JPH11205601A - Screening method and printed matter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a screening method with which gradation reproducibility can be improved by suppressing the generation of pseudo contour or tone jump even when high-definition printing is performed. SOLUTION: Gradation thinning is performed so that the gradation of a source image can be equal with a gradation capable of being expressed with microdots for forming one dot in advance and afterwards, screening is performed. Then, screening is performed to the respective pixels of the source image by allocating an all-linear pattern corresponding to the new gradation of that pixel. At such a time, all-linear patterns at mutually different angles are used for C, M, Y and K of the source image. Besides, since the mirror- inverted all-linear pattern is used at the interval of one pixel, the reproducibility of highlight part can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a screening method used for modulating the area of a multi-valued image. The present invention relates to a screening method capable of performing high-definition printing while maintaining gradation reproducibility even when output by an output device, and a printed matter printed by the screening method.

[0002]

2. Description of the Related Art A screening method is a method in which a multi-valued image is area-modulated and expressed as a binary image. An image obtained by the screening method is called a screening image. That is, in this specification, expressing a multi-valued image as a binary image is collectively called screening. Therefore, halftoning, which is widely used when printing a multi-valued image, is also a type of screening method for reproducing gradations according to the size of halftone dots.

A method of forming a screening image from a multivalued image to be printed (hereinafter, referred to as an original image) by using halftone dots will be described below.
Now, the resolution of the original image is 300 dpi and the resolution of the output device is
Assuming 1500 dpi, the size of one side of one pixel of the original image is
1/300 inch. And now, the halftone frequency (lines
/ Inch) is assumed to be 150 lines and one dot is formed by four pixels of 2 pixels × 2 pixels, the relationship between the pixels of the original image and the dots is as shown in FIG. FIG. 11 shows a case where the screen angle is 0 °.

As can be easily understood from FIG. 11, in this case, one pixel of the original image is developed into 5 × 5 micro dots. This is because the resolution of the output device is five times the resolution of the original image. In this case, the size of one microdot is 1/1500 inch on one side.
The microdot is the smallest unit for forming a halftone dot, and ink may or may not be placed on the microdot. In other words, the microdot takes on or off values. In the following,
The ON microdots are assumed to be filled with ink, and are shown by a black solid rectangle in the figure. The OFF microdots are assumed to be without ink, and are shown by a white rectangle in the figure.

By the way, as shown in FIG.
When 100 microdots of 10 × 10 are formed, since the microdots take two values as described above, the number of gradations that can be expressed by one halftone dot is different from the case where all the microdots are off. Up to when microdot is on 1
It becomes 01 gradation. As described above, the resolution of the output device is set to M (dp
i), when the number of halftone dots is L (dpi), the number of microdots forming one halftone dot is N × N, where N = M / L, and the number of gradations K that can be expressed at this time is K = N × N + 1 (1)

From the above, when the resolution M of the output device is fixed, if the number L of halftone dots is increased in order to perform high-definition printing, N is reduced by the increase in the number L of halftone dots. This means that the number of gradations that can be expressed by points decreases. In fact, if the resolution M of the output device is fixed and high-definition printing is performed, a pseudo-contour, that is, a contour that does not exist in the original image occurs in the printed image, or a floor called a tone jump. It has been confirmed that a phenomenon in which the tone flies extremely occurs, and the reproducibility of the tone is extremely reduced. Such pseudo contours and tone jumps are particularly likely to occur in an output device having a relatively small output resolution, for example, a digital printing device having an output resolution M of about 1200 to 2,000 dpi.

Accordingly, the present invention provides a screening method capable of suppressing the occurrence of false contours and tone jumps even when high-definition printing is performed, and improving the tone reproducibility, and such a screening method. It is an object of the present invention to provide a printed matter printed using the method.

[0008]

In order to achieve the above object, the screening method according to the first aspect of the present invention makes the gradation of the original image equal to the gradation that can be expressed by microdots that form one halftone in advance. It is characterized in that gradation thinning is performed so that screening is performed thereafter.

A screening method according to a second aspect is characterized in that, in the screening method according to the first aspect, the gradation thinning is performed by an error diffusion method.

A screening method according to a third aspect is characterized in that, in the screening method according to the first or second aspect, after thinning out the gradations, the gradations in the highlight portion and the shadow portion are further thinned out.

[0011] According to a fourth aspect of the present invention, in the screening method of the first, second or third aspect, the screening includes, for each pixel of the original image, a line pattern corresponding to a new gradation of the pixel. It is characterized by performing by assigning.

According to a fifth aspect of the present invention, there is provided the screening method according to the fourth aspect, wherein a parallel line pattern having different angles is used for each color of the original image.

According to a sixth aspect of the present invention, in the screening method of the fourth or fifth aspect, when a line pattern corresponding to a new gray level of the pixel is assigned to each pixel of the original image, It is characterized in that a line pattern that is mirror inverted for each pixel is used.

According to a seventh aspect of the present invention, there is provided a printed matter, wherein a line pattern corresponding to the gradation of the pixel is assigned to each pixel of the original image.

According to an eighth aspect of the present invention, there is provided the printed matter according to the seventh aspect, wherein a line pattern having an angle different from each other is used for each color of the original image.

According to a ninth aspect of the present invention, in the printed matter according to the seventh or eighth aspect, when a line pattern corresponding to a new gradation of the pixel is assigned to each pixel of the original image, every other pixel is used. And a mirror-reversed line pattern is used.

[0017]

Embodiments of the present invention will be described below with reference to the drawings. [First Embodiment] First, a first embodiment of the screening method according to the present invention will be described. FIG. 1 is a flowchart for explaining the screening method according to the first embodiment. First, an original image to be printed is prepared (step S1). This may be performed by a conventional method such as inputting an image of a document with a color scanner.
Here, the original image is image data of a print color of cyan (C), magenta (M), yellow (Y), and black (K), and each pixel of each image data of C, M, Y, and K is Each of them has a depth of 8 bits and is represented by 256 gradations. Therefore, in this case, the pixel value of each pixel directly represents the gradation.

Next, each of the C, M, Y, and K image data of the original image is subjected to gradation thinning to obtain a new gradation (hereinafter, referred to as a new gradation).
(Referred to as a new gradation) (step S2). At this time, the new gradation is made equal to a gradation that can be expressed by microdots forming one halftone dot. That is, the resolution of the output device is set to M (dp
i) When the halftone frequency is L (dpi) and N = M / L, the gradation is thinned out to the K gradation obtained by the above equation (1). For example, if N = 4, the original image of 256 gradations is thinned out to 17 gradations. FIG. 2 shows an example of the gradation of the original image in that case, that is, the pixel values of the original image and the new gradation after gradation thinning. In FIG. 2, the gradation of the original image is
Pixels 0 to 6 are all rounded to a new gradation of 0. The same applies to other cases. At this time, one halftone dot can express 17 gradations,
An example is shown in FIG. According to FIG. 3, for example, when the new gradation expresses 5, five micro dots are turned on as shown in FIG. 3 (f). The same applies to other cases. Further, the area ratios at the halftone dots expressing each gradation shown in FIG. 3 are as shown in FIG.

Various well-known methods can be used as a method for performing the gradation thinning, but it has been confirmed that it is desirable to use the error diffusion method. It has also been confirmed that when the error diffusion method is used, it is desirable to propagate the error two-dimensionally. Since the error diffusion method is well known, a detailed description is omitted.

After the gradation thinning is performed in this manner, screening is performed on each of the C, M, Y, and K image data (step S3). As a screening method, a well-known method such as a conventionally used halftoning method can be used. And after screening,
Printing is performed using the screening image (step S4).

As described above, in this screening method, the original image was screened as it was in the past, and the gradation of the original image was changed to a gradation that can be expressed in advance by microdots forming one halftone dot. Since gradation is thinned out so as to be equal and screening is performed afterwards, even when printing is performed on an output device having a relatively low resolution such as a digital printer, it is possible to suppress the occurrence of false contours and tone jumps, Thus, high-definition printing can be performed while maintaining gradation reproducibility. In fact, this has been confirmed by the present inventors.

[Second Embodiment] The above is the first embodiment. Next, the second embodiment will be described. As described above, as the screening in step S3 in FIG. 1, a commonly used halftoning method may be employed. However, according to the halftoning method, the C, M, Y, and K halftone Normally, the screen angles are different from each other. For example,
The halftone for the C version is 0 °, the halftone for the M version is 15 °, and the halftone for the Y version is 75.
The halftone angles of the color plates are different from each other, for example, the halftone angle of the K plate is 45 °. However, this can cause moiré in the printed image, referred to as a rosette pattern.

Therefore, this embodiment is the same as the first embodiment up to step S2 in FIG. 1. However, when performing the screening in step S3, a line-shaped pattern capable of expressing a predetermined gradation in advance is used. Have it ready
A line pattern corresponding to a new gradation of the pixel is assigned to each pixel of the original image.

For example, if the number of gradations of the original image is thinned out to 17 in step S2 in FIG. 1, a line pattern that can express a new gradation of the original image, that is, in this case,
A line pattern that can express 17 gradations is used. FIG. 5 shows an example of such a linear pattern. As shown in FIG. 5, it is apparent that all 17 gradations from 0 to 16 can be expressed by a line pattern formed of 4 × 4 16 microdots. In the parallel line pattern shown in FIG. 5, at the gradation 1, the microdot at the upper left corner is turned on, and as the gradation increases, the right microdot is sequentially turned on.
When the microdots in the first row are all turned on, then the leftmost microdots in the second row are turned on, so that the position of the microdot to be turned on is determined with the increase in gradation. Direction, in this case, increasing in the horizontal direction. For this reason, it is called a line pattern.

Accordingly, in the original image, the pixel G ₁ having the new gradation of 1 after the gradation is thinned out in step S2 in FIG. 1, the pixel G ₂ having the new gradation of 1 similarly, and the pixel G ₂ having the new gradation similarly. 2 pixels G
_3, and the likewise Shinkaicho four pixels of 2 pixels G ₄ is assumed to be continuous as shown in FIG. 6 (a), the pixel G ₁
And for the G ₂ is assigned ten thousand linear pattern of FIG. 5 (b), the ten thousand linear pattern shown in FIG. 5 (c) allocated to the pixel G _3, and G _4, the pixel G _1,
The screening images of G ₂ , G ₃ , and G ₄ are as shown in FIG.

The parallel line pattern shown in FIG.
It can be represented by a matrix as shown in FIG. The numerical values in the figure indicate the order of the microdots that are turned on. For pixels with a new gradation of j (j = 1,..., 16), the numerical value in the matrix is equal to or smaller than j. This indicates that a line pattern with dots turned on is assigned. Therefore, the line pattern shown in FIG. 5 (i) is assigned to the pixel having the new gradation of 8. Further, in the linear pattern shown in FIG.
As described above, since the number of microdots that are turned on increases in the horizontal direction, it will be referred to as a line pattern having an angle of 0 °.

By the way, a line pattern having an angle of 0 ° cannot be used for all of the original images C, M, Y and K. If the same line pattern is used for all the images of C, M, Y, and K, the printed image has a large density difference between a portion where the ink is applied and a portion where the ink is not applied, so that moire or the like does not occur. This is because the possibility that a desired pattern appears will increase.

Therefore, when printing in four colors of C, M, Y, and K, three more line-type patterns are used. Examples of such a linear pattern are shown in FIGS.
(D). These line patterns are examples of line patterns used when the gradation of the original image is thinned out to 17 gradations, and the meanings of numerical values in the matrix in these line patterns are the same as described above. Therefore, FIG.
The line pattern shown in (b) is a pattern in which the positions of microdots that are turned on are sequentially increased in the vertical direction from the left side as the gradation increases, and the angle 0 ° shown in FIG. Since the direction of the microdots that are turned on differs from that of the above-mentioned line pattern by 90 °, it will be referred to as a line pattern having an angle of 90 °. In addition, the line pattern shown in FIG. 7C is a pattern in which the positions of the microdots that are turned on sequentially increase from the upper left to the lower right as the gradation increases. Since the direction of the microdots to be turned on differs from that of the line pattern of 0 ° shown in FIG. 4) by 45 °, it will be referred to as a line pattern of 45 ° angle.
Similarly, the line pattern shown in FIG. 7D is a pattern in which the positions of the microdots that are turned on are sequentially increased from the upper right to the lower left as the gradation is increased. Since the direction of the microdot that is turned on differs from that of the line pattern having an angle of 0 ° shown in FIG.
° is called a linear pattern.

When the four types of line patterns are used as described above, line patterns having different angles can be used for C, M, Y, and K images. In addition,
Which angle of the line pattern is associated with which color image of C, M, Y, K is arbitrary.

As described above, in step S3 in FIG. 1, when screening is performed on the C, M, Y, and K images after the gradation thinning is performed, the C, M, Y, and K On the other hand, by assigning a line pattern corresponding to a new gradation of the pixel to each pixel by a line pattern having a different angle, the resolution is relatively high as in a digital printing machine. Even when printing with a low output device, the occurrence of false contours and tone jumps can be suppressed, which not only enables high-definition printing while maintaining gradation reproducibility, but also reduces the occurrence of moire. Can be suppressed.

[Third Embodiment] Next, a third embodiment will be described. The third embodiment is based on the above-described second embodiment.

It is known that, when printing is performed, if micro dots that are turned on are isolated, it is difficult for such micro dots to be inked and the reproducibility is poor. . Therefore, in FIG.
_{In the} line pattern assigned to the pixel indicated by ₂ , the microdots that are turned on are isolated, so when printed using this screening image, the microdots are poorly inked and the image is good. Is difficult to reproduce.

The third embodiment improves this, and the line pattern assigned to every other pixel is mirror-inverted. The mirror inversion may be performed in the same direction as the angle of the line pattern. Therefore, the angle 0
Since the mirror pattern is reversed in the horizontal direction with respect to the parallel line pattern having an angle of 90 °, the result is as shown in FIG. 8A.
The result is as shown in FIG. Similarly, a line pattern obtained by mirror-inverting a line pattern having an angle of 45 °, and an angle of −45
8 (c) and 8 (d) show the parallel line pattern obtained by mirror-inverting the parallel line pattern of °.

The details are as follows. Now, FIG.
A case where screening is performed using a line pattern having an angle of 0 ° shown in FIG. 7A will be described as an example. A pixel G ₁ having a new gray level of 1 after gray level thinning, and a pixel having a new gray level of 1 similarly When G _2, similarly Shinkaicho two pixels G _3, and similarly Shinkaicho four pixels of 2 pixels G ₄ is assumed to be continuous as shown in FIG. 9 (a), the pixel G Figure 8 for ₁
Assigned ten thousand linear pattern of gradation 1 ten thousand linear pattern mirrored shown in (a), and FIG. 5 for pixel G ₂
(B) assigned ten thousand linear pattern, assigned ten thousand linear pattern of gradation 2 ten thousand linear pattern mirrored shown in FIG. 8 (a) to the pixel G _3, Fig. 5 for the pixel G ₄ As in the case of assigning the line pattern shown in (c), the mirror-inverted line pattern is assigned every other pixel. Thus, the screening image of these pixels G ₁ , G ₂ , G ₃ , G ₄ is shown in FIG.
The result is as shown in FIG. The same applies to the case where a line pattern having another angle is used.

It will be apparent that by doing so, it is possible to prevent the isolated microdots from being turned on. This makes it possible to stabilize the application of the ink and improve the reproducibility of the highlight portion of the original image.

[Fourth Embodiment] Next, a fourth embodiment will be described. The fourth embodiment aims at improving the reproducibility of the highlight portion and the shadow portion of the original image, and the screening method is not limited. That is, the fourth embodiment can be applied to the case of halftoning which is usually performed, and can also be applied to the case of the second embodiment.

As described above, when the micro dots to be turned on are isolated, the reproducibility of the highlight portion is deteriorated. Further, when an off micro dot is isolated as shown in FIG. 8 (p), the dot may be crushed by a dot gain of an adjacent micro dot or the like, and the reproducibility of a shadow portion is poor. Become.

Therefore, in the fourth embodiment, when the tone of the original image is thinned to obtain a new tone in step S2 in FIG. 1, the tone in the highlight portion and the shadow portion is further thinned. .

The details are as follows. Here, a case will be described in which the fourth embodiment is applied to a case where a line pattern is assigned to each new gradation pixel and screening is performed as in the second embodiment. For example, when N = 4, the original image of 256 tones is thinned out to 17 tones in the first embodiment, but in the fourth embodiment, it is thinned out to 15 tones, for example. .

Then, as shown in FIG. 10, a line pattern to be allocated to the new gradation pixel is determined. That is, FIG.
The line pattern shown in FIG. 5B and the line pattern shown in FIG. 5P are not used. The same applies to the line pattern of other angles.

In the above, the case where the fourth embodiment is applied to the case of screening by allocating a line pattern to each new gradation pixel as in the second embodiment has been described. The same applies to the case of performing halftone dot conversion.

With this configuration, it is possible to avoid the isolation of the micro dots that are turned on and the isolation of the micro dots that are turned off, so that it is possible to improve the reproducibility of the image in the highlight portion and the shadow portion. .

[Brief description of the drawings]

FIG. 1 is a flowchart illustrating a screening method according to a first embodiment of the present invention.

FIG. 2 is a diagram for explaining an example of gradation thinning-out in step S2 of FIG. 1;

FIG. 3 is a diagram showing an example of a halftone dot capable of expressing 17 gradations.

FIG. 4 is a diagram showing an area ratio of a halftone dot expressing each gradation shown in FIG. 3;

FIG. 5 is a diagram showing an example of a line pattern used in the second embodiment.

FIG. 6 is a diagram for describing allocation of a line pattern to pixels of a new gradation in the second embodiment.

FIG. 7: Linear pattern at an angle of 0 °, linear pattern at an angle of 90 °, linear pattern at an angle of 45 °, and an angle of −45 °
FIG.

8 is a diagram showing a line pattern obtained by mirror-inverting each line pattern shown in FIG. 7;

FIG. 9 is a diagram for describing allocation of a line pattern to pixels of a new gradation in the third embodiment.

FIG. 10 is a diagram for explaining a fourth embodiment.

FIG. 11 is a diagram for explaining a relationship between pixels of an original image and halftone dots.

 ────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kazunari Iyoda 1-1-1 Ichigaya Kagacho, Shinjuku-ku, Tokyo Inside Dai Nippon Printing Co., Ltd.

Claims (9)

[Claims] 1. A screening method characterized in that a gradation of an original image is thinned out in advance so as to be equal to a gradation that can be expressed by microdots forming one halftone dot, and screening is performed thereafter. 2. The screening method according to claim 1, wherein the gradation thinning is performed by an error diffusion method. 3. The screening method according to claim 1, wherein after the gradation thinning-out, the gradations of a highlight portion and a shadow portion are further thinned out. 4. The screening according to claim 1, wherein the screening is performed by assigning a line pattern corresponding to a new gradation of the pixel to each pixel of the original image. Method. 5. A line-shaped pattern having different angles from each other for each color of an original image.
The screening method as described above. 6. The method according to claim 1, wherein when a line pattern corresponding to a new gradation of the pixel is assigned to each pixel of the original image, a line pattern that is mirror-inverted every other pixel is used. Item 6. The screening method according to item 4 or 5. 7. A printed material, wherein a line pattern corresponding to the gradation of the pixel is assigned to each pixel of the original image. 8. The printed matter according to claim 7, wherein line-shaped patterns having different angles from each other are used for each color of the original image. 9. When allocating a line pattern corresponding to a new gradation of the pixel to each pixel of the original image, a line pattern mirror-inverted every other pixel is used. The printed matter according to claim 7 or 8, wherein