#!/usr/bin/perl -w
# halftone.pl - Don Yang (uguu.org)
#
# Drop every other character.
#
# 2019-02-24

die "$0 <ratio>\n" unless $#ARGV >= 0;
my $ratio = shift @ARGV;
($ratio =~ /^\d+$/ && $ratio > 0) or die "Invalid ratio: $ratio\n";


my $y = 0;
while( my $line = <> )
{
   chomp $line;
   my $x = 0;
   my $output = "";
   foreach my $c (map {chr} unpack 'C*', $line)
   {
      if( ($x + $y) % $ratio == 0 )
      {
         $output .= $c;
      }
      else
      {
         $output .= " ";
      }
      $x++;
   }
   print $output, "\n";

   # Tweak to change the halftone direction.  If we simply did $y++,
   # the dots will appear in a diagonal pattern '/', but it seems more
   # pleasant to have the diagonal patterns look like '\'.
   $y = ($y + $ratio - 1);
}