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Crop a video using ffmpeg

Written by Solène, on 20 December 2019.
Tags: #ffmpeg

Comments on Mastodon

If you ever need to crop a video, which mean that you want to reduce the area of the video to a square of it to trim areas you don’t want.

This is possible with ffmpeg using the video filter crop. To make the example more readable, I replaced values with variables names:

  • WIDTH = width of output video
  • HEIGHT = height of output video
  • START_LEFT = relative position of the area compared to the left, left being 0
  • START_TOP = relative position of the area compared to the top, top being 0

So the actual commands look like

ffmpeg -i input_video.mp4 -filter:v "crop=$WIDTH:$HEIGHT:$START_LEFT:$START_TOP" output_video.mp4

If you want to crop the video to get a 320x240 video from the top-left position 500,100 the command would be

ffmpeg -i input_video.mp4 -filter:v "crop=320:240:500:100" output_video.mp4

Separate or merge audio and video using ffmpeg

Written by Solène, on 20 December 2019.
Tags: #ffmpeg

Comments on Mastodon

Extract audio and video (separation)

If for some reasons you want to separate the audio and the video from a file you can use those commands:

ffmpeg -i input_file.flv -vn -acodec copy audio.aac

ffmpeg -i input_file.flv -an -vcodec copy video.mp4

Short explanation:

  • -vn means -video null and so you discard video
  • -an means -audio null and so you discard audio
  • codec copy means the output is using original format from the file. If the audio is mp3 then the output file will be a mp3 whatever the extension you choose.

Instead of using codec copy you can choose a different codec for the extracted file, but copy is a good choice, it performs really fast because you don’t need to re-encode it and is loss-less.

I use this to rework the audio with audacity.

Merge audio and video into a single file (merge)

After you reworked tracks (audio and/or video) of your file, you can combine them into a single file.

ffmpeg -i input_audio.aac -i input_video.mp4 -acodec copy -vcodec copy -f flv merged_video.flv

How to remove a part of a video using ffmpeg

Written by Solène, on 02 October 2019.
Tags: #ffmpeg

Comments on Mastodon

If you want to remove parts of a video, you have to cut it into pieces and then merge the pieces, so you can avoid parts you don’t want.

The command is not obvious at all (like in all ffmpeg uses), I found some parts on differents areas of the Internet.

Split in parts, we want to keep from 00:00:00 to 00:30:00 and 00:35:00 to 00:45:00

ffmpeg -i source_file.mp4 -ss 00:00:00 -t 00:30:00 -acodec copy -vcodec copy part1.mp4
ffmpeg -i source_file.mp4 -ss 00:35:00 -t 00:10:00 -acodec copy -vcodec copy part2.mp4

The -ss parameter tells ffmpeg where to start the video and -t parameter tells it about the duration.

Then, merge the files into one file:

printf "file %s\n" part1.mp4 part2.mp4 > file_list.txt
ffmpeg -f concat -i file_list.txt -c copy result.mp4

instead of printf you can write into file_list.txt the list of files like this:

file /path/to/test1.mp4
file /path/to/test2.mp4