sub extract_crossing {
    my @x = @{ $_[0] }; my @y = @{ $_[1] };
    my @xcross =(); my @ycross =();
    for (my $i=0; $i<$#x; $i++) {
        my $k = ($y[$i] - $y[$i+1]) / ($x[$i] - $x[$i+1]);
        if($y[$i+1] * $y[$i] < 0) {
            my $xc = $x[$i+1] - $y[$i+1] / $k;
            push(@xcross, $xc);
        }
        if($x[$i+1] * $x[$i] < 0) {
            my $yc = $y[$i+1] - $x[$i+1] * $k;
            push(@ycross, $yc);
        }
    }
    return (@xcross, @ycross);
}