The viewing window is a conceptual boundary around a part of the model space. To produce the picture in the previous figure, the aircraft is drawn twice. The first time, the viewing window is on only the tail of the aircraft, and the second time, the viewing window is on the entire aircraft. Only those parts of the model space within the viewing window are visible in the picture assembled in presentation-page space. GpiSetViewingLimits is used to define the viewing window.