Live555源代码解读（11）

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．

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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)

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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．

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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！

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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：

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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。