Resize image to a maximum size on command line

Question

I have a very big collection of very big images, and I want to create a thumbnail gallery in which images are resized to a maximum-minimum-width (you will understand) of 180px.

For example: If the image is 2400x1800, it will be resized to 240x180 Else if it is 1600x1800, it will be resized to 180x202

Is it clear? The smallest size of the picture should be exactly 180, no matter if it is the width or height.

How is this possible with the command line? I know how to resize each file to a certain size, but not with those different sizes.

Answer 1

How about using ImageMagick's convertwhich has exactly such a minimal size option, see Image Geometry options?!

Copy & Paste snippet (bash syntax) -- please notice the ^ after the size specification:

for file in *.jpg; do
   echo -n Converting ${file}...
   convert -resize 180x180^ "$file" "th_$file"
   echo done
done

---

In addition, if you want to crop the resulting file to a quadratic shape around the center, you can use this script. The SIZE parameter in the first line specifies (surprise, surprise) the final size of the thumbnail.

SIZE=180
for file in *.jpg; do
   echo -n Converting ${file}...
   convert -resize ${SIZE}x${SIZE}^ "$file" temp.png
   convert -crop $(identify temp.png | awk -F'[ x]' -v SIZE=$SIZE '{ printf "%ux%u+%u+%u", SIZE, SIZE, ($3-SIZE)/2, ($4-SIZE)/2 }') temp.png "th_$file"
   echo done
done
rm temp.png

The script is not very optimized, as it runs two commands (identify and convert -crop) on the thumbnail. But as the thumbnail is only small, I think the speed is reasonable.