) {
      // This is a FU-A or FU-B NAL unit.  We need to find the start of the NAL unit's payload data:
      if (numBytesInFrame > 2) {
        sliceHeader = &frameStart[2];
        numBytesInNALUnitPayload = numBytesInFrame - 2;
      } else {
        // This NAL unit is unusable; we shouldn't have sent it
        numBytesInNALUnitPayload = 0;
      }
    }
    if (numBytesInNALUnitPayload > 0) {
      unsigned char slice_type = sliceHeader[0]&0x1F;
     fCurrentNALUnitEndsAccessUnit = False;
      if (fLastNALUnitEndsAccessUnit) {
        // This is the start of a new access unit
        if (fNextDeliverPresentationTime.tv_sec != 0 || fNextDeliverPresentationTime.tv_usec != 0) {
    // We have a saved 'next presentation time'.  Deliver the most recent frame with that presentation time now, before starting the new access unit
    if (fNumSavedNALUnits > 0) {
      doSpecialFrameHandling1(fNumSavedNALUnits, fSavedNALUnits, fSavedNALUnitSizes, fSavedNALUnitPresentationTimes, fNextDeliverPresentationTime);
      delete[] fSavedNALUnits;
      delete[] fSavedNALUnitSizes;
      delete[] fSavedNALUnitPresentationTimes;
    }
        }
      }
      //保存I帧
      if (nal_unit_type == 5) {
        saveFrameParameters(slice_type, fragmentationOffset, frameStart, numBytesInFrame, framePresentationTime);
      }
      fNextDeliverPresentationTime = framePresentationTime;
      fNextDeliverPresentationTime.tv_sec += fMaxPacketAge;
      fLastNALUnitEndsAccessUnit = fCurrentNALUnitEndsAccessUnit;
    }
  }
      }
    }
  }
}

// 这个函数在特定的时间间隔后被调用
void check_for_frame_to_send() {
    // 检查是否有帧需要发送
    if (!frames.empty()) {
        // 获取第一个帧
        auto frame = frames.front();
        // 检查这个帧是否是关键帧（GOP）
        if (frame->key_frame) {
            // 发送这个帧
            send_frame(frame);
            // 从队列中移除这个帧
            frames.pop();
        }
    }
}

// 假设我们有一个RTP包对象rtpPacket
RTPPacket rtpPacket;
// 获取当前的时间戳
uint32_t currentTimestamp = rtpPacket.getTimestamp();
// 假设我们希望实现2倍速播放，那么我们需要将时间戳减半
uint32_t newTimestamp = currentTimestamp / 2;
// 设置新的时间戳
rtpPacket.setTimestamp(newTimestamp);