live555学习笔记12－h264 rtp包的时间戳

十二 h264 rtp包的时间戳

这次我们一起来分析一下live555中是怎样为rtp包打时间戳的．就以h264为例吧．

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).
 if (fOurFragmenter != NULL) {
  H264VideoStreamFramer* framerSource = (H264VideoStreamFramer*) (fOurFragmenter->inputSource());
  // This relies on our fragmenter's source being a "H264VideoStreamFramer".
  if (fOurFragmenter->lastFragmentCompletedNALUnit()
    && framerSource != NULL && framerSource->pictureEndMarker()) {
   setMarkerBit();
   framerSource->pictureEndMarker() = False;
  }
 }

 setTimestamp(framePresentationTime);
}

函数中先检测是否是一个帧的最后一个包，如果是，打上'M'标记．然后就设置时间戳．这个间戳是哪来的呢？需看函数doSpecialFrameHandling()是被谁调用的，经查找，是被MultiFramedRTPSink::afterGettingFrame1()调用的．MultiFramedRTPSink::afterGettingFrame1()的参数presentationTime传给了doSpecialFrameHandling()．MultiFramedRTPSink::afterGettingFrame1()是在调用source的getNextFrame()时传给了source．传给哪个source呢？传给了H264FUAFragmenter,还记得暗渡陈仓那件事吗？所以H264FUAFragmenter在获取一个nal unit后调用了MultiFramedRTPSink::afterGettingFrame1()．也就是H264FUAFragmenter::afterGettingFrame1()调用了MultiFramedRTPSink::afterGettingFrame1()．
H264FUAFragmenter::afterGettingFrame1()是被它自己的source的afterGettingFrame1()调用的．H264FUAFragmenter的source是谁呢？是H264VideoStreamFramer，是在暗渡陈仓时传给H264FUAFragmenter的构造函数的．
H264VideoStreamFramer的afterGettingFrame1()是没有的，代替之的是MPEGVideoStreamFramer::continueReadProcessin()．它被MPEGVideoStreamParser暗中传给了StreamParser的构造函数．所以StreamParser在分析完一帧（或nal unit)之后，调用的就是MPEGVideoStreamFramer::continueReadProcessin()．以下即是证明：(补充：以下函数并不是在parser分析完一帧（或nal unit）之后调用，而是parser利用ByteStreamFileSuorce获取到原始数据后调用，然后MPEGVideoStreamFramer再调用Parser的parser()函数分析原始数据)

void StreamParser::afterGettingBytes(void* clientData,
  unsigned numBytesRead,
  unsigned /*numTruncatedBytes*/,
  struct timeval presentationTime,
  unsigned /*durationInMicroseconds*/)
{
 StreamParser* parser = (StreamParser*) clientData;
 if (parser != NULL)
  parser->afterGettingBytes1(numBytesRead, presentationTime);
}

void StreamParser::afterGettingBytes1(unsigned numBytesRead,
  struct timeval presentationTime)
{
 // Sanity check: Make sure we didn't get too many bytes for our bank:
 if (fTotNumValidBytes + numBytesRead > BANK_SIZE) {
  fInputSource->envir()
    << "StreamParser::afterGettingBytes() warning: read "
    << numBytesRead << " bytes; expected no more than "
    << BANK_SIZE - fTotNumValidBytes << "\n";
 }

 fLastSeenPresentationTime = presentationTime;

 unsigned char* ptr = &curBank()[fTotNumValidBytes];
 fTotNumValidBytes += numBytesRead;

 // Continue our original calling source where it left off:
 restoreSavedParserState();
 // Sigh... this is a crock; things would have been a lot simpler
 // here if we were using threads, with synchronous I/O...
 fClientContinueFunc(fClientContinueClientData, ptr, numBytesRead,
   presentationTime);
}

fClientContinueFunc就是MPEGVideoStreamFramer::continueReadProcessin()，而且我们看到时间戳被传入fClientContinueFunc．
然而，MPEGVideoStreamFramer::continueReadProcessin()中跟本就不理这个时间戳，因为这个时间戳是ByteStreamFileSource计算出来的，它跟本就不可能正确．

void MPEGVideoStreamFramer::continueReadProcessing(void* clientData,
  unsigned char* /*ptr*/,
  unsigned /*size*/,
  struct timeval /*presentationTime*/)
{
 MPEGVideoStreamFramer* framer = (MPEGVideoStreamFramer*) clientData;
 framer->continueReadProcessing();
}

看来真正的时间戳是在MPEGVideoStreamFramer中计算的，但是H264VideoStreamFramer并没有用到MPEGVideoStreamFramer中那些计算时间戳的函数，而是另外计算，其实H264VideoStreamFramer也没有自己去计算，而是利用H264VideoStreamParser计算的．是在哪个函数中呢？在parser()中！

unsigned H264VideoStreamParser::parse()
{
 try {
  // The stream must start with a 0x00000001:
  if (!fHaveSeenFirstStartCode) {
   // Skip over any input bytes that precede the first 0x00000001:
   u_int32_t first4Bytes;
   while ((first4Bytes = test4Bytes()) != 0x00000001) {
    get1Byte();
    setParseState(); // ensures that we progress over bad data
   }
   skipBytes(4); // skip this initial code

   setParseState();
   fHaveSeenFirstStartCode = True; // from now on
  }

  if (fOutputStartCodeSize > 0) {
   // Include a start code in the output:
   save4Bytes(0x00000001);
  }

  // Then save everything up until the next 0x00000001 (4 bytes) or 0x000001 (3 bytes), or we hit EOF.
  // Also make note of the first byte, because it contains the "nal_unit_type":
  u_int8_t firstByte;
  if (haveSeenEOF()) {
   // We hit EOF the last time that we tried to parse this data,
   // so we know that the remaining unparsed data forms a complete NAL unit:
   unsigned remainingDataSize = totNumValidBytes() - curOffset();
   if (remainingDataSize == 0)
    (void) get1Byte(); // forces another read, which will cause EOF to get handled for real this time
   if (remainingDataSize == 0)
    return 0;
   firstByte = get1Byte();
   saveByte(firstByte);

   while (--remainingDataSize > 0) {
    saveByte(get1Byte());
   }
  } else {
   u_int32_t next4Bytes = test4Bytes();
   firstByte = next4Bytes >> 24;
   while (next4Bytes != 0x00000001
     && (next4Bytes & 0xFFFFFF00) != 0x00000100) {
    // We save at least some of "next4Bytes".
    if ((unsigned) (next4Bytes & 0xFF) > 1) {
     // Common case: 0x00000001 or 0x000001 definitely doesn't begin anywhere in "next4Bytes", so we save all of it:
     save4Bytes(next4Bytes);
     skipBytes(4);
    } else {
     // Save the first byte, and continue testing the rest:
     saveByte(next4Bytes >> 24);
     skipBytes(1);
    }
    next4Bytes = test4Bytes();
   }
   // Assert: next4Bytes starts with 0x00000001 or 0x000001, and we've saved all previous bytes (forming a complete NAL unit).
   // Skip over these remaining bytes, up until the start of the next NAL unit:
   if (next4Bytes == 0x00000001) {
    skipBytes(4);
   } else {
    skipBytes(3);
   }
  }

  u_int8_t nal_ref_idc = (firstByte & 0x60) >> 5;
  u_int8_t nal_unit_type = firstByte & 0x1F;

  switch (nal_unit_type) {
  case 6: { // Supplemental enhancement information (SEI)
   analyze_sei_data();
   // Later, perhaps adjust "fPresentationTime" if we saw a "pic_timing" SEI payload??? #####
   break;
  }
  case 7: { // Sequence parameter set
   // First, save a copy of this NAL unit, in case the downstream object wants to see it:
   usingSource()->saveCopyOfSPS(fStartOfFrame + fOutputStartCodeSize,
     fTo - fStartOfFrame - fOutputStartCodeSize);

   // Parse this NAL unit to check whether frame rate information is present:
   unsigned num_units_in_tick, time_scale, fixed_frame_rate_flag;
   analyze_seq_parameter_set_data(num_units_in_tick, time_scale,
     fixed_frame_rate_flag);
   if (time_scale > 0 && num_units_in_tick > 0) {
    usingSource()->fFrameRate = time_scale
      / (2.0 * num_units_in_tick);
   } else {
   }
   break;
  }
  case 8: { // Picture parameter set
   // Save a copy of this NAL unit, in case the downstream object wants to see it:
   usingSource()->saveCopyOfPPS(fStartOfFrame + fOutputStartCodeSize,
     fTo - fStartOfFrame - fOutputStartCodeSize);
  }
  }

  //更新时间戳变量
  usingSource()->setPresentationTime();

  // If this NAL unit is a VCL NAL unit, we also scan the start of the next NAL unit, to determine whether this NAL unit
  // ends the current 'access unit'.  We need this information to figure out when to increment "fPresentationTime".
  // (RTP streamers also need to know this in order to figure out whether or not to set the "M" bit.)
  Boolean thisNALUnitEndsAccessUnit = False; // until we learn otherwise
  if (haveSeenEOF()) {
   // There is no next NAL unit, so we assume that this one ends the current 'access unit':
   thisNALUnitEndsAccessUnit = True;
  } else {
   Boolean const isVCL = nal_unit_type <= 5 && nal_unit_type > 0; // Would need to include type 20 for SVC and MVC #####
   if (isVCL) {
    u_int32_t first4BytesOfNextNALUnit = test4Bytes();
    u_int8_t firstByteOfNextNALUnit = first4BytesOfNextNALUnit
      >> 24;
    u_int8_t next_nal_ref_idc = (firstByteOfNextNALUnit & 0x60)
      >> 5;
    u_int8_t next_nal_unit_type = firstByteOfNextNALUnit & 0x1F;
    if (next_nal_unit_type >= 6) {
     // The next NAL unit is not a VCL; therefore, we assume that this NAL unit ends the current 'access unit':
     thisNALUnitEndsAccessUnit = True;
    } else {
     // The next NAL unit is also a VLC.  We need to examine it a little to figure out if it's a different 'access unit'.
     // (We use many of the criteria described in section 7.4.1.2.4 of the H.264 specification.)
     Boolean IdrPicFlag = nal_unit_type == 5;
     Boolean next_IdrPicFlag = next_nal_unit_type == 5;
     if (next_IdrPicFlag != IdrPicFlag) {
      // IdrPicFlag differs in value
      thisNALUnitEndsAccessUnit = True;
     } else if (next_nal_ref_idc != nal_ref_idc
       && next_nal_ref_idc * nal_ref_idc == 0) {
      // nal_ref_idc differs in value with one of the nal_ref_idc values being equal to 0
      thisNALUnitEndsAccessUnit = True;
     } else if ((nal_unit_type == 1 || nal_unit_type == 2
       || nal_unit_type == 5)
       && (next_nal_unit_type == 1
         || next_nal_unit_type == 2
         || next_nal_unit_type == 5)) {
      // Both this and the next NAL units begin with a "slice_header".
      // Parse this (for each), to get parameters that we can compare:

      // Current NAL unit's "slice_header":
      unsigned frame_num, pic_parameter_set_id, idr_pic_id;
      Boolean field_pic_flag, bottom_field_flag;
      analyze_slice_header(
        fStartOfFrame + fOutputStartCodeSize, fTo,
        nal_unit_type, frame_num, pic_parameter_set_id,
        idr_pic_id, field_pic_flag, bottom_field_flag);

      // Next NAL unit's "slice_header":
      u_int8_t next_slice_header[NUM_NEXT_SLICE_HEADER_BYTES_TO_ANALYZE];
      testBytes(next_slice_header, sizeof next_slice_header);
      unsigned next_frame_num, next_pic_parameter_set_id,
        next_idr_pic_id;
      Boolean next_field_pic_flag, next_bottom_field_flag;
      analyze_slice_header(next_slice_header,
        &next_slice_header[sizeof next_slice_header],
        next_nal_unit_type, next_frame_num,
        next_pic_parameter_set_id, next_idr_pic_id,
        next_field_pic_flag, next_bottom_field_flag);

      if (next_frame_num != frame_num) {
       // frame_num differs in value
       thisNALUnitEndsAccessUnit = True;
      } else if (next_pic_parameter_set_id
        != pic_parameter_set_id) {
       // pic_parameter_set_id differs in value
       thisNALUnitEndsAccessUnit = True;
      } else if (next_field_pic_flag != field_pic_flag) {
       // field_pic_flag differs in value
       thisNALUnitEndsAccessUnit = True;
      } else if (next_bottom_field_flag
        != bottom_field_flag) {
       // bottom_field_flag differs in value
       thisNALUnitEndsAccessUnit = True;
      } else if (next_IdrPicFlag == 1
        && next_idr_pic_id != idr_pic_id) {
       // IdrPicFlag is equal to 1 for both and idr_pic_id differs in value
       // Note: We already know that IdrPicFlag is the same for both.
       thisNALUnitEndsAccessUnit = True;
      }
     }
    }
   }
  }

  //注意！注意！注意！此处计算时间戳！！
  if (thisNALUnitEndsAccessUnit) {
   usingSource()->fPictureEndMarker = True;
   ++usingSource()->fPictureCount;

   // Note that the presentation time for the next NAL unit will be different:
   struct timeval& nextPT = usingSource()->fNextPresentationTime; // alias
   nextPT = usingSource()->fPresentationTime;
   double nextFraction = nextPT.tv_usec / 1000000.0
     + 1 / usingSource()->fFrameRate;
   unsigned nextSecsIncrement = (long) nextFraction;
   nextPT.tv_sec += (long) nextSecsIncrement;
   nextPT.tv_usec = (long) ((nextFraction - nextSecsIncrement)
     * 1000000);
  }
  setParseState();

  return curFrameSize();
 } catch (int /*e*/) {
  return 0; // the parsing got interrupted
 }
}

每当开始一个新帧时，计算新的时间戳．时间戳保存在fNextPresentationTime中，在usingSource()->setPresentationTime()中传给fPresentationTime.
哇，我们看到live555的类之间调用关系曲折复杂，的确有点不易维护啊！同时我写的也不够清析，自己看着都晕，如果把你搞晕了，这很正常哦！

fPresentationTime是64位的时间，经convertToRTPTimestamp转换为32的rtp时间戳，见函数：

u_int32_t RTPSink::convertToRTPTimestamp(struct timeval tv)
{
 // Begin by converting from "struct timeval" units to RTP timestamp units:
 u_int32_t timestampIncrement = (fTimestampFrequency * tv.tv_sec);
 timestampIncrement += (u_int32_t)(
   (2.0 * fTimestampFrequency * tv.tv_usec + 1000000.0) / 2000000);
 // note: rounding

 // Then add this to our 'timestamp base':
 if (fNextTimestampHasBeenPreset) {
  // Make the returned timestamp the same as the current "fTimestampBase",
  // so that timestamps begin with the value that was previously preset:
  fTimestampBase -= timestampIncrement;
  fNextTimestampHasBeenPreset = False;
 }

 u_int32_t const rtpTimestamp = fTimestampBase + timestampIncrement;
 
 return rtpTimestamp;
}

其实时间戳的转换主要就是把以秒为单位的时间，提升成按频率为单位的时间．也就是提升后，时间间隔不是以秒为单位，而是以1/fTimestampFrequency为单位，也就是1/9000秒。然后再强转为32。

live555学习笔记12－h264 rtp包的时间戳

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