The data to be used in computer graphics must be in a Cartesian coordinates system. If they are in some other coordinate system then we have to convert them in to Cartesian coordinates: Say, if we have an image to be converted. First we will take out its local coordinates based on the shape, size etc. Then, it is transferred in to a world coordinate reference system and later it is converted to normalized device coordinates for the device we are using. After that it is finally converted to the Cartesian Coordinates System.