Programming Web Graphics with Perl and GNU Softwar

Appendix A. A Simple PNG Decoder in Perl

The PNGObject Perl module will only parse the critical chunks, and will skip (and identify) ancillary chunks. You can use the PNGObject module to dump a listing of the chunks of a PNG file. It can be used like this:

use PNGObject;
my $png = new PNGObject;
$png->readPNG('some_image.png');

Here is the code for a simple PNG decoder module:

# -*- Perl -*- # PNGObject.pm # A Perl module that will decode the chunks of a PNG formatted file and # dump a printout of the various fields. # package PNGObject; use strict; # This hash defines the ordering of fields within the critical chunks # my %png_fields = ( # Critical chunks # header => [qw( width height bit_depth color_type compression filter interlace )], palette => [qw( red green blue )], image_data => [qw( data )] ); # This will create a string which can be used for # valid signature comparisons # my $png_signature = pack "H16", "89504E470D0A1A0A"; # A list of the standard ancillary chunks for use in a later # regular expression. # my $ancillary_chunks = join "|", qw( bKGD cHRM gAMA hIST pHYs sBIT tEXT tIME tRNS oFFs sCAL tIME ); # This hash may be referenced to determine the length of # each field (in bytes) for a particular chunk. # my %png_format = ( # Every chunk has an 8 byte header and a 4 byte trailer # all => { 'length' => 4, type => 4, # Chunk-specific data goes here # crc => 4 }, header => { width => 4, height => 4, bit_depth => 1, color_type => 1, compression => 1, filter => 1, interlace ...