1. A system comprising: a pen-based computer system for determining a position of a human viewable image on a media responsive to interaction between said image and said pen based computer system; wherein said determining comprises recognizing a substantially invisible position code; and wherein said pen-based computer is further operable to modify an operational parameter of said pen-based computer system responsive to said determining.
5. The system of claim 1 wherein a modification of said operational parameter causes said pen-based computer to produce a human-audible output corresponding to more than one human viewable images on said media.