DVD menu highlight is always rectangular

Hi all,

I'm trying to create some DVD menus but I can't figure out how to get the menu 
highlight to display my image.

I've created a four colour .png file with seven black arrows in it, with a 
transparent background.  I would like the arrows to become visible one by one 
as the menu selection changes.

spumux's automatic mode correctly identifies each arrow, but in the final menu 
I end up with a semi-transparent white box around where the arrow should be. 
It's as if it's using the bounding box instead of the image to create the menu 
highlight.

What am I doing wrong?  This is the XML being passed to spumux:

<subpictures>
   <stream>
     <spu end="00:00:00.0" force="yes"
       highlight="menu-buttons-highlighted.png"
       select="menu-buttons-selected.png"
       start="00:00:00.0"
       autooutline="infer"
       autoorder="rows"
       >
     </spu>
   </stream>
</subpictures>

But it's like the content of the .png files is being ignored (apart from 
successfully detecting the correct bounding box for each button's arrow image.)

Any suggestions as to how I can get a non-rectangular menu selection image?

Many thanks,
Adam.

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