CA1121056A - Character generating method and apparatus - Google Patents

Abstract

A font storage system for use in a typesetter having an electronically controlled character imaging device. The storage system, which preferably includes a floppy disk, has digital information stored thereon defining each character to be typeset by at least two outlines on a nor-malized X-Y grid. The digital information defining each character includes (1) digital numbers defining the X and Y coordinates of the initial start points of the outline and (2) digital numbers defining a plurality of straight line vectors extending successively along the character outlines. Each vector has a first digital number represent-ing the X coordinate distance and a second digital number representing the Y coordinate distance from one end of the vector to the other.

Description

BACKGROUND OF THE INYENTION

The present invention relates to the art of generat-ing alphanumeric characters or other symbols for reproduc-tion by a cathode ray tube (C~T), a laser beam scanner or other flying spot character imaging device which is capable of being electronically controlled. More particularly, the present invention concerns a font storage system for use in a character generator whereby a ~ont of characters or other symbols are stored in a digital code.
The field cf automated typesetting has experienced ever-accelerating advances since Ottmar Mergenthaler develop-ed the ~inotype~ machine for semi-automatically producing lines of type. The ~inotype machine and its progeny of "hot metal" typ.esetters have been called the first genera-tion of automatic typesetters. These typesetters ~ere re-fined over the years and are still in use in some locations.
The second generation of typesetters, which were pioneered by ~ene Higonnet and Louis Moyroud, among others, are called photo-mechanical typesetters, or simply phototype-setters. In these machines, one or more fonts of characters are arranged on a photographic negative. Selected characters are automatically projected through an optical system and positioned in a line on photographic film. ~ot only are these ;;
phototypesetters now less expensive than their first genera-tion parents, ~ut refinementsin the machines led to faster ~ .

, '':

speed, better quality and greater typographic flexibility.
Phototypesetters are currently enjoying a period o~ maximum use in the graphic arts industry, but are being impxoved upon by third generation machines: the so-called CRT (and laser) typesetters.
In CRT typesetters characters are electronically generated and written onto photographic film, thus eliminat-ing most of the mechanical movements characteristic of second generation phototypesetters. This change from mechanics to electronics is resulting in still aster speed and greater typographic flexibility, as well as less frequent adjustments and fe~,rer changes in "font dressings" or stored fonts which are necessary on all second generation typesetters. The CXT
typesetters are, as a rule, more e~pensive than their second generation counterparts so that, while they have become the dominant machines in the newspaper market, they are only just begir.nin~ to gain significance in non-newspa~er appli-cations. It is expecte~l, however r that the price of CRT
typesetters will come down as volume increases and ne~
mac~ines are developed to take advantage of advances in electronic circui. technology.
There are genera]ly t~o methods ~y which character fonts are stored in third generation typesetters. The so-called "analog" machines store the character masters on photographic film grids. These masters are scanned ~L2~g~5~;

with a flying spot scanner at the same time that the charac-ter is imaged in the appropriate size on the output CRT. A
second class of machines, the so-called "digital" machines, rely on character masters which have been coded in digital form and stored on some kind of digital storage medium in the machine. ~ith such digital machines the ability to store a large font library within the typesetter is limited only by the cost of providing a storage medium oE suitable size so that it is not normally necessary for the user to repeatedly "dress'' the machine by inserting new fonts. In addi~ion, the digital machines are at least twice as fast as the fastest analog (photographic store) machines and are capable of imaging cleaner, more uniform characters than the analog machines.
Originally, when digltal CRT typesetters were first introduced, the principal concern in preparing digital font masters was simply data ~eduction. In order to reproduce characters which were indistinguishable rom characters Lmaged from photographic masters or printed by cast type faces, it is necessary to encode each character with a relatively ~ine grid; i.e~, a "matri~" with a high resolu-tion or density of raster elements. At a minimum, and for small ch~racters,the grid may comprise ,O columns and 100 rows or 7,00Q raster elements. If the presence or absence of a portion of a character in each raster el~ment is represented by one bit, 7,Q00 bits of information are 5~

required to represent all el~ments or the grid. The U.S.
Patent No. 3,305,841 to Schwartz discloses a CRT typesetter in which the number of bits required to represent a character is compressed at least by a factor of 3 in every case, and by a factor of 5 or more iIl an average case. This data reduction is accomplished by identifying with a digital code the starting and ending points o the line segments (dark portions) of a character in each row or column of the grid. Thus, in a grid comprising 7,000 raster elements, the data required to define a character was reduced from 7,000 bits to approxlmately 1,500.
The U.S. Patent ~o. 3,471,848 to ~anber discloses an improvement on the above-noted system which permits an additional reduction in data. With this system, the start~
ing and ending points of a line segment within a row or ~;
column of the grid are encoded as an incremental increase or decrease from the starting and ending points, respectively, on a line segment in the previous row or column. Data compression is achieved because the numbers required to define the incremental addresses of a line segment are smaller than the numbers required to define the absolute addresses.
The Patent Nos. 3,30S,841 and 3,471,848 also dis-close a number of other techniques of data compression with digitally encoded characters:

LQ~6 ~ 1) The provision of a code which indicates the number of blank rows or columns on one side or the other (or both sides) of the character.

(2) The provision of a "line repeat" code which indicates that the line segment or segments in a row or column are at the same position(s) as the segment(s) of the previous row or column.
(3~ The provision of a code indicating that a selected start or end or a line segment address is to ~e repeated a prescribed number of times.
Notwithstanding the various techniques of data reduction, digital font masters produced in accordance with the teaching of the U.S. Paten~sNos. 3,305,841 and 3,471, ' 848 are appreciably more e~pensive than the photographic masters used in the analog CRT ty~esetters. There are ~;
two fundamental reasons for this:
~ 1) The digital machines size type by varying the spacing of strokes on the output tube. There are practi cal limits as to how far up and down an image can be sized in this fashion. Therefore, these machines have required several different master fonts in order to cover a com-plete range of output sizes.
(2) Digitizing type fonts is a tedious, ~ime consuming process. Character masters are first prepared on a standard grid and then scanned automatically to determine which raster points on the grid fall within the character. The resulting dot matrix is then "digitized"
in accordance with a particular code and stored in a machine readable form.
The U.S. Patent No. 4,029,947 to Evans et al. dis-closes a character encoding and decoding scheme for a CRT

typesetter which makes it possible to eliminate the first disadvantage noted above. This is accomplished by encoding the normalized character outllne (as distinguished from size-related character row or column line segments) with a series o successive slopes and curvatures from an initial starting point or points for the character. For this purpose, a large number or slo?es and curvatures are available for selection by the encoder, with each of such slopes and curvatures being identified by its individual binary code number.
Another character representat~.on scneme whlch treats characters in terms of normalized character outlines was used by the ~lodel 1601 CRT t~Ipesetter manufactured by SEACO Com-puter Display in Garland, Texas. This machine, which is dis closed in the Seybold Report, Vol. 1, Nos. 12 and 13 (Feb. 14 and 28, 1972~, stored the absolute coordinates of a number of points on the character outline. Data reduction was achie~ed because intermediate points on the outline b?tween stored points were considered to follow straight lines between the stored points.

- ~ \
~.2~S6 The SEACO 1~01 CRT typesetter, as well as the type-setter disclosed in the U.S. Patent ~o. 4,029,947, determine the data required for imaging the character over a range of point sizes from a single set of encoded character outline data by means of a calculation procedure, carried out either by software or hardware. In contrast, the CRT typesetters disclosed in the U.S. Patents Nos. 3,305,841 and 3,471,848 perform a minimum of calculation becaùse the information required to "stroke" successive line segments (i.e., the start and end addresses of each line segment~ are ~resent in the data.
Thus, while various digital character encoding schemes have been defined in the art for CRT typesetters, no scheme has been devised which optimally mee~s all the various requirements. These are: ~ ;
~ 1~ The encoding scheme should be conservative of space in di~ital memory.
(2~ A single set of data defining a character should be usable to generate character images in all point sizes

(3) The enc~ded data should be capable of being converted into the form required to control the C~T by a relatively simple and easy~to-automate computation procedure.

(4) The character encoding scheme should be defined by rules which are easily automated, so that the coded data may be generated from photomasters, raw dot matrices or from some other ccde by a disital computer.

2~0~

9 .
SUMMARY OF THE INVI~NTION
The present invention provides a digital encoding ; scheme for characters or 5ymb01s, and an associated font storage system, which meets all of the above-noted require-ments.
. According to the invention, characters are defined ~y encoding their outlir.~s on a normalized grid of first and second coordinates, as follows: ;
- (1) A starting point on a character outline is chosen and the first and second coordinates of this point are stored.
(2) One or more straight line vectors which extend ; successively along the character outline from the start ~oint, and closely ap~roximate the outline, are chosen. Each vec-tor is then represented by a first digital number defining the first coordinate distance, and a second digital number defining the second coordinate distance from one end of the vector to the ot~er. . : :~
The vector outline encoding sche~e according to the - present invention me.ets the four requirements set forth above.
This encoding scheme is, above all, conservative of space and m~ory. According to a preferred feature of the invention, th.e first and second digital numhers defining each vector are limited ~n size.. ~or e~ample, with a moderately high resolu- :
tion such as 432 units to the "em" square, they may be 4-bît numbers so that a vectox is represented by one ~yte (eight bits) of data. An analysis has shot~n that by far the : . , 2~
.

majority of Yectors required to define a character are with-in 15 units in tl,e first and second coordinate directions on the grid. The vector encoding scheme also inhexently provides incremental distances in both the first and second coordinate directions from the tip of the previous vector.
These incremental distances can be defined with less informa-tion than the absolute coordinates of a vector ~ip. In addi-tion, the start point and vector data are presented in a prescribed sequence ~hich, by itself, associates the data with s~ecific charac~er outlines. As a result of these three factors, the present encoding scheme compares favorably ~.~ith all the prior schemes of digitizing characters in the amount of data required to define a character, and in the comple~itv and speed of the hardware required to process this data.

Fur~hermore, a single set of character encoding data according to the inven~ion is usable to generate character images in all point sizes. It is necessary only to compute the intersections bet;lee~ each horizontal or vertical stroke and the character outLines to determine when the CRT or laser beam should be turned on or off. The straight line vectors defined by the encoded data make it possible to carry out this computation with a mini~mum o~ hard~are (or software) and at high speed.
Finally, the character encoding data accordi.ng to the invention may be derived automatically rom raw dot matrix ~n~ormation or from some other digitized code in a 2~
. V ~

relatively straight-forward way using a proyrammed digital computer. In particular, in accordance with a preferred method of encoding, the straight line vectors are chosen by first determining successive coordinate points on each outline for which the outline deviates less than a prescribed distance from a straight line drawn between these points. Once the outline points are determined, the first and second coordinate values of each successive point are subtracted from the first and second coordinate values of the previous point to determine the coordinate increments from point to point. These increments are then stored as the 4-bit first and second digital numbers defining each vector.
In accordance with an aspect of the invention there is provided a typesetter for the automatic generation of characters comprising a character imaging system for writing graphics qualit~ characters of any design on a print medium; a font storage system having digital data stored thereon defining each character to be imaged; and an electronic computation and control system, connecting :~
said font storage system with said character imaging system, for controlling said character imaging system in accordance with said digital data; said character imaging system including a flying spot scanning device for writing characters by means of a plurality of parallel scanning strokes of a scanning beam; and said computation and control system including: (a) means for producing a beam deflection signal, determinative of the amount of .~

5~j - lla -deflection of said scanning beam in the direction of each stroke, said beam deflection signal causing successive strokes of said beam to start substantially at the intercept point on one side Gf a character and to terminate substantially at the intercept point on the opposite side of said character.
In summary, the font storage system according to the present invention exhibits a combination of features which makes it uniquely suited for defining fonts of characters in digital form. Further features and advantages of this system will become apparent from the following detailed description, taken in conjunction with the various figures.

.~i. ,~

BRIEF DESCRIPTION OF Tl~E DRA~INGS

FIG. 1 is a normalized X,Y grid with the outline of an upper case "Q" superimposed thereon. The closest co-ordinate intersection points to the ou~line are also indi-cated.
FIG, 2 is a normalized X,Y grid similar to FIG. 1 in which certain intersection points representing the charac-ter outline have been deleted.
FIG. 3 is a normalized X,Y grid similar to FIGS. 1 and 2 in which additional intersection points have been deleted and straight line vectors between remainin~ points have been inserted in accordance ~itli the present invention. -~
FIG. 4 is a trial matrix used in the auto~atic selec~ion or vectors, in accordance with the present inven~
tion, to represent a character outline.
FIG. 5 is a flow chart indicating the steps which are taken in the automatic selection of vectors to represent a character outline.
FIGS. 6A-6E illustrate one preIerred format of digital data for the charzcter encoding sche~e according to ~he present invention.
FIG. 7 is a normalized X,~ grid with the outlines i~
of a representatiYe "character" defined by start points and vectors following t~e arrangement shown in the left-hand side of ~IG. 3.

"' :
.

FIG. 8 shows the actual coding for the character represented in FIG. 7 using the data format illustrated in FIG. 6.
FIGS. 9A-9D illustrate another preferred format of digital data for the character encoding scheme according to the present invention.
FIG. 10 illustrates a representative character superimposed on a normalized X-Y grid with the character outlines defined by start points and vectors following the arrangement shown in the right-hand side of FIG. 3.
FIG. 11 shows the actual coding for the character represented in FIG. 10 using the data format illustrated in FIG. 9.
FIG. 12 is a plan view of a hard-sectored floppy disk with sectors and tracks indicated.
FIG. 13 is a chart illustrating how the font and ``
character data are arranged (recorded) on a floppy disk.
FI5. 14 (appearing on the same sheet of drawings as figure 10) is a chart detailing the character look-up and wid~h file shown in FIG.13.
FIG. 15 shows an upper case "Q" as generated by vertical "strokes" on the face of a CRT.

- 12a -`' FIG. 16A shows a typical character having its outline bounded by straight line vectors which intercept vertical scan lines.
- FIG. 16B illustrates how the character of FIG.
16A is imaged in a particular character width by the vertical : scan lines.
FIG. 17A shows a typical character having its outline bounded by straight line vectors which in-tercept vertical scan lines.
FIG. 17B illustrates how the character o~ FIG. 17 ; is imaged in a particular character width by the vertical scan lines.
FIG. 18 illustrates how stroke end points (interrupt values) are determined by interpolation from encoded character data.
- 12aa -; ' ' ,' ' , ' : ' ': , ~ , '~ ~
:, - ~ . . , -12b-FIG. 19 illustrates how stroke end points (inte~cept values) are determined by averaging ~rom encoded character data.
FIG. 20 is a perspective view of a CRT typesetter with various elements shown in phantom.
FIG. 21 is a block diagram of the elements of the typesetter shown in FIG. 2Q.
FIGS. 22A and 22B are block and signal diagrams, respectively, showing the structure and operation of the character generator element of FIG. 21.
FIG. 23 sho~s the code converter element o~ FIG. 21 with its various inputs and outputs.
FIG. 24 is a block diagram of the elements of the code conver~er shown in FIGS. 21 and 23.
FIGo 25 is a block diagram oI the master controller element of the code converter shown in FIG. 24.
FIG. 26 is a geometric diagram illustrating the ~ector computation process carried out by the code converter.
FIG. 27 is a flot~ chart illustrating the operation -of the scaler element of tl~e code converter.
FIG. 28 is a geometric diagram illustrating the interpolation process c~rried out ~ the code ronverter.
~ G. 29 is a bloc~ diagra~ o~ the R~ addressing portion of the code converter.

-12c-FIG, 3a is a block di-grcl~ of the scale~ element of the code converter.
FIG. 31 is another flow chart illustrating the operation o~ the scaler eleme~t o~ the code converter.
FIG. 32 is a geometric diagram illustrating the averaging process carried ou~ by the code converter.

2~ 6 ~ESCRIPTION OF THE PREF~RRED EMBODIME~TS

The preferred embodiments of the present invention will now be described in detail. The first portion of this sec~ion is directed to the font storage system, with its novel and advantageous scheme for digitally encoding characters or symbols. The second portion concerns apparatus which is capable of imaging characters defined by the font storage system.
FIG. 1 shows, by way of e~ample, a greatly enlarged version of an upper case "Q" superimposed on a grid or matrix of horizontal and vertical lines. Each character or symbol that is recorded is located on such a grid. Horizontal and vertical resolution are indicated to be the same in ~IG.l, but this is not necessary. The characters may be of any width, and are situated on a "base line". E~ch charac'er or symbol is also considered to include a "white s?ace" about the character, and is fitted within character width edges called the left and right side bearings (LSB and RSB).
The lines in the grid shown in FIG. 1 may be represented ~umbered) by the X and Y coordinates of a Cartesian coordinate set~ ~ny point ~ithin the grid may be designated by the coordinates (~, Y) of the nearest intersection of a horizontal and ~ertical line. The left-most ~ertical edge of the charac-ter zone is designated X-0 and the horizontal base line is designated Y-a.
~hen a character, such as the upper case Q shown s~

in FIG. l, is to be digitally encoded it must first be plotted onto the grid in such a way that all values of X and Y are represented as integers. By eliminating fractional Yalues of the coordinates, the numbers representing X and Y
may be kept small. As shown in FIG. 1, the outlines of the character "Q" are plotted by choosing the closest intersection points on the grid. Each of these points may thus be repre-sented ~y its X,Y coordinates, where X and Y are integers.
It is therefore possible to completely define - i.e., digitally encode - the character by listing all of these coordinates, preferably in some ordered sequence. However, since the gxid ox matrix must have a sufficient line density to eliminate a jagged appearance of the character, even when the character is imaged in the largest point size, a definition or the c~arac~er in this manner would require an excessive amount of storage space. For eY~mple, for the upper case "Q" shown in FIG. 1 there are 267 outline coordinate points defined within a 60 x 80 matrix. If the matrix density is increased by a ~actor of 10 in each orthogonal direction (a more practical matrix for quality typesetters~ the character "Q" would have ~out 2,500 coordinate points. Since each coordinate in a 600 x 800 matrix requires 20 bits of data to define (10 bits each for X and Y~ one would require a~out 50R ~its to repre-sent the upper case "Q". Since a typical font has more than 100 characters, a typesetter ~ould have to have a high-speed memory with a ~apacity of about 60 million bits to store a single font in this type of code.

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Fig. 2 illustrates how the number of X, Y coordinate points deflning a character may be reduced by designating only the first and last points in a vertical or horizontal Iine (coordinate). The character "Q" has been divided in half in the ~igure. On the left side are the terminal outline p'oints of the vertical lines; and on the right side are the terminal outline points of the horizontal lines. By comparing ~igs. 1 and 2, it may b~ seen that the total number of coordinate points is substantially reduced. Wherever a vertical line of points appears in the character, as is the case along the left-hand side of the character, all the points intermediate the two end,points are deleted with the vertical outline code.
Similarly, wherever a horizontal line of points appears in the character, as is ,the case at the top of the character, the intermediate points are deleted with t:he horizontal outline ,code. Particularly i coordinate points are represented by relative distances from previous coordinate points, rather than by absolute coordinates, there is a considerable reduction in ~ '-the amount of data required to define the character. Such a representation would be substantiall~- the same as the charac-t~r encoding scheme disclosed in'the aforementioned U.S. Patent No. 3,305,84l to Schr.~artz and the U.S, Patent No. 3,471,848 to Manher~ , The present invention provides an encodin~ scheme which is even more conservative of storage space than the 5~

character representation shown in Fig. 2, and which may be utilized in a typesetter, wi~h a minimum of computational hardware, to image characters at high speed. Furthermore, this character encoding scheme may be automated in a straight-forward way using a programmed digital computer.
Figure 3 illustrates the encoding scheme according to the present invention. According to this scheme, the number of coordinate points along the character outlines is reduced still further, and it is assumed that these points are inter-connected by straight lines. Rather than specifying the ab-solute coordinates of these selected points around the character outline, the straight lines are represented as "vectors" by the number of coordinate units from one end of the vector to the other. The vectors are arranged in sequence, from head to tail, so that a new vector begins where a previous vector ends. A
series or string of such vectors, ~hich form an outline of the character, emanate from an initial "s~art point" which is gi~en in absolute coordinates.
For instance, as is shown in the left-half or Fig. 3, vectors proceed from left to right, with the convention that if two vectors commence from the same X coordinate, the lower-most ve_tor is listed first. Similarly, when a pair or pairs or start points are given, the lower pair and the lo~/er start point are listed first.
Thus, in Fig. 3 the start points ~ , Y and X , Y

are first gi~en in that order. Thereafter, the vectors e~anating ~rom these start points are listed in the ordex;
1, 2, 3, 4. The numbers defining these vectors are set forth in Table I:

TABLE I
Vector Number X Distance Y Distance _ When the vectors 3 and 4 have run out, it is neces-sary to define two ne-.r start points X3,Y3 and X4, Y4 before proceeding with new vectors. Otherwise, because the charac-ter data proceeds ,rom left to right, one would assume that there were no vectors or start points having X coordinate values in the X coordinate range- of the next two vectors.
After giving the start points X , Y and X ,Y the vectors are listed in the order 5, 6, 7, 8 using the conven-tion bottom-to-top. Further ~ectors are then listed in the ~
order left-to-right, bottom-to-top; iOe., in the order in ;;
which they "run out" as one proceeds to the right along the X a.~is.
Normally, start points occux in pairs; however, it is possible for two yectors to emanate from the same start point as illustrated by the vectors 9 and 10. In this case, it is convenient if the same start pointbe considered a "pair" of ' ' .

~;

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start points with identical values 50 that the vector 9 roceeds from the coordinate point X , Y and the vector 10 proceeds from the point X t Y .
The right~hand side of ~IG. 3 illustrates the same encoding scheme with a different convention. In this case, the vectors of a character are listed from top to bottom in an entire string following initial absolute coordinates of the upper-most point of a vector string. In the case of two start points having the same Y coordinate value, either point may be list~ first.
With the outline shown in the right-hand side of FIG.
3, the order of data is as follows: The start point X ,Y
and its vectors 11, 12, 13 and so on to the end or the strin~i he start point X , Y and so on to the end of the string;

the sta1-t point ~ , 'f ; the vectors 17 and 18; the start oint X , Y ; the vector 19 and so on.

Finally, as in the case of the staxt point X , Y and X , Y , a single point is defined as a "pair" o~ start points

6 6 X , Y and ~ , Y . First the point X , Y is listed with its ~ector 20; then the start point X , Y , is listed followed by the vector 21 and the other vectors or the stxing.
The vector 20 terminates at the end point 22. The vector string starting ~ith the vector 21 terminates at the end point 23. And the vecto string starting with the vector 11 termi-nates at the end point 24.

i6 There are two reasons why the start point an~ vector encoding scheme according to the present invention is more conservative of space in memory than the encoding scheme illustrated in ~ig. 2 and disclosed in the aforementioned U.S. Patents Nos. 3,305,841 and 3,471,848:
(1) Most characters, unlike the "Q" which was chosen for, illustration, include a number of strai-;ht lines in their outlines.
(2) Even curved surfaces can be represented with ad-equate acc~racy by a succession of straight line vectors of sufficient length that considerable data reduction is possible.
Experience has shown that the amount of data required to define a font of characters with the en~oding scheme ac-cording to the present invention is reduced, over the scheme disclosed in the patents Nos. 3,305,841 and 3,471,848,by about '~
a factor of 10.
A further advantage of the encoding scheme according to the present invention is that it lends itself to computer automation. That is, once the digital data defining a charac-ter has been reduced to .he format shown in Fig. 2, with either vertical or horizontal outlines, it may be converted into st~rt point and vector data using a simple, straight-~orward algorithm. Fig. 4 illustrates a typical c~lculation, and Fig. 5 such an algorit.~m which may be used to determine the length o a vector. , 5~

PIG. 4 shows a 15 x 15 trial matrix arranged in the upper right quadrant from a point (0,0~ which may be an in itial start point or the tip of a previous vector. The quadrant of the trial matrix assumes that a left-right vec-tor is to be defined which ~xtends upwardly (positive values of Y~. Clearly, the trial matrix may also be positioned in ~ne of the other auadrants depending upon the direction in which the vector extends.
Also, the size of the trial matrix corresponds to the maximum perrnissible length of a vector (in this case l5 units each in the X and Y directions, respec.ively). If the vectors are chosen to have a greater or lesser maximum length, the size of the matrix is adjusted accordingly.
In this example, the points 30 represent the actual digitized outline of the characte~r in the fonmat shown in FIG. 2. The line 32 is a proposed vector which must be tested to determine whether it comes sufficiently close to the most distant outline point to re~resent ~he o~tline. The coordinates X,Y define the current trial point for the tip of the vector 32. The coordinates of all of the outline points 30 are designated ~0, yO; xl, Yl; 15 15 accordance with their sequence along the X ax~s o the matrix.
As is shown in FIG, 5, the first outline ~oint to be tested is the point on the matr~x with the largest for~ard (in this case X~ component from the point (0,0). In FIG. 4, the first trial point X , Y is (15,9). The fourth trial T T
point, where ~ ,Y are coordinates (12,~) as shown in FIG.4, is tested a~ter ~it ~aîlure ~n the thxee prior trial points:
(15,9~, ~14,9~, and (13,9). The purpose of the algorithm ~3 to find the longest Yector that passes the it test.
The algorithm tests each lower valued outline point 30 ~with coordinates x,y) to detenmine whether a perpendicular distance ~ from ~hat point to the vector drawn from the initial point (~,0~ to X ,Y exceeds a preset fit constant . Initially, the coordinates x,y of the point 30 just prior to the trial point X ,Y are chosen and the test is performed. If the distance ~ is less t~an the constant (the tes-t is passed) the outline point 30 with the ne~t lowest value of X is hosen and the test is repeated. If the distance ~ exceeds the constant K (the,test failed) the test point X ,Y is abandoned and the next lowest value of X
is chosen.
When a t~ial point is found for which alL the out-line points 30 with lower X coordinates pass the test, or when the X coordinate X o the trial point has been reduced to one, the coordinates X ,Y are used in defining the vector.
The vector is then represented by the difference between the ,~
coordinates of the last previous vector tip (coordinate (0,0 in the trial matrix) and the coordinates of the chosen trial point X ,Y . That is, ~x, dy is set equal to X ,Y .
The perpendicular test distance ~ is determi.~ed for each point by simple geometry. Using similar triangles, we have:

Q~i~

X ' ~ = T - , and a y ~, T T

y a y -- y x Solving for ~ :

,, X T
: ~ = T (Y x - ) _ .

T T

~TABLE I value @ XT, YT] 6 6 (TABLE II value @ XT YT) ~

T y :
The values of ~ 2 2 and -- :
X + Y . X
T T T
may, of course, be calculated each time by a computer. ~ow-ever, since there are a limited number of X ,Y point~ in ~ 15 x 15 matrix, it is more convenient if all the possible solutions for these expressions be entered in a TABLE I ~nd a TABLE II, respectively, so that they may be quic~ly loo~ed up and retrieved frcm storage.

In addition, it should be noted that the preset fit constant may be chosen arbitrarily small so that the vectors come as close as desired ~o the actual character outline~
In a preferred embodiment the constant K is made dependent upon the slope of the trial vector so that near horizontal slopes may deviate more from the outline.

If T > 1, K = 0.5, and - X

Y
if T _ ~ 1, K = 1Ø
X
T
It will be appreciated that the algorit~m shown in FIG. 5 is e~txemely simple and may be carried out using a general purpose com~uter in which the vertical outline or horizontal outline poin s (per FIG. 2, left side and right side, respectively) are.stored. A pxogram for a par~icular computer may be developed from this algorithm using well-known progra~ing principais and techniques.
FIG. 4 shows a trial matrix in which the maximum permissible values of X and Y are lS units. A vector termi-nating anywhere within this matri.~ may oe defined by two 4-bit binary numbers: d~ and dy. An analysis has shown that, even ~ith a grid of moderately high resolution, by far the majority of vectors required to define a c~aracter fall within such a 15 x 15 matrix so that it is convenient, Si6 and results in da~a compression, if 8 bits (one byte) of data are used to define each vector....
According to the invention, therefore, the number of bits defining a vector is chosen to minimize the total da-ta content in a font of characters for a given resolution. The process of choosing the maximum vector length involves the following steps:
(1) The maximum point size of the characters to be generated by the typesetter is first determined.
C2) Given the maximum point size, a resolution is chosen which permits reproduction of the f.ne features in the largest characters.
(3) Given the resolution, th.e preset fit constant K
is chosen so that the vectors follow the curved character outlines ~Yith. sufficien~ accuracy that, when characters are r~produced in the largest point size, they will not appear to have a succession of "flats" on curved surfaces.
(4) Once the resolution and constant K are determined, it is possible to generate a statistica1 distribution of vectors of varying lengths ~or all characters in a font.
Such a vector length distribution will show the relative numbers of vectors at each of t~e permissible lengths (1 x 1,.
3 x 3, 7 .; 7, 15 x 15, 31 x 31, etc.~
- (5~ From this vector length distribution, a maximum vector length is chosen which minimizes the total quantity of data. If the maximum vector length is too short (e.g., 3 x 3 which can be defined with a total of 4 bits) the ;6 the definition of a character will requlre an excessive number of vectors and the data reduction will be minlmal.
Similarly, ix the maximum vector length is too long (e.g., 255 x 255 which can be deined by 16 bits~ the amount of data required to define short vectors is unnecessarily large, resulting in mini.mal data reduction.
FIG. 6 illustrates a preferred format for defining a character with left-right vectors (~IG. 3, left side).
These vectors are speci~ied in one quadrant by the X,Y coordi-nates of the e~d of the vector relative to the quadrant origin. Since outlines are traced from left to right across the character, only the two risht-hand quadrants are used.
Control codes permit quadrant selecti.on and curve initialisa-tion and completion. Start points are defined by their Y
values only, because the X position is implied by the coding.
~ "block" of data defining the character star~s with a "header word" A (comprising two 8-bit bytes) which ~ives the X coordinate of the character.left side bearing. This is followed by a "start point word" B giving the Y coordinate of the lowest start point in the first X grid line of the character. The word B is followed by a "vector byte" giving the values d~ and dy of a vector from that staxt point, and then another start point word D defining the nex_ lowest point. Still another start point w~rd E defines the highest point in the first X ~rid line and a vector byte F defines a vector from this start poirlt. If there are any start points within fifteen X units from the first grid line, these 0~6 ~ay be interspersed in their proper Y value sequence. The character data block continues with vector bytes, "control bytes" and start words C and terminates in an "end block byte" H denoting the end of the block.
FIGS 6B, 6C, 6D and 6E show the formats for the header word, start point word, vector byte and control byte, respecti-~ely. These formats axe drawn with the least signi- ~-ficant bit on the right. The significance of the symbols within these words and bytes are as follows:

Header ~ords:
.
X8X7X6~5x4x3x2xlxa ~ Left side bearing magnitude.

T - Test bit, may ~e used for detect-ing errors.
C - Chain bit indicates whether this word heads the final character block.
X - Kern bit, determines the direc-t1on of the left side bearing (away from or towards the previous character~.
N N2NlNQ - The number of start words on the first grid line sf the character.

Start Point ~ord: ;

Y Y Y Y Y Y Y Y Y Y - The vertical distance between the character base line and the start point (either positive or negative).
S - Undefined.
D - Down bit, determines in which of the two right-hand quadrants subsequent vector displacement will occur.

X X X X - The number of grid lines between the appearance of the ~7 start new line" controL code and the actual start points themselves.

Yector Byte:

Y Y Y Y - This value defines the vertical offset between the beginning and the end of a vector.
X X X X - This is the horizontal offset 3 2 1 ~
between the beginning and the end of a vector.
Control av te: `
Q O Q O - These bits, if set to zero, define a control byte.

M M M M - These four bits form a binary number (O to 15) which desig-nates a "control function".

Control func~ions: Control functions are required through-:
out the character block and are specified in the control byte with its four significant bits set to zero. This permits sixteen different functions to be defined by the numerical value of the remaining four bits.

O - Filler.
1 and 2 - Undefined.
3 - Start two outlines with no intermediate outlines.
4 - Start two outlines below existing ones.
5 - Start four outlines with no A intermediate outlines 6 - St~rt four outlines below existing ones.

7 - Replace an existing outline ~alue with a new value without changing the numerical order of values up the grid line ~i.e., end one-and start one outline~.

8 - Undefined.
3 - End block.

~l2~L~S6 la and 11~ - Undefined.
12 - End two outlines.
13 - End four outlines.
14 - Change direction. Subsequent ectors occur in other quadrant.
- Displacement by 16 u:lits in a vertica] direction with no horizontal movement.

FIGS. 7 and 8 illustxate how a character may be en-coded with the encoding scheme accord:ing to the present inven~ion usin~ the format illustrated in FIG. 6. In FI&. 7 a sim~le "character" has been drawn which contains a number of start points, end points and intervening vectors. The actual coding for this character is shown in FIG. 8, left column. The center column in FIG. 8 explains this ~odins and the right column shows the sequence in which the data would be ~rought in and used by the typesetter.
; FIG. 9 illustrates a preferred format for defining a character with up-do~n vectors (FIG. 3, right side). These vectors are specified in one quadrant by the X,Y coordinates of the end of tile vecto~ .clative to the quadrant origin.
Since outlines are traced from top to ~ottom down the charac-ter, only the two lower quadrants are used. As in the format illustrated in FIG. 6, control codes permit ~uadrant selec-tion and cuxve initialization and completion. r~ith this ; ~.

~2~5~i ..
-30- .

format the grid line Y-O is at the top of the character area and successive horizontal grid lines are given con-secutive Y numbers down the grid.
A ~lock of data defininq the character starts with a "Y data word" which gives the highest Y start coordinate of the character. This is followed by an "X data word"
defining the X start coordinate of an outline, and the ~ectors and controls for this outline.
All subsequent outlines are sequenced such that the starting point Y values are in increasing order; i.e., t~
Y value for each ne~t outline is equal to or ~reater than the Y value for the preceding outline. Thus, entire strinss or sequences of ~ectcrs are defined and completed before defining the ~e~t string. If t~o starting poin s have the same Y value, either poin~ may be listed first with its entire vector string.
; FIGS. 9B, 9C and 9D show the formats for the Y data word, X data word and the vector or control wo~d, respec-tively. These formats are drawn with the least significant bit on the right. The significance of the symbols within these words and bytes are as follows:

Y Data ~ord:
; Y - This data defines the vertical position of the start point.
K - Undefined.

X Data ~10rd:
XN - This data defines the horizontal position o a start point. Left side bearing (LSB) is defined as 0.
The sign bit defines the displacement of XN with respect to the LSB.
L - The L Bit defines the direction of the dx - o the îirst vector.
F - The ~ Bit or "Flare Bit" defines which .
- vector slope will be used by the decoder in extrapolating a character outline in the region o the grid immediately above ~ ~
the line Y~. `
E - The E Bit or "Extrapolation Bit" defines whsther extrapolation is or is not used in the region above the grid line YN.
B - T~e B Bit is the "Boundary On/O f Bit"
and defines whether the outline is the left-side (on) boundaxy or the right-side (off) boundary.

Vector/Control T~ord:
dydx - For all ~alues of dy greater than 0, ~his byte defines the slope of the vector out-line of the character fxom the start point (YN,XN~ or from the last vector end point.

All vectors are sequenced serially in the same sequence that they occur on the character outline. ~he initial vector is located in the MSB's of the word, the second in the LSB's.

Control Functions: For all values of dy=O, this byte defines a con~rol code. The specific control is dependent upon the - value of dx as indicated ~elow:
O - End of out3.ine. If located in MSB's, LSB's must be filled with zero's.
1 - Reverse the dx direction for the next vector.
2 - Defines that there are no displacement vectors applicable to the start point defined by the preceding Y and X Data Words.
This control will always be located in MSB's, the LS3's being filled -~ith æeros - to produce an "End of Outline" control code.
3 - Defines a vector with a horizontal displace ment of O units (a vertical vector) and a vertical displacement greater than 30 units.
The ne~t data byte defines a binary value of the vertical displacement. The data byte has a resultant range of vertical -33~

displacement of 0 to 255 inclusive, but it shall not be utilized between 0 and 30 inclusive.
4 - Deflnes a vector with a horizontal displace-ment of 1 unit and a vertical displacement of 3~ units.
S - Defines a vector with a horizontal displace-ment of 1 unit and a ~ertical displac~ment of 6a units.
- 6 - Defines a vector with a horizontal displace-ment of 1 unit and a vertical displacement of 120 units.
7 Defines a series of vectors wnich follow a concave outllne.
8 - Ditto the unction 7 for a convex outline.

9 - Ditto the function 7 for a straight outline.
- Defines whether the outline has a low or a high degree of concavity or convexity (this bit is sensed only if bits 7 or 8 indicate concavity or conve~ity).
11 - Defines a vector with a vertical displace-ment of 1 unit and a horizontal displacement greater than 255 units. The ne~t data byte defines the binary value of horizontal dis-placement in exc~ss of 25S units.

, . . , ., ~ - . .

12-14 - Undefined.
15 - Defines a vector with a horizontal dis-placement of 1 unit and a horizontal displacement greater than 15 units.
The neY.t data byte defines the binary value of the horizontal displacement.
FIGS. 10 and 11 illustrate how a character may be encoded with the encodin~ scheme according to the present invention using the for~at illustrated in FIG. 9. In FIG.

10 the character "A" contains a number of start points, end points and the intervening vectors. The actual coding for this character is shown in FIG. 11, :left column. The ri~ht column in FIG. 11 e~plains the nature o~ this c~ding.
FIG. 12 illustrates a conventional magnetic disk, called a "floppy disk", which has been removed from its cardboard jacket. The disk is about 8 inches in diameter and has a 1 1/2 inch center opening to permit rotation on , a spindle. The disk may be magnetically sensitive on one or ~, both sides so that the binary information may be recorded and stored on, and n*~Yed from one side or both sides.
The floppy disk shown in FIG. 12 is "hard sectored"
by 32 small holes spaced evenly around the center openins.
A 33rd hole is arranged mid~ay betwen t~ of the evenly placed holes to indicate a start point. The holes, which may be sensed by a photocell, divide the disk into 32 equal ~, ~2~

sectoxs (indicated by lines in FIG. 12 for purposes of illustration only). The disk is also divided concentri-cally into 77 circular tracks (also indicated by lines for purposes of illustration only). Thus, a location on the disk may be specified by track and sector, the numbers of a track and sector constituting an "address". Each addxess ~track and sec~or) on the disk is capable of storing up to 250 bytes of information.
~ FIG. 13 shows how one or more fonts of characters, which are encoded in accordance with the principles of the present invention, may be recorded on a floppy disk.
Two specific sectors on the disk on a speci ic track (e.g., on track 00, sectoxs 00 and al) are allocated to disk label and font index. The encoded character information may be stored, commencing at any other address on the disk.
- The disk label describes the contents of the disk in conventional Arabic letters, encoded in binary with a stan-dard code such as the American Standard Code ~or Informa-tion InterChange ~ASCII). The font index sives the initial address of each font recorded on the floppy disk. This font index may consist, for example, of a se~uence of double words, the first word defining the font number, and the second word the trac~ and sector address o' th~ start of the f~nt. Thus, i~ a user ~ishes to locate font num~er 126, he causes ~L2~5~

word defining the ~ont number, and the second word the track and sector address of the ~tart of the fontO Thus, if a user wishes to locate font number 126, he causes the computer to s~an the font index to find the initial address of that font.
The font information consists of a character look-up and width file, followed by blocks of data defining as many characters as there are in the font. The character data blocks may have the format shown in FIG. 6A or FIG.
9A or they may have some other suitable format for the encoded ch~racter data.
A typical look-up and width file is shown in FIG.
14. This file contains d~ta applicable to individual characters which are needed by a composition system. The character imagin~ system or tv~esetter ma};es no use of this information.
If three bytes are used to define the data for each ~ ;
character, U? to 83 characters may be described in one sector. Each character width group of ~hree bytes includes a character number, the character unit width and "flag bits", repsectively. The character number is related to the form of the character bv keyboard layout number. The unit width is the ~idth o the character in 1/5~ths of an "em"

æ~6 The flag bits are designated bits defining specific characteristics of the character. The flag bit 5 is the "B" bit denoting that the character is a base piece accent aligned with the lower portion of character which is not to be jumped when the upper case mode is envoked. Flag bit 5 is the "C" bit denoting that the character is a center-aligned piece accent, and flag bit 4 is the "D" ~it denot-ing that the character is a drawn display superior figure.
The character look-up and width file concludes ~ith a chain address containing the address of the next character width file sector or the first sector of the encoded charac-ter data.
Once digitized character infor~ation is encoded and stored on a flop~y disk, it must ~e read, interpreted and i~aged bv a typesetter onto photographic film. This charac-ter generation process will now be described for the character encoding sche~e set forth above in connection with ~IGS. 3-14 as arranged in the particular format shown in FIGS. 6-8. FIG. 15 illustrates the type of data required by a character generator to "stroke" a character (in this case again the "Q") by means of a CRT, laser ~eam or some other flying spot scanner. In particular, the charac-ter generator requires data in the form of intercept values on each output scan line. In the case of vertical scan lines, as shown in FIG. 15t these are the signed Y values of the on/off points on each scan line. The values are referenced to the character base line with the positive values of Y above, and negative values below the base line.
The top~most value of the highest imaged segment in a scan llne is flagged so that the character generator can immediately proceed to scan ~he next line.
In Fig. 15, in the first (left-most) scan line 40 the scanning beam is moved vertically upward and proceeds at a constant rate from the ~ase line. The beam remains off until it moves a distance Y0 from the base line. At this point, the beam is switched on and remains on until`it moves a distance Y1 from the base li~e. Thereafter, the scan may continue, with the beam switched off, until it reaches the top of the raster matrix. Preferably, however, the ~eam will lmmediately retrace to below Y2 or to the base line and proceed with the second scan line 420 ~his retrace is trisgered by associating and "end-of-the-line" flag with the data Yl.
The data sequence required by the character generator is therefore, Y0, Yl, Y2, Y3, Y4, YS, Y6, Y7, Y8, Y9, YlO, Yll, Y12, Y13, Yl4, Y15, etc., the end-o-the-line flag being in-dicated in this sequence by the italics. Since the data is stored and supplied to the typesetter in start point and vector outline format, the typesetter requires a "code con-verter" to convert this vector format into the intercept ~ormat illustrated in Fig. 15. The structural details of the code converter will depend upon the part~`cular vector format used ~for e~ample, the format illustrated in Figs.6-8, or the format illustrated in ~igs. 9~ and the particular intercept format (vertical or horizontal scan; single . . . .

s~
.

character or multiple characters per scan line). In the embodiment described below, the cod~ converter is capable of translating the format illustrated in Figs. 6-8 into a vertically scanned, single character intercept format.
In executing the translation from vector format into intercept format, the code converter should preferably be capable of performing scaling, interpolation, and averaging.
These three operations are illustrated in Figs. 16-19.
- Assuming that the output resolution (scan line density) of the character generator is fixed, characters must be hori-zontally scaled by adjustir.g the number of scan lines required to de ine a character. Figs. 16 and 17 illustrate this prin-ci~le, .~hereby the width of the character is varied by evenly distributing the necessarl num~er of scan lines across the character.
Vertical scaling may be accomplished either by analog hardware (e.g., a vertical deflection amplifier) or by digital hardware or soft.~are (e.g., by multiplying the intercept values YO, Yl, Y2...etc. by a digital scale factor).
For characters of larger point size it may be neces-sary to interpolate -to find the beam switch point on certain scan lines because the line density of the matrix or grid on which the character is encoded is i.nsufficient. In accor-dance with the preferred embod ~ent of the invention, straignt line interpolation is used to increase the digi-tized resolution. For e~ample, if the encoded character data corresponds to a 32 point character in the resolution of the character generator, it _s necessary to muptiply by more than two to achieve 72 point output. The vertical Y values are simply dou~led and the character generator multiplier makes the further adjustment. The code converter inserts three additional equally spaced vertical lines between each pair of digitization lines and uses a straight line inter-polation to estimate intercept values as shot~n in FIG. 18.
In this figure, the continuous lines are the original digi-tization resolutiorl and the dashed lines are the actditional interpolated positions. A "O" indicates a digitization point derived from vector decoding and an "X" indicates an interpo-lated point. If all of the additional lines were output at the constant output resolution, the character would appear four times the original size (e.g., 128 ~s 32). It is therefore possible to periodicall~ omit lines acxoss the character to produce ary width of character less than this size.
Below a certain point set width, an averaging techni-que may be used to reduce the ~nount of data. For small sizes, the amount o digitizated data will be in excess of that require~. To utilize all this information the code con~erter may pr~duce intercept ~alues that axe the arith-metical average of the digitization ~a~ues between output scan lines, as shown in FIG. 19. In this figure, the con-ti ~ us lines are the original digitization resolution and the dashed lines are the scan lines selected for output.

L2~56 "0" .i~dicates a digitization point derived from vector de-coding, an "~" indicates a vaiue used to calculate the average and a dashed "0" is the averaged output value of the code converter. As may be seen, the output value is calcu-lated from all intermediate digitization points as well as that of the previous output line. This averaging technique results in a displacement of the character by apprcximately hal an output scan resolution unit to the right.
Fig. 20 illustrates a third genera-tion (CRT) typesetter which may be designed to accept digiti.zed fonts encoded in accordance with the present invention. This machine com-prises one or more floppy disk read/write units (mounted on slides for ease of removal), a card framç containing a num~er of electronic boards, a cathode ray tu~e, a high voltage power supply unit for ~his CRT, and a photosensitive film transport mechanism for passing film past the face of the CRT into a take-up cassette~ The typesetter also includes the usual front panel controls and a paper tape reader.
Fig. 21 shows how the various elements of the type-well-known devices with the exception of the code converter and character generator which will be described in detail below.
The system is controlled by a central processor unit 5~ (an L.S.I. 3~05 Naked Milli Computer, produced by Computer Automation) either directly via its own data bus tmaxibus) 52 or indirectly ~ia a special data bus (auxiliary bus) 54.
The system opera~ion is determined by a program resident in a main memory 56 attached to the maxibus which may have up to 32 K X 16 bits of storage.
Operating instructions for the machine are received from three possible sources: a 300 C.P.S. paper tape reader 58, front panel controi 60, and an on-line interface 62. All of these elements are connected on the maxibus 52 as is a floppy disk read/write unit 64 which supplies the digitized fonts.
An auxiliary bus interface and auxiliary bus buffer 66 control the components attached to the auxiliary bus 54.
The inter~ace and control G6 is, in turn, controlled by tne CPU 50 via the maxibus 52.
A lor~ voltage power supply 6~ is connected to all of the electronic circuit boards to power and logic circuitry.
The components attached to the auxiliary bus 54 are responsibl~ for the generation of characters. The code con-verter 70 extracts condensed font data from a RA~I or PROM
font store 72 and processes it into an expanded, intercept format. A character ~enerator 74 recei~es this data and produces a beam switch signal on line 84 and analog vol-tages representing X and Y deflections OA a cathode ray tube. These analog voltages are amplified by video deflection amplifiers 76. Correction circuits in these amplifiers modify the analog signals to correct for the ~2~ ii6 CRT geometry. The characters are finally produced on a CRT 78 using electromagnetic defle.ction coils 80. The CRT
beam is switchcd on and off at the appropriate moments during scanning by the signal received on line 84 from the character generator 74. The electron beam is accelerated within the CRT by a high voltaye provided by the high voltage power supply 82.
Photosensitive paper or film is in contact with the CRT face, so that latent images axe formed of the characters.
A mechanical fil~ transport 36 advances the paper after each line of characters is complete. A stepper motor of the film transport receives power from a motor drive board 88 which is controlled by a leadin~ eontrQller board 90 attached to the au~iliary bus 54. The paper is fed into a light-tight take-up cassette which holas tne paper until it is developed. The paper is cut off with an electrically operated knife and then photographically processed.

As noted above, the computer 50 coordinates an~ con-trols the functions of the various elements of the system.
Ini~ially, the choice of font, point size, characters and ~ :~
character positions are read by the paper tape reader 58 and stored in the main memory 56. Thereafter, the encoded data defining the individual characters of the chosen font are read from a floppy disk by the read/write unit 64 and stored in the R~M 72. As the successive character bloc~s are read from the floppy disk, they are placed in specific locations in memory so that these blocks may be subsequently addressed as the characters are imaged. The R~ 72 therefore provides ready access to the compressed data defining the characters of a single fontO
On instructions rom the computer 50, the code con~
verter 70 receives encoded data for a single character on a need-to-know basis from the RA~ 72 and calculates the ~eam switching points for each successive raster line. The code converter also kee~s track of, and updates the ~ and Y
raster coordinates. To assist in the calculation of the ~eam switching points, a programma~le read-only memory tPROM) within the converter ser~res as a look-up table for the slope of each defined vector.
The characte~ imaging system comprising elements 74-90 images successive lines of chaxacters onto the photo-sensitive filmu On instructions from the computer 50 the imaging system advances the fiLm after each line is completed.

As noted above, all of the elements shown in FIG. 21, with the e~ception of the code converter 70 and character generator ~4, are of well known, routine or "off the shelf" designs or components. While the computer 50 i5 programmed, this software consists essentially o standard data moving and machine control instruction ln a given sequence. Conse~uently, this software is well within the skill of an average programmer.
Character generation operates as follows:

2~56 . .
The start point and Yector dat~ relating to the part of the character to be imaged in a Yertical scan line is addxessed (called) from the R~ 72 and is latched into the code converter input buffer. As each scan line is imaged, the sequential data defining start points and vec-tors for the next following line are called as required.
Since the vectors may, and normally do extend in the X
direction across a num~er of vertical scan lines, a new vector is calied only if the previously stored vector(s) are not sufficient to define the next scan line.
The calculation of the CRT beam switching points for the ne~t scan line then proceeds,using the slopesstored in the vector slope PRO~. As illustrated i~ ~IG. 22.~, the Y intercept ~ositions or values at which tne beam should be switched from off to on and from on to off are stored in a FIFO (first in, first out) register "stac~" 91o The Y inter-cept values for each scan line are sequentially entered into successive "Y registers" in the stack, the first or lowest Y value being placed in the lo~est Y register and successively higher Y values in successively higher registers. The upper-most Y value in the scan line is fl2gged with an ENDSC bit to indicate that the scan may be reset. The ou~put of the lowest Y register in the stac.~ is converted to an analog vallle by a d;gital to-analog converter 92 in the character genera-tor 74. The character generator also has a ramp generator g3 that produces a uniformly increasing output with time.

A comparator 94, connected to change the state of a flip-~lo~ "toggle" 95, tu~ns the CRT beam on or off when the ramp generator output reaches an analog value equal to the D-to-A output, and indexes the stack 91 to call up the next highest Y intercept value. If the ENDSC bit is on when a beam switch occurs so that a signal is present on line 96, the ramp generator 93 will be reset to produce a Y deflection volta~e just slightly lower than that of the next following Y intercept value. This avoids excessive flyback and in-creases the speed of the output. The CRT beam is therefore not reset to the baseline of the character or tlle base of the em square; ratner it is reset to the lo~est needed level for t~e next scan line, and does not have to be driven twice over space where it will not ~e turned on. ,-The ramp generator 93 is caused to rapidly reduce its output voltage at a constant rate when a signal is present at its ~lyback input. This ~lyback signal remains on until the output of the ramp generator has dropped below the lowest Y intercept value for the next scan line. The fly-back signal is produced by a logic circuit comprising an AND gate 97, inverter 98 and a flip-flop 99 which receive an input from the compar~tor 94 and the ENDSC signll on line 95.
The operation af the flyback logic i5 illustrated ; in FIG. 22B. This figuxe shows the CRT Y deflection voltage produced ~y t~e ramp generator 93 for several strokes of the .

~47- ; .

"Q" illustrated in FIG. 15. At the beginning of the first stroke 43, the Y intercept values Y6 and Y7 are entered ir.to the lowest and next lowest Y registers, respectively, in the FIFO stack 91. Because the output of the ramp generator starts at a point slightly below the analog voltage equivalent to Y~, the comparator g4 produces no output. However, when tne Y deflection voltage reaches the Y6 value, the comparator 94 produces a signal which switches the toggle 95 from off to on and calls up the next Y value, Y7, in the FIFO stac~ 91. The Y deflection voltase continues to ramp up untll it reaches a voltage equivalent to Y7. Because the ne~t Y value, Y8, is considerably lo~er than the Y deflection voltage, the compara-tor 94 cor.tinues to produce a signal until the ramp generator output has been reduced. Since an E~DSC bit is associated wlth Y7, a signal is present on line 96. The output of the com- -parator 94 and the signal on line 96 trigger the ~ND gate 97 and set the flip-flop 99 to produce a flyback signal. When the output of the ramp ~enerator 93 has fallen below the Y
value, the output of the comparator 9~ drops and resets the flip-flcp 99 through the inverter 98. This removes the fly-bacl~ sign21 and allcws he r~p generator to ramp up on the stro~e ~4. The Y deflection voltage will promptly reach the value, causing the comparator 94 to agaîn produce an out-put signal which s~Yitches the beam from off to on. The beam is switched off again when the Y deflection Yoltage reache~. Y9, switched on when it reaches Y10 and switched off again when it reaches YIl. Since an E~IDSC bit is associated with Yll, the flyback process is repeated to commence the stroke 45.
From this description of the operation, it will be understood that the lower and upper lLmits of beam travel in any particular stroke approximately correspond with the lowestand highest Y intercept values in that st.roke; that is, ~he lower and upper limits o the character intersections.
FIG. 23 specifies the various inputs and outputs of ~, the code converter 70. The signals to and from the auxiliary ~.
data bus ~4 are shown on the left, and the signals to and from the character generator 74 are shown on the right. These signals are defined as follows: .
XDB - 16 bit data word defining the charac-ter to be imaged are received in parallel from the R~l 72.
XBMS - 3 control inputs, whose states are determined by the computer 50~ initiate :~
and control operations in the code converter. .
XRST - A signal control input, originating ~rom the computer 50, is used to totally reset the code converter ir-respective of the states of any other signals.

CYCREQ - ~ata input occurs upon receipt of an ~5BS
CYCACK
signal. The code converter then assumes control of the handshake and supplies a signal on CYCREQ whenever it requires a data word. The word is latched when the data source responds with a signal on - CYCACK, and the CYCREQ signal is dropped.
EOC - ~en the code con~erter has completed processing a character it assumes an idle state until the character generator sends a signal on E~PTY, The code converter then supplies the signal on EOC until the XBMS
si~nal, indicatlng data input, is removed.
SDATA - 11 bit data words representing intercept values or beam switch points are passed to the character generator in serial form.
SERCK - The code converter generates a 5 MHz. cloc~ -signal, which is supplied to the character generator to synchronize the bits in the output data word (SDATA~.
ENDSC - If the output data word referred to the highest outline curve o~ the character at that point, a signal ;s passed to the chaxacter generator 74 on this line to end the scan (stroke), ~L~a 2~ ~
3~6 D~T~Q - The character generator requests data by DATAy supplying a signal on D~TRQ. The code con-verter responds with a si~nal on DATAV when an output data word is available, The data bits are then transmitted on SDATA through the next 11 clcck cycles and the signal on DATAV is dropped.
STEPDN The "white space" at the leading edae of a STEPUP
character ls scaled ~y the code converter. ~`
The width of the space is transmitted to the character generator as a series of pul.ses.
Each pulse corresponds to a movement OL one line scan (stroke). The side ~earing may be moved away from or ~oward the p evious character. The width of the space and the direction are specified in the character data.
Pulses appear on STEPU~ for an increasing side bearin~ and STEPDN for a kerned character. The pulses occur at the beginn~
ing of the character processing beLore any data ~ords are presented to the character genexator.
~5PTY - The c~aracter generator supplies an E~lPTY
signal when its output ~uffer is empty.
This is used by the code converter to determine when a character has been com-pletely drawn.

, .
FIG. 24 is a block diagram showing the elements of the code con~erter. The element la0, indicated as the "master controller", is broken down in FIG. 25. The con-troller 10~ receives 16 inputs from a control decoder 102 and four inputs corresponding to XB~IS (signals 0, 1, 2) and XRST. The decoder 102 generates the 7 control inputs from ~ signals, representing start words and control bytes, received from an input bu~fer 104. Data is latched into the input buffer from the 16 XDB lines.
The master controller, shown in FIG. 25, generates 46 output signals for controlling the operation of the code converter. These signals are applied to the various logi~
elements o the converte~, in a known manner, to gate and la~ch the sisnals in a prescribed sequence. The controller `~
comprises a state PROM 106 which determines the ne~t state of t~.2 code converter from the current state and the condi-tions on 16 control inputs. The state ~ROM is addressed bv 4 signals received from a multiple~er 108 and 5 signals received from a latcn 110~ The output of the state PRO~l is supplied to the latch 110 which, in turn, is connected to a state decoder 112 and a` "pseudo" state PROM 11~.
The pseudo state PROM 114 is capable of modifying its output state durin~ a processor cycle iI the current state and its control inputs force it. In addition to the state output from the latch 110, the pseudo state PROM
receives the 4 control signals principally from the decoder 102.

~.~ 2~

Of the 8 outputs of the pseudo stàte PROM 114, 5 are decoded by a pseudo state decoder to produce 24 control outputs.
Vector Processing: Five parameters are stored for vector processing. These are:
(1) Intercept value (11 bits): The intercept value, which is stored in the intercept store 120, is the Y value of successive vector ends around an outline. Thus -~
YO = ~Y start point (~XN, ~YN is the Nth vector) Y = Y ~ ~Y
. 1 o _ o Y = Y + ~Y
2 1 _ 1 ~.

Y = y ~ y N ~ N-l ~ 2) ~X value (4 bits): The ~X value, ~hich is stored in the ~X store 122, is the horizontal distance from the right-hand end of the current vector. Thus, for success-ive grid line calculations:
~ X = ~X (new vector starts here) X - 1 ) post decremented ~X

~X = 1 ~end of ~ector~.

s~

(3) ~Y value (5 bits~: The ~Y value, which is stored in the ~Y store 124, is the approximate vertical distance from the right-hand end of the current vector. The four most signi-ficant bits are taken as the input ~Y value and the least significant bit is introduced by a look-up table to improve accuracy.
- (4) Sign Bit tl bit): The sign bit, which is stored in the control bits store 126, is 0 for a vector in one (e.g., the upper) quadrart and one for a vector in the other (e.g., lower) quadrant.
(5) Valid Bit (1 bit): The valid bit, which is stored ln the control bits store 126, is 0 for an intercept value, ~hich is a new start point Y value without anv vector modification, and one for a modified intercept value which ma~-be used for calculating an output value.
With the exce~tion of the A, B and C bus loops ~hich : include the intercept store 120, an accumulator 128 and a correction store 130, the sign is ignored and positive values only are considered. The sign bit is in~roduced at the accumulator where appropriate.
Computation begins with a start point Y value loaded into the inter~ept store 120 and the ~ store 122 holding the displacement to the beginning of the first vector, and wi~h the valid bit set at zero. As each grid line is processed, - ~ 2~ ~ 56 the ~X store is decremented; when it reaches "1", it signals for a vector ~yte. The interce~.t store 120 is updated with the ~Y value and ~X and a~ are stored. The valid bit is set to 1 making the data available for output. This computation process is illustrated in FIG. 26. At suhsequent grid lines, the ~X store 122 is de_remented and ~Y is reduced b~ the out-put of a vector slope ~ROM 129. The PROM is addressed by ~X
and ~Y and outputs a normalized ~Y value,~y. ~y~ is inverted by an interpo~ation PRO~l 132 which in this mode is only acting as a complementing buffer. This output is then added to QY
by an adder 134 and restored in the ~Y store 124.
All the code converter stores are configured from 16 deep random access men~ories. The R~Ms are addressed in parailel from a 4 bit by 16 deep FI~O register as shown in FIG. 20. This register contains the RAM addresses for the current outlines in order of increasing intercept value. The FlFO is normally operated with its outputs connected to its inputs thereky recirculating the addresses. For every vector processing operation an address is clocked into the output register of the FIrO and the previous address is lo~ded into - the FIFO input.
New addresses at start points may be introduced into the loop ~rom the new address counter and added to the FIFO
stack. At end outline points the address is not reloaded into the FIFO and so is deleted from the stack, 5~

Initially the 4 bit new address counter is set to a maximum count of 15 and it is decremented as each start point occurs. Every R~M location which contains outline information (i.e., the address, occurs within the FIFO stack) has the "not vacant bit" set to a one. The not vacant bit (l bit), ~hich is stored in the control bitsstore 126, is 0 for an empty RAM location and one for an occupied location.
An end outline control code causes the not vacant bit to be returned to a,0.
~ hen 16 outlines occur in one chdracter, the new address counter will have decremented to zero. Any further start points must be preceded by at least an eq~lal number Oc end outline codes since no more than 16 outlines may be processed at one time ~ the ccde conve`xter. On receipt of such a start outline code the master controller sequentially addresses the R~1 locations, b~ decrementing the ne~ address counter, until an address with'the not vacant bit set to 0 is found. This address is then entered into the FIFO stac~ and used for the new outline.
The FIFO may consequently hold a variable len~th stack of non-sequential values which correspond to the RA~1 addresse~ of the current outlines. The o_der in ~hich start point codes and vector ccdes occur in the character data ensure that the addresses are entered into the stack and so presented to the RAMs in the correct order to provide increasing inter-cept values on output.

....

.
, The lowest outline latch is a,4 ~it xegister which ;' holds the R~M address value of the current lowest outline.
It is up-dated when outlines are started below the existing ones or when the existing lowest outline is ended and the next highest ~ecomes the lo~est. The latch ,output is continuously compared wit~ the current ~AM address and when they are iden-tical a con~l signal is sent to the master controller indicat-ing that a sc,an line has just been completed.
This RP~i addressing system provides a very fast and flexible method of cyclically processing a variable number of outlines whilemaintainmg a correct sequence with no over-heads at line ends.
Scaling: A value representing the character set width in points is loaded into a scaler 136 before vector processing is commenced. The job or the scaler is to hor~
zontally scale the character ~y determining the point at which Y values should be passed to the output buffer 13,8 for serial transmission to thè character generator. The scaler 136 informs the master controller 100 whether to compute the next grid line values or to output the current Y values. I
Y v~lues are to be placed in th2 cut2ut ~uffer, it supplies ~ither the interpolation address, or th~ averaging scaling ~' factor as ~ill be explained belo~
The scaler operates at a much higher resolution than the rest of the code converter to ensure high accuracy. It -- . .

uses 16 times the resolution of the vectors which is 4 tLmes the resolution necessary to interpolate the vectors Eor large point size expansion. If the vector resolution is X lines/em, the scaler works at 16X lines/em. To produce a character at a certain output size with a fixed output stroke resolution --may require W lines/em. Thus the scalex is approximating to the ~raction 16X/W which corresponds to the number of scaler lines ~etween each requi~doutput line. ~is is achieved by repeatedly selecting the integer below 16X/W and the integer akove 16X/W ;~
alterna'tely for differing num~ers of times. A~ four phase cycle is used with each integer occurring twice and with a differing num~er of repeats in e~ch phase. If the numhers of repeats are represented ~y the num~ers N , N , N and N and the integer ~elow 16X/~ ~y M, then the approximation can ~e stated as:

16X = ~Na xM) ~ ~1 x ~M+l~ + ~2 x ~ + (N3 x ~M+l~

Nl + N2 3 4 A special case occurs ~hen 16X~W is itsel~ an inte~er so only a single integer is used and the number of repeats lS
irrelevant.

The detail of the sczler is ~hown in FIG. 30. The set width register nolds the constant value of width supplied ~y the computer. T~is is used to address two PROM look up -58~

tables, One contains the numbers of lines (M) between each output line which are the integers below and above the required fraction. The least significant of the two bits which define the phase number (P~ is used in the address to select between the two integers for each set width valueO The other table contains the numbers of repeats (N). This is additionally addressed by both bits of the phase number allowing different numbers Or repeats in all four phases.
The output from the number of lines table is passed through an adder and split with the 4 least significant bits being held in the remainder latch and the four most signifi-cant bits being loaded into the line counter. The value (L) in the line counter corresponds to the number of lines at the vector resolution bet~.~een each successive output since the stripping oI the four least signi~icant bits effectively divides by 16. The o~tput from the number of repeats ta~le is loaded into the repeats counter when its count (R) reacl~es zero. Thus the value stored in the table is one less than the number of repeats required.
The operation o the scaler is shown by the flow diagram FIG. 31. The scaler is initialized at the beginning of each character and thereafter it is triggere~ into indivi-dual cycles on demand from the master controller which in turn senses the "output line" control signal.
The use of the scaler within the code converter S~

- s9 -processing operations is shown by the flow chart FIG. 27.
The scaler is cycled at the end of processing each grid line of the character and ater sending the ~alues for each out-put scan. The sensed state o the output line signal determines which loop is performed. It follows that every scaler cycle after a grid line calculation decrements the line counter and every scaier cycle after an output operation loads the line counter. At small point sizes ~he "no" loop is used more often since several grid lines occur between output lines. However, at large point sizes, the "yes"
loop is used ~ore of~en since several output lines occur between arid iines.
The interpolation address is simply supplied by the two most significant bits of t~e remander latch. ~his iden-tiies which of the interpolation lines is required.
The averaging scaling factor determines the "~ei~ht"
applied to ~y values in building up the correction term. The weighting depends upon tne total numbe~ of ~alues to be averaged and which particular ~y within the total is being processed. At the sma~l outpu~ sizesiat which averaging is used a very high accuracy s unnecessary. So only tw~ bits are used to define the total number of Yalues (the line counter input ignoring the least significant bit) and the output of the line counter determines which particular ~y is bein~ processed. A PROM look up ta~le is addressed by these six lines and 1 of 8 scaliny factors is selected, ~2~56 Interpolated Output: At point .sizes where inter-polation is used, the code canverter outputs Yalues calcu-lated ~rom straigh~ line interpolation between grid lines.
This interpolation process is illustrated in FIG. 28.
The intercept store 120 holds the absolute Y value o the end of the current vector. A ~Y store 124 holds the difference ~etween the intercept value and the Y value at the last grid line. The scaler 136 provides an interpolation address to the interpolation PROM 132, which is also supplied with ~y from the vector slope PROM 129. ~he output of the interpolation PRO~ 132, ~yt, is a proportion of ~y appropriate to the interpolation position. This is subtracted from ~Y
by the adder 134 and appears on the D bus. It is applied to the accumulator 128 via the A bus and the B bus carries the output of the intercept store 120. The C ~us transmits tne correct output value to the output buffer 138.
The output buffer holds the calculated value until the character generator signals that it is ready - to receive it. The serial transfer is then e~fected and the next output calculation can begin. If the value transerred is that for ~ `~
the highest current outline the code converter flags the character generator after the transfer on the ENDSC
control line.
Averaged output: At small point sizes, ~here there axe more than three grid lines between each output line, -61- ~

an aYeraying al~orit~n can be used to calculate output Y
values. The cor~ection stoxe 130 is used for this purpose.
This store holds a correction Yalue which is applied to the ~alue in the intercept store 120 to produce the output valueO
The averaging system ignores interpolation line addresses and only outputs on incegral grid line values.
The calculation is ~ased .on.the equatlon for the arithmetical mean o~ the values Y to ~ which is:

n - l Ym = [ l (Y - Y1~ + 2 (Y~ - Y2)-~.. + n(Yn-l~Yn)~ + Yn m = o n T~e expression in th.e s~uare brac~ets is t~le correc~
tion term. The averz~e is wor~ed out by considering the Y
values on each grid line and averaging these bet~een output- :. .
lines. Thus, n-l becomes the number of grid lines between output lines and the different terms are then the ~y outputs from the vec~or slope PROM 129.
The application of the equation is illustrated in FIG. 32 ~here the out~ut line at G3 is to be calculated. The intercept store contains the ~-alue Y or th.e yector end on ! G5 throu~hout the operation, Henc~; :
~- . . .

. : ..

ii6 Y = Y - aY ~intercept store minus Y store) o I ~y~ (vector slope PROM output on Gl?

1 2 ~Yl (vector slope PROM output on G2) 2 3 ~Y2 ~vectox slope PROM output on ~,3 n ~ 3 Average Y for lines Gol Gl , G2 13 Y0 3 Yl 2 The correction PRO~l 14a takes the ~y output of the vector slope PRC~ 129 and multiplies it by a factor approxi-mately equal to the aporopriate preceding fraction. This is selected ~y a smaller PROM - the factor selection PROM-in the scaler 135 which is addressed by the numDer of grid lines between out2ut lines (the divisor~ and the current line number (the dividend1. ~he three bit code allowing eight scaling factors is output by the factor selection PROM to the correction PROM.
The correction term is built up by adding the out-put of a correction PROM 140 into the correction store 130.
This store is cleared every time there is an output line and then starts building the correction for the next output. ~he PRO~l output on the ~ b~s is always added to the correction store ~utput on the A ~us hy the accumulator 128. The value in the correction store has its sign changed wherever t~8 outline changes its quadrant. The correction store is only eight bits but it ignores the least significant ~it of the C bus since at the s~all point sizes in which it operates such accuracy is unnecessary. Thus it is effectively nine bits and it has an overflow which lLmits it in the ~;
case of very great displacements.
The value held in the intercept stoxe 120 is not usually the Yn of the equation above but is the end of the current vector. So immediately before output, the correction store is adjusted ~y the current ~Y to allow for the dis- ;
crepancy.
The output value is finally calculated in the accumulator 128 by applying the correction store output on the A bus and the intercept st~ output on the B bus. The C
~us transmits the correct output value to the output buffer 138.
As explained above, the output bufer holds the c21-culated value until the character generator signals that it is ready to receive it. ~he serial transfer ~s then effected and the ne~t output calculation c2n begin. If the value transferred is that for the highest current outline the code converter flags the character generator after the transfex on the ENDSC control line.
r~hile there has been described what are belie~ed to he th~ preferred embodimentsof the invention, those skilled in the art will recognize that Yarious changes and ~odifica-tions may ~e made th~reto without departing from the spirit of the invention, and it is intended to claim all such embodiments as fall within the true scope of the invention.

Claims (5)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN
EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED
AS FOLLOWS:

1. A typesetter for the automatic generation of characters comprising a character imaging system for writ-ing graphics quality characters of any design on a print medium; a font storage system having digital data stored thereon defining each character to be imaged; and an elec-tronic computation and control system, connecting said font storage system with said character imaging system, for controlling said character imaging system in accordance with said digital data;
said character imaging system including a flying spot scanning device for writing characters by means of a plurality of parallel scanning strokes of a scanning beam;
and said computation and control system including:
(a) means for producing a beam deflection signal, determinative of the amount of deflection of said scanning beam in the direction of each stroke, said beam deflection signal causing successive strokes of said beam to start sub-stantially at the intercept point on one side of a character and to terminate substantially at the intercept point on the opposite side of said character.

2. The typesetter recited in claim 1, wherein said flying spot scanning device is a CRT.

3. The typesetter recited in claim 1, wherein said computation and control system further includes:
(b) means for randomly accessing said stored digital data and supplying said digital data in sequence;
and (c) means, adapted to receive said digital data in sequence, for converting said digital data into charac-ter intercept values for each scan line stroke of said scanning device; and wherein said signal producing means includes:
(1) means for producing an analog voltage representative of successive intercept values for each scan line;
(2) ramp generator means for producing a ramp voltage of substantially constant slope, said ramp generator means having a reset input causing said ramp voltage to rapidly return to a given value; and (3) means responsive to said analog voltage producing means and to said ramp generator means for applying a flyback signal to said reset input, said flyback signal causing said ramp voltage to return to said given value.

4. The typesetter recited in claim 3, wherein said given value of said ramp voltage corresponds to the lowest intercept value for the next successive scan line.

5. The typesetter recited in claim 1, wherein said means for applying a flyback signal includes:
(i) comparator means connected to said analog voltage producing means and to said ramp generator means for producing an output signal when the inputs thereto are sub-stantially equal;
(ii) logic means, connected to said comparator means, for generating a flyback signal when said ramp voltage is substantially equal to the analog voltage representative of the last intercept value for a scan line and for maintaining said flyback signal until said ramp voltage has fallen to a value corresponding to the lowest intercept value for the next successive scan line.