Update: Corrected a couple 1 off errors.

Update: Computes the upper half only. Makes a copy afterwards, flips vertically, and copies into bottom half.

Update: Draw lines when possible.

The following is constructed using MCE::Flow with 2 tasks (many providers and one writer). Spawning workers early is not necessary for this demonstration. I went with GD due to running faster than Imager. However, one can easily change to Imager inside the writer function.

use strict;
use warnings;

use MCE::Flow Sereal => 1;
use MCE::Queue  Fast => 1;
use GD;

# based on original code at http://www.alfrog.com/mandel.html
# karlgoethebier: initial code refactor, thanks perlmonks
# marioroy: parallelization via MCE::Flow (2 user tasks)
#           process upper half only  (reduces work for writer by 50%)
#           draw lines when possible (reduces work for writer by 56x)
# also see tips by BrowserUk at: 
# http://www.perlmonks.org/?node_id=1128885

# fasten your seat belt, enjoy the ride :)
my $width       = 1280 * 1;
my $height      = 1024 * 1;
my $iterations  = 20;

my $Q = MCE::Queue->new();
my $num_mandels = 3;
my $num_writers = 1; # must be 1

# init MCE
MCE::Flow::init {
   bounds_only  => 1,
   chunk_size   => 16,
   max_workers  => [ $num_mandels, $num_writers ],
   task_name    => [ 'tsk_mandel', 'tsk_writer' ],
   user_end     => sub {
      my ( $mce, $task_id, $task_name ) = @_;
      $Q->enqueue(undef) if ( $task_name eq 'tsk_mandel' );
   }
};

# compute mandelbrot (user tasks; sequence begin, end )
MCE::Flow::run_seq( \&mandel, \&writer, 0, $width - 1 );
MCE::Flow::finish;

# for MCE providers
sub mandel {
   my ( $mce, $chunk_ref, $chunk_id ) = @_;
   my ( $re_c, $im_c, $re_z, $im_z, $color, $last_color, $temp );
   my ( @ret, $middle );  $middle = int( $height / 2 );

   for my $x ( $chunk_ref->[0] .. $chunk_ref->[1] ) {
      for my $y ( 0 .. $middle ) {
         $re_c = ( $x - 3 * $width / 4 ) / ( $width / 3 );
         $im_c = ( $y - $height / 2 ) / ( $width / 3 );
         $re_z = $im_z = $color = 0;

         while ( 1 ) {
            $temp = $re_z;
            $re_z = $re_z * $re_z - $im_z * $im_z + $re_c;
            $im_z = 2 * $temp * $im_z + $im_c;
            ++$color;

            last if $re_z * $re_z + $im_z * $im_z > 4;

            if ( $color == $iterations ) {
               $color = 0;
               last;
            }
         }

         if ( $y && $color == $last_color ) {
            # increment line size, writer draws a line when > 0
            $ret[-1]++;
         }
         else {
            push @ret, $x, $y, $color, 0;
            $last_color = $color;
         }
      }
   }

   # freezing here to prevent double freezing/thawing
   $Q->enqueue( MCE->freeze( \@ret ) );
}

# for MCE consumer
sub writer {
   my ( $mce ) = @_;

   # init image and color palette
   my $image = new GD::Image( $width, $height );
   my @palette = $image->colorAllocate( 0, 0, 0 ); # black

   for ( 1 .. $iterations ) {
      my ( $r, $g, $b ) = map { int rand 255 } 1 .. 3;
      push @palette, $image->colorAllocate( $r, $g, $b );
   }

   # process draw requests
   while (1) {
      my $ret = $Q->dequeue;

      if (!defined $ret) {
         last unless --$num_mandels;
         next;
      }
      my $data = MCE->thaw( $ret );
      my $size = @{ $data };

      for ( my $i = 0; $i < $size; $i += 4 ) {
         if ( $data->[$i+3] ) {  # draw line if line_size > 0
            $image->line(
               $data->[$i],                    # x1
               $data->[$i+1],                  # y1
               $data->[$i],                    # x2
               $data->[$i+1] + $data->[$i+3],  # y2
               $palette[ $data->[$i+2] ]       # color
            );
         }
         else {                  # otherwise, set pixel
            $image->setPixel(
               $data->[$i],                    # x
               $data->[$i+1],                  # y
               $palette[ $data->[$i+2] ]       # color
            );
         }
      }
   }

   # copy upper half, flip vertically, then copy into bottom half
   my $middle = int( $height / 2 );
   my $temp = new GD::Image( $width, $middle );
   $temp->copy( $image, 0, 0, 0, 0, $width, $middle );
   $temp->flipVertical();

   $image->copy( $temp, 0, $middle + 1, 0, 0, $width, $middle );

   # save image
   open my $fh, '>mandelbrot.png' || return 1;
   binmode $fh;
   print $fh $image->png;
   close $fh;
}
[download]

The examples demonstrate using the Core MCE API and MCE::Flow.

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