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• FFMPEG Audio/Video out of sync after transcoding video in segments

  25 mai 2017, par Idan

  My system transcodes videos in a very specific way.

  1. separating the video and the audio
  2. transcoding the audio stream
  3. segmenting the video
  4. transcoding each of the segments of the video
  5. concat all segments back to 1 video
  6. merging the new transcoded video and audio back together

  While at 99% of the times the process works as it should and the result is a valid video+audio file. in 1% of the times, I get the video and audio out of sync.

  When investigated the issue I noticed that the sync issue appears in a specific segment/s. Meaning, if the video was sliced into 100 segments, the sync can be ok for the first 50 segments, then something happens and the audio or video gets an offset and goes out of sync. It can occur in any number of segments in one video.

  I guess it’s something to do with timestamps getting lost in the process and segments changing their length in the process but I have no idea how I can overcome it. Open for suggestions.

  The commands I use for each step (paths were shorten and may not match, not real issue there) :

  Segmenting the video :

  ffmpeg -fflags +genpts -i $INPUT_FILE -c copy -map 0:0 -flags -global_header -segment_time 10 -break_non_keyframes 0 -reset_timestamps 1 -segment_list segments.list -segment_list_type ffconcat -write_empty_segments 0 -segment_format mp4 -f segment seg-%d.mp4

  Transcoding audio :

  ffmpeg -i $INPUT_FILE  -vn -c:a aac -threads 1 -ac 2 -b:a 125588 audio.mp4

  Transcoding each of the segments :

  ffmpeg -y -i $f -an -vcodec libx264 -threads 4 -r 30 -pix_fmt yuv420p -crf 20 -preset:v fast -profile:v main -level:v 4.1 transcoded/$f

  Concat segments :

  ffmpeg -y -f concat -i segments.list  -c copy -movflags +faststart file_video.mp4

  Combine video and audio :

  ffmpeg -y -i file_video.mp4 -i file_audio.mp4 -c copy -shortest file_out.mp4

• FFmpeg - video frame cropped after rotating video ?

  20 juin 2021, par Md Yeamin

  I am using following command to rotate video :

  


  ffmpeg -i input.mp4 -c:v h264 -vf rotate=PI/2 -c:a copy output.mp4


  


  After processing the video, the frame get cropped. Please see the attached screenshot of the video for reference.

  


  Screenshot of original video


  


  Screenshot of rotated video


  


• Extracting parts of video and make a new video with them [on hold]

  13 février 2014, par Filippo

  I need to extract parts of a video and make a video (in order to make a summary).
  My videos are mostly mpg, sometimes avi.

  I'm using ffmpeg to do it :

  Extraction : ffmpeg -ss XX -t XX -i video.mpg -vcodec copy -acodec copy partX.mpg

  Concatenation : ffmpeg -i concat:part1.mpg|part2.mpg|... -vcodec copy -acodec copy summary.mpg

  I have two problems :

  • ffmpeg only let me set position by time reference and not by frame number. I will prefere to set it by frame number, but it's not a big deal
  • the resulting video is...bad. The transitions between extracted parts are not smooth at all.

  What is the best way to do that ?
  I guess I need some kind of non-linear editing library : gnonlin, gtreamer-editing-services (but no package for Fedora 16 ?), MLT...

  But how I can do that easily ? (in preference in python)

  I can't find any good example.

  Thanks.