Three commands:

identify [options] filename.jpg
display [options] filename.jpg
convert input_file.jpg [options] output_file.jpg

Options:

‐background black  # default to white, used for letterbox
‐gravity center
‐resize 1920x1080  # scale down to fit, preserve aspect ratio
‐extent 1920x1080 # scale up to fit, preserve aspect ratio

Example:

convert infile.jpg -gravity center -background transparent -resize 1920x1080 -extent 1920x1080 ou

Notes:

• order of the keywords is significant, -gravity and -background must come before -extent
• without -resize, with -extent only, the image will not be enlarged.

Example on multiplie files:

find . -name "*.jpg" -exec convert {} -gravity center -background transparent -resize 1920x1080 -