#!/usr/bin/perl -w
# Convert PGM image to text, outputting space for each white pixel and
# "X" for everything else.

use strict;

# Parse header lines.
my $line;
defined($line = <>) or die "Empty input.\n";
$line =~ /^P5/ or die "Not a PGM.\n";
defined($line = <>) or die "Incomplete header.\n";
$line =~ /^(\d+)\s+\d+/ or die "Missing image size.\n";
my $width = $1;
defined($line = <>) or die "Incomplete header.\n";
$line =~ /^255/ or die "Unsupported bit depth\n";

# Convert pixels.
my $x = 0;
while( $line = <> )
{
   foreach my $p (unpack "C*", $line)
   {
      print($p == 255 ? " " : "X");
      if( ++$x == $width )
      {
         print "\n";
         $x = 0;
      }
   }
}