求索阁

上一篇介绍了Live555如何实现录像功能，我录的是H264编码的视频文件。在《Live555的基本介绍》这一篇中，我介绍说把mp3文件放到live/mediaServer目录下，然后使用Live555流化，就可以通过vlc去点播该文件。那么我们录好的h264文件能否被Live555流化然后使用VLC点播呢？经试验，发现是不行的。

然后我就对比VLC去请求Live555流化mp3文件和h264文件的过程，发现了原因：在请求h264文件时返回的SDP信息中，总出现"a=range:npt=0-"，而请求mp3文件时返回的SDP信息中，npt后面是0到一个具体的数字，即指定了该ServerMediaSubsession的时间长度。在ServerMediaSession::generateSDPDescription函数中，我们找到与此相关的内容：

1  // Unless subsessions have differing durations, we also have a "a=range:" line:

2     float dur = duration();

3     if (dur == 0.0) {

4       rangeLine = strDup("a=range:npt=0-\r\n");

5     } else if (dur > 0.0) {

6       char buf[100];

7       sprintf(buf, "a=range:npt=0-%.3f\r\n", dur);

8       rangeLine = strDup(buf);

9     } else { // subsessions have differing durations, so "a=range:" lines go there

10       rangeLine = strDup("");

11     }

也就是说，duration函数返回值为0，才会出现"a=range:npt=0-"，表示此ServerMediaSession的持续时间未知（不是表示此ServerMediaSession的持续时间为0）。

1 float ServerMediaSession::duration() const {

2   float minSubsessionDuration = 0.0;

3   float maxSubsessionDuration = 0.0;

4   for (ServerMediaSubsession* subsession = fSubsessionsHead; subsession != NULL;

5        subsession = subsession->fNext) {

6     // Hack: If any subsession supports seeking by 'absolute' time, then return a negative value, to indicate that only subsessions

7     // will have a "a=range:" attribute:

8     char* absStartTime = NULL; char* absEndTime = NULL;

9     subsession->getAbsoluteTimeRange(absStartTime, absEndTime);

10     if (absStartTime != NULL) return -1.0f;

11

12     float ssduration = subsession->duration();

13     if (subsession == fSubsessionsHead) { // this is the first subsession

14       minSubsessionDuration = maxSubsessionDuration = ssduration;

15     } else if (ssduration < minSubsessionDuration) {

16     minSubsessionDuration = ssduration;

17     } else if (ssduration > maxSubsessionDuration) {

18     maxSubsessionDuration = ssduration;

19     }

20   }

21

22   if (maxSubsessionDuration != minSubsessionDuration) {

23     return -maxSubsessionDuration; // because subsession durations differ

24   } else {

25     return maxSubsessionDuration; // all subsession durations are the same

26   }

27 }

看一下ServerMediaSession::duration函数，发现ServerMediaSession的duration取决于各个ServerMediaSubsession的duration。那我们再看一下ServerMediaSubsession的duration函数:

1 float ServerMediaSubsession::duration() const {

2   // default implementation: assume an unbounded session:

3   return 0.0;

4 }

可以看到默认的实现是返回0，而对于H264视频文件对应的ServerMediaSubsession具体类是H264VideoFileServerMediaSubsession，在该类中没有找到覆盖duration的实现，因此对于H264视频文件duration函数返回0。而对于mp3文件，我们看一下MP3AudioFileServerMediaSubsession类中关于duration的实现:

1  float MP3AudioFileServerMediaSubsession::duration() const {

2    return fFileDuration; //返回的是fFileDuration，这个值由MP3FileSource得到

3  }

后来发现对于mkv文件，Live555也是支持点播的，那么我又去看了一下MatroskaFileServerMediaSubsession类关于duration的实现:

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

总之，对于mp3文件和mkv文件，druation函数都有具体的实现，而对于h264文件，使用的是默认的实现（返回0）。后来在官网找到这么一段说明:

这段话列出了Live555支持几种播放动作（包括暂停、点播、快进、倒放）的文件类型，Seeking即点播，从中我们可以看到支持的文件类型没有h264。但是后面又提示说了如果要使这些播放动作可以用于MPEG Transport Stream file（即ts流文件，后缀名是.ts），则必须有一个index file，还提示说可以使用MPEG2TransportStreamIndexer来产生index file。

然后我在testOnDemandRTSPServer.cpp中找到了对于.ts文件的处理

1  // A MPEG-2 Transport Stream:

2   {

3     char const* streamName = "mpeg2TransportStreamTest";

4     char const* inputFileName = "test.ts";

5     char const* indexFileName = "test.tsx";

6     ServerMediaSession* sms

7       = ServerMediaSession::createNew(*env, streamName, streamName,

8                       descriptionString);

9     sms->addSubsession(MPEG2TransportFileServerMediaSubsession

10                ::createNew(*env, inputFileName, indexFileName, reuseFirstSource));

11     rtspServer->addServerMediaSession(sms);

12

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

14   }

从中可以看到，对于.ts文件的流化，Live555还需要一个.tsx文件，这个就是对应于.ts文件的index file了。然后我在live/testProgs目录下找到了MPEG2TransportStreamIndexer.cpp

1 int main(int argc, char const** argv) {

2   // Begin by setting up our usage environment:

3   TaskScheduler* scheduler = BasicTaskScheduler::createNew();

4   env = BasicUsageEnvironment::createNew(*scheduler);

5

6

7   // Parse the command line:

8   programName = argv[0];

9   //if (argc != 2) usage();

10

11   char const* inputFileName = "test.ts";

12   // Check whether the input file name ends with ".ts":

13   int len = strlen(inputFileName);

14   if (len < 4 || strcmp(&inputFileName[len-3], ".ts") != 0) {

15     *env << "ERROR: input file name \"" << inputFileName

16      << "\" does not end with \".ts\"\n";

17     usage();

18   }

19

20   // Open the input file (as a 'byte stream file source'):

21   FramedSource* input

22     = ByteStreamFileSource::createNew(*env, inputFileName, TRANSPORT_PACKET_SIZE);

23   if (input == NULL) {

24     *env << "Failed to open input file \"" << inputFileName << "\" (does it exist?)\n";

25     exit(1);

26   }

27

28   // Create a filter that indexes the input Transport Stream data:

29   FramedSource* indexer

30     = MPEG2IFrameIndexFromTransportStream::createNew(*env, input);

31

32   // The output file name is the same as the input file name, except with suffix ".tsx":

33   char* outputFileName = new char[len+2]; // allow for trailing x\0

34   sprintf(outputFileName, "%sx", inputFileName);

35

36   // Open the output file (for writing), as a 'file sink':

37   MediaSink* output = FileSink::createNew(*env, outputFileName);

38   if (output == NULL) {

39     *env << "Failed to open output file \"" << outputFileName << "\"\n";

40     exit(1);

41   }

42

43

44

45   // Start playing, to generate the output index file:

46   *env << "Writing index file \"" << outputFileName << "\"...";

47   output->startPlaying(*indexer, afterPlaying, NULL);

48

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

50

51   return 0; // only to prevent compiler warning

52 }

53

54 void afterPlaying(void* /*clientData*/) {

55   *env << "...done\n";

56   exit(0);

57 }

这个程序演示了如何根据一个.ts文件产生对应的.tsx文件，只要同时具有.ts文件和.tsx文件就可以实现点播了。那现在如何把h264文件转成.ts文件呢？然后我又在live/testProgs目录下惊喜发现了testH264VideoToTransportStream.cpp文件，来看看

1 int main(int argc, char** argv) {

2   // Begin by setting up our usage environment:

3   TaskScheduler* scheduler = BasicTaskScheduler::createNew();

4   env = BasicUsageEnvironment::createNew(*scheduler);

5

6   // Open the input file as a 'byte-stream file source':

7   FramedSource* inputSource = ByteStreamFileSource::createNew(*env, inputFileName);

8   if (inputSource == NULL) {

9     *env << "Unable to open file \"" << inputFileName

10      << "\" as a byte-stream file source\n";

11     exit(1);

12   }

13

14   // Create a 'framer' filter for this file source, to generate presentation times for each NAL unit:

15   H264VideoStreamFramer* framer = H264VideoStreamFramer::createNew(*env, inputSource, True/*includeStartCodeInOutput*/);

16

17   // Then create a filter that packs the H.264 video data into a Transport Stream:

18   MPEG2TransportStreamFromESSource* tsFrames = MPEG2TransportStreamFromESSource::createNew(*env);

19   tsFrames->addNewVideoSource(framer, 5/*mpegVersion: H.264*/);

20  

21   // Open the output file as a 'file sink':

22   MediaSink* outputSink = FileSink::createNew(*env, outputFileName);

23   if (outputSink == NULL) {

24     *env << "Unable to open file \"" << outputFileName << "\" as a file sink\n";

25     exit(1);

26   }

27

28   // Finally, start playing:

29   *env << "Beginning to read...\n";

30   outputSink->startPlaying(*tsFrames, afterPlaying, NULL);

31

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

33

34   return 0; // only to prevent compiler warning

35 }

36

37 void afterPlaying(void* /*clientData*/) {

38   *env << "Done reading.\n";

39   *env << "Wrote output file: \"" << outputFileName << "\"\n";

40   exit(0);

41 }

这个程序又演示了如何将一个h264视频文件转成.ts格式的文件，这样把上面的两个例子程序结合起来，就可以实现H264文件的点播了。

在此，顺便说一下自己对Live555多线程编程的理解，Live555是基于事件驱动的单线程模式，每个TaskScheduler就对应一个Live555线程，那么在我们的程序中可以创建多个TaskScheduler来实现多线程。我们自己程序的其他线程和Live555线程的交互可以通过全局的flag变量或者调用triggerEvent函数。

作者：昨夜星辰