How to automate adding black pillars to create 16:9 aspect ratio images?

[Ofnuts]

I would look at the problem from a different point of view. If its for a TV, the final image size is fixed (1920*1080 or 3840*2160). So,

• Scale it so that its height is 1080 or 2160 (same as TV image prepared in first step), the width being scaled accordingly.
  
• Increase its size with a background
  

This is fairly easy to do in ImageMagick and can be done with a .BAT, the incantation being:

Code:

convert source.jpg -resize x400 -gravity center -background black -extent 800x400 -quality 85 result.jpg

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