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• FFmpeg - divide single .webm video into multiple videos (each representing part of the screen) specifying rows and columns count

  23 mai 2022, par salveiro

  Can FFmpeg divide one big rectangular video into x smaller rectangular ones ?
What would be a command for it ?

  


  Can we parametrize the command with number of rows and columns we desire ?

  


  Can we somehow prevent loosing pixel precision when command is provided with improper rows/column count for source video resolution ?

  


• How do I accurately extend the duration of a tiny video clip ?

  18 avril 2018, par joeycato

  I noticed that ffmpeg doesn’t always scale the duration of a clip to the desired value ( even accounting for milliseconds of precision loss.) For example, whenever I attempt to scale the duration of this tiny 67ms clip to 7 seconds :

  ffmpeg -i input.avi -filter:v "setpts=(7/0.067)*PTS" -an output.avi    

  the resulting video is only 3.5 seconds in duration.

  Not really sure why this is off by multiple seconds, but I suspect it has something to do with the input clip itself ( which is only 2 frames @ 29.97 fps ) So far, I’ve discovered that if I scale the duration to a much larger value first, then scale back from that to 7 seconds, it works fine :

  ffmpeg -i input.avi -filter:v "setpts=(1000)*PTS" -an input_scaled_high.avi
  
  ffmpeg -i input_scaled_high.avi -filter:v "setpts=(7/33.400033)*PTS" -an output.avi

  But I’m hoping I don’t need to resort to an intermediate file. Is it possible to solve this with a single ffmpeg call ?

  If it helps, here is the video file I’m working with : https://drive.google.com/drive/folders/1hI5Xo6kfAfMd8ZylM6XrPX5fuO88gncE?usp=sharing

  Note : I should mention that I don’t need sound at all in this experiment ( hence the -an )

• How to configure AVStream to write 29.97FPS files using FFmpeg

  14 mai 2018, par vtruant

  I’m trying to write mkv file using ffmpeg to encode in FFV1 and FLAC in NTSC format, but the frame rate shown in VLC and media info are not correct.

  Here is how I create and configure the output format context :

  AVOutputFormat  *outputFormat = av_guess_format("matroska", NULL, NULL);
  
  
  
  //Allocate an AVFormatContext for an output format.
  
  int err = avformat_alloc_output_context2(&_formatContext, outputFormat, NULL, filename);
  
  
  
  //Specify the codec of the outputFormat
  
  _formatContext->oformat->video_codec = _videoCodecContext->codec_id;
  
  
  
  //Create AVStream 
  
  AVStream *videoStream = avformat_new_stream(_formatContext, NULL);  
  
  
  
  //FrameDuration.value : 1001, FrameDuration.timescale : 30000
  
  videoStream->time_base = (AVRational){ (int)_frameDuration.value, (int)_frameDuration.timescale };  //1001 30000
  
  
  
  //Copy video stream parameters to the muxer
  
  err = avcodec_parameters_from_context(videoStream->codecpar, _videoCodecContext);
  
  
  
  //Open file for writing
  
  err = avio_open(&_formatContext->pb, filename, AVIO_FLAG_WRITE);            
  
  
  
  if (err >= 0) {
  
      //Write header
  
      err = avformat_write_header(_formatContext, &options);
  
  }

  Before writing the packet, I use this to convert PTS to the stream time_base

  // Rescale output packet timestamp values from codec to stream timebase
  
  av_packet_rescale_ts(inAVPacket, *inTimeStamp, [outputStream stream]->time_base);

  The thing is that the avformat_write_header method is changing the stream time_base from 30000/1001 to 1/1000, so PTS loose precision. In VLC inspector, the frame rate shown is 1000 fps and in MediaInfo 30.033 fps.

  The file is playing correctly and the video/audio sync is OK.

  Is there something to do to specify the file frame rate somewhere else ?
  Or a work around to avoid changing the time_base when calling avformat_write_header ?