How to make transparent color

[Jiang XU <jiang xu echostar com>]

I set up a pixmap as the background of a GtkWindow.  I want the widgets
that I put in that window will not cover the background, which means
they looks like transparent.
Fo example, I put a frame in the window, and it only shows the border
and does not cover the area of the background, so that you still can see
the background picture in that area

I wonder whether there are some easy way to do that?

Or, another way is that I get part of that background picture which will
be covered by the widgets and let them be the background picture of the
widgets.  But since I use a GdkPixmap to store the background, so how
can I easily create other GdkPixmaps of the areas which will be covered
by the widgets.

For example:
1.  I have a GdkPixmap *background.
2.  I set the GdkPixmap *background as the background picture of a
GtkWindow *mainwindow.
3.  I have a GtkFrame *frame1 which will be put in the GtkWindow
*mainwindow.

4.  I want  the area which will be covered by the frame1 looks like
transparent, so that I can see the background through the frame. (the
frame maybe only show the border).

How can I do that?
If somebody knows, please teach me ASAP

Thanks

Jiang