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• Audio fade in and looping

  30 juin 2020, par BIOStheZerg

  I'm trying to put some background music to a video with a person speaking. The background music should face in over a few seconds. The music is a short snippet, which loops really well. I tried to loop it via ffmpeg so that I have everything in one command :

  


  ffmpeg -i video_file.mp4 -f lavfi -i amovie=audio_file.wav:loop=0 -filter_complex '[0:a] volume=2.0 [voice]; [1:a] afade=in:st=0:d=5, volume=0.1 [soundtrack]; [voice][soundtrack] amix=inputs=2:duration=shortest' -codec:v copy out_file.mp4


  


  (this is a simplified version, the real one has a side chain compress as well, but that's irrelevant)

  


  The thing that doesn't work for me is the combination of the looping and the afade filter - every iteration of the background music fades in on its own. How could I do this such that only the first one fades in and the others just "continue" playing ? Is it possible to do it in a single command ?

  


  I know I could pre-loop a certain number of iterations based on the video length, but that would require creating a possibly large file (or lose audio quality via compression).

  


  Thanks !

  


• How to check when ffmpeg completes a video segment ?

  27 mai 2022, par Matthew Czarnek

  I'm using a ffmpeg Process from within my C# program. I start it off and run it in segmented mode.

  


  ffmpeg -i rtsp://127.0.0.1/axis-media/media.amp?resolution=1280x720 -c copy -map 0 -f segment -segment_time 21600 -strftime 1 -reset_timestamps 1 -segment_format flv "C:\REPLACE_ME_WITH_REAL_DIRECTORY\%Y-%m-%d_%H%M%S.flv"


  


  This creates a number of recording segments each in their own folder that are 6 hours long. I now need to be able to detect whether a file has started being written and whether it's completed being written as fast as possible to record it to a database. And this needs to work even in the face of crashes.

  


  I'm polling the folder and can detect that a file has started being written. But detecting whether a file has completed is much trickier. Possibly can be done by polling whether or not a file is being written to. Does ffmpeg have some sort of support for this ? Such as when it finishes a file it can launch another program or run a command ?

  


  I will occasionally scan through all the files and make sure that I record the ones that are there in case one is missed. But the more reliable, the better for this application.

  


• How to write a video stream containing B-frame and no DTS to a MP4 container ?

  14 février 2020, par SteveH

  I want to save a h264 video stream received from a RTSP source to a MP4 container.
  Not like other questions asked on SO, here the challenges I face are :

  • The stream contains B frames.

  • The stream has only PTS given by the RTP/RTCP.

  Here is the code I did

  //  ffmpeg
  
      pkt->data = ..;
  
      pkt->size = ..;
  
      pkt->flags = bKeyFrame? AV_PKT_FLAG_KEY : 0;    
  
      pkt->dts = AV_NOPTS_VALUE;
  
      pkt->pts = PTS;
  
  
  
      // PTS is based on epoch microseconds so I ignored re-scaling.
  
      //av_packet_rescale_ts(pkt, { 1, AV_TIME_BASE }, muxTimebase);
  
  
  
      auto ret = av_interleaved_write_frame(m_pAVFormatCtx, pkt);

  I received a lot of error messages like this :
  "Application provided invalid, non monotonically increasing dts to muxer ...".

  Result : the mp4 file is playable via VLC but the FPS is just a half of the original FPS and the video duration is incorrect (VLC shows a weird number).

  So how do I set correct DTS and PTS before sending to the container ?

  Update :
  I have tried some changes, though not successfully yet, I found that the reason of the frame rate drop is due to the muxer discards frames having incorrect DTS.
  Additionally, if I set start of PTS and DTS value too big, some players like VLC has to delay some time before showing video.