my $last_negativ=undef;
for(0..$#points_a)
{
  my $y_a=$points_a[$_][1];
  my $y_b=$points_b[$_][1];

  my $diff=$y_a-$y_b;

  my $negativ=0;
  $negativ=1 if($diff<0);

  if($diff!=0 and ( !defined($last_negativ) or $last_negativ != $negativ ) )
  {
    my $x=$points_a[$_][0];
    my $y=int(abs($diff)+$negativ?$y_a:$y_b);
    do_fill($x,$y,$negativ?$color_a:$color_b);
    $last_negativ = $negativ;
  }
}