live555学习笔记10-h264 RTP传输详解(2)
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十 h264 RTP传输详解(2)
上一章并没有把打开文件分析文件的代码找到,因为发现它隐藏得比较深,而且H264的Source又有多个,形成了连环计。所以此章中就将文件处理与H264的Source们并在一起分析吧。
从哪里开始呢?从source开始吧!为什么要从它开始呢?我就想从这里开始,行了吧?
FramedSource* H264VideoFileServerMediaSubsession::createNewStreamSource(unsigned /*clientSessionId*/,unsigned& estBitrate){estBitrate = 500; // kbps, estimate// Create the video source:ByteStreamFileSource* fileSource = ByteStreamFileSource::createNew(envir(),fFileName);if (fileSource == NULL)return NULL;fFileSize = fileSource->fileSize();// Create a framer for the Video Elementary Stream:return H264VideoStreamFramer::createNew(envir(), fileSource);}先创建一个ByteStreamFileSource,显然这是一个从文件按字节读取数据的source,没什么可细说的。但是,打开文件,读写文件操作的确就在其中。最终来处理h264文件,分析其格式,解析出帧或nal的应是这个source: H264VideoStreamFramer。打开文件的地方找到了,但分析文件的代码才是更有价值的。那我们只能来看H264VideoStreamFramer。
H264VideoStreamFramer继承自MPEGVideoStreamFramer,MPEGVideoStreamFramer继承自FramedFilter,FramedFilter继承自FramedSource。
啊,中间又冒出个Filter。看到它,是不是联想到了DirectShow的filter?或者说Photoshop中的filter?它们的意义应该都差不多吧?即插入到source和render(sink)之间的处理媒体数据的东东?如果这样理解,还是更接近于photoshop中的概念。唉,说实话,我估计自己说的也不全对,反正就这样认识吧,谬不了一千里。既然我们这样认识了,那么我们就有理由相信可能会出现多个filter们一个连一个,然后高唱:手牵着脚脚牵着手一起向前走...
H264VideoStreamFramer继承自MPEGVideoStreamFramer,MPEGVideoStreamFramer比较简单,它只是把一些工作交给了MPEGVideoStreamParser(又出来个parser,这可是个新东西哦,先不要管它吧),重点来看一下。
构造函数:
H264VideoStreamFramer::H264VideoStreamFramer(UsageEnvironment& env,FramedSource* inputSource,Boolean createParser,Boolean includeStartCodeInOutput): MPEGVideoStreamFramer(env, inputSource),fIncludeStartCodeInOutput(includeStartCodeInOutput),fLastSeenSPS(NULL),fLastSeenSPSSize(0),fLastSeenPPS(NULL),fLastSeenPPSSize(0){fParser = createParser ?new H264VideoStreamParser(this, inputSource,includeStartCodeInOutput) :NULL;fNextPresentationTime = fPresentationTimeBase;fFrameRate = 25.0; // We assume a frame rate of 25 fps, //unless we learn otherwise (from parsing a Sequence Parameter Set NAL unit)}由于createParser肯定为真,所以主要内容是创建了H264VideoStreamParser对象(先不管这个parser)。
其它的函数就没什么可看的了,都集中在所保存的PPS与SPS上。看来分析工作移到了H264VideoStreamParser,Parser嘛,就是分析器。分析器的基类是StreamParser。StreamParser做了不少的工作,那我们就先搞明白StreamParser做了哪些工作吧,并且可能为继承者提供什么样的调用框架呢?.....我看完了,呵呵。直接说分析结果吧:
StreamParser的主要工作是实现了对数据以位为单位进行访问。因为在处理媒体格式时,按位分析是很常见的情况。这两个函数skipBits(unsigned numBits)和unsigned getBits(unsigned numBits)很明显是基于位的操作。unsigned char* fBank[2]这个变量中的两个缓冲区被轮换使用。这个类中保存了一个Source,理所当然地它应该保存ByteStreamFileSource的实例,而不是FramedFilter的。那些getBytes()或getBits()最终会导致读文件的操作。从文件读取一次数据后,StreamParser::afterGettingBytes1()被调用,StreamParser::afterGettingBytes1()中做一点简单的工作后便调用fClientContinueFunc这个回调函数。fClientContinueFunc可能指向Frame的函数体也可能是指向RtpSink的函数体--因为Framer完全可以把RtpSink的函数体传给Parser。至于到底指向哪个,只能在进一步分析之后才得知。
下面再来分析StreamParser的儿子:MPEGVideoStreamParser。
MPEGVideoStreamParser::MPEGVideoStreamParser(MPEGVideoStreamFramer* usingSource,FramedSource* inputSource): StreamParser(inputSource,FramedSource::handleClosure, usingSource,&MPEGVideoStreamFramer::continueReadProcessing,usingSource),fUsingSource(usingSource){}
MPEGVideoStreamParser的构造函数中有很多有意思的东西。
首先参数usingSource是什么意思?表示正在使用这个Parser的Source? inputSource 很明确,就是能获取数据的source,也就是 ByteStreamFileSource。而且很明显的,StreamParser中保存的source是ByteStreamFileSource。从传入给StreamParser的回调函数以及它们的参数可以看出,这些回调函数全是指向的StreamParser的子类的函数(为啥不用虚函数的方式?哦,回调函数全是静态函数,不能成为虚函数)。这说明在每读一次数据后,MPEGVideoStreamFramer::continueReadProcessing()被调用,在其中对帧进行界定和分析,完成后再调用RTPSink的相应函数,RTPSink中对帧进行打包和发送(还记得吗,不记得了请回头看以前的章节)。
MPEGVideoStreamParser的fTo是RTPSink传入的缓冲指针,其saveByte(),save4Bytes()是把数据从StreamParser的缓冲把数据复制到fTo中,是给继承类使用的。saveToNextCode()是复制数据直到遇到一个同步字节串(比如h264中分隔nal的那一陀东东,当然此处的跟h264还不一样),也是给继承类使用的。纯虚函数parse()很明显是留继承类去写帧分析代码的地方。registerReadInterest()被调用者用来告诉MPEGVideoStreamParser其接收帧的缓冲地址与容量。
现在应该来分析一下MPEGVideoStreamFramer,以明确MPEGVideoStreamFramer与MPEGVideoStreamParser是怎样配合的。
MPEGVideoStreamFramer中用到Parser的重要的函数只有两个,一是:
void MPEGVideoStreamFramer::doGetNextFrame(){fParser->registerReadInterest(fTo, fMaxSize);continueReadProcessing();}
很简单,只是告诉了Parser保存帧的缓冲和缓冲的大小,然后执行continueReadProcessing(),那么来看一下continueReadProcessing():
void MPEGVideoStreamFramer::continueReadProcessing(){unsigned acquiredFrameSize = fParser->parse();if (acquiredFrameSize > 0) {// We were able to acquire a frame from the input.// It has already been copied to the reader's space.fFrameSize = acquiredFrameSize;fNumTruncatedBytes = fParser->numTruncatedBytes();// "fPresentationTime" should have already been computed.// Compute "fDurationInMicroseconds" now:fDurationInMicroseconds =(fFrameRate == 0.0 || ((int) fPictureCount) < 0) ?0 : (unsigned) ((fPictureCount * 1000000) / fFrameRate);fPictureCount = 0;// Call our own 'after getting' function. Because we're not a 'leaf'// source, we can call this directly, without risking infinite recursion.afterGetting(this);} else {// We were unable to parse a complete frame from the input, because:// - we had to read more data from the source stream, or// - the source stream has ended.}}先利用Parser进行分析(应该是解析出一帧吧?),分析完成后,帧数据已到了MPEGVideoStreamFramer的缓冲fTo中。计算出帧的持续时间后,调用FrameSource的afterGetting(),最终会调用RTPSink的函数。
看到这里,可以总结一下,其实看来看去,Parser直正被外部使用的函数几乎只有一个:parse()。
下面可以看H264VideoStreamParser了。其实也很简单,多了一些用于分析h264格式的函数,当然是非公开的,只供自己使用的。在哪里使用呢?当然是在parser()中使用。至于H264VideoStreamFramer前面已经说过了,没什么太多的东西,所以就不看了。总结起来也就是这样:RTPSink向H264VideoStreamFramer要下一帧(其实h264中肯定不是一帧了,而是一个NAL Unit),H264VideoStreamFramer告诉H264VideoStreamParser输出缓冲和容内的字节数,然后调用H264VideoStreamParser的parser()函数,parser()中调用ByteStreamFileSource从文件中读取数据,直到parser()获得完整的一帧,parser()返回,H264VideoStreamFramer进行一些自己的处理后把这一帧返回给了RTPSink(当然是以回调函数的方式返回)。
还有一个东西,H264FUAFragmenter,被H264VideoRTPSink所使用,继承自FramedFilter。它最初在RTPSink开始播放后创建,如下:
Boolean H264VideoRTPSink::continuePlaying(){// First, check whether we have a 'fragmenter' class set up yet.// If not, create it now:if (fOurFragmenter == NULL) {fOurFragmenter = new H264FUAFragmenter(envir(), fSource,OutPacketBuffer::maxSize,ourMaxPacketSize() - 12/*RTP hdr size*/);fSource = fOurFragmenter;}// Then call the parent class's implementation:return MultiFramedRTPSink::continuePlaying();}并且它取代了H264VideoStreamFramer成为直接与RTPSink发生关系的source.如此一来,RTPSink要获取帧时,都是从它获取的.看它最主要的一个函数吧:
void H264FUAFragmenter::doGetNextFrame(){if (fNumValidDataBytes == 1) {// We have no NAL unit data currently in the buffer. Read a new one:fInputSource->getNextFrame(&fInputBuffer[1], fInputBufferSize - 1,afterGettingFrame, this, FramedSource::handleClosure, this);} else {// We have NAL unit data in the buffer. There are three cases to consider:// 1. There is a new NAL unit in the buffer, and it's small enough to deliver// to the RTP sink (as is).// 2. There is a new NAL unit in the buffer, but it's too large to deliver to// the RTP sink in its entirety. Deliver the first fragment of this data,// as a FU-A packet, with one extra preceding header byte.// 3. There is a NAL unit in the buffer, and we've already delivered some// fragment(s) of this. Deliver the next fragment of this data,// as a FU-A packet, with two extra preceding header bytes.if (fMaxSize < fMaxOutputPacketSize) { // shouldn't happenenvir() << "H264FUAFragmenter::doGetNextFrame(): fMaxSize ("<< fMaxSize << ") is smaller than expected\n";} else {fMaxSize = fMaxOutputPacketSize;}fLastFragmentCompletedNALUnit = True; // by defaultif (fCurDataOffset == 1) { // case 1 or 2if (fNumValidDataBytes - 1 <= fMaxSize) { // case 1memmove(fTo, &fInputBuffer[1], fNumValidDataBytes - 1);fFrameSize = fNumValidDataBytes - 1;fCurDataOffset = fNumValidDataBytes;} else { // case 2// We need to send the NAL unit data as FU-A packets. Deliver the first// packet now. Note that we add FU indicator and FU header bytes to the front// of the packet (reusing the existing NAL header byte for the FU header).fInputBuffer[0] = (fInputBuffer[1] & 0xE0) | 28; // FU indicatorfInputBuffer[1] = 0x80 | (fInputBuffer[1] & 0x1F); // FU header (with S bit)memmove(fTo, fInputBuffer, fMaxSize);fFrameSize = fMaxSize;fCurDataOffset += fMaxSize - 1;fLastFragmentCompletedNALUnit = False;}} else { // case 3// We are sending this NAL unit data as FU-A packets. We've already sent the// first packet (fragment). Now, send the next fragment. Note that we add// FU indicator and FU header bytes to the front. (We reuse these bytes that// we already sent for the first fragment, but clear the S bit, and add the E// bit if this is the last fragment.)fInputBuffer[fCurDataOffset - 2] = fInputBuffer[0]; // FU indicatorfInputBuffer[fCurDataOffset - 1] = fInputBuffer[1] & ~0x80; // FU header (no S bit)unsigned numBytesToSend = 2 + fNumValidDataBytes - fCurDataOffset;if (numBytesToSend > fMaxSize) {// We can't send all of the remaining data this time:numBytesToSend = fMaxSize;fLastFragmentCompletedNALUnit = False;} else {// This is the last fragment:fInputBuffer[fCurDataOffset - 1] |= 0x40; // set the E bit in the FU headerfNumTruncatedBytes = fSaveNumTruncatedBytes;}memmove(fTo, &fInputBuffer[fCurDataOffset - 2], numBytesToSend);fFrameSize = numBytesToSend;fCurDataOffset += numBytesToSend - 2;}if (fCurDataOffset >= fNumValidDataBytes) {// We're done with this data. Reset the pointers for receiving new data:fNumValidDataBytes = fCurDataOffset = 1;}// Complete delivery to the client:FramedSource::afterGetting(this);}}
如果输入缓冲中没有数据,调用fInputSource->getNextFrame(),fInputSource是H264VideoStreamFramer,H264VideoStreamFramer的getNextFrame()会调用H264VideoStreamParser的parser(),parser()又调用ByteStreamFileSource获取数据,然后分析,parser()完成后会调用:
void H264FUAFragmenter::afterGettingFrame1(unsigned frameSize,unsigned numTruncatedBytes,struct timeval presentationTime,unsigned durationInMicroseconds){fNumValidDataBytes += frameSize;fSaveNumTruncatedBytes = numTruncatedBytes;fPresentationTime = presentationTime;fDurationInMicroseconds = durationInMicroseconds;// Deliver data to the client:doGetNextFrame();}然后又调用回H264FUAFragmenter::doGetNextFrame(),此时输入缓冲中有数据了,H264FUAFragmenter就进行分析处理.H264FUAFragmenter又对数据做了什么呢?
- live555学习笔记10-h264 RTP传输详解(2)
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