ffmpeg -i in.mkv -f lavfi -i color=c=black:s=1280x720 -filter_complex "scale=1080:720[v1];[v1][1:v]overlay=shortest=1:enable=gt(mod(t\,27)\,24)" -vcodec libx264 -pix_fmt yuv420p -r 24 -g 60 -b:v 2000k -shortest -preset ultrafast out.mkv