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 codesetParseState();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 timeif (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 otherwiseif (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 valuethisNALUnitEndsAccessUnit = 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 0thisNALUnitEndsAccessUnit = 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 valuethisNALUnitEndsAccessUnit = True;} else if (next_pic_parameter_set_id!= pic_parameter_set_id) {// pic_parameter_set_id differs in valuethisNALUnitEndsAccessUnit = True;} else if (next_field_pic_flag != field_pic_flag) {// field_pic_flag differs in valuethisNALUnitEndsAccessUnit = True;} else if (next_bottom_field_flag!= bottom_field_flag) {// bottom_field_flag differs in valuethisNALUnitEndsAccessUnit = 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; // aliasnextPT = 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。