How to concatenate videos with ffmpeg

This one works for me:

ffmpeg -i input1.mp4 -i input2.webm -i input3.mov \
-filter_complex "[0:v:0][0:a:0][1:v:0][1:a:0][2:v:0][2:a:0]concat=n=3:v=1:a=1[outv][outa]" \
-map "[outv]" -map "[outa]" output.mkv

Reference: https://trac.ffmpeg.org/wiki/Concatenate

It assumes the videos have the same size and TBR. To transpose a video when one is vertical and another is horizontal, see here.

When video TBRs are different, it may come up with the strange message:

"More than 1000 frames duplicated"

When this happens, try adding "-vsync 2" to the command. Reference: https://stackoverflow.com/questions/62822889/ffmpeg-filter-complex-concat-never-completes. It worked for me.

How to transpose a video with ffmpeg

ffmpeg -i input.mp4 -vf "transpose=1" output.mp4

or

ffmpeg -i input.mp4 -vf "transpose=clock" output.mp4

for one 90-deg clockwise rotation.

Reference: https://ostechnix.com/how-to-rotate-videos-using-ffmpeg-from-commandline/#:~:text=FFMpeg%20has%20a%20feature%20called,flip%20them%20vertically%20and%20horizontally.&text=Here%2C%20transpose%3D1%20parameter%20instructs,video%20by%2090%20degrees%20clockwise.

Combine image + audio file to make a video

Source: https://video.stackexchange.com/questions/5291/how-to-make-a-video-from-a-still-image-and-an-audio-file

I tried at least 5 commands from different sites, and only this one worked well enough for me - about 37x speed, waitable.

ffmpeg -loop 1 -f image2 -r 2 -i image.jpg -i audio.m4a -c:v libx264 -c:a copy -shortest video.mp4