Q:

FFMPEG Overlay 1080X1920 video over 1280X720 video at a particular vertical position

I have a 1080X1920 (vertical) video with alpha channel with a blank spot 500px north of the bottom.  This blank spot is 1080X607.
I have a 1280X720 sized video I want to position in that spot, so the 1280 video will have to be sized down to 1080 wide and then positioned 500px from the bottom and UNDER the video on top.
This is what I currently have, but it is failing beautifully.  
/usr/local/bin/ffmpeg  -i 1080x1920.mov -i 1280x720.MOV 
  -filter_complex "[1][0]overlay=main_w-overlay_w-0:main_h-overlay_h-500" 
  -c:v libx264 -profile:v high444 -pix_fmt yuv420p -level 3.1 -y  final.mp4

A:

You have to pad the rescaled 720p video with the correct margins and then carry out the overlay.
ffmpeg  -i 1080x1920.mov -i 1280x720.MOV 
  -filter_complex "[1]scale=1080:-1,pad=1080:1920:0:1920-500-607[1v];[1v][0]overlay=0:0" 
  -c:v libx264 -profile:v high444 -pix_fmt yuv420p -level 3.1 -y final.mp4
Note that high444 profile bitstream may not be compatible with many players.

