Graphical Online Pen Entry Systems

An example of a pen entry system is that developed by Littin . Littin's system allows the entry of formulae with a mouse or pen and tablet. Characters are recognised as they are drawn, using a feature-based character recogniser. Because of a low processing requirement for his parsing technique, the formula is parsed as the user writes. The output of the system is a linear format, such as a LISP-like prefix notation.

As explained later in Section , Littin's use of a modified SLR(1) parser puts a number of restrictions on the system. Users are limited in the order in which symbols for a particular formula can be entered, though the order is fairly reasonable. Editing is also limited to the modification or deletion of the most recently entered symbol. As a result, arbitrary editing of formulae with his system is impossible.

After a character written by a user has been recognised, it is morphed to a pre-defined ideal stroke shape. Because the formula is parsed as the user enters it, the morphing also rearranges the positions of the characters to the correct arrangement according to the parser's current understanding of them. The rearrangement is done so that enough room is left around the most recently entered symbols so that the user can still write around them.

Littin claims that moving characters to their correct positions as the person writes encourages them to write in straight lines. He also notes that the morphing of characters to what they have been recognised as and rearranging the formula in real time provides valuable feedback to the user on how the computer's interpretation of their formula is going.

When compared to other methods of formula entry, pen based systems have the advantage that, assuming they are well designed, they are more natural and intuitive to use. A system that allows the user to enter their formulae as they would by writing on a piece of paper has the advantage that it offers a style of interaction that is as similar as possible to writing with a pencil and paper, yet it also has the power of a computer available to perform computations on and manipulations of the formulae entered.

The disadvantage of pen based systems is that allowing freehand input of formulae means that the system must be able to deal with the sloppiness inherent in handwritten input, and ideally the arbitrary order in which users enter formulae. Littin's system avoids this by limiting the order in which users can enter symbols for their formulae, and restricts the editing of formulae to the most recently entered symbol. Ideally a system would allow a completely arbitrary symbol entry order.

Pen and tablets are also not common hardware for home users and writing neatly and quickly with a mouse is very difficult.