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I want to make a horizontal video get converted to vertical video using FFMPEG for automation purposes. I tried with this, which works for vertical to horizontal

    ffmpeg -i input.mp4 \
  -vf 'split[original][copy];[copy]scale=ih*16/9:-1,crop=h=iw*9/16,gblur=sigma=20[blurred];[blurred][original]overlay=(main_w-overlay_w)/2:(main_h-overlay_h)/2' \
  output.mp4

And tried to change the "16/9" and "9/16" parameters backwards so it makes the reverse procedure, but i get an error.

I don't really need the blur thing, because i want to put some images on the black boxes that will result from doing this process.

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To rotate, you can just edit the video metadata. For example the following command produce output that is shown 90 degree rotated in VLC, you may use -90, depending on orientetion of your video.

ffmpeg -i input.mp4 -metadata:s:v rotate=90 output.mp4

I don't know but if the player is inept to read and apply this, it mightn't work.

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For the blur effect command, you needed to swap a few widths with heights then rescale it so the original video fits in properly above the blurred video:

ffmpeg -i input.mp4 -vf 'split[original][copy];[copy]scale=-1:ih*(16/9)*(16/9),crop=w=ih*9/16,gblur=sigma=20[blurred];[blurred][original]overlay=(main_w-overlay_w)/2:(main_h-overlay_h)/2' output.mp4

If you want black bars instead, it was covered in this post, but for the tl;dr, it's this:

ffmpeg -i input.mp4 -vf "pad=iw:2*trunc(iw*16/18):(ow-iw)/2:(oh-ih)/2,setsar=1" -c:a copy output.mp4

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