[Enhanced] Bash stretch canvas up to 2880x2160 for portrait oriented photos

for f in *.*; do gm convert "$f" -background black -gravity center -extent "4320x2880<" -quality 75 "$f"; done

Enhanced version:

for f in *.*; do width=$(gm identify -format %w "$f"); if (($width != 4320)); then gm convert "$f" -background black -gravity center -extent 4320x2880 -quality 82 "$f"; fi; done

Hasselblad version:

for f in HX*.*; do width=$(gm identify -format %w "$f"); height=$(gm identify -format %h "$f"); if (($width < 2879)) || (($height < 2158)); then echo "$f" $width $height; fi; done

for f in HX*.*; do width=$(gm identify -format %w "$f"); height=$(gm identify -format %h "$f"); if (($width < 2879)) || (($height < 2158)); then gm convert "$f" -background black -gravity center -extent 2880x2160 -quality 100 "$f"; fi; done