JP2003143405A - Threshold arrangement correcting method and dot pattern data structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce stripes which are generated by the interference between output resolution and the number of screen lines, so-called single moire, in the vicinity of 50% of network percentage. SOLUTION: In processes of steps S2-S9, one noncontact point candidate position which is to be a noncontact point from contact points of network points is determined, in network point image data wherein about half of contact points of network points are in contact, when threshold arrangement where thresholds for regenerating gradation by a dot pattern is corrected. In a process of a step S100, one contact point candidate position is determined which is to be a contact point from noncontact points of the network points. In a process of a step S20, the threshold of the noncontact point candidate position selected by the processes of the step S2-S9, and the threshold of the contact point candidate position selected by the process of the step 100, are exchanged. By these processes, the threshold arrangement is corrected, and as the result, arrangement of contact positions of the network points is arranged.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image setter, a CTP (Computer to Plate) device, and a CTC (Computer).
to Cylinder) device, DDCP (Direct Digital Color P)
roof) system and other halftone dot image output devices in printing field devices, or a threshold value array correction method suitable for application to a device that outputs a continuous tone image such as an electrophotographic method or an inkjet method as a binary image or a multivalued image , And a dot pattern data structure.

[0002]

2. Description of the Related Art In the field of printing plate making, when an original image is reproduced as a printed matter, ink is placed on a printing paper sheet through a printing plate mounted on a printing machine to form an image.

As a method of reproducing an original image on a printed matter,
An area gradation reproduction method is known in which an image to be formed is a dot pattern composed of a plurality of halftone dots (also referred to as dots).

In this case, a dot pattern is also formed on the printing plate. The printing plate is produced through a film output from the imagesetter, or the C pattern is formed.
It is created directly by the TP device or the CTC device.

The dot pattern formed on the film or the printing plate is prepared in advance by an image processing device such as a workstation.

FIG. 10 shows a state in which a typical example of a dot pattern to be formed on the film or the printing plate is displayed on the display of the image processing apparatus.

Each halftone dot 1 is basically a halftone dot cell (also simply referred to as a cell) 2 formed of n × n (8 × 8 in FIG. 10) pixels. It is formed by a mass of blackened pixels inside.

The halftone dot 1 shown in FIG. 10 represents a so-called digital halftone dot inside the image processing apparatus. Actually, in the halftone dots formed on the printed matter, each halftone dot 1 is rounded due to so-called dot gain.

In the example of FIG. 10, the halftone dot% (also called halftone dot percentage) of the upper left halftone dot 1 is about 6.3 [%], and the halftone dot% of the upper right halftone dot 1 is about 18.8 [%. ], And the halftone% of the lower two halftone dots 1 is 50 [%], respectively.

The pitch of the halftone dot cells 2 is represented by the reciprocal of the screen ruling. The screen ruling is defined by the number of lines of halftone dot cells 2 included in one inch [line / inch].

Further, an image setter, a CTP device, a CT
The resolution of an output device such as a C device or DDCP system (referred to as output resolution), in the example of FIG. 10, the resolution of a dot pattern is defined by the number of pixels per inch [pixel / inch].

Conventionally, the inventor of the present application has found that undesired stripes generated by the interference between the output resolution and the screen ruling (hereinafter, since the stripes are generated in one plate without overlapping the plates, For the sake of convenience, in order to reduce single plate moire, the arrangement of blackened pixels in a so-called supercell, which is composed of a large number of halftone cells 2, or a threshold array (halftone dot array) for determining the arrangement of blackened pixels A technique for optimizing the point threshold data) has been proposed {for example, the technique disclosed in Japanese Patent Laid-Open No. 11-112814 (hereinafter referred to as the first technique)}.

Here, as a reference for the supercell itself and a technique for generating halftone dots in relation to this supercell, for example, "Book Name: Postscript Screening, Author: Peter Fink, Publisher: Co., Ltd. "Md-N Corporation, date of issue: August 11, 1994, first edition, first edition".

[0014]

By the way, the contact point of the halftone dot 1, for example, the contact point 3 to which the blackened pixels of the lower halftone dots 1 are connected in FIG. It is known that it exhibits a peculiar behavior called a tone jump due to the effect of a typical dot gain. That is, on an image (halftone image or dot pattern) consisting of printed halftone dots, the image density is halftone dot% of the area gradation.
It is known that the concentration fluctuates discontinuously over the concentration calculated from.

As a technique for eliminating or mitigating this tone jump, there is a technique disclosed in Japanese Patent Laid-Open No. 2001-189859 (hereinafter referred to as a second technique).

In the second technique, each halftone dot in the supercell does not contact at once with a certain gray level, but a halftone dot of 5%.
It is devised that the blackened pixels of the contact points 3 of each halftone dot 1 gradually come into contact with each other before and after the gradation of 0%.

According to this second technique, it is possible to eliminate or alleviate the tone jump. However, in this second technique, the spatial regularity of the contact points 3 of the halftone dots 1,
In other words, since the regularity on the dot pattern is not taken into consideration, if the contact points of the halftone dots forming the dot pattern are spatially regularly arranged, on the printed matter, the halftone% is about 50 [%]. For example, 50 ± 5 [%]
The inventor of the present application has found that the grayscale part of the image may be visually recognized as a single plate moire.

In particular, in the case of producing a halftone dot image (dot pattern) having a high screen ruling by lowering the resolution of the output machine, this single-plate moire has a resolution divided by the screen ruling (hereinafter,
Described as resolution / screen ruling. It was found by the inventor of the present application that when the value of (resolution) / screen ruling value is 10 or less, it becomes a problem.

The present invention has been made in view of the above problems, and a halftone dot threshold value array capable of eliminating or mitigating a single plate moire due to the arrangement of halftone dot contacts. It is an object to provide a correction method and a dot pattern data structure.

[0020]

A threshold value array correction method according to the present invention is a threshold value array in which threshold values for reproducing gradation are arranged by a dot pattern which is a set pattern of halftone dots composed of one or more blackened pixels. A step of determining at least one non-contact candidate position to be non-contact among the halftone dot contacts at a certain gradation, and a non-contact point of the halftone dots at the certain gradation. A step B for determining at least one contact candidate position to be a contact, and a threshold value for the non-contact candidate position selected in the step A in the threshold value array;
By replacing the threshold values of the contact point candidate positions selected in the B step with the C step of correcting the threshold value arrangement, the contact point arrangement of the halftone dots is corrected (the invention of claim 1) ).

According to the present invention, the arrangement of the contact positions of halftone dots on the dot pattern is changed by exchanging the threshold values in the predetermined threshold value array. By processing in this way, it is possible to eliminate or alleviate the so-called single-plate moire, which is a fringe generated due to the interference between the output resolution and the screen ruling.

In this case, in the steps A and B, the dot pattern in which about half of the contact points of the halftone dots in the dot pattern are in contact with each other is frequency-analyzed to obtain a specific frequency component, and then the non-contact point is contacted. The intensity of the specific frequency component at the candidate position is obtained, the intensity of the specific frequency component at the contact candidate position is obtained, and the non-contact candidate position and the contact candidate position are determined according to the magnitude of the obtained intensity. By doing so, the position of the threshold value to be replaced more specifically can be obtained (claim 2).
Invention described).

Also, in the present invention, when the unit of resolution of the dot pattern is [pixel / inch] and the unit of screen ruling of the halftone dot is [line / inch], the resolution of the dot pattern is screen line. The value of the quotient divided by the number is 1
Since the halftone dot image data having a value of 0 or less is applied to the threshold value array, the effect of eliminating or mitigating the single plate moire is large (the invention according to claim 3).

According to the present invention, in the data structure of a dot pattern which is a set pattern of halftone dots which are substantially uniform in size and which are arranged orthogonally at substantially equal intervals, the arrangement of the contact points of the halftone dots is irregular. It is arranged (the invention according to claim 4). By doing so, the effect of eliminating or mitigating single-plate moire is great.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows the basic construction of a plate making system 10 according to an embodiment of the present invention.

The plate making system 10 basically comprises an image input section 14, an image processing section 16, a halftone image data creating section (dot pattern creating section) 20, a threshold value array correcting section 22, and an image output device 24. Composed.

The plate making system 10 reproduces an image read from an original image 12 by an image input unit 14 such as a scanner on a film F or the like by a dot pattern which is a set pattern of halftone dots composed of one or more blackened pixels. System.

The halftone image data creation unit 20 and the threshold value array correction unit 22 indicate the functions achieved by a workstation or personal computer (not shown).

In the image input section 14, the reflected light or transmitted light from the original image 12 irradiated with light from the light source and transferred in the sub-scanning direction is guided to a photoelectric conversion element such as a linear image sensor and electrically. Main scanning is performed, and the image is converted into an image signal (pixel signal) which is an electric signal through the photoelectric conversion element. The converted image signal is converted into continuous tone image data, for example, 8-bit digital image data (also simply referred to as image data) DA having values 0, 1, ..., 255 by an A / D converter. .

The image input unit 14 is not limited to such a scanner, but may be an image recording disk (image recording medium) such as a DVD, a communication network, a digital camera, or the like.
As a result, any medium that outputs digital image data may be used.

The image data DA output from the image input unit 14 is subjected to color correction processing, sharpness processing, etc., as necessary, by the image processing unit 16, and image data G is obtained.
Is created. In reality, the image processing unit 16 is also composed of a personal computer.

In this embodiment, the image output device 24
, The output resolution is 1200 as an example.
It is assumed to be [pixel / inch].

As the resolution of the image output device 24, a value of about 900 to 5000 [pixel / inch] can be selected.

The image data G, which has been subjected to the predetermined processing by the image processing unit 16, is supplied to the halftone image data creating unit 20 as a gradation image data creating device. Incidentally, in recent years, as the image data G supplied to the halftone image data creating unit 20, there may be a case where the image processing is performed directly on the medium which outputs the digital image data as a result, such as the digital camera.

The halftone image data creating section 20 includes a comparing section 3
2, an address calculation unit 34, a threshold array storage unit 36 as a storage medium that stores a plurality of threshold arrays (same meaning as threshold data or threshold matrix) such as a supercell threshold template (note that the codes of the stored threshold arrays are stored). Is also a threshold array 36), and a network attribute input unit 38 as a selection unit for selecting a desired threshold array 36 from the stored threshold arrays 36.

The image data G supplied to the halftone image data creating section 20 is supplied to the comparison input of the comparing section 32. Further, the address calculation unit 34 calculates an address AD = AD (x, y) representing the addresses of the x-axis and the y-axis on the threshold data from the image data G.

The threshold value array 36 is a threshold value stored in the address AD designated by the address calculation unit 34 (in this case, 8 bits which take values 1, ..., 255 (correctly, 8
Although it is a value obtained by subtracting 1 from the bit, it is referred to as 8 bits for convenience. ) Threshold value} T and is supplied to the reference input of the comparison unit 32.

As the threshold array 36, a plurality of threshold arrays 3
Among them, those corresponding to the halftone attributes (screen ruling, halftone angle and halftone shape) designated by the halftone attribute input unit 38 are used. In this embodiment, as an example,
It is assumed that the screen ruling is 175 lines [lines / inch], the net angle is 15 °, and the net shape is a square shape. As a concrete threshold value array 36, a threshold value array obtained by applying the first technique can be adopted. In this embodiment, the value of the quotient obtained by dividing the output resolution by the screen ruling is 1200 ÷ 175≈6.9. Relatively low resolution and high screen ruling.

The number of screen lines is 85 and 175.
It is possible to select a value between 50 and 600 lines, such as lines, 300 lines, etc.

As described above, the super cell is composed of a plurality of halftone dot cells. Generally, in the field of halftone dot generation, a supercell is set on the pixel grid determined by the output resolution, the set supercell is divided into halftone dot cells, and a threshold value is set for each pixel in the divided halftone dot cells. Is generated to generate a threshold array 36, and the supercell in which the threshold is assigned to each halftone dot cell is a supercell threshold template, that is, the threshold array 3
6

By considering a super cell composed of a plurality of halftone dot cells, the screen ruling and halftone angle can be changed more finely, and a value closer to the specified screen ruling and halftone angle can be obtained. Has the advantage that

The pixel grid is a group of pixels which is a blackening unit. Therefore, the pixel grid may be imaged in such a manner that pixels are arranged vertically and horizontally at the output resolution.

In the comparing section 32, the magnitude comparison operation of G ≧ T → 1 (ON, blackening) and G <T → 0 (OFF, blank area, non-blackening, whitening) is performed for the image data G and the threshold value T. Then, halftone dot image data H as gradation image data indicating a dot pattern having a value 1 or a value 0 of the comparison calculation result is created.

The generated halftone image data H, that is, gradation image data, is supplied to the display unit 23 such as a CRT monitor and the like, and also to the exposure recording unit 26 constituting the image output device 24. Here, with the halftone image data H,
The image displayed on the display unit 23 is a grayscale image formed by a set pattern of halftone dot cells having dots composed of 0 or one or more blackened pixels, that is, a dot pattern.

In the exposure recording unit 26, the exposure recording unit 26
The photosensitive material M disposed inside is exposed and recorded by a laser beam (recording beam) which is turned on and off according to the halftone image data H to form a halftone image as a latent image on the photosensitive material M. The photosensitive material M on which the halftone image is formed is developed by the automatic developing machine 28, and the film F on which the visualized halftone image is formed is created. This film F
Is used as an original plate to prepare a printing plate, the created printing plate is mounted on a printing machine (not shown), and ink is applied to the mounted printing plate.

By placing the ink attached to the printing plate on the printing paper, it is possible to obtain a desired printed matter on which an image is formed by a dot pattern on the printing paper.

The present invention is not an image output device 24 which outputs a film F as an original plate, but an image output which can directly output a printing plate PP by halftone image data H as shown in FIG. It can also be applied to the CTP output device 24a which is a device. In the CTP output machine 24a,
The printing plate PP is directly obtained by scanning and recording the photosensitive material M with a laser beam (recording beam).

Further, the image output device 24 is not limited to a so-called scanning exposure device using a laser beam, but can be applied to a device for drawing a film, a printing plate or a printed matter by a surface exposure method or an ink jet method.

Furthermore, CTC (computer to cylinde
r) If the halftone image data H is supplied to the printing machine 24b, the CTC printing machine 24b scans and records the photosensitive material M wound around the cylinder based on the halftone image data H. Ink is attached to the printing plate, and the ink attached to the printing plate is placed on the printing paper,
It is possible to directly obtain the desired printed matter PM on which an image is formed on the sheet.

The halftone dot image data creation unit 2 in the example of FIG.
The threshold value array 36 forming 0 can be recorded in a recording medium 49 such as an optical disk which is a package medium such as a CDROM or a CDR and can be carried and carried. The threshold value array 36 can also be transmitted as data.

In this embodiment, the threshold array 36
Is stored in a storage medium such as a hard disk mounted on a personal computer or the like.

The above is the description of the basic configuration of the plate making system 10 according to the embodiment of the present invention.

Next, referring to the flow chart of FIG.
Threshold array 36 for eliminating or mitigating single plate moire
The correction method of is explained. The control subject of this flowchart is the threshold value array correction unit 22. The image generated by the threshold value array correction unit 22 can be displayed on the display unit 23.

First, in step S1, halftone image data H having halftone dot contacts is created. In this case, among the halftone dots forming the halftone dot image data H, halftone dot image data H (symbol H = Ha) in which approximately half of the halftone dot contacts are created.

As can be seen from the lower diagram of FIG. 10, the contact points (contact pixels) 3 of each halftone dot 1 whose halftone dot percentage is 50 [%] are half of the four upper, lower, left and right positions, that is, two places. There will be. A state in which one of the two points is in contact with another adjacent halftone dot 1 is a state in which approximately half of the contact points 3 of the halftone point 1 are in contact.

When the net shape is a square shape,
Basically, the halftone dot% of the halftone dot image data H in which all the contact points 3 of the halftone dot 1 are in contact is 50%, but the halftone dot percentage in which all the contact points 3 of the halftone dot 1 are in contact changes depending on the halftone dot shape. To do.

Here, the case where about half of the contact points 3 of the halftone dot 1 are in contact is (the number of halftone dots × 4/2) / 2 = (the number of halftone dot contacts) / 2, so in the end, the dots During the pattern,
This is a state in which there are approximately the same number of contact points 3 as the number of halftone dots 1.

Therefore, in the step of creating the halftone dot image data Ha having the contact points 3 of the halftone dots 1, the image data G having uniform pixel values corresponding to the size of the threshold array 36 is transferred from the threshold array correcting section 22 to the comparing section 32. And the address calculation unit 34.

The image data G having uniform pixel values and the threshold value T forming the threshold value array 36 are compared with each other, and the comparing section 3 described above is used.
By performing the comparison operation at 2, the halftone dot image data (dot pattern) Ha having the contact point 3 of the halftone dot 1 shown as an example in FIG. 3 is created.

Next, in step S2, the contact point distribution data Da consisting of the pixels of only the contact points 3 of the dot 1 shown in FIG. 4 is extracted from the dot image data Ha shown in FIG.

Here, a method of determining whether or not the pixel of interest is the contact point 3 (or the non-contact point candidate 3 because it is the pixel which should be the non-contact point) when the contact point distribution data Da is extracted will be described.

Considering the area 46 of 3 pixels × 3 pixels shown in FIG. 5, with respect to the target pixel *, the target pixel * is a blackened pixel and the pixels D and E on the left and right thereof are blackened pixels. , And the upper and lower pixels B and G are whitened pixels, the pixel of interest * is determined to be the contact point (non-contact point candidate) 3.

This time, for the sake of easy understanding, the contact point where the pixels are in contact with each other at an angle is excluded, but the contact point is detected with higher accuracy by taking the oblique pixel into consideration by the same method. You can also

Next, the contact distribution data Da shown in FIG. 4 is subjected to the low frequency component extraction process shown in step S3 (S3-1 to S3-4).

As can be seen from FIG. 4, contact distribution data D
a is image data in the position space (real space). Here, the image data on the position space means data on the coordinates defined on the xy plane.

First, in step S3-1, a two-dimensional fast Fourier transform (Fast) is applied to the contact distribution data Da.
Fourie Transformation, abbreviated FFT) processing,
Convert to data in frequency space (power spectrum distribution in frequency space).

Next, in step S3-2, the visual filter processing, which is the weighting processing by the human visual characteristic 65 shown in FIG. 6, is performed on the data in the frequency space. As can be seen from FIG. 6, the human visual characteristic 65 has a frequency of 0.8 (c
/ Mm), the characteristic having the maximum sensitivity is used.

Next, in step S3-3, a low pass filter (Low Pass) having the cutoff frequency as the screen frequency is used.
Filter, abbreviated as LPF) to extract low frequency components.

Next, in step S3-4, an Inverse Fast Fourie Transformation (abbreviated as IFF) is applied to the extracted low frequency components in the frequency space.
T) Process is performed to obtain low frequency components in the position space.

Incidentally, the above-mentioned steps S3-1 to S3-
The low frequency component extraction processing of No. 4 is performed by, for example, the above-described first
Is disclosed in the above technology.

Next, in step S4, the low frequency components in the position space are further subjected to FFT processing and decomposed into specific frequency components.

Next, in step S5, the intensities of the decomposed specific frequency components are compared and rearranged in order of intensity.

Then, in step S6, among the intensities of the rearranged specific frequency components, the IFFT is first performed for the specific frequency component having the highest intensity, that is, the first intensity.
Processing is performed and the intensity distribution in the position space is converted.

In practice, in the process of step S6, the IFFT process is performed on the specific frequency component having the highest intensity, that is, the maximum intensity among the intensities of the specific frequency components remaining at the present time, and again on the position space. The process of obtaining the intensity distribution is performed.

Next, in step S7, the intensity distribution on the position space is compared with the contact distribution data Da consisting of the contact (non-contact candidate) 3 shown in FIG. 4, and the position of each contact (non-contact candidate) 3 is compared. Calculate the intensity of.

Next, in step S8, the position of the contact (non-contact candidate) 3 having the maximum strength among the strength of each contact (non-contact candidate) 3 is determined as the non-contact candidate position to be made non-contact.

Next, in step S9, it is determined whether or not there is one non-contact candidate position. In general, the intensity distribution of the specific frequency on the position space in step S8 has a sinusoidal period, so a plurality of non-contact candidate positions are determined at this stage.

Then, again, with respect to the specific frequency component having the next highest intensity (that is, the second intensity) of the specific frequency component obtained in step S6, I of step S6
The FFT process is performed again.

In step S7 again, the intensity distribution in the position space for the second specific frequency component having the second intensity is compared with the plurality of non-contact candidate positions selected and determined in the previous step S8, and each non-contact candidate position is compared. The strength at the contact point candidate position is calculated.

Next, in step S8, the non-contact candidate having the maximum strength among the intensities of each non-contact candidate position is selected and determined as the non-contact candidate position to be made non-contact.

Similarly, in step S9, it is determined whether or not there is one non-contact candidate position.

If the number of non-contact candidate positions has not been narrowed down to one, the IFFT process is performed on the third specific frequency component having the third intensity in the same manner as described above, and then, in the following steps. The processes of steps S6 to S8 are repeated until the determination of S9 is established.

When the determination in step S9 is established, that is, when the number of non-contact candidate positions is one, the process proceeds to the contact candidate position determining process in the following steps S50 and S100.

When the judgment in step S9 is established, the selected 1 in the contact distribution data Da shown in FIG. 4 is selected.
The contact points 3 corresponding to the non-contact candidate positions are blackened pixels corresponding to whitened pixels.

Next, in step S50, contact candidate distribution data Db is created.

The contact point candidate distribution data Db corresponds to the dot image data Ha having the contact point 3 of the dot 1 shown in FIG.
Considering the region 46 of 3 pixels × 3 pixels shown in FIG. 5,
With respect to the pixel of interest *, the pixel of interest * is a whitened pixel and the pixels D and E on the left and right thereof are blackened pixels, and the pixels B above and below
When G is a whitened pixel, the target pixel * is determined to be a contact candidate.

If the target pixel * is a whitening pixel, the pixels B and G above and below it are blackening pixels, and the left and right pixels D and E are whitening pixels, the target pixel * is determined to be a contact candidate. To do.

Alternatively, the contact point candidate may be simply a blackened pixel in the peripheral portion of the halftone dot 1, or a blackened pixel in the peripheral portion of the halftone dot when the halftone dot% is limited within the range of 50% to 60%. it can.

In any case, it is preferable that the present invention is applied to a dot pattern in which the number of pixels of the contact points 3 forming the contact point distribution data Da and the number of pixels of contact point candidates forming the contact point candidate distribution data Db are substantially equal. .

Next, with respect to the contact point candidate distribution data Db, a process of selecting and determining a contact point candidate position to be the contact point 3 (blackened pixel) of the halftone dot 1 is performed.

That is, in the processing of steps S3 to S9, as a result, the contact exclusion pixel position of the halftone dot for changing one blackened pixel into a whitened pixel is determined, but in the processing of step S100, As a result, a process of determining a contact candidate position of a halftone dot for changing one whitening pixel into a blackening pixel is performed.

In the flowchart of FIG. 7, step S10 is performed.
The detailed flow of the process of 0 is shown. This step S
During the processing of 100, step S10 (S10-1, S10
-2, S10-3, S10-4) to S15 are the same as the above-described step S3 (S3-1, S3-2, S3).
-3, S3-4) to S9, which will be briefly described.

That is, step S10 (S10-1,
In the processing of S10-2, S10-3, S10-4), the low frequency component in the position space in the contact candidate distribution data Db is extracted.

Further, in the FFT processing of step S11, the extracted low frequency component is decomposed into specific frequency components.

In step S12, the specific frequency components are rearranged in order of strength.

In step S13, similar to step S6, the IFFT process is performed on the specific frequency component having the highest intensity, that is, the maximum intensity, among the remaining frequency components that have not been subjected to IFFT process at this time. , Again, the intensity distribution in the position space is obtained.

Then, in step S14, step S
Similar to 7, the intensity distribution on the position space is compared with the contact candidate distribution data Db to calculate the intensity of each contact candidate.

Then, in step S15, the position of the contact candidate having the smallest strength among the intensities of the contact candidates is determined as the contact candidate position to be the contact. The process of step S15 is different from the process of step S8 in that the contact candidate having the minimum strength instead of the maximum strength is selected as the contact candidate position.

Next, in step S16, it is determined whether or not there is one contact candidate position. Normally, the intensity distribution on the position space in step S8 has a sinusoidal period, and therefore, at this stage, a plurality of contact candidate positions are selected.

Therefore, again, with respect to the specific frequency component having the next highest intensity (that is, the second intensity) of the specific frequency component obtained in step S11, step S13 is performed again.
IFFT processing is performed.

Thereafter, by the same processing procedure, step S
In the process of 16, when the contact candidate positions are not narrowed down to one, the processes of steps S13 to S15 are repeated until the determination of step S16 is established.

When the determination in step S16 is established, that is, when the number of contact candidate positions is one, the process proceeds to the next step S20.

When the determination in step S16 is established, one selected contact candidate position in the contact candidate distribution data Db is set as a pixel position where a whitened pixel should be a blackened pixel.

Next, in step S20, step S9
By replacing the threshold value of the contact position (non-contact candidate position) and the threshold value of the contact candidate position selected and determined in step S16, the current threshold value array 36 is corrected.

According to the halftone image data H generated by the corrected threshold value array 36, the arrangement position of the contact point 3 of the halftone dot 1 is corrected.

Next, in order to confirm the correction effect of the contact arrangement of the halftone dot 1, in the processing of step S21, step S1
The image data G with uniform pixel values supplied to the comparison unit 32 and the address calculation unit 34 in the processing of 1 is supplied to the comparison unit 32 and the address calculation unit 34.

Then, the image data G having uniform pixel values and the threshold value T which constitutes the corrected threshold value array 36 are compared,
By performing the comparison calculation in the above-mentioned comparison unit 32, halftone dot image data Hc similar to the halftone dot image data (dot pattern) Ha having the contact points 3 of the halftone dot 1 shown in FIG. 3 is created, and in step S22, Step S for evaluation of single plate moire
The contact distribution data Dc is created by the same processing as the processing of 2.

In the processing of step S23, this contact distribution data Dc is visually inspected to confirm whether single-plate moire appears. Instead of visual inspection, it is also possible to compare the intensity distributions in the position space after the low frequency component extraction processing of step S3 before and after the correction of the threshold array 36.

If it is evaluated that the single plate moiré appears, the steps S1 to S3 are repeated until the evaluation becomes good.
The processing up to S22 is repeated.

FIG. 8 shows the halftone dot image data after the threshold array correction corresponding to the halftone dot image data (dot pattern) Ha having the contact points of the halftone dots 1 shown in FIG. 3 when the evaluation in step S23 is good. (Dot pattern) Hc is shown.

FIG. 9 shows contact point distribution data Dc consisting of the contact points 3 of the dot 1 extracted from the dot image data Hc shown in FIG. 8 by the algorithm shown in FIG.

In the corrected contact distribution data Dc generated by the corrected threshold value array 36, the periodicity corresponding to the single plate moire is eliminated or relaxed as compared with the contact distribution data Da shown in FIG. Is understood.

As described above, according to the above-described embodiment,
When the threshold value array 36 in which the threshold value T for reproducing the gradation is arranged by the dot pattern which is a set pattern of the halftone dots 1 composed of one or more blackened pixels is corrected, a certain gradation (for example, the dot 1 In the halftone dot image data Ha) in which about half of the contact points of No. 1 are in contact with each other, in the contact point 3 of the halftone dot 1, one non-contact point candidate position to be made non-contact is determined (step A to step S2). , A step (B step) of step S100 of determining one contact candidate position to be the contact point 3 among the non-contact points of the halftone dot 1 at the certain gradation, and steps S2 to S9.
By replacing the threshold value T of the non-contact candidate position selected in the process of and the threshold value T of the contact candidate position selected in the process of step S100 in the process of step S20, the position of the contact 3 of the halftone dot 1 is arranged. I am trying to fix it.

By creating the threshold value array 36 in which the threshold value T is replaced by the above procedure, the so-called single plate moire, which is a fringe generated by the interference between the output resolution and the screen ruling, which occurs near the contact point of the halftone dots, is eliminated. Or it can be reduced.

By performing the processes in steps S2 to S9 and step S100 a plurality of times respectively, in other words, by selecting and determining a plurality of non-contact candidate positions and contact candidate positions, the single plate moire is determined. The effect of eliminating or mitigating

The threshold value array 36 thus determined is
It is possible to record on a recording medium 49 such as an optical disk and supply it to the market.

Similarly, by setting new parameters (screen ruling, halftone angle, output resolution, halftone shape, etc.) by the halftone attribute input section 38, the corrected threshold value array 36 corresponding to this parameter is set. Can be determined almost automatically.

Usually, when performing color printing, it is necessary to prepare plates for four colors of C (cyan), M (magenta), Y (yellow), and K (black). Four different angles depending on the algorithm (typically 0 ° (eg Y
Plate), 15 ° (for example, C plate), 45 ° (for example, M plate)
Plate), 75 ° (for example, K plate)}, and a threshold array 36 for four plates is created.

In the strength calculation process at each non-contact candidate position in step 7 and the strength calculation process at each contact candidate position in step S14, only the contact position is focused, but the threshold value in step S20 is set. The low frequency components in the entire dot pattern may be taken into consideration when performing the position replacement process.

That is, in the above-mentioned method, only the distribution of the contact points 3 (FIG. 4) is considered when calculating the non-contact candidate positions and the contact point candidate positions, but the distribution of the entire dot pattern (FIG. 3) is taken into consideration. It is better to put it in. That is, the low-frequency components of the distribution of the contact points 3 as well as the low-frequency components for the entire dot pattern implemented by the first technique are calculated for each candidate position and taken into consideration when determining the candidate position. This improves the contact distribution and does not deteriorate the dot pattern.

Further, the algorithm based on the flowchart shown in FIG. 2 may be recursively applied in the halftone dot range where the contact point 3 of the halftone dot 1 exists. For example, 50% → 4
By applying this algorithm in sequence for each net% of 8% → 52% → 49% → 51% → 48.5%, the threshold placement correction that can eliminate or alleviate single plate moire around 50% A later threshold array 36 can be obtained.

Further, when determining the non-contact candidates, for example, the candidate pixels for which the halftone dot shape is extremely broken such that the pixels on the four sides of the blackened pixels are whitened pixels, that is, so-called isolated pixels are not selected. It is preferable. The same applies when determining contact candidates.

In the above-described embodiment, 2
However, the present invention is not limited to binary halftone dot image data H, but output values of "0, 1,
It can also be applied to multivalued halftone dot image data such as four-valued, eight-valued, etc. that take values of "2, 3".

According to the plate making system 10 of FIG. 1 in which the threshold value array 36 created by the above procedure is set, it has been difficult to create the plate due to the single plate moiré, for example, the resolution is 1200 [pixels / inch]. ], The screen frequency is 1
Even if the output condition of 75 [lines / inch], generally, the output condition of output resolution / number of lines is 10 or less,
There is an advantage that a film F or the like on which an image as a dot pattern with almost no single plate moire is formed can be created.

In the above-described embodiment, the method of determining the arrangement position of the threshold value in the gradation image reproducing method using the threshold value array 36 is described as an example, but the present invention is the gradation near the contact point of halftone dots. What is the threshold array 36
Is determined to be the optimum arrangement. As can be easily guessed by those skilled in the art, the threshold value array 36 determined by this technique is used as a sub-pixel of Z × Z dots for each pixel of the grayscale image. Needless to say, the present invention can be applied to other gradation reproduction techniques such as a density pattern method in which the density of each pixel is reproduced by the area ratio of blackened pixels in the sub-matrix in correspondence with the matrix.

Further, in the above-described embodiment, the halftone dots by the so-called AM screen that expresses the lightness and darkness by the size of the halftone dot (halftone dots of substantially uniform size are orthogonally arranged at substantially equal intervals). .) As an example, but also in a gradation reproduction method using an arrangement method other than halftone dots, for example, in an FM screen in which dots of the same size are arranged irregularly and shades are expressed by the density of the dots, Needless to say, various configurations can be adopted without departing from the scope of the present invention, such as being applicable to reduction of low-frequency components generated in association with the threshold array.

In the case of the FM screen, in the low-pass filter processing, only the visual characteristics of humans are used, so that the filtering by the low-pass filter corresponding to the screen frequency in the halftone dot period is unnecessary. Is.

[0129]

As described above, according to the present invention, the so-called single plate moiré, which is a fringe caused by the interference between the output resolution and the screen ruling due to the arrangement of the contact points of the halftone dots, is eliminated or alleviated. can do.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a plate making system to which a threshold value array according to an embodiment of the present invention is applied.

FIG. 2 is an overall flowchart showing an example of a threshold value replacement procedure.

FIG. 3 is a diagram showing a dot pattern on a position space represented by halftone dot image data.

FIG. 4 is a diagram showing the contact points of halftone dots with respect to the dot pattern of FIG.

FIG. 5 is a diagram illustrating an example of an algorithm for extracting a contact point (non-contact point candidate) of a halftone dot.

FIG. 6 is a diagram used for explaining human visual characteristics.

FIG. 7 is a flowchart provided for explaining determination of a contact candidate position.

FIG. 8 is a diagram showing a dot pattern on a position space after the contact positions of halftone dots have been replaced.

9 is a diagram showing the contact points of halftone dots with respect to the dot pattern of FIG.

FIG. 10 is a diagram generally illustrating a dot pattern.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Halftone dot 2 ... Halftone dot cell 3 ... Contact point 10 ... Plate making system 12 ... Original image 14 ... Image input unit 16 ... Image processing unit 20 ... Halftone image data creating unit 22 ... Threshold value array correction unit 24 ... Image output device 26 ... exposure recording unit 28 ... automatic developing machine 32 ... comparing unit 34 ... address calculating unit 36 ... threshold array (threshold array storing unit) 38 ... net attribute inputting unit 46 ... area 49 ... recording media Da, Dc ... contact distribution data Db ... Contact candidate distribution data F ... Film G ... Image data H ... Halftone dot image data Ha ... Halftone dot image data M at which about half of the halftone dots are in contact M ... Photosensitive material PM ... Printed matter PP ... Plate

Claims (4)

[Claims] 1. A method of correcting a threshold value array in which threshold values for reproducing a gradation by a dot pattern, which is a set pattern of halftone dots composed of one or more blackened pixels, are arranged. A step of determining at least one non-contact candidate position to be non-contact among halftone dots, and at least one contact candidate position to be non-contact among the halftone dots at a certain gradation. The process B to be determined and the threshold value of the non-contact candidate position selected in the process A in the threshold value array and the threshold value of the contact candidate position selected in the process B are exchanged to correct the threshold value array C. And a step of correcting the contact arrangement of the halftone dots. 2. The threshold value array correction method according to claim 1, wherein in the step A and the step B, a dot pattern in which approximately half of contact points of the halftone dots in the dot pattern are in contact is identified by frequency analysis. After obtaining the frequency component, the intensity of the specific frequency component at the non-contact candidate position is obtained, and the intensity of the specific frequency component at the contact candidate position is obtained, and the non-contact candidate is determined according to the magnitude of the obtained intensity. A method for correcting a threshold value array, which comprises determining a position and the contact point candidate position. 3. The threshold value array correction method according to claim 1, wherein the unit of resolution of the dot pattern is [pixel / inch] and the unit of screen frequency of the halftone dot is [line / inch]. The threshold array correction method is characterized in that the value of the quotient obtained by dividing the resolution of the dot pattern by the screen ruling is 10 or less. 4. In a data structure of a dot pattern, which is a set pattern of halftone dots that are substantially uniform in size and are orthogonal to each other and are arranged at substantially equal intervals, the arrangement of the contact points of the halftone dots is irregular. A dot pattern data structure that is characterized by: