scalable fonts not scaling

[bwkeck3 gmail com]

(sorry for empty message)

Hello,

I've got an old perl script for printing plaintext files in monospace.
Its arguments allow specifying the width of lines of text in characters
& millimeters (simultaneously & independently).  This works because it
uses Courier via the normal PostScript operators (scalefont in
particular).

I've been trying to rewrite it using Pango, but I can't get the scaling
to work accurately.  Hoping for some assistance from the list, though I
see that most of the traffic is about Gtk, & I'm interested in
PostScript.

I've golfed it down to the following script (independent of
Gtk/PostScript) ...

      #!/usr/bin/perl
      use Pango;
      use constant PANGO_SCALE => 1024;
      my $fontpat = 'Nimbus Mono L Regular';
      my $fontdesc = Pango::FontDescription->from_string($fontpat);
      my $fontmap = Pango::Cairo::FontMap->new;
      my $context = $fontmap->create_context;
      my $layout = Pango::Layout->new($context);
      my $text = 'A' x 50;
      for my $set_size (1 .. 10) {
        $fontdesc->set_size($set_size * PANGO_SCALE);
        $layout->set_font_description($fontdesc);
        $layout->set_text($text);
        my $textwidth = ($layout->get_size)[0] / PANGO_SCALE;
        print "$set_size $textwidth\n";
      }

The output is ...

     $set_size  $textwidth
         1          50
         2         100
         3         100
         4         150
         5         200
         6         250
         7         300
         8         300
         9         350
        10         400

Why is this so non-linear when Nimbus Mono L is supposedly scalable?
And when my old script works via CUPS to a non-postscript printer.

Thanks for any help,
Brian Keck