Alpha / transparency in a gtk_drawing_area

["Daniel Haude" <dunno stoptrick com>]

Hello fellow developers,

currently I'm trying to design my first custom widget. The widget is  
supposed to
graphically represent a rectangular area within a larger area (the widget
boundaries). The user is supposed to be able to drag, resize, and rotate  
the
rectangle with the mouse, but until the changes are committed I want the
original size/position of the rectangle to remain visible as a  
semi-transparent
shape. Just think of having a semi-transparent rectangle in the top layer  
of a
drawing program such as inkscape while dragging around an opaque one with  
the
mouse underneath.

Is there any way to do this "natively", or do I have to compute the four  
colors
involved (background, background behind transparent, opaque behind  
transparent,
opaque) and the corresponding polygons myself? I've tried to look it up in  
the
inkscape code but quickly got lost.

Thanks,
--Daniel