Hi I have read in a book that : " In this chapter it is assumed that all objects lie within the boundaries of the display devise coordinate system . In practise it is often necessary to apply a viewing transformation to accomplish this ." I think that every object could be displayed in the display device coordinate system . But why does this book say that it is often necessary to apply a viewing transformation ?? Thanks

Without any more context, I'm going to guess that the author could have been referring to local coordinates, that is coordinates that are in terms of an origin relative to an object rather than the screen or the 'world.' For example, if I have a message box and inside of that there is a button, I might express the button's coordinates relative to the message box, rather than relative to the entire screen.