1. Command ffmpeg -i input.mp4 -filter_complex "[0:v]scale=1080:-2,gblur=sigma=20[bg];[0:v]scale=1080:-2[fg];[bg][fg]overlay=(W-w)/2:(H-h)/2,crop=1080:1920,format=yuv420p[v]" -map "[v]" -map 0:a? -c:v libx264 -crf 20 -preset medium -c:a aac -b:a 128k -movflags +faststart out.mp4 2. Filtergraph in words: the source is scaled to the target width twice, once into the blurred background and once as the sharp foreground, then overlaid centred. Blurring happens on the small frame first, before any upscale, so the band does not stretch. The final crop pins the frame to exactly 1080x1920. 3. Captions: none given, so no ASS step is needed. 4. Verify ffprobe -v error -select_streams v:0 -show_entries stream=width,height,codec_name -of csv=p=0 out.mp4 should print 1080,1920,h264. Then ffprobe -v error -show_entries format=duration -of csv=p=0 out.mp4 should match the source within one frame, and ffmpeg -v error -i out.mp4 -f null - should print nothing at all. 5. Pitfalls here: odd source height producing 1080x1918 unless the final crop pins it, and an audio map that drops audio if the source has none. The -map 0:a? form only maps audio when it exists. 6. This command does not do semantic framing. The crop window is geometric, so a subject that moves across the frame still needs a fixed band or an editor.