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• Use ffmpeg libraries to convert stream formats

  17 septembre 2021, par Syrinx

  I'm attempting to write a small program and link it to a minimal set of ffmpeg libraries, like libavformat and whatever other libraries I need.

  


  I am looking for documentation to get me started, or maybe a quick fix to the example program I am using.

  


  I know ffmpeg (the project) provides example programs to help developers get started. I'm using the transcoding example, as it's close to my end goal, but it exits during init with an error about an audio issue.

  


  Here I am using the transcoding example program that come with ffmpeg v4.4, on Ubuntu 18.04. My input source has one video channel (h264) and one audio channel (pcm_mulaw).

  


  $ LD_LIBRARY_PATH=../../dist/lib ./transcoding rtsp://ip-camera/stream out.flv
...
  Stream #0:0: Video: h264, yuv420p, 1280x720, q=2-31, 20 tbn
  Stream #0:1: Audio: pcm_mulaw, 8000 Hz, 0 channels, s16
auto_resampler_0 @ 0x55da787fa140] [SWR @ 0x55da787fa5c0] Rematrix is needed between mono and 0 channels but there is not enough information to do it
[auto_resampler_0 @ 0x55da787fa140] Failed to configure output pad on auto_resampler_0


  


  In libswresample/swresample.c :

  


  320      if ((!s->out_ch_layout || !s->in_ch_layout) && s->used_ch_count != s->out.ch_count && !s->rematrix_custom) {
321          av_log(s, AV_LOG_ERROR, "Rematrix is needed between %s and %s "
322                 "but there is not enough information to do it\n", l1, l2);
323          ret = AVERROR(EINVAL);
324          goto fail;
325      }


  


  I'd really like it if I could make the transcoding example program work (fix it, or maybe use it appropriately if I am misunderstanding something). But short of that, where should I look for documentation about using the ffmpeg libraries ?

  


  I don't even care about the audio. If I can just disable the audio, I would be happy with that solution. I tried tracking the "-an" option to ffmpeg (the program) to see how it does that in source code, but the options handling is a mess and I can't distinguish the parts of the code that I need from all the noise.

  


  ffmpeg has web pages like this that aren't useful at all. There is documentation in the source code that looks like it should be viewed as HTML, but I don't see it exported anywhere. "make doc" generates a very small set of man pages that are insufficient to get me started.

  


• Playing RTSP in WPF application with low latency using FFMPEG / FFMediaElement (FFME)

  22 mars 2019, par Paboka

  I’m trying to use FFMediaElement (FFME, WPF MediaElement replacement based on FFmpeg) component to play RSTP live video in my WPF application.

  I have a good connection to my camera and I want to play it with minimum available latency.

  I’ve reduced the latency by changing ProbeSize to its minimal value :

  private void Media_OnMediaInitializing(object Sender, MediaInitializingRoutedEventArgs e)
  
  {
  
    e.Configuration.GlobalOptions.ProbeSize = 32;
  
  }

  But I still have about 1 second of latency since the very beginning of the stream. I mean, when I start playing, I have to wait for 1 second till the video appears and then I have 1s of latency.

  I’ve also tried to change following parameters :

  e.Configuration.GlobalOptions.EnableReducedBuffering = true;
  
  e.Configuration.GlobalOptions.FlagNoBuffer = true;
  
  e.Configuration.GlobalOptions.MaxAnalyzeDuration = TimeSpan.Zero;

  but it gave no result.

  I measured time-interval between FFmpeg output lines (the number in the first column is the time, elapsed from the previous line, ms)

   ----     OpenCommand: Entered
  
     39     FFInterop.Initialize: FFmpeg v4.0
  
      0     EVENT START: MediaInitializing
  
      0     EVENT DONE : MediaInitializing
  
    379     EVENT START: MediaOpening
  
      0     EVENT DONE : MediaOpening
  
      0     COMP VIDEO: Start Offset:      0,000; Duration:        N/A
  
     41     SYNC-BUFFER: Started.
  
    609     SYNC-BUFFER: Finished. Clock set to 1534932751,634
  
      0     EVENT START: MediaOpened
  
      0     EVENT DONE : MediaOpened
  
      0     EVENT START: BufferingStarted
  
      0     EVENT DONE : BufferingStarted
  
      0     OpenCommand: Completed
  
      0     V BLK: 1534932751,634 | CLK: 1534932751,634 | DFT:    0 | IX:   0 | PQ:     0,0k | TQ:     0,0k
  
      0     Command Queue (1 commands): Before ProcessNext
  
      0        Play - ID: 404 Canceled: False; Completed: False; Status: WaitingForActivation; State: 
  
     94     V BLK: 1534932751,675 | CLK: 1534932751,699 | DFT:   24 | IX:   1 | PQ:     0,0k | TQ:     0,0k

  So, the most the process of "sync-buffering" takes the most of the time.

  Is there any parameter of FFmpeg which allows reducing a size of the buffer ?

• Fast movie creation using MATLAB and ffmpeg

  24 février 2018, par hyiltiz

  I have some time series data that I would like to create into movies. The data could be 2D (about 500x10000) or 3D (500x500x10000). For 2D data, the movie frames are simply line plot using plot, and for 3D data, we can use surf, imagesc, contour etc. Then we create a video file using these frames in MATLAB, then compress the video file using ffmpeg.

  To do it fast, one would try not to render all the images to display, nor save the data to disk then read it back again during the process. Usually, one would use getframe or VideoWriter to create movie in MATLAB, but they seem to easily get tricky if one tries not to display the figures to screen. Some even suggest plotting in hidden figures, then saving them as images to disk as .png files, then compress them using ffmpeg (e.g. with x265 encoder into .mp4). However, saving the output of imagesc in my iMac took 3.5s the first time, then 0.5s after. I also find it not fast enough to save so many files to disk only to ask ffmpeg to read them again. One could hardcopy the data as this suggests, but I am not sure whether it works regardless of the plotting method (e.g. plot, surf etc.), and how one would transfer data over to ffmpeg with minimal disk access.

  This is similiar to this, but immovie is too slow. This post 3 is similar, but advocates writing images to disk then reading them (slow IO).