EXCERCISE 3.7 :

3.7 The mapping from a point in object or world coordinates to one in screen coordinates is well defined. It is not invertible because we go from three di-mensions to two dimensions. Suppose, however, that we are working with a two-dimensional application. Is the mapping invertible? What problem can arise if you use a two-dimensional mapping to return to a position in object or world coordinates by a locator device?