I'm trying to add a 16:9 intro to videos for my site using ffmpeg. So far I have the following filter which I found here:

ffmpeg -i intro.mp4 -i input.mp4 -filter_complex "[0][1]scale2ref=iw:ih[intro][main];[intro]drawbox=t=fill[intro-bg];[0][intro-bg]scale2ref=iw:ih:force_original_aspect_ratio=decrease:flags=spline[intro][intro-bg];[intro-bg][intro]overlay=x='(W-w)/2':y='(H-h)/2'[intro-resized]; [intro-resized][0:a][main][1:a]concat=n=2:v=1:a=1:unsafe=1[v][a]" -map '[v]' -map '[a]' -movflags +faststart -acodec aac -vcodec h264_nvenc -cbr true -b:v 1000k -bt 1000k -y -2pass true -threads 32 -profile:v high -r 30 -preset medium output.mp4

It works great unless the main video isn't 16:9. If it's a portrait video it will just be stretched out to fill the screen.

Does anyone know how i can adjust the filter to add black bars down the side of the video to keep it the correct aspect ratio? I'm struggling to get my head around the filters.

Thanks in advance

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