A window and its associated presentation space have three regions that control where drawing takes place in the window. These regions ensure that the application does not draw outside the boundaries of the window or intrude into the space of an overlapping window.
旼컴컴컴컴컴컴컴쩡컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� 쿝egion 쿏escription � 쳐컴컴컴컴컴컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� 쿢pdate Region 쿟his region represents the area of the window� � 퀃hat needs to be redrawn. This region changes� � 퀇hen overlapping windows change their z-order� � 쿽r when an application explicitly adds an � � 쿪rea to the update region to force a window � � 퀃o be painted. � 쳐컴컴컴컴컴컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� 쿎lip Region 쿟his region and the visible region determine � � 퀇here drawing takes place. Applications can � � 쿬hange the clip region to limit drawing to a � � 쿾articular portion of a window. Typically, a � � 쿾resentation space is created with a clip � � 퀁egion equal to NULL, which makes this region� � 쿮quivalent to the update region. � 쳐컴컴컴컴컴컴컴탠컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴� 쿣isible Region 쿟his region and the clip region determine � � 퀇here drawing takes place. The system changes� � 퀃he visible region to represent the portion � � 쿽f a window that is visible. Typically, the � � 퀆isible region is used to mask out � � 쿽verlapping windows. When an application � � 쿬alls the WinBeginPaint function in response � � 퀃o a WM_PAINT message, the system sets the � � 퀆isible region to the intersection of the � � 퀆isible region and the update region to � � 쿾roduce a new visible region. Applications � � 쿬annot change the visible region directly. � 읕컴컴컴컴컴컴컴좔컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴컴�Whenever drawing occurs in a window's presentation space, the output is clipped to the intersection of the visible region and clip region.
The clip region includes the overlapped part of the back window, but the visible region excludes that portion of the back window. The system maintains the visible region to protect other windows on the screen; the application maintains the clip region to specify the portion of the window in which it draws. Together, these two regions provide safe and controllable clipping.
To further control drawing, both the system and the application manipulate the update region. For example, if a window's position is switched from back to front, positions front to back, several changes occur in the regions of both windows. The system adds the lower-right corner of the new front window to that window's visible region. The system also adds that corner area to the window's update region.