GB2157119A

GB2157119A - Halftone dot generation in image reproduction

Info

Publication number: GB2157119A
Application number: GB08508282A
Authority: GB
Inventors: Kunio Ikuta
Original assignee: Dainippon Screen Manufacturing Co Ltd
Current assignee: Dainippon Screen Manufacturing Co Ltd
Priority date: 1984-04-06
Filing date: 1985-03-29
Publication date: 1985-10-16
Also published as: JPS60213170A; GB8508282D0; DE3511977A1

Abstract

To convert an original with graduated tones to half-tone dots, without a conventional screen memory storing a 2-dimensional array of threshold values for the elements of a dot, the co-ordinates W, V of the scanning head of a reproducer are transformed to the rotated axes X',Y' of elements in the corresponding dot. Concentric zones in the dot, such as diamonds or circles distinguished by a single parameter, have uniform reference density; calculation of this zone parameter from the co-ordinates X',Y' of any element in the dot allows an algorithm or a look-up table to provide a reference density to the element. The measured density signal for each element of the original is converted by a suitable function to an intermediate density which is compared with the reference density for the element. If the reference density is lower the element is recorded dark; if higher the element is light. To save time a group of elements on the scanning heads may be recorded simultaneously. <IMAGE>

Description

SPECIFICATION Halftone dot generation in image reproduction Field of the invention The present invention relates to halftone dot generation in image reproduction, and more particularly to a method and apparatus for electronic halftone dot generation in an image reproduction, wherein no screen memory is required.

Background of the invention It is well known in image reproduction processing that halftone dots are formed by means of a contact screen, and recently such have come to be produced electronically by means of a halftone dot generator. In this connection, Japanese Patent Laid Open No. 55-6393 or U.K.Patent Application No. 8503431 discloses a method which comprises the steps of preparing a screen memory storing threshold levels with addresses corresponding to the positions of a plurality of sub-cells composing haltone dots, reading one of the threshold levels stored in an address corresponding to a signal representative of the present recording position, obtaining a recording beam control signal by comparing the density value of the present pixel with the read out threshold, and controlling a recording beam for recording a corresponding sub-cell on a photosensitive film according to the conrol signal. This kind of method, while somewhat satisfactory, necessitates a plurality of screen memories together with equipment for successively using one screen memory at a time, while driving a plurality of recording beams to attain a sufficient recording speed.This requires complex circuitry and is relatively costly.

Summary of the invention Accordingly, it is an object of the present invention to provide an improved method of and apparatus for producing halftone dots.

Another object is to provide a method of and apparatus for producing halftone dots, wherein no screen memory is necessary.

Briefly, in accordance with the present technique, a control signal for driving recording beams is obtained by comparing a halftone dot reference density value D as a function of the recording position of a recording drum [D = f(x, y)j and an intermediate value Ds obtained from a density value S obtained in turn from an input scanning means. The intermediate value Ds is employed because density value S output from the input scanning means is not comparable in magnitude and proportion to the reference density value D; therefore, the two values must be coordinated through the intermediate value Ds. The intermediate value Ds corresponds to the density value S through a function Ds = g(s).

By implementing the above measures, halftone dots can be produced without a screen memory.

The above and other objects and features of this invention can be appreciated more fully from the following detailed description when read with reference to the accompanying drawings.

Brief description of the drawings Figure 1 shows the density contours of a halftone dot.

Figure 2 shows the relation between density and intermediate values of a halftone dot.

Figures 3(A) and 3(B) are representations of screen angle used to describe the invention.

Figure 4 is a block diagram of an embodiment of the present invention.

Figures 5(A)-5(F) show several different density contours of halftone dots.

Figures 6(A)-6(F) are diagrams of calculation circuits for generating function signals respectively corresponding to the density contours shown in Figure 5(A)-5(F).

Figure 7 is a circuit diagram of a square root calculator used in the invention.

Figure 8 shows several relations between density and intermediate halftone density values.

Detailed description ofa preferred embodiment of the invention In a method of generating halftone dots in accordance with the invention, a reference value D is compared with a particular value Ds (an intermediate value) corresponding to the density value of an original halftone to generate a control signal. The signal controls a recording beam such that a corresponding point of a photosensitive film is blackened when Ds > D or whitened when Ds < D. Whereas the values D and Ds to be compared have conventionally been halftone density values, in the present invention they are as follows.

Figure 1 shows a coordinate system having x and y directions and an origin at P (0,0), on which squareshape density contours do to dn (do < d, ... < dn) are displayed. Generalizing the square contours do to d as D, the value D can be expressed by the equation: D=lxl+lyl ----- (1).

In equation (1), since the value D can be regarded as a density contour of a halftone dot used for image reproduction, it can act as a reference value to be stored in a conventional screen memory. It should be noted in this connection, however, that the variation of an image density value S obtained from an original via an input scanning means (not shown in Figure 1 ) does not correspond to the value D representative of the area (density) variation of a halftone dot; therefore, they cannot, as they are, be compared directly.

Accordingly, the density value S is modulated by a function g(s) to be a value Ds (called the "intermediate value" hereinafter) comparable with the value D.

Although the above describes only a halftone dot composed of a square pattern, halftone dots of other patterns can also be correspondingly expressed as follows. Ina regard to examples shown in Figure 5(a) to 5(F), a halftone dot of a square pattern [Figure 5(A)] can be expressed as: D=lxiilyl -----(2); a halftone dot of a chain pattern [Figure 5(B)] can be expressed as: D=aixt+blyt (3); a halftone dot of a circle pattern [Figure 5(C)] can be expressed as: D = #x + y2 (4); a halftone dot of an ellipse pattern [Figure 5 (D)] can be expressed as: D = V(x2/a2) + (y2Ib2) (5); a halftone dot of a barrel pattern (Figure 5(E)] can be expressed as: D = V (I x + a)2 + ( y + a)2 -----(6) where a > 0; and a halftone dot of a spindle pattern [Figure 5(F)] can be expressed as: D = ( x - a)2 + ( yl - a)2 (7) where a > 0.

The value D can be found from an x, y coordinate system. Therefore, a signal for transforming the above x, y coordinate system to that of the actual recording film must be generated. Thus, the coordinate system (V, W) (where V is the main scanning direction value and W is the sub-scanning direction value) of the present recording point is detected by a device provided in an image reproducing system. The coordinate system (V, W) corresponds to another coordinate system (x', y') of halftone dots through a screen angle (3 as follows: x' = - W sinO + V cosO ----- (8) y' = W cos # + V sin # as shown in Figures 3(A) and 3(B). Assuming that a halftone dot comprises 2m halftone sub-cells in the subscanning direction x', the coordinate system (x0, yo) of a halftone dot originating at P (0, 0) arranged as shown in Figure 3(A) can be expressed by the following equations: x0 = y' - 2mN (9) Y0 = y' - 2mN wherein N is an integer.

Since the coordinate values x' and y' respectively comprise a modulo 2mN, equations (9) can further be expressed as: x0 = x' mode 2m -----(1 O) Yo = Y' mod 2m The coordinate system (x, y) which originates at the center of a halftone dot has a relation to the above-mentioned coordinate system (x0, yo) expressible by the equations: lXl = lXo - mi ----- (11) = = y0 - ml By relating the equations (10) and (8) to the equation (11), there can be found a relation between the coordinate system (x, y) and that (x', y') of the recording film.

In Figure 4, which is a block diagram of an embodiment of the present invention, a recording drum 1 is rotated by a main scanning direction motor 5, to which a rotary encoder 4 is coaxially connected.

The rotary encoder 4 outputs a one revolution pulse signal rand a multiple pulse signal N every one revolution of the recording drum 1,the latter signal N being multiplied buy a PLL circuit 14 and input to a counter 15.

The counter 15 counts the pulse number of the output signal of the PLL circuit 14 to output a signal representative of the main scanning recording position W. The count number of the counter 15 is reset by the one revolution pulse signal revery one revolution of the recording drum 1. On the other hand, a recording head 6 is fed along a feed screw 8 in the sub-scanning direction by a sub-scanning direction motor 7. The recording head 6 is equipped with a detector 11 for detecting the sub-scanning direction position thereof with a linear encoder 10.

The output signal of the detector 11 is input to a counter 13, which outputs a signal representative of the sub-scanning direction position V of the recording head 6. The count number of the counter 13 is reset upon each return of the recording head 6 to the starting point by a recording head return indication signal output from a recording head return detector 12.

The thus-obtained position signals V and Ware applied to a coordinate rotator 16, which transforms them according to the equation (8) to corresponding position signals' and y' through a coordinate rotation by a screen angle 0. Then the coordinare system (x0, yo) of a halftone dot can be obtained by a computation circuit 17 by respectively subtracting from the bits of signals representing the positions x' and y' the maximum multiples of the modulos 2m as indicated in the equations (9). The computation circuit 17 performs a computation of the equations (11) to obtain absolute values Ixi and iyi corresponding to the values x0 and Yo and then performs a computation of the equation (1 ) to output a corresponding reference value D to a comparator 19.

In the meantime, a density signal S obtained by scanning an original is input to a table memory 18, which produces said intermediate value Ds by modulating the density signal S by said function g(s) and applying the same to the comparator 19.

The comparator 19 compares the intermediate value Ds with said reference value D. For instance when D S Ds, the recording head 6 exposes a corresponding point of a photosensitive film 2 with the use of an internal beam controller thereof (not shown). In this connection, the following system is employed in order to deal with exposing a plurality of recording lines simultaneously, as shown in Figure 3(A). That is, adders 231 to 23-n respectively sum up the x' direction value output from the coordinate rotator 16 and values expressive of cosO to n cosO respectively stored in cosine value holders 20.1 to 20-n to compute the x' direction values of the plural recording points.

On the other hand, adders 241 to 24-n respectively sum up the y' direction value output from the coordinate rotator 16 and values expressive of sinZ to nsinZ respectively stored in sine value holders 21 1 to 21., to compute the y' direction values of the plural recording points. The thus-obtained coordinate values of thex' and y' directions are input to computation circuit 17, to 17, equ ivalent of said computation circuits 17 o, by which coordinate values the computation circuits 17, to 17., respectively output reference values D1 to Dn to comparators 1 9-i to 19-n.The comparators 19-1 to 19-n respectively compare the reference values D1 to Dn with the intermediate value Ds input from the table memory 18 to generate signals for controlling recording beams.

It should be noted incidentally that by superimposing random numbers output from a random number generator onto the intermediate value Ds by means of an adder 26, the possibility of the appearance of moiré effect caused by an interference between a screen pattern and the recording beam can be reduced.

Figures 6(A) to 6(F) show embodiments of the computation circuit 17 for obtaining the reference value D corresponding to said equations (2) to (7). Figure 6(A) shows an embodiment for obtaining a halftone dot of a square pattern as in Figure 5(A), in which the reference value D is obtained by summing up values Ixi and lyi by an adder 30.

Figure 6(B) shows an embodiment for obtaining a halftone dot of a chain pattern as in Figure 5(B), in which the reference value D is obtained by at first multiplying values Ixl and lyl by a and b (where a, b are integers) respectively with multipliers 31 and 32, and summing up both the products with an adder 33.

Figure 6(C) shows an embodiment for obtaining a halftone dot of a circle pattern as in Figure 5(C), in which the reference value D is obtained by at first multiplying valuesx and y by themselves (squaring) respectively with multipliers 34 and 35, secondly summing up both the products with an adder 36, and then extracting the square root of the sum with a square root calculator 37.

Figure 6(D) shows an embodiment for obtaining a halftone dot of an ellipse pattern as in Figure 5(D), in which the reference value D is obtained by first multiplying values x and y by coefficients lia and lib respectively with multipliers 38 and 39, secondly multiplying the products by themselves respectively (squaring) with multipliers 40 and 41, thirdly summing up the outputs of the multipliers 40 and 41 with an adder 42, and then extracting the square root of the sum with a square root calculator 43.

It is clear in view of the above examples that halftone dots of a barrel pattern as in Figure 5(E) and a spindle pattern as in Figure 5(F) can be obtained by respectively using circuits as shown in Figures 6(E) and 6(F).

Each of the square root calculators as shown in Figures 6(C) to 6(F) can be embodied either by a table memory which outputs a value D when a value D2 is input thereto, or a circuit as shown in Figure 7.

In the circuit of Figure 7, a value D2is input to comparators 61a, 61b, 61.... as in input value A, while a first comparison value K (n2, (n - 1)2, (n - 2)2...) is input to the comparators 61a, 61b, 61c... respectively as input values B. The comparators 61a, 61b and 61C output, for example, an "H", or high, signal to AND-gates 62b, 62c and 62d composing a selector respectively when A 'B. In this case, the output of the comparator 61 which compares the value D2 with the highest value n2 is input directly to a subsequent AND-gate.

The outputs of the comparators 61,, 616 and 61,... are also input via respective inverters 63at 63b and ......

to the AND-gates 62b, 62y and 62d, which each is to output an "H" or "L" (low) signal to the subsequent AND-gates 64a, 64b and 64c... respectively according to their inputs. At this juncture, the AND-gate corresponding to a comparator to which the maximum of the first comparison value K satisfying the inequality D2 ~ K is input outputs only an "H" signal to the subsequent AND-gates The AND-gates 64a, 64b and 64C respectively received a second comparison value 1 at (n, n - 1, n - 2...), and the second comparison value 1 applied to the AND-gate to which "H" signal is input is output as a reference value D via an OR-gate 65.

The purpose of employing a table memory or the square root calculator as shown in Figure 7 in the computation circuit 17 is as follows.

The capacity of the table memory employed for the above use must be as large as that of a conventional two-dimensional screen memory for storing one halftone dot pattern. However, in practice, a plurality of two-dimensional screen memories are needed to cope with a variety of screen patterns, namely the variation of screen ruling. Taking the halftone dot of a circle pattern as shown in Figure 5(C), for example, it is required to have several density distribution curves as shown in Figure 8, besides that shown in Figure 2. The halftone dots of the patterns as shown in Figures 5(D), (E) and (F) are of course required to have a variation of density distribution curves corresponding to the variation of the coefficients a and b as can be understood by the equations (9), (10) and (11).

As opposed to using a conventional two-dimensional screen memory of a greater capacity for memorizing all the necessary halftone dot patterns, employing a one-dimensional table memory or a square root memory as shown in Figure 7 in the computation circuit 17 requires less memory capacity.

It should be further noted that the square root calculator requires a one-dimensional increase of the comparators, AND-gates and so on with an increase of the input value as opposed to the fact that the table memory requires a two-dimensional increase of the number of the addresses with the increase of the input value. Thus, the advantage of the square root calculator over that of a table memory increases with greater input values.

As mentioned above, in comparison with using a conventional two-dimensional screen memory for storing halftone sub-cell thresholds, this invention which is capable of one-dimensionally storing an equivalent reference value representative of a density contour of a halftone dot, is useful for reducing data capacity to be stored, especially when halftone dot density is high.

In this disclosure, there is shown and described only the preferred embodiment of the invention, but, as aforementioned, it is to be understood that the invention is capable of use in various other combinations and environments and is capable of changes or modifications within the scope of the inventive concept as expressed herein.

Claims

1. A method for forming halftone dots by a recording beam controlled by a recording beam control signal obtained by comparing a value corresponding to the density value of an original with a reference value corresponding to each of a plurality of halftone sub-cells composing a halftone dot, comprising the steps of:: (a) converting the coordinate value of a recording point on a recording film to a corresponding coordinate value of a coordinate system obtained by rotating the coordinate system of the recording film by a screen angle; (b)converting the coordinate value obtained in step (a) to a corresponding coordinate value of a coordinate system (x, y) originating at the center of a halftone dot and computing a reference value D = f(x, y) of a halftone dot of a particular pattern according to the coordinate value of the coordinate system (x, y); (c) obtaining an intermediate value Ds from a density value S obtained from an input scanning point of the original such that the intermediate value Ds corresponds to the reference value D; (d) comparing the intermediate value Ds with the reference value D; and (e) in response to step (d), generating the recording beam control signal.

2. A method for forming halftone dots by a plurality of recording beams controlled by recording beam control signals obtained by comparing a value corresponding to the density of an original image with a reference value corresponding to each of a plurality of halftone sub-cells composing a halftone dot, comprising the steps of:: (a) converting the coordinate value of one of the recording points on a recording film to a corresponding coordinate value of a coordinate system obtained by rotating the coordinate system of the recording film by a screen angle; (b) converting the coordinate value obtained in step (a) to a corresponding coordinate value of a coordinate system (x, y) originating at the center of a halftone dot and computing reference values for each of the recording points expressed by a function D = f(x, y) of a halftone dot of a particular pattern according to the coordinate values of the coordinate system (x, y);; (c) obtaining an intermediate value Ds for each of the recording points from a density value S measured from one of the input scanning points of the original image such that the intermediate value Ds of each of the present recording points corresponds to the reference value D of a corresponding one of the recording points; and (d) generating recording beam conrol signals for the plural recording beams by comparing the intermediate value Ds of each of the recording points with the reference value D of the corresponding one of the recording points.

3. A method as recited in claim 2 in which the reference value D of each of the recording points is computed according to an equation D = Ixl + lyl.

4. A method as recited in claim 2 in which the reference value D of each of the recording points is computed according to an equation D = aixi + blyl.

5. A method as recited in claim 2 in which the reference value D of each of the recording points is computed according to an equation D = Vx2 + y2.

6. A method as recited in claim 2 in which the reference value D of each of the recording points is computed according to an equation D = V(x21a2) + (y2/b2)

7. A method as recited in claim 2 in which the reference value D of each of the recording points is computed according to an equation D = V (Ixl+a)2 + (lyl+a)2

8. A method as recited in claim 2 in which the reference value D of each of the recording points is computed according to an equation D = (lxi -a)2 + (lyl-a)2.

9. An apparatus for forming halftone dots by a recording beam controlled by a recording beam control signal obtained by comparing the density value of an original with a reference value corresponding to each of a plurality of halftone sub-cells composing a halftone dot comprising: (a) first means for generating a signal representing the coordinate value of a recording point of a recording film; (b) second means for converting a coordinate value output from the first computation means to a corresponding coordinate value of a coordinate system obtained by rotating the coordinate system of the recording point of the recording film buy a screen angle;; (c) third means for converting a coordinate value output from the second computation means to a corresponding coordinate value of a coordinate system (x, y) originating at the center of a halftone dot and computing a reference value D = f(x, y) of a halftone dot of a particular pattern according to the coordinate value of the coordinate system (x, y); (d) fourth means for obtaining an intermediate value Ds from a density value S measured from an input scanning point of the original image such that the intermediate value Ds corresponds to the reference value D; and (e) fifth means for generating a recording beam control signal by comparing the intermediate value Ds with the reference value D.

10. An apparatus for forming halftone dots by a recording beam conrolled by a recording beam control signal obtained by comparing a value corresponding to the density value of an original image with a reference value corresponding to each of a plurality of halftone sub-cells composing a halftone dot, comprising:: (a) first means for generating a signal representing the coordinate value of one of a plurality of the recording points of a recording film; (b) second means for converting the coordinate value output from the first computation means to a corresponding coordinate value of a coordinate system obtained by rotating the coordinate system of the recording points of the recording film by a screen angle; (c) third means for converting the coordinate value output from the second computation means to a corresponding coordinate value of a coordinate system (x, y) originating at the center of a halftone dot and computing a reference value for each of the recording points expressed by a function D - f(x, y) of a halftone dot of a certain pattern according to the coordinate values of the coordinate system (x, y);; (d) fourth means for obtaining an intermediate value Ds for each of the recording points from a density value S measured from one of the input scanning points of the original image such that the intermediate value Ds of each of the recording points corresponds to the reference value D of corresponding one of the recording points; and (e) fifth means for generating recording beam control signals for the plural recording beams by comparing the intermediate value Ds of each of the recording points with the reference value D of a corresponding one of the recording points.

11. An apparatus as recited in claim 9 or 10 in which the fourth means is a table memory storing various values of the intermediate value D and addressable by corresponding density values S.

12. An apparatus as recited in claim 9 or 10 in which the third means computes the reference value D according to an equation D = Ixl + ly.

13. An appartus as recited in claim 9 or lOin which the third means computes the reference valueD according to an equation D = aixi + bill.

14. An apparatus as recited in claim 9 or 10 in which the third means includes means for computing the reference value D according to an equation D=X/X2+ y2

15. An apparatus as recited in claim 9 or 10 in which the third means includes means for computing the reference value D according to an equation D = ç (x21a2) + (y21b2).

16. An apparatus as recited in claim 9 or 10 in which the third means includes means for computing the reference value D according to an equation D = V xl+a)2 + (!yr ca)2.

17. An apparatus as recited in claim 9 or 10 in which the third means includes means for computing the reference value D according to an equation D = (Ixi-a)2 + (iyi-a)2.

18. An apparatus as recited in claim 14 in which the square root calculating means comprises: (a) a plurality of comparators for outputting high level signals when D2 > K, wherein K = n7, (n-1 )2, (n-2)2...

(b) a plurality of first AND-gates of which respective inputs are responsive to the output of a corresponding one of the comparators and inverted outputs respectively of other comparators; (c) a plurality of second AND-gates of which respective inputs are responsive to the output of a corresponding one of the first AND-gates and a fixed value; and (d) an OR-gate of which inputs are responsive to the outputs of all the second AND-gates.

19. An apparatus as recited in claim 15 in which the square root calculating means comprises: (a) a plurality of comparators for outputting high level signals when D2 > K, wherein K = n2, (n-1)2, (n-2)2...

20. An apparatus as recited in claim 16 in which the square root calculating means comprises: (a) a plurality of comparators for outputting high level signals when D2 > K, wherein K = n2, (n-1)2, (n-2)2...

21. An apparatus as recited in claim 17 in which the square root calculating means comprises: (a) a plurality of comparators for outputting high level signals when D2 > K, wherein K = n2, (n-1)2, (n-2)2...

(b) a plurality of first AND-gates of which respective inputs are responsive to the output of a corresponding one of the comparators and inverted outputs respectivley of other comparators; (c) a plurality of second AND-gates of which respective inputs are responsive to the output of a corresponding one of the fi rst AN D-gates and a fixed value; and (d) an OR-gate of which inputs are responsive to the outputs of all the second AND-gates.

22. An apparatus as recited in claim 14 in which the square root calculating means comprises a table memory storing various values of the square number D2 addressable by the reference value D.

23. An apparatus as recited in claim 15 in which the square root calculating means comprises a table memory storing various values of the square number D2 addressable by the reference value D.

24. An apparatus as recited in claim 16 in which the square root calculating means comprises a table memory storing various values of the square number D2 addressable by the reference value D.

25. An apparatus as recited in claim 17 in which the square root calculating means comprises a table memory storing various values of the square number D2 addressable by the reference value D.