QRate - iterators for encoding


Dealing with the iterators for encoding in QRate.

We already saw that assembling the PDF is done based on an iterator function that is supposed to provide PNG images:

sub encode ($input, $output) {
   my $data = compress(path($input)->slurp_raw, 9) or die "compress()\n";
   return 0;

As we can see, there are two nested iterators at play:

  • qrcoder returns the QR codes as PNG images, based on a right-sized slice of data provided by
  • slicer, which takes the overall data and slices it accordingly.

Function qrcoder_it is a factory function, i.e. one that returns a function to be called later:

sub qrcoder_it ($it) {
   my $qrcode = Imager::QRCode->new(
      size          => 8,
      margin        => 2,
      mode          => '8-bit',
      version       => 1,
      level         => 'H',
      casesensitive => 1,
      lightcolor    => Imager::Color->new(255, 255, 255),
      darkcolor     => Imager::Color->new(0, 0, 0),
   return sub {
      my $data = $it->() // return;
      my $img = $qrcode->plot($data)->to_paletted;
      my $retval;
      $img->write(data => \$retval, type => 'png')
        or die "Failed to write: " . $img->errstr;
     return $retval;

As we can see, the whole point is to call the inner iterator to get the next chunk of data, and using Imager::QRCode to do the right magic. The returned value is a PNG image in memory.

Last, let’s take a look at the other factory function, i.e. the input data slicer:

use constant LINES_PER_SLICE => 16;

sub slicer_it ($data) {
   my @encoded = split m{\n}mxs, encode_base64($data);
   my $n_slice = 0;
   return sub {
      return unless @encoded;
      my @payload = splice @encoded, 0, LINES_PER_SLICE;
      my $header = $n_slice++ . (@encoded ? '+' : '.');
      return join "\n", $header, @payload, '';

Again, it’s a factory because it returns a sub that can be called to gather all the different pieces. In addition to slicing, ths function also does some framing, i.e. it puts a small header on top of each slice to keep track of which slice we are dealing with. In particular, we’re adding a sequence number and an indicator of wheter more slices are in line (with the + mark) or not (the final slice has marker .). This will come handy later, when we will reassemble the whole thing (possibly from unsorted scannerized pictures).

The choice to fit 16 lines per slice comes from the limit of a High-redundancy QR code for binary data, which for Imager::QRCode is set at 1268 bytes as we saw in a previous post

So… we’re done with the encoding!

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