1. An apparatus for providing input to a computer from an off-screen medium comprising: a handheld pen; a single linear array of optical detectors disposed in said pen; input logic means coupled to said linear array and to said computer for selectively configuring said single linear array of detectors in one of a plurality of input modes including at least a two-dimensional input mode to optically read information from said off-screen medium into said computer, said two-dimensional input mode being defined as relative motion of said single linear array of detectors in at least two noncolinear directions with respect to said off-screen medium; wherein said off-screen medium comprises a grid, said grid comprising in turn a plurality of horizontal lines and a plurality of diagonal lines, said horizontal lines being detected by said input logic means through said single linear array of optical detectors to detect relative motion of said apparatus with respect to said off-screen medium in a direction perpendicular to said horizontal lines, detection of relative movement with respect to said diagonal lines being interpreted by said input logic means to indicate motion of said apparatus parallel to the said horizontal lines, so that said apparatus is operable as a mouse input device, said horizontal lines of said grid comprising a plurality of horizontal lines of a first predetermined thickness and spaced apart from each other by a dimension less than the optical length of said single linear array of detectors, and wherein said plurality of diagonal lines comprise a plurality of diagonal line segments disposed between each of said horizontal lines, said diagonal line segments having a second thickness, said second thickness being distinguishable by said input logic means from said first thickness, whereby a universal input device for said computer is provided.