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Date: Sun, 31 May 2015 22:08:06 -0700
From: Ethan Furman <ethan@stoneleaf.us>
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Just for funsies, I did a slight rewrite using my scription[1] library:

--8<------------------------------------------------------------------------------
#!/usr/bin/python3
"""
Convert scanned images: contrast is increased, size is enlarged,
format is changed from jpeg to png.
"""

from __future__ import print_function   # only needed if running in 2
from glob import glob
from scription import *

@Command(
         threshold=Spec('cutoff value for white vs black', OPTION, type=int, default=66),
         verify=Spec('confirm conversion by viewing on screen', FLAG),
         )
def image_convert(verify, threshold):
     for count, input in enumerate(sorted(glob('*.JPG')), start=1):
         output = '{0:02d}.png'.format(count)
         # only prints if --verbose on commandline
         print('Going to convert {} to {}'.format(input, output))
         while True:
             # Execute automatically parses into a list
             attempt = Execute(
                     'convert -threshold {}% {} {}'.format(threshold, input, output),
                     interactive='echo',
                     )
             if attempt.returncode:
                 raise SystemExit(attempt.returncode)
             if not verify:
                 break
             attempt = Execute(
                     'display_the_png {}'.format(output),
                     interactive='echo',
                     )
             if attempt.returncode:
                 raise SystemExit(attempt.returncode)
             if get_response('Does the image look good?'):
                 break
             threshold = get_response("New threshold value:", type=int)
         # conversion looks good, or manual verification skipped
         print('going to print {0}'.format(output))
         attempt = Execute(
                 'lpr -o fit-to-page -o media=A4 {}'.format(output),
                 interactive='echo',
                 )
         if attempt.returncode:
             raise SystemExit(attempt.returncode)

Main()
--8<------------------------------------------------------------------------------

Opinions about the usability of the above script, both as the script writer and as the user, welcomed.

--
~Ethan~


[1] yes, everyone apparently writes their own command-line processor -- this one is mine.  ;)