#!/usr/bin/perl

use GD;

my $i = GD::Image->newFromJpeg("cave.jpg",1);

my $b = $i->colorAllocate(255,255,255);

for my $x (0..639) {
    for my $y (0..479) {
        $i->setPixel($x,$y,$b) if ($y % 2 ? $x+1:$x) % 2
    }
}

open $x, ">", "out2.jpg";
binmode $x;
print $x $i->jpeg();
close $x;