[cairo] shadow of transparent png image

[Dusan Halicky]

Hi

I have transparent png image. Can I somwhow in cairo draw it's shadow?
I mean not rectangular shadow of picture, but shadow of it's contents.
I was thinking about another surface - copy of original - but darken
to black color and displayed behind original surface. However I don't
know how to make this "darken" effect in cairo. Thanks.

I want something like this:

http://img216.imageshack.us/img216/5742/shadowvn1.png