This is, again, is mostly a note to my future self. 🙂
Occasionally, I need to process a bunch of image files in a batch. Most often it’s about resizing them, so they fit into a given format of at most x ⨉ y pixels, or precisely into, say a square format. Here’s how I do it using ImageMagick and a bit of Ruby code:
Processing a single file
ImageMagick comes with convert (it’s linked to …/bin/magick on my machine), as command line tool for processing image files in a whole lot of ways. In order to resize a single file so that it ends up as a square image in a given number of pixels one can use the following on the command line (I use zsh on macOS):
convert input_file.jpg -resize 1200x1200 -background White -gravity center -extent 1200x1200 output_file.jpg
This command
- reads ‘input_file.jpg’,
- creates a new image with 1200⨉1200 pixels,
- a white background,
- resizes the input image as needed,
- puts it in the center of the new image
- and saves it as ‘output_file.jpg’.
Batch processing files
For this step, I use a Ruby script (of course this can also be done in zsh, bash, Python etc.):
images = Dir['file_pattern*.jpg']
images.each do |fn|
`convert #{fn} -resize 1200x1200 -background White -gravity center -extent 1200x1200 #{fn.gsub(/\./, '_res.')}`
end
Tip: Be sure to use different names for the input and output file names.
