Stroke Regrouping

The strokes entered by the user are initially grouped by the stroke grouping algorithm, described in Section 4.3.3. The algorithm tries all possible grouping of strokes, ranking each based on confidences returned by the character recogniser. As the success of this algorithm relies on the accuracy of the character recogniser, it is possible that strokes will be incorrectly grouped.

From a study of new users using the system with a character recogniser trained to a style of writing similar to theirs, 13% of the characters they wrote were misgrouped by the automatic stroke grouping process. This does improve if the user has trained the character recogniser.

A simple and effective method is required to fix grouping errors. There are two possible situations that the user has to correct:

• Strokes that should be recognised as a single character are grouped as parts of separate characters, or

• Strokes that should be recognised as part of separate characters are grouped into a common character.

The user can correct both of these types of errors by entering modify stroke groups mode. In this mode, drawing with the pen will temporarily mark out a line. Upon finishing the line, any strokes that were touched by that line are forced into a group of their own, possibly causing a regrouping of other strokes. The temporary line then disappears, and the system automatically reruns the character recogniser on all affected groups. This technique has been called ``squiggle select'', as the user can either draw a line or squiggle over the strokes they want grouped together. SGI Inperson  uses the same technique for selecting objects in a multi-user collaborative white-board application.

Drawing a line through the strokes to be grouped is better than circling or drawing a box around the strokes. A single line is able to target specific strokes easily, and is easier to draw. If there is a group of closely spaced strokes, it is easier to pick out individual ones with a single line.

  

Figure 4.6: Modifying stroke groupings.

(a) Initial grouping.

(b) The user indicates that two strokes should be grouped together.

(c) The system displays the regrouped and re-recognised characters.

(d) The user indicates that two strokes should form their own group.

(e) The final result.

Figure  shows the modify stroke groups mode being used to correct the two types grouping errors. Figure 4.6(a) shows the initial state, in which the strokes in the ``='', the ``4'' and the ``2'' are not correctly grouped.

First, the user draws a line through the two strokes of the ``='' that should be combined into a single group, as shown in Figure 4.6(b). Figure 4.6(c) shows the result after the pen was lifted. Note that the temporary line has disappeared and the ``='' has now been correctly recognised.

To split the ``4'' and the ``2'' apart, the user draws a line through one or more strokes that should be split off from the larger group. In Figure 4.6(d), a line is drawn through the two strokes of the ``4''. A line through the ``2'' would have had the same effect. Figure 4.6(e) shows the final formula, with the strokes now correctly grouped and recognised.