Transparency/background color and iperender -png

Hi all!

I would like to generate some PNG icons from IPE files using iperender.
However, it seems that iperender always uses some kind of
half-transparent background pattern (a mixture of transparent and white
pixels).

To reproduce, just draw a simple rectangle (stroke only, black), and use
iperender -png rectangle.ipe rectangle.png

Is there some way to get a completely transparent background?

Or is this a cairo problem (since iperender uses cairo)?

I'm using the ipe-7.0.10-2 package on Ubuntu 10.10 amd64.

Best regards,
Martin