FFMpeg to resize any video to fit 1080x1920 vertical, without cropping, instead by shrinking and adding blurred borders?

Question

I found some posts explaining how to turn any video horizontal by adding blurred borders using FFMpeg, but I want to convert videos to vertical 1080x1920. I don't want it to enlarge the video, nor crop if a dimension is bigger than either 1080 or 1920 dimension. Instead, I want it to shrink the video until it fits fully inside 1080x1920, and then I want it to add blurred borders to the empty areas.

This is the snippet I found, but when I tried reversing the numbers, it actually cropped the video.

ffmpeg -I input.mp4 -lavfi "[0:v]scale=1920*2:1080*2,boxblur=luma_radius=min(h\,w)/20:luma_power=1:chroma_radius=min(cw\,ch)/20:chroma_power=1[bg];[0:v]scale=-1:1080[ov];[bg][ov]overlay=(Ww)/2:(Hh)/2,crop=w=1920:h=1080" output.mp4

Answer 1

Simple method:

ffmpeg -i input.mp4 -filter_complex "[0:v]boxblur=40,scale=1080x1920,setsar=1[bg];[0:v]scale=1080:1920:force_original_aspect_ratio=decrease[fg];[bg][fg]overlay=y=(H-h)/2" -c:a copy output.mp4

"Simple" because it forces the background to 1080x1920 and ignores aspect ratio. So the background it will looked stretched, but it is blurred so much nobody will care or notice.