JP2002211097A - Method for processing generation of halftone dots using number of plural screen lines - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for reproducing halftone dots capable of improving reproducing stability of a highlighted part or a shadow part without reducing quality of an entirety in a printing system in which a reproduction of the dots of the highlighted part or the shadow part uses an unstable plate with low resolving power. SOLUTION: The quality of the entirety is not reduced by using the number of more rough screen lines in the highlighted part or the shadow part of an image, using the halftone dots of the number of the plurality of the screen lines of a reference in another part except the above and forming the dots formed by changing the number of the plurality of screen lines to suit for a distribution of a gradation value of an original image.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a screening process for image information performed during plate making.

[0002]

2. Description of the Related Art In flexographic printing, original image data such as a photograph is converted into halftone data at an arbitrary screen ruling using a raster image processor (hereinafter referred to as "RIP") at the time of plate making, and the data is used for plate making. In general, printing is performed by outputting a masking film, superposing and developing on a photocurable resin plate, and developing. In addition, in the flexo CTP (Computer To Plate) that has recently appeared, a mask layer provided on a photocurable resin plate is patterned by a laser drawing device without using a masking film, and the printing plate is exposed and developed. Is going.

[0003]

In a printing method using a plate having a low resolution, such as flexographic printing, a minute halftone dot cannot be formed stably. Unevenness due to missing halftone dots or poor inking occurs. As a method of reproducing a highlight portion without unevenness, a method of performing tone correction without using minute halftone dots that cannot be reproduced stably is used. However, this method has a problem that the minimum density is higher than other printing methods such as gravure printing.

In order to maintain the stability of halftone dot reproduction, a method of reducing the number of lines is conceivable. However, there is a problem that the resolution of the entire image is reduced and the quality is reduced.

The present invention has been made in view of such problems, and in a printing method using a plate having a low resolution and an unstable reproduction of a halftone dot in a highlight portion, the overall quality is reduced. It is another object of the present invention to provide a halftone dot reproduction method that improves the reproduction stability of a highlight portion.

[0006]

The basic idea of the present invention to solve the above-mentioned problem is to form a halftone portion having a high screen ruling in a halftone portion in order to improve the overall image quality of a print reproduction image. However, in a highlight portion or a shadow portion, which is an image portion where it is necessary to form a halftone dot smaller than the minimum dot size that can be stably formed on the plate material,
This is to form halftone dots with a reduced screen ruling. The relatively high screen ruling used in the halftone portion is set as the reference screen ruling. For this reference screen ruling, determine the halftone dot area ratio threshold determined by the chemical or physical properties of the plate printing process, such as the resolution of the plate material, and set the minimum to be reproduced in the highlight or shadow part of the original image. From the halftone dot area ratio or the maximum halftone dot area ratio, the second screen coarser than the reference screen ruling applied to the highlight portion or the shadow portion
Alternatively, a third screen ruling is obtained, and the two or more kinds of screen rulings are determined by determining whether the reference screen ruling is applicable according to the distribution of the gradation value data of the original image. The problem is solved by a halftone dot film or a printing plate formed by using two or more screen rulings as a result, or a printing method using them. Further, the above object is solved by a halftone generation method for creating the above halftone dots and a RIP device equipped with a function for realizing the halftone dots.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the flow of the procedure of a halftone dot generation processing method of the present invention will be schematically described with reference to the flowchart of FIG. First, the number of screen lines (reference line number) of the entire plate for halftoning the plate data is set. This can be done by setting the number of lines normally used in the printing plate line, except in special cases (S1). Next, based on the size of the dot diameter reproducible on the printing plate determined by the printing plate material to be used, a critical halftone dot area ratio at the reference number of lines is obtained (S2).

For example, an image setter with a resolution of 1750 dp
In the case of i, the resolution of the plate material is 29 μm with respect to the reference line number of 175 lines, the size of one side of one basic dot handled by the RIP device is 1/1750 inch = 25.4 mm / 1750 = 14.5 μm. This is a dot diameter of 29μ that can be generated stably with plate material.
less than m. Since the resolution of the plate material is 29 μm, a halftone dot having a size of 2 dots vertically and horizontally is the minimum halftone dot when this plate material is used. This corresponds to a dot of 4% from the relationship between the resolution of the imagesetter and the number of reference lines. Therefore, the halftone dot area ratio threshold is calculated as 4%.

Next, the number of lines (applied to a dot less than 4%) for forming a dot in the highlight area is determined from the minimum dot area ratio to be reproduced in the highlight portion (S).
3). For example, if the minimum halftone dot area ratio to be reproduced in the highlight portion is 4%, the number of lines in the highlight region is also 1
75 lines may be used. However, for example, when it is desired to reproduce from halftone dots of 1%, the screen ruling is set to be coarser.

In the case of the above example, if it is desired to reproduce the highlight from halftone dots of 1%, the line 87.5 is appropriate. 8
This is because if the number of lines is 7.5, the size of a halftone dot of 1% of the number of lines corresponds to the size of 4% of a halftone dot of 175 lines.

A halftone generation process is performed based on the reference number of lines, the halftone dot area threshold, and the number of lines in the highlight area (S4).

FIG. 2 is an explanatory diagram illustrating the concept of halftone dot processing of the present invention. FIG. 2 (A) shows one of the 175 lines.
5 shows a halftone dot pattern. Although the halftone dot can be created by the RIP device, the halftone dot becomes thick on the printing plate as shown in FIG. 2B due to the tone correction performed to stabilize the halftone dot reproduction. Therefore, in the halftone dot generation method according to the present invention, as shown in FIG.
Do halftone processing as 7.5 lines.

Next, the screening process in step S4 of FIG. 1 will be described. Hereinafter, an example of a halftone dot generation method applicable to the method of the present invention will be described with reference to FIG.

A screening process is performed on the input image data using 175 lines (FIG. 3). A search is performed in the horizontal direction from the upper left pixel of the image data, and pixels having a dot area ratio equal to or less than the dot area ratio threshold (4%) are extracted from the pixels (FIG. 3). Four adjacent pixels, with the pixel at the upper left, are combined and processed with half the number of lines (87.5 lines) (FIG. 3). The halftone dot area ratio for four pixels processed with half the number of lines may be determined as an average value of the original halftone dot area ratios of those pixels, or may be determined as a median value. Subsequently, a search is performed in the horizontal direction, and similar processing is performed on pixels having a dot area ratio of 4% or less. (Processing is not performed on the area already processed with the 87.5 line.) When the end of the line is reached, the process is repeated with the next line.

If the pixels on the right and bottom ends of the image data or the pixels already processed on the right side have a dot area ratio of 4% or less, the processing is performed together with the pixels on the right side. (If there is no pixel to be matched, no processing is performed. FIG. 3)

The above-described screening process is an example of the screening process for two types of lines performed in step S4 in FIG. 1, and other procedures may be used.

By directly supplying a high-resolution binary image (binary image having the resolution of an image setter) forming a halftone dot pattern obtained by the screening process to a RIP device, an image setter or a CTP device, a desired halftone image can be obtained. A film or printing plate screen is obtained. Alternatively, the above-described screening processing function may be provided in the RIP device.

The method of the present invention has been described above with reference to specific examples. The above specific example is a useful application example of the present invention to a printing method in which the stability of minute dots in a highlight portion is difficult. Conversely, in a printing method having a problem in the stability of halftone dot formation in the shadow portion, a halftone dot area threshold is provided in the shadow portion by the method disclosed in the present invention, and the halftone dot area ratio equal to or larger than the threshold value is set. It is also possible to make the screen ruling of the shadow area coarse.

If a computer program for performing the screening process in step S4 on the given document image data is installed in an existing RIP device, the RIP device has the halftone dot generation processing method of the present invention.

[0020]

By applying the halftone generation processing method of the present invention to a printing method having a problem in the stability of halftone reproduction in a highlight part (or shadow part), a high resolution halftone part can be obtained. In the highlight area (or shadow area) where halftone dots cannot be reproduced stably, stable gradation reproduction is realized by using halftone dots with coarser screen ruling, resulting in an excellent overall A remarkable effect that a printed material with tonal reproduction can be obtained can be obtained. Since the method of the present invention can be applied to a general printing method in which a printing plate having poor resolution has to be used, the application range is wide.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a flow of a procedure according to the present invention.

FIG. 2 is a diagram showing an example of a screening process of the present invention.

FIG. 3 is a diagram illustrating a specific example of a screening process in step S4.

Claims (6)

[Claims] 1. A method of using two or more screen rulings of a reference screen ruling and a coarser screen ruling according to a halftone dot area ratio threshold with respect to a reference screen ruling,
A halftone generation processing method for forming halftone dots. 2. A halftone film having halftone dots formed by selectively using two or more types of screen rulings of a standard screen ruling and a coarser screen ruling according to a halftone dot area ratio threshold with respect to the standard screen ruling. A printing method using a printing plate. 3. A halftone film or printing plate formed by selectively using two or more types of screen rulings, ie, a standard screen ruling and a coarser screen ruling, and printed matter produced using them. 4. A reference screen ruling, a halftone dot area ratio threshold value with respect to the reference screen ruling, the halftone dot area ratio threshold and a minimum halftone dot area ratio or a maximum to be reproduced in a highlight portion or a shadow portion of a document image. From the screen ruling coarser than the reference screen ruling determined from the dot area ratio,
A halftone generation method for forming halftones by selectively using halftone dots having a reference screen ruling and halftones having a ruling lower than the standard screen ruling according to the distribution of gradation values of pixels of a document image. 5. A halftone dot area ratio threshold with respect to a reference screen ruling is determined from the chemical or physical properties of the printing plate printing process, and a minimum halftone dot area ratio or a minimum dot area ratio desired to be reproduced in a highlight portion or a shadow portion of an original image. A screen ruling that is coarser than the reference screen ruling applied to the highlight portion or the shadow portion is determined from the maximum halftone dot area ratio, and the two or more types of screen rulings are selectively used according to the halftone dot ratio threshold. A printing method using a dot film or printing plate having dots. 6. A RIP device having a function of realizing the halftone dot generation processing method according to claim 1.