Shall I call you Honza or dolik? I use the Ubuntu ppa you maintain regularly, and thank you for the time you you invested for the community.
Your reply is closer to what I was actually seeking. A little question, as Upp will alway mask and transfer the changed Rect or possibly combined area to the screen, what's the point of cache the Ctrl image? I should be able to draw on a blank image representing the Ctrl Rect, and draw only the part that's changed, Upp will mask off other parts before it transfer to the screen anyways.