JPH1084477A - Dot-forming method and storage medium for it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize stable gradation reproduction in a super cell system by lighting up a pixel, so that a difference between a maximum deviation and a minimum deviation is a specific rate within a specific range of a dot area rate, with respect to the deviation among density values, calculated at a plurality of density measurement positions. SOLUTION: A plurality density measurement positions are set in a respective unit area and in the case of lighting up pixels within the repetitive unit area, depending on an almost uniform dot area rate, the picture elements are lighted up, so that a difference between a maximum deviation and a minimum deviation within a range of the dot area rate of about 20%-80% is at most 0.3%, with respect to the deviation among density values calculated at a plurality of density measurement positions. The area of a prescribed shape including a plurality of dot areas is used for one repetitive unit area, and a threshold level assigned to each picture element in the repetitive unit area is compared with an image signal in the method of forming dots and fluctuation of the density deviation is less in comparison to the conventional devices and stable gradation reproduction is realized.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for forming a halftone dot by comparing a threshold value assigned to each pixel in a repeating unit area with an image signal.

[0002]

2. Description of the Related Art When forming a halftone dot, it is common to form a halftone dot signal by comparing a threshold value assigned in a repetitive unit area with a multi-tone image signal. As a method of defining a repeating unit area, a method in which a halftone dot area in which one halftone dot is formed is defined as one repeating unit area, and a method in which a larger area including a plurality of halftone areas is defined as one repeating unit area. There is a method.

FIG. 1 shows that one halftone area (also called “halftone cell”) is converted into one repetition unit area (“repetition block”).
FIG. In this scheme, the four corners of each halftone dot region coincide with the corners of the pixel grid. Therefore, the entire image plane can be covered by repeatedly applying the halftone dot area in a tile shape. However,
In this method, the number of screen lines that can be realized (“screen number”
There is a problem that the screen angle (also called "screen angle") and the screen angle (also called "screen angle") are considerably limited. This is because the four corners of each halftone dot region must match the corners of the pixel grid.

FIG. 2 is an explanatory diagram showing a method in which a wide area including a plurality of halftone dot areas is used as one repetition unit area. In this example, an area including 4 × 4 halftone areas is 1
One repetition unit area (also referred to as a “repetition block”). Such a repeating unit area including a plurality of halftone dot areas is generally called a “supercell”. Although the four corners of the supercell correspond to the corners of the pixel grid, the four corners of each halftone dot region do not necessarily correspond to the corners of the pixel grid. In the supercell method, 1
Since there is flexibility in the number of halftone dot regions constituting one supercell, the number of halftone lines and halftone angle can be more freely realized by the so-called rational tangent method. Here, the rational tangent method is a method of forming a halftone dot such that the tangent (tan) of the halftone angle is a rational number.

[0005]

By the way, a plurality of halftone dot regions formed in a supercell are not formed with a fixed number of pixels, but usually include a different number of pixels. FIG. 3 and FIG. 4 are explanatory diagrams showing examples of a conventional supercell. 3 shows an example in which the screen angle is 0 degrees, and FIG. 4 shows an example in which the screen angle is 45 degrees. In the example shown in FIG. 3, one supercell has a halftone area of 11 × 11 pixels and an 11 × 11 pixel.
A halftone area of 10 pixels, a halftone area of 10 × 11 pixels,
And a halftone dot area of 10 × 10 pixels. Therefore, 1
The plurality of halftone dot regions included in one supercell do not always have the same number of pixels. As a result, even when an image having the same halftone dot area ratio is reproduced, the number of lighting pixels included in each halftone dot differs depending on the halftone dot. This is shown more clearly in FIG. That is, when the dot area ratio is 50%, in the super cell of FIG.
A halftone dot composed of pixels, a halftone dot composed of 4 × 5 pixels, and a halftone dot composed of 5 × 4 pixels are formed. When such a dot image is observed, it may be observed as unevenness. Further, if the manner of occurrence of unevenness differs depending on the dot area ratio, there is a problem that stable gradation reproducibility cannot be obtained.

The present invention has been made to solve the above-mentioned problems in the prior art, and has as its object to provide a technique capable of realizing stable gradation reproducibility in a supercell system.

[0007]

In order to solve at least a part of the above-described problems, a first aspect of the present invention is to provide a region having a predetermined shape including a plurality of halftone dot regions as one repeating unit region. The method for forming a halftone dot by comparing a threshold value assigned to each pixel in the repeating unit area with an image signal, wherein a plurality of density measurement positions are set in the repeating unit area. When the pixels in the repeating unit area are turned on according to the uniform dot area ratio, the dot area ratio is about 20% to about the deviation between the density values calculated at the plurality of density measurement positions. About 80%
The pixel is illuminated such that the difference between the maximum value and the minimum value of the deviation in the range is approximately 0.3% at most.

In this case, since the fluctuation of the density deviation becomes small, stable gradation reproducibility can be realized.

In the first aspect, the density value may be:
It is preferable that the brightness is calculated by averaging the lighting data indicating the lighting state of each pixel by a predetermined weighting function in a predetermined range including a plurality of halftone areas around the density measurement position.

[0010] The halftone dot is about 15 ° or about 75 °.
It is preferable to have a screen angle of °.

According to a second aspect of the present invention, a region having a predetermined shape including a plurality of halftone dot regions is used as one repetition unit region, and a threshold value assigned to each pixel in the repetition unit region is compared with an image signal. A storage medium for storing a software program for implementing a process of determining a threshold distribution used in a method of forming halftone dots, wherein a plurality of density measurement positions are set in the repetitive unit area, and a substantially uniform halftone dot is set. When the pixels in the repeating unit area are turned on according to the dot area ratio, the halftone dot area ratio is about 20% to about 8 with respect to the deviation between the density values calculated at the plurality of density measurement positions.
A threshold value is assigned to the repetition unit area so as to realize a halftone dot forming process of lighting a pixel so that the difference between the maximum value and the minimum value of the deviation in a range of 0% is at most about 0.3%. A software program for executing processing is stored.

If halftone dots are formed using the threshold value determined by such a software program, the variation of the density deviation is reduced, as in the first invention, so that stable gradation reproducibility can be realized. Can be.

[0013]

Other Embodiments of the Invention The present invention includes the following other embodiments. The first aspect uses a region having a predetermined shape including a plurality of halftone dot regions as one repetition unit region, and compares a threshold value assigned to each pixel in the repetition unit region with an image signal. An apparatus for forming, when the pixels in the repeating unit area are turned on according to the substantially uniform halftone dot area ratio, calculation is performed at a plurality of density measurement positions set in the repeating unit area. Regarding the deviation between the density values, the halftone dot area ratio is about 20% to about 80%.
The difference between the maximum value and the minimum value of the deviation in the range of% is
A memory for storing a threshold distribution for turning on the pixels so as to be at most about 0.3%, and a comparison for generating a halftone signal representing a halftone dot by comparing a threshold read from the threshold distribution with an image signal. And a container.

According to a second aspect, there is provided a program supply device which, when executed by a microprocessor of a computer system, supplies a software program for realizing each step or each means of the invention through a communication path.

[0015]

Next, embodiments of the present invention will be described based on examples. FIG. 5 is a flowchart showing a processing procedure according to the embodiment of the present invention. In step S1,
The lighting order of the pixels is set for each dot cell in the repeating block. FIG. 6 is an explanatory diagram showing a repetition block SC as a target in this embodiment. The repetition block SC includes 5 × 5 halftone cells HC. In this embodiment, the position of each halftone cell in the repeating block is
It is specified by a two-digit number. This two-digit number is called a “dot cell number”. That is, the halftone cell HC00 having the halftone cell number “00” is located at the upper left corner of the repetition block SC, and the halftone cell HC44 having the halftone cell number “44”.
Is located at the lower right end of the repetition block SC. In step S1, for each pixel in each halftone cell HC in such a repetition block SC, an independent lighting order is set for each halftone cell HC. FIG. 7 shows one halftone cell HC
It is explanatory drawing which shows an example of the arrangement | sequence of the lighting order in. The lighting order is set for each pixel in the halftone cell HC so that the value gradually increases from the center to the periphery.

In step S2 of FIG. 5, the lighting order data of each halftone cell in the repetition block SC is read into the main memory, and a plurality of density measurement points are set in the repetition block SC. FIG. 8 is an explanatory diagram showing the positions and types of measurement points. FIG. 8A shows the positional relationship between the repetition block SC and the halftone cells HC00 to HC44.
(B) shows the position of the measurement point. In FIG. 8B, for reference, halftone dots formed when the halftone dot area ratio is 50% are shaded. As measurement points, mountain-side measurement points P00 to P44 located at the center of each halftone dot cell
And valley-side measurement points B00 located at the four corners of each halftone cell
To B44 are set. The last two numbers of the codes (for example, “P00” and “P44”) of the mountain-side measurement points indicate the numbers of the halftone cells (FIG. 8A) including the mountain-side measurement points. Also, the sign of the valley-side measurement point (for example, “B0
The last two numbers of “0” and “B44”) are set to the same numbers as the hill-side measurement points at the upper left of the valley-side measurement points. In addition, since the repetition block SC is repeatedly applied in a tile shape, an equivalent measurement point (for example, B44, B40, etc.) appears a plurality of times at the measurement points on the sides of the repetition block SC.

In step S2, an adjacency table showing the adjacency between the halftone cells is further created. FIG. 9 is an explanatory diagram showing an adjacency table showing an adjacency relationship between the halftone cells. For example, in FIG. 9 (A), the number of the halftone cell having an adjacent relationship with the 22nd halftone cell HC22 near 8 is:
32, 31, 21, 11, 11, 12, 13, 23, and 33. The adjacent relation table shown in FIG. 9B is a table in which, for each halftone cell, the numbers of halftone cells having adjacent relations near eight are registered.

In step S3 of FIG. 5, a lighting bitmap area used for determining the threshold distribution in the repetition block SC is secured in the main memory. The lighting bitmap is a 1-bit deep memory area in which 1 is set for pixels to be turned on and 0 is set for pixels that are not turned on. Using this lighting bit map, density measurement described later is performed. In step S3, the lighting bitmap data is all initialized to 0. FIG.
FIG. 4 is an explanatory diagram showing an area of a lighting bit map. The lighting bitmap area is a wide area having an extended width for density measurement around a rectangular area (shown by a broken line) circumscribing the repetition block SC. That is, when measuring the density at the density measurement point (see FIG. 8B) existing at the corner of the repetitive block SC, it is necessary to know the lighting state in a certain range around the density measurement point. Therefore, as the lighting bit map, a wide area provided with a predetermined extension width around the repetition block SC is used. Preferably, the expansion width is set to a value corresponding to a few halftone dot pitch.

The lighting bitmap is composed of a repeating block S
Since there is an area wider than C, there is a position in the lighting bitmap where a plurality of equivalent pixels exist. Such an equivalent plurality of pixels are associated with each other so as to be lit simultaneously. In this embodiment, the association of a plurality of equivalent pixels is called "nesting", and the information is called "nesting information". FIG. 11 is an explanatory diagram showing the contents of the nesting information. The pixel at the coordinates (100, 120) shown in FIG. 11A is the pixel of the lighting order 1 of the halftone cell HC00, as shown in FIG. 11B. FIG.
As indicated by black circles in 1 (A), there are three other pixels equivalent to this pixel in the lighting bitmap. The nesting information correlates the coordinates of these four pixels. On the other hand, in the access information of the lighting bitmap (information indicating the lighting order of each pixel and the presence / absence of lighting), the lighting order corresponding to the pixels of one repetition block SC is stored. Therefore, when setting the lighting state (1 or 0) of each pixel in the lighting bitmap, the same lighting state is always set for a plurality of pixels associated by the nesting information.

In step S4 of FIG.
Based on the data prepared in S3, a repetition block S
The threshold of each pixel in C is determined. FIG. 12 is a flowchart showing a detailed procedure of step S4. In step S11, an initial lighting position in the repetition block SC is set, and the lighting is performed. The initial lighting position can be set to a pixel having a lighting order of 1 in an arbitrary halftone cell.
For example, the dot cell HC22 at the center of the repetition block SC
Of the lighting order 1 and the pixel of the lighting order 1 of the halftone dot cell HC00 at the upper left corner of the repetition block SC can be selected as the initial lighting position.

In step S12, the density is measured at each measurement point shown in FIG. 8B, and the density measurement value at the measurement point affected by the lighting is updated. FIG. 13 is an explanatory diagram showing a method for measuring the concentration. The density value D at the measurement point is integrated by multiplying the weighting function W (x, y) centered on the measurement point by the lighting bitmap data B (x, y), and is integrated by the integral value of the weighting function W. This is a normalized value. In other words, the density value D is obtained by converting the lighting bitmap data B (x, y) in a certain range including a plurality of halftone cells (hereinafter referred to as a “measurement mask”) into a weighting function W (x, y). ). As the distribution of the weight function W (x, y), it is preferable to employ a weight value distribution approximating a Gaussian distribution. Also, the range of the weight function W (x, y) (measurement mask)
Is preferably about 4.25 times the halftone dot pitch. The density value D obtained in this way is calculated using a densitometer having an aperture (opening) of the size of the measurement mask.
The value is substantially equal to the density value obtained by measuring the density of the halftone dot image.

In step S12 of FIG. 12, first, in FIG.
At each measurement point shown in FIG. 13B, a density value is calculated according to the method shown in FIG. Then, for the measurement point at which the measured density value has been changed, the measured density value is updated.

FIG. 14 is an explanatory diagram showing an example of the result of the measured density value. The horizontal axis in FIG. 14A indicates the position of the measurement point, and the vertical axis indicates the density value. Although the peak-side measurement points and the valley-side measurement points are alternately arranged on the horizontal axis, these axes are separately shown for convenience of illustration.
In the example of FIG. 14, the mountain side measurement point P22 of the halftone cell HC22
Indicates the lowest density value.

In step S13, a plurality of lighting candidates to be turned on next are selected from the vicinity of the measurement point showing the lowest density value. FIG. 15 is an explanatory diagram illustrating an outline of a lighting candidate selection method. In FIG. 15, it is assumed that the peak-side measurement point Pmn or the valley-side measurement point Bmn of the halftone cell HCmn indicates the lowest density value among the measurement points.

If the peak measurement point Pmn indicates the lowest density value, a predetermined number (for example, three) of non-lighted pixels in the halftone dot cell HCmn are selected in accordance with the lighting order, and lighting candidates are selected. And FIG. 16 is an explanatory diagram illustrating a specific method of selecting lighting candidates. FIG. 16 shows a dot cell HC.
22 shows a case in which three lighting candidates are selected. Among the non-lighted pixels in the halftone cell HC22, the three pixels with the fastest lighting order have the lighting order of 102, 10
3,105 pixels. Therefore, these three pixels are selected as lighting candidates.

When the halftone dot area ratio is close to 100% and a predetermined number of unlit pixels are not present in the halftone cell HCmn, the remaining lighting candidates are selected from adjacent halftone cells. I do. At this time, the adjacency table shown in FIG. 9B is used. That is, first, a lighting candidate is selected from halftone cells near the target halftone cell HCmn (halftone cells in any of the left, right, upper, and lower),
If the number is insufficient, a lighting candidate is selected from the other eight neighboring halftone dot cells.

On the other hand, when the valley-side measurement point Bmn indicates the lowest density value, it is desired to select a lighting candidate from the vicinity of the valley-side measurement point Bmn. Therefore, in this case, as shown in FIG. 15C, a predetermined number of pixels are selected from the unlit pixels in the four halftone cells around the valley-side measurement point Bmn according to the lighting order. , Lighting candidates.

In step S14 of FIG.
The pixels of the lighting candidates selected in step 13 are temporarily lit one by one,
While each lighting candidate is lit, the density measurement is performed at each measurement point. FIG. 17 shows calculation results of density values obtained in a state where each of the three lighting candidates is turned on. As illustrated in FIGS. 17B to 17D,
In the distribution of measured density values obtained when each lighting candidate is turned on, a density deviation is obtained. Here, the density deviation is a difference between the highest density value and the lowest density value. The fact that the density deviation is small means that the unevenness of the image is small. Therefore, in step S15 of FIG. 12, a lighting candidate with the smallest density deviation is selected from among a plurality of lighting candidates.
The next lighting position is determined. To the pixel determined as the next lighting position, the lighting order among all the pixels in the repetition block SC is assigned as a threshold value of the pixel. That is, if the lighting position is the m-th lit pixel in the repetitive block SC, (m-1) is assigned as the threshold value.

In step S16, the repetition block SC
It is determined whether or not the assignment of the threshold value has been completed for all the pixels in. If the processing has not been completed, the process returns to step S12, and the processing of steps S12 to S16 described above is repeated. When the assignment of the thresholds for all the pixels is completed, in step S17, the thresholds for all the pixels included in the repetition block are stored in a storage device such as a hard disk. Thereafter, in step S18, the threshold is normalized as necessary. For example, when the total number of pixels in the repetition block is N, thresholds 0 to (N-1) are assigned to N pixels in steps S11 to S17. If it is desired to finally set the range of this threshold value to 0 to 254, each of these threshold values is set to 254 / (N−
Integer conversion may be performed by multiplying by 1).

As described above, in the above embodiment, when a plurality of lighting candidates are selected and these are temporarily lit,
The lighting candidate with the smallest density deviation is adopted as the next lighting position. As a result, the density deviation can be kept small, and the unevenness of the image can be reduced.

FIG. 18 is a block diagram showing the configuration of a halftone dot forming apparatus to which the above-described embodiments are applied. This halftone dot forming apparatus includes a CPU 30 and a main memory (ROM and ROM).
AM) 32, a floppy disk drive 34, an SPM memory 36, a sub-scanning address counter 38, a main scanning address counter 40, and a comparator 42. The dot forming apparatus also includes an exposure device (not shown) for recording a dot image on a recording medium with a light beam. The SPM memory 36 stores a threshold pattern in the super cell. The floppy disk device 34 stores a plurality of types of threshold patterns created according to the above-described embodiment, and one of them is selected and transferred to the SPM memory 36.

In the above embodiment, the repetition block SC
The various functions for assigning thresholds in the computer system are controlled by the CPU (microprocessor) 30 of the computer system.
Is realized by executing a software program. Software programs (application programs, computer programs) for realizing the above functions are transferred from a portable storage medium (portable storage medium) such as a floppy disk or CD-ROM to a main memory or an external storage device of the computer system. You.
Alternatively, the program may be supplied from the program supply device to the computer system via a communication path.

The sub-scanning address counter 38 receives a sub-scanning start signal Rx and a sub-scanning clock signal Cx. The sub-scanning start signal Rx is a signal that generates one pulse when the sub-scanning coordinates of the light beam are reset to the initial position. The sub-scanning clock signal is a signal that generates one pulse each time the sub-scanning coordinates of the light beam are updated. The sub-scanning address counter 38 outputs these signals R
x and Cx, the sub-scanning coordinates of the light beam in the repetition unit block are generated and stored in the SPM memory 36.
As a sub-scanning address. Similarly, the main scanning address counter 40 generates main scanning coordinates of the light beam in the repeating unit block according to the main scanning start signal Ry and the main scanning clock signal Cy, and stores the generated main scanning coordinates in the SPM memory 36 as a main scanning address. Supply. One threshold value Ss is read from the threshold pattern in the SPM memory 36 in accordance with the addresses given from the two address counters 38 and 40, and is supplied to the comparator 42.

The comparator 42 compares the threshold value Ss with the input image signal Im, and generates a binary output (exposure signal, halftone signal) according to the comparison result. The levels of the binarized output are as follows. When Ss <Im: H level (exposure, lighting); When Im ≦ Ss: L level (non-exposure, non-lighting). When the input image signal Im is in the range of 0 to 255, the range of the threshold value Ss is 0 to 254.

An exposure device (not shown) exposes a photosensitive recording medium (eg, a photosensitive film) with a light beam in accordance with the binarized output, thereby forming a dot image on the recording medium. In this way, a halftone image of each color plane of YMCK is created, and these halftone images are overprinted with inks of the respective colors, whereby a multicolor print can be obtained.

FIGS. 19 to 21 are graphs showing the density deviation of halftone dots formed by using the threshold values created according to this embodiment in comparison with the conventional example. FIG. 19 shows that the output resolution is 4000 dpi and the screen ruling (screen ruling) is 400.
1pi, and indicates the density deviation of the halftone dot image when the screen angle (halftone angle) is about 15 °, about 45 °, and about 75 °, respectively. FIG. 20 shows a case where the output resolution is 4000 dpi and the number of screen lines is 350 lpi.
1 means that the output resolution is 4000 dpi and the number of screen lines is 310 lp
The case of i is shown.

It can be seen that under most of the conditions shown in FIGS. 19 to 21, the density deviation of the embodiment is smaller than the density deviation of the conventional example. However, when the screen angle is 45 °, the density deviation in the conventional example may be smaller than the density deviation in the embodiment, as in the example of FIG. This is considered for the following reasons. When the screen angle is 45 °, the side of a halftone dot (a group of lighting pixels) is parallel to the scanning direction, so that the shape of the halftone dot can be made geometrically simple. Therefore, since the shape of the halftone dot is easily formed neatly, the density deviation is reduced even in the prior art. On the other hand, in the case of the embodiment, the lighting state in the range of the measurement mask (FIG. 13) is averaged by the weighting function W (x, y) to obtain the density value, and the lighting position is set so that the deviation of the density value becomes small. Is determined. As described above, in the case of the embodiment, the lighting position is determined according to the weighted average of the lighting state in a certain range, and as a result, the halftone dot may not have a geometrically simple shape. The density deviation may be smaller in the conventional example.

As described above, the effect of the embodiment becomes remarkable when the screen angles are about 15 ° and about 75 °. 19 to 21, the screen angle is about 15 °.
And the graph of about 75 °, the dot area ratio is about 20% to about 8%.
Difference between the maximum value and the minimum value of the density deviation in the range of 0% △
Is shown. In an embodiment, this difference △ is at most 0.3
2%, with a minimum of 0.18%. On the other hand, in the conventional example, the difference △ is 1.52% at the maximum and 0.39% at the minimum. Thus, in this embodiment, the halftone dot area ratio is about 20%.
% In the range of about 80% to about 80%, the difference 、 of the density deviation is at most about 0.3%, and is in the range of about 0.3% to about 0.1%. The difference 濃度 of the density deviation in such an embodiment is suppressed to a value smaller than 0.4% which is the minimum value of the difference in the conventional example. In the embodiment, the absolute value of the density deviation is about 0.95% at the maximum, and is about 1% or less.

When the screen angles shown in FIGS. 19 to 21 are about 15 ° and about 75 °, the dot area ratio is about 30 °.
The difference △ between the maximum value and the minimum value of the density deviation in the range of about% to about 70% is 0.28% at the maximum in the embodiment and 0.
15%. Therefore, the dot area ratio is about 30% to about 70%.
%, The difference △ between the maximum value and the minimum value of the density deviation in the example is at most about 0.3%, and is about 0.3% to about 0.1%.
% Range. In the embodiment, the absolute value of the density deviation is about 0.95% at the maximum, and is about 1% or less. As described above, since the variation in the density deviation is smaller in the embodiment than in the conventional example, it is possible to realize more stable gradation reproduction as compared with the conventional example.

FIG. 22 is a table showing combinations in which the combinations of the output resolution and the screen ruling are equivalent. 4000d
At output resolutions in the range of pi to 1000 dpi, the equivalent screen rulings are grouped in three rows. For example, a halftone image having an output resolution of 4000 dpi and a screen ruling of 400 lpi has an output resolution of 3000 dpi.
i, a screen image with a screen ruling of 300 lpi, or an output resolution of 2400 dpi with a screen ruling of 240 lpi
It is equivalent to the dot image of i. Here, the meaning of “equivalent” means that the same density deviation as that shown in FIGS. 19 to 21 is obtained. Therefore, the same relationship as that shown in FIGS. 19 to 21 can be obtained for the halftone images of various output resolutions and screen rulings as shown in FIG.

It should be noted that the present invention is not limited to the above Examples and Embodiments, but can be implemented in various modes without departing from the gist thereof.
For example, the following modifications are possible.

(1) In this embodiment, one repetition block SC is subjected to the threshold value allocation processing. However, a wider area including a plurality of repetition blocks SC may be adopted as the processing target area. It is possible.

(2) As the measurement points, points other than those shown in FIG. 8B can be adopted. For example, all the pixels in the repetition block can be used as measurement points. Alternatively, it is also possible to arrange measurement points at fixed intervals at positions different from the peak-side measurement points and the valley-side measurement points in FIG. Also, the repetition block SC
The measurement points may be randomly arranged in the area. By arranging the measurement points at regular intervals, there is an advantage that the correlation between the density deviation and the unevenness of the image can be increased.

(3) Weight function W for performing density measurement
As (x, y), various types other than those shown in FIG. 13 can be used. For example, a weight function described in FIG. 5 of Japanese Patent Publication No. 61-27683 disclosed by the present applicant may be used.

(4) In step S15 of FIG.
As a criterion for determining the next lighting position, the criterion that “the density deviation is minimized” has been adopted, but other criterion can be adopted. For example, a lighting candidate that maximizes the density value at the lowest density measurement point obtained in step S12 can be adopted as the next lighting position. At this time, a limitation may be added that the next lighting position is adopted only when the density value at the lowest density measurement point does not become the highest density value. Alternatively, other evaluation criteria (for example, the smaller the peripheral length of a halftone dot (lighted portion) is better) may be considered together with the density deviation of a plurality of lighting candidates.

(5) As a method for selecting a plurality of lighting candidates in step S13 in FIG. 12, a method other than the method shown in FIGS. FIG. 23 is an explanatory diagram showing another method of selecting a lighting candidate. In this method, the region is not divided for each halftone dot cell as shown in FIG. 23 (A), and the mountain region (H1) as shown in FIGS. 23 (B) and (C).
To H9) and a valley region (V1 to V4, etc.). The mountain region is a region including pixels that are turned on when the dot area ratio is about 50% or less. The valley region has a dot area ratio of about 5
It is an area composed of pixels that are not lit when it is 0% or less.

As shown in FIG. 23 (B), when the lowest density measurement point exists in the valley region V1, lighting candidates are selected from the peak regions H1, H2, H4, and H5 adjacent to the valley region V1. select. The reason why a lighting candidate is selected from adjacent mountain regions is that, in general, the shape of a halftone dot becomes clearer when pixels in the mountain region are turned on first than in a valley region.
When the halftone dot area ratio approaches 50%, these peak regions H
In some cases, a sufficient number of lighting candidates cannot be selected from among 1, H2, H4, and H5. In this case, a lighting candidate is selected from the valley regions near the mountain region. For example, when there are not enough lighting candidates in the mountain region H5,
Lighting candidates are selected from the valley regions V1, V2, V3, and V4 in the vicinity.

On the other hand, as shown in FIG. 23C, when the lowest density measurement point exists in the mountain region H5, a plurality of lighting candidates are selected from the mountain region H5. When a sufficient number of lighting candidates cannot be selected in the mountain region H5, the surrounding mountain regions H2, H4, H6, H8, H1,
A lighting candidate is selected from H3, H7, and H9. When the halftone dot area ratio approaches 50%, a sufficient number of lighting candidates may not be selected from these mountain regions. In this case, a lighting candidate is selected from a valley region in the vicinity. For example, when there are not enough lighting candidates in the mountain region H6, the lighting candidates are selected from the valley regions V2 and V4 in the vicinity.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a method of using one halftone dot area as one repetition unit area.

FIG. 2 is an explanatory diagram showing a method of setting a wide area including a plurality of halftone dot areas as one repetition unit area.

FIG. 3 is an explanatory diagram showing an example of a conventional supercell.

FIG. 4 is an explanatory diagram showing an example of a conventional supercell.

FIG. 5 is a flowchart showing a processing procedure according to the embodiment of the present invention.

FIG. 6 is an explanatory diagram showing a repetition block SC targeted in this embodiment.

FIG. 7 is an explanatory diagram showing an example of an arrangement of lighting order in one dot cell HC.

FIG. 8 is an explanatory diagram showing the positions and types of measurement points.

FIG. 9 is an explanatory diagram showing an adjacency table showing an adjacency relationship between halftone cells;

FIG. 10 is an explanatory diagram showing an area of a lighting bit map.

FIG. 11 is an explanatory diagram showing the contents of nesting information.

FIG. 12 is a flowchart showing a detailed procedure of step S4.

FIG. 13 is an explanatory diagram showing a method for measuring the concentration.

FIG. 14 is an explanatory diagram showing an example of a result of a measured density value.

FIG. 15 is an explanatory diagram showing a lighting candidate selection method.

FIG. 16 is an explanatory diagram showing a specific method of selecting lighting candidates.

FIG. 17 is an explanatory diagram showing a calculation result of a density value obtained in a state where three lighting candidates are turned on.

FIG. 18 is a block diagram illustrating a configuration of a halftone dot forming apparatus.

FIG. 19 is a graph showing a comparison between the density deviation of the embodiment and the density deviation of the conventional example.

FIG. 20 is a graph showing a comparison between the density deviation of the embodiment and the density deviation of the conventional example.

FIG. 21 is a graph showing a comparison between the density deviation of the example and the density deviation of the conventional example.

FIG. 22 is an explanatory diagram showing an example in which a combination of an output resolution and a screen ruling is equivalent.

FIG. 23 is an explanatory view showing another method of selecting a lighting candidate.

[Explanation of symbols]

 Reference Signs List 30 CPU 32 Main memory 34 Floppy disk device 36 SPM memory 38 Subscanning address counter 40 Main scanning address counter 42 Comparator

Claims (4)

[Claims] 1. A region having a predetermined shape including a plurality of halftone dot regions is used as one repetition unit region, and a threshold value assigned to each pixel in the repetition unit region is compared with an image signal, so that halftone dots are obtained. Forming a plurality of density measurement positions in the repeating unit area, and lighting the pixels in the repeating unit area according to a substantially uniform halftone dot area ratio. Regarding the deviation between the density values calculated at the measurement position, the difference between the maximum value and the minimum value of the deviation in the range of the dot area ratio of about 20% to about 80% is at most about 0.3%. A halftone dot forming method, wherein a pixel is turned on at the same time. 2. The halftone dot forming method according to claim 1, wherein the density value indicates a lighting state of each pixel in a predetermined range including a plurality of halftone areas around the density measurement position. A halftone dot forming method calculated by averaging lighting data with a predetermined weight function. 3. The method according to claim 1, wherein the halftone dots have a screen angle of about 15 ° or about 75 °. 4. A region having a predetermined shape including a plurality of halftone dot regions is used as one repetition unit region, and a threshold value assigned to each pixel in the repetition unit region is compared with an image signal. A storage medium for storing a software program for realizing a threshold distribution determination process used in a forming method, wherein a plurality of density measurement positions are set in the repetitive unit area, and a substantially uniform halftone dot area ratio is obtained. Accordingly, when the pixels in the repeating unit area are turned on, regarding the deviation between the density values calculated at the plurality of density measurement positions, the halftone dot area ratio is in the range of about 20% to about 80%. A process of assigning a threshold value within the repetitive unit area is performed so as to realize a halftone dot forming process of lighting a pixel so that the difference between the maximum value and the minimum value of the deviation is about 0.3% at most. Storage medium characterized by storing a shift ware program.