To generate a variety of different views of a single picture, we can generate different view of a picture by applying the appropriate scaling and translation. While doing this, we have to identify the visible part of the picture for inclusion in the display image. This selection process is not straight forward. Certain lines may lie partly inside the visible portion of the picture and partly outside. These lines cannot be omitted entirely from the display image because the image would become inaccurate. This is illustrated in figures shown below.

The process of selecting and viewing the picture with different views is called Windowing, and a Process which divides each element of the picture into its visible and invisible portions, allowing the invisible portion to be discarded is called Clipping.

We know that the picture is stored in the computer memory using any convenient Cartesian coordinate system, referred to as World Coordinate System (WCS). However, when picture is displayed on the display device it is measured in Physical Device Coordinate System (PDCS) corresponding to the display device.

The World Coordinate System (WCS) is infinite in extent and the device display area is finite. Therefore, to perform a viewing transformation we select a finite world coordinate area for display called a Window. An area on a device to which a window is mapped is called a Viewport. The window defines what is to be viewed; the viewport defines where it is to be displayed, as shown in the figure (b).