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    croak "Unknown parameter $_" unless exists $acceptable->{$_};
    $self->set($_, $args{$_});
  }
  $self->x(352) unless $args{x};
  $self->y(288) unless $args{y};

  # Kowtow to the metadata bodging into filenames world
  $self->suffix('.png');

  $self;
}

sub type {
  'png';
}

sub write_images {
  my $self = shift;
  $self->section(0);
  $self->offset(0);
  $self->datum_length($self->calculate_datum_length());
  my $type = $self->type;
  my $filename_generator
    = $self->filename_generator || \&default_filename_generator;

  my @filenames;

  my ($filename, $state);
  while (!$self->done()) {
    my $image = $self->generate_next_image;
    ($filename, $state) = &$filename_generator($state);
    $filename .= $self->suffix;
    $image->write(file => $filename, type=> $type);
    push @filenames, $filename;
  }
  @filenames;
}

# package method
sub read_files {
  my $class = shift;
  # This is intentionally a "sparse" array to avoid some "interesting" DOS
  # possibilities.
  my $length;
  my %got;
  foreach my $file (@_) {
    my $img = new Imager;
    $img->open(file => $file) or carp "Can't read '$file': " . $img->errstr;
    my $payload = $class->extract_payload($img);
    my $datum;
    my $section;
    ($section, $datum) = unpack "wa*", $payload;
    if ($section == 0) {
      # Oops. Strip off the length.
      ($length, $datum) = unpack "wa*", $datum;
    }
    $got{$section} = $datum;
  }
  carp "Did not find first section in files @_" unless defined $length;

  my $data = join '', map {$got{$_}} sort {$a <=> $b} keys %got;
  substr ($data, $length) = '';

  $data;
}

1;
__END__

=head1 NAME

Acme::Steganography::Image::Png - hide data (badly) in Png images

=head1 SYNOPSIS

  use Acme::Steganography::Image::Png;

  # Write your data out as RGB PNGs hidden in the image "Camouflage.jpg"
  my $writer = Acme::Steganography::Image::Png::RGB::556FS->new();
  $writer->data(\$data);
  my @filenames = $writer->write_images("Camouflage.jpg");
  # Returns a list of the filenames it wrote to

  # Then read them back.
  my $reread =
     Acme::Steganography::Image::Png::RGB::556->read_files(@files);

=head1 DESCRIPTION

Acme::Steganography::Image::Png is extremely ineffective at hiding your
secrets inside Png images.

There are 4 implementations

=over 4

=item Acme::Steganography::Image::Png::FlashingNeonSignGrey

Blatantly stuffs your data into greyscale PNG files with absolutely no attempt
to hide it.

=item Acme::Steganography::Image::Png::RGB::556

Stuffs your data into a sample image, using the low order bits of each colour.
2 bytes of your data are stored in each pixel, 5 bits in Red and Green, 6 in
Blue. It produces a rather grainy image.

=item Acme::Steganography::Image::Png::RGB::323

Also stuffs your data into a sample image, using the low order bits of each
colour. Only 1 byte of your data is stored in each pixel, 3 bits in Red and
Blue, 2 in Green. To the untrained eye the image looks good. But the fact
that it's PNG will make anyone suspicious about the contents.

=item Acme::Steganography::Image::Png::RGB::556FS

Stuffs your data into a sample image, using the low order bits of each colour.
2 bytes of your data are stored in each pixel, 5 bits in Red and Green, 6 in
Blue. Changing the value of pixels to store data is adding error to the image,
in this case rather a lot of error. To attempt to conceal some of the
graininess Floyd-Steinberg dithering is used to spread the errors around. It's