JPH11157132A - Printing method for digital gradational image, and printed matter where gradational image is expressed - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve print quality in producing printed matter expressed by dots. SOLUTION: An entire image Z0 made up of an aggregate of picture elements having a prescribed tonal value, respectively, is prepared as image data. A first dot image Z1 where tonal values are expressed by dot size is produced by executing AM screening on the entire image Z0 and a second dot image Z2 where tonal values are expressed by dot density is produced by executing FM screening. Concerning each of the picture elements forming the entire image Z0, an area suited to the AM screening and an area suited to the FM screening are defined by referring to the tonal values or spatial frequencies, and masks M1, M2 are prepared. A dot image Z1* where only the area suited to the AM screening is extracted, is prepared using the mask M1 and a dot image Z2* where only the area suited to the FM screening is extracted, is prepared using the mask M2. An entire dot image Z3 is obtained by synthesizing the two dot images.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for printing a digital gradation image and a printed matter expressing the gradation image.
In particular, the present invention relates to a method of printing a digital gradation image by a method in which an AM screening method and an FM screening method are combined.

[0002]

2. Description of the Related Art In the field of general commercial printing, when expressing a color image, a digital halftone technique of expressing a digital gradation image as a binary image is used. Usually, a digital gradation image expressed by four colors of CMYK is prepared as image data composed of a set of pixels having a predetermined gradation value for each color component plate, and based on each image data. In this case, a screen for arranging halftone dots is performed, and a halftone image is produced for each color component plate. The halftone image thus produced is a binary image composed of a halftone dot portion and a background portion, and the size or arrangement density of the halftone dot is determined according to the tone value of the pixel of the original image. By performing the modulation, pseudo gradation expression becomes possible.

A method of expressing gradation by modulating the size of a halftone dot has been used for a long time in the field of commercial printing, and the conventional general halftone printing is mostly based on this method. It is. Normally, grid lines having the same pitch are defined vertically and horizontally, and individual halftone dots are arranged so that the center position of the halftone dot is located at the intersection. Although the size of each halftone dot differs based on the tone value of each pixel of the original image, the arrangement pitch of each halftone dot is constant. In general commercial printing, the interval between grid lines (ie, the pitch of halftone dots) is set to about 0.145 mm (1/175 inch). In the case of a color image, screening is performed for each of the CMYK plates, a plate making film is prepared for each of the CMYK plates, and a plate is prepared for each plate based on the plate making film. Printing will be performed using the plate. However, if the screen angle of each plate (the arrangement angle of the grid line which is the basis of the dot arrangement) is equal, interference fringes (partially due to the optical interaction of the halftone dots of each plate formed on the printed matter). Since a so-called moiré or rosetta pattern is observed, a method of shifting the screen angle for each CMYK plate is usually employed.

[0004] On the other hand, a method of expressing gradation by modulating the arrangement density of halftone dots has been gradually developed since the theoretical presentation was made in Germany in 1983, and has been partially achieved. Is used in the field of commercial printing. This method is generally called FM (Frequency Modulation) screening. In contrast to this, the conventional method of performing tone expression by modulating the size of the halftone dot is AM (Amplitude Modulation) screening. ) Called screening. In the FM screening, tone expression is usually performed by randomly arranging halftone dots of the same size at a density based on the tone value of each pixel of the original image. In the case of a color image, screening is also performed for each of the CMYK plates, a plate making film is prepared for each of the CMYK plates, and a plate is prepared for each plate based on the plate making film. Printing is performed using a color printing plate. Since the pitch of each halftone dot is random, there is no occurrence of interference fringes as in the case of AM screening.

[0005]

The above-described AM screening method and FM screening method each have problems to be solved. First, the problem of the AM screening method is the occurrence of interference fringes such as moiré and rosette patterns as described above. In the AM screening, each halftone dot is arranged at a predetermined pitch.
A regular pattern is formed for each plate of MYK. As a result of overlaying these regular patterns,
Interference fringes will appear. So, for each color plate,
Although a method of suppressing the occurrence of interference fringes by changing the pitch of each halftone dot is also known, it cannot be said that this is an efficient countermeasure because the load of the calculation performed on the image data increases. Of course, it is possible to suppress the generation of interference fringes by shifting the screen angles of the four plates to each other, but it is not possible to completely suppress the interference fringes. Therefore, at present, the screen angle is set so that interference fringes are generated only for relatively light colors such as the Y plane, and a compromise is made by making the overall interference fringes inconspicuous. For this reason, for example, moire is likely to be observed in a striped pattern portion or the like. There is also a problem that the resolution is lower than that of the FM screening method.

On the other hand, according to the FM screening technique, the problem of interference fringes does not occur at all. Since each halftone dot arrangement is originally random and has no regularity, interference fringes do not occur in principle. In addition, since the degree of freedom in the arrangement position of the halftone dots is higher than in the AM screening,
There is also an advantage that higher resolution can be obtained. However, when observed with human eyes, there is a demerit that a so-called "smoky feeling" or "roughness" is given, and the natural texture is impaired.
The reason for giving the impression of "smoky feeling" is that the arrangement of the halftone dots is random, so there is a partial difference in density,
This is considered to be because it is recognized as a noise component. For example, when an image in which the entire surface is solid-colored with the same color at a uniform density is printed by the FM screening method, the halftone dot density is not necessarily uniform over the entire surface, and the fine portion is based on random arrangement. As a result, a difference in shading occurs, and a “smoky feeling” appears. On the other hand, the cause of giving the impression of “roughness” is that the size of the halftone dots is the same, and in particular, from halftone to the highlight part, individual halftone dots are recognized as grains, giving a grainy feeling. Conceivable.

Accordingly, an object of the present invention is to provide a method for further improving print quality when producing a printed matter in which a digital gradation image is represented by halftone dots.

[0008]

According to a first aspect of the present invention, there is provided a digital gradation image printing method for printing a digital gradation image on a predetermined medium, wherein the method has a predetermined gradation value. An image data preparation step of preparing image data composed of a set of pixels; and an area division step of dividing an image represented by the prepared image data into an area belonging to a first group and an area belonging to a second group. AM for image data of an area belonging to the first group.
A halftone image forming step of performing screening, performing FM screening on image data of an area belonging to the second group, and forming a halftone image in which halftone dots are arranged based on the tone value of each pixel; And a printing step of performing printing based on the halftone dot image.

(2) A second aspect of the present invention is a digital gradation image printing method for printing a digital gradation image on a predetermined medium, the image comprising a set of pixels having a predetermined gradation value. An image data preparing step of preparing data; an area dividing step of dividing an entire image represented by the prepared image data into an area belonging to a first group and an area belonging to a second group; Creates a first halftone image in which halftone dots having a size corresponding to the tone value of a pixel are arranged at a predetermined pitch based on the image data in each of the regions belonging to the first region. In the halftone image creation step, for each area belonging to the second group, halftone dots having a predetermined size are arranged at a density corresponding to the gradation value of the pixel based on the image data in each area. Second
A second halftone image creating step of creating halftone images of
Combining the halftone image and the second halftone image to form an overall halftone image corresponding to the overall image, and a printing step of performing printing based on the overall halftone image And, it is made to do.

(3) A third aspect of the present invention is the above-mentioned second aspect.
In the digital gradation image printing method according to the aspect, the first halftone image creation step and the second halftone image creation step are performed in an overlapping manner for all the regions constituting the entire image, In the creating step, by combining an area belonging to the first group of the first halftone image with an area belonging to the second group of the second halftone image,
An entire halftone dot image is created.

(4) The fourth aspect of the present invention is the above-mentioned first aspect.
In the digital gradation image printing method according to the third aspect, after the area belonging to the first group and the area belonging to the second group are defined in the area dividing step, the area satisfying the predetermined reference area is satisfied. The non-consideration area is recognized as a non-consideration area, and the non-consideration area is processed as a part of another area including the non-consideration area, so that a process of eliminating the non-consideration area is performed.

(5) The fifth aspect of the present invention is the above-mentioned first aspect.
In the digital gradation image printing method according to the fourth to fourth aspects, in the image data preparation stage, image data composed of a set of pixels each having a predetermined gradation value is prepared for each plate of each color component, and An n-version image data is prepared, and a common region division is performed on the n-version image data at the region division stage.

(6) The sixth aspect of the present invention is the above-mentioned fifth aspect.
In the method for printing a digital gradation image according to the embodiment,
A grayscale image is created by synthesizing the image data of the plate, and common area division is performed based on the grayscale image.

(7) A seventh aspect of the present invention is the above-mentioned first aspect.
In the digital gradation image printing method according to the fourth to fourth aspects, in the image data preparation stage, image data composed of a set of pixels each having a predetermined gradation value is prepared for each plate of each color component, and In this method, image data of n versions is prepared, and in the area dividing step, separate area division is performed for each of the n versions of image data.

(8) The eighth aspect of the present invention is the above-mentioned first aspect.
In the digital gradation image printing method according to the seventh to seventh aspects, the group to which the pixel belongs is determined in the region division stage according to the gradation value of the pixel.

(9) The ninth aspect of the present invention is the above-mentioned eighth aspect.
In the digital gradation image printing method according to the aspect, a predetermined threshold value is set for the gradation value of the pixel, and the pixels having the gradation value equal to or larger than the threshold value belong to the second group. Pixels having a tone value less than the value belong to the first group.

(10) A tenth aspect of the present invention is the method for printing a digital gradation image according to the ninth aspect, wherein
Of the halftone dots of various sizes prepared for expressing the gradation value by the size, to the halftone dots of almost the same size as the halftone dot prepared for expressing the gradation value by the density The associated gradation value is set as a threshold value.

(11) According to an eleventh aspect of the present invention, in the digital gradation image printing method according to any one of the first to seventh aspects described above, the spatial frequency of the gradation value possessed by the pixel in the region division stage is The group to which the user belongs is determined accordingly.

(12) According to a twelfth aspect of the present invention, in the digital gradation image printing method according to the eleventh aspect, a predetermined threshold value is set for a spatial frequency of a gradation value of a pixel. Pixels having a spatial frequency higher than the threshold value belong to the second group, and pixels having a spatial frequency lower than the threshold value belong to the first group.

(13) According to a thirteenth aspect of the present invention, in the digital gradation image printing method according to the eleventh or twelfth aspect, a spatial filter is applied to the image data prepared in the image data preparation stage. For each pixel, obtain a parameter value indicating a difference in tone value between each adjacent pixel, create a reference image having this parameter value as a pixel value, and This is to define the area division.

(14) According to a fourteenth aspect of the present invention, in the digital gradation image printing method according to any of the first to seventh aspects described above, in the area dividing step, the gradation value of the pixel and the The group to which the user belongs is determined according to both the spatial frequency of the tone value and the spatial frequency.

(15) According to a fifteenth aspect of the present invention, in the method for printing a digital tone image according to the fourteenth aspect, a first threshold value is set for a tone value of a pixel, A second threshold value is determined for the spatial frequency of the tone value having
Pixels having a gradation value less than the threshold value and having a spatial frequency less than the second threshold value belong to the first group, and the other pixels belong to the second group. It was done.

(16) According to a sixteenth aspect of the present invention, in the digital gradation image printing method according to any one of the first to fifteenth aspects, the region belonging to the first group and the second group have After different dot gain corrections are performed on the areas to which they belong, halftone dots are formed.

(17) According to a seventeenth aspect of the present invention, a printed matter in which a gradation image is expressed is produced by the digital gradation image printing method according to the first to sixteenth aspects. It is.

(18) According to an eighteenth aspect of the present invention, in a printed matter in which a grayscale image is expressed by a number of halftone dots, a gradation is expressed by a halftone dot size in some areas. In another partial area, the gradation is expressed by the density of halftone dots.

(19) According to a nineteenth aspect of the present invention, in a printed matter in which the gradation image according to the eighteenth aspect is expressed, a highlight portion having a brightness equal to or higher than a predetermined threshold value is shaded. The gradation is expressed by the size of the point,
For shadows whose brightness is less than this threshold,
The gradation is represented by the density of the halftone dots.

(20) According to a twentieth aspect of the present invention, in a printed matter in which the gradation image according to the eighteenth aspect is expressed, a portion having a spatial frequency equal to or higher than a predetermined threshold value is a halftone dot. Is expressed by the density of the image, and the portion where the spatial frequency is lower than the threshold value is expressed by the size of the halftone dot.

(21) According to a twenty-first aspect of the present invention, in the printed matter in which the gradation image according to the eighteenth aspect is expressed, the brightness is equal to or more than a predetermined threshold value and the spatial frequency is set to a predetermined value. Is smaller than the threshold value, the gradation is expressed by the size of the halftone dot, and the other parts are expressed by the density of the halftone dot. .

(22) According to a twenty-second aspect of the present invention, in the printed matter in which the gradation images according to the seventeenth to twenty-first aspects are expressed, the printed matter is composed of an n plate using n inks. For each plate, there is a portion where the gradation is expressed by the size of the halftone dot and a portion where the gradation is expressed by the density of the halftone dot.

[0030]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below based on an embodiment shown in the drawings.

§1. Conventional Screening Techniques Here, for convenience of explanation, the basic techniques of AM screening, which has been generally used, and FM screening, which has recently become popular, will be briefly described. In a general digital color gradation image, for example,
As shown in FIG. 1, a pixel array is defined for each of the four CMYK plates, and a predetermined pixel value is defined for each pixel. When the gradation value for one pixel is expressed by 8 bits, each pixel has any gradation value in the range of 0 to 255.

In the AM screening method, each pixel is represented by a dot having a size corresponding to the gradation value. For example, pixels P (1,1), C on the C plane shown in FIG.
P (1,2), P (1,3), ..., P (2,1), ...
Are the halftone dots Q (1,1) and Q as shown in FIG.
(1,2), Q (1,3),..., Q (2,1),. For example, gradation value = 255
Are given by the largest halftone dot, and the pixels given the tone value = 128 are represented by half the size of halftone dot. Here,
An example in which one pixel is represented by one halftone dot will be described. A plurality of pixels may be expressed by one halftone dot (in this case, for example, the size of the halftone dot based on the average of the gradation values of each pixel) May be determined), or one pixel may be represented by a plurality of halftone dots. In this AM screening, each halftone dot is arranged such that the center position is located at the intersection of the grid lines arranged vertically and horizontally, as indicated by the dashed line in FIG. Usually, the horizontal pitch dx and the vertical pitch d of this grid line
y is constant and dx = dy = 0.145 mm (1/1)
75 inches). As a result, a large number of halftone dots having different sizes are arranged at the same vertical and horizontal pitches.

Note that one halftone dot is not always formed as a continuous area having one unity, and one halftone dot may be formed as a set of smaller dots. For example, instead of using halftone dots as shown in Fig. 3 (a), halftone dots consisting of a set of small dots as shown in Fig. 3 (b) should be used as halftone dots in AM screening. Is also possible. In the example shown, small dots are formed in 24 central portions of a 10 × 10 matrix, and a halftone dot corresponding to a size of 24% when the maximum halftone dot size is 100%. It is shown.

On the other hand, in the FM screening method, each pixel is represented by a halftone dot having a density corresponding to the gradation value. For example, when a pixel represented by a halftone dot having a size of 24% as shown in FIG. 3 is represented by the FM screening method, the result is as shown in FIG. In both the example of FIG. 3 (b) and the example of FIG.
The four dots are the same in that they are arranged in the same manner, and all represent pixels having a tone value of 24%. However, in the halftone dots in the AM screening shown in FIG. 3B, small dots are gathered in the central portion, and one halftone dot is formed by the set of small dots, whereas the halftone dots shown in FIG. In the screening, small dots form independent dots,
The dot arrangement is random.

Although the examples of the AM screening and the FM screening have been described above with reference to the illustrated example, in the AM screening, the gradation value of each pixel is represented by the size (area) of a halftone dot. , FM
In the screening, the tone value of each pixel is represented by the density of halftone dots. Therefore, as described above, each screening technique has its own problems.

A serious problem in AM screening is the generation of interference fringes. As shown in FIG. 2, the arrangement pitch of the individual halftone dots becomes constant. In addition, in order to reduce the load of the operation for preparing the digital image data, usually,
The halftone dot arrangement pitch for each of the CMYK plates is also set equal. Therefore, a regular pattern having the same period is formed for each of the CMYK plates, and interference fringes appear as a result of overlapping these patterns.
However, it is possible to suppress the generation of interference fringes such as moiré by devising the setting of the screen angle when overlapping the plates. According to an experiment conducted by the present applicant, moiré occurs when two plates are overlapped with a screen angle shifted by 15 °.
No moiré occurred when staggered.

Therefore, if three plates are superimposed,
The screen angle of each plate is, for example, 0 °, 30 °, 6
If the angle is set to 0 °, the amount of deviation of the screen angle between the plates can be set to 30 °, so that the occurrence of moire can be suppressed. However, in general color printing, four planes of CMYK are required, so that the deviation of the screen angle between the planes cannot be made 30 °.
Therefore, usually, for example, as shown in the upper part of FIG.
Screen angle of plate is 0 °, screen angle of M plate is 1
5 °, the screen angle of the K plane is set to 45 °, and the screen angle of the C plane is set to 75 °. In such a setting, as shown in the lower part of FIG. 5, the deviation of the screen angle between the Y plate and the M plate becomes 15 °, and moire occurs between the two plates. Is 15 °, and moire occurs between the two.
However, since the Y plate has a relatively light color, moiré becomes relatively inconspicuous, and such a setting can be said to be a more preferable choice in the related art. However, even if it is inconspicuous, it is a fact that moire is occurring, and does not provide the best solution.

On the other hand, if the method of FM screening is adopted, as shown in FIG. 4, since the halftone dots are arranged at random, no interference fringes such as moire are generated. However, when observed with the human eye, noise components due to partial differences in density are observed as "misty feeling", and the graininess of halftone dots from the halftone to the highlight part is observed as "graininess". As described above, there is a disadvantage that the natural texture is impaired.

§2. Screening Method According to the Present Invention The basic idea of the present invention is that A is used for a part of one image.
M screening is performed, halftone dots having a size corresponding to the gradation value of the pixel are arranged at a predetermined pitch, and FM screening is performed on the remaining part, and a halftone dot having a predetermined size is determined as a pixel. Is arranged at a density corresponding to the gradation value of. Very common images contain high brightness highlights and low brightness shadows,
In addition, there are also a portion that gives a smooth impression due to a gradual change in gradation and a portion that gives a complicated impression due to a sharp change in gradation. For this reason, general images include A
This means that a portion where M screening is preferable and a portion where FM screening is preferable are included. The basic idea of the present invention is to improve the print quality by applying an appropriate screening method to each part of one image.

Here, a procedure for printing based on a digital gradation image as shown in FIG. 6 will be described. Although the illustrated image is shown as a binary image due to restrictions of the electronic application, an image having a gradation is actually prepared. For example, when the gradation value of one pixel is represented by 8-bit information, each pixel has a total of 256 gradation values from 0 to 255. In addition, here, for convenience of explanation, an example in which the present invention is applied to printing using only one color ink will be described. However, in fact, as described in §3 described below, the present invention is applied to color printing. In practical use, it is more useful in color printing.

First, a digital gradation image as shown in FIG. 6 is prepared in a computer as image data. Specifically, a pixel array in which many pixels are arranged vertically and horizontally is defined,
An 8-bit gradation value is defined for each pixel. Then, this image is stored in an area (AM) belonging to the first group.
An operation of dividing into a region to be screened) and a region belonging to the second group (a region to be subjected to FM screening) is performed. Here, for example,
It is assumed that the image shown in FIG. 6 has been subjected to region division as shown in FIG. In the example shown in FIG. 7, the white region is a region belonging to the first group, and the hatched region is a region belonging to the second group.

Such area division can be performed manually according to the judgment of the operator. For example, FIG.
May be displayed on a display, a range of each area is specified using a pointing device such as a mouse, and an input specifying which group belongs to each area may be performed. A somewhat skilled operator can recognize which part is suitable for AM screening and which part is suitable for FM screening. However, the inventor of the present application has found a specific method for determining an objective criterion for discriminating a part to be subjected to AM screening and a part to be subjected to FM screening. If this objective criterion is used, the area division can be executed as an arithmetic processing by a computer, and the automatic processing can be performed without bothering a skilled operator. Here, for convenience of explanation, a specific method of this area division will be described in detail in §4.

When the area division as shown in FIG. 7 is completed,
As shown in FIG. 8, AM screening is performed on a region belonging to the first group, and FM screening is performed on a region belonging to the second group. In other words, for each pixel in the region belonging to the first group, halftone dots of a size corresponding to the gradation value are arranged at a predetermined pitch, and the pixels in the region belonging to the second group are arranged. For each pixel, a process of arranging halftone dots of a predetermined size at a density according to the gradation value is performed. Focusing on the individual regions, the regions belonging to the first group are shown in FIG.
A first halftone image is created as shown in FIG. 9A, and a second halftone image is created for an area belonging to the second group as shown in FIG. 9B. . Here the first
Is an image obtained by the AM screening, and is an image in which the gradation value is expressed by the size of the halftone dot. The second halftone image is an image obtained by the FM screening, and is an image in which the gradation value is expressed by the density of the halftone dots.

A plate making film is produced based on the whole halftone image obtained by synthesizing the first halftone image and the second halftone image, and a printing plate is produced based on the plate making film. If printing is performed, the printing process according to the present invention is completed. The original image as shown in FIG. 6 is expressed as a gradation image on the printed matter, and the gradation expression method is different for each region. That is, for some regions (regions belonging to the first group), gradation is expressed by the size of halftone dots, while other regions (regions belonging to the second group). Area), the gradation is represented by the density of the halftone dots.

In practice, it is preferable to perform the AM screening and the FM screening in an overlapping manner on the entire area of the original image shown in FIG. 6, and extract and synthesize only necessary parts. . This procedure is shown in FIG. First, the entire original image is prepared as an entire image Z0, and AM screening is performed on the entire region of the entire image Z0.
A first halftone image Z1 is created. Similarly, the whole image Z0
Is performed on the entire area of the image to generate a second halftone image Z2. Subsequently, a region M is divided into the entire image Z0, and a mask M1 for extracting only a region belonging to the first group and a mask M2 for extracting only a region belonging to the second group are formed. (In the illustrated example, regions corresponding to the white portions of the masks M1 and M2 are extracted.) Then, the first halftone image Z1
Are extracted using the mask M1 to obtain the first halftone image Z1 ^*, and only the necessary portions in the second halftone image Z2 are extracted using the mask M2. As two halftone images Z2 ^* , the halftone images Z1 ^* and Z2 ^* are combined to obtain an entire halftone image Z3. A plate making film may be prepared based on the whole halftone dot image Z3 thus obtained, a printing plate may be prepared using the plate making film, and printing may be performed.

§3. More Practical Embodiments Here, some more practical embodiments for implementing the present invention will be described.

<3.1> Dot Gain Correction In general, a halftone dot printed matter obtained through a printing plate process or a printing process has a phenomenon in which the tone expressed by the halftone dot differs from the original tone value. Are known. The dot gain correction is a correction performed to correct such a phenomenon. For example, as shown in FIG. 11A, suppose that a pattern corresponding to a halftone dot having a diameter D is formed on a printing plate and printing is performed in an attempt to form a halftone dot having a diameter D on a printed material. However, due to the characteristics of the ink and paper used, bleeding of the ink occurs at the time of printing. In practice, as shown in FIG. Suppose that it is expected that a point will be formed. As described above, when it is expected that a halftone dot larger by Δr is formed due to ink bleeding,
In consideration of the bleeding of the ink, the gradation values of the pixels of the original image may be corrected to be smaller in advance.

Such a phenomenon is caused not only by the bleeding of the ink at the time of printing but also by the corrosive conditions when the printing plate is formed by etching or the like. Therefore, in general, the gradation values of the pixels of the original image are corrected in consideration of the unique conditions in the printing plate process and the printing process. This is a correction generally called dot gain correction. For example, as shown in FIG. 12, when the original gradation value is plotted on the horizontal axis and the halftone dot area actually obtained on the printed matter is plotted on the vertical axis, a linear relationship like graph A is originally obtained. It is assumed that the relationship as shown in the graph B is actually obtained although it must be performed. In such a case, graph B
, Dot gain correction is performed on the image data constituting the original image.

In the conventional printing method, this dot gain correction is generally performed for all images in common.
This is because the degree of ink bleeding on the paper is almost unchanged for any part of the image. However, in the present invention, the situation is different because a part of the image is processed by the AM screening and the remaining part of the image is processed by the FM screening. This is because different dot gain corrections need to be performed when performing AM screening and when performing FM screening. For example, in the case of the AM screening, the bleeding width Δr of the ink generated for a halftone dot having a large diameter D1 as shown in FIG.
(b) shows the same bleed width Δr as the bleed width Δr of the ink generated for a halftone dot having a small diameter D2 as shown in FIG.
However, the ratio as the amount of change to the size of the original halftone dot differs greatly. On the other hand, in the case of FM screening, since the sizes of the halftone dots are all the same,
There is no difference in the ratio of the fluctuation amount for each halftone dot.

Under such circumstances, the function characteristics corresponding to the graph B in FIG. 12 are greatly different between the case where the AM screening is performed and the case where the FM screening is performed. Therefore, when performing dot gain correction in the present invention, dot gain correction based on different function characteristics is performed on image data of an area to be subjected to AM screening and image data of an area to be subjected to FM screening, respectively. Need to do. For example, in the case of the procedure shown in FIG. 10, the dot gain correction performed when obtaining the first halftone image Z1 and the dot gain correction performed when obtaining the second halftone image Z2 based on the entire image Z0. Then, it is sufficient to perform the processing separately using different functions.

<3.2> Deletion of Small Area In the present invention, each area constituting the original image is divided into an area belonging to the first group and an area belonging to the second group. Although the work of the area division can be performed based on an instruction by a manual operation of an operator, in practice, it is preferable to execute the work as an automatic calculation process using a computer based on some objective criterion. . For a specific example of this objective region segmentation method,
It will be described in §4. Here, a description will be given of handling of a minute area generated when such automatic area division is performed.

For example, it is assumed that the result of performing the automatic region segmentation calculation process described in §4 on the original image shown in FIG. 6 results in the segmentation result shown in FIG. In this example, a white area in the figure is an area belonging to the first group, and a hatched area is an area belonging to the second group. However, when the division mode of FIG. 14 is compared with the division mode of FIG. 7, it can be seen that the former includes minute regions g1 to g4. When an automatic division process using a computer is executed based on some objective criterion, such a small area may be defined in some cases. However, in general, even if a fine division for forming such a minute area is performed and the screening method is switched,
The advantage of the present invention of improving the image quality on printed matter is not so remarkable. Conversely, quality degradation is caused by noise components generated by the switching.

Therefore, in practice, when the automatic region division is performed, it is preferable to perform a process of identifying a region less than a predetermined reference area as a non-consideration region and deleting the region. In order to delete the non-consideration area, a process of eliminating the non-consideration area by treating it as a part of another area including the non-consideration area may be performed. For example, in the example illustrated in FIG. 14, since the areas of the minute regions g1 to g4 are less than the reference area, consider a case where these are recognized as non-considered regions. In this case, the non-consideration areas g1 and g2 belong to the second group, but disappear by being treated as a part of another area including them (that is, an area belonging to the first group). And conversely, the non-consideration area g
3 and g4 are areas belonging to the first group,
They disappear by treating them as a part of another area including them (that is, an area belonging to the second group). As a result, the area division shown in FIG. 14 is modified to the area division shown in FIG. 7 and is simplified.

<3.3> Application to Color Image For convenience of explanation, an example in which the present invention is applied to a gradation image printed using one color ink has been described. Is suitable for application to color images. When the present invention is applied to a color image, in the stage of preparing image data, image data composed of a set of pixels each having a predetermined gradation value is prepared for each color component plate, and a total of n plates is prepared. Prepare image data, and for each edition,
The screening and the FM screening may be performed for each individual region.

For example, the example shown in FIG. 15 shows an embodiment in which separate and independent area divisions are defined for each of the four CMYK plates used in general printing. The area dividing method described in §4 described below is separately and independently performed for each image data of each CMYK version, a unique area division is performed for each of the CMYK versions, and each area is divided according to the basic method described in §2. What is necessary is just to create an entire halftone dot image for each plate and produce a plate making film for each plate. For example, with respect to the C plane image data, based on the C plane area division, the AM screening is performed on a part of the area and the FM screening is performed on another part of the area. In this way, by defining an independent area division for each edition, it is possible to switch the optimal screening for each edition. Therefore, the finally obtained image on the printed matter is a four-color image using four colors of ink.
There is an area in which gradation is represented by the size of a halftone dot for each edition, and an area in which gradation is expressed by the density of halftone dots. The area configuration will be different.

However, in the case of a general color image, the characteristics of each region do not show a large difference for each plate. For example, a highlight portion as a color image is a highlight portion for each plate, and a shadow portion as a color image is a shadow portion for each plate in many cases. In addition, characteristic portions such as a smooth portion and a complicated portion are common in each edition. For this reason, in an area division stage, a common area division may be performed for a plurality of n-version image data. As described above, when the common area division is performed, there is an advantage that the calculation load is reduced. In this case, a grayscale image may be created by synthesizing a plurality of n versions of image data, and the area division defined based on this grayscale image may be used as a common area division.

For example, FIG. 16 shows an example of a method for obtaining a common area division for the four CMYK plates. First,
A grayscale image is generated based on the four versions of CMYK. For example, a grayscale image is generated by a set of pixels in which the tone value of each pixel of the 4th plate is multiplied by a predetermined coefficient for each color, and the average value of the resulting product is used as the pixel value. A method described in §4 may be applied to this grayscale image to obtain a common area division. For both versions, AM screening or FM screening is performed based on this common area division. Therefore, the finally obtained image on the printed matter is composed of four plates using four color inks, and a region where the gradation is expressed by the size of the halftone dot for each plate, and a halftone dot There are areas where gradation is expressed by density, and the area configuration is the same for each plate.

§4. Automatic Region Division Processing As described above, in implementing the present invention, it is necessary to divide an original image into regions. This area division can be performed based on an instruction from a skilled operator. However, in practice, it is preferable to determine some objective criterion and perform automatic area division using this objective criterion. Here, three modes of the automatic area division will be described.

<4.1> Region Division Based on Tone Value As the objective criterion for performing region division, the simplest criterion is a criterion relating to the tone value of each pixel. For example, when the gradation value D is expressed by 8 bits, as shown in FIG. 17A, the minimum gradation value D (L) = 0, and the maximum gradation value D
(H) = 255, and each pixel is 2 from 0 to 255
It has one of the 56 gradation values. As mentioned in §1, when performing AM screening,
As shown in FIG. 7 (b), individual pixels are expressed by arranging halftone dots (only the center point and the outline are shown in the figure) of a size corresponding to each gradation value at a predetermined pitch. In the case of performing screening, individual pixels are represented by arranging halftone dots of a fixed size as shown in FIG. 17C at a density corresponding to each gradation value. According to an experiment performed by the inventor of the present application, as shown in FIG.
A predetermined threshold value D (th) is set for this gradation value, and the region composed of pixels having a gradation value smaller than the threshold value is classified into a first group, and the gradation value equal to or larger than the threshold value is classified If the area consisting of the pixels belonging to the first group is classified into a second group, the area belonging to the first group is subjected to the AM screening, and the area belonging to the second group is subjected to the FM screening. It was confirmed that the effect of the present invention of improving image quality was obtained.

In the example shown in the figure, the threshold value D (th) is set around a gradation value of 64. As a result, the original image has an area composed of pixels having a gradation value of less than 64 and a gradation value of 64. Is divided into a region consisting of 64 or more pixels, and the former region is represented by gradation by changing the size of a halftone dot,
For the latter area, gradation is expressed by changing the density of halftone dots.

FIG. 18 is a diagram showing the characteristics of an image on a printed material printed by the above-described method. FIG. 18 (a)
Include the gradation values D (L) to which each pixel constituting the original image has
D (H) and corresponding lightness B on printed matter
(H) to B (L) are shown as scales. In the case of printed matter, the gradation value and lightness have the opposite relationship, and the lightness decreases as the gradation value increases, because the amount of ink increases.
The lightness increases as the tone value decreases, because the amount of ink decreases. Therefore, on the scale shown in FIG. 18A, the minimum value D (L) of the gradation value is the maximum value B (H) of the lightness.
, And the maximum value D (H) of the gradation values is the minimum value B of the lightness.
(L). Now, the threshold value D (t
The lightness threshold corresponding to h) is defined as B (th), and a bright portion on the printed matter whose brightness is equal to or greater than the threshold B (th) is referred to as a highlight portion, and the brightness is determined as the threshold B (th). If a dark portion less than is referred to as a shadow portion, an image on a printed material printed by the above-described method has a gradation expressed by the size of a halftone dot in a highlight portion and a shadow portion in the image. Means that the gradation is represented by the density of the halftone dots.

As described in §1, FM screening has the advantage that interference fringes such as moiré do not occur, but has the drawback of deteriorating image quality due to the impression of “smoky feeling” or “graininess”. In particular, there is a characteristic that the “smoked feeling” and “grainy feeling” are remarkable in a highlight portion. Therefore, as shown in FIG. 18, the image is roughly divided into a highlight portion and a shadow portion with the threshold value B (th) as a boundary, the AM portion is subjected to the AM screening, and the “ The overall image quality can be improved by suppressing the "smoked feeling" and "roughness", performing FM screening on the shadow portion, and suppressing interference fringes peculiar to AM screening.

The threshold value D (th) of the gradation value or the threshold value B (th) of the lightness can be set as appropriate for each individual image. Among the halftone dots of various sizes prepared for expressing the tone value by the size (expressed by the AM screening) (for example, of the halftone dots of various sizes shown in FIG. 17B), A halftone dot having a size substantially the same as the size of a halftone dot (for example, a halftone dot of a certain size shown in FIG. 17C) prepared for expressing a tone value by density (expressed by FM screening). By setting the associated gradation value as a threshold,
It has been found that a sense of discomfort can be reduced at the boundary between the area subjected to AM screening and the area subjected to FM screening, and a high-quality image can be obtained. For example, in the case of the example shown in FIG.
The size of the halftone dot of AM screening corresponding to
Since the size of the halftone dot for screening is almost the same as the size of the halftone dot, the threshold value D (th) may be set near this gradation value 64. With such a setting, when the gradation value gradually increases from 0, the size of the halftone dot also gradually increases (AM screening), and when the gradation value reaches 64, the size of the halftone dot increases. Instead of saturating, the arrangement density of the halftone dots increases with the increase of the gradation value (FM screening).
The discontinuous impression in the vicinity of the gradation value 64 at which the switching from the AM screening to the FM screening is performed is eliminated.

<4.2> Based on Spatial Frequency of Tone Value
As an objective criterion for performing the area division, a method based on the spatial frequency, not the gradation value itself, will be described here. For example, as shown in FIGS. 19 (a) and (b),
Consider a graph in which the horizontal axis indicates a one-dimensional pixel position and the vertical axis indicates the gradation value of each pixel. Although the gradation value of a pixel changes spatially, as shown in the graph of FIG. 19A, in a region where the spatial frequency of the gradation value of the pixel is low, the spatial gradation change is gentle. This gives the impression of a “smooth part”. In contrast, FIG.
As shown in the graph of (b), the region where the spatial frequency of the gradation value of the pixel is high is a part where the spatial gradation change is abrupt, and gives an impression as a “complex part”. .
For example, in the case of an image including a portrait, a cheek portion or the like is generally a smooth portion having a low spatial frequency, and a hair portion or the like is generally a complex portion having a high spatial frequency.

Therefore, as shown in FIG. 20A, on a scale from the lowest spatial frequency f (L) to the highest spatial frequency f (H), as shown in FIG. A value f (th) is set in advance, a region including pixels having a spatial frequency less than the threshold value f (th) is classified into a first group, and a gradation value equal to or more than the threshold value f (th) is classified. If the area consisting of the pixels belonging to the first group is classified into a second group, the area belonging to the first group is subjected to the AM screening, and the area belonging to the second group is subjected to the FM screening. It was confirmed that the effect of the present invention of improving image quality was obtained. After all, on a printed matter, for a “complex part” in which the spatial frequency is equal to or higher than the threshold f (th), the gradation is expressed by the density of the halftone dots, and the spatial frequency is equal to the threshold f (th).
As for the “smooth portion” which is less than the gray level, the gradation is represented by the size of the halftone dot.

As described above, when the screening method is switched based on the spatial frequency of the gradation value of each pixel, AM screening is performed on “smooth portions” where “smoky feeling” and “graininess” are conspicuous. "Complex part" where interference fringes are noticeable
Is subjected to FM screening, so that there is an advantage of improving the image quality as a whole. In addition, since the FM screening can obtain a higher resolution than the AM screening, applying the FM screening to a “complex part” is also effective in increasing the resolution.

As a method of obtaining the spatial frequency of the gradation value of each pixel, for example, FFT (Fast Fou
An operation method such as rie transfomer) is known, and a spatial frequency value for each pixel can be obtained by performing an FFT operation on image data constituting an original image. However, since the FFT calculation has a heavy calculation load, in practice, a calculation for applying a spatial filter to image data is performed, and for each pixel, a parameter value indicating a difference in tone value between each pixel and an adjacent pixel is calculated. It suffices to determine the value and use this parameter value as a value indicating the spatial frequency.

FIG. 21 is a diagram showing the principle of the operation for operating this spatial filter. For example, consider the case where the spatial frequency of a pixel P located at the center of the original image (a part occupying 9 pixels of the original image) shown in FIG. Here, the spatial frequency of the central pixel P is calculated by dividing eight adjacent pixels P around it.
It will be determined with reference to 1 to P8. Now, the gradation value of the central pixel P is represented by P and the gradation values of the surrounding pixels P1 to P8 are represented by the same symbols as P1 to P8, respectively, and the parameter value PP for the central pixel P is represented by FIG. Defined by the formula shown in c). The numerator of the equation in FIG. 21C is the gradation value P of the center pixel P and the pixels P1 to P
8 shows the sum of the differences from the gradation values P1 to P8 of the image No. 8, and k of the denominator is a constant for normalization. In short, the parameter value PP indicates the difference in the gradation value between the pixel P and the pixel adjacent thereto, and the larger this value is, the larger the gradation difference is generated between the pixel P and the adjacent pixel.

When the parameter value PP is obtained by performing the operation shown in FIG. 21 (c) on the pixel P shown in the center of FIG. 21 (a), as shown in FIG. 21 (b) , A reference image having the parameter value PP as a pixel value is defined. In FIG. 21 (b), only the parameter value PP for the center pixel is defined, but the parameter values can be defined for the other pixels in the same manner (strictly speaking, Y row X A reference image having a pixel array of (Y-2) rows and (X-2) columns can be defined for an original image having a column pixel array). The reference image defined in this way can be said to be an image having, as a pixel value, a parameter value indicating a spatial frequency of a gradation value of each pixel constituting the original image. Therefore, if a predetermined threshold value for this parameter value is set as the threshold value f (th) of the spatial frequency, it is possible to perform region division based on this reference image. That is, among the pixels constituting the reference image, a region including pixels having a pixel value less than the threshold value is defined as a first group, and a region including pixels having a pixel value equal to or greater than the threshold value is defined as a second group. Can be classified.

For example, if this technique is applied to the procedure shown in FIG. 10, the above-mentioned spatial filter is applied to the entire image Z0 to define the reference image Zr. Then, of the pixels constituting the reference image Zr, a mask M1 (a white portion thereof) is created based on an area composed of pixels having a pixel value less than the threshold value, and a pixel having a pixel value equal to or greater than the threshold value is created. (The white portion of the mask M2) may be created based on the region consisting of.

In the above-mentioned example, as shown in FIG. 21A, eight pixels P1 to
The spatial filter for calculating the difference between the gradation value of P8 and P8 is applied.
A spatial filter for calculating the difference between the tone values of P4, P5 and P7 may be applied. Alternatively, a spatial filter for calculating the difference between the gradation values of more than eight adjacent pixels may be applied. In any case,
The operation of applying the spatial filter is lighter in load than the operation of the FFT, and is therefore a very practical operation as a method for obtaining a spatial frequency.

<4.3> The relationship between the gradation value and its spatial frequency
Region Division Based on Both Finally, a method of performing region division according to both the gradation value of a pixel and the spatial frequency of this gradation value will be described.
For example, as shown in FIG. 22, the axis of the spatial frequency is defined in the horizontal direction. In this example, the left end of the figure shows the lowest spatial frequency f (L), and the right end of the figure shows the highest spatial frequency f (L).
(H) is shown. In other words, the spatial frequency is lower and the “smooth state” is shown on the left side of the figure, and the spatial frequency is higher and the “complex state” is shown on the right side of the figure. on the other hand,
The axis of the gradation value is defined in the vertical direction. In this example, the upper end of the figure shows the minimum gradation value D (L), and the lower end of the figure shows the maximum gradation value D (H). In addition, as mentioned above,
The gradation value on the image data and the lightness on the printed matter are integrated front and back, and an axis of the lightness can be defined in the vertical direction of the drawing. In this example, the upper end of the figure shows the highest brightness B (H), and the lower end of the figure shows the lowest brightness B (L). In other words, the image on the printed matter is brighter toward the upper side of the figure and darker toward the lower side of the figure.

Here, the threshold value f (t
h), a threshold value D (th) or B (th) is set on the gradation value axis or the lightness axis, and the two-dimensional area having the spatial frequency and the gradation value (or lightness) as parameters is defined. As shown in the figure, the image is divided into four areas of attributes A to D. For example, in the region of the attribute A, the spatial frequency has a threshold value f (t
h) and the tone value is less than the threshold value D (th) (or the brightness is equal to or more than the threshold value B (th)). The present inventor has defined such four attributes,
A region consisting of pixels belonging to the attribute A is classified into a first group and subjected to AM screening, and attributes B, C,
It was confirmed that a high-quality printed material could be obtained by classifying the region consisting of pixels belonging to D into the second group and subjecting it to FM screening. In other words, on this printed matter, for a portion where the brightness is equal to or more than the threshold value B (th) and the spatial frequency is less than the threshold value f (th), the gradation is determined by the size of the halftone dot. Is expressed, and in the other parts, the gradation is expressed by the density of the halftone dots.

Such a classification method is described in §4.1 described above.
Alternatively, the classification method described in §4.2 is slightly modified. That is, according to the method described in §4.1, the pixels belonging to the attribute B are pixels smaller than the threshold value D (th). It should be targeted, but because of high spatial frequency, it is classified into the second group and is targeted for FM screening. As a result, it is possible to suppress the occurrence of interference fringes such as moire and to obtain the advantage of increasing the resolution. On the other hand, according to the method described in §4.2, the pixels belonging to the attribute C are pixels smaller than the threshold value f (th). Although it should be targeted, it is classified into the second group and is subjected to FM screening because of its high tone value. Thereby, generation of interference fringes such as moire can be suppressed. Also, since the gradation value is high, FM
The granularity peculiar to the screening is not noticeable.

As described above, if the area is divided based on both the gradation value and its spatial frequency, finer classification can be performed.

As described above, the present invention has been described based on the illustrated embodiments. However, the present invention is not limited to these embodiments, and can be implemented in various other forms. In particular, the present invention is applicable to a case where a gradation image is printed using only one color ink and a case where a color gradation image is printed using a plurality of color inks. In short, the basic idea of the present invention is to perform gradation expression by AM screening on a part of one image and to perform gradation expression by FM screening on the remaining part, deviating from this basic idea. Unless otherwise specified, the present invention may be implemented in any manner. In the above-described embodiment, a printing plate is produced based on a plate making film and printing is performed. However, the present invention is directed to a direct printing plate system or a plateless printing system capable of performing printing without using a plate making film. It is also applicable to printing systems.

[0077]

As described above, according to the method of printing a digital gradation image according to the present invention, the AM of each image
Since halftone prints are produced by selecting either screening or FM screening, print quality can be improved.

[Brief description of the drawings]

FIG. 1 is a diagram showing a pixel array defined for each of four versions of CMYK.

FIG. 2 is a plan view showing an example of halftone dots obtained by performing an AM screening on image data having the pixel arrangement shown in FIG.

FIG. 3 is a plan view showing an example of a dot configuration in AM screening.

FIG. 4 is a plan view showing an example of a dot configuration in FM screening.

FIG. 5 is a plan view showing an example of setting a screen angle for four CMYK plates in a conventional AM screening.

FIG. 6 is a diagram showing an example of an original image to which a printing method according to the present invention is applied.

FIG. 7 is a diagram showing an example of area division defined based on the original image shown in FIG.

8 is a diagram showing an example in which each area has an AM based on the area division shown in FIG.
It is a figure showing work which performs screening or FM screening.

9 is a diagram showing a first halftone image obtained by the AM screening shown in FIG. 8 and a second halftone image obtained by the FM screening.

FIG. 10 is a diagram showing a procedure of a more specific method of mixing AM screening and FM screening on an original image.

FIG. 11 is a plan view showing ink bleeding in a printing process.

FIG. 12 is a graph showing a general concept of dot gain correction.

FIG. 13 is a plan view showing the ratio of the spread width of ink to the size of a halftone dot.

FIG. 14 is a diagram illustrating a process of deleting a minute area at the time of area division.

FIG. 15 is a diagram illustrating a method of defining a region division for each of the plates constituting a color image.

FIG. 16 is a diagram showing a method of defining a region division common to each plate constituting a color image.

FIG. 17 is a conceptual diagram showing an example of a halftone dot arranged according to each gradation value and showing a concept of performing area division based on a threshold value D (th) of the gradation value.

FIG. 18 is a conceptual diagram for performing region division based on a lightness threshold B (th).

FIG. 19 is a graph showing a spatial frequency of a gradation value of a pixel.

FIG. 20 is a conceptual diagram for performing region division based on a spatial frequency threshold value f (th).

FIG. 21 is a diagram showing the principle of an operation for applying a spatial filter for obtaining a spatial frequency of a gradation value.

FIG. 22 is a conceptual diagram of performing region division based on both a tone value of a pixel and its spatial frequency.

[Explanation of symbols]

dx, dy: Halftone dot arrangement pitch D, D1, D2: Halftone dot diameter g1 to g4: Non-consideration area M1, M2: Mask P: Pixel / pixel value P1 to P8: Pixel / pixel value PP: Parameter value Q: Halftone dot Z0 ... Overall image Z1 ... First halftone image (AM screening) Z2 ... Second halftone image (FM screening) Z1 ^* ... First halftone image Z2 ^* ... Necessary part extracted only necessary part The second halftone image Z3: the entire halftone image Δr: the bleed width of the ink

Claims (22)

[Claims] 1. A method of printing a digital gradation image on a predetermined medium, comprising: preparing image data consisting of a set of pixels having a predetermined gradation value; An area dividing step of dividing the image shown into an area belonging to a first group and an area belonging to a second group; and performing AM screening on image data of the area belonging to the first group. Performing an FM screening on image data of an area belonging to the second group, and generating a halftone image in which halftone dots are arranged on the basis of the gradation value of each pixel; And a printing step of printing based on the halftone dot image. 2. A method for printing a digital gradation image on a predetermined medium, comprising: preparing image data consisting of a set of pixels having a predetermined gradation value; An area dividing step of dividing the entire image shown into an area belonging to a first group and an area belonging to a second group; and for each area belonging to the first group, image data in each area. A first halftone image creating step of creating a first halftone image in which halftone dots having a size corresponding to the gradation value of a pixel are arranged at a predetermined pitch based on A second halftone image in which halftone dots having a predetermined size are arranged at a density corresponding to the gradation value of a pixel based on image data in each area for each individual area Creating a point image; and the first halftone dot Combining an image and the second halftone image to create an entire halftone image corresponding to the entire image, a printing step of printing based on the entire halftone image, and A method for printing a digital gradation image, comprising: 3. The method of printing a digital gradation image according to claim 2, wherein the first halftone image creating step and the second halftone image creating step are overlapped for all the regions constituting the entire image. In the whole halftone image creating step, the area belonging to the first group of the first halftone image and the area belonging to the second group of the second halftone image are synthesized. A digital halftone image printing method, wherein an entire halftone image is created. 4. The method for printing a digital gradation image according to claim 1, wherein in the area dividing step, the area belonging to the first group and the second
After defining a region belonging to the group, a region less than the predetermined reference area is recognized as a non-considered region, and this non-considered region disappears by treating it as a part of another region including it. A method for printing a digital gradation image, comprising: 5. The method for printing a digital gradation image according to claim 1, wherein, in the image data preparation step, image data composed of a set of pixels each having a predetermined gradation value is converted into each color. A step of preparing a total of n versions of image data for each component plate, and performing a common area division on the n versions of image data in the area division step. Method. 6. A method for printing a digital gradation image according to claim 5, wherein a grayscale image is created by combining n-version image data, and a common area division is performed based on the grayscale image. A method for printing a digital gradation image. 7. The digital gradation image printing method according to claim 1, wherein in the image data preparation step, image data composed of a set of pixels each having a predetermined gradation value is converted into each color image data. A digital gradation image which is prepared for each plate of the component so as to prepare image data consisting of a total of n plates, and performing a separate region division for each of the n plates of image data in a region dividing step. Printing method. 8. The method for printing a digital gradation image according to claim 1, wherein a group to which the pixel belongs is determined in the region division step according to a gradation value of the pixel. Printing method of digital gradation image. 9. The method of printing a digital gradation image according to claim 8, wherein a predetermined threshold value is set for a gradation value of the pixel, and a pixel having a gradation value equal to or larger than the threshold value is set to a second value. A method of printing a digital gradation image, wherein the method belongs to a group and pixels having a gradation value less than a threshold value belong to a first group. 10. The method for printing a digital gradation image according to claim 9, wherein the gradation value is represented by a density among halftone dots of various sizes prepared for expressing the gradation value by the size. A digital gradation image printing method, characterized in that a gradation value associated with a halftone dot having substantially the same size as a halftone dot prepared for expression is set as a threshold value. 11. The digital gradation image printing method according to claim 1, wherein, in the region division step, a group to which the pixel belongs according to the spatial frequency of the gradation value of the pixel is determined. Characteristic digital gradation image printing method. 12. The digital gradation image printing method according to claim 11, wherein a predetermined threshold value is set for a spatial frequency of a gradation value of the pixel, and a pixel having a spatial frequency equal to or higher than the threshold value is determined. 2. A method for printing a digital gradation image, wherein pixels belonging to a second group belong to a first group and pixels having a spatial frequency lower than a threshold value belong to a first group. 13. The method for printing a digital gradation image according to claim 11, wherein an operation of applying a spatial filter to the image data prepared in the image data preparation stage is performed, and each pixel is individually processed. A digital gradation method comprising: obtaining a parameter value indicating a difference in a gradation value from an adjacent pixel; creating a reference image having the parameter value as a pixel value; and defining a region division based on the reference image. How to print the image. 14. The method of printing a digital gradation image according to claim 1, wherein in the area dividing step, both the gradation value of the pixel and the spatial frequency of the gradation value are used. A digital gradation image printing method, wherein a group to which the image belongs is determined. 15. The method of printing a digital gradation image according to claim 14, wherein a first threshold value is set for a gradation value of the pixel, and a second threshold is set for a spatial frequency of the gradation value of the pixel. A threshold value, pixels having a tone value less than a first threshold value and having a spatial frequency less than a second threshold value belong to a first group,
A method of printing a digital gradation image, wherein other pixels belong to a second group. 16. The method for printing a digital gradation image according to claim 1, wherein an area belonging to the first group and an area belonging to the second group are different from each other. A method for printing a digital gradation image, wherein halftone dots are formed after performing dot gain correction. 17. A printed matter representing a gradation image produced by the printing method according to claim 1. Description: 18. A printed matter in which a gradation image is expressed by a number of halftone dots, wherein a gradation is expressed by a halftone dot size in a part of an area, and a gradation is expressed by a halftone dot in another part of the area. Is a printed matter on which a gradation image is represented, in which gradation is represented by the density of halftone dots. 19. The printed matter according to claim 18, wherein a gray scale is expressed by a size of a halftone dot in a highlight portion having a brightness equal to or higher than a predetermined threshold value, and the brightness is set to the threshold value. A printed matter in which a gradation image is represented, wherein a gradation is represented by a density of halftone dots for a shadow portion of less than. 20. The printed matter according to claim 18, wherein, for a portion where the spatial frequency is equal to or higher than a predetermined threshold value, a gradation is expressed by halftone dot density, and the spatial frequency is lower than the threshold value. Is a printed matter on which a gradation image is represented, wherein gradation is represented by the size of a halftone dot. 21. The printed matter according to claim 18, wherein, for a portion where the brightness is equal to or more than a predetermined threshold value and the spatial frequency is lower than the predetermined threshold value, the gradation is determined by the size of the halftone dot. And a printed matter in which a gray scale image is expressed in the other parts, in which the gray scale is expressed by the density of halftone dots. 22. The printed matter according to any one of claims 17 to 21, wherein the printed matter is composed of n plates using n colors of ink, and the gradation is expressed by the size of a halftone dot for each plate. A printed matter on which a gradation image is expressed, characterized by having a shaded portion and a portion where gradation is represented by the density of halftone dots.