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• FFMpeg : how to merge video and audio and tail align video to audio length ?

  28 septembre 2021, par Bobby

  I have a video file and an audio file to merge together. Audio duration is shorter than video. I want to align them at tail and cut off the head part of the video.

  


  Graph illustration :

  


  [xxxx------] video
    [------] audio
    [======] merged


  


  Where "x" parts of the video is removed.

  


  The duration of the video/audio is unknown but the audio duration is guaranteed to be shorter than video. The reason of it being unknown is because I'm generating video dynamically and pipe to FFmpeg, so it is not possible to know the duration beforehand.

  


• Cutting HEVC video results in parts stating with no video (black image) but correct audio for a few seconds

  8 juin 2020, par Oliver

  i have large HEVC video files and want to cut out parts of them. I use ffmpeg for that with the following command :

  



  ffmpeg -i input.mp4 -ss 01:18:47.040 -c:v copy -c:a copy -to 01:18:42.640 output.mp4


  



  i know that there can be problems when copy is used and no re-encoding, that is why i checked before that the frames specified in the -ss and -to parameters where keyframes (i-frames).

  



  So iam expecting to get a clean cut at the beginning and at the end. But what i get is 10 seconds of black image with normal audio in the beginning until the video starts. To make this clear : 10 seconds of video are missing in the beginning but the audio is fine.

  



  Btw : when moving the -ss part before the -i there seem to be no problems with the video (at least in the beginning). I know about the differences of putting -ss before and after the -i, but i also know, that using -ss after the -i should be slower (and was more accurate in the past) but should not make any differences in the final result. That is why I am so irritated.

  


• How to generate a video by looping an image and then concat with another very long video without re-encoding using ffmpeg ?

  23 mars 2021, par Linghao Chen

  I have an image and a very long video (1.5h).
Now I want to generate a heading video about 5 seconds by looping the image, and then concatenate it with the long video. Since the video is long, I don't want to re-encode the videos.
I have tried to generate the heading video using

  


  ffmpeg -loop 1 -i image.png -c:v libx264 -t 5 -pix_fmt yuv420p head.mp4


  


  and then

  


  ffmpeg -f concat -i list.txt -c:v copy concat.mp4


  


  where the list.txt contains

  


  file head.mp4
file longvideo.mp4


  


  I have tried these operations on two machines.
On one machine, the concatenated video has no audio. Moreover, it stucks at 5-10 seconds and directly jump to 11s.
On the other machine, the video and audio are not synchronized.

  


  To provide more information, I have checked the codecs of the two videos by

  


  ffprobe -v error -select_streams v:0 -show_entries stream=codec_name \
  -of default=noprint_wrappers=1:nokey=1 xxxxx.mp4


  


  Both of them are h264.

  


  My question is, how to generate it correctly with ffmpeg ? If it is hard using ffmpeg, is there any method to do it fast ? As far as I know, Adobe Premiere and Final Cut Pro do not export as fast as I expect because they re-encode the videos.