#!/usr/bin/env perl

use 5.010;
use strict;
use warnings;

# --------------------------------------

use GD;

my @white = ( 0xFF, 0xFF, 0xFF );
my @black = ( 0x00, 0x00, 0x00 );

my $size   = shift @ARGV || 400;
my $offset = $size / 2;
my $height = 0.9 * $offset;
my $width  = 0.1 * $offset;

my $img = GD::Image->new( $size, $size, 1 );
$img->saveAlpha( 1 );
$img->alphaBlending( 1 );

# background color: black & transparent
my $bg  = $img->colorAllocateAlpha( @black, gdAlphaTransparent );
$img->fill( 0, 0, $bg );

# a simple polygon
my $p = GD::Polygon->new();
$p->addPt( $offset + $width, $offset +       0 );
$p->addPt( $offset +      0, $offset - $height );
$p->addPt( $offset - $width, $offset +       0 );
$p->addPt( $offset +      0, $offset +  $width );

# polygon color: black & opaque
my $clr = $img->colorAllocateAlpha( @black, gdAlphaOpaque );
$img->setAntiAliased( $clr );
$img->filledPolygon( $p, gdAntiAliased );

# for testing, use script name for PNG file
my $png_file = "$0.png";
open my $fh, '>:raw', $png_file or die "could not open $png_file: $!\n
+";
print {$fh} $img->png() or die "could not print to $png_file: $!\n";
close $fh or die "could not close $png_file: $!\n";

__END__
[download]

I changed the background colour to transparent white and it seems that the transparency is anti-aliasing in the complete opposite of what it should be. That is, opaque where it should be transparent and vice versa.

#!/usr/bin/env perl

use 5.010;
use strict;
use warnings;

# --------------------------------------

use GD;

my @white = ( 0xFF, 0xFF, 0xFF );
my @black = ( 0x00, 0x00, 0x00 );

my $size   = shift @ARGV || 400;
my $offset = $size / 2;
my $height = 0.9 * $offset;
my $width  = 0.1 * $offset;

my $img = GD::Image->new( $size, $size, 1 );
$img->saveAlpha( 1 );
$img->alphaBlending( 1 );

# background color: white & transparent
my $bg  = $img->colorAllocateAlpha( @white, gdAlphaTransparent );
$img->fill( 0, 0, $bg );

# a simple polygon
my $p = GD::Polygon->new();
$p->addPt( $offset + $width, $offset +       0 );
$p->addPt( $offset +      0, $offset - $height );
$p->addPt( $offset - $width, $offset +       0 );
$p->addPt( $offset +      0, $offset +  $width );

# polygon color: black & opaque
my $clr = $img->colorAllocateAlpha( @black, gdAlphaOpaque );
$img->setAntiAliased( $clr );
$img->filledPolygon( $p, gdAntiAliased );

# for testing, use script name for PNG file
my $png_file = "$0.png";
open my $fh, '>:raw', $png_file or die "could not open $png_file: $!\n
+";
print {$fh} $img->png() or die "could not print to $png_file: $!\n";
close $fh or die "could not close $png_file: $!\n";

__END__
[download]

How can I fix this?