JPH08160601A - Half-toning method for image - Google Patents

Abstract

PURPOSE: To improve various characteristics such as the dot gain characteristic and the rough deposit feeling of an image. CONSTITUTION: A threshold matrix is divided into multiple rectangular regions having multiple spots respectively. At least one of the multiple rectangular regions differs in size from other rectangular regions. A uniform threshold is allocated to multiple spots contained in each rectangular region, and different threshold values are allocated for multiple rectangular regions respectively. A relatively large threshold matrix is divided into multiple intermediate rectangular regions, and the multiple intermediate rectangular regions may be divided into multiple rectangular regions in different classification respectively.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of halftoning multi-tone image data using a threshold matrix.

[0002]

2. Description of the Related Art In color offset printing, an original image such as a photographic image is separated into four color separation images corresponding to four color inks of yellow, magenta, cyan, and black, and the color separation image of each ink is printed. A printed material is created by overlapping.

The halftone image on the printing plate corresponding to each ink is expressed by the size of minute dots (dots) on which the ink is placed. The halftone dots are arranged in a grid pattern at regular intervals, and the size of each halftone dot changes according to the density of the original image. In other words, reproduction of an image with halftone dots is a method of expressing image density by amplitude modulation. On the other hand, in recent years, a method called “FM screening” that expresses the density of an image by frequency modulation has been put into practical use. In FM screening,
The size of the dots on which the ink is placed is fixed, and the appearance frequency of the dots changes according to the density of the image. In FM screening, small dots are dispersed and arranged as compared with the conventional halftone dots, so that the original image can be reproduced with high resolution. Further, since the appearance of dots has no periodicity, it has an advantage that moire and rosette patterns are less likely to occur even when multicolor printing is performed.

However, since the size of each dot is small in the FM screening, the dot gain characteristic (change of dot area ratio) when making a printing plate from a halftone film and the dot gain characteristic at the time of printing are the same as those of the conventional halftone dot. Is quite different from. That is, in the FM screening, the dot gain tends to be smaller in the highlight region (low density region) in the image than in the conventional halftone dot.
On the other hand, in the halftone area to the shadow area (high density area), the dot gain becomes larger than that of the conventional halftone dot.

In FM screening, it is possible to adjust the size of one unit of dot. For example, when recording a halftone image (binary image) created by FM screening on a recording medium such as a photosensitive film, a predetermined number of light spots (for example, 2 × 2 spots) is set as a minimum dot unit. be able to.

[0006]

However, if the dot unit is made smaller, the dot gain in the shadow area becomes larger, and there is a problem that the white spots are crushed in black. On the other hand, when the dot unit is increased, there is a problem that a reproduced image has a feeling of graininess and the reproducibility of the detail is deteriorated.

The present invention has been made to solve the above-mentioned problems in the prior art, and provides a method of halftoning an image capable of improving various characteristics such as dot gain characteristics and graininess of an image. The purpose is to do.

[0008]

In order to solve the above problems, an image halftoning method according to a first aspect of the present invention comprises: (a) a threshold matrix, a plurality of rectangles each having a plurality of spots; While dividing into areas
Dividing the threshold matrix so that at least one of the plurality of rectangular areas has a different size from other rectangular areas; and (b) assigning equal thresholds to the plurality of spots included in each rectangular area. At the same time, a step of determining a threshold array in the threshold matrix by assigning different thresholds to each of the plurality of rectangular areas, and (c) reading the thresholds from the threshold matrix to obtain a multi-tone image. Halftoning the multi-tone image data by comparing it with the data.

By dividing the threshold matrix so that at least one of the plurality of rectangular areas has a size different from that of the other rectangular areas, and by assigning equal thresholds to the plurality of spots included in each rectangular area. During the halftoning of the image, spots of different sizes are sequentially recorded in dot units. Therefore, by adjusting the thresholds assigned to each rectangular area,
It is possible to improve various characteristics such as dot gain characteristics and graininess of an image.

In the method of halftoning an image according to a second aspect, the step (b) includes a step of setting a minimum threshold value for a minimum rectangular area among a plurality of rectangular areas.

If the minimum threshold value is set for the minimum rectangular area, the minimum rectangular area is recorded at the lowest density, so that the graininess of the image can be suppressed in the highlight area.

In the method of halftoning an image according to a third aspect, the step (b) comprises a step of setting a maximum threshold value for a maximum rectangular area among a plurality of rectangular areas.

If the maximum threshold value is set for the maximum rectangular area, the maximum rectangular area remains as a blank area until the density becomes the highest, so that the dot gain in the shadow area can be reduced.

In the method of halftoning an image according to claim 4, in the step (a), the threshold matrix is divided into N intermediate rectangular regions (N is an integer of 2 or more) of equal size. And dividing the plurality of intermediate rectangular areas into M (M is an integer of 2 or more) rectangular areas in different sections, thereby forming N × M rectangular areas in the threshold matrix. And the step (b) includes a step of sequentially setting the smallest N threshold values for the smallest rectangular area in each intermediate rectangular area.

The plurality of intermediate rectangular areas are divided into M rectangular areas in different sections, and the smallest N threshold values are sequentially set for the smallest rectangular area in each intermediate rectangular area. By doing so, the smallest rectangular area in each intermediate rectangular area is sequentially recorded as the image density increases, so that dots randomly appear in the highlight area. Therefore, when a color image is reproduced by superimposing halftone images of a plurality of colors, moire due to interference is unlikely to occur.

In the image halftoning method according to the fifth aspect, the step (b) includes a step of sequentially setting the largest N threshold values for the largest rectangular area in each intermediate rectangular area. Including.

If the largest N threshold values are sequentially set for the largest rectangular area in each intermediate rectangular area, the white spots will be random when the image density becomes close to 100%. Remain in the correct position. Therefore, when a color image is reproduced by superimposing halftone images of a plurality of colors, moire due to interference is unlikely to occur.

[0018]

【Example】

A. Difference between halftone dot and FM screening: FIG. 1 is an explanatory diagram showing a change in dot shape of a conventional halftone dot (square dot). Further, FIG. 2 is an explanatory diagram showing changes in the dot shape of FM screening. 1 and 2 show how halftone dots and FM dots change in accordance with the density in a square image area of the same size. In the halftone dot shown in FIG. 1, the area of one dot portion grows as the density increases, whereas in the FM screening shown in FIG. 2, the frequency of appearance of the dots (that is, as the density increases). The frequency of appearance) is higher (of course, the area of the dot part is also increasing).

FIG. 3 is a graph showing the dot gain characteristics of halftone dots and FM screening. The halftone dots have different dot gain characteristics depending on the screen ruling, the dot shape, and various other printing conditions.
75 lines / inch square dot with a resolution of 4000d
An example of characteristics in the case of recording with a pi recording device is shown. In addition, the dot gain characteristic of FM screening is
Using a recording device with a resolution of 4000 dpi, the minimum dot is fixed to 3 × 3 spots, and other printing conditions are 17
This is the characteristic when it is the same as the case of the 5-line halftone dot. Here, the "spot" is the smallest unit that is blackened in the recording apparatus. Since the width of one spot is about 6.35 μm at 4000 dpi, the width of one dot having a size of 3 × 3 spots is about 20 μm.

As can be seen from FIG. 3, in the FM screening, the dot gain in the highlight area (low halftone area) is smaller than that of the halftone dot, and the shadow area (high halftone area).
The dot gain is larger than the halftone dot. The printing machine is adjusted so as to print an image with good reproducibility on the premise of the dot gain characteristics of halftone dots. Therefore, even when halftoning (binarization) of an image is performed using FM screening, it is preferable that the dot gain characteristic be as close as possible to the conventional halftone dot. For this purpose, it is preferable to increase the dot size in FM screening.

On the other hand, one of the features of FM screening is that an original image can be reproduced with high resolution. From this point, it is preferable to reduce the dot size. In addition, since the graininess of the image occurs when the dot size is increased, it is preferable to reduce the dot size also from this point.

The size of the dots to be recorded is determined by the threshold array in the threshold matrix. In consideration of the above-mentioned various characteristics, the various embodiments shown below are FM
The threshold array in the threshold matrix used for screening is devised.

B. First embodiment of threshold matrix: FIG.
FIG. 3 is an explanatory diagram showing a first embodiment of a threshold value matrix of the present invention and a change in the shape of dots generated thereby. As shown in FIG. 4A, this threshold matrix (also called “dither matrix”) is 10 × 10.
It has a spot size, and is divided into 4: 3: 3 along the main scanning direction Y and the sub-scanning direction X, respectively.
FIG. 5 is an explanatory diagram showing details of the threshold matrix shown in FIG. The area of 10 × 10 spots is divided into nine rectangular areas surrounded by thick lines in the figure, and the same threshold value is assigned to a plurality of spots included in each rectangular area. 99 is assigned as a threshold to the largest rectangular area of 4 × 4 spots. There are four rectangular areas of 4 × 3 spots of intermediate size, and the threshold value is 1
3, 36, 59 and 82 are assigned respectively.
Further, there are four rectangular areas each having the smallest 3 × 3 spot, and 0, 23, 46, and 69 are respectively assigned.

In this specification, both 4 × 3 spots and 3 × 4 spots are referred to as “4 × 3 spots”. That is, the term “n × m spot” is
It simply means the size of the rectangular area.

When halftoning an image, the threshold matrix is applied to the image so that such threshold matrices are spread in the image plane without overlapping and without making a gap. That is, the threshold matrix is tiled on the image plane. Then, the multi-tone image data is halftoned by comparing the image data with the threshold values in the threshold matrix.

FIGS. 4B to 4D show how the shape of the dots printed changes as the image density increases. In the embodiment of the present invention, when recording a halftone image in the recording apparatus, the multi-tone image data ID
And the threshold TD satisfy the following inequality, the spot is blackened. Threshold TD <Image data ID (1)

That is, the value of the threshold TD is the image data ID.
When it is less than, the spot is blackened. Therefore, a rectangular area having a small threshold is blackened at a low image density, and a rectangular area having a large threshold is blackened at a high image density.
What is described in the lower part of each of FIGS. 4B to 4D is the value of the blackened area ratio of the halftone image. For example, in FIG. 4B, only one 3 × 3 region having a threshold value of 0 is blackened, and 9 of 100 spots in the threshold matrix are blackened. Therefore, the area ratio of the blackened portion of this halftone image is 9%.

The minimum threshold value (= 0) is assigned to the 3 × 3 spot area having the smallest size. On the other hand, the maximum threshold value (= 99) is assigned to the 4 × 4 spot area having the largest size. An intermediate threshold value is assigned to the intermediate size 4 × 3 spot region.
Therefore, as shown in FIG. 4B, 3 × 3 spots are blackened in the highlight region (low density region), and as shown in FIG. 4C, 3 × 3 spots and 4 × 3 spots are formed in the halftone region. Is blackened, and 4 × 4 spots remain as white spots in the shadow region (high density region) as shown in FIG.

If different thresholds are assigned to 100 spots in the threshold matrix, 100 spots will be used.
The gradation can be expressed smoothly. On the contrary,
As shown in FIG. 5, if a region of 10 × 10 spots is divided into nine rectangular regions and equal thresholds are assigned to a plurality of spots included in each rectangular region, only 10 gradations can be expressed. is there. However, the threshold matrix shown in FIG. 5 has an advantage that a better dot gain characteristic can be obtained as compared with the case where different thresholds are assigned to 100 spots.

The number of gradations can be increased by increasing the size of the threshold matrix shown in FIG. Further, in the example of FIG. 5, discrete threshold values such as 0, 13, 23, ... Are assigned to each rectangular area, but 0, 1, 2, 3 ,.
You may make it allocate continuous threshold values, such as. For example, when the image data is represented by 8 bits,
If a threshold matrix having at least 255 rectangular areas is created and continuous threshold values of 0 to 254 are assigned to each rectangular area, 256 gradations can be smoothly expressed.

FIG. 6 is a graph showing the dot gain characteristic with respect to the threshold value matrix of the first embodiment. FIG.
As shown in (B), since the 3 × 3 spot area, which is the smallest rectangular area in the highlight area (low-density area), is blackened, the dot gain characteristic (dotted line) of the first embodiment in the highlight area is indicated. ) Is the size of 1 dot is 3 × 3
This is almost the same as when the spot is constant (broken line). on the other hand,
As shown in FIG. 4D, the shadow area (high density area)
Since the largest rectangular area, 4 × 4 spot area, remains as a blank area, the dot gain characteristic (dotted line) of the first embodiment in the shadow area shows that the size of one dot is 3 × 3 spots ( An intermediate characteristic between the dotted line) and the conventional halftone dot (solid line) is shown.

That is, in the threshold matrix shown in FIGS. 4 and 5, since the maximum threshold value (= 99) is assigned to the largest rectangular area of the plurality of rectangular areas, the shadow matrix is shadowed as compared with the conventional FM screening. The dot gain in the area is small. Therefore, there is an effect that the white areas are less likely to be blackened by being crushed in the shadow area.

Further, in the threshold matrix shown in FIGS. 4 and 5, since the minimum threshold value (= 0) is assigned to the minimum rectangular area, the dot size in the highlight area remains relatively small. A halftone image obtained by FM screening tends to have graininess in the highlight region, and the larger the dot size, the more noticeable the graininess. In the threshold value matrix of FIGS. 4 and 5, since the dot size in the highlight area is suppressed to be small, there is an effect that the graininess of the image can be suppressed.

FIG. 7 is an explanatory diagram showing another threshold value matrix in the first embodiment and changes in the shape of the dots generated thereby. As shown in FIG. 7A, this threshold matrix is obtained by changing the division method of the threshold matrix shown in FIG. 4A, and the size of the rectangular area to which each threshold is assigned is shown in FIG. Same as A).
Therefore, the threshold matrix shown in FIG.
It has the same characteristics as the threshold matrix shown in FIG.

FIG. 8 is an explanatory diagram showing still another threshold value matrix in the first embodiment and changes in the shape of the dots generated thereby. As shown in FIG. 8A, this threshold value matrix has a region of 20 × 20 spots and four intermediate rectangular regions M of a size of 10 × 10 spots.
It is divided into R1 to MR4, and each 10 × 10 spot region is divided into nine. Also, each 10 × 10
The manner of division within the spot area also differs.
As a result, the 20 × 20 threshold matrix is divided into 36 rectangular areas.

The threshold matrix shown in FIG. 8A is 400
Since it has 3 spots, the maximum value of the threshold is 399
Is set to When the thresholds in FIG. 8A are arranged in descending order, they become 399, 383, 366, 350, ..., The four largest thresholds are the respective intermediate rectangular regions MR1 to MR.
It is assigned to the 4 × 4 spot area of R4. This can be understood from the dot shape shown in FIG. Therefore, the threshold matrix of FIG. 8A has almost the same characteristics as the threshold matrix shown in FIG. 4A in the shadow area.

Further, when the threshold values in FIG. 8A are arranged in ascending order, they are 0, 9, 18, 28 ... And the four smallest threshold values are all assigned to the 3 × 3 spot area. This can be understood from the dot shape shown in FIG. Therefore, the threshold matrix of FIG. 8A has almost the same characteristics as the threshold matrix shown in FIG. 4A even in the highlight area.

As shown in FIG. 8A, each 10 ×
If the 10 spot areas are divided in different ways, dots appear to appear randomly, as can be seen from FIGS. 8B and 8C, so that halftone images of a plurality of colors are superimposed. When the color images are reproduced by matching, there is an advantage that moire due to interference hardly occurs.

In the method of partitioning a relatively large threshold matrix as shown in FIG. 8A, in general, it is partitioned into N (N is an integer of 2 or more) intermediate rectangular areas of equal size, and N It is possible to divide each of the intermediate rectangular regions into M rectangular regions (M is an integer of 2 or more) in different sections.

C. Second embodiment of threshold matrix: FIG. 9
FIG. 8 is an explanatory diagram showing a second embodiment of the threshold value matrix of the present invention and a change in the shape of dots generated thereby. This threshold matrix has a size of 10 × 14 spots and is divided into nine rectangular areas in total. There is one rectangular area with the largest 5x4 spot,
All others are rectangular areas of 5 × 3 spots.

The minimum threshold value (= 0) is assigned to the 5 × 3 spot area. On the other hand, the maximum value of the threshold (= 13
9) is allocated to the maximum size 5 × 4 spot area. Therefore, as shown in FIGS. 9B and 9C, 5 × 3 spots are blackened in the highlight region (low density region) and the halftone region, and the shadow region (high density region) is darkened as shown in FIG. 9D. In the area), 5 × 4 spots remain as white spots.

FIG. 10 is a graph showing the dot gain characteristic when the threshold matrix of the second embodiment shown in FIG. 9A is used. The threshold matrix shown in FIG. 9A is
This is the same as the first embodiment shown in FIG. 4A in that the minimum value of the threshold is assigned to the rectangular area of the minimum size.
Therefore, the characteristic of the highlight region is close to the characteristic regarding the threshold matrix shown in FIG. Further, since the maximum value of the threshold value is assigned to the 5 × 4 spot area, the dot gain in the shadow area is also relatively small, and therefore, it is possible to reduce the collapse of the blank area.

FIG. 11 is an explanatory diagram showing another threshold value matrix in the second embodiment and changes in the shape of the dots generated thereby. As shown in FIG. 11 (A),
This threshold matrix is obtained by changing the division method of the threshold matrix shown in FIG. 9A, and the size of the rectangular area to which each threshold is assigned is the same as that in FIG. 9A. Therefore, the threshold matrix shown in FIG. 11A has the same characteristics as the threshold matrix shown in FIG. 9A.

FIG. 12 is an explanatory diagram showing still another threshold value matrix in the second embodiment and changes in the shape of dots generated thereby. As shown in FIG. 12A, this threshold matrix divides the area of 20 × 28 spots into four intermediate rectangular areas of the size of 10 × 14 spots, and further divides each of the 10 × 14 spot areas into nine areas. It is divided into. Further, the manner of division within each 10 × 14 spot area also differs. As a result,
The 20 × 28 threshold matrix is divided into 36 rectangular areas.

The threshold matrix shown in FIG. 12A is 56.
Since it has 0 spots, the maximum threshold value is 55.
It is set to 9. When the thresholds in FIG. 12A are arranged in descending order, they are 559, 538, 518, 497, ..., The four largest thresholds are all assigned to the 5 × 4 spot area, which is the largest rectangular area. This can be understood from the dot shape shown in FIG. Further, when the threshold values in FIG. 12A are arranged in ascending order, they are 0, 15, 31, 46, ...
Each of the two thresholds is assigned to the 5 × 3 spot area, which is the smallest rectangular area. This can be understood from the dot shape shown in FIG. Therefore, FIG.
The threshold matrix of (A) has almost the same characteristics as the threshold matrix shown in FIG.

As shown in FIG. 12A, if the four 10 × 14 spot areas are divided in different ways, moire due to interference of halftone images of a plurality of colors in the color image. Has the advantage of being less likely to occur.

D. Third embodiment of threshold matrix: FIG.
3A and 3B are explanatory diagrams showing a third embodiment of the threshold value matrix of the present invention and changes in the shape of dots generated thereby. This threshold matrix has a size of 15 × 13 spots and is divided into 10 rectangular areas in total. Nine of the 10 rectangular areas are 5 × 4 spot areas, and the remaining one is a 5 × 3 spot area. The minimum value of the threshold (= 0) is the minimum rectangular area 5 ×
Allocated to 3 spot areas, other threshold is 5 ×
It is assigned to four spot areas. Therefore, FIG.
As shown in FIG. 13B, 5 × 3 spots are blackened in the highlight region (low density region), and 5 × 4 spots are blackened in the halftone region as shown in FIG. 13C. In addition, FIG.
As shown in (D), it is 5 in the shadow area (high density area).
× 4 spots remain as blank areas.

FIG. 14 is a graph showing the dot gain characteristic when the threshold matrix of the third embodiment shown in FIG. 13 (A) is used. The threshold value matrix shown in FIG. 13A is that the maximum value of the threshold value is assigned to the rectangular area of the maximum size, and the minimum value of the threshold value is assigned to the rectangular area of the minimum size. This is the same as the first embodiment shown in FIG. 9A and the second embodiment shown in FIG.
Therefore, the dot gain characteristics of the highlight area and the shadow area show characteristics close to those of the first and second embodiments, respectively. Therefore, as in the first and second embodiments, it is possible to reduce the crushing of the white void portion in the shadow region. In addition, since the unit of blackening in the halftone area is relatively large at 5 × 4 spots, there is a feature that the dot gain in the halftone area is reduced.

FIG. 15 is an explanatory diagram showing another threshold value matrix in the third embodiment and changes in the shape of the dots generated thereby. As shown in FIG.
This threshold matrix is obtained by changing the division method of the threshold matrix shown in FIG. 13A, and the size of the rectangular area to which each threshold is assigned is the same as that in FIG. 13A. Therefore, the threshold matrix shown in FIG. 15A has the same characteristics as the threshold matrix shown in FIG.

FIG. 16 is an explanatory view showing still another threshold value matrix in the third embodiment, and FIG. 17 is shown in FIG.
FIG. 6 is an explanatory diagram showing changes in the shape of dots generated by the threshold value matrix of No. 6; As shown in FIG. 16, this threshold matrix has an area of 30 × 26 spots of 15 × 1.
The image is divided into four intermediate rectangular areas each having a size of 3 spots, and each 15 × 13 spot area is divided into 10 areas. Further, the manner of division within each 15 × 13 spot area also differs. As a result, 30 × 26
The threshold matrix of is divided into 40 rectangular areas.

Since the threshold matrix shown in FIG. 16 has 780 spots, the maximum threshold value is set to 779. When the thresholds in FIG. 16 are arranged in descending order, they are 779, 759, 738, 718, ..., But the four largest thresholds are all assigned to the 5 × 4 spot area. This can be understood from the dot shape shown in FIG. Further, when the threshold values in FIG. 16 are arranged in ascending order, they are 0, 15, 31, 46, ..., But the four smallest threshold values are all assigned to the 5 × 3 spot area. This can be understood from the dot shape shown in FIG. Therefore, the threshold matrix shown in FIG. 16 has substantially the same characteristics as the threshold matrix shown in FIG.

As shown in FIG. 16, four 15 ×
If the 13 spot areas are divided in different ways, there is an advantage that moiré is less likely to occur in a color image due to interference of halftone images of a plurality of colors.

E. Fourth embodiment of threshold matrix: FIG.
8A and 8B are explanatory diagrams showing a fourth embodiment of the threshold value matrix of the present invention and a change in the shape of dots generated thereby. This threshold matrix has a size of 10 × 10 spots, is divided into 4: 3: 3 along the main scanning direction Y, and is divided into 3: 4: 3 along the sub scanning direction X. . That is, the area of 10 × 10 spots is divided into nine rectangular areas in total. There is one largest rectangular area of 4 × 4 spots, four rectangular areas of intermediate size 4 × 3 spots, and four smallest rectangular areas of 3 × 3 spots.

The minimum value (= 0) of the threshold is 3 of the minimum size.
It is assigned to the × 3 spot area. On the other hand, the maximum threshold value (= 99) is assigned to the intermediate size 4 × 3 spot region. An intermediate threshold value (44) is assigned to the maximum size 4 × 4 spot area.
Therefore, as shown in FIG. 18B, the 3 × 3 spot is blackened in the highlight region (low-density region), and FIG.
As shown in FIG. 18, the 4 × 4 spots and the 4 × 3 spots are blackened in the halftone area, and the 4 × 3 spots remain as blank areas in the shadow area (high density area) as shown in FIG. 18D.

FIG. 19 is a graph showing the dot gain characteristic when the threshold value matrix of the fourth embodiment shown in FIG. 18A is used. In the threshold matrix shown in FIG. 18A, since the 4 × 4 spots and the 4 × 3 spots are blackened in the halftone region, there is an advantage that the dot gain in the halftone region can be reduced. Further, since the minimum value of the threshold is assigned to the rectangular area of the minimum size, there is an advantage that it is possible to suppress the rough feeling of the image in the highlight area.

FIG. 20 is an explanatory diagram showing another threshold value matrix in the fourth embodiment and changes in the shape of dots generated thereby. As shown in FIG.
This threshold matrix is obtained by changing the division method of the threshold matrix shown in FIG. 18A, and the size of the rectangular area to which each threshold is assigned is the same as that in FIG. 18A. Therefore, the threshold matrix shown in FIG. 20A has the same characteristics as the threshold matrix shown in FIG.

FIG. 21 is an explanatory diagram showing still another threshold value matrix in the fourth embodiment and changes in the shape of dots generated thereby. As shown in FIG. 21 (A), this threshold matrix divides a 20 × 20 spot area into four intermediate rectangular areas each having a size of 10 × 10 spots, and further divides each 10 × 10 spot area into nine areas. It is divided into. Further, the manner of division within each 10 × 10 spot area also differs. As a result,
The 20 × 20 threshold matrix is divided into 36 rectangular areas.

The threshold matrix shown in FIG. 21A is 40.
Since it has 0 spots, the maximum threshold value is 39
It is set to 9. When the thresholds in FIG. 21A are arranged in the descending order, they become 399, 387, 375, 362, ..., But the four largest thresholds are all assigned to the 4 × 3 spot area. This can be understood from the dot shape shown in FIG. In addition, FIG.
When the thresholds in are arranged in ascending order, 0, 9, 18, 2
However, the four smallest threshold values are all assigned to the 3 × 3 spot area, which is the smallest rectangular area. This can be understood from the dot shape shown in FIG. Further, in the maximum size 4 × 4 spot area, intermediate threshold values (142, 158, 174, 191) are set.
Has been assigned. Therefore, the threshold matrix shown in FIG. 21A has almost the same characteristics as the threshold matrix shown in FIG.

As shown in FIG. 21 (A), if the four 10 × 10 spot regions are divided in different ways, the moire due to the interference of the halftone images of a plurality of colors in the color image. Has the advantage of being less likely to occur.

F. Fifth embodiment of threshold matrix: FIG.
2A and 2B are explanatory diagrams showing a fifth embodiment of the threshold value matrix of the present invention and a change in the shape of dots generated thereby. This threshold matrix has a size of 10 × 10 spots, is divided into 3: 4: 3 along the main scanning direction Y, and is divided into 4: 3: 3 along the sub scanning direction X. . That is, the area of 10 × 10 spots is divided into nine rectangular areas in total. There is one largest rectangular area of 4 × 4 spots, four rectangular areas of intermediate size 4 × 3 spots, and four smallest rectangular areas of 3 × 3 spots.

The minimum value (= 0) of the threshold is 4 of the intermediate size.
It is assigned to the × 3 spot area. On the other hand, the maximum threshold value (= 99) is assigned to the minimum size 3 × 3 spot area. An intermediate threshold value (42) is assigned to the maximum size 4 × 4 spot area. Therefore, as shown in FIG. 22B, the 4 × 3 spot is blackened in the highlight region (low density region), and
As shown in (C), in the halftone area, 4 × 4 spots and 4
The × 3 spot is blackened, and as shown in FIG. 22D, the 3 × 3 spot remains as a blank area in the shadow region (high-density region).

FIG. 23 is a graph showing the dot gain characteristic when the threshold matrix of the fifth embodiment shown in FIG. 22 (A) is used. In the threshold matrix shown in FIG. 22,
Since the minimum value of the threshold is assigned to the rectangular area having the intermediate size, the dot gain in the highlight area can be increased, and the dot thinning can be reduced. Also, in the halftone area, 4 × 4 spots and 4
Since the × 3 spot is blackened, there is an advantage that the dot gain in the halftone region can be reduced.

FIG. 24 is an explanatory diagram showing another threshold value matrix in the fifth embodiment and changes in the shape of the dots generated thereby. As shown in FIG. 24 (A),
This threshold matrix is obtained by changing the division method of the threshold matrix shown in FIG. 22 (A), and the size of the rectangular area to which each threshold is assigned is the same as that in FIG. 22 (A). Therefore, the threshold matrix shown in FIG. 24A has the same characteristics as the threshold matrix shown in FIG.

FIG. 25 is an explanatory diagram showing still another threshold value matrix in the fifth embodiment and changes in the shape of dots generated thereby. As shown in FIG. 25 (A), this threshold matrix divides a 20 × 20 spot area into four intermediate rectangular areas having a size of 10 × 10 spots, and further divides each 10 × 10 spot area into nine pieces. It is divided into. Further, the manner of division within each 10 × 10 spot area also differs. As a result,
The 20 × 20 threshold matrix is divided into 36 rectangular areas.

The threshold value matrix shown in FIG.
Since it has 0 spots, the maximum threshold value is 39
It is set to 9. When the thresholds in FIG. 25A are arranged in descending order, the thresholds are 399, 390, 380, 371, ... The four largest thresholds are all assigned to the 3 × 3 spot area. This can be understood from the dot shape shown in FIG. In addition, FIG.
When the thresholds in are arranged in ascending order, 0, 12, 25,
37, but the four smallest thresholds are all 4 ×
It is assigned to three spot areas. This is shown in FIG.
It can be understood from the dot shape shown in (B). Furthermore, an intermediate threshold value (1
40, 156, 173, 189) are assigned. Therefore, the threshold matrix shown in FIG. 25A has almost the same characteristics as the threshold matrix shown in FIG.

As shown in FIG. 25 (A), if the four 10 × 10 spot areas are divided in different ways, moire due to interference of halftone images of a plurality of colors is obtained in the color image. Has the advantage of being less likely to occur.

G. Sixth embodiment of threshold matrix: FIG.
6A and 6B are explanatory diagrams showing a sixth embodiment of the threshold value matrix of the present invention and changes in the shape of dots generated thereby. This threshold matrix has a size of 10 × 10 spots and is divided into 4: 3: 3 along the main scanning direction Y and the sub scanning direction X, respectively. That is, 1
The area of 0 × 10 spots is divided into nine rectangular areas in total. The largest rectangular area of 4x4 spots is 1
There are four rectangular areas of 4 × 3 spots of intermediate size, and four rectangular areas of the smallest 3 × 3 spot.
There is

The minimum value (= 0) of the threshold is 4 of the maximum size.
It is assigned to the × 4 spot area. On the other hand, the maximum threshold value (= 99) is assigned to the minimum size 3 × 3 spot area. Therefore, as shown in FIG. 26B, the 4 × 4 spot is blackened in the highlight region (low density region), and as shown in FIG.
The × 3 spot and the 3 × 3 spot turn black, and FIG. 26 (D)
As shown in (3), 3 × 3 spots remain as blank areas in the shadow area (high-density area).

FIG. 27 is a graph showing the dot gain characteristic when the threshold matrix of the sixth embodiment shown in FIG. 26 (A) is used. In the threshold matrix shown in FIG. 26A, since the minimum value of the threshold is assigned to the rectangular area having the maximum size, the dot gain in the highlight area can be increased and the dot thinning can be reduced. . However, since the size of the dots in the highlight area is large, the graininess of the image is larger than in the other embodiments. The threshold matrix of the sixth embodiment is preferably used when recording a so-called halftone negative image (reverse image). On the other hand, it is preferable to use the threshold matrix of the first to fifth embodiments described above when recording a halftone positive image. That is, if the threshold value matrix of the sixth embodiment is used for recording a halftone negative image, the same effect as when the threshold value matrix of the first embodiment is used for recording a halftone positive image can be obtained.

FIG. 28 is an explanatory diagram showing another threshold value matrix in the sixth embodiment and changes in the shape of the dots generated thereby. As shown in FIG. 28 (A),
This threshold matrix is obtained by changing the division method of the threshold matrix shown in FIG. 26A, and the size of the rectangular area to which each threshold is assigned is the same as that in FIG. Therefore, the threshold matrix shown in FIG. 28A has the same characteristics as the threshold matrix shown in FIG.

FIG. 29 is an explanatory diagram showing still another threshold value matrix in the sixth embodiment and changes in the shape of the dots generated thereby. As shown in FIG. 29 (A), this threshold matrix divides a 20 × 20 spot area into four intermediate rectangular areas each having a size of 10 × 10 spots, and further divides each 10 × 10 spot area into nine areas. It is divided into. Further, the manner of division within each 10 × 10 spot area also differs. As a result,
The 20 × 20 threshold matrix is divided into 36 rectangular areas.

The threshold matrix shown in FIG. 29A is 40.
Since it has 0 spots, the maximum threshold value is 39
It is set to 9. When the thresholds in FIG. 29 (A) are arranged in descending order, they are 399, 389, 380, 370, ..., But the four largest thresholds are all assigned to the 3 × 3 spot area. This can be understood from the dot shape shown in FIG. In addition, FIG. 29 (A)
When the thresholds in are arranged in ascending order, 0, 17, 33,
50, but the four smallest thresholds are all 4 ×
It is assigned to four spot areas. This is shown in FIG.
It can be understood from the dot shape shown in (B). Therefore, the threshold matrix shown in FIG. 29A has almost the same characteristics as the threshold matrix shown in FIG.

As shown in FIG. 29 (A), if four 10 × 10 spot areas are divided in different ways, moire due to interference of halftone images of a plurality of colors in a color image. Has the advantage of being less likely to occur.

In each of the above embodiments, an example of a relatively small threshold value matrix has been described, but a larger threshold value matrix may be used for expressing 256 gradations, for example. In practice, a large threshold matrix of about 256 × 256 spots is used. Generally, L1 x L2
(L1 and L2 are integers) A threshold matrix with spot size is used.

H. Apparatus Configuration and Operation: FIG. 30 is a block diagram showing the configuration of an image recording apparatus to which the embodiment of the present invention is applied. This image recording apparatus uses multi-tone image data I
An image memory 20 for storing D, address generators 24, 26 for respectively generating a sub-scanning address (X address) and a main scanning address (Y address) of the image plane, and L1 ×
Threshold matrix memory 3 for storing L2 threshold matrix
0, address generators 32 and 34 for generating sub-scanning addresses (x addresses) and main scanning addresses (y addresses) in the L1 × L2 threshold matrix, respectively, and L1 × L2
It is provided with offset generators 36 and 38 for respectively generating a sub-scanning direction offset (X offset) and a main scanning direction offset (Y offset) of the threshold value matrix, a comparator (comparator) 40, and an output device 50.
The image memory 20 and the offset generators 36, 38
A color component designating signal Sc indicating any one of a plurality of color components is given to the above by a controller (for example, CPU) not shown.

From the image memory 20, the color component designation signal S
The multi-gradation image data ID of the color component corresponding to c is read according to the X address and the Y address. Further, the offset generators 36 and 38 output the offsets of the color components according to the color component designation signal Sc.

FIG. 31 shows L1 × L2 applied to each color component.
It is explanatory drawing which shows the method of setting the offset of a threshold value matrix to a respectively different value. Here, in an example in which the color image data is composed of four color components of Y (yellow), M (magenta), C (cyan), and K (black), the offset of each color component with respect to the origin O of the image plane. OY, OM, OC, and OK are shown. "256
× 256 ”is one L1 ×
It corresponds to the L2 threshold matrix, and this L1 is on the image plane.
The xL2 threshold matrix is applied repeatedly. The Y component offset OY (X _Y , Y _Y ) is (0, 0). Further, the M component offset OM (X _M , Y _M ), the C component offset OC (X _C , Y _C ), and the K component offset O
K (X _K , Y _K ) are set to different values.
These offsets can be set to arbitrary values.

The sub-scanning coordinate x in the L1 × L2 threshold matrix is the X address output from the X address generator 24 (that is, the sub-scanning coordinate of the image plane) and the X offset output from the X offset generator 36. Is generated by the x-address generator 32 in response to Specifically, the effective bit of the value obtained by subtracting the X offset from the X address is x.
It becomes an address. Here, the effective bit is L1 × L2
It is the number of bits indicating the size of the threshold value matrix in the x direction, and the effective bits in the x direction in the case of FIG. 31 are 8 bits. The main scanning coordinate y in the L1 × L2 threshold matrix is similarly generated by the y address generator 34. FIG.
In the case of 1, the effective bits in the y direction are also 8 bits.

The threshold value TD stored in the threshold value matrix memory 30 is read according to the address given from the address generators 32 and 34, and compared with the multi-tone image data ID by the comparator 40. The comparator 40 generates a recording signal RS indicating ON / OFF of each spot according to the comparison result, and supplies the recording signal RS to the output device 50. The output device 50 is, for example, a recording scanner for plate making, and records a halftone image of each color component on a recording medium such as a photosensitive film. Regular patterns are not conspicuous in the halftone images of each color component created in this way, and interference patterns such as moire and rosette patterns are also present in the color images obtained by overprinting these halftone images. There is a feature that it does not occur.

The present invention is not limited to the above embodiments, but can be implemented in various modes without departing from the scope of the invention.

[0081]

As described above, according to the invention described in claim 1, various characteristics such as the dot gain characteristic and the rough feeling of the image are adjusted by adjusting the threshold values to be respectively assigned to the plurality of rectangular areas. Can be improved.

According to the second aspect of the present invention, if the minimum threshold value is set for the minimum rectangular area, the minimum rectangular area is recorded at the lowest density. In, it is possible to suppress the graininess of the image.

According to the third aspect of the present invention, since the maximum rectangular area remains as a blank area until the density becomes the highest, the dot gain in the shadow area can be reduced.

According to the invention described in claim 4, it seems that dots appear at random in the highlight area. Therefore, when a color image is reproduced by superimposing halftone images of a plurality of colors, The effect is that moire due to interference is unlikely to occur.

According to the invention described in claim 5, when the area ratio of the blackened portion of the image is close to 100%, the white spots appear to be left at random positions. When a color image is reproduced by superimposing halftone images, moire due to interference is less likely to occur.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a change in dot shape of a conventional halftone dot (square dot).

FIG. 2 is an explanatory diagram showing changes in dot shape in FM screening.

FIG. 3 is a graph showing dot gain characteristics of halftone dots and FM screening.

FIG. 4 is an explanatory diagram showing a first embodiment of the threshold value matrix of the present invention and changes in the shape of dots generated thereby.

FIG. 5 is an explanatory diagram showing details of a threshold matrix shown in FIG.

FIG. 6 is a graph showing dot gain characteristics with respect to the threshold matrix of the first embodiment.

FIG. 7 is another threshold matrix in the first embodiment,
Explanatory drawing which shows the change of the shape of the dot produced by this.

FIG. 8 is an explanatory diagram showing still another threshold value matrix in the first embodiment and changes in the shape of dots generated by the threshold value matrix.

FIG. 9 is an explanatory diagram showing a second embodiment of the threshold value matrix of the present invention and changes in the shape of dots generated thereby.

10 is a graph showing dot gain characteristics when the threshold matrix of the second embodiment shown in FIG. 9 is used.

FIG. 11 is an explanatory diagram showing another threshold value matrix in the second embodiment and changes in the shape of dots generated by the threshold value matrix.

FIG. 12 is an explanatory diagram showing still another threshold value matrix in the second embodiment and changes in the shape of dots generated by the threshold value matrix.

FIG. 13 is a third embodiment of the threshold value matrix of the present invention;
Explanatory drawing which shows the change of the shape of the dot produced by this.

FIG. 14 is a graph showing dot gain characteristics when the threshold matrix of the first embodiment shown in FIG. 13 is used.

FIG. 15 is an explanatory diagram showing another threshold value matrix in the third embodiment and changes in the shape of dots generated by the threshold value matrix.

FIG. 16 is an explanatory diagram showing still another threshold value matrix in the third embodiment.

17 is an explanatory diagram showing changes in the shape of dots generated by the threshold matrix shown in FIG.

FIG. 18 is a fourth embodiment of the threshold matrix of the present invention;
Explanatory drawing which shows the change of the shape of the dot produced by this.

FIG. 19 is a graph showing dot gain characteristics when the threshold matrix of the fourth embodiment shown in FIG. 18 is used.

FIG. 20 is an explanatory diagram showing another threshold value matrix in the fourth embodiment and changes in the shape of dots generated by the threshold value matrix.

FIG. 21 is an explanatory diagram showing still another threshold matrix in the fourth embodiment and changes in the shape of dots generated by the threshold matrix.

FIG. 22 is a fifth example of the threshold value matrix of the present invention;
Explanatory drawing which shows the change of the shape of the dot produced by this.

FIG. 23 is a graph showing dot gain characteristics when the threshold matrix of the second embodiment shown in FIG. 22 is used.

FIG. 24 is an explanatory diagram showing another threshold value matrix in the fifth embodiment and changes in the shape of dots generated by the threshold value matrix.

FIG. 25 is an explanatory diagram showing still another threshold value matrix in the fifth embodiment and changes in the shape of dots generated thereby.

FIG. 26 is a sixth embodiment of the threshold value matrix of the present invention;
Explanatory drawing which shows the change of the shape of the dot produced by this.

27 is a graph showing dot gain characteristics when the threshold matrix of the first embodiment shown in FIG. 26 is used.

FIG. 28 is an explanatory diagram showing another threshold value matrix in the sixth embodiment and changes in the shape of dots generated by the threshold value matrix.

FIG. 29 is an explanatory diagram showing still another threshold value matrix in the sixth embodiment and changes in the shape of dots generated by the threshold value matrix.

FIG. 30 is a block diagram showing the configuration of an image recording apparatus to which an embodiment of the present invention is applied.

FIG. 31 is an explanatory diagram showing a method of setting the offsets of the L1 × L2 threshold matrix applied to each color component to different values.

[Explanation of symbols]

 20 ... Image memory 24, 26 ... Address generator 30 ... Threshold matrix memory 32, 34 ... Address generator 36, 38 ... Offset generator 40 ... Comparator 50 ... Output device ID ... Multi-gradation image data RS ... Recording signal Sc ... Color component designation signal TD ... Threshold

Claims (5)

[Claims] 1. A method for halftoning multi-tone image data, comprising: (a) dividing a threshold matrix into a plurality of rectangular areas each having a plurality of spots, and at least one of the plurality of rectangular areas. One is to divide the threshold matrix so as to have a different size from other rectangular areas, and (b) assign equal thresholds to a plurality of spots included in each rectangular area, and Assigning different thresholds to each, determining the threshold array in the threshold matrix; and (c) reading the thresholds from the threshold matrix and comparing with the multi-tone image data. And a step of halftoning the gradation image data. 2. The method of halftoning an image according to claim 1, wherein the step (b) comprises a step of setting a minimum threshold value for a minimum rectangular area of the plurality of rectangular areas. Image halftoning method. 3. The image halftoning method according to claim 1, wherein the step (b) sets a maximum threshold value for a maximum rectangular area of the plurality of rectangular areas, An image halftoning method comprising: 4. The method of halftoning an image according to claim 1, wherein the step (a) comprises: N threshold values of equal size to each other (N is 2).
Dividing the plurality of intermediate rectangular regions into M (M is an integer of 2 or more) rectangular regions by different divisions, respectively. To form N × M rectangular areas, and step (b) includes the step of sequentially setting the smallest N threshold values for the smallest rectangular area in each intermediate rectangular area. , Image halftoning method. 5. The image halftoning method according to claim 4, wherein the step (b) sequentially sets the largest N threshold values for the largest rectangular area in each intermediate rectangular area. A method for halftoning an image, the method including a step.