In what follows, the accent will lie on the Human Output Channel (HOC) aspects of pen-based interaction. The CIM aspects will occasionally be touched upon. For now it suffices to mention the fact that in pen-based computing the concept of ``electronic ink'' as a typical fundamental data type, differing substantially from other basic data types such as ``pixels'', ``vectors'' and ``sound''. As such the ink object has a number of typical properties. It refers to a pen-tip trajectory, possibly including pen angle and axial pen force, and it has a translation along the x and y axis, a scale, and a slant. Further rendering attributes are color, line thickness and brush. In a limited number of areas, also the concept of ``invisible ink'', e.g., for pen-up movements, is needed. Aspects of electronic ink processing will be dealt with elsewhere. As regards computer output modalities (COM), there are no fundamental problems in using state-of-the art computer graphics in rendering electronic ink. Modeling user interfaces in pan-based COM is sometimes referred to as Pen User Interface (PUI) design, similar to the well-known term Graphical User Interface (GUI) design.
For the time being, however, we will return to the HOC aspects of pen-based user interaction.
The mouse is basically a selection instrument for selection objects, and menu items. Some continuous control like the dragging of screen objects can be performed by the user after some training but continuous control is not a strong point of the mouse. Also drawing and sketching are very difficult. With a pen, however, the same user-actions can be performed as with a single-button mouse, but there are additional functions which are typical for the pen. More elaborate data entry is possible, in the form of linguistic data (text) or graphical data (ink). Also, the user has a more direct control over objects on the screen: there is a higher degree of Direct Manipulation, similar as in finger touch screens, but with a much higher spatial resolution in case of the pen.
A basic distinction in pen input is between (i) Textual Data Input, (ii) Command Input, (iii) Graphical Input. Signature Verification will not be considered in this context.