How to make my own widget semi-transparent?

[Zhe Su <james su gmail com>]

Hi,
  I'm writing a program which uses cairo to paint some customized
semi-transparent widgets.These widgets might overlap with each other.
However I didn't find a way to make the widgets truely semi-transparent. If
a widget is covered by another widget, then the covered area will always be
erased by the upper widget.
  A test program and its screenshot is attached. The program draws a window
with two fixed widgets, both are painted with cairo. One is semi-transparent
green another is blue. No matter if composite extension is enabled or not,
the result is not what I expect.
  Anyone know how to implement real semi-transparent widgets with current
gtk?

Regards
James Su