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• ffmpeg/libav - how to wirte video files with valid pts

  9 mai 2017, par Kai Rohmer

  I’m currently trying the write out a real time rendered video into a h264 encoded file. After reading a lot of (mostly) old samples and the few class references they call a documentation, I manager to write my video file and I’m also able to read it. Unfortunately, I need some metadata for each frame but I’m not having a constant frame rate. So my intension was to start with the presentation timestamps to "frametime" during recoding. But after all I tried I get no pts while reading the the file (pts stays -9223372036854775808). Before wring a lot of code, here are the basics steps I’m doing. I’m probably using the wrong container or I’m missing to set a flag and you will notice it right away.

  // open a AVFormatContext
  
  avformat_alloc_output_context2(&m_FormatContext, nullptr, "avi", m_FileName.c_str());
  
  
  
  // open a stream
  
  m_VideoStream = avformat_new_stream(m_FormatContext, avcodec_find_encoder(AV_CODEC_ID_H264));
  
  
  
  // setup the codec context (including bitrate, frame size, ...)
  
  m_CodecContext = m_VideoStream ->codec;
  
  m_CodecContext->coder_type = FF_CODER_TYPE_VLC;
  
  m_CodecContext->time_base = AVRational{1, 120}; // I expect 20-60 Hz
  
  m_CodecContext->pix_fmt = AV_PIX_FMT_YUV420P;
  
  m_CodecContext->color_range = AVCOL_RANGE_JPEG;
  
  ...
  
  av_opt_set(m_CodecContext->priv_data, "preset", "ultrafast", 0);
  
  av_opt_set(m_CodecContext->priv_data, "tune", "zerolatency,fastdecode", 0);
  
  
  
  
  
  // set the same time_base to the stream
  
  m_VideoStream ->time_base = m_CodecContext->time_base;
  
  
  
  // open the codec
  
  avcodec_open2(m_CodecContext, m_CodecContext->codec, nullptr);
  
  
  
  // open file and write header
  
  avio_open(&m_FormatContext->pb, m_FileName.c_str(), AVIO_FLAG_WRITE);
  
  avformat_write_header(m_FormatContext, nullptr);
  
  
  
  // then in a loop:
  
  // render frame, convert RGBA to YUV frame, set the frames pts (timestamp is double application time in seconds)
  
  frameToEncode.pts = int64_t(timestamp / av_q2d(m_VideoStream->time_base));
  
  av_init_packet(m_EncodedPacket);
  
  avcodec_encode_video2(m_CodecContext, m_EncodedPacket, frameToEncode, &got_output);
  
  
  
  // check packet infos
  
  //m_EncodedPacket->pts equals frameToEncode.pts
  
  m_EncodedPacket->dts = AV_NOPTS_VALUE; // also tried incrementing numbers, or zero
  
  m_EncodedPacket->stream_index = m_Stream->index;
  
  m_EncodedPacket->duration = 0;
  
  m_EncodedPacket->pos = -1;
  
  m_EncodedPacket->flags = 0;
  
  m_EncodedPacket->flags |= AV_PKT_FLAG_KEY; // read that somewhere
  
  
  
  // write the packet to stream
  
  av_interleaved_write_frame(m_FormatContext, m_EncodedPacket);
  
  
  
  
  
  // after the loop
  
  // I encode delayed frames and write the trailer
  
  av_write_trailer(m_FormatContext); 

  Thats pretty much it. I’m not getting what is missing. Since I have some meta data per frame I tried to add side data to each package but this data also disapered after reading from file. If decode the packets directly (instead of writing them to file, the data is there)

  I’m quite sure the problem is with the encoding. I managed to decode the big buck bunny movie in which case i got valid pts values.

  Thanks a lot for your help !

• C# execute external program and capture (stream) the output

  22 mars 2017, par Roberto Correia

  I’m making a program to work with some video files.

  I’m using the ffmpeg executable to merge several files in a single file.
  This command takes several minutes to finish, so, I need a way to "monitor" the output, and show a progress bar on GUI.

  Looking at the following stackoverflow topics :

  • How to parse command line output from c# ?
  • Process.start : how to get the output ?
  • How To : Execute command line in C#, get STD OUT results

  I made this code :

  Process ffmpeg = new Process
  
  {
  
    StartInfo = 
  
    {
  
      FileName = @"d:\tmp\ffmpeg.exe",
  
      Arguments = "-f concat -safe 0 -i __sync.txt -c copy output.mp4",
  
      UseShellExecute = false,
  
      RedirectStandardOutput = true,
  
      CreateNoWindow = true,
  
      WorkingDirectory = @"d:\tmp"
  
    }
  
  }
  
  
  
  ffmpeg.EnableRaisingEvents = true;
  
  ffmpeg.OutputDataReceived += (s, e) => Debug.WriteLine(e.Data);
  
  ffmpeg.ErrorDataReceived += (s, e) => Debug.WriteLine($@"Error: {e.Data}");
  
  ffmpeg.Start();
  
  ffmpeg.BeginOutputReadLine();
  
  ffmpeg.WaitForExit();

  When I run this code, the ffmpeg start to merge files, I can see the ffmpeg process on Windows Task Manager, and if I wait long enough, the ffmpeg finish the job without any error. But, the Debug.WriteLine(e.Data) is never called (no output on Debug window). Tried to change to Console.WriteLine too (again, no output).

  So, after this, I tried this another version :

  Process ffmpeg = new Process
  
  {
  
    StartInfo = 
  
    {
  
      FileName = @"d:\tmp\ffmpeg.exe",
  
      Arguments = "-f concat -safe 0 -i __sync.txt -c copy output.mp4",
  
      UseShellExecute = false,
  
      RedirectStandardOutput = true,
  
      CreateNoWindow = true,
  
      WorkingDirectory = @"d:\tmp"
  
    }
  
  }
  
  
  
  ffmpeg.Start();
  
  while (!ffmpeg.StandardOutput.EndOfStream)
  
  {
  
    var line = ffmpeg.StandardOutput.ReadLine();
  
    System.Diagnostics.Debug.WriteLine(line);
  
    Console.WriteLine(line);
  
  }
  
  ffmpeg.WaitForExit();

  Again, the ffmpeg is started without any error, but the C# "hangs" on While (!ffmpeg.StandardOutput.EndOfStream) until ffmpeg is finished.

  If I execute the exact command on Windows prompt, a lot of output text is showed with progress of ffmpeg.

• h264 : don’t sync pic_id between threads.

  3 avril 2017, par Ronald S. Bultje

  h264 : don’t sync pic_id between threads.
  This is how the ref list manager links bitstream IDs to H264Picture/Ref
  
  objects, and is local to the producer thread. There is no need for the
  
  consumer thread to know the bitstream IDs of its references in their
  
  respective producer threads.
  In practice, this fixes tsan warnings when running fate-h264 :
  WARNING : ThreadSanitizer : data race (pid=19295)
  
    Read of size 4 at 0x7dbc0000e614 by main thread (mutexes : write M1914) :
  
      #0 ff_h264_ref_picture src/libavcodec/h264_picture.c:112 (ffmpeg+0x0000013b3709)
  
  [..]
  
    Previous write of size 4 at 0x7dbc0000e614 by thread T2 (mutexes : write M1917) :
  
      #0 build_def_list src/libavcodec/h264_refs.c:91 (ffmpeg+0x0000013b46cf)
  

  • [DH] libavcodec/h264_picture.c