This is really more of a reminder for me than anything else.

Convert is a tool from the same stable as Mogrify, and is an absolute life-saver when working with large batches of images.

Just the other day I was using it to crop some 2000 jpegs which were scans of pages from a rather thick book. The odd and even pages were offset, by which I mean that the space on the outside of the page was greater than on the inside, and I wanted to crop the pages so that they looked pretty even.

Not wanting to do this all by hand, I wrote a Python shell script to process the files.

#!/usr/bin/env python3

import subprocess
import os
import sys

path_to_jpegs = "/home/dani/big fat book/pages/"

path_to_output = path_to_jpegs + 'cropped/'

print('starting...')

file_path = path_to_jpegs

# loop through all the images in the specified folder
for filename in os.listdir(file_path):
    if ('.jpg' in filename):    
        print('@@@ processing file: ' + path_to_jpegs + filename)

        # file number is needed in order to determine of the file is an odd or even page
        dot_pos = filename.find('.')
        file_number = int(filename[:dot_pos])
        
        # switch coords depending if this file is even or odd
        # left page
        if (file_number % 2 == 0):
            #coords = '1940x2720+966+240'
            coords = '1940x2720+400+238'
        # right page
        else:
            #coords = '1940x2720+400+238'
            coords = '1940x2720+966+240'
        
        subprocess.call(['convert', '-crop', coords, path_to_jpegs + filename, path_to_output + filename])

If you are lucky or smart enough to use Ubuntu, then you have a wealth of tools sitting just under your keyboard that you might know owt about.

Two of those are the convert and mogrify tools, which come from the Imagemagick stable. I use these quite frequently for batch conversion jobs.

Here is an example of what these tools can do, specifically mogrify which acts on batches of files.

I have a folder load of transparent images that the client wants on a white background, so I open the command line, navigate to that folder (or just open the command line in that folder directly using Nemo instead of Nautilus) and type the following incantation:

mogrify -format jpg -quality 100 -fill white -opaque none *.png

If we break this down into individual components, then

mogrify this is the command itself

-format jpg The output format, in this case jpg

-quality 100 for jpgs specifies the compression or quality levels. 100 is tops, 1 is rubbish.

-fill white the colour of the fill which will be determined by the next command.

-opaque none deals with the alpha (transparency) channel, by filling it with a colour, which has handily just been specified.

*.png simply tells mogrify to grab any file that ends in “.png”