Live555學習之(七)---------- Live555實現h264視頻的點播

　　上一篇介紹了Live555如何實現錄像功能，我錄的是H264編碼的視頻文件。在《Live555的基本介紹》這一篇中，我介紹說把mp3文件放到live/mediaServer目錄下，然后使用Live555流化，就可以通過vlc去點播該文件。那么我們錄好的h264文件能否被Live555流化然后使用VLC點播呢？經試驗，發現是不行的。

　　然后我就對比VLC去請求Live555流化mp3文件和h264文件的過程，發現了原因：在請求h264文件時返回的SDP信息中，總出現"a=range:npt=0-"，而請求mp3文件時返回的SDP信息中，npt后面是0到一個具體的數字，即指定了該ServerMediaSubsession的時間長度。在ServerMediaSession::generateSDPDescription函數中，我們找到與此相關的內容：

 // Unless subsessions have differing durations, we also have a "a=range:" line:
    float dur = duration();
    if (dur == 0.0) {
      rangeLine = strDup("a=range:npt=0-\r\n");
    } else if (dur > 0.0) {
      char buf[100];
      sprintf(buf, "a=range:npt=0-%.3f\r\n", dur);
      rangeLine = strDup(buf);
    } else { // subsessions have differing durations, so "a=range:" lines go there
      rangeLine = strDup("");
    }

　　也就是說，duration函數返回值為0，才會出現"a=range:npt=0-"，表示此ServerMediaSession的持續時間未知（不是表示此ServerMediaSession的持續時間為0）。

float ServerMediaSession::duration() const {
  float minSubsessionDuration = 0.0;
  float maxSubsessionDuration = 0.0;
  for (ServerMediaSubsession* subsession = fSubsessionsHead; subsession != NULL;
       subsession = subsession->fNext) {
    // Hack: If any subsession supports seeking by 'absolute' time, then return a negative value, to indicate that only subsessions
    // will have a "a=range:" attribute:
    char* absStartTime = NULL; char* absEndTime = NULL;
    subsession->getAbsoluteTimeRange(absStartTime, absEndTime);
    if (absStartTime != NULL) return -1.0f;

    float ssduration = subsession->duration();
    if (subsession == fSubsessionsHead) { // this is the first subsession
      minSubsessionDuration = maxSubsessionDuration = ssduration;
    } else if (ssduration < minSubsessionDuration) {
    minSubsessionDuration = ssduration;
    } else if (ssduration > maxSubsessionDuration) {
    maxSubsessionDuration = ssduration;
    }
  }

  if (maxSubsessionDuration != minSubsessionDuration) {
    return -maxSubsessionDuration; // because subsession durations differ
  } else {
    return maxSubsessionDuration; // all subsession durations are the same
  }
}

　　看一下ServerMediaSession::duration函數，發現ServerMediaSession的duration取決於各個ServerMediaSubsession的duration。那我們再看一下ServerMediaSubsession的duration函數:

float ServerMediaSubsession::duration() const {
  // default implementation: assume an unbounded session:
  return 0.0;
}

　　可以看到默認的實現是返回0，而對於H264視頻文件對應的ServerMediaSubsession具體類是H264VideoFileServerMediaSubsession，在該類中沒有找到覆蓋duration的實現，因此對於H264視頻文件duration函數返回0。而對於mp3文件，我們看一下MP3AudioFileServerMediaSubsession類中關於duration的實現:

 float MP3AudioFileServerMediaSubsession::duration() const {
   return fFileDuration;　　　　　　　//返回的是fFileDuration，這個值由MP3FileSource得到
 }

　　后來發現對於mkv文件，Live555也是支持點播的，那么我又去看了一下MatroskaFileServerMediaSubsession類關於duration的實現:

float MatroskaFileServerMediaSubsession::duration() const { return fOurDemux.fileDuration(); }

　　總之，對於mp3文件和mkv文件，druation函數都有具體的實現，而對於h264文件，使用的是默認的實現（返回0）。后來在官網找到這么一段說明:

　　這段話列出了Live555支持幾種播放動作（包括暫停、點播、快進、倒放）的文件類型，Seeking即點播，從中我們可以看到支持的文件類型沒有h264。但是后面又提示說了如果要使這些播放動作可以用於MPEG Transport Stream file（即ts流文件，后綴名是.ts），則必須有一個index file，還提示說可以使用MPEG2TransportStreamIndexer來產生index file。

　　然后我在testOnDemandRTSPServer.cpp中找到了對於.ts文件的處理

 // A MPEG-2 Transport Stream:
  {
    char const* streamName = "mpeg2TransportStreamTest";
    char const* inputFileName = "test.ts";
    char const* indexFileName = "test.tsx";
    ServerMediaSession* sms
      = ServerMediaSession::createNew(*env, streamName, streamName,
                      descriptionString);
    sms->addSubsession(MPEG2TransportFileServerMediaSubsession
               ::createNew(*env, inputFileName, indexFileName, reuseFirstSource));
    rtspServer->addServerMediaSession(sms);

    announceStream(rtspServer, sms, streamName, inputFileName);
  }

　　從中可以看到，對於.ts文件的流化，Live555還需要一個.tsx文件，這個就是對應於.ts文件的index file了。然后我在live/testProgs目錄下找到了MPEG2TransportStreamIndexer.cpp

int main(int argc, char const** argv) {
  // Begin by setting up our usage environment:
  TaskScheduler* scheduler = BasicTaskScheduler::createNew();
  env = BasicUsageEnvironment::createNew(*scheduler);


  // Parse the command line:
  programName = argv[0];
  //if (argc != 2) usage();

  char const* inputFileName = "test.ts";
  // Check whether the input file name ends with ".ts":
  int len = strlen(inputFileName);
  if (len < 4 || strcmp(&inputFileName[len-3], ".ts") != 0) {
    *env << "ERROR: input file name \"" << inputFileName
     << "\" does not end with \".ts\"\n";
    usage();
  }

  // Open the input file (as a 'byte stream file source'):
  FramedSource* input
    = ByteStreamFileSource::createNew(*env, inputFileName, TRANSPORT_PACKET_SIZE);
  if (input == NULL) {
    *env << "Failed to open input file \"" << inputFileName << "\" (does it exist?)\n";
    exit(1);
  }

  // Create a filter that indexes the input Transport Stream data:
  FramedSource* indexer
    = MPEG2IFrameIndexFromTransportStream::createNew(*env, input);

  // The output file name is the same as the input file name, except with suffix ".tsx":
  char* outputFileName = new char[len+2]; // allow for trailing x\0
  sprintf(outputFileName, "%sx", inputFileName);

  // Open the output file (for writing), as a 'file sink':
  MediaSink* output = FileSink::createNew(*env, outputFileName);
  if (output == NULL) {
    *env << "Failed to open output file \"" << outputFileName << "\"\n";
    exit(1);
  }



  // Start playing, to generate the output index file:
  *env << "Writing index file \"" << outputFileName << "\"...";
  output->startPlaying(*indexer, afterPlaying, NULL);

  env->taskScheduler().doEventLoop(); // does not return

  return 0; // only to prevent compiler warning
}

void afterPlaying(void* /*clientData*/) {
  *env << "...done\n";
  exit(0);
}

　　這個程序演示了如何根據一個.ts文件產生對應的.tsx文件，只要同時具有.ts文件和.tsx文件就可以實現點播了。那現在如何把h264文件轉成.ts文件呢？然后我又在live/testProgs目錄下驚喜發現了testH264VideoToTransportStream.cpp文件，來看看

int main(int argc, char** argv) {
  // Begin by setting up our usage environment:
  TaskScheduler* scheduler = BasicTaskScheduler::createNew();
  env = BasicUsageEnvironment::createNew(*scheduler);

  // Open the input file as a 'byte-stream file source':
  FramedSource* inputSource = ByteStreamFileSource::createNew(*env, inputFileName);
  if (inputSource == NULL) {
    *env << "Unable to open file \"" << inputFileName
     << "\" as a byte-stream file source\n";
    exit(1);
  }

  // Create a 'framer' filter for this file source, to generate presentation times for each NAL unit:
  H264VideoStreamFramer* framer = H264VideoStreamFramer::createNew(*env, inputSource, True/*includeStartCodeInOutput*/);

  // Then create a filter that packs the H.264 video data into a Transport Stream:
  MPEG2TransportStreamFromESSource* tsFrames = MPEG2TransportStreamFromESSource::createNew(*env);
  tsFrames->addNewVideoSource(framer, 5/*mpegVersion: H.264*/);
  
  // Open the output file as a 'file sink':
  MediaSink* outputSink = FileSink::createNew(*env, outputFileName);
  if (outputSink == NULL) {
    *env << "Unable to open file \"" << outputFileName << "\" as a file sink\n";
    exit(1);
  }

  // Finally, start playing:
  *env << "Beginning to read...\n";
  outputSink->startPlaying(*tsFrames, afterPlaying, NULL);

  env->taskScheduler().doEventLoop(); // does not return

  return 0; // only to prevent compiler warning
}

void afterPlaying(void* /*clientData*/) {
  *env << "Done reading.\n";
  *env << "Wrote output file: \"" << outputFileName << "\"\n";
  exit(0);
}

　　這個程序又演示了如何將一個h264視頻文件轉成.ts格式的文件，這樣把上面的兩個例子程序結合起來，就可以實現H264文件的點播了。

　　在此，順便說一下自己對Live555多線程編程的理解，Live555是基於事件驅動的單線程模式，每個TaskScheduler就對應一個Live555線程，那么在我們的程序中可以創建多個TaskScheduler來實現多線程。我們自己程序的其他線程和Live555線程的交互可以通過全局的flag變量或者調用triggerEvent函數。我們來看看Live555官方對於此問題的解答：