colorless/transparent of drawing area

[nil nil <ztwinsen yahoo com hk>]

Hi everybody,
    
      I'm faced a problem of rendering a image. I can
now render a image to screen throught gdk. But the
image is background is not colorless, it's something
grey in color(drawing area?). I have seen
someone else have rendered pixmap's background into
colorless by gnome_pixmap_new_from_file, but I want to
do by gdk. My question is: How can I turn the drawing
area into colorless by gdk?

      I've suffered this problem for a long while and
have no idea how to solove it. plz help! any
suggestion / comment is welcome. 

thx
ZTwinsen

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