What is the correct way to merge two ASCII art files side by side while preserving alignment?

Question

art_file (cat -A output):

 .::""-,                      .::""-.$
/::     \                    /::     \$
|::     |   _..--""""--.._   |::     |$
'\:.__ /  .'              '.  \:.__ /$
 ||____|.' _..---"````'---. '.||____|$
 ||:.  |_.'                `'.||:.  |$
 ||:.-'`       .-----.        ';:.  |$
 ||/         .'       '.        \.  |$
 ||         / '-.   '. \\       |.  |$
 ||:.     _| '   \_\_\\/(        \  |$
 ||:.\_.-' )     ||   m `\.--._.-""-;$
 ||:.(_ . '\ __'// m ^_/ /    '.   _.`.$
 ||:.  \__^/` _)```'-...'   _ .-'.'    '-.$
 ||:..-'__  .'        '. . '      '.      `'.$
 ||:(_.' .`'        _. ' '-.         '.   . ''-._$
 ||:. :   '.     .'          '.  . ' ' '.`       '._$
 ||:.  :    '. .'     .::""-: .''.        ' .   . ' ':::''-.$
 ||:. .'    ..' .    /::     \    '.        . '.    /::     \$
 ||:.'    .'      '. |::     |    _.:---""---.._'   |::     |$
 ||.      :          '\:.__ /   .'    -.  .-    '.   \:.__ /$
 ||:     : '.       . ||____|_.'    .--.  .--.    '._||____|$
 ||:'.___:   '.   .'  ||:.  |      (    \/    )      ||:.  |$
 ||:___| \     '. :   ||:.  |       '-.    .-'       ||:.  |$
 [[____]  '.     '.-._||:.  |      __  '..'  __      ||:.  |$
            '.    :   ||:.  |     (__\ (\/) /__)     ||:.  |$
              '.  :   ||:.  |        `  \/  `        ||:.  |$
                '-:   ||:.  |           ()           ||:.  |$
                   '._||:.  |________________________||:.  |$
                      ||:___|'-.-'-.-'-.-'-.-'-.-'-.-||:___|$
                      [[____]                        [[____]$

caption_file (cat -A output):

$
$
  _________                     .__          $
 /   _____/____    _____ ______ |  |   ____  $
 \_____  \\__  \  /     \\____ \|  | _/ __ \ $
 /        \/ __ \|  Y Y  \  |_> >  |_\  ___/ $
/_______  (____  /__|_|  /   __/|____/\___  >$
        \/     \/      \/|__|             \/ $
___________              __                  $
\__    ___/___ ___  ____/  |_                $
  |    |_/ __ \\  \/  /\   __\               $
  |    |\  ___/ >    <  |  |                 $
  |____| \___  >__/\_ \ |__|                 $
             \/      \/                      $
$
$

I am trying to merge art_file with caption_file side by side. So far I have tried two methods:

1. using pr -Jmt art_file caption_file

    .::""-,                      .::""-.
   /::     \                    /::     \
   |::     |   _..--""""--.._   |::     |        _________                        .__
   '\:.__ /  .'              '.  \:.__ /        /   _____/____    _____ ______ |  |     ____
    ||____|.' _..---"````'---. '.||____|        \_____  \\__    \  /     \\____ \|  | _/ __ \
    ||:.  |_.'                `'.||:.  |        /          \/ __ \|  Y Y  \  |_> >  |_\  ___/
    ||:.-'`       .-----.        ';:.  |       /_______  (____  /__|_|  /   __/|____/\___  >
    ||/         .'       '.        \.  |                 \/     \/      \/|__|             \/
    ||         / '-.   '. \\       |.  |       ___________                __
    ||:.     _| '   \_\_\\/(        \  |       \__      ___/___ ___  ____/  |_
    ||:.\_.-' )     ||   m `\.--._.-""-;         |      |_/ __ \\  \/  /\   __\
    ||:.(_ . '\ __'// m ^_/ /    '.   _.`.       |        |\  ___/ >    <  |  |
    ||:.  \__^/` _)```'-...'   _ .-'.'    '-.           |____| \___  >__/\_ \ |__|
    ||:..-'__  .'        '. . '      '.      `'.                      \/      \/
    ||:(_.' .`'        _. ' '-.         '.   . ''-._
    ||:. :   '.     .'          '.  . ' ' '.`       '._
    ||:.  :    '. .'     .::""-: .''.        ' .   . ' ':::''-.
    ||:. .'    ..' .    /::     \    '.        . '.    /::     \
    ||:.'    .'      '. |::     |    _.:---""---.._'   |::     |
    ||.      :          '\:.__ /   .'    -.  .-    '.   \:.__ /
    ||:     : '.       . ||____|_.'    .--.  .--.    '._||____|
    ||:'.___:   '.   .'  ||:.  |      (    \/    )      ||:.  |
    ||:___| \     '. :   ||:.  |       '-.    .-'       ||:.  |
    [[____]  '.     '.-._||:.  |      __  '..'  __      ||:.  |
               '.    :   ||:.  |     (__\ (\/) /__)     ||:.  |
                 '.  :   ||:.  |        `  \/  `        ||:.  |
                   '-:   ||:.  |           ()           ||:.  |
                      '._||:.  |________________________||:.  |
                         ||:___|'-.-'-.-'-.-'-.-'-.-'-.-||:___|
                         [[____]                        [[____]
   

2. paste art_file caption_file

 .::""-,                      .::""-.
/::     \                    /::     \
|::     |   _..--""""--.._   |::     |        _________                     .__
'\:.__ /  .'              '.  \:.__ /        /   _____/____    _____ ______ |  |   ____
 ||____|.' _..---"````'---. '.||____|        \_____  \\__  \  /     \\____ \|  | _/ __ \
 ||:.  |_.'                `'.||:.  |        /        \/ __ \|  Y Y  \  |_> >  |_\  ___/
 ||:.-'`       .-----.        ';:.  |       /_______  (____  /__|_|  /   __/|____/\___  >
 ||/         .'       '.        \.  |               \/     \/      \/|__|             \/
 ||         / '-.   '. \\       |.  |       ___________              __
 ||:.     _| '   \_\_\\/(        \  |       \__    ___/___ ___  ____/  |_
 ||:.\_.-' )     ||   m `\.--._.-""-;         |    |_/ __ \\  \/  /\   __\
 ||:.(_ . '\ __'// m ^_/ /    '.   _.`.       |    |\  ___/ >    <  |  |
 ||:.  \__^/` _)```'-...'   _ .-'.'    '-.    |____| \___  >__/\_ \ |__|
 ||:..-'__  .'        '. . '      '.      `'.                    \/      \/
 ||:(_.' .`'        _. ' '-.         '.   . ''-._
 ||:. :   '.     .'          '.  . ' ' '.`       '._
 ||:.  :    '. .'     .::""-: .''.        ' .   . ' ':::''-.
 ||:. .'    ..' .    /::     \    '.        . '.    /::     \
 ||:.'    .'      '. |::     |    _.:---""---.._'   |::     |
 ||.      :          '\:.__ /   .'    -.  .-    '.   \:.__ /
 ||:     : '.       . ||____|_.'    .--.  .--.    '._||____|
 ||:'.___:   '.   .'  ||:.  |      (    \/    )      ||:.  |
 ||:___| \     '. :   ||:.  |       '-.    .-'       ||:.  |
 [[____]  '.     '.-._||:.  |      __  '..'  __      ||:.  |
            '.    :   ||:.  |     (__\ (\/) /__)     ||:.  |
              '.  :   ||:.  |        `  \/  `        ||:.  |
                '-:   ||:.  |           ()           ||:.  |
                   '._||:.  |________________________||:.  |
                      ||:___|'-.-'-.-'-.-'-.-'-.-'-.-||:___|
                      [[____]                        [[____]

Both of them mess up the alignment of the second file, with paste generating a somewhat better output. So my questions are:

1. Using either paste or pr can I generate desired output? Some option(s) I am overlooking, perhaps?
2. If neither of them are the correct tools for the job, other than writing a new program, what pre-existing solution can I use?

Answer 1

The trouble is each line has a different length. The easiest solution is to give a large enough width to pr:

pr -mtw 150 art_file caption_file

If you want the caption text to get closer, I suggest

awk '
  l<length && NR<=n{l=length}
  NR!=FNR{
    printf "%-"l"s", $0
    getline line < "caption"
    print line
  }
' n="$(wc -l < caption)" art art

• n is the number of lines of the caption file.
• l is the length of the longest line between the first n lines of the art file.
• printf right-pads the art file with spaces so that it all its lines have l length.
• getline then gets a line from the caption file and prints it next to the just printed art line.

Note that you can add or subtract to the value of l in printf to ad-hoc adjust the spacing.

 .::""-,                      .::""-.
/::     \                    /::     \
|::     |   _..--""""--.._   |::     |                _________                     .__
'\:.__ /  .'              '.  \:.__ /                /   _____/____    _____ ______ |  |   ____
 ||____|.' _..---"````'---. '.||____|                \_____  \\__  \  /     \\____ \|  | _/ __ \
 ||:.  |_.'                `'.||:.  |                /        \/ __ \|  Y Y  \  |_> >  |_\  ___/
 ||:.-'`       .-----.        ';:.  |               /_______  (____  /__|_|  /   __/|____/\___  >
 ||/         .'       '.        \.  |                       \/     \/      \/|__|             \/
 ||         / '-.   '. \\       |.  |               ___________              __
 ||:.     _| '   \_\_\\/(        \  |               \__    ___/___ ___  ____/  |_
 ||:.\_.-' )     ||   m `\.--._.-""-;                 |    |_/ __ \\  \/  /\   __\
 ||:.(_ . '\ __'// m ^_/ /    '.   _.`.               |    |\  ___/ >    <  |  |
 ||:.  \__^/` _)```'-...'   _ .-'.'    '-.            |____| \___  >__/\_ \ |__|
 ||:..-'__  .'        '. . '      '.      `'.                    \/      \/
 ||:(_.' .`'        _. ' '-.         '.   . ''-._
 ||:. :   '.     .'          '.  . ' ' '.`       '._
 ||:.  :    '. .'     .::""-: .''.        ' .   . ' ':::''-.
 ||:. .'    ..' .    /::     \    '.        . '.    /::     \
 ||:.'    .'      '. |::     |    _.:---""---.._'   |::     |
 ||.      :          '\:.__ /   .'    -.  .-    '.   \:.__ /
 ||:     : '.       . ||____|_.'    .--.  .--.    '._||____|
 ||:'.___:   '.   .'  ||:.  |      (    \/    )      ||:.  |
 ||:___| \     '. :   ||:.  |       '-.    .-'       ||:.  |
 [[____]  '.     '.-._||:.  |      __  '..'  __      ||:.  |
            '.    :   ||:.  |     (__\ (\/) /__)     ||:.  |
              '.  :   ||:.  |        `  \/  `        ||:.  |
                '-:   ||:.  |           ()           ||:.  |
                   '._||:.  |________________________||:.  |
                      ||:___|'-.-'-.-'-.-'-.-'-.-'-.-||:___|
                      [[____]                        [[____]