pygtk: transparent pixmaps

[Jachym Cepicky <jachym cepicky centrum cz>]

Hallo,

I try to script some app. using pygtk. One of the tasks is to draw some
data on the image background. I would like to use approach like follows:

    pixbuf =  gtk.gdk.pixbuf_new_from_file("tmp.ppm")
    pixmap = gtk.gdk.Pixmap(widget.window, width, height)

    for line in lines:
        pixmap.draw_line(line[x], line[y]...)

    drawingarea.window.draw_pixbuf(...,pixbuf,0,0,0,0,.....)
    drawingarea.window.draw_drawable(..., pixmap, ....)

Result is, the background image (pixbuf) appears, but only shortly, than
it is overlaid by *black* (non-transparent) pixmap.

So my question: How to perform task like this with python?

Thank you very much for hints

Jachym


P.S.: yes, I read the archive, however I did not found satisfying answare,
which would be clear for me :-(
-- 
Jachym Cepicky
e-mail: jachym cepicky centrum cz
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