DirectDraw manages clip lists by using the DirectDrawClipper object. A clip list is a series of rectangles that describes the visible areas of the surface. A DirectDrawClipper object can be attached to any surface. A window handle can also be attached to a DirectDrawClipper object, in which case DirectDraw updates the DirectDrawClipper clip list with the clip list from the window as it changes.
Although the clip list is visible from the DirectDraw HAL, DirectDraw calls the HAL only for blitting with rectangles that meet the clip list requirements. For instance, if the upper-right rectangle of a surface was clipped and the application directed DirectDraw to blit the surface onto the primary surface, DirectDraw would have the HAL do two blits, the first being the upper-left corner of the surface, and the second being the bottom half of the surface.
The HAL considers the clip list for overlays only if the overlay hardware can support clipping and if destination color keying is not active. Most of today's hardware does not support occluded overlays unless they are subject to destination color keying. This can be reported to DirectDraw as a driver capability, in which case the overlay will be turned off if it becomes occluded. Under these conditions, the HAL does not consider clip lists either.