JPH04126251A - Method for calculating dot gain and method for correcting dot - Google Patents

Abstract

PURPOSE:To precisely calculate a dot gain by calculating the dot gain on the basis of a thickening width measured in advance and applying the result of this calculation to the correction of a dot to find out an increase in a percent dot area. CONSTITUTION:A test chart TC including a predetermined line width is printed (P0), a thickening width 'q' on one side of the printed line is measured (P2) in advance, the enlarged area 'd' of each dot DT in a reference region CR which specifies a 100 percent dot is computed (P3) when each dot DT before printing is enlarged by the thickening width 'q' throughout the peripheral length, and a dot gain f(k) is calculated from said enlarged area 'd'. For example, when printing is performed by using a dot that is a square dot in a shape, the dot DT is thickened with the thickening width 'q' throughout the entire periphery of the dot DT, and the dot area is increased. An increase in the dot area can be calculated from the entire peripheral length of the dot DT and the thickening width 'q'. Further, the thickening width 'q' which occurs during a printing process can be easily and correctly measured by printing a test chart including a fine line and measuring the line width of the printed fine line with a microscope and the like.

Description

[Detailed description of the invention] [Industrial application field]

The present invention provides a method for calculating dot gain that occurs when printing a halftone image in a printing process, and applying the dot gain calculation to two printing processes with different conditions.
The present invention relates to a method of correcting the halftone dot area before printing in one printing process based on the calculated deviation between dot gains. Note that in the present invention, printing refers to proof printing. Including simple proofing one-shot printing, etc., and halftone dots before printing, etc.
It also includes halftone dots on a printing plate, halftone dots on an exposed film, and halftone dots as an image signal separated into one color.

[Conventional technology 1] In the printing process, in order to obtain the consent of the orderer before printing a large amount of printed matter, a proof is made using, for example, a flatbed proofing machine.The orderer judges that the result of the zero proof is good. Only after that, actual printing is performed using an offset rotary press or the like. In order to match the results of proof printing and final printing, particular attention is paid to the control of dot gain among various control factors in the printing process. Dot gain refers to the rate at which the halftone dots after printing are enlarged compared to the halftone dots before printing when printing a halftone image, and the enlargement of the halftone dot area is expressed as a percentage (%). Two types of dot gain are known: physical dot gain, which expands the area to which printing ink adheres, and optical dot gain, which occurs due to the diffusion of light hitting the surface of the printed material. By the way, due to the difference in printing conditions between the proofing machine that performs proof printing and the offset rotary press that performs main printing (large-volume printing), for example, the printing pressure of the offset rotary press is higher, The physical dot gain is thicker than the proof print, and the result does not match the proof print if it is left alone. This is mainly due to the collapse of the halftone dots due to the difference in printing pressure, and this drawback, i.e. In order to reduce the deviation in dot gain between the two printing processes, conventional measures have been taken such as using highly fluid ink in the proofing machine or adjusting the degree of baking of the printing plate for actual printing. Such deviations in dot gain occur not only between proof printing and final printing, but also between proofing machines and printing machines with different printing conditions.On the other hand, dot gain as an index of print finish management is First, print a gray scale, etc., and measure the density of the print result with a reflection densitometer.The obtained density is converted to a halftone dot area ratio, and this is calculated as the halftone dot area ratio before printing. The dot gain is determined by comparing with For example, even if you change the fluidity of the ink in the proof to make the results of the proof and the final print match, dot gain depends on the area of the halftone dots, so the results of both prints should be completely consistent. Therefore, in many cases, it is necessary to judge the quality of printed matter by inferring the final state of printed matter from the proof printing results. In different printing processes, such as proof printing and final printing, there is a problem in that it requires skill to infer the results of the actual printing from the results of the proof printing and judge its quality. A reflection densitometer is used to determine the dot gain, and the measurement of the dot area ratio by this reflection densitometer is easily affected by the printing density of the printed matter (and the accuracy of the dot area ratio of the measurement result is lacking). This is because the printing density of printed matter varies depending on the ink density, the degree of paper absorption, etc. even if the dot area ratio is the same.Furthermore, when using a reflection densitometer, the optical dot gain must be adjusted. Therefore, it is not possible to identify and extract substantial halftone dot expansion, that is, physical dot gain. Also from this point of view, it has not been possible to match the printing results obtained by two printing processes with different printing conditions. [Means for Solving the Problems 1] Therefore, the technical means of the present invention for solving the above problems has the following configuration. First, the first configuration, as illustrated in FIG. 1(A),
Dot gain f (k) is calculated from the difference in the area of halftone dots before and after halftone image printing in the printing process (po) (Pl)
A method, comprising: printing a test chart TC including lines of a predetermined line width (PO);
Then, measure the thickness q on one side of the printed line in advance.
2), When each halftone dot DT before printing is expanded by the measured thickening width q over its circumference, the enlarged area d of each halftone dot DT in the reference region CR that defines a 100% halftone dot is Calculate the dot gain f from the enlarged area d.
, (k). Further, the second configuration, as illustrated in FIG. 1(B),
The dot gain calculation (PO to P3) in the first configuration is applied to each of the first printing process and the second printing process whose printing conditions are different from the first printing process, and the first printing process is performed. The first dot gain f1 (
k) and the second dot gain f2(k) in the second printing process are calculated in advance (pH, Pl2), and the difference between the first dot gain fl(k) and the second dot gain f2(k) is calculated in advance. Find f (k) to the deviation (P
l3), in the second printing step so that the halftone dots of the image printed by the first printing step are matched with the halftone dots of the image printed by the second printing step. The area of halftone dots before printing is corrected in advance by the deviation Δf(k) (P
l4). Note that the first configuration in the second configuration above. As the second printing process, for example, the relationship between proof printing and actual printing, printing using a plurality of different printing machines, etc. can be considered. C action] According to the first configuration, the one-sided thickening width q of the printed line of the test chart TC is measured in advance, and the dot gain f is measured in advance.
Focusing on the fact that (k) occurs due to area expansion over the circumference of the halftone dot DT, the physical dot gain f(k) is accurately determined in order to calculate the substantial enlarged area d of the halftone dot DT. I can do it. Note that the substantial enlarged area d is the enlarged area of the halftone dot DT in the reference region CR that defines a 100% halftone dot. On the other hand, according to the second configuration, the dot gain calculation based on the first proofing is applied to each of the first printing process (for example, proof printing) and the second printing process (for example, main printing) that have different printing conditions. Therefore, the dot gains fl(k) and f2(k) obtained in both printing processes are highly accurate. Therefore, dot gain fl(k)
, f2(k) By correcting the halftone dots DT before printing in the second printing process by the deviation Δf (k) between them, the results of printing in the second printing process can be compared to those in the first printing process. It is possible to match the results of printing by the method, and no special skill is required. [Example 1] In order to further clarify the structure and operation of the present invention described above, FIGS. 1 and 2 (A
) is a block diagram showing a schematic configuration of a direct prepress system to which the method of the present invention is applied, FIG. 2(B) is a block diagram showing an outline of a halftone correction circuit, and FIG. FIG. 2 is a block diagram showing a printing process. As shown in the figure, the direct plate making system includes an editing system 1 that edits images and characters, a halftone dot forming device 3 that has a halftone correction function, and a printing plate making device 5 that creates printing plates.
(Figure 2 (A)). The Wfi system 1 includes a function OPI for inputting a WI image document ID by color separation using a scanner, a function OP2 for inputting a text document CD using a scanner or a keyboard, and a function ED for editing data of these images or characters. (Figure 3). While looking at the full-color image on the editing system 1, the operator operates the keyboard and various pointing devices to combine image data and character data, decide on their arrangement, and edit and create image data for the entire manuscript. . The image data after g collection is 8 bits (256 gradations)
The zero image data outputted as a density signal is outputted to the halftone dot forming device 3 according to an operator's instruction. The halftone dot forming device 3 includes a well-known CPU I, ROM 12,
A terminal interface 1 includes an arithmetic and logic operation circuit having a RAM 14 and mediates signals with a terminal 15.
7. Image data interface 19 that mediates signals with the editing system 1; Lookup table L for dot correction
A halftone correction memory 20 that stores T, a dot generator 22 that generates halftone dots according to the corrected image data, and a bus 25 that interconnects these elements and circuits.
Equipped with etc. The halftone correction memory 2°, which is the main part of halftone correction, is
As shown in Figure CB), the upper 8 bits of the address
The output data from the PUI 1 is connected to the output of a latch 31 that can be specified. On the other hand, the output data of the image data interface 19 is input to the lower 8 bits of the address. A data bus of the halftone correction memory 20 is connected to a dot generator 22. Therefore, the halftone correction memory 20 outputs pre-stored data (corrected image data) based on the image data output from the image data interface 19 in the page whose upper address is specified by the CPU II. This results in a multi-page lookup table. The halftone dot forming device 3 prepares the contents of the lookup table LT in advance as will be described later, and calculates the separately measured line thickening width (hereinafter simply referred to as thickening width) q and the set number of screen lines. Based on p, dot correction is performed on the image data input from the editing system l by referring to the lookup table LT. Note that the thickening width q and the number of screen lines p are input by the operator from the terminal 15. Ru. The plate making device 5 includes an acousto-optic modulator 35 that controls on/off of a laser beam for plate making according to an on/off signal output from the dot generator 22. Function D to directly form printing plates by turning on and off laser light
o (Figure 3) is realized. As a result, a printing plate is obtained, and printing PR is performed using the developed printing plate obtained by performing development PD on this plate. Next, details of dot gain calculation will be explained. The thickening of halftone dots (physical dot gain) during printing occurs because the ink on the halftone dots protrudes from the halftone dot area6.For example, if you print with square dots , as shown in FIG. 4, the halftone dot DT is thickened by a thickening width q over the entire circumference of the halftone dot DT. As a result, the halftone dot area increases. This thickening width q does not depend on the original dot area ratio and is almost constant. Therefore, the increase in the halftone dot area due to the thickening of the line can basically be calculated from the total circumferential length of the halftone dot DT and the thickening width q. Note that the thickening width q generated in the printing process can be easily and accurately measured by printing a test chart including thin lines and measuring the line width of the printed thin lines using a microscope or the like. Next, taking a square dot as an example, the dot gain f
The calculation for obtaining (k) will be explained. The total circumferential length of the halftone dot DT varies depending on the size of the halftone dot DT before printing with respect to the maximum area of the halftone dot DT, that is, the reference area CR that defines a 100 percent halftone dot. As shown in FIGS. 4 and 5, if the size of the halftone dots before printing, that is, the halftone dots DT treated as image data, is within 1/2 of the area of the reference region CR, The entire circumference of the halftone dot DT is involved in dot gain. On the other hand, as shown in FIG. 6, when the area of the halftone dot DT becomes larger than 1/2 of the area of the reference region CR, the halftone dot DT is responsible for the dot gain.
become part of the surroundings. Furthermore, as shown in FIG. 5, the area of the halftone dot DT is expanded due to the dot gain, and the area of the halftone dot DT is expanded to cover the reference area CR.
If the area protrudes from the dot DT and overlaps with the adjacent halftone dot DT, it is necessary to remove this protruding area as it does not contribute to the dot gain. Therefore, when calculating the dot gain, the area ratio of the halftone dot DT is It is necessary to classify the cases depending on the size of the thickening width q. The cases are roughly divided into [1] cases where the area (k - S) of the halftone dots DT before printing is less than 1/2 of the area S of the reference area CR;
] Similarly, the former [I] is divided into (i) a case where the dot area remains below 1/2 of the area S of the reference region CR even with dot gain (see Fig. 4). ) and (if) the case where the dot gain exceeds 1/2 of the area S (Fig. 5). The calculation of dot gain will be explained below according to this case classification. In the following explanation, the total circumference length of halftone dot DT is Ll, the area of halftone dot DT increased by dot gain is d, and the increment in halftone dot area ratio is The dot gain is expressed as f(k
), and the operator "°" represents the power, rSQ
RTJ indicates the square root, respectively. Note that the area S of the reference region CR is the number of screen lines (lines/inch)
As p, 5=((25,4÷p)=2) ... (1
) is required. Here, the screen line number p is defined as the halftone dot center distance of 1 (unit: mm), as shown in Figure 4.
It is expressed as p=25.4/l, and is defined as the number of lines per inch connecting the centers of halftone dots. [I] (i) Area of halftone dot DT before printing (k-3
) is less than 1/2 of the area S of the reference region CR and remains below 1/2 of the area S even with the dot gain, that is, k<0. 5 and 5QRT (k-S) + 2q≦5QRT (S/2) In this case, the total circumferential length of the halftone dot DT is L=4.5QRT (k-S) (see Figure 4), The expansion of the halftone dot area on the entire circumference is q-L, and the area q at the four corners of the halftone dot DT is
``Considering that the dot area increases by 2, the increment d of the halftone dot area
is d=49-8QRT(k-8)+4q"2=4q (S
QRT (k-S) +q) ...(2). [T] (i i) Area of halftone dot DT before printing (k
・If S) is less than 1/2 of the area S of the reference region CR, and exceeds 1/2 of the area S due to the dot gain,
In other words, when k(0,5 and 5QRT (k-3)+2q>5QRT (S/2)), then considering the area of the portion of the reference region CR excluding the halftone dots after enlargement, the fifth As shown in the figure, its area is 4.r"2. That is, the increment d of the halftone dot area is -5-4.r"2 for d=s-. Here, r is the fifth As shown in the figure, the following relationship is satisfied: 5QRT (S/2) = r + q + 5QRT (k - S)/2 Using this relationship to find the increment d of the halftone dot area, d = 4q (SQ
RT (2S)-3QRT (k -5)-3(1
-2・5QRT (2k)+2k)-2-4q“2
... (3). [I I] If the area (k-3) of the halftone dot DT before printing is equal to or larger than 1/2 of the area S of the reference region CR, that is, k
In the case of ≧0.5, the total area of the area excluding the area of the halftone dot DT from the reference area CR before the area is enlarged by dot gain is:
(S-k -S), and the length of one side of each area that can be considered a right triangle is 5QRT((S-k -S)/2)
Therefore, among each side of halftone dot DT, adjacent halftone dot D
The length of the side that can be expanded without overlapping with T is 5QRT
The halftone dot area, which becomes (S-k -S), expands with a width q on each side, and at both ends, as shown in FIG. q“2)
/2 exists. Considering these points, the increment d in halftone area is: d = 4q - 5QRT (-8 in S-) - 4q
“ 2=4q (SQRT (S-to-3)
Q). In addition, as shown in FIG. 7, the halftone dot area is approximately in the reference area C.
When the height of the remaining area becomes equal to R and becomes smaller than the thickening width q, that is, (SQRT(S-k -S))/2
In the case of <q, it is also suitable to use -8 (5) for d=s- instead of the above calculation formula. Above, we have explained the relationship between the thickening width q and the increment d in the dot area by taking the square dot-shaped dot DT as an example, but dividing the increment d in the dot area by the area S of the reference area CR By doing this, the increment in the dot area ratio, that is, the dot gain f (k) can be determined. This calculation of dot gain f (k) is applied to the proof printing by the proofing machine PR and the actual printing by the printing machine RO shown in FIG. In other words, test chart TC (Figure 1 (A))
is printed by each of the proofing machine PR and the printing RO, and from the printing results, the one-sided thickening width of each thin line is measured in advance, and the above-mentioned dot gain calculation is performed using the equations (1) or 4) using these thickening widths. Do this. Figure 8 shows the calculated dot gain fl(
k) and the dot gain f2(k) in the actual printing.
) is determined, and this is also shown in FIG. Then, in order to correct the area of halftone dots before actual printing, correction data for the image data is created using the deviation Δf (k). The corrected halftone dot area ratio (this is called the printing halftone dot area ratio) k′ for the halftone dot area ratio k as image data
Figure 9 shows the results obtained. In Figure 9, - point #l
! The characteristic before correction is indicated by BL, and the characteristic after correction is indicated by solid line CL. The calculations of the dot gains fl(k) and f2(k), the deviation Δf(k), and the printing halftone area ratio are performed using the CPU I, ROM 12, and RAM 1 in the halftone dot forming circuit 3 shown in FIG.
This will be done in 4. The final correction data shown in FIG. 9 thus obtained is stored in one of the look-up tables LT in the halftone correction memory 20 shown in FIG. Note that in reality, a plurality of pieces of correction data are prepared according to printing conditions such as the number of screen lines, and each piece of data is stored in the lookup table LT.
ing. Therefore, when the orderer approves the proof printing result produced by the proofing machine PR and the actual printing is performed by the printing machine RO, the image data is corrected as follows. When the operator inputs the conditions from terminal 15, C
The PU 11 outputs the corresponding upper address to the halftone correction memory 20. Therefore, the lookup table LT corresponding to this upper address is selected. The image data input from the editing system 1 is a density signal of 256 gradations, and is a signal that corresponds one-to-one to the area ratio k of each halftone dot. be. Therefore 1. When the halftone correction memory 20 refers to the lookup table LT based on this image data, halftone correction is performed based on the above-mentioned halftone area ratio. The image data subjected to halftone dot correction in this manner is output to the dot generator 22. The dot generator 22 generates an on/off signal for each minute dot forming a halftone dot according to the corrected image data, and outputs this signal to the acousto-optic modulator 35 of the printing plate making device 5 to generate a printing plate. It directly outputs the . In the dot correction method using the dot forming bag fi3 of this embodiment configured as described above, the increase in the dot area ratio (dot gain') fl is calculated in each of the proof printing and the actual printing.
(k), f2 (k), [Il(i),
[Il (i i), [I Il conditions]
It is calculated accurately based on each one-sided thickening width q, and the deviation Δf (k) is canceled out, that is, the halftone dots of the printed image in the final printing match the halftone dots in the printed image in the proof printing. The inverse characteristics determined in the table are stored in advance in the form of a look-up table LT. By referring to this lookup table LT, the dot area ratio as image data is corrected. Therefore, compared to the conventional technique of calculating dot gain indirectly from the gray scale reflection density, it becomes possible to calculate dot gain with much higher accuracy. In addition, there is no need to measure the reflection density at each step of the gray scale, and the dot gain f ( k) can be determined with high accuracy. As a result, the calculation accuracy can be improved and the printing result of the actual printing can be brought closer to the printing result of the proof printing. Furthermore, in this embodiment, since the look-up table LT is used to correct the halftone dots, there is no need to perform complicated calculations when correcting the halftone dots (the correction can be performed at high speed. Although the embodiment has been described, the present invention is not limited to this embodiment. For example, instead of the square-shaped halftone dots, various halftone dots such as chain dots, round dots, parent-child dots, and grain dots may be used. Of course, the present invention can be implemented in various ways without departing from the spirit of the present invention, such as a configuration in which calculation is performed for each halftone dot and correction is made instead of a configuration in which a look-up table is referred to. In the example, we explained dot gain calculation and halftone correction in printing using a direct plate-making system that directly creates a printing plate, but in order to perform color proofing before printing, it is also possible to directly print a color image to a multicolor printer using halftone dots. It can also be applied to correcting halftone dots in an electronic color proofing machine that outputs.In this case, it can be applied to correcting enlargement due to ink bleeding during printing.On the other hand, it can be applied to correcting enlargement due to ink bleeding during printing. Color proofing machines that use a photosensitive material such as silver halide film for printing and do not produce dot gain during image reproduction even when using halftone dots may simulate the expansion of the halftone dot area due to printing. The halftone correction method of the present invention can be applied to enlarge dots according to the dot gain during printing.In addition, it can be applied not only to the relationship between a proofing machine and a printing machine, but also to two printing machines with different printing conditions. The present invention can be applied. Effect 1 of the Invention As detailed above, according to the dot gain calculation method and halftone dot correction method of the present invention, the increase in the halftone dot area ratio, that is, the dot gain, can be It is calculated based on the pre-measured thickening width and applied to halftone correction.Therefore, it is possible to directly determine the increase in halftone area ratio, compared to conventional techniques that indirectly calculate dot gain from grayscale reflection density. In addition, there is no need to measure the reflection density at each step of the gray scale, and dot gain can be calculated with high accuracy by simply calculating the dot area ratio. In addition to improving halftone correction accuracy, it is possible to easily match printing results obtained in two printing processes with different printing conditions.

[Brief explanation of the drawing]

Figures 1 (A) and (B) are block diagrams illustrating the basic configuration of the present invention, and Figure 2 (A) shows a schematic configuration of a direct prepress system to which a halftone correction method as an embodiment is applied. 2(B) is a block diagram showing the configuration of the halftone correction memory 20, FIG. 3 is a block diagram showing the plate making process in this direct plate making system, FIGS. 4, 5, and 5. Figures 6 and 7 are explanatory diagrams showing how to obtain the dot gain, respectively. Figure 8 is a graph illustrating the dot gain determined by calculation. Figure 9 is a graph showing the dot area ratio and printing dot area ratio as image data. ′ is a graph illustrating the relationship with ′. l... Editing system 3... Halftone forming device 5... Plate making device 11... CPU 12... ROM 14...
・RAM l 5...Terminal 17...
- Terminal interface l9... Image data interface 20... Memory for halftone correction 22... Dot generator 35... Acousto-optic modulator CR... Reference area DT... Halftone dot LT... Look up table

Claims (1)

[Claims] 1. A method for calculating dot gain from the difference in halftone dot area before and after halftone image printing in a printing process, comprising: printing a test chart including lines of a predetermined line width; When each halftone dot before printing is expanded by the measured thickening width over its circumference, the width of each halftone dot in the reference area that defines a 100% halftone dot is calculated in advance. A dot gain calculation method comprising: calculating an enlarged area and calculating a dot gain from the enlarged area. 2. Applying the dot gain calculation according to claim 1 to each of a first printing process and a second printing process whose printing conditions are different from the first printing process, A first dot gain in the second printing process and a second dot gain in the second printing process are calculated in advance, and a deviation between the first dot gain and the second dot gain is calculated, The area of the halftone dots before printing in the second printing step so that the halftone dots of the printed image obtained by the printing step are matched with the halftone dots of the printed image obtained by the second printing step. A halftone dot correction method characterized in that: is corrected in advance by the deviation amount. 3. The halftone dot correction method according to claim 2, wherein the first printing step is proof printing and the second printing step is actual printing.