[Therion] How to see a bitmap on a surface grid

[Footleg]

I am trying to create a surface model with a bitmap overlaid on it.

Using a 600 x 600 pixel bitmap file, I have done the following based
on an example found in the mailing list archive:

survey Surface -title "Test Surface Image"

surface
   bitmap bitmap.jpg [0 0 45000 32000 600 600 46000 33000]

   grid-units meter
   grid 45000 32000 100 100 10 10
   grid-flip none #(none)/vertical/horizontal
 730 730 725 700 715 710  715  700  660  640
 740 740 710 710 760 760  720  675  660  630
 760 750 720 720 765 700  685  670  695  670
 740 750 730 750 735 700  690  695  730  720
 735 750 755 780 775 720  740  730  760  740
 760 760 760 760 740 735  765  760  750  710
 780 775 770 755 740 730  780  775  755  745
 800 790 770 785 775 755  755  780  775  765
 800 795 785 825 800 760  755  780  800  780
 815 805 800 840 805 775  765  780  810  800

endsurface

endsurvey Surface

I get a surface model, but in the Loch viewer I only see a grey surface.
If I comment out the bitmap line then I get the same model, but the
file size of the lox file is much smaller. So my bitmap must be being
included, but it is not visible. What am I missing?

Footleg