I never used GD before today, so I made a little program to compare the output given by GD and Image::Magick. Here is the code:

#!/usr/bin/perl

use strict;
use warnings;

use GD;
use Image::Magick;

use vars qw( $default_orientation $default_width $default_height $file
+name );

$default_orientation = 'vertical';
$default_width = 150;
$default_height = 100;
$filename = 'image.jpg';

thumb_with_imagemagick($filename, 'im.jpg');
thumb_with_gd($filename, 'gd.jpg');

exit;

sub thumb_with_imagemagick {
        my ($infile, $outfile, $image, $error, $x, $y,
            $size, $format, $width, $height);

        ($infile, $outfile) = @_;

        $image = Image::Magick->new;
        $error = $image->Read($infile);

        if (!$error)
        {
                ($x, $y, $size, $format) = $image->Ping("$infile");
                ($width, $height) = thumbnail_dimensions($x, $y);
                $image->Scale(width=>$width, height=>$height);
                $error = $image->Write($outfile);
        }

        return $error;
}

sub thumb_with_gd {
        my ($infile, $outfile, $image, $error, $x, $y,
            $width, $height, $thumb);

        ($infile, $outfile) = @_;

        $image = GD::Image->new($infile);
        ($x, $y) = $image->getBounds();
        ($width, $height) = thumbnail_dimensions($x, $y);
        $thumb = new GD::Image($width, $height);
        $thumb->copyResized($image, 0,0 , 0,0 , $width,$height , $x,$y
+);

        open(IMAGE, '>', $outfile);
        print IMAGE $thumb->jpeg;
        close(IMAGE);
}


sub thumbnail_dimensions {
        my ($x, $y, $ratio, $height, $width);

        ($x, $y) = @_;

        $ratio = $x / $y;

        if ($default_orientation eq 'height') {
                $height = $default_height;
                $width = int($height * $ratio);
        }
        else
        {
                $width = $default_width;
                $height = int($width / $ratio);
        }

        return $width, $height;
}
[download]

I made some tests over few images and I still prefer the thumbnails produced by ImageMagick.

Ciao, Valerio