Abstract:
To relax the graphical constraints on the precision of the handwriting that is required for accurate computerized interpretation of handwritten text, the text that is to be interpreted is written in accordance with this invention using symbols that are exceptionally well separated from each other graphically. These symbols preferably define an orthographic alphabet to reduce the time and effort that is required to learn to write text with them at an acceptably high rate. Furthermore, to accommodate &#34;eyes-free&#34; writing of text and the writing of text in spatially constrained text entry fields, the symbols advantageously are defined by unistrokes (as used herein, a &#34;unistroke&#34; is a single, unbroken stroke).

Description:
This is a continuation of application Ser. No. 08/132,401, filed Oct. 6, 1993 now abandoned. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to systems for interpreting handwritten text and, more particularly, to computerized interpreted text entry systems that are especially well-suited for &#34;eyes-free&#34; (e.g., &#34;heads-up&#34;) applications and for other applications where it is inconvenient or impractical to spatially distinguish between successive, manually entered alphanumeric characters. Another aspect of this invention relates to relaxing the constraints on the graphical precision of the handwriting that is required for accurate computerized recognition of handwritten text. 
     BACKGROUND OF THE INVENTION 
     The keyboard has been the input device of choice for entering text and other alphanumeric data (collectively referred to herein as &#34;text&#34;) into most computer systems. Experts use &#34;touch-typing&#34; techniques to enter text at relatively high rates, while novices often employ &#34;hunt and peck&#34; techniques to perform the text entry function at more modest rates. 
     Modern computers permit of many different form factors, some of which are not particularly well matched to the use of a keyboard for text entry purposes. For example, there are very small &#34;pocket size&#34; and even smaller personal computers, as well as computers that have very large (e.g., wall size) user interfaces to facilitate collaborative interaction. In these miniaturized and very large scale computer systems, the keyboard commonly is supplemented or even replaced by an alternative text entry mechanism, such a stylus or similar pointing device, that can be manually manipulated by the user to &#34;handwrite&#34; graphical symbols that represent the desired text, together with an appropriately programmed processor for translating the handwritten symbols into corresponding alphanumeric computer codes. 
     The challenge, of course, for these interpreted text entry systems is to accurately interpret the handwritten symbols in the face of the graphical variations that may exist among different instances of most any handwritten symbol when each of the instances is written by a different user or even when all of the instances are written by the same user. High speed and/or &#34;eyes free&#34; handwriting tend to make it even more difficult to meet this challenge because they generally detract from the graphical precision with which the symbols are written (i.e., they usually increase the &#34;sloppiness&#34; of the handwriting). 
     Clearly, the characters of ordinary Roman alphabets are not reliably distinguishable from each other in the face of rapid or otherwise sloppy writing. For example, many pairs of letters in the English alphabet (such as &#34;r&#34; and &#34;v,&#34; &#34;a&#34; and &#34;d,&#34; &#34;N&#34; and &#34;W,&#34; and &#34;g&#34; and &#34;q&#34;) tend to blur together when they are written quickly, without paying close attention to their subtle graphical distinctions. Accordingly, it will be evident that the performance of interpreted text entry systems could be improved if all text was entered using characters that are well separated from each other in &#34;sloppiness space.&#34; 
     This &#34;sloppiness space&#34; notion can best be understood by recognizing that each alphanumeric symbol is defined by some number of features (say, d features). Thus, each symbol nominally resides at a unique point in a d-dimensional space which is referred to herein as &#34;sloppiness space.&#34; From this it follows that the amount of overlap, if any, that occurs in the positioning within this d-dimensional space of the normal variants of the symbols of a given alphabet determines how well separated those symbols are in sloppiness space. If there is little, if any, overlap between the variants of different symbols, the symbols are &#34;well separated from each other in sloppiness space.&#34; 
     Ordinary shorthand systems are attractive for interpreted text entry because they can be used to write text at high speed. Unfortunately, however, known shorthand systems use symbols that are at least as difficult to recognize as cursive writing. Nevertheless, it should be noted that there are both orthographic and phonetic shorthand systems for writing text at an atomic level (i.e., the character level in the case of orthographic systems and the phoneme level in the case of phonetic systems). As is known, orthographic systems use conventional spelling and have one symbol for each letter of the native alphabet (e.g., the English alphabet). They, therefore, are relatively easy to learn, but they provide only a modest speedup over ordinary cursive. Phonetic systems (such as the well known Gregg and Pitman systems), on the other hand, employ phonetic spelling, and sometimes use special phonetic alphabets to do so. This makes them more difficult to learn, but it also explains why they generally are preferred when speed is of paramount concern. This indicates that there is a trade off between speed and ease of learning that comes into play when designing a stylus compatible alphabet for interpreted text entry systems. 
     SUMMARY OF THE INVENTION 
     To relax the graphical constraints on the precision of the handwriting that is required for accurate computerized interpretation of handwritten text, the text is written in accordance with this invention using symbols that are exceptionally well separated from each other graphically. These symbols preferably define an orthographic alphabet to reduce the time and effort that is required to learn to write text with them at an acceptably high rate. Furthermore, to accommodate &#34;eyes-free&#34; writing of text and the writing of text in spatially constrained text entry fields, the symbols advantageously are defined by unistrokes (as used herein, a &#34;unistroke&#34; is a single, unbroken stroke). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Still additional features and advantage of this invention will become apparent when the following detailed description is read in conjunction with the attached drawings, in which 
     FIG. 1 illustrates the basic strokes of a unistroke alphabet in each of their recommended orientations; 
     FIG. 2 shows how the symbols of a unistroke alphabet that is based on the strokes and orientations that are shown in FIG. 1, together with a directional parameter, suitably are mapped onto the alphanumeric characters of the English alphabet and some of the more common punction marks and control functions; 
     FIG. 3 is a simplified functional block diagram of a system for interpreting text that is handwritten through the use of a stylus or similar pointing device and the unistroke alphabet shown in FIG. 2; 
     FIG. 4 shows a suitable user interface for an interpreted text entry system that embodies the present invention; and 
     FIG. 5 illustrates an alternative input mechanism for writing unistroke symbols. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While the invention is described in some detail hereinbelow with reference to certain illustrated embodiments, it is to be understood that it is not limited to those embodiments. On the contrary, the intent is to cover all modifications, alternatives and equivalents falling within the spirit and scope of the invention as defined by the appended claims. 
     Turning now to the drawings, and at this point especially to FIGS. 1 and 2, it will be seen that there is a unistroke alphabet (FIG. 2) that is composed of just five different strokes (FIG. 1) that are written in up to four different rotational orientations (0°, 45°, 90° and 135°) and in one of two opposite directions (stoke direction is more easily captured electronically than in standard mechanical writing systems. This provides 40 unique symbols (5×4×2), which is more than ample to encode the 26 alphabetic characters of, for example, the English alphabet. 
     Advantageously, the number of different strokes that are used to define the unistroke alphabet is minimized, so that the strokes can be selected to be geometrically well separated from each other in sloppiness space. For the same reason, a substantial angular offset (e.g., at least 45° and preferably 90°) or directional distinction (opposing directions) is provided to distinguish between geometrically like strokes that represent different alphanumeric characters. As a general rule, the symbols of the unistroke alphabet are mapped onto the alphanumeric character set so that the graphical correlation between the two alphabets is as close as possible. In practice, however, some of the geometrically simpler unistrokes, such as the straight line symbols, may be reserved for assignment to the more common alphanumeric characters, such as `e,` `a,` `t,` `i` and `r,` in the interest of facilitating higher speed text entry. 
     In keeping with the goal of minimizing the size of the unistroke alphabet, lower and upper case versions of any given alphabetic character are entered in the illustrated embodiment through the use of the same unistroke symbol. To that end, shifts from lower case characters to upper case characters and vice-versa are signaled on a character-by-character basis or on a block basis by entering unistroke symbols that are assigned to the signaling task (see FIG. 2) or by actuating and deactuating a button, such as a button on the barrel of the stylus that is used to write the unistroke symbols (see FIG. 5), to emulate the function of a standard a shift key. Similarly, shifts from a text entry mode to a numeric data entry mode and vice-versa may be signaled on a character-by-character or block basis by entering a unistroke control symbol that is assigned to that task (see FIG. 2) or by operating an appropriate button (if necessary, a multi-click methodology may be used to distinguish between commands that are entered through the operation of any given button, such as the button shown in FIG. 5). When unistroke symbols are used for control functions of the foregoing type, character-by-character commands suitably are signaled by entering the appropriate control symbol just once (i.e., a singlet) while block commands are signaled by entering the control symbol twice (i.e., a doublet). 
     In operation, a user typically employs an electronic pen or a stylus 31 (sometimes collectively referred to herein as a &#34;stylus&#34;) to handwrite one after another of the unistroke symbols that are needed to formulate an intended text on a suitable writing surface 32. In accordance with the present invention, provision is made (a) for sensing the engagement and disengagement of the stylus 31 with the writing surface 32 at the beginning and end, respectively, of each unistroke symbol, and (b) for converting the geometric path that the stylus 31 traverses while the symbol is being written into a correspondingly ordered sequence of x-y coordinates. To that end, as shown in FIG. 3, an active pressure sensitive writing surface 32 that has a sufficiently fined grained, two dimensional array of position sensors can be employed. Alternatively, as shown in FIG. 5, and active stylus 51 may be employed for performing the engagement/disengagement sensing functions and for converting the motion of the stylus 51 as it is drawn across the writing surface 52 into a corresponding sequence of x-y coordinates. Or, some of those functions (such as the contact sensing) may be performed by the stylus, while others (such as the stylus tracking) are being performed by the writing surface. 
     In any event, the path the stylus traverses while it is being drawn across the writing surface is converted into an ordered list of x-y coordinates at a sufficiently high spatial sampling frequency to satisfy the Nyquist sampling criterion for all unistroke symbols. 
     As shown in FIG. 3, the x, y coordinate pairs that describe the path that the stylus 31 traverses while it is being drawn across the writing surface are serially loaded into a buffer 35; starting with the coordinates at which the stylus 31 is engaged with the writing surface 32 and concluding with the coordinates at which the stylus 32 is disengaged therefrom. The buffer 35 is cleared in preparation for each unistroke symbol, such as in response to a state signal indicating that the stylus 31 has been brought into pressure contact with the writing surface 32. Thus, the buffer 35 accumulates an ordered list of x-y coordinate pairs to represent each unistroke symbol that is written. 
     A recognition unit 36 is triggered when the stylus 31 is disengaged from the writing surface 32 to identify (if possible) the unistroke symbol that best fits the ordered list of x-y coordinates that have been accumulated by the buffer 35 and to read out the corresponding alphanumeric character code from a table lockup memory (not shown). The character code, in turn, is loaded into a character buffer 38 so that it can be edited if desired and then into a display buffer/character generator 39 for writing the corresponding alphanumeric character on a display 40. 
     The relatively wide separation of the unistroke symbols in sloppiness space reduces the probability of obtaining erroneous or ambiguous results from the recognition process. In practice, the tests that are conducted to identify the unistroke symbol that best fits any given ordered list of x-y components are dictated in substantial part by the geometric details of the particular unistroke alphabet that is employed. For example, the unistroke alphabet shown in FIG. 2 lends itself to the following recognition methodology: 
     1. Accumulate ordered list of coordinate pairs, (x1, y1), . . . , (xn, yn), to characterize stroke; 
     2. Filter list to remove noise and to otherwise smooth characterization of stroke; 
     3. Test for straight lines (if straight line, find slope, use slope as index for performing character lookup function, then exit); 
     4. If not a straight line, normalize the stroke to fit in a square bounding box. 
     Then compute the following 6 features: 
     dx=xn-x1 (i.e., the displacement between the origin and terminus of the stroke as measured on the x-axis), 
     dy=yn-y1 (i.e., the displacement between the origin and terminus of the stroke as measured on the y-axis), 
     1/2-dx=x(n/2)-x1 (i.e., the displacement between the origin and the geometric midpoint of the stroke as measured on the x-axis), 
     1/2-dy=y(n/2)-y1 (i.e., the displacement between the origin and the geometric midpoint of the stroke as measured on the y-axis), 
     |dx|=|x2-x1|+|x3-x2|+. . . +|xn-x(n-1)| (i.e., the cumulative length of the stroke as projected onto the x-axis); and 
     |dy|=|y2-y1|+|y3-y2|+. . . +|yn-y(n-1)| (i.e., the cumulative length of the stroke as projected onto the y-axis); 
     5. Find the unistroke whose features are closest to the ones computed in step #4, using Table 1 below; 
     6. If step #5 gives the answer of `u` or `o`, determine whether the slope vector of the stroke rotates in a clockwise direction or a counterclockwise direction to decide whether the stroke is a `u` or an `o`, respectively. 
     
                       TABLE 1______________________________________dx         dy      1/2-dx   1/2-dy |dx|                                    |dy|______________________________________a      0.0     -1.0    0.0    -0.5   0.0   1.0b      0.0     1.0     1.0    0.5    2.0   1.0c      0.0     -1.0    -1.0   -0.5   2.0   1.0d      0.0     1.0     -1.0   0.5    2.0   1.0e      -1.0    0.0     -0.5   0.0    1.0   0.0f      -1.0    1.0     -1.0   0.0    1.0   1.0g      1.0     -1.0    0.5    0.0    1.0   1.0h      1.0     1.0     1.0    0.0    1.0   1.0i      0.0     1.0     0.0    0.5    0.0   1.0j      -1.0    1.0     0.0    1.0    1.0   1.0k      1.0     -1.0    0.5    -0.5   1.0   1.0l      1.0     1.0     0.0    1.0    1.0   1.0m      -1.0    0.0     -0.5   -1.0   1.0   2.0n      1.0     0.0     0.5    -1.0   1.0   2.0o      -0.5    0.5     -1.0   1.0    1.5   1.5o      -1.0    0.0     -0.5   1.0    2.0   2.0p      0.0     1.0     1.0    0.5    2.0   2.0q      0.0     1.0     -1.0   0.5    2.0   2.0r      1.0     1.0     0.5    0.5    1.0   1.0s      -1.0    1.0     -0.5   0.5    2.0   1.0t      1.0     0.0     0.5    0.0    1.0   0.0u      -1.0    0.0     -0.5   1.0    1.0   2.0v      1.0     0.0     0.5    1.0    1.0   2.0w      1.0     1.0     0.5    0.5    1.0   3.0x      1.0     0.0     0.5    1.0    2.0   2.0y      -1.0    1.0     -0.5   0.5    1.0   1.0z      1.0     1.0     0.5    0.5    2.0   1.0bkspc  -1.0    -1.0    -0.5   -0.5   2.0   1.0______________________________________ 
    
     As will be seen, there are alternate characterizations in Table 1 for the unistroke symbol that is assigned to the character `o` (see FIG. 2) because it has been found that that particular symbol is often written in open form (i.e., without completing the overlapping tail of the stroke). Also, the cumulative lengths of the x-axis projection of the unistroke symbols for `s` and `z` are specified as having nominal values of 2.0, but these specifications are subject to change because the available evidence indicate that 3.0 might work better in practice. 
     A Modula-3 program that implements the above-described recognition methodology for unistroke symbols that are characterized by streams or files of ordered x,y coordinate pairs is appended hereto. 
     As previously pointed out, a user actuatable button (such as the button on the barrel of the stylus 51 in FIG. 5) and/or special unistroke symbols that are dedicated to specified control functions (such as the unistroke control symbols shown in FIG. 2) can be used to provide the additional differentiation that is needed by the recognition unit 36 to accurately interpret unistroke symbols that are used to encode multiple alphanumeric symbols (such as lower case and upper case alphabetic symbols and/or alphabetic symbols and numeric symbols). As will be understood, the additional differentiation that is provided by these button triggered control signals and/or symbolic flags enables the recognition unit 36 to switch from one lookup table to another for the decoding of successive unistroke symbols. 
     Turning now to FIG. 4, it now will be evident that unistroke symbols are especially well suited for &#34;eyes-free&#34; handwriting because each symbol is defined by a single stroke which, in turn, is delimited by moving the stylus 31 into and out of contact with the writing surface 32. Eyes-free operation is not only important for the sight impaired, but also for &#34;heads-up&#34; writing while transcribing text, taking notes while visually observing events, etc. Furthermore, unistroke symbols may be written one on top of another because they are interpreted in the order in which they are written and they are unambiguously differentiated from each other by being defined by different strokes. Thus, the symbols may be employed to advantage for handwriting text into computers that have small text entry fields, such as the computer 41 with its text entry field 42. Indeed, the use of spatially overlapping symbols for text entry tends to be less tiring than ordinary handwriting because it does not require wrist movement. Spaces between words typically are indicated by a tap of the stylus 31 against the writing surface 32, so the dot-like symbol that is produced by such a tap suitably is discriminated from the other unistrokes by the relatively small size of the dot-like symbol. 
     If desired, the user interface 43 for the computer 41 may include one or more &#34;soft-keys&#34;, such as at 44, for entering compositional characters and/or control codes through the use of a point and select methodology. Additionally, the user interface 43 may have a text editing field 46 for displaying the text of the message that is being entered and for sensing any gesture based editing commands that the user may enter. 
     Advantageously, the tactile feedback that a user receives while using unistroke symbols for entering text for computer interpretation in accordance with this invention is similar to the usual tactile sensation of using pen and paper to handprint text. To this end, the origin of a unistroke symbol suitably is defined by sensing the point at which the contact pressure between the stylus 31 and the writing surface 32 first exceeds a threshold level, while the terminus of such a symbol by the point at which the contact pressure betwen the stylus 31 and the writing surface 32 is confirmed to have dropped below the threshold level. 
     CONCLUSION 
     In view of the foregoing, it now will be evident that the present invention provides a method computerized interpretation of handwritten text with substantial accuracy at modest processing cost. Expert users can employ unistrokes for writing text at high speed. In accordance with this invention, the unistroke symbols are readily discriminable from each other, even when imperfectly formed. Moreover, the unistroke text entry technique of this invention is well suited for writing text through the use of a stylus and the like, including for applications involving &#34;eyes-free&#34; text entry. Conceivably, this invention could be extended to facilitate the recognition of text that is written in three dimensional space using unistroke symbols that are characterized by order lists of x, y, z coordinate triplets. In some applications, the &#34;pointer&#34; that is used to write the unistroke symbols may be a finger or the like of the writer, such as when the unistroke symbols are written on a touch sensitive screen. 
     For additional details, Goldberg et al., &#34;Touch-Typing with a Stylus,&#34; CSL-93-1, Xerox Corporation Palo Alto Research Center, May 1993 hereby is incorporated by reference. That paper also appeared in the Proceedings of INTERCHI &#39;93, Conference on Human Factors in Computing Systems, Apr. 24-29, 1993, Amsterdam, pp. 80-87.