source: trunk/npemap.org.uk/scripts/tile-processing/makeSmaller.pl @ 83

#!/usr/bin/perl
#
use List::Util qw[min max];
use Image::Magick;

my $tilesize = 125;
my $scale = 3;

my $outdir = "scaled$scale";

if (! -d $outdir) {
    mkdir $outdir or die "Unable to create dir '$outdir'"
}

my @files = glob("*/tile-*.jpg");
my %tiles ;

my $minx, $miny, $maxx, $maxy;

foreach my $f (@files) {
  $f =~ m/-(\d+)-(\d+)\./;
  my ($x, $y) = ($1 +0, $2 +0);
  $tiles{$x} ||= {};
  $tiles{$x}{$y} = $f;

}


$maxx = max(keys %tiles);
$maxy = max(map { max(keys %{$tiles{$_}})} keys %tiles);

# start at 0, if there are any tiles in a WxW square, render a scale model 
# of it

foreach my $x (0..($maxx/$scale)) {
    foreach my $y (0..($maxy/$scale)) {
      my $filename = sprintf("%s/%03d/%03d.jpg", $outdir,$x,$y);
      if ((! -f $filename) && any_tiles($x, $y, \%tiles) ) {
         my $out = Image::Magick->new(size => ($tilesize * $scale). "x".($tilesize *$scale));
         $out ->ReadImage('xc:white');
 foreach my $i (0..($scale -1)) {
     foreach my $j (0..($scale -1)) {
         if (defined $tiles{($x*$scale)+$i}{($y*$scale)+$j}) {
            my $image = Image::Magick->new;
            print $image->Read($tiles{($x*$scale)+$i}{($y*$scale)+$j});
            print $out->Composite(image=>$image, x=>($i*$tilesize), y=>((($scale -1)-$j)*$tilesize));
            undef $image;
        }
     }
 }
 $out->Resize("${tilesize}x$tilesize");
 $out->Write("jpg:$filename");
 undef $out;
          print "($x,$y)\n";
        
      }
    }
}

use Data::Dumper;

print "($maxx, $maxy)\n";

sub any_tiles {
 my ($x, $y, $tiles) = (shift, shift, shift);
 foreach my $i (0..($scale -1)) {
     foreach my $j (0..($scale -1)) {
         if (defined $$tiles{($x*$scale)+$i}{($y*$scale)+$j}) {
             return 1;
         }
     }
 }
 return 0;
}