How to realize the undo function in a sketchbook application?

["hamb ka" <hamb_ msn com>]

Hi, all

I am now writing a sketchbook application, and by using multiple GdkPixmaps 
I want to implement the Redo and Undo functions. It is simple to record down 
every stroke a user made, but if the strokes are overlapping each other, the 
later pixmaps would cover the previous one. And since there is no 
transparent color in gtk+, the option of changing color in graphics contexts 
is not viable.
Please give me a suggestion on it...
Thanks :)

Regards,
hamb

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