"non-detructive" line drawing?

[Phil Cummins <cummins jamstec go jp>]

Hello,
  
  I'm developing an application using GtkPerl in which I
load a pixmap into the DrawingArea widget, and then use the
pointer to draw a polyline on top of it. But it is important that 
I be able to erase, if desired, segments of the polyline. I
know from past (non-gtk) experience that a simple way to do this 
is to do the drawing via a boolean exlusive-or of the pixel 
values. Then to erase the line, all you have to do is re-draw it.
Can any one please tell me if there is a way to change
the line-drawing mode for the DrawingArea widget so that
it uses such an exclusive-or operation? Or is there another
way to erase polyline segments? Note that I want to avoid
having to re-draw the pixmap everytime an erase operation
occurs, as this would be too time-consuming.

   Thanks.
-- 
-- 
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Phil R Cummins                                 cummins jamstec go jp
Frontier Research for Subduction Dynamics  tel(i/d):81/0-468-67-3393
JAMSTEC, 2-15 Natsushima-cho               fax(i/d):81/0-468-67-3409
Yokosuka 237-0061, Japan