ImageMagick: how to minimally crop an image to a certain aspect ratio?

Question

With imagemagick, I'd like to crop an image, in a minimal fashion, so that it fits a given aspect ratio.

Example: given an image of, say, 3038 x 2014 px, I want to crop it to have a 3:2 aspect ratio. The resulting image would then be 3021 x 2014 px, cropped from the, say, center of the original image.

So looking for a command looking something like convert in.jpg -gravity center -crop_to_aspect_ratio 3:2 out.jpg.

Answer 1

Imagemagick 7.0.7.22 and above

-crop 3:2 works since January 6th, 2018.

JPG

magick convert in.jpg -gravity center -crop 3:2 out.jpg 

Warning/reminder: if you don't use -gravity center, you will get two output files:

PNG

As fmw42 points out, PNG files store the virtual canvas size. +repage is recommended.

magick convert in.png -gravity center -crop 3:2 +repage out+repage.png 

GIMP, IrfanView, Chrome and Windows Explorer don't show any difference, but Imagemagick knows:

magick identify out*png out_stndrd.png PNG 252x168 314x168+31+0 8-bit sRGB 78557B 0.000u 0:00.000 out+repage.png PNG 252x168 252x168+0+0 8-bit sRGB 78529B 0.000u 0:00.000 

Imagemagick 6.9.9-34 and above

JPG

convert in.jpg -gravity center -crop 3:2 out.jpg 

PNG

convert in. -gravity center -crop 3:2 +repage out.png 

Imagemagick 6.9.9-33 / 7.0.7.21 and below

Note: you need to add magick before any convert for v7.

1. Specific target resolution

If your goal at the end is to have a certain resolution (for example 1920x1080) then it's easy, using -geometry, the circumflex/hat/roof/house symbol (^) and -crop:

convert in.jpg -geometry 1920x1080^ -gravity center -crop 1920x1080+0+0 out.jpg 

To loop over multiple jpg files:

for i in *jpg   do convert "$i" -geometry 1920x1080^ -gravity center -crop 1920x1080+0+0 out-"$i" done 

2. Aspect ratio crop only

If you want to avoid scaling, you have to calculate the new length of the cropped side outside of Imagemagick. This is more involved:

aw=16 #desired aspect ratio width... ah=9 #and height in="in.jpg" out="out.jpg"  wid=`convert "$in" -format "%[w]" info:` hei=`convert "$in" -format "%[h]" info:`  tarar=`echo $aw/$ah | bc -l` imgar=`convert "$in" -format "%[fx:w/h]" info:`  if (( $(bc <<< "$tarar > $imgar") )) then   nhei=`echo $wid/$tarar | bc`   convert "$in" -gravity center -crop ${wid}x${nhei}+0+0 "$out" elif (( $(bc <<< "$tarar < $imgar") )) then   nwid=`echo $hei*$tarar | bc`   convert "$in" -gravity center -crop ${nwid}x${hei}+0+0 "$out" else   cp "$in" "$out" fi 

I'm using 16:9 in the examples, expecting it to be more useful than 3:2 to most readers. Change both occurrences of 1920x1080 in solution 1 or the aw/ah variables in solution 2 to get your desired aspect ratio.

Photo credit: Anders Krusberg / Peabody Awards

Answer 2

Recent versions of Imagemagick (since 6.9.9-34) have an aspect crop. So you can do:

Input:

convert barn.jpg -gravity center -crop 3:2 +repage barn_crop_3to2.png 

The output is 400x267+0+0. But note that the +repage is needed to remove the virtual canvas of 400x299+0+16, because PNG output supports virtual canvas. JPG output would not need the +repage, since it does not support a virtual canvas.