JP2002101297A - Apparatus and method for determining dot parameter value - Google Patents

Abstract

PROBLEM TO BE SOLVED: To determine dot parameter values such that generation of moire fringes can be avoided. SOLUTION: The dot parameter value determining apparatus 100 comprises a section 30 for selecting temporary dot parameter values among candidate dot parameter values stored at a candidate parameter storing section 20, a section 40 for calculating moire based on selected dot parameters, a section 50 for making a decision whether calculated moire falls within an allowable level or not, and a section 60 for employing the temporary dot parameter values as definite values if the moire falls within the allowable level wherein the candidate parameter selecting section 30 selects new temporary dot parameter values if the moire deviates from the allowable level.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dot parameter value determining apparatus and a dot parameter value determining method for determining a dot parameter value representing the number of lines and the angle of a dot.

[0002]

2. Description of the Related Art Generally, a color image in a printed matter is
The halftone images of Y (yellow), M (magenta), C (cyan) and K (black) color plates are created by being superimposed on each other by a printing machine.

[0003] Y, M, C, K constituting this color image
The halftone image of each color plate is, for example, 0 °, 15 °,
It is composed of halftone dots arranged at an angle of 45 ° and 75 °. In these halftone images, the gradation of the image is represented by the area modulation of each halftone dot. Note that the arrangement of the halftone dots in the halftone image is defined not only by the angle but also by the number of lines representing the density of the line composed of the halftone dots. In the color image in which the halftone images of the respective Y, M, C, and K color plates are superimposed, MCK excluding the Y color plate having a lighter color is included.
Due to the overlap of the plates, a characteristic rosette pattern occurs in the printed image.

Conventionally, it has been known that a high quality color image can be obtained if this rosette pattern is generated finely. The conditions under which this rosette pattern is generated have been analyzed for a long time. For example, as described in Japanese Patent Application Laid-Open No. 2-58176, the arrangement of halftone dots is regarded as a wave, and the halftone dots arranged at an angle of 15 ° overlap with the halftone dots arranged at an angle of 75 °. The primary moiré component generated by the above method is obtained, and the primary moiré component and the halftone dots arranged at an angle of 45 ° are not overlapped to generate secondary moiré, in other words, the primary moiré component and the angle of 45 °. The rosette pattern is formed by setting the number of lines and the angle of the halftone dots so that the wavenumber components of the halftone dots coincide with those of the halftone dots.

[0005] Incidentally, as a combination of the angles of the halftone dots used in printing, a combination of (15 °, 45 °, 75 °) is generally used. It is important that the relative angles are separated by 30 degrees, and the combinations of the angles of the halftone dots are not limited to the above combinations. For example,
It is well known that a rosette pattern occurs even in a combination such as (22.5 °, 52.5 °, 82.5 °). In addition, since the halftone dots are generally arranged in a square lattice shape, they are arranged at the above-described angle and also at an angle obtained by adding ± 90 ° or 180 ° to the angle. For this reason, the halftone dot angle may be referred to as an angle of 0 ° to 90 °, or may be referred to as an angle obtained by adding ± 90 ° or 180 ° to the angle of 0 ° to 90 °.

[0006]

In recent years, digital devices have become widespread in the field of printing, and when a dot parameter value representing the number of lines and the angle of a dot is set, the set dot parameter is set. Image data representing a halftone image composed of halftone dots having the number of lines and the angle represented by the value is generated. Then, the halftone image represented by the image data is output by a digital output device such as an underprint film processor or a high-resolution proofer.

However, when a dot image is output by such a digital output device, even if a dot parameter value representing the number of lines and the angle of a dot which causes a rosette pattern is set, a streak pattern is obtained. It is known that moiré fringes may occur. Such streak-like moiré stripes
This also occurs even if the M, C, and K color plates are individually output.

Conventionally, in order to avoid such streak-like moiré fringes, a variety of halftone dot parameter values are set to output various halftone images, and a setting for obtaining the highest quality halftone image is set. The work of recruiting is underway. Such a work places a burden on the operator and causes waste of the underlay film and the like, which causes a delay in the printing work and an increase in cost.

In view of the above circumstances, the present invention can determine a halftone dot parameter value so that the occurrence of moire fringes can be avoided. The purpose is to provide a decision method.

[0010]

According to the present invention, there is provided a halftone dot parameter value determining apparatus for obtaining a tentative halftone dot parameter value for a halftone dot parameter value representing the number of lines and the angle of a halftone dot. A value acquisition unit, and a moiré calculation unit that calculates a moiré generated when a halftone dot having the number of lines and an angle represented by the provisional halftone dot parameter value acquired by the provisional parameter value acquisition unit is output by a predetermined output device. A determination unit that determines whether the moiré calculated by the moiré calculation unit is a moiré within an allowable level,
When the moiré calculated by the moiré calculator is determined to be a moiré within an allowable level by the determiner, a final parameter value adopter that adopts a provisional halftone parameter value as a final halftone parameter value is provided. The provisional parameter value acquisition unit is to acquire a new provisional halftone dot parameter value when the determination unit determines that the moiré calculated by the moiré calculation unit is a moiré outside the allowable level. It is characterized by.

Here, the "dot parameter value" may be a value of one parameter or a combination of values of a plurality of parameters.

[0012] The halftone parameter value determining apparatus of the present invention includes:
For a halftone dot having a screen ruling and an angle represented by the provisional halftone parameter value, a moiré calculation unit is provided for calculating the moire at the time of output by the output device. There is no need to output images. Further, since the determination unit that determines whether or not the moire is within the allowable level is provided, it is possible to determine the halftone dot parameter value so as to avoid the generation of the streak-like moiré fringes.

In the halftone parameter value determining apparatus according to the present invention, the temporary parameter value acquiring section selects and acquires a temporary halftone parameter value from a plurality of halftone parameter values prepared in advance. Is preferred.

Further, the halftone parameter value determining apparatus of the present invention comprises a parameter value calculating section for calculating a plurality of halftone parameter values each of which satisfies a predetermined precondition independent of the condition indicating the permissible level. It is also preferable that the provisional parameter value acquisition unit selectively acquires a provisional halftone parameter value from a plurality of halftone parameter values calculated by the parameter value calculation unit. "

In determining dot parameter values, it is necessary to consider various conditions in addition to moire at the time of output, and the number of dot parameter values satisfying such various conditions is limited to a finite number. For this reason, the provisional parameter values can be easily obtained by calculating and storing the finite number of halftone dot parameter values in advance.

Further, the halftone parameter value determining apparatus according to the present invention further comprises the step of: "if the moiré calculated by the moiré calculating unit is determined to be a moiré outside the allowable level, the allowable range of the halftone parameter value is determined. The provisional parameter value acquiring unit includes a setting unit that sets the allowable value set by the range setting unit when the moiré calculated by the moiré calculation unit is determined to be a moiré outside the allowable level. A new provisional halftone dot parameter value is obtained within the range. "

According to the halftone dot parameter value determining apparatus having such a range setting unit, a new provisional halftone parameter value is obtained within the allowable range set by the range setting unit. By appropriately setting the allowable range of the dot parameter value, the new provisional dot parameter value becomes a more appropriate dot parameter value than the previous time. As a result, the dot parameter value finally reaches a value at which the determination unit can determine that the moire is within the allowable level.

The halftone parameter value determining method of the present invention that achieves the above object includes a tentative parameter value set obtaining step of obtaining a tentative halftone parameter value for a halftone parameter value representing the number of lines and the angle of a halftone dot. A moiré calculating step of calculating a moiré generated when a halftone dot having the number of lines and an angle represented by the provisional halftone dot parameter value obtained in the provisional parameter value set obtaining step is output by a predetermined output device, and a moiré calculating step The determination process of determining whether or not the moiré calculated in the above is a moire within an allowable level, and the provisional process when the moire calculated in the moire calculation process is determined to be a moire within an allowable level. And a step of adopting a final parameter value set in which the halftone dot parameter value is adopted as the final halftone parameter value. Of when to have been determined in the determination process is moire, and repeating the moire calculation process and determination process acquires the dot parameter values of the new provisional returns to tentative parameter value acquisition process.

The method of determining a dot parameter value according to the present invention will be described only in its basic form. However, this is simply to avoid duplication, and the method of determining a dot parameter value according to the present invention will be described. The method includes not only the above-described dot parameter value determining method in the basic mode but also various types of dot parameter value determining methods corresponding to the respective modes of the above-described dot parameter value determining apparatus.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following describes the dot parameter values.

In a digital output device, an image is output as a set of pixels arranged at a predetermined resolution, and the dots themselves forming a halftone image are also output as a set of pixels.
In general, an array of halftone dots constituting a halftone image in a digital output device is defined as a supercell including a plurality of halftone dots. When such a supercell is used, a halftone dot image is easily created by repeating the supercell, and there is an advantage that the degree of freedom of the angle and the number of lines of the halftone dot is high.

There are two typical types of supercells. One is a parallel halftone type supercell in which the sides of the supercell obliquely intersect with the arrangement direction of the pixels of the output device, and the halftone dots in the supercell are arranged in parallel with the side of the supercell. The other is a diagonal halftone dot type supercell in which the sides of the supercell are parallel to the pixel arrangement direction of the output device, and the halftone dots in the supercell are arranged diagonally to the side of the supercell. It is.

FIG. 1 is a diagram showing a super-cell of the parallel halftone dot system.

In this parallel halftone dot system, the supercell 1
Is tilted by an angle θ of a halftone dot with respect to the pixel array 2 in the output device, and each of the four corners 1a of the supercell 1 matches one of the pixels. Therefore, the angle θ of the halftone dot is calculated based on the rational number b / a.

[0025]

(Equation 1)

## EQU2 ## Also, side 1 of supercell 1
The number of pixels t corresponding to the length of b is calculated based on a predetermined integer p.

[0027]

(Equation 2)

The number Rs of halftone dots is calculated based on the number q of the halftone dots 3 arranged on the side 1b of the supercell 1 and the resolution Ro of the output device (the unit is the number of lines).

[0029]

(Equation 3)

## EQU2 ##

That is, in this parallel halftone method, if the resolution Ro of the output device is known, the angle θ of the halftone dot and the number of lines Rs
Is represented by a combination of four integer values a, b, p, and q, and this combination is an example of a dot parameter value according to the present invention.

FIG. 2 is a diagram showing a supercell of the oblique halftone dot system.

In the oblique halftone dot system, the halftone dots are arranged at an angle θ with respect to the supercell 4, and each of the four corners 4a of the supercell 4 coincides with any pixel. I have. In addition, since each of these four corners 4a also matches one of the square points of the square lattice formed by the boundary lines between the halftone dots 5, the angle θ of the halftone dot is also expressed by equation (1). You. The number of halftone dots T corresponding to the length of the side 4b of the supercell 4 is calculated based on a predetermined integer s.

[0034]

(Equation 4)

Thus, the number Rs of halftone dots is calculated based on the number r of pixels arranged on the side 4b of the supercell 4 and the resolution Ro of the output device.

[0036]

(Equation 5)

## EQU3 ##

That is, in the skewed halftone dot system, if the resolution Ro of the output device is known, the angle θ of the halftone dot and the number of lines Rs
Is represented by a combination of four integer values a, b, r, and s, and this combination is also an example of the dot parameter value according to the present invention.

In general, the number of lines and angles such as "175 LPI" and "15 °" of typical halftone dots used in analog devices are used as they are in digital devices. However, halftone parameter values representing the angle and the number of halftone dots used in digital equipment are generally a combination of integer values, and the angle and the number of halftone dots represented by such halftone dot parameter values are , Are limited to discrete values. For this reason, the substantial number of lines and angles of halftone dots used in digital equipment are 1
It is an approximate value for the number of lines and angle as a name, such as 72.7 LPI and 15.2 °. The number of lines and the angle as a name are so-called ideal values of the number of lines and the angle of the halftone dot.

Hereinafter, embodiments of the present invention will be described.

FIG. 3 is a block diagram showing an embodiment of a dot parameter value determining apparatus according to the present invention.

The dot parameter value determining apparatus 100 shown in FIG.
Is a parameter candidate selection unit 30 as an example of a provisional parameter value acquisition unit according to the present invention, a moiré calculation unit 40, a determination unit 50, and a parameter determination unit 60 as an example of a confirmed parameter value adoption unit according to the present invention. It has.

The halftone parameter value determining device 10
0 includes a parameter candidate storage unit 20.

Further, the dot parameter value determining device 1
00 includes a parameter candidate calculation unit 10 which is an example of a parameter value calculation unit according to the present invention, and also includes a range setting unit 70.

As will be described below, the dot parameter value determining apparatus 100 executes one embodiment of the dot parameter value determining method of the present invention.

FIG. 4 is a flowchart showing an embodiment of a method for determining a dot parameter value according to the present invention. Hereinafter, the flowchart of FIG. 4 will be described with reference to FIG.

First, the parameter candidate calculation unit 10 supplies the resolution Ro of the output device, the angle of halftone dots and the ideal value θ of the number of lines.
r, Rsr and allowable errors δθ, δRs are input (FIG. 4).
Step S101). The parameter candidate calculation unit 10
A plurality of halftone dot parameter value candidates corresponding to the methods shown in FIGS. 1 and 2 are calculated based on these input values and equations (1), (3), and (5) (FIG. 4). Step S10
2). The plurality of candidates calculated by the parameter candidate calculation unit 10 are stored in the parameter candidate storage unit 20, and one of the stored plurality of candidates is set by the parameter candidate selection unit 30 as a provisional halftone dot parameter value. It is selectively obtained (step S103 in FIG. 4). Here, it is assumed that the halftone dot parameter value at which the angle and the number of lines closest to the ideal values of the angle and the number of lines are obtained as the initial values θr and Rsr.

Thereafter, the moiré calculating section 40 calculates the moiré at the time of output based on the provisional halftone dot parameter value (step S104 in FIG. 4).

The moire can be calculated as a moire vector as described below.

The fundamental frequency vectors Rox, Roy of the pixel array output by the output device are represented by Rox = (Ro, 0) and Roy = (0, Ro). Further, a fundamental frequency vector R of an array of halftone dots
Rs1 = (Rs · cosθ, Rs · sinθ) Rs2 = (− Rs · sinθ, Rs · cosθ) Then, the frequency vector (Moiré vector) M of the Moiré fringe is M = (i · Rs1 + j · Rs2) − (m · Rox + n · Roy) for four arbitrary integers i, j, m, and n (6) And the magnitude | M | of the moire vector M is | M | = {[(i ² + j ² ) Rs ² -2} (m + n) icos θ + (nm) jsin θ｝ RsRo + (m ² + n ² ) Ro ² ] (7)

The smaller the magnitude | M | of the moiré vector M is, the longer the period becomes and conspicuous moiré fringes are generated. Therefore, the integers i, j,
The combination of m and n is important. Since a halftone dot is composed of a plurality of pixels, large moire fringes generally occur when i, j> m, n. Further, even when the magnitude | M | of the moiré vector M is small,
When the values of the integers i, j, m, and n are all large, the ratio of halftone dots and pixels that contribute to moiré is small, resulting in weak moiré.

In the moire calculating section 40 shown in FIG. 3 and in step S104 shown in FIG. 4, the integers i and i are set within a range where strong moire fringes with large intervals are generally expected to occur.
The magnitude | M | of the moire vector M is calculated by a threshold (7) for every combination of j, m, and n,
The calculated values are sorted in the order in which the size | M | of the moiré vector M is small (in other words, the order of the intervals of the moiré fringes is large).

For example, if the provisional halftone dot parameter values represent the angle and the number of lines of 23.2 degrees and 172.7 LPI, respectively, and the resolution of the output device is 96 lines / mm, the top 10 Is calculated as shown in Table 1 below. However, theoretically, there are four types of combinations of integers i, j, m, and n that cause a certain moiré fringe interval. In the following, one of those combinations is selected. I do. (Table 1) ij mn Moire fringe interval (mm) -------------------------- 24--20 -2-1 0.368 -6 -13 -10 0.333 5 13 10 0.262 24 21 21 0.237 1971 1 0.236 1871 1 0.217 198 111 0.196 -18 -8 -1- 1 0.185 -11 -25 -20 0.153 -25 -20 2-1 -1 0.149 Also, for example, the angle and the number of lines represented by the provisional halftone parameter values are 23.2 degrees and 172, respectively. In the case of 0.7 LPI and the resolution of the output device is 96 lines / mm, the upper nine calculation results are as shown in Table 2 below. (Table 2) ij mn Moire fringe spacing (mm) ------------------------------------ 2 -14 0 1 1.001 -12 -16- 110.70813161-10.1851-140010.167-2150-0-10.15912151-1-10.150-12-17-17-110.139-2 13 0 −1 0.135 14 3 1 0 0.131 As described above, when the moiré is calculated in the moiré calculation unit 40 shown in FIG. 3 and in step S104 shown in FIG. 4, as described below, the determination unit 50 is used. It is determined whether or not the moire is within the allowable level (step S10 in FIG. 4).
5).

In general, moire fringes with an interval of 0.4 mm or less are very difficult to identify with the naked eye. For example, in the case of the calculation results shown in Table 1, the interval between moire fringes is at most 0,1.
Since it is 368 mm, in this case, it is determined that the moire is within the allowable level. Then, the parameter determining unit 60 shown in FIG. 3 adopts the halftone parameter value used as a precondition for the calculation as the final value of the halftone parameter value (Step S106 in FIG. 4).

On the other hand, in the case of the calculation results shown in Table 2, moiré fringes having an interval of 1 mm or more are generated, and it is determined that the moiré is out of the allowable level. And
The permissible range of the dot parameter value is calculated and set by the range setting unit 70 shown in FIG. 3 (step S1 in FIG. 4).
07). The range setting unit 70 directly sets an allowable range for the integer value itself included in the halftone parameter value, or indirectly sets the allowable range for the angle or the number of lines of the halftone dot by indirectly setting the halftone parameter value. It is conceivable to set an allowable range for. Here, it is assumed that an allowable range is set for the number of halftone dots as an example.

The range setting unit 70 substitutes the combination of the integers i, j, m, and n that produce the maximum moiré into the above-described equation (7), and then calculates the number Rs of halftone dots and the frequency of the moiré fringe. Is calculated. An allowable range is set based on the calculation result.

FIG. 5 is a graph showing the relationship between the number of halftone dots and the frequency of moiré fringes.

The vertical axis of this graph indicates the frequency of moiré, and the horizontal axis indicates the number of halftone dots.

A curve 71 shown in this graph represents the relationship between the number of halftone dots and the frequency of moiré fringes. When the number of halftone dots is about 172 LPI, the moiré frequency becomes minimal.
When the number of halftone dots is about 175 LPI or more, the frequency of moire is about 2 lines / mm or more. The interval between moire fringes having a frequency of about 2 lines / mm or more is a small interval of about 0.5 mm or less. From such a calculation result, the allowable range of the number of halftone dots is set to 175 LPI or more so that the moire frequency is about 2 lines / mm or more.

As described above, when the allowable range of the halftone frequency is set in the range setting unit 70 shown in FIG. 3 and in step S107 in FIG. 4, a plurality of candidates stored in the parameter candidate storage unit 20 are stored. From among them, a dot parameter value that satisfies the allowable range of the number of lines is acquired by the parameter candidate selection unit 30 as a new temporary dot parameter value (step S103 in FIG. 4).

Thereafter, the above procedure is repeated to determine and determine a halftone dot parameter value such that the moire at the time of output by the output device is a moire within an allowable level. When halftone dots based on the halftone parameter values determined in this way are output from the output device, the occurrence of the above-described streak-like moire fringes is avoided.

When the above procedure is repeated a predetermined number of times, if a halftone parameter value that results in a moire within an allowable level cannot be obtained, a halftone parameter value that gives a moire closest to the allowable level is obtained. , Are adopted as final values by the parameter determining unit 60.

Next, a second embodiment will be described.

The configuration of the second embodiment is the same as the configuration of the halftone parameter determining apparatus shown in FIG. 3, but in the second embodiment, the halftone parameter of each of the M, C, and K color planes Values are determined simultaneously.

In the parameter candidate calculation unit 10, M, C,
The candidates for the halftone parameter values for each of the K color planes are calculated, and the calculated candidates are stored in the parameter candidate storage unit 20. Then, the provisional halftone parameter values for each of the M, C, and K color plates are stored in the parameter candidate selection unit 30.
Is selected in consideration of the moiré among the M, C, and K color plates. In the moiré calculation unit 40,
The moire at the time of output is calculated based on the halftone parameter values of each of the M, C, and K color planes.
A determination is made for moire for each of the C and K color plates. When the moire is within the allowable level for all of the M, C, and K color plates, the parameter determination unit 60
The provisional halftone dot parameter value of each of the K color plates is adopted as the final value.

According to the second embodiment, M, C, K
M, C, K after considering the moire between each color plate
For each color plate, a halftone parameter value is obtained such that the moire at the time of output is within the allowable level.

The parameter candidate storage unit 20 described above
Since the dot parameter value candidates stored in are not dependent on the resolution of the output device, the amount of calculation can be reduced by reusing these candidates for a plurality of output devices having different resolutions.

In the above-described embodiment, the parameter candidate calculation unit 10 calculates a candidate for a halftone parameter value. The halftone parameter determination apparatus of the present invention employs means for calculating a candidate for a halftone parameter value. Without having, the calculated candidate may be obtained from outside.

In the above-described embodiment, the moiré calculating section 40 calculates the moiré using the moiré vector. However, the moiré calculating section according to the present invention calculates the moiré using a method other than the moiré vector. It may be.

[0070]

As described above, according to the present invention,
The halftone dot parameter value can be determined so as to avoid generation of streak-like moiré fringes.

[Brief description of the drawings]

FIG. 1 is a diagram showing a super-cell of the parallel halftone dot system.

FIG. 2 is a diagram illustrating a supercell in a skewed halftone dot system.

FIG. 3 is a configuration diagram illustrating an embodiment of a halftone parameter value determining apparatus according to the present invention.

FIG. 4 is a flowchart illustrating an embodiment of a method for determining a dot parameter value according to the present invention.

FIG. 5 is a graph showing the relationship between the number of halftone dots and the frequency of moiré fringes.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Parameter candidate calculation part 20 Parameter candidate storage part 30 Parameter candidate selection part 40 Moire calculation part 50 Judgment part 60 Parameter determination part 70 Range setting part 100 Halftone parameter value determination device

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5B057 AA11 BA02 BA23 CA01 CA07 CA12 CA16 CB01 CB07 CB12 CB16 CC01 CE16 CH01 CH08 CH18 5C077 LL03 LL19 MP02 MP08 NN06 NN07 PP33 PQ08 PQ12 PQ20 PQ22 SS05 TT08

Claims (5)

[Claims] 1. A provisional parameter value acquisition unit for acquiring a provisional halftone parameter value for a halftone parameter value representing the number of lines and an angle of a halftone dot, and a provisional halftone parameter acquired by the provisional parameter value acquisition unit. A moiré calculating unit that calculates moiré generated when a halftone dot having a line number and an angle represented by a value is output by a predetermined output device; and whether the moiré calculated by the moiré calculating unit is a moiré within an allowable level. A determination unit for determining whether or not the moiré calculated by the moiré calculation unit is a moire within an allowable level; A provisional parameter value adopting unit that adopts the following formula: wherein the provisional parameter value acquiring unit allows the moiré calculated by the moiré calculating unit If it is determined by the determination unit that the moire outside the bell, dot parameter value determination device, characterized in that it is intended to obtain the dot parameter values of the new interim. 2. The method according to claim 1, wherein the provisional parameter value acquiring section selects and acquires a provisional halftone parameter value from a plurality of halftone parameter values prepared in advance. Dot parameter value determination device. 3. A parameter value calculation unit for calculating a plurality of halftone parameter values each satisfying a predetermined precondition independent of a condition indicating the allowable level, wherein the provisional parameter value acquisition unit includes 2. The halftone parameter value determining apparatus according to claim 1, wherein a tentative halftone parameter value is selectively acquired from a plurality of halftone parameter values calculated by the calculation unit. 4. A range setting unit that sets an allowable range for a halftone dot parameter value when the moiré calculated by the moiré calculating unit is determined to be a moiré outside an allowable level, The provisional parameter value obtaining unit, when the moiré calculated by the moiré calculating unit is determined by the determination unit to be a moiré outside the allowable level, a new moiré within the allowable range set by the range setting unit. The halftone parameter value determination device according to claim 1, wherein a temporary dot parameter value is obtained. 5. A provisional parameter value set obtaining step for obtaining a provisional halftone parameter value for a halftone dot parameter value representing a line frequency and an angle of a halftone dot, and a provisional mesh obtained in the provisional parameter value set obtaining step. A moiré calculating step of calculating moiré generated when a halftone dot having a line number and an angle represented by a point parameter value is output by a predetermined output device, and the moiré calculated in the moiré calculating step is a moiré within an allowable level. A determination step of determining whether or not there is, and when the moire calculated in the moire calculation step is determined to be a moire within an allowable level in the determination step, a provisional halftone parameter value is determined to determine a halftone dot. A fixed parameter value set adopting step for adopting as a parameter value, wherein the moiré calculated in the moiré calculating step is a moiré outside an allowable level. If it is determined it is,
Returning to the provisional parameter value obtaining step to obtain a new provisional halftone parameter value and repeating the moiré calculation step and the determination step;