Computer Graphics How do the results of Exercise 3.7 apply to picking?

(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 dimensions 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?)