JP2006086785A - Printing method for picture consisting of dots and method and apparatus for converting picture - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a half-tone picture having good picture quality in the whole region ranging from a high intensity section to a low intensity section in an original picture. <P>SOLUTION: The original picture 12 consisting of the aggregation of pixels P1-P9, each having a pixel value in a range of Pmin-Pmax is converted into the half-tone picture 23 consisting of many dots. A threshold Pth (Pmin<Pth<Pmax) is defined. The dots D11-D15, each having an area complying with the pixel value are placed (AM (Amplitude Modulation) screening) at a pixel position having a pixel value of Pth or higher so that a large area may be obtained as the pixel value is higher. The dots D16-D19 having the same area as that of the dot D15 to be placed at the pixel position having the pixel value Pth are placed (FM (Frequency Modulation) screening) with a probability of P/Pth at the picture position having a pixel value P of less than Pth so that the higher probability may be obtained as the pixel value P is higher. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for printing an image composed of halftone dots, and more particularly to a technique for converting an original image having continuous gradation into a halftone image expressed by a large number of halftone dots.

  In offset printing, letterpress printing, and the like, when a continuous tone image given as an original image is printed, it is necessary to perform a screening process for converting the image into a halftone image. This is because in normal offset printing and letterpress printing, a plurality of types of ink having different densities cannot be used properly according to the density information of the original image. Also, in gravure printing, the density information of the original image is represented by individual halftone dots (in the case of gravure printing, it is generally called “cell”, but the “halftone dot” referred to in the present application is for such gravure printing. In this case, it is necessary to perform a screening process and a conversion process to a halftone image. A halftone image is basically a binary image showing the distinction between a portion to which ink is applied and a portion to which ink is not attached, so it is not necessary to prepare a plurality of types of ink having different densities when performing printing. . However, in order to maintain the gradation information expressed on the original image even after the continuous tone image given as the original image is converted into a halftone image, the halftone image uses the gradation information of halftone dots. It must be expressed as area or density. Eventually, the halftone image obtained by the conversion can be said to be a pseudo gradation image (pseudo gray image) representing a pseudo continuous tone.

  Screening processing for converting an original image into a halftone image is roughly divided into AM screening processing and FM screening processing. The AM screening process is a process that literally modulates the density information (shading information) of the image, and the pixel value (density) of the original image is reflected as the area of the halftone dots. . On the other hand, the FM screening process is a process for literally frequency-modulating density information (shading information) of an image, and the pixel value (density) of the original image has a certain area. This is reflected as the distribution density in the plane of the halftone dots.

Recently, a screening process combining this AM screening process and FM screening process has also been proposed. For example, Patent Document 1 below discloses a technique in which an original color image is expressed in an HSL color system, AM screening processing is applied to a portion with high lightness, and FM screening processing is applied to a portion with low lightness. Has been. Further, in Patent Document 2 below, when a large-diameter glitter pigment layer is formed on a base material and used for printing with a normal ink on the upper surface for use in applications such as a cosmetic material, For the pigment layer, a technique is disclosed in which an original image is subjected to FM screening processing to express a picture, and for a normal ink layer, an original image is subjected to AM screening processing to express a picture.
JP-A-8-79547 JP 2003-341297 A

  The AM screening process and the FM screening process described above have advantages and disadvantages. First, the AM screening process employs a method of reducing the area of the halftone dots to be arranged when expressing the low density area of the original image. Therefore, in the actual printing process, the ink in the low density area is sufficiently applied to the paper. The problem of not being transferred arises. In particular, in the case of gravure printing, each halftone dot is obtained by transferring ink filled in a concave cell to the paper side, but in the case of ink containing a bright pigment with a large particle size, the cell When the opening area of the cell becomes small, the glittering pigment is not satisfactorily filled in the cell or clogged in the cell, so that a situation in which the pigment is not sufficiently transferred to the paper side tends to occur. On the other hand, in the FM screening process, the density information is expressed by the density of halftone dots. However, as the distribution density of the halftone dots decreases, the spatial resolution of the image decreases, and deterioration of the image quality cannot be avoided.

  In the aforementioned Patent Documents 1 and 2, a technique using both of them is disclosed so that the advantages of AM screening and FM screening can be utilized respectively. However, in order to solve the above-mentioned problem, it is not always sufficient. The effect cannot be obtained, and printing with good image quality cannot be performed for all regions from the high density portion to the low density portion of the original image.

  Therefore, the present invention has an object to provide an image conversion method and apparatus capable of converting all regions from a high density portion to a low density portion of an original image into a halftone image having good image quality. It is another object of the present invention to provide a method for printing an image composed of halftone dots using such an image conversion method.

(1) According to a first aspect of the present invention, in the method for printing an image composed of halftone dots,
The AM screening is applied to the high density part of the image, and the FM screening is applied to the low density part.

(2) According to a second aspect of the present invention, there is provided a printing method for expressing an image composed of a set of pixels having pixel values within a range of Pmin to Pmax by a large number of halftone dots.
A predetermined threshold value Pth (where Pmin <Pth <Pmax) is determined, and a halftone dot having an area corresponding to the pixel value is arranged at a pixel position having a pixel value equal to or greater than Pth, and less than Pth A halftone dot having a predetermined area is arranged at a pixel position having a pixel value with a probability corresponding to the pixel value.

(3) According to a third aspect of the present invention, there is provided a printing method for expressing an image composed of a set of pixels having pixel values in the range of Pmin to Pmax by a large number of halftone dots.
A predetermined threshold value Pth (where Pmin <Pth <Pmax) is determined, and an area corresponding to the pixel value has a larger area at a pixel position having a pixel value equal to or greater than Pth as the pixel value increases. A pixel position having a pixel value Pth with a probability corresponding to the pixel value such that the larger the pixel value is, the larger the pixel value is. A halftone dot having the same area as the halftone dot to be arranged is arranged.

(4) According to a fourth aspect of the present invention, in the method for printing an image composed of halftone dots according to the third aspect described above,
At a pixel position having a pixel value P less than Pth, a halftone dot having the same area as the halftone dot to be arranged at the pixel position having the pixel value Pth is arranged with a probability of P / Pth. Is.

(5) According to a fifth aspect of the present invention, in the method for printing an image composed of halftone dots according to the third or fourth aspect described above,
Printing is performed using ink containing a bright pigment, and the diameter of the halftone dot to be arranged at the pixel position having the pixel value Pth is set to be sufficiently larger than the average particle size of the bright pigment. is there.

(6) A sixth aspect of the present invention is a printing method for expressing an image composed of a set of pixels having pixel values in the range of Pmin to Pmax by a large number of halftone dots.
The generating points are arranged at a random pitch on the two-dimensional plane. For each point on the two-dimensional plane, the generating point closest to each other is defined as the closest generating point, and the same generating point is the closest generating point. The two-dimensional plane is divided into a plurality of unit regions so that one unit region is constituted by a set of points
The image to be printed is superimposed on this two-dimensional plane, and the pixel value on the image corresponding to each mother point position is defined as the pixel value of the mother point,
A predetermined threshold value Pth (where Pmin <Pth <Pmax) is determined, and in a unit region to which a mother point having a pixel value equal to or greater than Pth belongs, the occupation ratio with respect to the unit region increases as the pixel value increases. As described above, a halftone dot having an occupation ratio corresponding to the pixel value is arranged, and a unit area to which a mother point having a pixel value less than Pth belongs has a higher probability as the pixel value increases. A halftone dot having an occupation ratio corresponding to the pixel value Pth is arranged with a probability corresponding to the pixel value.

(7) According to a seventh aspect of the present invention, in the method for printing an image composed of halftone dots according to the sixth aspect described above,
When defining the pixel value on the image corresponding to each mother point position as the pixel value of the mother point, the interpolation value using the pixel values of a plurality of pixels adjacent to the mother point position was obtained. The interpolation value is the pixel value of the mother point.

  (8) In the eighth aspect of the present invention, a printed material or a printing plate used for printing the printed material is prepared by the method for printing an image composed of halftone dots according to the first to seventh aspects described above. Is.

(9) A ninth aspect of the present invention is a printing plate or printed matter having a halftone dot,
For the high density portion of the image, the density is expressed by the area of the halftone dots, and for the low density portion, the density is expressed by the density of the halftone dots.

(10) In a tenth aspect of the present invention, in a printing plate or printed matter having a halftone dot,
An image is formed by two types of areas, a high density part and a low density part. In the high density part, a large number of halftone dots having an area within the range of the maximum area to the minimum area are arranged, and individual halftone dots are arranged. In the low density portion, a large number of halftone dots having the minimum area are arranged, and the density is represented by the density of each halftone dot.

(11) In an eleventh aspect of the present invention, a halftone image in which an original image composed of a set of pixels having pixel values in the range of Pmin to Pmax is represented as a set of halftone dots each having a predetermined area. In the image conversion method of converting to
Setting a predetermined threshold value Pth (where Pmin <Pth <Pmax);
For a pixel having a pixel value equal to or greater than Pth, a step of converting into a halftone dot having an area corresponding to the pixel value so that the larger the pixel value, the larger the area;
For a pixel having a pixel value less than Pth, it is determined whether or not to perform halftone dot arrangement with a probability corresponding to the pixel value so that the larger the pixel value is, the higher the probability is. When the determination is made, the pixel is converted into a halftone dot having the same area as the halftone dot to be converted for the pixel having the pixel value Pth, and a decision is made not to perform the halftone dot arrangement. If so, converting the pixel into an empty area without halftone dots,
Is executed by a computer.

(12) A twelfth aspect of the present invention is a halftone image in which an original image composed of a set of pixels having pixel values in the range of Pmin to Pmax is represented as a set of halftone dots each having a predetermined area. In the image conversion method of converting to
The generating points are arranged at a random pitch on the two-dimensional plane. For each point on the two-dimensional plane, the generating point closest to each other is defined as the closest generating point, and the same generating point is the closest generating point. Dividing the two-dimensional plane into a plurality of unit regions so that one unit region is constituted by a set of points
Superimposing the image to be printed on this two-dimensional plane and defining the pixel value on the image corresponding to each mother point position as the pixel value of the mother point;
Setting a predetermined threshold value Pth (where Pmin <Pth <Pmax);
In a unit area to which a mother point having a pixel value equal to or greater than Pth belongs, a halftone dot having an occupation ratio corresponding to the pixel value is arranged so that the larger the pixel value, the larger the occupation ratio for the unit area. And the stage of
In a unit region to which a mother point having a pixel value less than Pth belongs, it is determined whether or not to perform halftone dot arrangement with a probability corresponding to the pixel value so that the larger the pixel value, the higher the probability. When it is determined that dot placement is to be performed, when it is determined that a halftone dot having an occupation ratio corresponding to the pixel value Pth is placed in the unit area and that halftone dot placement is not performed. Converting the unit area into an empty area without halftone dots;
Is executed by a computer.

(13) According to a thirteenth aspect of the present invention, in the image conversion method to an image composed of halftone dots according to the eleventh or twelfth aspect described above,
For pixels having a pixel value P less than Pth, it is determined that halftone dot placement is to be performed with a probability of P / Pth.

(14) A fourteenth aspect of the present invention is a halftone image in which an original image composed of a set of pixels having pixel values in the range of Pmin to Pmax is represented as a set of halftone dots each having a predetermined area. In the image conversion method of converting to
An original image input stage for inputting the original image;
A halftone dot position defining step for defining a halftone dot placement position for placing individual halftone dots;
A threshold value setting stage for setting a predetermined threshold value Pth (where Pmin <Pth <Pmax);
For each halftone dot placement position, a halftone dot placement stage for placing a halftone dot having a predetermined area as necessary,
A halftone image output stage for outputting an image composed of a set of arranged halftone dots as a halftone image;
To the computer,
The halftone dot placement stage for one halftone dot placement position C is:
Recognizing a pixel value P on the original image corresponding to the halftone dot arrangement position C;
Comparing the pixel value P with a threshold value Pth;
As a result of comparison, when P ≧ Pth, a step of arranging a halftone dot having an area corresponding to the pixel value P at the halftone dot arrangement position C;
If P <Pth as a result of the comparison, it is determined whether or not halftone dot placement is to be performed with a probability of P / Pth, and if it is determined that halftone dot placement is to be performed, the threshold value Pth is determined. Arranging a halftone dot having an area corresponding to the halftone dot arrangement position C;
It is made up by.

(15) A fifteenth aspect of the present invention is a halftone image in which an original image composed of a set of pixels having pixel values in the range of Pmin to Pmax is represented as a set of halftone dots each having a predetermined area. In the image conversion method of converting to
An original image input stage for inputting the original image;
A unit region definition stage for defining a unit region for arranging individual halftone dots;
A threshold value setting stage for setting a predetermined threshold value Pth (where Pmin <Pth <Pmax);
For each unit region, a halftone dot placement stage for placing halftone dots having a predetermined area as necessary,
A halftone image output stage for outputting an image composed of a set of arranged halftone dots as a halftone image;
To the computer,
The halftone dot placement stage for one unit region U is
Recognizing a pixel value P on the original image corresponding to the unit area U;
Comparing the pixel value P with a threshold value Pth;
As a result of the comparison, when P ≧ Pth, a step of arranging a halftone dot having an occupation ratio corresponding to the pixel value P in the unit region U;
If P <Pth as a result of the comparison, it is determined whether or not halftone dot placement is to be performed with a probability of P / Pth, and if it is determined that halftone dot placement is to be performed, the threshold value Pth is determined. Arranging a halftone dot having an occupation ratio according to the unit area U;
It is made up by.

(16) According to a sixteenth aspect of the present invention, in the image conversion method to an image composed of halftone dots according to the fifteenth aspect described above,
At the unit region definition stage, generating points are arranged at random pitch on the 2D plane, and for each point on the 2D plane, the closest generating point is defined as the closest generating point. A two-dimensional plane is divided into a plurality of unit areas so that one unit area is constituted by a set of points whose points are nearest neighbors, and indefinite unit areas having different shapes and sizes are defined. It is what I did.

  (17) According to a seventeenth aspect of the present invention, there is provided a program for causing a computer to execute the image conversion method to an image composed of halftone dots according to the above-described eleventh to sixteenth aspects, and the program is read by a computer. It can be recorded on a possible recording medium and distributed.

(18) According to an eighteenth aspect of the present invention, a halftone image in which an original image composed of a set of pixels having pixel values in the range of Pmin to Pmax is expressed as a set of halftone dots each having a predetermined area. In the image conversion device for converting to
An original image input unit for inputting an original image;
A halftone dot placement position defining section for defining halftone dot placement positions for placing individual halftone dots;
A threshold value setting unit for setting a predetermined threshold value Pth (where Pmin <Pth <Pmax);
For each halftone dot placement position, a halftone dot placement section for placing a halftone dot having a predetermined area,
A halftone image output unit for outputting an image composed of a set of arranged halftone dots as a halftone image;
Provided,
When the halftone dot placement unit places a halftone dot at one halftone dot placement position C,
Processing for recognizing the pixel value P on the original image corresponding to the halftone dot arrangement position C;
A process of comparing the pixel value P with a threshold value Pth;
As a result of comparison, when P ≧ Pth, a process of arranging a halftone dot having an area corresponding to the pixel value P at the halftone dot arrangement position C;
If P <Pth as a result of the comparison, it is determined whether or not halftone dot placement is to be performed with a probability of P / Pth, and if it is determined that halftone dot placement is to be performed, the threshold value Pth is determined. A process of arranging a halftone dot having an area corresponding to the halftone dot arrangement position C;
Is to do.

(19) According to a nineteenth aspect of the present invention, a halftone image in which an original image composed of a set of pixels having pixel values in the range of Pmin to Pmax is represented as a set of halftone dots each having a predetermined area. In the image conversion device for converting to
An original image input unit for inputting an original image;
A unit area definition section for defining a unit area for arranging each halftone dot;
A threshold value setting unit for setting a predetermined threshold value Pth (where Pmin <Pth <Pmax);
For each unit region, a halftone dot placement unit for placing halftone dots having a predetermined area,
A halftone image output unit for outputting an image composed of a set of arranged halftone dots as a halftone image;
Provided,
When the halftone dot placement unit places halftone dots in one unit region U,
Processing for recognizing the pixel value P on the original image corresponding to the unit region U;
A process of comparing the pixel value P with a threshold value Pth;
As a result of the comparison, when P ≧ Pth, a process of arranging a halftone dot having an occupation ratio according to the pixel value P in the unit region U;
If P <Pth as a result of the comparison, it is determined whether or not halftone dot placement is to be performed with a probability of P / Pth, and if it is determined that halftone dot placement is to be performed, the threshold value Pth is determined. A process of arranging a halftone dot having an occupation ratio according to the unit area U;
Is to do.

(20) According to a twentieth aspect of the present invention, in the image conversion device to an image composed of halftone dots according to the nineteenth aspect described above,
The unit region definition unit places the generating points at a random pitch on the two-dimensional plane, and for each point on the two-dimensional plane, defines the generating point that is closest to each other as the closest generating point. It has a function to execute a process of dividing a two-dimensional plane into a plurality of unit regions so that one unit region is constituted by a set of points having points as nearest neighbors, and the shape and size are different from each other An undefined unit area is defined.

  According to the present invention, the AM screening process is performed on the high density part of the original image, and the FM screening process is performed on the low density part, so that the AM screening process and the FM screening process are selectively used. Since a halftone image consisting of dots is obtained, a halftone image with good image quality can be obtained for all regions from the high density portion to the low density portion of the original image.

  Hereinafter, the present invention will be described based on the illustrated embodiments.

<<< §1. General screening process >>>
Here, first, in order to facilitate understanding of the invention, a basic concept of general screening processing will be briefly described. As already described, the screening process is an arithmetic process for converting an original image having continuous tone into a halftone image. FIG. 1 is a block diagram showing a basic concept of a general screening process. As illustrated, the image conversion apparatus 100 is an apparatus that should be called a screening processing apparatus that performs a screening process, and has a function of converting a given original image 10 (continuous gradation image) into a halftone image 20 (pseudo gradation image). Have Both the original image 10 and the halftone image 20 are data composed of a two-dimensional pixel array, and each of the original image 10 and the halftone image 20 includes a set of pixels each having a predetermined pixel value (image density). However, while the pixels constituting the original image 10 are defined with pixel values within a predetermined range necessary for expressing multi-level gradation, the pixels constituting the halftone image 20 are One of the binary pixel values will be defined.

  For example, in the case of the original image 10 having an 8-bit gradation value, any pixel value within the range of 0 to 255 is defined for each pixel, but the pixels constituting the halftone image 20 are It has only one pixel value of two values indicating whether ink is deposited or not deposited on the final print. The binary information of the pixels constituting the halftone image 20 is represented by 0 or 1 bits on the computer. Here, for convenience of explanation, black (ink ink) is used instead of 0 or 1. Will be described using pixel values of white (no ink).

  FIG. 2 is a principle diagram of a general AM screening process performed by the image conversion apparatus 100. Here, for convenience of explanation, a process of converting a monochrome original image 11 composed of four pixels into a halftone image 21 is shown. In the illustrated example, the original image 11 is composed of four pixels P1 to P4 having an 8-bit pixel value in the range of 0 to 255. Here, the pixel P1 has a pixel value 255, the pixel P2 has a pixel value 120, the pixel P3 has a pixel value 30, and the pixel P4 has a pixel value 0. The pixel P1 having the maximum pixel value 255 shows the highest density, and the pixel P4 having the lowest pixel value 0 shows the lowest density (in the figure, for convenience, the density of each pixel constituting the original image 11 is hatched density) ).

  On the other hand, the halftone image 21 is composed of a plurality of halftone dots. In the example shown in the drawing, 16 meshes made up of a 4 × 4 matrix are prepared as the dot arrangement position C, and each halftone dot is arranged at the center of each cell. In the example shown in FIG. 2, since the relationship between the pixel pitch L1 of the original image 11 and the halftone dot pitch L2 of the halftone image 21 is L1 = 2 × L2, one pixel on the original image 11 is , Corresponding to the four dot arrangement positions C on the halftone image 21. For example, the pixel P1 on the original image 11 is represented on the halftone image 21 by four halftone dots D1 arranged at the upper left.

  A characteristic of the AM screening process is that one pixel on the original image is expressed by a halftone dot having an area corresponding to the pixel value. In the example illustrated in FIG. 2, the pixel P1 having the pixel value 255 is represented by four halftone dots D1, and the pixel P2 having the pixel value 120 is represented by four halftone dots D2, and the pixel value 30 Is represented by four halftone dots D3. The pixel P4 having the pixel value 0 is theoretically expressed by four halftone dots D4. However, since the halftone dot D4 is a halftone dot having an area of 0, substantially, The pixel P4 is expressed as an empty area where there is no halftone dot.

  A table 15 showing pixel values of the original image described in the lower part of FIG. 2 is an original corresponding to each halftone dot arrangement position C (position of individual cells arranged in 4 × 4) in the halftone image 21. The pixel value on the image is shown. As described above, in the illustrated example, since one pixel corresponds to four halftone dot arrangement positions, the same pixel value is arranged in four adjacent cells in the table 15. The area of each halftone dot arranged on the halftone image 21 corresponds to each pixel value on the table 15. That is, as the pixel value on the table 15 is larger, a halftone dot having a larger area is arranged.

  For example, when the pixel value and the halftone dot area are set to have a linear relationship, if the area of the halftone dot D1 on the halftone image 21 is 1, the area of the halftone dot D2 is 120/255 and the halftone dot D3. The area is 30/255, and the area of the halftone dot D4 is 0/255. However, in practice, when ink is transferred onto paper, halftone dots with the expected area are not always formed. Therefore, in practice, pixel values and pixel values are considered in consideration of various correction factors. In many cases, the relationship with the dot area is set nonlinearly.

  In the example shown in FIG. 2, the halftone dot D1 corresponding to the pixel P1 having the maximum pixel value 255 is not large in the mesh of the halftone dot arrangement position C, but is sized so that a slight gap can be secured around it. However, this is because the convenience for performing the gravure printing is taken into consideration. In gravure printing, a gravure cell composed of physical depressions is formed on a plate surface, ink is filled into the gravure cell, and the filled ink is transferred to the paper side. Accordingly, each halftone dot formed on the paper surface corresponds to each gravure cell on the plate surface, and it is necessary to secure a so-called bank area between adjacent gravure cells. The reason why a slight gap is secured around the halftone dot D1 shown in FIG. 2 is to form a region that becomes the bank.

  Of course, when there is no need to secure a bank area such as offset printing or letterpress printing, the halftone dot D1 corresponding to the pixel P1 having the maximum pixel value 255 is formed to fill the halftone dot arrangement position C. It doesn't matter. In this case, adjacently arranged halftone dots D1 are in contact with each other to form a fused region. In the case of gravure printing, the halftone dots D1 to D3 shown in FIG. 2 are generally called “cells”. However, in this specification, “cells” in such gravure printing are also referred to as “halftone dots”. I will call it. In other words, the term “halftone dot” in the present application includes “cells” in gravure printing.

  On the other hand, FIG. 3 is a principle diagram of a general FM screening process performed by the image conversion apparatus 100, and a monochrome original image 11 consisting of four pixels is converted into a halftone image 22 as in the example shown in FIG. The process to do is shown. In the halftone image 22 as well, 16 meshes made of a 4 × 4 matrix are prepared as the dot arrangement position C, and each halftone dot is arranged at the center of each cell. Therefore, one pixel on the original image 11 corresponds to four halftone dot arrangement positions C on the halftone image 22. For example, the pixel P1 on the original image 11 is arranged on the upper left on the halftone image 22. This is expressed by the four halftone dots D1.

  A feature of the FM screening process is that one pixel on the original image is expressed by halftone dots arranged at a density corresponding to the pixel value. In the example shown in FIG. 3, each halftone dot D1 constituting the halftone image 22 is a halftone dot having the same area, and the halftone dot D1 (maximum pixel value 255) on the halftone image 21 shown in FIG. Halftone dot having an area corresponding to). However, a halftone dot is not necessarily arranged at each halftone dot arrangement position C on the halftone image 22. In the illustrated example, the pixel P1 having the pixel value 255 is represented by four halftone dots, but the pixel P2 having the pixel value 120 is represented by two halftone dots and has a pixel value of 30. P3 is represented by one halftone dot, and a pixel P4 having a pixel value of 0 is represented by zero halftone dots.

  Similar to the table 15 shown in FIG. 2, the table 15 indicating the pixel values of the original image described in the lower part of FIG. 3 is a pixel on the original image corresponding to each halftone dot arrangement position C in the halftone image 22. The value is shown. The arrangement density of each halftone dot arranged on the halftone image 22 corresponds to each pixel value on the table 15. That is, as the pixel value on the table 15 is larger, halftone dots are arranged with a larger arrangement density.

  For example, when the pixel value and the halftone dot arrangement density are set to have a linear relationship, if a halftone dot is arranged at a density of 1 in the area corresponding to the pixel P1, the density corresponding to the pixel P2 is 120 / At 255, a halftone dot D1 having the same size may be arranged at a density of 30/255 in the region corresponding to the pixel P3 and at a density of 0/255 in the region corresponding to the pixel P4. However, as described above, in practice, the relationship between the pixel value and the halftone dot arrangement density is often set nonlinearly in consideration of various correction factors.

  Eventually, when FM screening processing is performed, the decision of whether or not to place a halftone dot at each halftone dot placement position C is made, but in a process using a computer, This determination can be made by probability calculation using random numbers. For example, in the case of the example shown in FIG. 3, with reference to individual pixel values shown on the table 15 indicating the pixel values of the original image, the probability of arranging halftone dots is (reference pixel value / maximum pixel value). As such, an alternative decision may be made. In this example, since the maximum pixel value is 255, in the table 15 of FIG. 3, it is determined that a halftone dot is placed at a halftone dot placement position to which the pixel value 255 is assigned with a probability of 255/255. As a result, halftone dots are arranged at all four locations. On the other hand, a halftone dot arrangement position to which the pixel value 120 is assigned is assigned a pixel value of 0 with a probability of 120/255, and a halftone dot arrangement position to which the pixel value of 30 is assigned is assigned a pixel value of 0 with a probability of 30/255. In the example shown in the figure, halftone dots are placed at two locations, one location, and zero location, respectively, with a probability of 0/255. Has been.

  The basic principles of the conventional general AM screening process and FM screening process have been described above. As described above, these conventional screening processes have advantages and disadvantages. That is, the AM screening process has a problem that the ink in the low density region is not sufficiently transferred to the paper in the actual printing process. For example, in the example shown in FIG. 2, the pixel P3 (pixel value 30) in the low density region on the original image 11 is represented by four halftone dots D3 on the halftone image 21, Since the area of the halftone dot D3 is smaller than the area of the halftone dots D1 and D2, there is a possibility that the transfer of the ink onto the paper surface becomes insufficient. In particular, in the case of gravure printing using ink containing a bright pigment with a large particle size, the bright pigment is not filled well into the cell or clogged in the cell, and it is fully transferred to the paper side. It is easy to happen. On the other hand, the FM screening process has a problem that the spatial resolution of an image decreases as the distribution density of halftone dots decreases. For example, in the case of the example shown in FIG. 3, in the lower half area of the halftone image 22, a lot of blank portions in which no halftone dots are arranged are conspicuous, which causes a graininess during observation.

<<< §2. Principle of screening process according to the present invention >>
The screening process according to the present invention combines the above-described AM screening process and FM screening process into a halftone image having good image quality for all regions from the high density part to the low density part of the original image. It is intended to be converted. FIG. 4 is a principle diagram showing the basic concept of the screening process according to the present invention. The basic concept of the present invention is to apply AM screening processing to the high density portion of the original image and FM screening processing to the low density portion.

  As shown in FIG. 4, the original image is composed of a set of pixels having pixel values within the range of the minimum pixel value Pmin to the maximum pixel value Pmax. Therefore, a predetermined threshold value Pth is determined so as to satisfy the condition of Pmin <Pth <Pmax, and the position of a pixel having a pixel value equal to or greater than Pth (high density portion of the original image) depends on the pixel value. A halftone dot having a predetermined area is arranged (corresponding to the AM screening process), and the position of a pixel having a pixel value less than Pth (low density portion of the original image) has a predetermined area with a probability corresponding to the pixel value. Is arranged (corresponding to FM screening process).

  FIG. 5 is a principle diagram showing an example of the AM / FM mixed screening process according to the present invention. In this example, a process of converting a monochrome original image 12 composed of 9 pixels into a halftone image 23 is shown. In the illustrated example, the original image 12 is composed of nine pixels P1 to P9 having 8-bit pixel values in the range of 0 to 255.

  On the other hand, the halftone image 23 is composed of a plurality of halftone dots. In the example shown in the drawing, as the halftone dot arrangement position C, 36 squares made of a 6 × 6 matrix are prepared, and each halftone dot is arranged at the center of each square. In the example shown in FIG. 5 as well, the relationship between the pixel pitch L1 of the original image 12 and the halftone dot pitch L2 of the halftone image 23 is L1 = 2 × L2, as in the examples shown so far. One pixel on the original image 12 corresponds to four halftone dot arrangement positions C on the halftone image 23.

  The table 16 showing the pixel values of the original image shown in the lower part of FIG. 5 is an original corresponding to each halftone dot arrangement position C (the position of each cell arranged in 6 × 6) in the halftone image 23. The pixel value on the image is shown. As shown in the figure, the pixel values of 255, 180, 120, 90, 60, 45, 30, 15, 0 are assigned to the pixels P1 to P9 constituting the original image 12, and the minimum pixels constituting the original image The value Pmin = 0 and the maximum pixel value Pmax = 255.

  In the example shown in FIG. 5, the threshold value Pth is set to Pth = 60. Therefore, according to the basic principle of the present invention described above, the AM screening process is applied to the pixels P1 to P5 having the pixel value equal to or greater than the threshold value Pth among the pixels constituting the original image 12, and the individual pixels. A halftone dot having an area corresponding to the pixel value of is arranged. That is, the pixels P1 to P5 having a pixel value equal to or greater than the threshold value Pth are pixels belonging to the high density portion shown in FIG. 4, and the area corresponding to the pixel value is larger as the pixel value is larger. It is expressed by arranging halftone dots with.

  On the other hand, for the pixels P6 to P9 whose pixel values are less than the threshold value Pth, FM screening processing is applied, and halftone dots having a predetermined area are arranged with a probability corresponding to the pixel value of each pixel. Will be. That is, the pixels P6 to P9 having a pixel value less than the threshold value Pth are pixels belonging to the low density portion shown in FIG. 4, and the probability corresponding to the pixel value is larger as the pixel value is larger. This is expressed by arranging a halftone dot having a predetermined area (specifically, a halftone dot having the same area as the halftone dot to be arranged at the pixel position having the pixel value Pth).

  Looking at the halftone image 23 shown in FIG. 5, halftone dots are arranged at all the halftone dot arrangement positions corresponding to the pixels P <b> 1 to P <b> 5 corresponding to the high density portion of the original image 12. I understand that. This is because the arrangement density of halftone dots is always 100% because the AM screening process is applied in the high density portion. However, the areas of halftone dots are different. As shown in the figure, each of a halftone dot D11 corresponding to the pixel P1, a halftone dot D12 corresponding to the pixel P2, a halftone dot D13 corresponding to the pixel P3, a halftone dot D14 corresponding to the pixel P4, and a halftone dot D15 corresponding to the pixel P5. The area is gradually getting smaller. This is because the area of each halftone dot corresponds to the pixel value of the corresponding pixel.

  For example, when the pixel value and the halftone dot area are set to have a linear relationship, assuming that the area of the halftone dot D11 is 1, the area of the halftone dot D12 is 180/255, and the area of the halftone dot D13 is 120/255. The area of the halftone dot D14 may be 90/255, and the area of the halftone dot D15 may be 60/255. Of course, as described in §1, the relationship between the pixel value and the halftone dot area is not necessarily linear, and the relationship in which the halftone dot area decreases as the pixel value decreases is satisfied. If it is enough.

  On the other hand, halftone dots having the same area are arranged with a predetermined probability at the halftone dot arrangement positions corresponding to the pixels P6 to P9 corresponding to the low density portion of the original image 12. That is, the halftone dots D16, D17, D18, and D19 that are arranged are all halftone dots having the same area as the halftone dot D15. The pixels are arranged with a probability corresponding to the pixel value of the pixel.

  Specifically, with regard to the halftone dot arrangement position corresponding to the pixel P6 having the pixel value 45, the halftone dot D16 is arranged only in three of the four places, and the pixel P7 having the pixel value 30 is arranged. As for the corresponding halftone dot arrangement position, the halftone dot D17 is arranged only in two of the four places, and the halftone dot arrangement position corresponding to the pixel P8 having the pixel value 15 is out of the four places. The halftone dot D18 is arranged only at one location, and the halftone dot D19 is arranged only at 0 out of 4 locations corresponding to the pixel P9 having the pixel value 0.

  In order to perform such a halftone dot arrangement, a halftone dot to be arranged at a pixel position having the pixel value Pth at a pixel position having a pixel value P less than the threshold value Pth with a probability of P / Pth. A halftone dot having the same area may be arranged. Then, regarding the low density portion, the relationship between the pixel value and the dot density is a linear relationship. However, the relationship between the pixel value and the halftone dot density does not necessarily have to be a linear relationship, and it is sufficient if the relationship that the halftone dot density decreases as the pixel value decreases is satisfied.

  The halftone image 23 thus obtained can be said to be an image in which the density is expressed by the area of halftone dots for the high density portion and the density is expressed by the density of the halftone dots for the low density portion. More specifically, this image includes a high density portion (a region corresponding to the pixels P1 to P5 in the example of FIG. 5) and a low density portion (a region corresponding to the pixels P6 to P9 in the example of FIG. 5). An image is formed by two types of areas, and in the high density portion, halftone dots D11 having a maximum area to halftone dots D15 having a minimum area are arranged, and the density is expressed by the area of each halftone dot. In the low density portion, halftone dots D16 to D19 having the minimum area are arranged, and the density is expressed by the density of each halftone dot.

  Now, when the halftone image 23 shown in FIG. 5 is observed, it can be understood that the grayscale information on the original image 12 is reproduced as it is. That is, the density of the pixels on the original image 12 gradually decreases as it goes to the pixels P1 to P9, but the portion of the pixels P1 to P5, which is the high density portion, is a halftone image 23 on the halftone image 23. This is expressed by points D11 to D15, and a decrease in density is expressed as the area of the halftone dots gradually decreases. On the other hand, the portions of the pixels P6 to P9 which are low density portions are represented by halftone dots D16 to D19 on the halftone image 23, and the density decreases as the density of the halftone dots gradually decreases. Is expressed.

  Here, paying attention to the size of the halftone dot arranged on the halftone image 23, the halftone dot having the smallest area is a halftone dot having a size equivalent to the halftone dot D15 corresponding to the threshold value Pth. Yes, no smaller halftone dots are used. This is a feature not found in the conventional AM screening process shown in FIG. That is, a small halftone dot such as the halftone dot D3 on the halftone image 21 shown in FIG. 2 is not seen on the halftone image 23 shown in FIG. This means that the present invention solves the problem of the conventional AM screening process that “if there is a small halftone dot, ink transfer failure occurs”.

  In particular, when gravure printing is performed using ink containing a bright pigment having a relatively large particle diameter, the diameter of the halftone dot D15 (gravure cell) to be arranged at the pixel position having the pixel value Pth is: If it is set to be sufficiently larger than the average particle diameter of this glittering pigment, the glittering pigment will not be filled well into the cell or clogged in the cell, and it will be transferred sufficiently to the paper side. You can prevent the situation that is not.

  On the other hand, when attention is paid to the spatial resolution of halftone dots arranged on the halftone image 23, the arrangement density of halftone dots cannot be denied in the low density portion, but in the high density portion, the individual halftone dot arrangement is reduced. The arrangement probability of halftone dots with respect to the position is 100%, and a sufficient spatial resolution is ensured. This means that the problem of the conventional FM screening process that “the spatial resolution of the image is reduced” is alleviated by the present invention.

<<< §3. Image conversion method and image conversion apparatus according to the present invention >>
FIG. 6 is a flowchart showing the procedure of the image conversion method according to the present invention. This procedure is based on the principle described in §2 and represents an original image composed of a set of pixels having pixel values in the range of Pmin to Pmax, and is represented as a set of halftone dots each having a predetermined area. This is an image conversion procedure for converting to a tone image, and is actually executed by a computer.

  First, in step S1, an original image composed of a set of pixels having pixel values within the range of Pmin to Pmax is input to the computer. Subsequently, in step S2, the halftone dot placement position is defined. In the examples described so far, the halftone dot placement position has been described as a grid of the matrix where the halftone dot is to be placed. However, in practice, this halftone dot placement position is defined by setting parameters such as the number of lines and the angle. Is done by. The number of lines is a parameter indicating the number of halftone dot arrangement positions per unit length, and the angle is a parameter indicating the slope of the matrix constituting the halftone dot arrangement positions described above. Since the method of setting these parameters and defining the halftone dot arrangement position is already a well-known matter, detailed description thereof is omitted here.

  In the subsequent step S3, a predetermined threshold value Pth indicating the boundary between the high density part and the low density part is set. Here, as the threshold value Pth, any value may be set as long as it satisfies the condition of Pmin <Pth <Pmax. However, in practice, there is no problem in transferring the ink. A pixel value corresponding to a halftone dot having a minimum area is preferably set as the threshold value Pth. By doing so, it is possible to expand as much as possible the high-density portion that is the target of the AM screening process that can ensure a sufficient spatial resolution.

  The above steps S1 to S3 are procedures that should be called the preparation stage of the conversion process, and the actual image conversion process is executed by the following steps S4 to S11. The purpose of these procedures is to convert a pixel having a pixel value equal to or greater than the threshold value Pth into a halftone dot having an area corresponding to the pixel value so that the larger the pixel value, the larger the area. For a pixel having a pixel value less than the threshold value Pth, it is determined whether or not to perform halftone dot arrangement with a probability corresponding to the pixel value so that the larger the pixel value, the higher the probability. If it is determined to perform the conversion, the pixel is converted into a halftone dot having the same area as the halftone dot to be converted for the pixel having the pixel value Pth, and the halftone dot arrangement is not performed. Is determined, the pixel is converted into an empty area without a halftone dot. Actually, for a pixel having a pixel value P less than the threshold value Pth, it may be determined that halftone dot arrangement is performed with a probability of P / Pth.

  First, in step S4, a process of recognizing a pixel value P (x, y) on the original image corresponding to the halftone dot arrangement position C (x, y) is performed. Here, the halftone dot arrangement position C (x, y) indicates a halftone dot arrangement position indicated by the X coordinate x and the Y coordinate y in the XY coordinate system. For example, in the example shown in FIG. 5, one square on the halftone image 23 is shown, and the pixel value P on the original image corresponding to the halftone dot arrangement position C in the first row and first column is 255. Become. In step S5, the recognized pixel value P (x, y) is compared with the threshold value Pth.

  If the recognized pixel value P (x, y) is equal to or larger than the threshold value Pth, the process proceeds from step S5 to step S6, and a halftone dot (A) having an area corresponding to the pixel value P (x, y) ( For example, in the example shown in FIG. 5, a process of arranging the halftone dot D11) at the halftone dot arrangement position C (x, y) is performed.

  On the other hand, if the recognized pixel value P (x, y) is less than the threshold value Pth, the process proceeds from step S5 to step S7, and first, a uniformly distributed random number R within a range of 0 ≦ R ≦ Pth. Is generated. In step S8, the generated random number R is compared with the pixel value P (x, y). If R <P (x, y), the process branches to step S9, where R ≧ P (x, y If y), the process branches to step S10. The determination in step S8 is to determine whether or not to perform halftone dot placement with a probability of P (x, y) / Pth. That is, when the random number R that satisfies the above conditions is generated, the probability of branching from step S8 to step S9 is P (x, y) / Pth.

  Step S9 is processing when it is determined that the halftone dot arrangement is performed. In this case, a halftone dot having an area corresponding to the threshold value Pth (halftone dot D15 in the case of the example shown in FIG. 5). Is placed at the halftone dot placement position C (x, y). On the other hand, step S10 is a process performed when it is determined that the halftone dot placement is not performed. In this case, the halftone dot placement is not performed at the halftone dot placement position C (x, y), and a blank area is obtained. It becomes.

  In step S11, the above-described processing from step S4 is repeatedly executed until all the halftone dot arrangement positions are completed. Then, in the last step S12, a process of outputting an image made up of a set of arranged halftone dots as a halftone image is executed.

  FIG. 7 is a block diagram showing the basic configuration of the image conversion apparatus 100 according to the present invention. This apparatus 100 is an apparatus for executing the procedure shown in the flowchart of FIG. 6, and an original image 10 composed of a set of pixels having pixel values in the range of Pmin to Pmax is converted into a network having a predetermined area. It has a function of converting to a halftone image 20 expressed as a set of points.

  As illustrated, the image conversion apparatus 100 includes an original image input unit 110 that inputs an original image 10, a halftone dot placement position definition unit 120 that defines a halftone dot placement position for placing each halftone dot, A threshold value setting unit 130 for setting a threshold value Pth (where Pmin <Pth <Pmax), a halftone dot placement unit 140 for placing halftone dots having a predetermined area for each halftone dot placement position, And a halftone image output unit 150 that outputs an image composed of a set of arranged halftone dots as a halftone image 20. However, in practice, each of these components is configured by incorporating a predetermined program into a computer, and this image conversion apparatus 100 is configured by a combination of computer hardware and software. Is.

  The original image input unit 110 is a component for executing step S1 of FIG. 6, the halftone dot position defining unit 120 is a component for executing step S2, and the threshold setting unit 130 is , A component for executing step S3. The halftone dot arrangement unit 140 is a component for executing steps S4 to S11 in FIG. 6, and the halftone image output unit 150 is a component for executing step S12.

<<< §4. Halftone image actually obtained >>>
Next, the result of the conventional general AM / FM screening process and the result of the screening process according to the present invention will be compared with the actually obtained halftone image. 8 is an enlarged view of a halftone image obtained by a conventional general AM screening process, FIG. 9 is an enlarged view of a halftone image obtained by a conventional general FM screening process, and FIG. It is an enlarged view of the halftone image obtained by the AM / FM mixed screening process according to the present invention. In any case, in the upper diagram (a), all pixel values P are 250, in the middle diagram (b), all pixel values P are 100, and in the lower diagram (c), all pixel values P are 50. This is a conversion result for an original image (so-called solid image).

  When the conventional AM screening process is performed, as shown in FIG. 8 (c), when the pixel value of the original image becomes small, the area of the halftone dot also becomes small accordingly, which may cause an ink transfer failure or the like. is there.

  On the other hand, when the conventional FM screening process is performed, as shown in FIGS. 9A to 9C, unless the original image has the maximum pixel value, a halftone dot missing region (no halftone dot is arranged). Area) occurs, the spatial frequency is lowered, and the image quality is lowered, such as giving a grainy feeling during observation.

  On the other hand, when the AM / FM mixed screening process according to the present invention is performed, these problems can be solved as shown in FIGS. 10 (a) to (c). That is, the halftone dots shown in FIG. 10 (c) have the same area as the halftone dots shown in FIG. 10 (b), and there is little possibility of poor ink transfer in the low density portion. In the halftone images shown in FIGS. 10 (a) and 10 (b), there are no halftone dot areas, and a sufficient spatial frequency can be secured at least for these density areas. Degradation of image quality can be prevented.

<<< §5. Embodiments and modifications in which halftone dot arrangement is irregular >>
In the embodiments described so far, the halftone dot arrangement positions are regularly defined, so that the halftone dots on the created halftone image have periodicity. Of course, with respect to the low concentration portion to which FM screening is applied, since there is a region in which halftone dots are not arranged with a predetermined probability, the periodicity of each halftone dot is considerably lost, but high concentration to which AM screening is applied. As for the portion, since the halftone dots are always arranged at a constant pitch, it has complete periodicity. For example, in the halftone image 23 shown on the right side of FIG. 5, since the meshes at which the halftone dots are arranged are arranged at a constant pitch L2, the halftone dots D11 to D15 to which AM screening is applied have the pitch L2. It will take a periodic arrangement. The halftone dots D16 to D18 to which FM screening is applied do not have complete periodicity, but some periodicity still remains. Such periodicity of halftone dots causes generation of moire fringes and rosette patterns, and in the case of a continuous gradation pattern, it also causes generation of density jump called so-called jumping.

  In order to eliminate such an adverse effect, the present inventor has disclosed a novel screening technique for making the dot arrangement irregular in Japanese Patent Application No. 2004-155653 and Japanese Patent Application No. 2004-254037. The printing method and the image conversion method according to the present invention can be used in combination with the novel screening technique disclosed in the above specification. Therefore, an embodiment in which both are used together will be described below.

  FIG. 11 is a plan view in which halftone image 23 shown in the right side of FIG. 5 has a halftone dot arrangement position C (mesh) and halftone dots extracted, and a mother point M is displayed at the center of each mesh. Each halftone dot arrangement position C is literally a grid for arranging halftone dots, and its substance is a rectangular closed region. Therefore, here, the halftone dot arrangement position C is referred to as a unit region U. Each unit region U is configured by a square centered on the generating point M. Therefore, FIG. 11 can be said to be a plan view showing an example in which unit areas U for halftone dot arrangement are defined based on generating points M arranged at a constant pitch L2. As described above, since the unit region U is defined based on the regularly arranged generating points M and the halftone dots are arranged in the unit region U, periodicity appears in the halftone dot arrangement. Is inevitable.

  On the other hand, FIG. 12 is a plan view showing an example in which unit areas U for dot arrangement are defined based on generating points M arranged at a random pitch on a two-dimensional plane. As described above, the unit areas U defined based on the randomly arranged generating points are indefinite areas having different shapes and sizes. Therefore, the halftone dots arranged in each unit area U are also irregular, and the halftone dot arrangement is also irregular. In FIG. 12, as an example, a halftone dot D21 arranged in the unit area U21 and a halftone dot D22 arranged in the unit area U22 are shown.

  Now, in order to define the irregular unit region U as shown in FIG. 12, the following procedure may be performed. First, the generating points M are arranged at a random pitch on a two-dimensional plane. For this purpose, for example, random numbers x and y are generated using a computer, and the mother point M may be plotted at a position where the generated random number (x, y) is a coordinate value. However, if the position of the generating point M is completely random, the area of the unit region U defined on the basis of the generating point M may become too large, which may lead to undesirable results in practice. is there. Therefore, as a more preferable arrangement method of the mother point M, the following method proposes the following method.

  First, as shown in FIG. 11, generating points M are arranged at a constant pitch. In FIG. 11, the generating points M are arranged in a square lattice shape. However, in practice, it is preferable to arrange the generating points M at the center position of a regular hexagon that is laid without gaps on a two-dimensional plane. Subsequently, the individual mother points M are moved randomly. The movement is performed for each individual mother point M in a random direction by a random distance. Specifically, the moving direction and moving distance for each mother point M may be determined based on random numbers generated using a computer. At this time, if the upper limit of the movement distance is determined, it is possible to leave some regularity in the arrangement of each mother point even after the movement process for all the mother points M is completed. In addition, favorable results are obtained.

  If the generating points M arranged at a random pitch can be obtained in this way, subsequently, a process of determining the unit regions U based on the respective generating points M is performed. This process is a set of points where the nearest mother point is defined as the nearest mother point for each point on the two-dimensional plane where the mother point is placed, and the same mother point is the nearest mother point. Thus, a process of dividing the two-dimensional plane into a plurality of unit regions may be performed so that one unit region is configured. Such a process is a process generally known as “Voronoi division”. In other words, all points on the two-dimensional plane will belong to the power of one of the generating points, but the generating point of which generating point is closest to me. It will be put under the power of the mother point. Each unit area U obtained by this Voronoi division processing is an area indicating the power range of each individual mother point, and the boundary line between two adjacent unit areas is the respective mother area in the two unit areas. The position is equidistant from the point.

  When the unit area U as shown in FIG. 12 can be defined on the two-dimensional plane by the above procedure, the original image to be printed is superimposed on the two-dimensional plane, and the image corresponding to each mother point position is superimposed on the two-dimensional plane. The pixel value is defined as the pixel value of the base point. For example, the original image 12 shown on the left side of FIG. 5 is overlaid on the unit region group shown in FIG. 12, and the pixel value on the image corresponding to each mother point M is defined as the pixel value of the mother point M. Will be done. At this time, if necessary, an interpolation calculation using pixel values of a plurality of pixels adjacent to the position of each mother point M may be performed, and the obtained interpolation value may be used as the pixel value of the mother point. For example, in the example shown in FIG. 12, a total of 36 generating points M are defined, whereas the original image 12 shown on the left side of FIG. 5 is composed of a total of 9 pixels. As described above, when the resolution of the generating point is higher than the resolution of the image, an interpolation calculation may be performed between adjacent pixels to determine a pixel value for a specific generating point.

  Thereafter, as in the embodiments described so far, a predetermined threshold value Pth (where Pmin <Pth <Pmax) is defined, and the unit region to which the mother point having a pixel value equal to or greater than Pth belongs is assigned. The halftone dot arrangement based on the AM screening process may be performed, and the halftone dot arrangement based on the FM screening process may be performed on the unit region to which the mother point having a pixel value less than Pth belongs. Specifically, a unit area to which a mother point having a pixel value equal to or greater than Pth belongs has an occupancy ratio according to the pixel value so that the occupancy ratio for the unit area increases as the pixel value increases. In a unit region to which a base point having a pixel value less than Pth belongs is assigned, a probability corresponding to the pixel value is increased according to the pixel value so that the larger the pixel value is, the higher the probability is. A halftone dot having an occupied ratio may be arranged.

  For example, consider the case where the minimum pixel value Pmin = 0, the maximum pixel value Pmax = 255, the threshold value Pth = 60, and the maximum occupation ratio 90%. Here, the maximum occupancy 90% indicates that only halftone dots having an area corresponding to 90% of the area of the unit region can be allocated to each unit region at the maximum. The remaining 10% portion is a bank area formed between halftone dots (if the bank area is unnecessary, the maximum occupation ratio may be set to 100%). After all, in the case of the above setting, a halftone dot having a size of (90 × (P / 255))% of the area of the unit region is arranged in the unit region to which the mother point having a pixel value P of 60 or more belongs. do it. For a unit region to which a mother point having a pixel value P of less than 60 belongs, a halftone dot (90 × (60/255))% of the area of the unit region with a probability of P / 60 ( In other words, a halftone dot having an occupation rate corresponding to the threshold value Pth may be arranged.

  In this embodiment, the size of each halftone dot is determined not as an absolute value based on the pixel value on the original image, but as a relative value of the occupation ratio with respect to the unit area. Therefore, the minimum halftone dot area finally formed on the halftone image cannot be determined as an absolute value, and it is possible to completely suppress the generation of small halftone dots that may cause ink transfer failure. Can not. However, since the minimum value of the halftone dot area finally formed on the halftone image can be determined as a relative value (occupation ratio with respect to the unit area), the generation of small halftone dots that may cause ink transfer failure The effect of reducing can be sufficiently expected.

  Based on the principle described in §5, the procedure of an image conversion method for converting an original image into an image composed of halftone dots is almost the same as the method shown in the flowchart of FIG. However, indefinite unit regions having different shapes and sizes are defined as the halftone dot arrangement positions defined in step S2. Specifically, as described above, the mother points are arranged at a random pitch on the two-dimensional plane, and for each point on the two-dimensional plane, the closest mother point is defined as the closest mother point, By dividing a two-dimensional plane into a plurality of unit regions so that one unit region is constituted by a set of points having the same generating point as the nearest neighbor point, irregular shapes having different shapes and sizes from each other A unit area can be defined.

  In step S4, the pixel value P (x, y) on the original image corresponding to the predetermined unit region U (x, y) is recognized. In step S6, the pixel value P (x, y) is recognized. A process of arranging a halftone dot having a corresponding occupation rate in the unit area U (x, y) is performed. On the other hand, in step S9, a process of arranging a halftone dot having an occupation ratio according to the threshold value Pth in the unit area U (x, y) is performed.

  The configuration of the image conversion apparatus based on the principle described in §5 is almost the same as the configuration shown in the block diagram of FIG. A unit region definition section that can define different irregular unit regions is provided. For example, the unit region defining unit arranges generating points at a random pitch on a two-dimensional plane, and defines the closest generating point as the closest generating point for each point on the two-dimensional plane, Each unit region can be defined by executing a process of dividing the two-dimensional plane into a plurality of unit regions so that one unit region is constituted by a set of points having the same generating point as the closest generating point. That's fine. When the halftone dot arrangement unit 140 arranges halftone dots in one unit area U, the halftone dot arrangement unit 140 recognizes the pixel value P on the original image corresponding to the unit area U, and uses the pixel value P as a threshold value Pth. If P ≧ Pth as a result of comparison, a process of arranging a halftone dot having an occupation ratio corresponding to the pixel value P in the unit area U is performed. If P <Pth, It is determined whether or not halftone dot placement is to be performed with a probability of P / Pth, and when it is determined that halftone dot placement is to be performed, a halftone dot having an occupation ratio corresponding to the threshold value Pth is determined as the unit. The processing to be arranged in the area U may be executed.

  As mentioned above, although this invention was demonstrated based on embodiment shown in figure, this invention is not limited to these embodiment, In addition, it can implement in a various aspect.

  For example, in the above-described embodiment, only the handling of a monochrome original image has been described. However, the present invention can also be applied to a color original image. A general color print is configured by printing four color inks of CMYK in a superimposed manner. In this case, the processing based on the above-described embodiment may be executed for each individual color. In order to prevent the occurrence of moiré, etc., when defining the halftone dot arrangement position in step S2 in FIG. 6, it is common to define a different halftone dot arrangement position for each color.

  In the embodiment shown in FIG. 5, the halftone dot placement position is defined so that the relationship between the pixel pitch L1 of the original image 12 and the halftone dot pitch L2 of the halftone image 23 satisfies L1> L2. The halftone image 23 in which one pixel on the original image corresponds to a plurality of halftone dots is obtained. Of course, the definition of the halftone dot arrangement position is such that the relationship between both pitches is L1 = L2 or L1 <L2. It is also possible to perform. When the relationship between the two pitches is defined so that L1 <L2, one halftone dot corresponds to a plurality of pixels on the original image, and therefore, on the original image corresponding to one halftone dot arrangement position. As the pixel value of these pixels, an average value of pixel values of a plurality of pixels may be used.

  In the embodiment shown in FIG. 5, for a plurality of halftone dot arrangement positions where the corresponding pixels on the original image are the same, the pixel values for the same corresponding pixels are referred to in common. A handling that takes into account the pixel values of adjacent pixels is also possible. For example, for the pixel P5 located in the center portion of the original image 12, four halftone dot arrangement positions (mesh) correspond to the halftone image 23. Therefore, in the above-described embodiment, when each halftone dot is placed in each of these four cells, processing using the pixel value 60 of the pixel P5 is performed, and as a result, the same area is obtained in the four cells. A halftone dot D15 is arranged. Instead, among the four cells, the pixel value 255 of the diagonally adjacent pixel P1 is slightly taken into consideration for the upper left cell, and the pixel value 120 of the diagonally adjacent pixel P3 is slightly changed for the upper right cell. Taking into account, for the lower left cell, slightly consider the pixel value 30 of the diagonally adjacent pixel P7, and for the lower right cell, slightly consider the pixel value 0 of the diagonally adjacent pixel P9, A predetermined process may be executed. When handling in consideration of the pixel value of the adjacent pixel, weighting according to the distance to the adjacent pixel may be performed.

It is a block diagram which shows the basic concept of a general screening process. It is a principle figure showing an example of general AM screening processing. It is a principle figure showing an example of general FM screening processing. It is a principle figure which shows the basic concept of the AM / FM mixed screening process which concerns on this invention. It is a principle figure which shows an example of the AM / FM mixed screening process which concerns on this invention. It is a flowchart which shows the procedure of the image conversion method which concerns on this invention. 1 is a block diagram showing a basic configuration of an image conversion apparatus according to the present invention. It is an enlarged view of the halftone image obtained by the conventional general AM screening process. It is an enlarged view of the halftone image obtained by the conventional general FM screening process. It is an enlarged view of the halftone image obtained by the AM / FM mixed screening process according to the present invention. It is a top view which shows the example which defined the unit area | region U for halftone dot arrangement | positioning based on the mother point M arrange | positioned with a fixed pitch. It is a top view which shows the example which defined the unit area | region U for halftone dot arrangement | positioning based on the mother point M arrange | positioned with a random pitch.

Explanation of symbols

10 to 12 ... Original images 15 to 16 ... Tables 20 to 23 indicating pixel values of the original image ... Halftone image 100 ... Image conversion device (screening processing device)
110 ... Original image input unit 120 ... Halftone dot arrangement position definition unit 130 ... Threshold setting unit 140 ... Halftone dot arrangement unit 150 ... Halftone image output unit C ... Halftone dot arrangement position (cell)
C (x, y) ... halftone dot arrangement positions D1 to D22 existing at coordinates (x, y) ... halftone dots L1 to L2 ... pitch of pixels and halftone dots M ... mother points P1 to P9 ... pixels constituting the original image P (x, y) ... Pixel Pmin existing at coordinates (x, y) ... Minimum pixel value Pmax ... Maximum pixel value Pth ... Threshold values S1 to S12 ... Steps U, U21, U22 in the flowchart ... Unit region

Claims (20)

  An image printing method comprising halftone dots, wherein AM screening is applied to a high density portion of an image and FM screening is applied to a low density portion. In a printing method for expressing an image composed of a set of pixels having pixel values within a range of Pmin to Pmax by a large number of halftone dots,
A predetermined threshold value Pth (where Pmin <Pth <Pmax) is determined, and a halftone dot having an area corresponding to the pixel value is arranged at a pixel position having a pixel value equal to or greater than Pth, and less than Pth A method of printing an image composed of halftone dots, characterized in that a halftone dot having a predetermined area is arranged at a pixel position having a pixel value with a probability corresponding to the pixel value. In a printing method for expressing an image composed of a set of pixels having pixel values within a range of Pmin to Pmax by a large number of halftone dots,
A predetermined threshold value Pth (where Pmin <Pth <Pmax) is determined, and an area corresponding to the pixel value has a larger area at a pixel position having a pixel value equal to or greater than Pth as the pixel value increases. A pixel position having a pixel value Pth with a probability corresponding to the pixel value such that the larger the pixel value is, the larger the pixel value is. A method for printing an image composed of halftone dots, characterized in that halftone dots having the same area as the halftone dots to be arranged are arranged. The printing method according to claim 3,
A halftone dot having the same area as a halftone dot to be arranged at a pixel position having a pixel value Pth is arranged at a pixel position having a pixel value P less than Pth with a probability of P / Pth. A method for printing an image composed of halftone dots. The printing method according to claim 3 or 4,
Printing is performed using ink containing a glitter pigment, and the diameter of the halftone dot to be arranged at the pixel position having the pixel value Pth is set to be sufficiently larger than the average particle diameter of the glitter pigment. A method for printing an image composed of halftone dots. In a printing method for expressing an image composed of a set of pixels having pixel values within a range of Pmin to Pmax by a large number of halftone dots,
The generating points are arranged at a random pitch on the two-dimensional plane, and for each point on the two-dimensional plane, the closest generating point is defined as the closest generating point, and the same generating point is the closest generating point. The two-dimensional plane is divided into a plurality of unit regions so that one unit region is constituted by a set of points
The image is superimposed on the two-dimensional plane, and pixel values on the image corresponding to individual mother point positions are defined as pixel values of the mother points,
A predetermined threshold value Pth (where Pmin <Pth <Pmax) is determined, and in a unit region to which a mother point having a pixel value equal to or greater than Pth belongs, the occupation ratio with respect to the unit region increases as the pixel value increases. As described above, a halftone dot having an occupation ratio corresponding to the pixel value is arranged, and a unit area to which a mother point having a pixel value less than Pth belongs has a higher probability as the pixel value increases. A method for printing an image composed of halftone dots, characterized in that halftone dots having an occupation ratio corresponding to a pixel value Pth are arranged with a probability corresponding to the pixel value. The printing method according to claim 6, wherein
When defining the pixel value on the image corresponding to each mother point position as the pixel value of the mother point, it is obtained by performing an interpolation operation using pixel values of a plurality of pixels adjacent to the mother point position. A method for printing an image composed of halftone dots, wherein the interpolated value is used as the pixel value of the base point.   A printing material printed by the printing method according to claim 1 or a printing plate used for printing the printing material.   A printing plate or printed matter characterized in that the density is expressed by the area of halftone dots for the high density portion and the density is expressed by the density of the halftone dots for the low density portion.   An image is formed by two types of regions of a high density portion and a low density portion, and in the high density portion, a large number of halftone dots having an area within the range of the maximum area to the minimum area are arranged, and individual meshes are arranged. The printing plate is characterized in that the density is expressed by the area of the dots, and in the low density part, a large number of halftone dots having the minimum area are arranged, and the density is expressed by the density of each halftone dot Or printed matter. An image conversion method for converting an original image composed of a set of pixels having pixel values within a range of Pmin to Pmax into a halftone image expressed as a set of halftone dots each having a predetermined area,
Setting a predetermined threshold value Pth (where Pmin <Pth <Pmax);
For a pixel having a pixel value equal to or greater than Pth, a step of converting into a halftone dot having an area corresponding to the pixel value so that the larger the pixel value, the larger the area;
For a pixel having a pixel value less than Pth, it is determined whether or not to perform halftone dot arrangement with a probability corresponding to the pixel value so that the larger the pixel value is, the higher the probability is. When the determination is made, the pixel is converted into a halftone dot having the same area as the halftone dot to be converted for the pixel having the pixel value Pth, and a decision is made not to perform the halftone dot arrangement. If so, converting the pixel into an empty area without halftone dots,
An image conversion method into an image composed of halftone dots, characterized by causing a computer to execute. An image conversion method for converting an original image composed of a set of pixels having pixel values within a range of Pmin to Pmax into a halftone image expressed as a set of halftone dots each having a predetermined area,
The generating points are arranged at a random pitch on the two-dimensional plane, and for each point on the two-dimensional plane, the closest generating point is defined as the closest generating point, and the same generating point is the closest generating point. Dividing the two-dimensional plane into a plurality of unit regions so that one unit region is constituted by a set of points,
Superimposing the image on the two-dimensional plane, and defining pixel values on the image corresponding to individual mother point positions as pixel values of the mother points;
Setting a predetermined threshold value Pth (where Pmin <Pth <Pmax);
In a unit area to which a mother point having a pixel value equal to or greater than Pth belongs, a halftone dot having an occupation ratio corresponding to the pixel value is arranged so that the larger the pixel value, the larger the occupation ratio for the unit area. And the stage of
In a unit region to which a mother point having a pixel value less than Pth belongs, it is determined whether or not to perform halftone dot arrangement with a probability corresponding to the pixel value so that the larger the pixel value, the higher the probability. When it is determined that dot placement is to be performed, when it is determined that a halftone dot having an occupation ratio corresponding to the pixel value Pth is placed in the unit area and that halftone dot placement is not performed. Converting the unit area into an empty area without halftone dots;
An image conversion method into an image composed of halftone dots, characterized by causing a computer to execute. The image conversion method according to claim 11 or 12,
A method for converting an image into a halftone dot image, wherein a decision is made to place a halftone dot with a probability of P / Pth for a pixel having a pixel value P less than Pth. An image conversion method for converting an original image composed of a set of pixels having pixel values within a range of Pmin to Pmax into a halftone image expressed as a set of halftone dots each having a predetermined area,
An original image input stage for inputting the original image;
A halftone dot position defining step for defining a halftone dot placement position for placing individual halftone dots;
A threshold value setting stage for setting a predetermined threshold value Pth (where Pmin <Pth <Pmax);
For each halftone dot placement position, a halftone dot placement stage for placing a halftone dot having a predetermined area as necessary,
A halftone image output stage for outputting an image composed of a set of arranged halftone dots as a halftone image;
To the computer,
The halftone dot placement stage for one halftone dot placement position C is:
Recognizing a pixel value P on the original image corresponding to the halftone dot arrangement position C;
Comparing the pixel value P with the threshold value Pth;
As a result of comparison, when P ≧ Pth, a step of arranging a halftone dot having an area corresponding to the pixel value P at the halftone dot arrangement position C;
If P <Pth as a result of the comparison, it is determined whether or not halftone dot placement is to be performed with a probability of P / Pth, and if it is determined that halftone dot placement is to be performed, the threshold value Pth is determined. Arranging a halftone dot having an area corresponding to the halftone dot arrangement position C;
An image conversion method into an image composed of halftone dots, characterized by comprising: An image conversion method for converting an original image composed of a set of pixels having pixel values within a range of Pmin to Pmax into a halftone image expressed as a set of halftone dots each having a predetermined area,
An original image input stage for inputting the original image;
A unit region definition stage for defining a unit region for arranging individual halftone dots;
A threshold value setting stage for setting a predetermined threshold value Pth (where Pmin <Pth <Pmax);
For each unit region, a halftone dot placement stage for placing halftone dots having a predetermined area as necessary,
A halftone image output stage for outputting an image composed of a set of arranged halftone dots as a halftone image;
To the computer,
The halftone dot placement stage for one unit region U is
Recognizing a pixel value P on the original image corresponding to the unit area U;
Comparing the pixel value P with the threshold value Pth;
As a result of the comparison, when P ≧ Pth, a step of arranging a halftone dot having an occupation ratio corresponding to the pixel value P in the unit region U;
If P <Pth as a result of the comparison, it is determined whether or not halftone dot placement is to be performed with a probability of P / Pth, and if it is determined that halftone dot placement is to be performed, the threshold value Pth is determined. Arranging a halftone dot having an occupation ratio according to the unit area U;
An image conversion method into an image composed of halftone dots, characterized by comprising: The image conversion method according to claim 15, wherein
At the unit region definition stage, generating points are arranged at a random pitch on the two-dimensional plane, and for each point on the two-dimensional plane, the closest generating point is defined as the closest generating point, and the same generating point is set. The two-dimensional plane is divided into a plurality of unit regions so that one unit region is constituted by a set of points whose points are nearest neighbors, and indefinite unit regions having different shapes and sizes are defined. An image conversion method into an image composed of halftone dots, characterized in that:   The program for making a computer perform the image conversion method in any one of Claims 11-16, or the computer-readable recording medium which recorded the said program. An image conversion apparatus for converting an original image composed of a set of pixels having pixel values within a range of Pmin to Pmax into a halftone image expressed as a set of halftone dots each having a predetermined area,
An original image input unit for inputting an original image;
A halftone dot placement position defining section for defining halftone dot placement positions for placing individual halftone dots;
A threshold value setting unit for setting a predetermined threshold value Pth (where Pmin <Pth <Pmax);
For each halftone dot placement position, a halftone dot placement section for placing a halftone dot having a predetermined area,
A halftone image output unit for outputting an image composed of a set of arranged halftone dots as a halftone image;
With
When the halftone dot placement unit places halftone dots at one halftone dot placement position C,
Processing for recognizing the pixel value P on the original image corresponding to the halftone dot arrangement position C;
A process of comparing the pixel value P with the threshold value Pth;
As a result of comparison, when P ≧ Pth, a process of arranging a halftone dot having an area corresponding to the pixel value P at the halftone dot arrangement position C;
If P <Pth as a result of the comparison, it is determined whether or not halftone dot placement is to be performed with a probability of P / Pth, and if it is determined that halftone dot placement is to be performed, the threshold value Pth is determined. A process of arranging a halftone dot having an area corresponding to the halftone dot arrangement position C;
An image conversion apparatus for converting an image composed of halftone dots, which has a function of performing the above. An image conversion apparatus for converting an original image composed of a set of pixels having pixel values within a range of Pmin to Pmax into a halftone image expressed as a set of halftone dots each having a predetermined area,
An original image input unit for inputting an original image;
A unit area definition section for defining a unit area for arranging each halftone dot;
A threshold value setting unit for setting a predetermined threshold value Pth (where Pmin <Pth <Pmax);
For each unit region, a halftone dot placement unit for placing halftone dots having a predetermined area,
A halftone image output unit for outputting an image composed of a set of arranged halftone dots as a halftone image;
With
When the halftone dot placement unit places halftone dots in one unit region U,
Processing for recognizing the pixel value P on the original image corresponding to the unit region U;
A process of comparing the pixel value P with the threshold value Pth;
As a result of the comparison, when P ≧ Pth, a process of arranging a halftone dot having an occupation ratio according to the pixel value P in the unit region U;
If P <Pth as a result of the comparison, it is determined whether or not halftone dot placement is to be performed with a probability of P / Pth, and if it is determined that halftone dot placement is to be performed, the threshold value Pth is determined. A process of arranging a halftone dot having an occupation ratio according to the unit area U;
An image conversion apparatus for converting an image composed of halftone dots, which has a function of performing the above. The image conversion apparatus according to claim 19, wherein
The unit region definition unit arranges the generating points at a random pitch on the two-dimensional plane, and defines the closest generating point as the closest generating point for each point on the two-dimensional plane. The unit has a function of executing a process of dividing the two-dimensional plane into a plurality of unit regions so that one unit region is constituted by a set of points whose points are nearest neighbors, and each has a shape and a size. An image conversion apparatus for converting images into halftone dots, characterized in that different irregular unit regions are defined.