Since I wasn't able to set the mode (s.a.), I now draw only the outlines of columns of pdf page which works without a full refresh in most cases. In addition drawing a curve on the screen (for ink annotations) almost never provokes a full refresh.
Do you know of any parameters (e.g. number of changed pixels, size of changed area (in the sense of a convex hull)) which determine whether a full refresh is performed or not in the AUTO mode? Is switching black to white different from white to black?
PS: I created a thread for my program in the pdf section, if you like to test it:
https://www.mobileread.com/forums/sho...d.php?t=194502
(it uses Qt, which needs ~25Mb so you'll need a reformatted reader or Link2SD).