diff --git a/coord_tripler.pl b/coord_tripler.pl
index 4344f43..9b6cf22 100644
--- a/coord_tripler.pl
+++ b/coord_tripler.pl
@@ -9,8 +9,15 @@ no warnings "experimental::signatures";
 
 my $coords_regex = qr/^\s*(-?\d+)\s*,\s*(-?\d+)\s*$/;
 
-my %regions_to_tiles;
-my %tiles_to_regions;
+my %regions;
+my %tile_to_regions;
+my %bounds =
+(
+	nw_min => "Inf",
+	nw_max => "-Inf",
+	sw_min => "Inf",
+	sw_max => "-Inf"
+);
 
 my $input_file = shift;
 open (my $input_fh, $input_file);
@@ -18,29 +25,104 @@ open (my $input_fh, $input_file);
 my $output_file = shift // "$input_file.out";
 open (my $output_fh, ">$output_file");
 
+$regions{holes}{bg} = '#000000';
 while (my $line = <$input_fh>)
 {
 	my @fields = split '; ', $line;
-	print $output_fh (shift @fields) . "; "; #region name
-	print $output_fh (shift @fields) . "; "; #background colour
-	my @expanded_tile_specs = ();
-	foreach my $coords (@fields)
+	my $region = (shift @fields); #region name
+	$regions{$region}{bg} = (shift @fields); #background colour
+	expand_coords(\%regions, \%tile_to_regions, $region, \%bounds, @fields);
+}
+
+fill_holes(\%regions, \%tile_to_regions, \%bounds);
+foreach my $region (keys %regions)
+{
+	say $output_fh ("$region; $regions{$region}{bg}; " . make_output_region_line(\%regions, $region));
+}
+
+sub expand_coords($regions_hashref, $tile_to_region_hashref, $region, $bounds, @coord_list)
+{
+	foreach my $coords (@coord_list)
 	{
 		do { carp "Skipping bad spec: $coords"; next; } unless $coords =~ $coords_regex;
 		my $nw_base = $1 * 3;
 		my $sw_base = $2 * 3;
 
-		my @new = ("$nw_base,$sw_base");
-		push @new, ($nw_base+1) . "," . ($sw_base);
-		push @new, ($nw_base-1) . "," . ($sw_base);
-		push @new, ($nw_base) . "," . ($sw_base+1);
-		push @new, ($nw_base) . "," . ($sw_base-1);
-		push @new, ($nw_base+1) . "," . ($sw_base-1);
-		push @new, ($nw_base-1) . "," . ($sw_base+1);
+		push @{$regions_hashref->{$region}{tiles}{$nw_base}}, $sw_base;
+		push @{$regions_hashref->{$region}{tiles}{$nw_base+1}}, $sw_base;
+		push @{$regions_hashref->{$region}{tiles}{$nw_base-1}}, $sw_base;
+		push @{$regions_hashref->{$region}{tiles}{$nw_base}}, $sw_base+1;
+		push @{$regions_hashref->{$region}{tiles}{$nw_base}}, $sw_base-1;
+		push @{$regions_hashref->{$region}{tiles}{$nw_base+1}}, $sw_base-1;
+		push @{$regions_hashref->{$region}{tiles}{$nw_base-1}}, $sw_base+1;
+
+		$tile_to_region_hashref->{$nw_base}{$sw_base} = $region;
+		$tile_to_region_hashref->{$nw_base+1}{$sw_base} = $region;
+		$tile_to_region_hashref->{$nw_base-1}{$sw_base} = $region;
+		$tile_to_region_hashref->{$nw_base}{$sw_base+1} = $region;
+		$tile_to_region_hashref->{$nw_base}{$sw_base-1} = $region;
+		$tile_to_region_hashref->{$nw_base+1}{$sw_base-1} = $region;
+		$tile_to_region_hashref->{$nw_base-1}{$sw_base+1} = $region;
+
+		$bounds{nw_min} = $nw_base - 1 if $bounds{nw_min} > $nw_base - 1;
+		$bounds{nw_max} = $nw_base + 1 if $bounds{nw_max} < $nw_base + 1;
+		$bounds{sw_min} = $sw_base - 1 if $bounds{sw_min} > $sw_base - 1;
+		$bounds{sw_max} = $sw_base + 1 if $bounds{sw_max} < $sw_base + 1;
+	}
+}
+
+sub fill_holes($regions_hashref, $tile_to_region_hashref, $bounds)
+{
+	for(my $nw = $bounds->{nw_min}; $nw <= $bounds->{nw_max}; $nw++)
+	{
+		for(my $sw = $bounds->{sw_min}; $sw <= $bounds->{sw_max}; $sw++)
+		{
+			if($nw % 3 != 0 && $nw % 3 == $sw % 3)
+			{
+				my %neighbouring_regions;
+				$neighbouring_regions{$tile_to_region_hashref->{$nw+1}{$sw}}++ if exists $tile_to_region_hashref->{$nw+1}{$sw};
+				$neighbouring_regions{$tile_to_region_hashref->{$nw-1}{$sw}}++ if exists $tile_to_region_hashref->{$nw-1}{$sw};
+				$neighbouring_regions{$tile_to_region_hashref->{$nw}{$sw+1}}++ if exists $tile_to_region_hashref->{$nw}{$sw+1};
+				$neighbouring_regions{$tile_to_region_hashref->{$nw}{$sw-1}}++ if exists $tile_to_region_hashref->{$nw}{$sw-1};
+				$neighbouring_regions{$tile_to_region_hashref->{$nw+1}{$sw-1}}++ if exists $tile_to_region_hashref->{$nw+1}{$sw-1};
+				$neighbouring_regions{$tile_to_region_hashref->{$nw-1}{$sw+1}}++ if exists $tile_to_region_hashref->{$nw-1}{$sw+1};
 
-		push @expanded_tile_specs, (join "; ", @new);
+				my $total_neighbours = 0;
+				map {$total_neighbours += $_} (values %neighbouring_regions);
+				next if $total_neighbours < 6; # Must be surrounded to be filled
+				# Each neighbouring region will always provide 2,4 or 6 neighbouring tiles
+				my $found_region;
+				foreach my $region (keys %neighbouring_regions)
+				{
+					if($neighbouring_regions{$region} > 3) # == 4 or 6
+					{
+						$found_region = $region;
+					}
+				}
+				if(!$found_region) # Neighboured by 3 regions each providing 2 tiles
+				{
+					$found_region = (keys %neighbouring_regions)[int(rand(3))];
+				}
+				push @{$regions_hashref->{$found_region}{tiles}{$nw}}, $sw;
+				$tile_to_region_hashref->{$nw}{$sw} = $found_region;
+			}
+		}
+	}
+}
+
+sub make_output_region_line($regions_hashref, $region)
+{
+	my $line = "";
+	foreach my $nw (keys %{$regions_hashref->{$region}{tiles}})
+	{
+		foreach my $sw (@{$regions_hashref->{$region}{tiles}{$nw}})
+		{
+			$line .= "$nw,$sw; ";
+		}
 	}
-	say $output_fh (join "; ", @expanded_tile_specs);
+	chop($line);
+	chop($line);
+	return $line;
 }
 
 close $input_fh;