Method for producing halftone dots in a halftone plate recording apparatus

A method for producing halftone dots in a halftone plate recording apparatus is disclosed. A photosensitive material is scanned with a purality of light beams and a plurality of minute dots in lattice-like arrangement are exposed in the range of the halftone dots areas of the halftone plate. Distribution of the quantity of light beams for recording the minute dots is gradually reduced from the center to the periphery thereof, and the intensity of exposure light beams on the minute dots is controlled by the degree of conformity of the minute dots or the pattern of their arrangement with the halftone dot areas.

BACKGROUND OF THE INVENTION 
The present invention relates to a method for producing a halftone dot in a 
halftone plate picture image from an original picture having a continuous 
tone required for manufacturing printings, and more particularly relates 
to a dot generator which forms an individual halftone dot by collecting a 
plurality of minute dots. 
Methods of such types described above have been known by the Japanese 
Patent Publication Nos. 52-33523 and 52-49361, particularly the latter 
disclosing a method in which a screen angle of a halftone plate is taken 
rational tangent is adopted to avoid occuring mutual moire-image 
interference fringe among four color plates of cyan, magenta, yellow and 
black for multi-color printing and each of the screen angles is different 
one another. 
FIG. 1 shows an example which adopts a screen angle of tan 
.theta.=1/3corresponding to 15.degree., and other three screen angles are 
tan .theta.=0, tan .theta.=1/1, and tan .theta.=3/1 which do not generate 
moire-image interference fringes. 
They are screen patterns of which screen angles of FIG. 1a, 1b, 1c, and 1d 
are corresponding to 0.degree., 15.degree., 45.degree. and 75.degree. 
respectively, and of which lengths of the longitudinal and lateral sides 
are equal. 
In the above method halftone dots may be output on the entire picture image 
by means of referencing addresses which incliment sequentially in the main 
scanning direction by a memory device of relative small capacity. 
That is, any of directions of longitudinal and lateral side of the memory 
construction of the square or the rectangular image in which the screen 
pattern is written agrees with the direction in which the addresses are 
referenced. Additionally, in general, there occurs no case that the 
address-referencing is performed passing over or applies data at the same 
coordinates position twice. 
In this method, to produce a halftone dot having a screen angle of 
15.degree. a memory device having a capacity of covering relative wide 
area including more than several halftone dots therein (examples shown in 
FIG. 1b and 1d are cases of ten halftone dots) is provided and a diagonal 
screen pattern is written therein. 
As easily understood from FIGS. 1a to 1d, each of those screen pattern 
written in this memory device may be connected continuously with it 
opposite sides. 
However, in case that a screen patterns for multi-color printing are unable 
to take screen angles of 0.degree., 15.degree., 45.degree. and 75.degree. 
exactly due to a certain restriction of printing, content of an original 
picture, etc., a moire-image interference fringe may be produced. 
In order to eliminate such moire-image interference fringe, the number of 
screen line needs varying, e.g., the number of minute dot included in a 
halftone dot needs varying while using different screen pattern, or the 
diameter of the minute dot or the distance between minutes dots needs 
varying while using same screen pattern. 
In the former method in which the screen pattern is varied, providing a set 
of four color screen patterns corresponding to variety of screen number is 
very troublesome. 
Therefore, it is further convenient to apply the latter method in which the 
diameter of the minute dot or the distance between minute dots may be 
varied by using a zoom lens or an interchangeable lens in addition to 
varying the scanning pitch or the scanning clock in using the same screen 
pattern. 
Particularly, alteration of the scanning pitch corresponding to the number 
of screen line brings disadvantages such as the scanning mechanism becomes 
complicated, adjustments of the distance between the light beams, the 
diameter and the intensity thereof, etc. are required. Additionally, 
another disadvantage is that a relationship between a coordinate system in 
which character, ruled line, etc. are quantized and a coordinate system in 
which pictorial pattern is quantized can not be held constant, thus a 
picture containing both character and the like and pictorial pattern can 
not be recorded at the same time in scanning. Moreover, it is impossible 
to record a plural set of halftone picture plates in a same size having 
different number of scanning lines based on a same picture informations 
stored in a recording means such as magnetic disc etc. 
As one of the data which suggests a method improving the above-described 
disadvantages, there is a treatise "New Development in Scanner Technology" 
described on page 251 of the "Progress of Technical Association of the 
Graphic Arts" published in 1981. 
Here, the treatise discloses a method in which a square memory pattern of 
which a side in length a fundamental period of a halftone dot is 
referenced in an oblique direction which is not required to agree with a 
direction of a side of a screen pattern. 
With respect to the above-described method a brief description will be 
given hereinafter. 
FIG. 2 shows a square screen pattern representing information of a halftone 
dot, of which one side is a fundamental period of the halftone dot. 
FIG. 3 is a view illustrating coordinate transformation in case that the 
square screen pattern memory in which said screen pattern is written is 
referenced sequentially in a oblique direction. 
Directions of X and Y axes are coordinate axes of address of said memory, 
agreeing with a direction of fundamental period of a halftone dot. u and v 
denotes a scanning direction and a scanning pitch direction respectively. 
As shown in FIG. 3, assuming that an angle between the scanning direction 
and the X axis of the coordinate system is .theta., the following equation 
of coordinate transformation are established, that is; 
EQU X=u cos .theta.-v sin .theta. 
EQU Y=u sin .theta.+v cos .theta. (1) 
here, assuming that the interval between the minute dots forming a halftone 
dot is p, and putting u=mp, v=np, then, the above equations become as 
follows; 
EQU X=mp cos .theta.-np sin .theta. 
EQU Y=mp sin .theta.+np cos .theta. (2) 
Assuming that the coordinates of the subsidiary scanning are constant 
during one period of the main scanning operation, the afore-described 
equations become as follows; 
EQU X=mp sin .theta.+C.sub.1 
EQU Y=mp sin .theta.+C.sub.2 ( 3) 
where, 
EQU C.sub.1 =-np sin .theta. 
EQU C.sub.2 =-np cos .theta.tm (3') 
The latter equations (3') do not vary during one period of the main 
scanning operation. 
At each of the beginning points of the respective main scanning operation, 
C.sub.1 and C.sub.2 are previously calculated to be set as X=C.sub. and 
Y=C.sub.2, and from which every time the main scanning operation advances 
and interval of a space between each of the minute points for forming a 
halftone dot, if p cos .theta. is added to an addresses of the X 
coordinate and p sin .theta. to that of the Y coordinate, addresses to 
which each of screen patterns at respective times can be referenced. 
In this case if each of the addresses of X coordinate and that of Y 
coordinate of the afore-mentioned screen pattern are made to Nth power of 
2, in the course of address computation of the above described expression 
3 by basing upon binary rotation, even if there occurs overflow to carry 
in the computation, the screen pattern memory can shift endlessly from the 
left to the right vice versa by neglecting the carried portion. 
In this case if N is 6 and a screen pattern of more than 256 gradations by 
64.times.64 addresses is used, a sufficiently smooth screen pattern can be 
obtained. However, in the case of accuracy of caluculation being not high, 
even if it is desired to set screen angle .theta.=15.degree., when p cos 
.theta. and p sin .theta. are calculated, because of rounded errors caused 
by digital calculation, the same repetition occurs relatively in short 
time interval. 
Accordingly, in order to avoid the afore-mentioned disadvantage even if one 
side of the screen pattern memory of square shape is 6 bits (2.sup.6 =64), 
accuracy of calculation of X and Y coordinates should be extremely high. 
The accuracy is such as an accuracy of a degree which can sufficiently 
calculate the number of necessary halftone dots to the size of the largest 
size that can be output. For example, size of the larger side is 30 
inches, the number of screen lines is 175/inch, the number becomes 
175.times.30 .times.64=336,000&gt;2.sup.18. Accordingly, if any number larger 
than 18 bits and multipliers of 1 byte (8 bits) is convenient, an address 
may be set to a degree of 24 bits or 32 bits. 
Whether a dot should be exposed or not can be determined by comparing a 
screen level obtained by referencing the screen pattern memory basing on X 
and Y addresses obtained as afore-described with a picture image level. 
Collection of plurality of thus exposed minute dots form an individual 
halftone dot. 
In the above described method a halftone picture of any described screen 
angle can be output using the same screen pattern only by changing the 
screen angle .theta., and further, by varying value of p, a halftone dot 
of any given screen line number can be output. That is, as p is put 
smaller, times of adding operation necessary for X and Y addresses to 
attain to the fundamental period of the screen pattern are increased, so 
that even if the main scanning clock and the scanning pitch are fixed, a 
large halftone dot can be output. 
Basing upon the above described technique, a feeding mechanism, a main 
scanning clock generating circuit etc. can be simplified. Further, even if 
there are any variations in feeding pitch, adjustment and/or modification 
regarding distance between beams, size of the beams, intensity of the 
beams etc. which have been necessary in those conventional methods are 
unnecessary. 
However, in this method there is also a serious disadvantage as follows; 
that is, in this method a screen level D of which coordinates are obtained 
by using the pitch p which corresponds to a desired screen angle .theta. 
and the desired screen line number is compared with the picture image 
level, and according to the result whether exposure should be performed or 
not is determined. Accordingly, even in the case of an identical contour 
line configuration being referenced by basing upon the degree of 
conformity of a contour line configuration corresponding to the picture 
image level E on the screen pattern to the reference coordinates, there 
occurs dispersion in the number of exposed minute dots. 
FIGS. 4a and 4b show one of the examples of the above described cases. FIG. 
4a shows a case in which the degree of conformity is good, and FIG. 4b 
shows another case in which the degree of conformity is not good. Here in 
FIGS. 4a and 4b, there are shown cases in which exposing operation is 
carried out when the screen level D is higher than the picture image level 
E which corresponds to light and darkness of the pictorial pattern to be 
reproduced. 
As described the above, even if it is identical with the picture image 
level, depending upon the degree of conformity of the coordinates which 
refer to the screen pattern, there comes out considerable difference in 
the number of the minute dots to be exposed. Such dispersion in the number 
of these exposed minute dots results in occurrence of fatal blurs in the 
halftone plate for printing. 
Of course, a method in which merely the intensity of an exposure light beam 
is varied stepwisely is disclosed in the specification of the U.S. Pat. 
No. 4,025,189. The object of the invention disclosed in the patent is not 
to pursue subtle control of an area of a halftone dot, but lies in 
consequently forming a halftone dot (soft dot) of which quantity of silver 
at the peripheral edge portion is less than that of the central portion 
thereof. 
In the plate-making process, when it is desired to make a part or the whole 
of halftone dots of a film once having been exposed and developed smaller, 
the dot etching is performed. It is very convenient that in this case even 
if the halftone dot is made smaller by performing etching, in a soft dot 
there remains considerable quantity of silver in the central portion. 
The method disclosed in the afore-mentioned U.S. Pat. No. 4,025,189 is that 
when the intensity of an exposure light beam is calculated, if a pair of 
values aligned in the scanning pitch direction of a screen pattern memory 
are accessed simultaneously, and differences obtained by comparing each of 
the values with the level of the same picture image are put as a and b 
respectively, the results are as follows; that is, if both a and b are 
positive values, the intensity is 100% of the intensity of the light beam, 
if both are negative, the intensity is 0% of that of the light beam, and 
in the case of a and b being different signs, the intensity of the 
exposure light beam is calculated basing upon the following equation, 
(a+b)/(a-b)=intensity of exposure light beam. 
However, this method is on the premise that in case of multiprintings, the 
memory referencing method disclosed in the afore-described Japanese Patent 
Publication No. 52-49361 is applied. Accordingly, there is no description 
in the publication with respect to excellency in the degree of conformity 
of the coordinates to which the screen pattern in question is referenced, 
and dispersion in the number of minute points. 
In addition, there is another disadvantage that calculation for determining 
the intensity of the exposure light beam is quite complicate. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to improve disadvantages of those 
conventional methods in which dispersion in the number of minute dots 
basing upon the degree of conformity, as described the above, results from 
shape and configuration of each of halftone dots and dimension of an area 
thereof so as to control the intensity of exposure light beam to be 
emitted thereto, and to reduce the dispersion in the dimension of the 
halftone dot area basing upon the degree of conformity by varying the 
intensity of the exposure light beam stepwisely. 
That is, the object of the present invention is to make it possible to 
subtly control an area of a halftone dot by varying the intensity of an 
exposure light beam stepwisely, and solve problems caused by the degree of 
conformity.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 5a to 5d show examples illustrating subtle changes of the halftone dot 
area in the case of 12 minute dots in which the intensity of the exposure 
light beam is varied stepwisely. FIG. 5a show an example of distribution 
of the intensities of the exposure light at the minute dots in the case of 
the maximum intensity of the exposure light being set as 1, and cases in 
which the intensities of the exposure lights are set to 0, 1/4, 2/4, 3/4 
and 4/4. 
The distribution of the intensity of the minute dots shown in FIG. 5a 
presents Gaussian distribution, so that at the maximum, and a position at 
which the intensity of light is a half of that of the central portion is 
determined to be a half of the distance to the adjacent minute point. 
A contour line configuration of the quantity of light being 30%, when the 
intensities of light beam at both A and B dots of the halftone dot model 
shown in FIG. 5b are varied by using such beams as the afore-mentioned 
(when over the considerably wide range the quantity of light at all the 
peripheral portions is 4/4, the quantity of light at the central portion 
of the halftone dot is determined to be 100%), and a sectional view of 
distribution of the quantity of light at X--X' position is shown in FIG. 
5c. From FIG. 5c it will be understood that according to variation of the 
intensity of light, the contour line is gradually swollen. 
In addition, it can also be understood that the slope of a slanting surface 
which traverses the contour line of the quantity of light of 30% is more 
gentle in the case of the intensity of light at B dots being defined to be 
1/4, 2/4, and 3/4 in comparison with the case in which the intensity of 
light of the same dot is defined to be 0/4 by reviewing the sectional view 
at the X--X' position of distribution of the quantity of light. This means 
that the halftone dot becomes soft. 
Further, change of area of the contour line configuration of the quantity 
of light of 10%, 30% and 50% in the case of the intensity of the exposure 
light at A and B dots is shown in FIG. 5d. 
At each of the cases the area generally becomes larger in accordance with 
increase of the intensity of light. Especially, in the case of the contour 
line configuration of 30%, the area increases approximately linearly. 
In the case of any advantage being obtained by the degree of conformity 
(i.e., the halftone dot becomes larger) by utilizing the above-described 
result, even if the total number of the minute dots to be exposed are 
large, when the intensity of the exposure light at the periphery is 
attenuated, and the case of losing advantage by the degree of conformity 
(the halftone dot becomes smaller), it is possible to reduce variation of 
shapes of the halftone dots and dispersion of areas of the halftone dots 
by making the intensity of light beam at the peripheral edge portions 
approximately equal degree to that of the central portion. 
In the following two methods for strengthening the intensity of exposure 
light beam at the peripheral edge portions of a halftone dot in comparison 
with that of the central portion thereof will be described, for example. 
It goes without saying that these methods are applicable both in a single 
beam system and/or a multi-beam system in which each of a plurality of 
beams is independently controlled. 
The first method of the afore-mentioned is a method for varying the 
intensity of exposure light beam in which calculation is carried but by 
using the screen level D and the picture image level E, and according to 
the magnitude of the result of the calculation, the intensity of the 
exposure light beam is to be varied. In this method the following 
technique is utilized that even if a screen pattern written in a memory is 
to be set as the largest (or the smallest) in the center point of the 
halftone dot, it is usual to take a small (or large) value as being away 
from the point. 
In this method when the contour line configuration F on the screen pattern 
corresponding to the picture image just conforms within the limits, as 
shown in FIG. 6a, difference between the screen level D and the picture 
image level E at the both ends, and in the case shown in FIG. 6b 
difference between the screen level D and the picture image level E at the 
ends is considerably large. Considering the above, basing on largeness of 
difference between the screen level D and the picture image level E, the 
intensity of the exposure light is controlled. Of course, when value of 
difference becomes negative, irrespective of largeness of absolute value 
thereof, the intensity of the exposure light is defined to zero (no 
exposure is emitted). 
It is one of the methods to proportionate the largeness of the difference 
between them at this time to the intensity of the exposure light; however, 
it is possible to select width for making the exposure light soft not 
necessarily by depending on proportional relation, but by varying 
stepwisely, by setting a constant maximum or minimum, or by 
proportionating to value of squared difference or that of squared root. 
FIG. 7 there is shown a block diagram of the method illustrating one 
embodiment in which the constant maximum and the minimum limits are 
provided. 
In FIG. 7 the reference number 1 is an address generator, 2 is a screen 
pattern memory into which, for example, the screen level as shown in FIG. 
2 is written at a predetermined address, 3 is a subtractor, 4 and 6 are 
comparators, 5 is a maximum value setup circuit consisting of a digital 
switch etc., 7 is a minimum value setup circuit and 8 is a NAND gate. 9, 
10 and 11 are tri-state gates and 12 is a digital/analog converter. 
In FIG. 7 an analog/digital converted picture image signal n by an 
appropriate sampling pitch is input to a negative terminal of the 
subtractor 3 as a reference value, while basing upon clock pulse t, an 
address designating signal to a table memory (screen pattern memory) 2 is 
output from the address generator 1, and from the predetermined address of 
the table memory 2 a numeric value m corresponding to the afore-mentioned 
screen level D is input to a positive terminal. 
Value m-n operated by the subtracter 3 is input to the comparators 4 and 6, 
respectively, and in the comparator 4 the value m-n is compared with the 
maximum setup value V.sub.max, and in the other comperator 6 the value is 
compared with a minimum setup value V.sub.min having been set up in the 
minimum value setup circuit 7, respectively. 
For example, when the value m-n operated by the subtractor 3 is equal to 
the maximum setup value V.sub.max, or in the case of the value is larger 
than the maximum setup value V.sub.max, the gate 9 is opened by an output 
from the comparator 4, and a high level signal H is output through the 
gate 9. By an output signal from the comparator 6 the gate 11 is opened, 
and a lower level signal L corresponding to the intensity of light beam of 
zero is output. Further, when the value m-n is smaller than the maximum 
setup value V.sub.max and larger than the minimum setup value V.sub.min, 
the gate 10 is opened by an output signal from the NAND gate 8, then the 
value m-n is output through the gate 10, and the light beam is modulated 
so that the intensity of the light beam corresponds to the value m-n. 
In addition, practically the same number of light beam modulation control 
circuits as that of light beams to be recorded, as one of them being shown 
in FIG. 7, are arranged in parallel. 
The gist of the second method is that addressing and referencing are 
carried out in a density of integral multiples of the density 
corresponding to the distance between the minute dots, a result obtained 
by comparing the screen level D in an area of which one side corresponds 
to the distance between the minute dots and the picture image level E (if 
it is exposed, the result is 1, and not exposed, then the result is 0) are 
added, and in proportion to the added result the intensity of the exposure 
light beam is output. 
In FIG. 8 one example in which a reference distance is set to 1/2 of the 
distance between two minute dots (density is two times, area is four 
times) is shown. Each of the minute dots is exposed with the intensity of 
the exposure light which is proportional to a sum of the compared results 
at four dots in the periphery thereof. 0/4 shows a case in which a result 
of no exposure at the four dots in the perphery is obtained, 1/4 shows a 
case in which a result of only one dot being exposed in the periphery is 
obtained, 2/4 shows a case in which a result of two dots being exposed in 
the periphery is obtained, 3/4 shows a case in which a result that three 
dots are exposed in the periphery is obtained, and 4/4 shows a case in 
which a result that all four dots are exposed is obtained. 
According to the method, a disadvantage that addressing and comperison for 
referencing must be carried out in a closer density to the twice thereof, 
however, there are excellent advantages as follows; 
(1) logic is simple, and this method can improve processing speed and count 
up to four; 
(2) the periphery of each of the halftone dots becomes soft; 
(3) the shape of the contour line configuration can be reflected in good 
condition; and 
(4) to some extent both levels of the maximum and the minimum points are 
stabilized. 
In the first method when difference between the levels is small, there are 
some cases in which light beam is exposed by an intermediate intensity, 
even at portions other than the outermost edge portion. 
There is shown a block diagram illustrating an example for practicing the 
present method in FIG. 9. The reference numbers 13 and 14 are address 
generators of the left and the right sides, respectively. 15 is a screen 
pattern meory, 16 and 17 are comparators, respectively. 18 is an adder, 19 
is an adding register 20 is a digital/analog converter. 
In FIG. 9 an picture image signal n converted from analog to digital with a 
suitable sampling pitch is input into the comparators 16 and 17 as the 
reference value. While, basing on the reference clock pulse t.sub.1 (this 
clock pulse is in synchronism with a modulation clock puse t.sub.2 
frequency of twice thereof which will be described hereinafter), from the 
address generator 13 and 14, a pair of right and left address designation 
signals to the screen pattern memory 15 are output. 
The screen pattern memory 15 inputs each of screen level signals m.sub.L 
and m.sub.R which corresponds to the respective left and right addressing 
signals to the comparators 16 and 17, respectively. In the comparator 16 n 
is compared with m.sub.L, and in the comparator 17 n is compared with 
m.sub.R, and respectively inputs 1 to an adder 18 when the result requires 
to be exposed, and inputs 0 in the case of no exposure being required. 
The adder 18 adds both compared results, and inputs the added result to an 
adding register 19. The adding register 19 counts the compared results 
twice per each of twice the reference clock, that is, after all it counts 
the compared results four times, and any of the values from 0 to 4 is 
transferred to the digital/analog converter 20 in synchronism with the 
modulation clock t.sub.2, and after then it is cleared. 
As described heretofore, according to the present invention, a fixed 
scanning pitch system can be adopted for varying the screen line number, 
and simplification of the feeding mechanism can be realized. In addition, 
there is another advantage that even if there is any variation in the 
feeding pitch, adjustment of the distance between the beams, size of the 
beams and the intensity of the light beam which have been conventionally 
required is unnecessary. 
Further, when the present invention is applied to an output device of a 
layout scanner, relationship between coordinate system in the case of the 
character, ruled lines etc. being quantized (usually finer than the screen 
pitch and wider than the distance between the minute points), and 
coordinate system regarding the pictorial pattern (scanning pitch) becomes 
constant, so that simultaneous existence of the pictorial pattern, the 
characters, the ruled lines etc. and simultaneous output thereof can be 
facilitated. 
Further, basing upon the identical picture image information stored in a 
magnetic disc etc., several sets of same sized halftone plate picture 
images each of which the screen line number is different can be output. 
It is further advantage of the present invention that dispersion in shape 
of each of halftone dots and dispersion of the number of the minute dots 
which are resulted from goodness of the degree of conformity, can be 
diminished to prevent printings from being blurred, and for the reason 
that the dots become soft, dot-etching can be also carried out. Thus, many 
excellent effects can be obtained. 
Although the present invention has been described and shown with reference 
to the preferred embodiments illustrated in the acoompanying drawings, 
various changes and modifications can be made thereto by those skilled in 
the art without departing from the scope of the present invention.