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• Thread safety of FFmpeg when using av_lockmgr_register

  12 août 2013, par Stocastico

  My application uses FFmpeg to read video streams. So far, I ensured thread safety by defining my own global lock and looking for all the methods inside FFmpeg libraries which are not thread safe.
  This makes the code a bit messy, so while looking for better ideas I found this answer, but apparently I couldn't make use of the suggestions.
  I tried testing it in my own environment, but I always get critical heap error. Here's the test code

  class TestReader
  
  {
  
  public:
  
   TestReader( std::string sVid )
  
   {
  
     m_sVid = sVid;
  
     m_cVidPtr.reset( new VideoReader() );
  
   }
  
  
  
   ~TestReader()
  
   {}
  
  
  
   void operator() ()
  
   {
  
      readVideoThread();
  
   }
  
  
  
   private:
  
   int readVideoThread()
  
   {
  
     m_cVidPtr->init( m_sVid.c_str() );
  
     MPEGFrame::pointer cFramePtr;
  
  
  
     for ( int i=0; i< 500; i++ )
  
     {
  
       cFramePtr = m_cVidPtr->getNextFrame();
  
     }
  
  
  
     return 0;
  
   }
  
   boost::shared_ptr<videoreader> m_cVidPtr;
  
   std::string m_sVid;
  
  };
  
  
  
  /*****************************************************************************/
  
  int lockMgrCallback(void** cMutex, enum AVLockOp op)
  
  {
  
   if (nullptr == cMutex)
  
     return -1;
  
  
  
   switch(op)
  
   {
  
   case AV_LOCK_CREATE:
  
     {
  
       *cMutex = nullptr;
  
       boost::mutex* m = new boost::mutex();
  
       *cMutex = static_cast(m);
  
       break;
  
     }
  
   case AV_LOCK_OBTAIN:
  
     {
  
       boost::mutex* m =  static_cast(*cMutex);
  
       m->lock();
  
       break;
  
     }
  
   case AV_LOCK_RELEASE:
  
     {
  
       boost::mutex * m = static_cast(*cMutex);
  
       m->unlock();
  
       break;
  
     }
  
   case AV_LOCK_DESTROY:
  
     {
  
       boost::mutex * m = static_cast(*cMutex);
  
       delete m;
  
       break;
  
     }
  
   default:
  
     break;
  
   }
  
   return 0;
  
  }
  
  
  
  int testFFmpegMultiThread( std::string sVideo )
  
  {
  
   if ( ::av_lockmgr_register( &amp;lockMgrCallback ) )
  
   {
  
     std::cout &lt;&lt; "Could not initialize lock manager!" &lt;&lt; std::endl;
  
     return -1;
  
   }
  
   TestReader c1(sVideo);
  
   TestReader c2(sVideo);
  
   boost::thread t1( c1 );
  
   boost::thread t2( c2 );
  
  
  
   t1.join();
  
   t2.join();
  
  
  
   return 0;
  
  }
  
  </videoreader>

  The classes VideoReader and MPEGFrame are just wrappers and have always worked perfectly in single threaded scenarios, or in multi-threaded scenario managed using my own global lock.
  Am I missing something obvious ? Can anybody point me to some working code ? Thanks in advance

• How can I change a video frame rate with FFmpeg keeping the same total number of frames ?

  28 mai 2017, par Nuno

  I’ve been searching for an answer here on Stack Overflow and googling everywhere... even though it seems like it should be a very simple command line to me, I just can’t find an answer anywhere.

  I would like to change the frame rate of a video from 23.976fps to 24fps with FFmpeg, lossless and keeping the total number of frames.

  To make it simpler :

  Let’s say I have a 25fps video with a total lenght of 100 frames.

  How can I change it’s frame rate to 50fps, with FFmpeg, lossless and keeping the same total lenght of 100 frames ?

  This was so far the best solution I came across with (which can be found here) :

  Extract the frames as rawvideo :

  ffmpeg -i input.mov -f rawvideo -b 50000000 -pix_fmt yuv420p -vcodec
  rawvideo -s 1920x1080 -y temp.raw

  Recreate the video with new framerate :

  ffmpeg -f rawvideo -b 50000000 -pix_fmt yuv420p -r 24 -s 1920x1080 -i
  temp.raw -y output.mov

  Note 1 : I had to remove "-b 50000000" when recreating the video with the new frame rate, in order to get it to work properly.

  It did exactly what I intended it to do, but I’m still wondering if there is any simpler way to do this ? I’ve tried to pipe them together in one line only, as suggested in the same post, but couldn’t get it to work.

  Note 2 : Even though it does exactly what I wanted it to do, I’ve just later realized there is quality loss using this method, which I would prefer to avoid.

  Thanks everyone in advance !

• How can I change a video frame rate with FFmpeg, lossless and keeping the same total number of frames ?

  28 novembre 2019, par Nuno

  I’ve been searching for an answer here on Stack Overflow and googling everywhere... even though it seems like it should be a very simple command line to me, I just can’t find an answer anywhere.

  I would like to change the frame rate of a video from 23.976fps to 24fps with FFmpeg, lossless and keeping the total number of frames.

  To make it simpler :

  Let’s say I have a 25fps video with a total lenght of 100 frames.

  How can I change it’s frame rate to 50fps, with FFmpeg, lossless and keeping the same total lenght of 100 frames ?

  This was so far the best solution I came across with (which can be found here) :

  Extract the frames as rawvideo :

  ffmpeg -i input.mov -f rawvideo -b 50000000 -pix_fmt yuv420p -vcodec
  rawvideo -s 1920x1080 -y temp.raw

  Recreate the video with new framerate :

  ffmpeg -f rawvideo -b 50000000 -pix_fmt yuv420p -r 24 -s 1920x1080 -i
  temp.raw -y output.mov

  Note 1 : I had to remove "-b 50000000" when recreating the video with the new frame rate, in order to get it to work properly.

  It did exactly what I intended it to do, but I’m still wondering if there is any simpler way to do this ? I’ve tried to pipe them together in one line only, as suggested in the same post, but couldn’t get it to work.

  Note 2 : Even though it does exactly what I wanted it to do, I’ve just later realized there is quality loss using this method, which I would prefer to avoid.

  Thanks everyone in advance !