Inverting PDF colors (negative) using Perl PDF::API2

Question

Like from title I'm trying to write a script that inverts the colors of a bunch of PDF, by using Perl and PDF::API2. I'm not very familiar with perl, I've modified a brief script I found here on stackoverflow, from this post, with the help of chatGPT

how to change all colours in a PDF to their respective complimentary colours; how to make a PDF negative

The code I've come to is the following:

use strict;
use warnings;
use PDF::API2;
use PDF::API2::Basic::PDF::Utils;

my $dirname = '.';
my $filename;
opendir(DIR, $dirname) or die "Could not open $dirname\n";

mkdir("inverted") unless -d "inverted";

while ($filename = readdir(DIR)) {
  print("$filename\n");
  next unless $filename =~ /\.pdf$/; # Skip files that are not PDFs
  my $pdf = PDF::API2->open($filename);

  for my $n (1..$pdf->pages()) {
    my $p = $pdf->openpage($n);

    $p->{Group} = PDFDict();
    $p->{Group}->{CS} = PDFName('DeviceRGB');
    $p->{Group}->{S} = PDFName('Transparency');

    my $gfx = $p->gfx(1);  # prepend
    $gfx->fillcolor('white');
    $gfx->rect($p->mediabox());
    $gfx->fill();

    $gfx = $p->gfx();  # append
    $gfx->egstate($pdf->egstate->blendmode('Difference'));
    $gfx->fillcolor('white');
    $gfx->rect($p->mediabox());
    $gfx->fill();
  }
  $pdf->saveas("inverted/$filename");
}

closedir(DIR);

It seems to partially work sometimes, some pages are correctly inverted; however sometimes the first half of the page is not inverted, it remains white, like in this pic:

Page partially inverted

I'd like to fix this, I'd really need a simple script that perform this job, I've also written a script that after that join all the pdf files from multiple files into a single PDF. If anyone has an idea on how to fix it I'll be grateful, I could also upload the result on github if anyone needs this (it's a question has been asked other times too, but I haven't found a script nor in python or other languages that performs this work well, except for a couple of scripts that relies on docker and nodejs in order to install them)

• I've tried working with chatGPT to fix the issue, but it has no idea on how to do this (yes, I know, I shouldn't rely on it, but this is the first time I use Perl)

Answer 1

I am debugging this and am confident it has to do with rotation of the pages but I do not understand the details of the problem yet. However, I have this workaround for the test file:

• Rotate it 90 degrees with pdftk , then apply the perl script, then rotate it back 90 degrees with pdftk:

   $ pdftk test.pdf cat 1-endLeft output test2.pdf $ # run perl script to invert the colors in test2.pdf $ pdftk test2.pdf cat 1-endRight output test3.pdf

After this test3.pdf seems to be correctly inverted. This workaround might also work for the other files you have.