sub _thick_line {
    my ($image, $thickness, $color, $x1, $y1, $x2, $y2) = @_;

    my $angle  = atan2 $x2-$x1, $y2-$y1;
    my $mirror = atan2 $y1-$y2, $x2-$x1; # perpendicular angle

    my $dx = $thickness*sin($mirror)/2;
    my $dy = $thickness*cos($mirror)/2;

    $image->polygon(
        aa => 1,
        color => $color,
        points => [
            [$x1 + $dx, $y1 + $dy], # top left
            [$x2 + $dx, $y2 + $dy], # top right
            [$x2 - $dx, $y2 - $dy], # bottom right
            [$x1 - $dx, $y1 - $dy], # bottom left
        ],
    );
}