Applies imagemagick's -trim to a set of images, but applies the same crop to all of them together (making sure to cut nothing untrimmable off from any)

trim-together.py

#!/usr/bin/env python

'''
Author: Yotam Gingold <yotam (strudel) yotamgingold.com>
License: Public Domain [CC0](http://creativecommons.org/publicdomain/zero/1.0/)
Description: Applies imagemagick's -trim to a set of images, but applies the same crop to all of them together (making sure to cut nothing untrimmable off from any).
URL: https://gist.github.com/yig/7770ec597c595ae4bf1d0de4ca5a65cf
'''

from __future__ import print_function, division
import subprocess

import os, sys

def eprint(*args, **kwargs):
    print(*args, file=sys.stderr, **kwargs)

def usage():
    eprint( "Applies imagemagick's -trim to a set of images, but applies the same crop to all of them together (making sure to cut nothing untrimmable off from any)." )
    eprint( 'Usage:', sys.argv[0], "-suffix.png|--auto path/to/image1 path/to/image2 ..." )
    sys.exit(-1)

if len( sys.argv ) < 2: usage()

suffix = sys.argv[1]
paths = sys.argv[2:]

if os.path.exists( suffix ):
    eprint( "The first parameter should be a suffix or --auto. It exists as a file, which is probably an error." )
    usage()

for image in paths:
    if not os.path.exists( image ):
        eprint( "File not found:", image )
        usage()

top, left, right, bottom = None, None, None, None

## Gather outpaths
outpaths = []
for image in paths:
    base, ext = os.path.splitext( image )
    outpath = base + ( suffix if suffix != "--auto" else "-trim" + ext )
    outpaths.append( outpath )

## Gather dimensions
for image, outpath in zip( paths, outpaths ):
    output = subprocess.check_output( [ "convert", "-verbose", "-trim", image, outpath ], stderr = subprocess.STDOUT )
    
    last_line = output.rstrip('\n').split('\n')[-1]
    
    new_width, new_height = [ int(x) for x in last_line.split('=>')[-1].split()[0].split('x') ]
    new_x,     new_y      = [ int(x) for x in last_line.split('=>')[-1].split()[1].split('+')[1:] ]
    
    # print( "width:", new_width )
    # print( "height:", new_height )
    
    if top is None or new_y < top: top = new_y
    if left is None or new_x < left: left = new_x
    if bottom is None or new_y + new_height > bottom: bottom = new_y + new_height
    if right is None or new_x + new_width > right: right = new_x + new_width

## Apply crop
crop_parameter = "%sx%s+%s+%s" % ( right-left, bottom-top, left, top )
for image, outpath in zip( paths, outpaths ):
    command = [ "convert", "-crop", crop_parameter, image, outpath ]
    print( ' '.join( command ) )
    subprocess.check_output( command, stderr = subprocess.STDOUT )


print( "If you want to replace the original images, you may wish to follow up with a command like: rename -f -d '-trim' *-trim.png" )