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？
[cpp] view plain copyFramedSource* 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：[cpp] view plain copyH264VideoStreamFramer::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。
[cpp] view plain copyMPEGVideoStreamParser::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：
[cpp] view plain copyvoid 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()：
[cpp] view plain copyvoid 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：[cpp] view plain copyBoolean 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：[cpp] view plain copyvoid 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 happen envir() << "H264FUAFragmenter::doGetNextFrame(): fMaxSize (" << fMaxSize << ") is smaller than expected/n"; } else { fMaxSize = fMaxOutputPacketSize; } fLastFragmentCompletedNALUnit = True; // by default if (fCurDataOffset == 1) { // case 1 or 2 if (fNumValidDataBytes - 1 <= fMaxSize) { // case 1 memmove(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 indicator fInputBuffer[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 indicator fInputBuffer[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 header fNumTruncatedBytes = 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：
[cpp] view plain copyvoid 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？新闻热点
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