Three-Dimensional Viewing
Three dimensional objects are created using modelling coordinate system. The modelled objects are then placed in locations specified in the scene with respect to the world coordinate system. You can view the scene from any orientation and this is what you call as the view of the scene. Viewing coordinates are the coordinates generated with respect to the frame of reference of the viewer, or you may call it the frame of reference created by the position of camera. Viewing transformation is a composite transformation, which transforms the scene from world coordinate system to viewing coordinate system. To understand what viewing coordinate system and the viewing transformation are, let us consider a simple example. Suppose you have a vacant room and you want to organize its interior to make it an office. Chairs, tables, shelves, etc. are created somewhere else (in some factory) using some coordinate system. Let that be treated as a modelling coordinate system. Then these items will be brought in the room (Transform to locations in the room). Finally every object will be placed in appropriate position. This means the modelled objects are now placed in the world coordinate system. Now you view it from various angles and then become satisfied with the arrangements. When you are watching it from various angles, each time you change your position, the relative positions of the objects in the room also change. Objects closer to you become farther when you go on the other side of the room. This is viewing coordinate system and your eyes make an automatic transformation of objects from world coordinate system to viewing coordinate system.