A window receives a WM_PAINT message whenever its update region is not NULL. A window procedure responds to a WM_PAINT message by calling the WinBeginPaint function, drawing to fill in the update areas, then calling the WinEndPaint function.
The WinBeginPaint function returns a handle to a presentation space that is associated with the device context for the window and that has a visible region equal to the intersection of the window's update region and its visible region. This means that only those portions of the window that need to be redrawn are drawn. Attempts to draw outside this region are clipped and do not appear on the screen.
If the application maintains its own presentation space for the window, it can pass the handle of that presentation space to WinBeginPaint, which modifies the visible region of the presentation space and passes the presentation-space handle back to the caller. If the application does not have its own presentation space, it can pass a NULL presentation-space handle and the system will return a cached-micro presentation space for the window. In either case, the application can use the presentation space to draw in the window.
The WinBeginPaint function takes a pointer to a RECTL structure, filling in this structure with the coordinates of the rectangle that encloses the area to be updated. The application can use this rectangle to optimize drawing, by drawing only those portions of the window that intersect with the rectangle. If an application passes a NULL pointer for the rectangle argument, the application draws the entire window and relies on the clipping mechanism to filter out the unneeded areas.
After the WinBeginPaint function sets the update region of a window to NULL, the application does the necessary drawing to fill the update areas. If an application handles a WM_PAINT message and does not call WinBeginPaint, or otherwise empty the update region, the application continues to receive WM_PAINT messages as long as the update region is not empty.
After the application finishes drawing, it calls the WinEndPaint function to restore the presentation space to its former state. When a cached-micro presentation space is returned by WinBeginPaint, the presentation space is returned to the system for reuse. If the application supplies its own presentation space to WinBeginPaint, the presentation space is restored to its previous state.