I managed to install this package via my linux package manager along with the library.

Here is something to get you started from first principles:

#!/usr/bin/perl
# by bliako
# for https://perlmonks.org/?node_id=11138293
# on 01/11/2021
use strict;
use warnings;

use Graphics::Magick;

my($image, $status);
$image = Graphics::Magick->new;
$status = $image->Read('a.jpg');

my $W = $image->Get('width');
my $H = $image->Get('height');
my $maxW = 800;
my $maxH = 200;

my ($newW, $newH, $factor) = trans($W, $H, $maxW, $maxH);
my $AR = $W / $H;
my $newAR = $newW / $newH;

print "($W x $H) => ($newW x $newH)\n";
print "original aspect ratio: $AR, new aspect ratio:$newAR\n";
$image->Resize(width => $newW, height => $newH);
$image->write("resized.jpg");

#testme();

# given original dimensions and new max dimensions
# it returns (newW, newH, resize_factor)
sub trans {
  my ($W, $H, $maxW, $maxH) = @_;

  my ($arW, $arH);

  my $factor = ($arW=($W/$maxW)) < ($arH=($H/$maxH)) ? $arH : $arW;

  my $newW = int($W/$factor);
  my $newH = int($H/$factor);
  return ($newW, $newH, $factor);
}

# this does a brute-force testing on sizes 10-1000
# for 800x200 max dim
sub testme {
  my $maxW = 800;
  my $maxH = 200;
  for my $W (map { $_*10 } 1..100){
    for my $H (map { $_*10 } 1..100){
      my ($newW, $newH, $factor) = trans($W, $H, $maxW, $maxH);
      my $AR = $W / $H;
      my $newAR = $newW / $newH;
      die "failed test: aspect ratios not the same ($AR -> $newAR) for
+ ($W x $H) => ($newW x $newH)"
        if ($AR-$newAR) > 10E-1;
      die "failed test: new width exceeds maximum for ($W x $H) => ($n
+ewW x $newH)"
        if $newW > $maxW;
      die "failed test: new height ($newH) exceeds maximum for ($W x $
+H) => ($newW x $newH)"
        if $newH > $maxH;
    }
  }
}
[download]

Output:

(2560 x 1536) => (333 x 200)
original aspect ratio: 1.66666666666667, new aspect ratio:1.665
[download]

bw, bliako

my $newimg = $img->scale(
    xpixels => 800,
    ypixels => 200,
    type => 'min',
);
[download]

I used convert a lot in the past and at the beginning I always felt dumb when thing didn't go like documented.

Then I realized that the tool is a Frankenstein'ish monster of various incompatible tools for different formats sewed together.

The docs are accordingly a Babylonian mishmash where different terminologies mix and getting things done require a lot of try and error.

Perl modules providing a wrapper can only attempt to compensate this.

Cheers Rolf
_{(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery}

> Babylonian Frankenstein'ish monster

No doubt, sounds like why it's spelled magick with an icK :-)

forked from ImageMagick in 2002

... but still does use the braindead ImageMagick way of NOT raising errors, but instead returning error codes. Shouldn't have been too hard to fix that after forking. (See also Re^2: Pixel setting oddity - Image:Magick, Re^7: Use of uninitialized value $pic)

Alexander

--
Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)

since there was a bit of time before the replies started coming I've started on a similar path using: gm convert -resize '800x200>' inputFile outputFile based on a combination of ImageMagick and GraphicsMagicK CLI documentations
https://www.imagemagick.org/script/command-line-processing.php#geometry
and
http://www.graphicsmagick.org/GraphicsMagick.html#details-resize

www.graphicsmagick.org/perl.html

See here for a command line alternative. Or here for a cheap GUI. See also for a solution for free. Less or more all the stuff i mentioned is scriptable/capable for batch processing etc. I forgot the ugly details.

G'day melutovich,

I can't find "graphicsmagick" on CPAN; please provide a link to this package. (You may find "What shortcuts can I use for linking to other information?" useful.)

You may find "What shortcuts can I use for linking to other information?" useful

Thanks for the reminder Ken,
I'd forgotten quite a few of the links and they will save a lot of time linking to CPAN modules!

G'day Bod,

You're welcome.

To be honest, except for links within this site (e.g. [id://11123653|Bod] above; and [id://43036] for the shortcuts links I posted), and very basic external URLs (e.g. [URL]), I rarely use those shortcuts.

I normally check the webpage and section first; my guesses are generally good but I'm not infallible. When writing up my post, I'll type "["; Alt-Tab to the webpage and copy the address bar; Alt-Tab again and paste; type "|"; Alt-Tab again and copy the title; Alt-Tab again and paste; type "]" and I'm done.

This potentially seems like a lot of work but, with over a decade of muscle memory at my fingertips, this is a two second task. Furthermore, I'll never provide a link to splice labelled split, or vice versa, again.

— Ken

Re^2: graphicsmagick perl shrink image to size

I used ImageMagick based stuff for a long time. But i switched to GD for most of my stuff, because it also allows for easy acceptable pixel manipulation, even if it's painfully slow.

The basic calculation to calculate the new image dimension goes something like this, no matter which image manipulation module you end up using (from memory, untested):

# Check if we are limited by height or width of the source image
my $factor = $sourcewidth / $targetwidth;
if($sourceheight / $factor > $targetheight) {
    $factor = $sourceheight / $targetheight;
)
my $newwidth = int($sourcewidth / $factor);
my $newheight = int($sourceheight / $factor);
[download]

You can then, for example, use the copyResized method from GD:

$newpic->copyResized($sourcepic,
                              $destx, $desty, # DEST X Y
                              0, 0, # SRC X Y
                              $newwidth, $newheight, # DEST W H
                              $sourcewidth, $sourceheight, # SRC W H
                          );
[download]

It's also worth noting that there might be a lot more pixel massaging needed to be done, depending on what those resulting images are used for. For example, i have a use case where i have to print user provided logos onto thermal paper. I ended up having to implement multiple algorithms for transforming color images to black&white pixels from which the user can select. Here is the printAddGreyscaleImage() from my PageCamel Framework PrintProcessor.pm:

Dithering by pixel manipulation is painfully slow (it can take multiple seconds per logo). But the software usually runs for days on end and has only a few different logos to deal with, so i did a simple in-memory caching. Once the system is up and running for a few minutes, no more slowdown.