live555学习笔记12

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。


原文链接： http://blog.csdn.net/nkmnkm/article/details/6948834