I'm not sure exactly what you mean by "pen"? But assuming you mean palette index, then this will get you started.

It takes an image name as it's command line argument and outputs a table showing the palette indexes, the rgb values at those indexes and the count of pixels using that index. It will be easy to modify to just display the index/rgb of a particular pixel:

#! perl -slw
use strict;
use GD;

my $img = GD::Image->new( $ARGV[0] ) or die $!;

my %palette;
my( $x, $y ) = $img->getBounds;
for my $yi ( 0 .. $y ) {
    ++$palette{ $img->getPixel( $_, $yi ) } for 0 .. $x;
}

for my $i ( sort{ $a <=> $b } keys %palette ) {
    printf "index:%3d (%02x:%02x:%02x) n:%d\n",
        $i, $img->rgb( $i ), $palette{ $i };
}
__END__
C:\test>imgInspect.pl v3d_usb_perf.gif
index:  0 (ff:ff:ff) n:274410
index:  1 (00:00:7f) n:4909
index:  2 (7f:7f:7f) n:80
index:  3 (00:00:ff) n:14025
index:  5 (3f:3f:3f) n:2390
index:  6 (00:ff:00) n:12507
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Finally, a graphics system that lets you at the values in the GIF, rather than pretranslating them back to colours. Thanks for pointing me in the right direction. However, I thought it better to go with this:

#! perl -slw
use strict;
use GD;

my $img = GD::Image->new( $ARGV[0] ) or die $!;

my %palette;
my $total= 0;
my( $x, $y ) = $img->getBounds;
for my $yi ( 0 .. $y-1 ) {
    ++$palette{ $img->getPixel( $_, $yi ) } for 0 .. $x-1;
}

for my $i ( sort{ $a <=> $b } keys %palette ) {
    $total += $palette{ $i };
    printf "index:%3d (%02x:%02x:%02x) n:%d\n",
        $i, $img->rgb( $i ), $palette{ $i };
}
print( " $x * $y = ", $x*$y, " =?= $total\n");
[download]


M.

Good catch on my bounds error. ++