Viewing Transformation is the mapping of coordinates of points and lines that form the picture into appropriate coordinates on the display device.

  World coordinate system (WCS) is the right handed cartesian co-ordinate system where we define the picture to be displayed.   A finite region in the WCS is called the Window. The corresponding coordinate system on the display device where the image of the picture is displayed is called the device coordinate system. Mapping the window onto a subregion of the display device called the viewport is called the Viewing Transformation. Normalized device coordinate (NDC) is the display area of the virtual display device corresponding to a unit square. The lower left handed corner is the origin of the coordinate system. Mapping the window in the world coordinate space to viewport in NDC space is called the Normalization Transformation, N.

where is the scale in x and y directions, given by

From P. Asokarathinam -- see details 27.11.1996