Like the other guy, you are thinking about regions (which is what the X11 DAMAGE / WM_PAINT / etc stuff use). Dirty rectangles is a method used in older games where the game kept track of -usually- sprites on screen and whenever something changed (e.g. a sprite moving) that part of the screen was marked as dirty (often implemented as a list of non-overlapping rectangles, hence the name). Dirty rects also flows only in one direction.