JP6745185B2 - Threshold matrix creating method, threshold matrix creating apparatus, and program - Google Patents

Description

The present invention relates to a threshold matrix creating method, a threshold matrix creating apparatus, and a program, and more particularly to a threshold matrix creating technique used in halftone processing for converting a continuous tone image into a dot pattern that is a binary image.

In the printing field, frequency modulation screens have been adopted as well as amplitude modulation screens characterized by a combination of line number, angle, and dot shape. The amplitude modulation screen is called an AM (Amplitude Modulation) screen. The frequency modulation screen is called an FM (Frequency Modulation) screen.

As described in Patent Document 1 and Patent Document 2, when the FM screen is used in offset printing, there is a problem that roughness is conspicuous in terms of image quality. Further, in the FM screen, the dot gain becomes large and the image reproduction becomes unsuccessful at the time of printing, when outputting a film which is an intermediate step of printing, or when outputting a printing plate by a CTP (Computer to Plate) device. Problems such as becoming stable occur.

In order to solve such a problem of the FM screen, in Patent Document 1, for example, when an output system that cannot stably reproduce 1×1 pixel dots on the highlight side is used, dots are reliably generated on the highlight side. A minimum threshold dot size is set to 2×2 pixels so as to be reproducible, and a threshold matrix generating method capable of generating a binary image with reduced graininess in halftone and less dot gain is disclosed.

As a specific example of the output system, for example, any one of printing field devices such as a film setter, a plate setter, a CTP device, a CTC (Computer to Cylinder) device, a DDCP (Direct Digital Color Proof) system, or an offset printing machine, or An appropriate combination of these may be used.

The method of creating the threshold matrix described in Patent Document 1 will be outlined. In the method of creating the threshold matrix described in Patent Document 1, first, a white noise pattern is generated with a halftone dot percentage of 50%. Next, the white noise pattern is transformed into the frequency domain by the fast Fourier transform, further subjected to the pattern frequency band filter, and then transformed into the spatial domain by the fast inverse Fourier transform. The data in the space area thus obtained is binarized to define a dot pattern, and the blackened area in this dot pattern is set as a dot candidate position.

Next, the number of new dots of the minimum dot size in each halftone dot percentage is determined. Then, in each halftone percentage, first, the determined number of new dots of the minimum dot size is set in the dot candidate position, and then the dots for the remaining number of pixels are attached to the existing dots in the dot candidate position and set. Repeat to set the threshold for each halftone dot percentage.

Patent No. 4168033 JP, 2005-252888, A

In the case of the method described in Patent Document 1, the dot pattern with a dot percentage of 50% determines the perimeter of the dot pattern of almost all dot percentages including at least each dot percentage in the halftone region. However, the perimeter of the halftone dot pattern belonging to the highlight area is determined by the set minimum dot size.

However, it is desirable that the perimeter of the dot pattern for each halftone dot can be determined more freely. Generally, the longer the perimeter of the dot pattern, the smaller the individual dots, and the less visible the roughness, and the thinner the ink film thickness, which reduces ink consumption and improves ink cost reduction and drying. In addition, the color reproduction range is expanded. On the other hand, as the perimeter of the dot pattern becomes longer, the ink film thickness becomes thinner, so that the ink inking property and the printing durability are reduced, and it becomes more susceptible to blanket piling, double-dip, etc., and the fluctuation of dot gain also increases. Therefore, printability is reduced.

It is necessary to balance each of these contradictory quality items and adjust to an appropriate dot pattern perimeter, but the optimal balance of each quality item is considered to differ depending on each halftone dot, so each halftone dot percentage will be different. In, it is desirable that the perimeter of the dot pattern can be freely determined.

The present invention has been made in view of the above circumstances, and a method of creating a threshold matrix that can freely determine the perimeter of a dot pattern for each halftone dot according to the characteristics of the output system, the required quality, and the like. An object of the present invention is to provide a threshold matrix creating device and a program.

In order to solve the problems, the following invention modes are provided.

A threshold matrix creating method according to the first aspect is a threshold matrix creating method for converting a continuous tone image into a dot pattern that is a binary image, and an image quality evaluation value of a dot pattern corresponding to a threshold arrangement in the threshold matrix. A step of setting an evaluation function for calculating, a step of setting a perimeter limit value of the dot pattern for each threshold value with respect to at least a part of the threshold value, and a perimeter length limit value of at least a part of the threshold value or less and And a step of determining a pixel placement position of the threshold value based on the image quality evaluation value from among the threshold value candidate pixels that are candidates of the threshold placement position.

According to the first aspect, it is possible to freely set the perimeter of the dot pattern for each halftone dot pattern corresponding to the threshold and create a threshold matrix that achieves a target balance between image quality and printability. it can.

In the threshold value matrix creating method of the first aspect as the second aspect, the step of setting the perimeter length limit value is performed for each threshold value belonging to at least a part of the threshold sections in the entire threshold range from the minimum threshold value to the maximum threshold value. The configuration may be such that a perimeter length limit value is set.

As a third aspect, in the threshold matrix creating method of the first aspect or the second aspect, a step of setting a minimum dot size that is the number of constituent pixels of the minimum size dot, and a total threshold range from the minimum threshold to the maximum threshold In at least one of a highlight section that is a threshold range that is greater than or equal to the minimum threshold and less than or equal to the first threshold, and a shadow section that is a threshold range that is greater than or equal to the first threshold and less than or equal to the maximum threshold, the perimeter limit value is The step of determining the arrangement position of the threshold value so as to arrange the dots of the minimum dot size by not applying the constraint of the perimeter limitation based on the above.

As a fourth aspect, in the threshold value matrix creating method of the third aspect, the threshold values included in the halftone interval that is a threshold value range that is equal to or greater than the third threshold value that is greater than the first threshold value and equal to or less than the fourth threshold value that is less than the second threshold value are included. It is possible to adopt a configuration in which the restriction of the perimeter limit based on the perimeter limit value is applied to each of them, and the threshold value is arranged in the pixels having the perimeter length limit value or less.

As a fifth aspect, in the threshold matrix creating method of the fourth aspect, a highlight transition section that is a threshold range between the first threshold and the third threshold and a shadow transition that is a threshold range between the fourth threshold and the second threshold. In at least one of the transition sections, the perimeter length non-restriction processing that determines the placement position of the threshold value to arrange the dots of the minimum dot size without applying the perimeter length restriction constraint, and the perimeter length limit value or less It is possible to adopt a configuration in which a perimeter length limiting process that determines the arrangement position of the threshold value based on the image quality evaluation value among the threshold value candidate pixels is performed in combination.

As a sixth aspect, in the method of creating the threshold matrix of the fifth aspect, a step of setting a plurality of threshold sections that is a threshold range in which a unit threshold is a consecutive threshold number that is a multiple of the minimum dot size, and a plurality of threshold values A step of setting a threshold number for arranging dots of the minimum dot size by perimeter length non-limitation processing for each of the sections may be included.

As a seventh aspect, in the threshold matrix creating method according to any one of the third aspect to the sixth aspect, an evaluation function applied to a part of the halftone dot percentages in at least one of the highlight section and the shadow section, and a part of the halftone dots It is possible to adopt a configuration in which an evaluation function applied to a mesh percentage other than the percentage is different.

In the threshold matrix creating method according to any one of the first to seventh aspects as an eighth aspect, when there is no threshold candidate pixel that is equal to or less than the perimeter limit value, the pixel with the smallest perimeter is set as the threshold. It can be configured to be a candidate pixel.

As a ninth aspect, in the threshold matrix creating method according to any one of the first to eighth aspects, a step of setting the number of lines of the amplitude modulation screen, and a step of setting a perimeter limit value for each threshold from the number of lines And can be configured to include.

In the threshold matrix creating method according to any one of the first to ninth aspects as a tenth aspect, in addition to the perimeter limitation based on the perimeter limitation value, a first additional constraint condition based on the perimeter of each pixel is added. The configuration may include a step of setting and a step of limiting candidates for the arrangement position of the threshold value based on the perimeter of each pixel according to the first additional constraint condition.

In the threshold matrix creating method according to any one of the first to tenth aspects as an eleventh aspect, in addition to the perimeter limitation based on the perimeter limitation value, a step of setting a priority order based on the perimeter of each pixel, , Setting threshold candidate pixels based on the perimeter of each pixel according to the priority order.

As a twelfth aspect, in the threshold value matrix creating method of the tenth aspect, in the halftone interval which is a threshold range of the third threshold value to the fourth threshold value in the entire threshold range from the minimum threshold value to the maximum threshold value, the periphery of each pixel is The configuration may include a step of dividing the first additional constraint condition based on the length into a plurality of different threshold sections.

In the threshold value matrix creating method according to the eleventh aspect as the thirteenth aspect, in the halftone interval which is the threshold range of the third threshold value to the fourth threshold value in the entire threshold value range from the minimum threshold value to the maximum threshold value, the periphery of each pixel is The configuration may include a step of dividing the rule of the priority order based on the length into a plurality of different threshold sections.

As a fourteenth aspect, in the threshold value matrix creating method according to any one of the first to thirteenth aspects, a step of setting a second additional constraint condition based on the arrangement of the threshold-set pixels around the threshold-unset pixels; According to the second additional constraint condition, the step of limiting candidates for the threshold value arrangement based on the arrangement of the threshold value-set pixels around the threshold-unset pixel can be included.

As a fifteenth aspect, in the threshold value matrix creating method of the fourteenth aspect, a step of setting a third additional constraint condition based on the perimeter of the surrounding threshold-set pixels, a second additional constraint condition, and a third additional constraint According to the condition, a step of limiting candidates for threshold placement based on the placement of threshold-set pixels around the threshold-unset pixels and the perimeter of the surrounding threshold-set pixels may be included.

A threshold value matrix creating device according to a sixteenth aspect is a threshold value matrix creating device that creates a threshold value matrix that converts a continuous tone image into a dot pattern that is a binary image. An evaluation function setting unit that sets an evaluation function for calculating the image quality evaluation value, a perimeter length limit value setting unit that sets a perimeter limit value of the dot pattern for each threshold value for at least some threshold values, and at least one A threshold based on the image quality evaluation value is selected from among the threshold candidate pixel determination unit that determines a pixel whose perimeter is less than or equal to the perimeter limit value as a candidate for the threshold position, and the threshold candidate pixel that is a candidate for the threshold position. A threshold placement determining unit that determines the placement position of the.

In the threshold value matrix creating device of the sixteenth aspect, the same matters as the matters specified in the second to fifteenth aspects can be appropriately combined. In that case, the element of the step specified in the method of creating the threshold matrix can be understood as a processing unit or a functional unit that is a unit that performs the corresponding processing or operation.

A program according to a seventeenth aspect is a program for causing a computer to realize a function of creating a threshold value matrix for converting a continuous tone image into a dot pattern which is a binary image, the dot corresponding to an arrangement of threshold values in the threshold value matrix. A function to set the evaluation function for calculating the image quality evaluation value of the pattern, a function to set the perimeter limit value of the dot pattern for each threshold with respect to at least some thresholds, and a surrounding for at least some thresholds. A function of setting a pixel having a length limit value or less as a candidate for a threshold arrangement position, and a function of determining a threshold arrangement position based on an image quality evaluation value from among threshold candidate pixels which are candidates for the threshold arrangement position. It is a program to be realized.

In the program of the seventeenth aspect, the same matters as the matters specified in the second to fifteenth aspects can be appropriately combined. In that case, the element of the step specified in the method of creating the threshold matrix can be understood as the element of the program that realizes the function of the process or operation corresponding thereto.

According to the present invention, a balance between image quality and printability, which are contradictory quality items, can be freely determined for each halftone dot, and a threshold matrix that can obtain a dot pattern that achieves a target balance can be created.

FIG. 1 is a diagram showing an example of a dot pattern. FIG. 2 is a diagram showing another example of the dot pattern. FIG. 3 is a flowchart showing the outline of the method of creating the threshold matrix according to the embodiment. FIG. 4 is an explanatory diagram showing the correspondence between halftone dot percentages and threshold values. FIG. 5 is a chart showing a setting example of how to reduce the number of new dots of the minimum dot size in the highlight transition section. FIG. 6 is a graph showing a setting example of the perimeter length limit value. FIG. 7 is a diagram showing an example of a dot pattern corresponding to the threshold 131 in a matrix size of 20×20 pixels. FIG. 8 is a diagram in which the perimeter of each pixel when a dot is arranged in each pixel for which a threshold value has not been set is added to the dot pattern of FIG. 7. FIG. 9 is a table showing the adjacency relationship between non-threshold-set pixels and threshold-set pixels, the perimeter of the non-threshold pixels, and the perimeter increase when dots are placed in the non-threshold pixels. FIG. 10 is a block diagram showing the functional configuration of the threshold value matrix creating device according to the embodiment of the present invention. FIG. 11 is a flowchart showing a first example of parameter setting processing in the threshold value matrix creating device. FIG. 12 is a flowchart showing an example of threshold setting processing in the threshold matrix creating device. FIG. 13 is a flowchart showing the flow of the ascending threshold setting process. FIG. 14 is a flow chart when the threshold values are alternately set from the highlight side and the shadow side. FIG. 15 is a graph showing an example of the dot pattern perimeter of the threshold matrix created according to this embodiment. FIG. 16 is a diagram showing an example of the AM screen when the angle of the halftone dots is 0 degree. FIG. 17 is a diagram showing an example of halftone dots having an angle of halftone dots of 0 degree. FIG. 18 is a diagram showing an example of halftone dots having an angle of 15 degrees. FIG. 19 is a diagram showing an example of an AM screen composed of circular halftone dots. FIG. 20 is a diagram showing an example of circular halftone dots. FIG. 21 is a graph showing an example of the perimeter length per square inch when the output resolution is 2400 dpi (dot per inch). FIG. 22 is a flowchart showing a second example of the parameter setting process in the threshold matrix creating device. FIG. 23 is a flowchart showing a third example of the parameter setting process in the threshold matrix creating device. FIG. 24 is a flowchart showing a fourth example of the parameter setting process in the threshold matrix creating device. FIG. 25 is an explanatory diagram showing an example of surrounding dot arrangement restrictions for pixels having a peripheral length of 2. FIG. 26 is an explanatory diagram showing an example of a surrounding dot arrangement restriction for a pixel having a peripheral length of 2. FIG. 27 is an explanatory diagram showing an example of a surrounding dot arrangement restriction for a pixel having a peripheral length of 3. FIG. 28 is an explanatory diagram showing an example of surrounding dot arrangement restrictions for pixels with a peripheral length of 2. FIG. 29 is an explanatory diagram showing an example of a surrounding dot arrangement restriction for a pixel having a peripheral length of 3. FIG. 30 is a flowchart showing a fifth example of parameter setting processing in the threshold value matrix creating device. FIG. 31 is an explanatory diagram showing an example of constraint conditions based on the perimeter of surrounding dots for a pixel of perimeter 3. FIG. 32 is an explanatory diagram showing an example of a constraint condition based on the perimeter of surrounding dots for a pixel having a perimeter of 3. FIG. 33 is a flow chart showing a sixth example of parameter setting processing in the threshold value matrix creating device. FIG. 34 is a block diagram showing an example of the hardware configuration of the threshold value matrix creating device. FIG. 35 is a diagram showing a configuration example of a printing system.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[About the perimeter of the dot pattern]
First, the perimeter of the dot pattern will be briefly described. FIG. 1 is a diagram showing an example of a dot pattern. The dot pattern 10A illustrated in FIG. 1 includes 16 1×1 pixel dots 14 in a pixel region of 12×12 pixels. The 1×1 pixel dot 14 is a dot having a size of vertical 1 pixel×horizontal 1 pixel. It is assumed that one pixel is a square having a side length L _u . L _u depends on the output resolution of the output system. For example, when the output resolution is 2400 dpi, L _u is about 10.58 micrometers [μm]. dpi (dot per inch) is a unit representing the number of dots per inch. One inch is 25.4 millimeters [mm].

In FIG. 1, the pixels displayed by filling in gray are the dot-on black pixels, and the pixels displayed in white are the dot-off white pixels. The total area of the black pixels of the dot pattern 10A is 16×L _u ² =16L _u ² . The perimeter of the dot pattern 10A is the total length of the black and white boundaries. The black-and-white boundary length is the length of the boundary line between the white pixel and the black pixel. The dot perimeter of the 1×1 pixel dot 14, that is, the dot perimeter is 4L _u . The dot perimeter is the length of the black and white boundary of one dot. The perimeter of the dot pattern 10A in which 16 1×1 pixel dots 14 are arranged is 16×4L _u =64L _u . The "perimeter of the dot pattern" is the sum of the perimeters of the dots in the dot pattern, and is also referred to as the "perimeter of the dot pattern".

FIG. 2 is a diagram showing an example of another dot pattern. The dot pattern 10B illustrated in FIG. 2 includes four 2×2 pixel dots 18 in a pixel region of 12×12 pixels. The 2×2 pixel dot 18 indicates that the dot has a size of 2 vertical pixels×2 horizontal pixels. In FIG. 2, the pixels displayed by filling in gray are the dot-on black pixels, and the pixels displayed by the outline are the dot-off white pixels. The 2×2 pixel dot 18 is a dot having a side length of 2L _u and a dot circumference length of 8L _u . The total area of the black pixels of the dot pattern 10B is 16×L _u ² . The perimeter of the dot pattern 10B is 4×8L _u =32L _u .

The dot pattern 10A shown in FIG. 1 and the dot pattern 10B shown in FIG. 2 have the same total area of black pixels. That is, the dot pattern 10A and the dot pattern 10B have the same total area of black pixels per unit area with 12×12 pixels as the unit area, and have the same halftone dot percentage. On the other hand, the circumferential length of each of the dot patterns, while the perimeter of the dot pattern 10A is 64L _u, the perimeter of the dot pattern 10B is 32L _u, the dot pattern towards the perimeter of the dot patterns 10A Twice longer than the perimeter of 10B.

When the dot percentage is the same, the larger the dot pattern perimeter becomes, the smaller each dot becomes and the less visible the roughness becomes, but the dot gain increases and the printability deteriorates.

The present disclosure provides a means capable of freely setting the perimeter of a dot pattern for each halftone percentage to create a desired threshold matrix so as to satisfy the required balance between print quality and printability. The threshold matrix is a matrix that converts a continuous tone image into a dot pattern that is a binary image, and is used for halftone processing by the dither method. The threshold matrix is also called a dither matrix.

[Method of Creating Threshold Matrix According to Embodiment]
Hereinafter, a method of creating a threshold matrix according to the embodiment will be described. FIG. 3 is a flowchart showing the outline of the method of creating the threshold matrix according to the embodiment. The main body that executes the steps of the process shown in the flowchart of FIG. 3 is, for example, a computer that functions as a threshold matrix creation device (not shown). By executing the program, the computer can function as a threshold matrix creating device.

The threshold value matrix creating method according to the present embodiment includes a step of setting parameters (step S1) and a step of sequentially setting threshold values based on the image quality evaluation value under the perimeter restriction (step S2). It should be noted that in the creation of the threshold matrix, “setting the threshold” means arranging the threshold in pixels in the matrix. That is, setting the threshold value means determining a pixel to which the threshold value is to be arranged from pixels in which the threshold value is not set in the threshold value matrix, and disposing the threshold value at the position of the pixel.

The perimeter limit is one of constraint conditions when setting a threshold value. Pixels in which the perimeter of the dot pattern is equal to or less than the perimeter limit value are candidates for the pixels in which the threshold is arranged. The image quality evaluation value is used as a determination index for determining the position of the pixel in which the threshold value is to be arranged, from the viewpoint of which pixel should have the threshold value to obtain good image quality.

<Explanation of Step S1>
In step S1, the threshold matrix creating device sets various parameters necessary for creating the threshold matrix. The parameters necessary for creating the threshold matrix include the size of the threshold matrix, the evaluation function for calculating the image quality evaluation value, and the initial arrangement. The size of the threshold matrix is called “matrix size”. The initial arrangement means a dot arrangement corresponding to a particular halftone dot ratio which is an initial dot arrangement when the arrangement of each threshold value is sequentially determined.

Further, in step S1, the threshold matrix creation device sets a perimeter limit value for each threshold with respect to at least some thresholds. The perimeter length limit value is a total limit value of the perimeter lengths in the dot patterns of the entire matrix size. The perimeter limit value is a constraint condition for determining threshold candidate pixels that are candidates for pixels in which thresholds are arranged. Among the pixels for which the threshold value is not set, the pixels having the perimeter length limit value or less can be the threshold value candidate pixels.

Further, in step S1, the threshold value matrix creating device sets the minimum dot size so that dots can be reliably reproduced in highlight, and sets how to reduce the number of new dots of the minimum dot size from highlight to halftone. To do.

The minimum dot size is the number of constituent pixels of an isolated dot having a minimum size that can be stably formed by an output system (not shown). The minimum dot size is represented by the number of pixels with the size of one pixel defined by the output resolution of the output system as a unit. The output system may have various configurations. For example, the output system is a system including a platesetter and an offset printing machine that prints using a printing plate output from the platesetter.

The minimum dot size is determined depending on the output system used. When the number of pixels of the minimum size isolated dot that can be stably formed by the output system is 2×2 pixels, the minimum dot size is set to 2×2 pixels. In this case, the number of pixels of the minimum dot size is “4”.

Hereinafter, as a specific example, a case where the minimum dot size is 2×2 pixels and the matrix size is 20×20 pixels will be described as an example. In this example, assuming that the minimum threshold value is threshold value 1 and the maximum threshold value is threshold value 400, 400 threshold values from threshold value 1 to threshold value 400 are arranged in each pixel of a 20×20 pixel matrix.

First, an example of setting a highlight threshold will be described. The highlight threshold value refers to a threshold value that belongs to the gradation region on the highlight side with halftone dot percentage of 50% or less. A threshold value that belongs to the gradation range on the shadow side exceeding halftone dot percentage 50% is called the shadow threshold value.

FIG. 4 is an explanatory diagram showing the correspondence between halftone dot percentages and threshold values. Threshold value 1 which is the minimum threshold value corresponds to 0% of halftone dot, and threshold value 400 which is the maximum threshold value corresponds to 100% of halftone dot. The threshold value 200 corresponds to a halftone dot percentage of 50%. The relationship between each of the other threshold values and the halftone dot value is associated by the calculation of linear interpolation.

As shown in FIG. 4, the entire threshold range from the minimum threshold to the maximum threshold is divided into a plurality of threshold sections according to the size of the threshold. FIG. 4 shows an example divided into six sections: a highlight section, a highlight transition section, a halftone section, a shadow transition section, and a shadow section. The halftone section can be divided into two sections with a halftone area of 50% as a boundary, a highlight halftone section which is a highlight-side halftone section, and a shadow halftone section which is a shadow-side halftone section. ..

The highlight section in the example of FIG. 4 is a section from threshold 1 to threshold 16. When p and q are integers and p<q, the expression "threshold p-threshold q" or the expression "threshold q-threshold p" indicates a threshold range from the threshold p to the threshold q. means. In addition, the section from the threshold value p to the threshold value q may be referred to as “threshold value section p to q”.

The initial arrangement on the highlight side is set, for example, as a dot arrangement of halftone dots corresponding to the threshold value 8. A threshold range equal to or less than the threshold value 8, that is, a section from the threshold value 1 to the threshold value 8 is called an extreme highlight section. In FIG. 4, the pole highlight section is described as “pole HL section”. HL is a shorthand notation for highlight.

The highlight transition section is a section from threshold 17 to threshold 64. The halftone section is a section of threshold values 65 to 337. Among the halftone intervals, the interval between the threshold value 65 and the threshold value 200 is the highlight intermediate tone interval.

The section setting on the highlight side and the section setting on the shadow side have a symmetrical structure with a halftone dot percentage of 50% as a boundary. The section between the threshold value 385 and the threshold value 400 is the shadow section. The initial arrangement on the shadow side is set, for example, as a dot arrangement of halftone dots corresponding to the threshold value 393. A section between the threshold value 393 and the threshold value 400 is a polar shadow section. In FIG. 4, the polar shadow section is described as “polar SD section”. SD is a shorthand notation for shadow. The section between the threshold value 337 and the threshold value 384 is the shadow transition section. The interval between threshold 201 and threshold 336 is the shadow halftone interval.

The highlight transition section is a threshold section that transitions from the highlight section to the highlight halftone section. The shadow transition section is a threshold section that transitions from the shadow section to the shadow halftone section.

FIG. 5 is a chart showing a setting example of how to reduce the number of new dots of the minimum dot size in the highlight transition section. The highlight transition section is set with a unit section for determining the arrangement of thresholds. The threshold number of unit intervals is determined by a multiple of the minimum dot size. Here, since the number of pixels of the minimum dot size is “4”, the threshold number of the unit section is set to 16, which is a multiple of 4, as an example. That is, the highlight transition section is divided into three sections, that is, a threshold section 17 to 32, which is a unit section having a threshold number of 16, a threshold section 33 to 48, and a threshold section 49 to 64.

For the threshold sections 17 to 32, the number of new dot thresholds per unit section is set to "12". The new dot threshold number means the threshold number for generating new dots of the minimum dot size. Since the minimum dot size of this example is “4”, the number of new dot thresholds of the minimum dot size is 12 means that three new dots of the minimum dot size are arranged in the threshold section 17 to 32. To do.

Similarly, for the threshold sections 33 to 48, the number of new dot thresholds per unit section is set to 8. The new dot threshold number of 8 means that two new dots having the minimum dot size are arranged in the threshold sections 33 to 48.

Also, for the threshold sections 49 to 64, the new dot threshold number per unit section is set to 4. When the number of new dot thresholds is 4, it means that one new dot having the minimum dot size is arranged in the threshold sections 49 to 64.

That is, according to the setting shown in FIG. 5, the number of new dots of the minimum dot size generated in the threshold sections 17 to 32 is 3, the number of new dots of the minimum dot size generated in the threshold sections 33 to 48 is 2, and the threshold value is 2. The number of new dots of the minimum dot size generated in the sections 49 to 64 is one, and the number of new dots of the minimum dot size is gradually reduced as the threshold value increases.

When setting the threshold value of the highlight transition section in step S2, according to the setting shown in FIG. 5, of the 16 threshold values belonging to the threshold sections 17 to 32, 12 threshold values do not apply the perimeter limit. The threshold is set so that three new dots having the minimum dot size are arranged, and the remaining four thresholds are set under the constraint condition that the perimeter length limit value is applied.

In addition, among the 16 threshold values belonging to the threshold sections 33 to 48, 8 threshold values are set so that two new dots of the minimum dot size are arranged without applying the perimeter limitation, and the remaining threshold values are set. The eight thresholds are set under the constraint condition that the perimeter length limit value is applied.

Of the 16 thresholds belonging to the threshold sections 49 to 64, 4 thresholds are set so that one new dot of the minimum dot size is arranged without applying the perimeter limit, and the remaining 12 thresholds are set. The threshold value of is set under the constraint condition that the perimeter length limit value is applied.

Regarding each of the unit sections shown in FIG. 5, which threshold is actually assigned to the specified number of new dot thresholds, that is, which threshold is to be arranged as the threshold of a new dot of the minimum dot size Can have various settings.

For example, among the 16 threshold values belonging to the threshold sections 17 to 32, 12 threshold values of threshold values 17 to 28 are arranged as threshold values of new dots of the minimum dot size, and the remaining 4 threshold values of 29 to 32 are set. As for the thresholds, the thresholds are assigned so that the thresholds are arranged under the constraint condition that the perimeter length limit value is applied. Further, among the 16 thresholds belonging to the threshold sections 33 to 48, 8 thresholds of the thresholds 33 to 40 are arranged as thresholds of new dots of the minimum dot size, and the remaining 8 thresholds of 41 to 48 are arranged. The threshold value is set such that the threshold value is arranged under the constraint condition that the perimeter length limit value is applied. Of the 16 threshold values belonging to the threshold sections 49 to 64, the four threshold values of the threshold values 49 to 52 are arranged as the threshold values of the new dot of the minimum dot size, and the remaining 12 threshold values of the threshold values 53 to 64 are: The threshold assignment is set so that the thresholds are arranged under the constraint condition that the perimeter length limit value is applied.

All highlight sections with a threshold value of 16 or less are arranged as threshold values for forming new dots of the minimum dot size. It should be noted that for the highlight section having a threshold value of 16 or less, the number of new dot threshold values per unit section of the threshold section 1 to 16 can be set to 16 according to the chart of FIG.

For the halftone section having a threshold value of 65 or more, the setting of the new dot threshold value number as shown in FIG. 5 can be omitted. For each of the threshold values in the halftone section of the threshold value 65 to the threshold value 336, the threshold value is arranged under the constraint condition that the perimeter length limit value is applied. The constraint condition to which the perimeter length limit value is applied refers to a condition that the threshold value is arranged so that the perimeter length of the dot pattern is equal to or less than the perimeter length limit value. The constraint condition to which the perimeter length limit value is applied may be simply referred to as “perimeter length limit”.

Regarding the threshold value that determines the arrangement under the constraint condition that the perimeter length limit value is applied in each threshold section, the perimeter length limit value is set as a parameter in step S1.

In the case of this example, the perimeter limit is not applied to the highlight sections of the threshold sections 1 to 16, so that the parameter setting of the perimeter limit value is unnecessary. In the threshold sections 17 to 32, the perimeter limit value is set as a parameter for each of the threshold values 29 to 32. In the threshold sections 33 to 48, the perimeter limit value is set as a parameter for each of the threshold values 41 to 48. In the threshold sections 49 to 64, the perimeter limit value is set as a parameter for each of the threshold values 53 to 64. For the halftone sections equal to or greater than the threshold section 65, the perimeter length limit value is set as a parameter for all the threshold values.

The perimeter length limit value of this example is a limit value of the total perimeter length in a matrix size of 20×20 pixels. The perimeter is expressed in units of the length of one side of one pixel defined by the output resolution of the output system. For example, considering the perimeter limit value of the threshold 29, the thresholds up to the threshold 28 are all thresholds for new dots of the minimum dot size, and there are seven new dots of the minimum dot size. Is 8×7=56.

Therefore, the perimeter limit value of the threshold value 29 is set to any value in the range of 52 or more and 60 or less. However, in practice, the threshold value 29 is set as a threshold value for expanding a dot aggregate by adding a black pixel at a position where one side of a pixel is in contact with a dot having an existing minimum dot size, or an existing minimum dot size. Since the black pixel of 1×1 pixel dot isolated from the dot of 1 is set as the threshold for adding, the perimeter of the dot pattern of the threshold 29 can be only 58 or 60. Absent.

FIG. 6 is a graph showing a setting example of the perimeter length limit value. The horizontal axis represents the dot percentage and the vertical axis represents the perimeter length limit value. Since the halftone dot percentage on the horizontal axis has a one-to-one correspondence with the threshold value as described in FIG. 4, the halftone dot percentage can be converted into the threshold value.

In FIG. 6, the perimeter limit value of each halftone percentage is shown for the entire range of the halftone dot percentage from 0% to 100%. However, when the threshold value is set, the perimeter length limit value is not applied. It is not necessary to set the perimeter length limit value for the part, a part of the shadow transition section, and the threshold value of the shadow section. Further, it is not necessary to set all the perimeter length limit values for each threshold in advance, but the perimeter length limit values may be set for some threshold values, and the perimeter length limit values for other threshold values may be calculated by interpolation. ..

<Explanation of Step S2>
When the setting of various parameters is completed in step S1 of FIG. 1, the threshold value matrix creating device proceeds to the process of step S2. In step S2, the threshold matrix generation device sequentially determines the arrangement position of each threshold according to the set parameters. Here, a method of setting each threshold value on the highlight side will be described using the specific example shown in FIG.

[About threshold setting for highlight section]
In the highlight section of the threshold values 1 to 16, each threshold value is set so that the dots of the minimum dot size are sequentially arranged based on the image quality evaluation value without the limitation of the peripheral length. “Without perimeter length restriction” means “without applying the constraint condition of perimeter length limitation by the perimeter length limit value”.

As the image quality evaluation value, for example, an evaluation value for quantitatively evaluating "roughness" is used. As the image quality evaluation method, a known method such as the method described in JP-A-2005-253052 can be adopted. As a first specific example of the image quality evaluation method, for example, when the thresholds are set in ascending order, a new dot is provisionally added to the already determined dot arrangement, and the binary value of the dot arrangement including the added new dot is used. A method of convolving an image with a human visual function and calculating a standard deviation can be adopted. Adding a new dot means adding a black pixel. When the thresholds are set in descending order, the dots are temporarily deleted from the already determined dot arrangement, and the human visual function is convolved with the binary image of the dot arrangement in which the dots have been deleted, and the standard deviation is calculated. .. Temporarily deleting a dot means deleting a black pixel from an existing dot arrangement, that is, changing a black pixel to a white pixel. Deleting dots from the dot arrangement of black pixels corresponds to adding white pixels in the white dot arrangement which is the arrangement form of white pixels.

The standard deviation calculated in this way is an example of the image quality evaluation value. When the image quality evaluation method according to the first specific example is adopted, a function for convolving a binary image showing a dot arrangement with a visual function and calculating a standard deviation corresponds to one form of the “evaluation function”.

As a second specific example of the image quality evaluation method, for example, when the thresholds are set in ascending order, a new dot is provisionally added to the already determined dot arrangement, and the binary value of the dot arrangement including the added new dot is set. A method of calculating an evaluation value by applying a visual function in a frequency space and integrating the image after frequency conversion can be adopted. Two-dimensional fast Fourier transform can be used for frequency conversion. When setting the threshold values in descending order, temporarily delete dots from the already determined dot arrangement, frequency-convert the binary image of the dot arrangement with the deleted dots, and then apply the visual function in the frequency space. And integrate. The integral value thus calculated is an example of the image quality evaluation value. When the image quality evaluation method according to the second specific example is adopted, a function for frequency-converting a binary image indicating a dot arrangement, applying a visual function in a frequency space, and integrating to calculate an integrated value is “evaluation function”. It corresponds to one form.

Standard deviation, which is an image quality evaluation value calculated from the evaluation function of the image quality evaluation method according to the first specific example, or integration, which is an image quality evaluation value calculated from the evaluation function of the image quality evaluation method according to the second specific example. The smaller the value, the smaller the roughness.

In setting the threshold value based on the image quality evaluation value illustrated above, in the case of ascending order, a new dot is actually added to a pixel having a smaller image quality evaluation value, and the threshold value is set at the pixel position of the added new dot. To do. In the case of the descending order, dots are actually deleted from the pixels having the smaller image quality evaluation value, and the threshold is set at the pixel position of the deleted dots.

For example, when the dot arrangement in which two dots of 2×2 pixels, which are the minimum dot size, are arranged as the initial arrangement, the threshold values 8 to 1 are set in descending order and the threshold values 9 to 16 are set in ascending order. Will be done.

The extreme highlight section of the highlight section is different from the image quality evaluation value described in the first concrete example or the second concrete example because there are few dots and it is difficult to obtain a good distributed arrangement even by using a visual function. It is preferable to use the evaluation value of. That is, it is preferable that the evaluation function applied to the dot percentage corresponding to the extreme highlight interval in the highlight interval and the evaluation function applied to the dot percentage other than the extreme highlight interval be different.

Specifically, for the extreme highlight section, a value obtained by convolving a potential image such as a Gaussian function into a binary image with a dot arrangement is used as an image quality evaluation value, and in ascending order, a pixel with a small image quality evaluation value is the most distant from other dots. Add a new dot as if it were a distant pixel. On the other hand, in descending order, a pixel having a large image quality evaluation value is regarded as a pixel having the shortest distance from other dots, and the dot is deleted. A method of setting a threshold value in such an extreme highlight section is known as a void and cluster method. The function for calculating the image quality evaluation value used in the void and cluster method corresponds to one form of the evaluation function.

[About threshold setting for highlight transition section]
In the highlight transition section of the threshold value 17 to the threshold value 64, new dots of the minimum dot size of the number defined in each threshold value section described in FIG. Set the threshold value under the perimeter limit. “Under the perimeter length limitation” means that the constraint condition of the perimeter length limitation based on the perimeter length limit value is applied. Setting the threshold value under the perimeter limit means arranging the threshold value on the pixels having the perimeter length equal to or less than the perimeter limit value. When there is a margin of “4” or more in the perimeter length limit value of the threshold value that should be set under the perimeter limit, a new isolated dot of 1×1 pixel dot can be added.

For example, first, in the threshold value sections 17 to 32, 12 threshold values of threshold values 17 to 28 are arranged as threshold values of new dots having the minimum dot size, and the remaining 4 threshold values of 29 to 32 are peripheral lengths. The threshold value is arranged under the constraint condition that the limit value is applied.

Next, in the threshold sections 33 to 48, eight thresholds 33 to 40 are arranged as thresholds for new dots of the minimum dot size, and perimeter limit values are set to eight thresholds 41 to 48. Place the threshold under the applied constraints.

Then, in the threshold sections 49 to 64, the four thresholds of the thresholds 49 to 52 are arranged as the thresholds of the new dots of the minimum dot size, and the perimeter limit value is applied to the 12 thresholds of the thresholds 53 to 64. Place thresholds under constraints.

[Regarding threshold settings for highlight halftone section]
In the highlight halftone section of the threshold value 65 to the threshold value 200, all the threshold values are set under the perimeter limit. FIG. 7 is an example of a dot pattern having a matrix size of 20×20 pixels and a threshold value 131. The dot pattern shown in FIG. 7 is composed of 131 black pixels. Black pixels in FIG. 7 are pixels for which threshold values have been set. White pixels are pixels for which a threshold value has not been set. Pixels for which thresholds have been set are referred to as "threshold set pixels". Pixels for which a threshold value has not been set are referred to as “threshold value unset pixels”. The perimeter of the dot pattern shown in FIG. 7, that is, the total sum of the black and white boundaries is 222.

FIG. 8 is a diagram in which the perimeter of each pixel when a dot is arranged in each pixel for which a threshold value has not been set is added to the dot pattern of FIG. 7. The perimeter of each pixel for which a threshold value has not been set has a value of 0, 1, 2, 3, or 4. Note that in the example of FIG. 8, there are no pixels with a perimeter of 0.

The “perimeter in each pixel when dots are arranged in each pixel”, which is the numerical value given to each pixel for which a threshold value is not set in FIG. 8, is the length of the boundary at which each pixel contacts another white pixel. The perimeter of each pixel for which a threshold has not been set may be referred to as "perimeter of a pixel for which no threshold has been set" or simply "perimeter of pixel". It should be noted that the method of counting the perimeters of the pixels located at the ends of the matrix area is specified from the relationship with the adjacent pixels in a state where the dot patterns of the same matrix area are arranged so as to be spread in the vertical and horizontal directions. When the pixel position in the matrix area shown in FIG. 8 is expressed as pixel (i, j) using the row number i and the column number j, for example, the upper side of the pixel (1, 3) is spread over the upper side of FIG. It contacts the lower side of the pixels (20, 3) in the same matrix area arranged side by side. Since the pixel (20, 3) is a white pixel, the perimeter of the pixel (1, 3) is “3”. Similarly, the right side of the pixel (20, 20) is in contact with the left side of the pixel (20, 1) in the same matrix area arranged side by side so as to be spread over the right side of FIG. Further, the lower side of the pixel (20, 20) is in contact with the upper side of the pixel (1, 20) in the same matrix region arranged side by side so as to be spread over the lower side of FIG. 8. Since the pixel (20, 1) is a white pixel and the pixel (1, 20) is a black pixel, the perimeter of the pixel (20, 20) is “2”.

FIG. 9 shows the adjacency relationship between the threshold-unset pixel and the threshold-set pixel, the perimeter of the threshold-unset pixel, and the perimeter increase when a dot is placed in the non-threshold pixel.

When there is no threshold-set pixel at pixel positions adjacent to each other in the four directions of the threshold-unset pixel in the four directions, the perimeter of the same pixel when a dot is placed in the threshold-unset pixel is four. Is. Such a pixel for which a threshold value has not been set is called a pixel having a perimeter of 4. When a dot is arranged in a pixel having a perimeter of 4, the perimeter increment is "+4". Note that arranging dots in pixels for which thresholds have not been set has the same meaning as making white pixels for which thresholds have not been set into black pixels, that is, arranging thresholds for pixels for which thresholds have not been set.

When a threshold-set pixel exists at an adjacent pixel position in any one of the four directions of the threshold-unset pixel, that is, the threshold-unset pixel is separated from the one threshold-set pixel. When they are in contact with each other, when a dot is arranged in this threshold-unset pixel, the perimeter of the same pixel is 3. Such threshold-unset pixels are referred to as pixels with a perimeter of 3. When a dot is arranged in a pixel having a perimeter of 3, the perimeter increment is "+2".

In the case where a threshold value set pixel exists at a pixel position adjacent to each of two directions of four directions of the threshold value unset pixel, that is, the threshold value unset pixel is two threshold set pixels. When the borders are in contact with each other, the perimeter of the pixel in which the threshold value is not set is 2 when the dot is arranged in the pixel. Such threshold-unset pixels are referred to as pixels with a perimeter of 2. When dots are arranged in pixels with a perimeter of 2, the perimeter increment is "0".

In the case where the threshold-set pixels are present at pixel positions adjacent to each of the three directions of the upper, lower, left, and right four directions of the threshold-unset pixels, that is, the threshold-unset pixels are three threshold-set pixels. In the case where the borders are in contact with each other, the perimeter of the pixel in which the threshold value is not set is 1 when the dot is arranged in the pixel. Such a pixel for which a threshold value has not been set is called a pixel having a perimeter of 1. When a dot is arranged in a pixel with a perimeter of 1, the perimeter increment is "-2". A minus perimeter increment means that the perimeter is reduced, and a perimeter increment "-2" means that the perimeter is reduced by two.

If threshold-set pixels are present at pixel positions adjacent to the threshold-unset pixels in each of the four directions, that is, if the threshold-unset pixels are adjacent to the four threshold-set pixels, When a dot is arranged in this threshold-unset pixel, the perimeter of the same pixel is 0. Such a pixel for which a threshold value has not been set is called a pixel having a peripheral length of 0. When a dot is arranged on a pixel having a perimeter of 0, the perimeter increment is "-4".

For each pixel for which a threshold value has not been set, the perimeter when dots are arranged in each pixel can be calculated according to the rules shown in FIG. The sum of the perimeters of the pixels on which the dots are placed is the perimeter of the dot pattern for the entire matrix size.

Here, the relationship between the perimeter in each pixel for which the threshold value is not set, which is the numerical value shown in FIG. 8, and the “sum of the perimeters in the entire matrix size” will be described. First, the sum of the perimeters of the pixels for which thresholds have already been set in which dots have already been arranged is equal to the sum of the perimeters for the entire matrix size. Then, the change in the total sum of the peripheral lengths in the entire matrix size when the dots are arranged in the pixels for which the threshold value is not set and in which the dots have not been arranged yet is as follows.

For example, when dots are arranged in pixels with a perimeter of 0, the total perimeter of the entire matrix size is reduced by 4. Note that there is no pixel corresponding to a pixel having a perimeter of 0 in FIG. Further, "when dots are arranged in pixels with a perimeter of 0" is synonymous with "when a threshold is set for pixels for which a threshold of 0 is not perimeter is set."

Similarly, if dots are arranged in pixels with a perimeter of 1, the total perimeter is reduced by 2. When dots are arranged in pixels with a perimeter of 2, the total perimeter does not change. When dots are arranged in pixels with a perimeter of 3, the total perimeter increases by 2. When dots are arranged in pixels with a perimeter of 4, the total perimeter increases by 4.

Therefore, it is possible to search for a pixel in which the total sum of the perimeters of the entire matrix size is equal to or less than the perimeter limit value based on "the perimeter of each pixel when dots are arranged in each pixel". Pixels that are less than or equal to the perimeter length limit value are pixels that are candidates for threshold placement. Pixels that are candidates for threshold position are called threshold candidate pixels.

For example, when the perimeter length limit value of the threshold value 132 is 220 or 221, the pixel with the perimeter length 1 in FIG. 8 is the threshold candidate pixel of the threshold value 132. In FIG. 8, a pixel having a perimeter of 1 is a pixel highlighted by a combination of diagonal lines and gray filling.

When the perimeter length limit value of the threshold value 132 is 222 or 223, the pixels with the perimeter length 1 and the pixels with the perimeter length 2 in FIG. In FIG. 8, a pixel having a perimeter of 2 is a pixel highlighted by a combination of halftone dots and gray filling.

When the perimeter limit value of the threshold value 132 is 226 or more, all the pixels for which the threshold value is not set become the threshold value candidate pixels. In this case, new dots, which are 1×1 pixel dots, can be added.

As described above, in the present embodiment, only the pixels having the perimeter length limit value or less are set as the threshold placement candidates, and the pixel positions where the thresholds are placed are determined from the threshold candidate pixels based on the image quality evaluation value.

[Configuration of threshold matrix creation device]
FIG. 10 is a block diagram showing the functional configuration of the threshold matrix creation device 30 according to the embodiment of the present invention. The threshold matrix creation device 30 is a device that creates a threshold matrix that converts a continuous tone image into a dot pattern that is a binary image.

The threshold matrix creating device 30 automatically creates a threshold matrix according to a matrix creating condition including various parameters necessary for creating the threshold matrix. The threshold value matrix creating device 30 of the present embodiment includes a parameter setting unit 32, a threshold value setting processing unit 34, and a threshold value matrix storage unit 36. The threshold matrix creation device 30 also includes an input device 40 and a display device 42 that function as a user interface.

The threshold matrix creation device 30 can be realized by a combination of computer hardware and software. Software is synonymous with "program." The threshold matrix creation device 30 is configured to include one or a plurality of CPUs (Central Processing Units), and one or a plurality of programs stored in a program storage unit (not shown) provided in the threshold matrix creation device 30. It operates by being loaded into the CPU. Further, some or all of the processing functions of the threshold value matrix creating device 30 may be realized by an integrated circuit.

The threshold matrix generation device 30 may be connected to an output system (not shown). “Connected” means a relationship capable of transmitting information, and may be contact connection or non-contact connection. Connection is a term that includes, for example, contact connection between corresponding terminals, wire connection, wireless connection, optical communication connection, or an appropriate combination thereof. Further, the connection also includes a form of network connection which is connected via a telecommunication line (not shown). The network can be a local area network, or a wide area network, or a combination thereof.

The input device 40 is means for the user to perform an operation of inputting various information. The input device 40 receives an input operation of various information from a user. The information input by the user from the input device 40 includes parameters and operation commands that are conditions for creating the threshold matrix. The input device 40 can adopt various configurations such as a keyboard, a mouse, a touch panel, a trackball, or an operation button, and may be an appropriate combination of these.

As the display device 42, for example, display devices according to various display methods such as a liquid crystal display and an organic EL (Organic Electro-Luminescence) display can be used. Operations such as inputting various parameters and inputting instructions to the threshold value matrix creating device 30 can be performed using the input device 40 and the display device 42. The user can input various information using the input device 40 while confirming the content displayed on the screen of the display device 42, and can operate the threshold matrix creation device 30. Further, the user can confirm the threshold matrix created by the threshold matrix creating device 30 through the screen of the display device 42.

The threshold matrix creation device 30 may include at least one of a media interface unit (not shown) and a communication interface unit (not shown). The media interface unit reads information from an external storage medium (not shown) and writes information to the external storage medium. As the external storage medium, for example, a portable medium such as an optical disk or a memory card can be adopted. The threshold matrix creating device 30 may acquire information on the parameters necessary for creating the threshold matrix from an external storage medium (not shown). Further, the threshold matrix creation device 30 may acquire information on parameters necessary for creating the threshold matrix through a communication interface unit (not shown). The communication interface unit exchanges signals according to a wired or wireless communication standard.

The parameter setting unit 32 is a processing unit that sets various parameters related to the attributes of the threshold matrix to be created. The parameter setting unit 32 includes a matrix size setting unit 50, an evaluation function setting unit 52, a minimum dot size setting unit 54, an initial placement setting unit 56, a threshold section setting unit 58, a new dot number condition setting unit 60 for the minimum dot size, and A perimeter length limit value setting unit 62 is included.

The matrix size setting unit 50 sets the size of the threshold matrix to be created. The matrix size can be represented by the number of M×N pixels arranged in M rows and N columns. Each of M and N may specify any integer of 1 or more. M and N may have the same value or different values. The user can specify a desired matrix size from the input device 40.

A numerical range that can be set for each of M and N may be set in advance. In addition, selection candidates of settable matrix sizes may be prepared in advance. The user may be allowed to select a desired matrix size from a plurality of selection candidates prepared in advance. Alternatively, the size of the threshold matrix may be set to one specific type in advance. The M×N pixel area that is a two-dimensional array of M rows and N columns set by the matrix size setting unit 50 is called a “matrix area”.

The evaluation function setting unit 52 sets an evaluation function for calculating the image quality evaluation value of the dot pattern corresponding to the threshold arrangement. The dot pattern corresponding to the arrangement of a certain threshold value t is a dot arrangement in which pixels in which a threshold value of t or less is set in the matrix area are black pixels. t represents an integer. A black pixel is a pixel that forms a dot, and may be rephrased as a “dot-on pixel” or a “pixel with a dot”. Pixels in which dots are not formed are referred to as “white pixels” with respect to black pixels. The white pixels may be rephrased as “dot-off pixels” or “dot-free pixels”.

The image quality evaluation value is an index for quantitatively evaluating the rough texture in the dot pattern. As an example of the evaluation function, for example, a function of convolving a human visual function with a binary image showing a dot pattern and calculating a standard deviation can be adopted. As another example of the evaluation function, it is possible to adopt a function of converting the frequency of the dot pattern and then applying a visual function in the frequency space to calculate an integrated value. The smaller the image quality evaluation value calculated according to these evaluation functions, the smaller the roughness. As the evaluation function, an evaluation function according to a known method can be adopted in addition to the functions exemplified here. Depending on the definition of the evaluation function, the larger the image quality evaluation value, the smaller the roughness may be evaluated.

The evaluation function setting unit 52 can set different evaluation functions depending on the size of the threshold. For example, in the extreme highlight gradation area, since there are few dots and a good distributed arrangement cannot be obtained even by using a visual function, another evaluation function that does not use a visual function is set for the extreme highlight section. ..

The minimum dot size setting unit 54 sets the minimum dot size, which is the number of pixels of isolated dots of the minimum size that can be stably formed by an output system (not shown). The information on the minimum dot size may be input from the input device 40, or may be acquired from the output system via a communication interface unit (not shown).

The initial placement setting unit 56 sets an initial placement that is a dot placement corresponding to a specific halftone percentage that is an initial condition for sequentially determining the placement of each threshold value. The initial arrangement setting unit 56 of the present example sets the initial arrangement on the highlight side, which is a dot arrangement corresponding to a certain halftone dot belonging to the gradation region of highlight, and a certain halftone dot belonging to the gradation region of shadow. And an initial placement on the shadow side, which is a dot placement corresponding to.

The initial arrangement on the highlight side is preferably a dot arrangement in which one or a plurality of dots having the minimum dot size are arranged. The initial arrangement on the shadow side can be defined as a dot arrangement in which the same number of white pixels as the initial arrangement on the highlight side is arranged so as not to overlap the black pixels in the initial arrangement on the highlight side. ..

The threshold section setting unit 58 sets a section that divides the entire threshold range from the minimum threshold to the maximum threshold into a plurality of threshold sections. The threshold interval is a threshold range of continuous thresholds. The threshold section setting unit 58 determines the number of thresholds included in each threshold section. The “threshold number” means the number of thresholds. The threshold section setting unit 58 sets the threshold value of the boundary of each threshold section, as described in FIG. In the example of FIG. 4, the threshold 16 that is the threshold of the boundary of the threshold section is an example of the “first threshold”. The threshold value 385 is an example of a “second threshold value”. The threshold value 65 is an example of a “third threshold value”. The threshold value 336 is an example of a “fourth threshold value”.

The threshold section setting unit 58 performs section setting for further dividing each section of the highlight transition section and the shadow transition section into a plurality of unit sections. The threshold number of this unit section is a multiple of the minimum dot size. The number of thresholds in each of the highlight transition section and the shadow transition section is a multiple of the minimum dot size.

The new dot number condition setting unit 60 for the minimum dot size sets how to reduce the new dot number for the minimum dot size per unit section as illustrated in FIG.

The perimeter length limit value setting unit 62 sets the perimeter length limit value of the dot pattern for each threshold value for at least a part of the threshold values in the entire threshold range from the minimum threshold value to the maximum threshold value. The user can specify an arbitrary perimeter length limit value from the input device 40. The perimeter length limit value setting unit 62 can set the perimeter length limit value of the dot pattern corresponding to each threshold value arrangement for each threshold value in the entire threshold value range.

However, the threshold value matrix creating apparatus 30 of this example performs a process of determining the threshold value placement without applying the constraint by the perimeter length limit value to the highlight section and the shadow section. Further, with respect to some thresholds in the highlight transition section and some thresholds in the shadow transition section, the processing of determining the arrangement of the thresholds is performed without applying the constraint by the perimeter limit value. That is, it is not necessary to set the perimeter length limit value for the highlight section and the shadow section, and for some thresholds of the highlight transition section and some thresholds of the shadow transition section. The perimeter limit value may not be set for a highlight section and a shadow section that are not substantially used, and for some thresholds of the highlight transition section and some thresholds of the shadow transition section.

The threshold matrix storage unit 36 stores the threshold matrix of the matrix size set by the matrix size setting unit 50. The threshold matrix storage unit 36 can store the data of the incomplete threshold matrix generated in the process of the threshold setting processing by the threshold setting processing unit 34. The incomplete threshold matrix refers to a matrix in the process of being created that includes pixels for which thresholds have not been set. The threshold value matrix storage unit 36 also stores a threshold value matrix created through the threshold value setting processing of the threshold value setting processing unit 34.

The threshold value setting processing unit 34 includes a threshold value setting order control unit 70, a threshold value candidate pixel determination unit 72, an image quality evaluation value calculation unit 74, and a threshold value arrangement determination unit 76. The threshold setting order control unit 70 determines the order of thresholds for performing the process of determining the arrangement of thresholds in the entire threshold range. The arrangement of the thresholds is sequentially determined according to the order set by the threshold setting order control unit 70. In this example, an example in which the threshold value on the highlight side and the threshold value on the shadow side are set alternately will be described.

The threshold value candidate pixel determination unit 72 determines a threshold value candidate pixel that is a candidate for the pixel position where the threshold value to be set is arranged among the threshold value unset pixels in the matrix area. When applying the perimeter limitation, the threshold candidate pixel determination unit 72 calculates the perimeter of each pixel as described in FIG. 8 based on the already determined dot arrangement, and the perimeter limitation set for each pixel. A threshold candidate pixel is determined based on the value. The threshold value candidate pixel determination unit 72 sets, as the threshold value candidate pixel, a pixel in which the sum of the perimeters is equal to or less than the perimeter length limit value among the threshold-unset pixels.

The image quality evaluation value calculation unit 74 calculates the image quality evaluation value of the dot pattern using the evaluation function set by the evaluation function setting unit 52. For example, when the arrangement of the threshold value t is to be decided in ascending order, the dots to be added by the threshold value t for newly deciding the arrangement position are added to the dot arrangement up to the threshold value t-1 where the arrangement is already fixed. The image quality evaluation value is calculated for the dot pattern that is the dot arrangement when placed in the temporary threshold value arrangement pixel. The dot pattern corresponding to the dot percentage of the threshold value t is in a relationship of including all the dot patterns corresponding to the dot percentage of the threshold value k−1 that is one gradation smaller. The constraint condition that the arrangement of each threshold value is determined while maintaining the relationship that the dot pattern corresponding to the dot percentage corresponding to the threshold value t includes the dot pattern corresponding to the dot percentage corresponding to the threshold value k−1 is referred to as “stacking constraint”. The threshold value matrix creating device 30 determines the arrangement position of the threshold value using the image quality evaluation value, and creates the threshold value matrix that satisfies the stacking constraint.

That is, both the highlight side and the shadow side, when determining the threshold candidate pixels that are candidates for the threshold position, the threshold candidate pixels are determined excluding the threshold-set pixels that have already been threshold-set. To be done.

The image quality evaluation value calculation unit 74 calculates the image quality evaluation value for each dot pattern by changing the provisional threshold value arrangement pixels, and stores the calculated image quality evaluation value in association with each dot pattern.

The threshold value arrangement determination unit 76 determines the arrangement of threshold values based on the image quality evaluation value calculated by the image quality evaluation value calculation unit 74. The threshold value arrangement determination unit 76 determines a “temporary threshold value arrangement pixel” that is a dot pattern having the best image quality evaluation value as a threshold value arrangement pixel, and sets a threshold value in the determined threshold value arrangement pixel.

FIG. 11 is a flowchart showing a first example of parameter setting processing in the threshold matrix creating device 30. The flowchart of FIG. 11 is an example of the processing content of step S1 described in FIG.

In step S12 of FIG. 11, the matrix size setting unit 50 sets the size of the threshold matrix to be created. In the case of the example shown in FIG. 4, a matrix size of 20×20 pixels is set. The matrix size setting step of step S12 is an example of the step of setting the size of the threshold matrix.

In step S14, the evaluation function setting unit 52 sets the evaluation function used to calculate the image quality evaluation value. The evaluation function setting unit 52 uses the first evaluation function used when evaluating the image quality of the dot pattern of the extreme highlight and the polar shadow, and the image quality of the dot pattern of the gradation range excluding the extreme highlight and the polar shadow. And a second evaluation function used for. The evaluation function setting step of step S14 is an example of the step of setting the evaluation function.

In step S16, the minimum dot size setting unit 54 sets the minimum dot size. In the example of FIG. 4, 2×2 pixels are set as the minimum dot size. The minimum dot size setting step of step S16 is an example of the step of setting the minimum dot size.

In step S18, the initial placement setting unit 56 sets the first initial placement, which is the initial placement on the highlight side, and the second initial placement, which is the initial placement on the shadow side. In the example of FIG. 4, the dot arrangement of the threshold value 8 is set as the first initial arrangement, and the dot arrangement of the threshold value 393 is set as the second initial arrangement. The initial placement setting step of step S18 is an example of the step of setting the initial placement.

In step S20, the threshold section setting unit 58 sets the threshold section as illustrated in FIG. In addition, the threshold section setting unit 58 sets the unit section described in FIG. The threshold section setting unit 58 determines the threshold number of unit sections based on the minimum dot size. The threshold section setting step of step S20 is an example of the step of setting the threshold section.

In step S22, the minimum dot size new dot number condition setting unit 60 sets the reduction method of the minimum dot size new dot number in the high latency transition section, as illustrated in FIG. The new dot number condition setting step of the minimum dot size of step S22 is an example of the step of setting the condition defining the number of new dots of the minimum dot size.

In step S24, the perimeter length limit value setting unit 62 sets perimeter length limit values for at least some of the threshold values in the entire threshold range. In the case of this example, the perimeter length limit value is set for each of the thresholds of the highlight transition section, the threshold of the halftone section, and the thresholds of the shadow transition section. The perimeter length limit value setting step of step S24 is an example of the step of setting the threshold perimeter limit value of the dot pattern for each threshold value for at least some threshold values. The order of performing steps S12 to S24 may be appropriately changed.

FIG. 12 is a flowchart showing an example of threshold setting processing in the threshold matrix creating device 30. The flowchart of FIG. 12 is an example of the processing content of step S2 described in FIG.

FIG. 12 shows the processing procedure of the threshold value on the highlight side in the entire threshold value range. The threshold value on the highlight side is a threshold value with a halftone dot percentage of 50% or less, and refers to a threshold value in the threshold range of threshold values 1 to 200 in the example of FIG.

As shown in FIG. 12, the highlight side threshold value setting process includes a step of setting a threshold value of a highlight section (step S32), a step of setting a threshold value of a highlight transition section (step S34), and a highlight intermediate step. Setting a threshold value of a key interval (step S36).

In step S32, the threshold setting processing unit 34 sets the thresholds 8 to 1 in descending order from the dot arrangement of the threshold 8 which is the initial arrangement. Further, the threshold value setting processing unit 34 sets each threshold value of the threshold values 9 to 16 in ascending order from the dot arrangement of the threshold value 8. Regarding the highlight section, as described above, the arrangement position of each threshold value is determined so that a new dot of the minimum dot size is arranged without the limitation of the peripheral length. A publicly known void-and-cluster method can be adopted as a method for determining a pixel in which each threshold value of the extreme highlight section is to be arranged in the highlight section. Literatures describing the void and cluster method include, for example: Robert A. Ulichney, “The void-and-cluster method for dither array generation,” IS&T/SPIE Symposium on Electronic Imaging and Science, San Jose, CA, vol. . 1913, 332-343, (1993).

The threshold setting step of the highlight section in step S32 is an example of the step of deciding the arrangement position of the threshold so as to dispose the restriction of the perimeter length restriction based on the perimeter length limit value and arrange the dots of the minimum dot size.

In step S34, the threshold value setting processing unit 34 sets each threshold value of the highlight transition section in ascending order from the smallest value. At that time, the threshold setting processing unit 34 sets each threshold under the constraint of the number of new dot thresholds of the minimum dot size in each unit section described in FIG. That is, with respect to the threshold value of the number of new dot thresholds defined in each unit section, the perimeter length non-limitation process is performed to determine the placement position of the threshold value so that the dots of the minimum dot size are placed without applying the perimeter length limitation constraint. .. On the other hand, for the remaining thresholds other than the threshold value of the new dot threshold number defined in each unit section, the perimeter length that determines the placement position of the threshold value based on the image quality evaluation value among the threshold value candidate pixels that are less than or equal to the perimeter length limit value. Perform restriction processing.

The setting of each threshold value in the highlight transition section is performed by combining the perimeter length non-limiting process and the perimeter length limiting process in accordance with the constraint condition of the number of new dots of the minimum dot size illustrated in FIG.

In step S36, the threshold value setting processing unit 34 sets each threshold value in the highlight halftone section in ascending order from the smallest value. At that time, the threshold value setting processing unit 34 sets, for each threshold value, a pixel having a perimeter length limit value or less as a threshold value candidate pixel, and sets a threshold value to a pixel having the best image quality evaluation value among the threshold value candidate pixels.

FIG. 13 is a flowchart showing the flow of the ascending threshold setting process. The flowchart of FIG. 13 is executed by the threshold setting processing unit 34.

First, in step S42, the threshold setting order control unit 70 specifies the threshold Th(k) to be set. k is a variable that represents a threshold index. For example, k in the case of targeting the threshold values 9 to 200 in FIG. 4 can take any integer in the range of 9≦k≦200. For example, the initial value may be k=9.

In step S43, the threshold setting order control unit 70 determines whether to apply the perimeter limit when setting the threshold Th(k). The threshold value Th(k) belongs to the highlight section, or the threshold value Th(k) belongs to the highlight transition section and corresponds to the threshold value assigned to the new dot threshold value number of the minimum dot size described in FIG. In this case, the threshold setting order control unit 70 determines that the perimeter length limitation is not applied and moves to step S46. In step S46, the threshold value setting processing unit 34 arranges the threshold value Th(k) so as to arrange the dots of the minimum dot size without the perimeter length limitation. The position at which the dot of the minimum dot size is arranged is determined based on the image quality evaluation value calculated by the image quality evaluation value calculation unit 74.

On the other hand, in step S43, when the threshold Th(k) belongs to the highlight transition section and does not correspond to the threshold assigned to the new dot threshold number of the minimum dot size described in FIG. 5, threshold setting order control The unit 70 determines that the perimeter limit is applied, and moves to step S48.

In step S48, the threshold candidate pixel determination unit 72 sets the pixels having the perimeter length limit value or less as the threshold candidate pixels. The threshold candidate pixel determination step of step S48 is an example of the step of determining threshold candidate pixels.

In step S50, the image quality evaluation value calculation unit 74 calculates the image quality evaluation value of the dot pattern in which one pixel among the threshold value candidate pixels is temporarily added as a black pixel to the dot arrangement of the threshold value Th(k−1). The image quality evaluation value calculation unit 74 stores the position of the threshold value candidate pixel, which is temporarily blackened, and the calculated image quality evaluation value in association with each other.

In step S52, the threshold setting processing unit 34 determines whether or not the image quality evaluation values of all threshold candidate pixels have been calculated. If the determination in step S52 is No, the process proceeds to step S54.

In step S54, the image quality evaluation value calculation unit 74 changes the threshold candidate pixel to be turned into a black pixel, and returns to step S50.

If the determination in step S52 is Yes, the process proceeds to step S56. In step S56, the threshold placement determining unit 76 places the threshold Th(k) on the threshold candidate pixel having the best image quality evaluation value.

After step S56 or step S46, the process proceeds to step S58. In step S58, the threshold setting order control unit 70 determines whether to set the next threshold. If the next threshold to be set remains, the determination in step S58 becomes Yes and the process proceeds to step S60.

In step S60, the threshold setting order control unit 70 increments the value of k, which is an index, and sets "k+1" as a new value of k, and the process returns to step S42.

On the other hand, in the determination of step S58, if there is no next threshold to be set, the flowchart of FIG. 13 ends.

[Regarding threshold setting order]
The description so far has described only the threshold value setting method from highlight to halftone. Actually, the threshold value setting on the highlight side from the highlight to the halftone of highlight and the threshold value setting on the shadow side from the shadow to the halftone of shadow are alternately performed.

In the example of FIG. 4, first, in a highlight section with a threshold of 16 or less and a shadow section with a threshold of 385 or more, new dots of 2×2 pixels, which is the minimum dot size, are alternately added (when set in ascending order) or deleted. (When setting in descending order) and set the threshold value. Next, in the highlight transition section of the threshold values 17 to 64 and the shadow transition section of the threshold values 384 to 337, new dots of 2×2 pixels are alternately added to set the threshold values, and the threshold values are alternately set under the perimeter limit. Set. After that, in the highlight halftone section of the threshold value 65 to the threshold value 200 and the shadow halftone section of the threshold value 336 to the threshold value 201, the threshold value is alternately set under the perimeter limitation, and finally the threshold value 201 is set to create the threshold value matrix. Complete.

That is, the threshold value 8 which is the initial arrangement on the highlight side and the threshold value 393 which is the initial arrangement on the shadow side are set to start respectively, and the threshold value 8→the threshold value 7→the threshold value 6→the threshold value 5→the threshold value 393→the threshold value 394→the threshold value 395. → threshold 396 → threshold 4 → threshold 3 → threshold 2 → threshold 1 → threshold 397 → threshold 398 → threshold 399 → threshold 400 → threshold 9 → threshold 10 → threshold 11 → threshold 12 → threshold 392 → threshold 391 → threshold 390 → threshold 389→...threshold value 17→threshold value 18→threshold value 19→threshold value 20→threshold value 384→threshold value 383→threshold value 382→threshold value 381→...threshold value 29→threshold value 372→threshold value 30→threshold value 371→...threshold value 65→ Arrangement positions of all thresholds are determined in the order of threshold 336→threshold 66→threshold 335→... threshold 199→threshold 200→threshold 201.

From the threshold value 9 above the threshold value 8 which is the initial arrangement on the highlight side to the threshold value 200, each threshold value is set in the direction of increasing the threshold value and black pixels are added. From the lower threshold 392 to the threshold 201, each threshold is set in the direction of decreasing the threshold and white pixels are added. As to the thresholds 1 to 8, as described above, each threshold is set in the direction of decreasing the threshold and black pixels are deleted, and for the thresholds 400 to 393, each of the thresholds is increased in the direction of increasing the threshold. Set and delete white pixels.

FIG. 14 is a flow chart when the threshold values are alternately set from the highlight side and the shadow side.

In step S71, the threshold setting processing unit 34 sets the threshold for the highlight section. In step S71, one threshold value to be set that belongs to the highlight section is selected, and the arrangement position of the threshold value is determined.

Next, in step S72, the threshold setting processing unit 34 sets a threshold for the shadow section. In step S72, one threshold value to be set that belongs to the shadow section is selected, and the arrangement position of the threshold value is determined.

Next, in step S73, the threshold setting order control unit 70 determines whether or not the setting of the thresholds belonging to each of the highlight section and the shadow section has been completed. When there is an unset threshold in at least one of the highlight section and the shadow section, the process returns to step S71, the threshold to be set is changed, and the processes of step S71 and step S72 are repeated.

When the setting of the threshold values belonging to each of the highlight section and the shadow section is completed, the determination in step S73 becomes Yes, and the process proceeds to step S74.

In step S74, the threshold setting processing unit 34 sets the threshold for the highlight transition section. In step S74, one threshold value to be set belonging to the highlight transition section is selected, and the arrangement position of the threshold value is determined.

In step S75, the threshold setting processing unit 34 sets a threshold for the shadow transition section. In step S75, one threshold to be set belonging to the shadow section is selected, and the arrangement position of the threshold is determined.

Next, in step S76, the threshold setting order control unit 70 determines whether or not the setting of the thresholds belonging to each of the highlight transition section and the shadow transition section has been completed. If an unset threshold value exists in at least one of the highlight transition section and the shadow transition section, the process returns to step S74, the threshold to be set is changed, and the processes of steps S74 and S75 are repeated.

When the setting of the thresholds belonging to each of the highlight transition section and the shadow transition section is completed, the determination in step S76 is Yes, and the process proceeds to step S77.

In step S77, the threshold value setting processing unit 34 sets a threshold value for the highlight halftone section. In step S77, one threshold value to be set that belongs to the highlight halftone section is selected, and the arrangement position of the threshold value is determined.

In step S78, the threshold value setting processing unit 34 sets a threshold value for the shadow halftone section. In step S78, one setting target threshold value belonging to the shadow halftone section is selected, and the arrangement position of the threshold value is determined.

Next, in step S79, the threshold setting order control unit 70 determines whether or not the setting of the thresholds belonging to each of the highlight halftone section and the shadow halftone section has been completed. If an unset threshold exists in at least one of the highlight halftone section and the shadow halftone section, the process returns to step S77, the threshold to be set is changed, and the processes of steps S77 and S78 are repeated.

When the setting of the threshold values belonging to each of the highlight halftone section and the shadow halftone section is completed, the determination in step S79 becomes Yes, and the flowchart in FIG. 14 ends.

[Advantages of form in which threshold value on highlight side and threshold value on shadow side are set alternately]
According to the method of creating the threshold value matrix in which the threshold values of highlight and shadow are alternately set, the image quality of the highlight side net and the shadow side net in the threshold matrix can be designed to be equal. Therefore, a negative printing plate and a positive printing plate can be made with the same image quality by using the threshold matrix obtained by this method.

On the other hand, if you want to prioritize the image quality on the highlight side, such as when the use of the threshold matrix is limited to negative or positive, first set the threshold values from highlight to halftone sequentially. After setting, the threshold values from the halftone to the shadow may be sequentially set. That is, first, a threshold value of 0% to 50% may be set, and then a threshold value of 50% to 100% may be set. In this case, the shadow side does not perform the process corresponding to the addition (or deletion) of the new dot of the minimum dot size on the highlight side. That is, no shadow section or shadow transition section corresponding to the highlight section or highlight transition section on the highlight side is provided.

On the contrary, if it is desired to prioritize the image quality on the shadow side, first, the threshold values from the shadow to the halftone may be sequentially set, and then the threshold values from the halftone to the highlight may be sequentially set. In this case, the shadow section and the shadow transition section are provided, but the highlight section and the highlight transition section need not be provided.

[Example of perimeter of dot pattern obtained by threshold matrix created according to this embodiment]
FIG. 15 is a graph showing an example of the dot pattern perimeter of the threshold matrix created according to this embodiment. According to the present embodiment, the perimeter of a halftone can be arbitrarily designed as shown in FIG. 15 depending on how to set the perimeter limit value. If it is difficult to set an appropriate perimeter limit value from the beginning, use the model data of the perimeter limit value as a reference to provisionally set the perimeter limit value and create a threshold matrix. However, the setting of the perimeter length limit value may be changed by appropriately editing the model data according to the result of the test printing.

For example, a graphical user interface for displaying a graph as shown in FIG. 15 on the screen of the display device 42 and accepting an operation for correcting the curve shape of the graph from the input device 40 may be provided.

[Example of how to determine the perimeter length limit value]
A method of creating a threshold matrix according to the present disclosure sets a perimeter length limit value for at least a part of threshold values as one of parameters that is a threshold matrix creation condition, and sets a perimeter length limit value indicated by the perimeter length limit value. Create a threshold matrix with.

An arbitrary value can be set as the perimeter length limit value. As an example of how to determine the perimeter length limit value that is preferably printable, the perimeter length that is a reference determined from the AM line number can be used as a guide. Specifically, an example is given in which the number of AM lines is designated, the perimeter length corresponding to the designated number of AM lines is output as a perimeter length limit value, and a matrix is created under the perimeter limit. The number of AM lines is the number of screen lines of the AM net and indicates the number of halftone dots per inch.

[Explanation of method of calculating perimeter from AM line number]
A method of calculating the perimeter length per square inch will be described, where R is the given number of AM lines. The unit of R is "lpi (line per inch)". The line per inch is a unit indicating the number of halftone dots per inch.

In FIG. 16, the black halftone dot 80, which is a block of black pixels, is referred to as a “halftone dot”, and the case where the angle of the halftone dot is 0 degrees is shown for the sake of simplicity. The halftone dot angle is synonymous with the screen angle and refers to the angle with the vertical axis or the horizontal axis of the screen. As shown in FIG. 16, since the number of halftone dots is R per 1 inch in each of the vertical and horizontal directions, there are R ² halftone dots per square inch.

The length of one side of one halftone dot is a millimeter [mm], and the halftone dot shape is regarded as a square. First, an equation for obtaining a from R and D is derived with the halftone dot percentage being D [%].

Since the area of one halftone dot is a ² square millimeter [mm ² ], the halftone dot area per square inch is R ² *a ² square millimeter [mm ² ]. In addition, "*" represents a calculation symbol of multiplication. Since the area of 1 square inch is (25.42) ² [mm ² ], the halftone dot percentage D [%] is 100*R ² *a ² /(25.42) ² . That is, a is obtained from R and D by the following [Formula 1].

a=(25.4/R)*sqrt(D/100)... [Equation 1]
Here, sqrt(x) represents the square root of x.

Since the perimeter of one halftone dot is 4*a [mm], if the perimeter of 1 square inch is L [mm], then L=4*R ² *a, and from [Equation 1], L is obtained from R and D by the following [Formula 2].

L=4*25.4*R* sqrt (D/100)... [Equation 2]
The unit of L in [Equation 2] is millimeters [mm], but the perimeter length limit value when creating the threshold matrix needs to be given as the perimeter with the length of one pixel of the threshold matrix as a unit. , Convert to that length.

When the resolution of the threshold matrix, that is, the output resolution is r[dpi], the length of one pixel is 25.4/r millimeters [mm]. Therefore, the peripheral length in units of the length of one pixel is Lp. Lp=L*r/25.4, that is, [Equation 3] below.

Lp=4*R*r* sqrt (D/100)... [Equation 3]
By using [Equation 3], the resolution r of the AM screen having the number of lines R and the perimeter Lp per square inch in the halftone dot D can be obtained.

Although the case where the angle of the halftone dot is 0 degree is shown in FIG. 16, the peripheral length Lp can be obtained by [Equation 3] regardless of the angle of the halftone dot. Although Lp in [Equation 3] is the perimeter of each side of the halftone dot as a straight line, the halftone dot in actual CTP platemaking is formed as a cluster of pixels arranged in a grid pattern as shown in FIG. ..

Therefore, it is necessary to obtain the perimeter of the grid-shaped halftone dots as the perimeter of the halftone dots assuming an actual output system. The grid-like halftone dots are halftone dots composed of clusters, which are clusters of grid-like pixels.

The perimeter of the grid of dots varies depending on the angle of the dots. For example, in the halftone dots shown in FIG. 17 (halftone dot angle 0°) and the halftone dots shown in FIG. 18 (halftone dot angle 15°), even if the length a of one side considered to be a straight line is the same, The length of one side at the halftone dots is different. Therefore, the perimeter of the halftone dot shown in FIG. 17 and the perimeter of the halftone dot shown in FIG. 18 are different.

A method for obtaining the length of one side of a grid-shaped halftone dot from the length a of one side regarded as a straight line of halftone dots will be described with reference to FIG. In FIG. 18, the halftone dots are inclined by 15 degrees, but it is generalized that they are inclined by the angle θ. However, the angle θ of the halftone dots is assumed to be θ≦45°. As shown in FIG. 18, the center of the halftone dot is “O”, the angle between the diagonal line of the substantially square halftone dot and the vertical axis of the screen (the halftone dot angle) is θ, and one side of the straight line is the oblique side. Let each vertex of the triangle be A, B, C and ∠ABC be α. The notation of ∠ABC means a corner ABC and represents a corner between the line segment AB and the line segment BC.

From FIG. 18, it can be seen that the length of one side of the grid-shaped halftone dot is given by a*sinα+a*cosα. Since ∠CBO=45°, ∠ABO=α+45°, and ∠ABO=90°−θ, so α=45°−θ. Therefore, the length of one side of the grid-like halftone dot. Can be calculated from a and θ by the following [Equation 4].

a*{sin(45°−θ)＋cos(45°−θ)} ・・・[Equation 4]
When the angle θ exceeds 45°, the length of one side of the grid-like halftone dot can be calculated from a and θ in the same manner.

Summarizing the case where the angle θ is 45° or less and the case where it exceeds 45°, the length of one side of the grid-shaped halftone dot is obtained by the following [Equation 5].

a*{sin(abs(45°−θ))+cos(abs(45°−θ))} [Equation 5]
Here, abs(x) represents the absolute value of x.

The perimeter of one halftone dot is 4*a*{sin(abs(45°−θ))+cos(abs(45°−θ))}. The perimeter L per square inch is L=4*R ² *a*{sin(abs(45°−θ))+cos(abs(45°−θ))}.

Therefore, from [Equation 1], L is obtained from R, D, and θ by the following [Equation 6].

L=4*25.4*R* sqrt(D/100)*{sin(abs(45°−θ))+cos(abs(45°−θ))} [Equation 6]
Further, the perimeter Lp in the unit of the length of one pixel of the resolution r[dpi] is given by the following formula.

Lp=4*R*r* sqrt(D/100)*{sin(abs(45°−θ))+cos(abs(45°−θ))} [Equation 7]
When the halftone dot shape is a square, the perimeter can be obtained by the above [Equation 2], [Equation 3], [Equation 6], or [Equation 7]. Although a square is generally used as a halftone dot shape, a circle with less noticeable tone jump may be used.

FIG. 19 shows an example of circular halftone dots. Hereinafter, a method for obtaining the perimeter of a halftone dot having a circular shape will be described.

First, assuming that the radius is a [mm], the area of one halftone dot is π*a ² [mm ² ], so the halftone dot area per square inch is π*R ² *a ² [mm ² ]. Become. π is the pi.

The dot percentage D is 100*π*R ² *a ² /(25.4) ² . Therefore, a is calculated from R and D using the following [Equation 8].

a=(25.4/R)* sqrt(D/(100*π))... [Equation 8]
Since the perimeter of one halftone dot is 2*π*a [mm], the perimeter L per square inch is L=2*π*R ² *a [mm]. Therefore, from [Equation 8], L is obtained from R and D using the following [Equation 9].

L=2*25.4*R* sqrt (π*D/100)... [Equation 9]
Further, the perimeter Lp in the unit of the length of one pixel of the resolution r [dpi] is Lp=L*r/25.4, that is, the following [Formula 10].

Lp=2*R*r* sqrt (π*D/100)... [Equation 10]
[Equation 9] and [Equation 10] are the perimeters that regard the halftone dot shape as a smooth circle, but the halftone dots in actual CTP platemaking etc. are formed as clusters of pixels arranged in a grid pattern as shown in FIG. To be done. Therefore, the perimeter of this grid-shaped halftone dot is obtained.

As shown in FIG. 20, the perimeter of one circular halftone dot having a radius of a [mm] is given by 8*a. Therefore, the perimeter L per square inch is L=8*R ² *a, and from [Equation 8], L is obtained from R and D using the following [Equation 11].

L=8*25.4*R* sqrt(D/(100*π))... [Equation 11]
Further, the perimeter Lp in the unit of the length of one pixel of the resolution r[dpi] is given by the following formula.

Lp=8*R*r* sqrt (D/(100*π))... [Equation 12]
When the halftone dot shape is circular, the perimeter does not depend on the halftone dot angle.

Comparing the two equations [Equation 2] and [Equation 9] or the two equations [Equation 3] and [Equation 10], the circular halftone shape has the same halftone percentage as the halftone dot shape is square. Perimeter is short. That is, it is understood that the circular halftone dots are less affected by the dot gain than the square halftone dots.

Even in the case of grid-shaped halftone dots, comparing two expressions of [Equation 6] and [Equation 11], or comparing two equations of [Equation 7] and [Equation 12], the circular shape is generally more square. However, the perimeter at the same net percentage is short. That is, it can be seen that the circular halftone dots are less affected by the dot gain than the square halftone dots. However, depending on the angle of the halftone dots, the circumference of the square may be shorter than that of the circle at the same halftone percentage.

In the above description, the equation [Equation 2], [Equation 3], [Equation 6], and [Equation 7] for determining the perimeter when the halftone dot shape is a square with the halftone dot of the black pixel as the “halftone dot” And Equations [Equation 9], [Equation 10], [Equation 11], and [Equation 12] for determining the perimeter when the halftone dot shape is circular.

However, the halftone dot percentage in which the halftone dot shape of the black pixel is square or circular is in the range of 0 to 50%, and the halftone dot shape of the black pixel is not square or circular in the halftone dot percentage range of 50 to 100%. ..

With a dot percentage of 50 to 100%, the dot shape of the white pixel becomes a square or a circle, so the dot of the white pixel is defined as a “dot”, the dot percentage of the white pixel is defined as D, and the dot shape is square. In the case of, the perimeter may be obtained from the expressions [Equation 2], [Equation 3], [Equation 6], and [Equation 7]. Alternatively, when the shape of the halftone dot of the white pixel is circular, the perimeter is calculated from the formulas [Equation 9], [Equation 10], [Equation 11], and [Equation 12] with the halftone dot percentage of the white pixel being D. Good. Here dot percentage of _{D B} of the dots of black pixels, a relationship of _D W = _{100-D B} when the dot percentage of the halftone dot of the white pixel to _{D W.}

[Example of changing halftone dot shape by halftone dot percentage]
When calculating the perimeter length limit value from the AM line number, the perimeter length limit value may be calculated by changing the halftone dot shape depending on the halftone dot percentage. For example, at a dot percentage of 0 to 40%, the perimeter of a circular dot is set as a perimeter limit value, and at a dot percentage of 40 to 50%, the perimeter is gradually changed from a circular to a square dot perimeter. If the dot percentage is 50% or more, the perimeter of square dots may be used as the perimeter limit value.

Also, for example, the perimeter of circular dots is set as the perimeter length limit value for halftone dot percentage 0 to 30% and halftone dot percentage 70 to 100%, and perimeter length limit value for halftone dot percentage 30 to 40% and halftone dot percentage 60 to 70%. For example, the perimeter may be gradually changed from the circular shape to the perimeter of the square, and the perimeter of the square halftone dot may be set as the perimeter limit value at a dot percentage of 40 to 60%.

[Specific example of perimeter calculated from the number of AM lines]
FIG. 21 is a graph showing an example of the perimeter per square inch obtained from [Equation 7] and [Equation 12] when the output resolution is 2400 dpi. The graph shown by the thin solid line in FIG. 21 shows the perimeter when the number of AM lines is 175, the halftone dot shape is square, and the halftone dot angle is 0°. The graph shown by the alternate long and short dash line in FIG. 21 shows the perimeter when the number of AM lines is 175 and the halftone dot shape is circular. The graph shown by the thick solid line in FIG. 21 shows the perimeter when the number of AM lines is 300 and the angle of the halftone dots having a square dot shape is 0°. In the graph shown by the broken line in FIG. 21, the number of AM lines is 175, the halftone dot shape is circular when the dot percentage is 0 to 30% and the dot percentage is 70 to 100%, and the dot dot shape is when the dot percentage is 40 to 60%. Indicates the perimeter of the square, and gradually changes from the circular perimeter to the square perimeter at a mesh percentage of 30 to 40% and a mesh percentage of 60 to 70%.

Perimeter L and perimeter Lp calculated from [Equation 2], [Equation 3], [Equation 6], and [Equation 7], and [Equation 9], [Equation 10], [Equation 11], and Since the perimeter L and perimeter Lp calculated from [Equation 12] or the perimeter illustrated in FIG. 21 are perimeters per square inch, these values are used when creating the threshold matrix. It is necessary to convert to the perimeter length limit value of, that is, the limit value of the total sum of the perimeter lengths in the matrix size.

Since the number of pixels per square inch at the resolution r [dpi] is r ² , if the matrix size is N ² , the perimeter length limit value may be N ² /r ² times the perimeter length per square inch. .. Note that N of N ² indicating the matrix size is the number of pixels on one side of the threshold matrix.

[Flow chart of configuration for setting perimeter limit value from number of AM lines]
FIG. 22 is a flowchart showing a second example of the parameter setting process in the threshold value matrix creating device 30. In the flowchart of FIG. 22, steps that are the same as or similar to the steps described in the flowchart of FIG. 11 are assigned the same step numbers, and description thereof is omitted. The flowchart of FIG. 22 may be adopted instead of the flowchart of FIG. 11.

The flowchart of FIG. 22 includes a step of setting the AM line number (step S23). The AM line number setting step of step S23 may be performed before the perimeter length limit value setting step of step S24. The user can input information on the desired number of lines and the angle of the halftone dot from the input device 40. As a method of inputting information, an arbitrary number of lines may be numerically input, or a desired number of lines may be selected from selection candidates prepared in advance. The parameter setting unit 32 sets the number of lines and the angle of the halftone dot according to the information input from the input device 40. The parameter setting unit 32 of the threshold matrix generation device 30 includes an AM line number setting unit (not shown).

In the perimeter length limit value setting step of step S24, the perimeter length limit value setting unit 62 follows the calculation method described with reference to FIGS. It includes an arithmetic processing unit that calculates the perimeter length per inch and calculates the sum of the perimeter lengths in the matrix size.

The perimeter length limit value setting unit 62 sets the perimeter length limit value based on the sum of the perimeter lengths calculated from the number of AM lines. The perimeter length limit value setting unit 62 may set the total sum of the calculated perimeter lengths as the perimeter length limit value, or sets a value corrected by multiplying the calculated total perimeter length by a coefficient as the perimeter length limit value. You may. The coefficient by which the sum of the perimeters is multiplied may be one of the parameters input from the input device 40, or a predetermined coefficient determined by a program in advance may be used.

[About additional constraints]
To further improve printability, additional perimeter limits may be provided based on the perimeter of each pixel. The parameter setting unit 32 of the threshold matrix creation device 30 may include an additional constraint condition setting unit (not shown) that sets additional constraint conditions.

<Example 1 of additional constraint conditions>
For example, in the example shown in FIG. 8, pixels with a perimeter of 4 may be excluded from the threshold candidates. Due to this limitation, a new dot of 1×1 pixel dot cannot be generated. The constraint condition that pixels with a perimeter of 4 are excluded from the threshold candidate pixels is an example of a first additional constraint condition as an additional perimeter limit. The parameter setting unit 32 of the threshold matrix creation device 30 may include a first additional constraint condition setting unit (not shown) as one form of the additional constraint condition setting unit. The first additional constraint condition setting unit sets the first additional constraint condition.

FIG. 23 is a flowchart showing a third example of the parameter setting process in the threshold matrix creating device 30. In the flowchart of FIG. 23, steps that are the same as or similar to the steps described in the flowchart of FIG. 11 are given the same step numbers, and description thereof is omitted. The flowchart of FIG. 23 can be adopted instead of the flowchart of FIG.

The flowchart of FIG. 23 includes a step (step S25) of setting a first additional constraint condition based on the perimeter of each pixel. Although FIG. 23 shows an example in which the first additional constraint condition setting step of step S25 is performed after the perimeter length limit value setting step (step S24), the order of performing the first additional constraint condition setting step is particularly Not limited.

In step S25, the parameter setting unit 32 sets the first additional constraint condition based on the perimeter of each pixel. For example, as the first additional constraint condition, a condition of excluding a pixel having a perimeter of 4 from the threshold candidate pixels is set.

The user can input information for specifying the first additional constraint condition from the input device 40. As a method of inputting information, for example, a desired condition may be selected from selection candidates prepared in advance. The parameter setting unit 32 sets the first additional constraint condition according to the information input from the input device 40. Note that the flowchart of FIG. 23 may include the AM line number setting step (step S23) described in FIG.

When the flowchart of FIG. 23 is executed and the first additional constraint condition is set in the threshold matrix creation device 30, the threshold value setting process limits the threshold candidate pixels according to the first additional constraint condition.

For example, between steps S48 and S50 in the flowchart of FIG. 13, a step of limiting candidates of threshold position positions based on the perimeter of each pixel according to the first additional constraint condition is added. The threshold value candidate pixel determination unit 72 determines, as the threshold value candidate pixels, only the pixels satisfying the first additional constraint condition among the pixels having the peripheral length limit value or less, based on the peripheral length constraint value and the first additional constraint condition. .. Note that the step of limiting the threshold candidate pixels according to the first additional constraint condition may be included in the process of step S48.

<Example 2 of additional constraint conditions>
Further, the priority order may be set for the pixels that are candidates for setting the threshold based on the perimeter of each pixel, and the threshold candidate pixels may be preferentially set based on the perimeter of each pixel. For example, first, a pixel with a perimeter of 0 is searched for, and if there is a pixel with a perimeter of 0, it is set as a threshold candidate pixel. If there is no pixel with a perimeter of 0, then a pixel with a perimeter of 1 is searched for. Then, if there is a pixel with a perimeter of 1, it may be used as a threshold candidate pixel, and if there is no pixel, a pixel with a perimeter of 2 or 3 may be searched for as a threshold candidate pixel.

A pixel with a perimeter of 0 is the first priority, a pixel with a perimeter of 1 is the second priority, pixels with a perimeter of 2 and 3 are the third priority, and so on. Providing a priority order based on the length is an example of a constraint condition. The parameter setting unit 32 of the threshold matrix creation device 30 may include a priority order setting unit (not shown), which is one form of the additional constraint condition setting unit. The priority order setting unit sets the priority order based on the perimeter of the pixel.

FIG. 24 is a flowchart showing a fourth example of parameter setting processing in the threshold matrix creating device 30. In the flowchart of FIG. 24, steps that are the same as or similar to the steps described in the flowchart of FIG. 11 are assigned the same step numbers, and description thereof is omitted. The flowchart of FIG. 24 can be adopted instead of the flowchart of FIG.

The flowchart of FIG. 24 includes a step (step S26) of setting a priority order based on the perimeter of each pixel. Although FIG. 24 shows an example in which the priority order setting step of step S26 is performed after the perimeter length limit value setting step (step S24), the order of performing the priority order setting step is not particularly limited.

In step S26, the parameter setting unit 32 sets the priority order based on the perimeter of each pixel. The user can input information for specifying the priority order based on the perimeter of the pixel from the input device 40. The parameter setting unit 32 sets the priority order according to the information input from the input device 40. Note that the flowchart of FIG. 24 may include the AM line number setting step (step S23) described in FIG.

When the flowchart of FIG. 24 is executed and the priority order based on the perimeter of each pixel is set in the threshold value matrix creation device 30, the threshold value setting process searches the threshold value candidate pixels according to this priority order.

For example, between steps S48 and S50 in the flowchart of FIG. 13, a step of setting threshold candidate pixels based on the perimeter of each pixel according to the priority order is added. The threshold value candidate pixel determination unit 72 determines a threshold value candidate pixel from the pixels having the perimeter length limit value or less based on the perimeter length limit value and the priority order according to the priority order. The step of narrowing down the threshold candidate pixels in order of priority may be included in the process of step S48.

[About subdivision of threshold setting rule]
As described in the example of the additional constraint condition described above, by providing an additional perimeter limit or by providing a priority order, the halftone interval in which the rule for setting the threshold candidate pixels changes is subdivided, and each threshold interval is divided. You may change the rule of. For example, in the example of FIG. 4, in the range of the threshold value 65 to the threshold value 128 among the halftone sections of the threshold value 65 to the threshold value 200, the addition of the isolated new dot of 1×1 pixel is not limited by the perimeter of each pixel. On the other hand, in the range of the threshold value 129 to the threshold value 200, it is possible to remove the addition of 1×1 pixel isolated new dots by providing a limit to remove the pixels having the perimeter of 4 from the threshold value candidates.

The process of setting the plurality of threshold sections in which the first additional constraint condition and the rule of the priority order are different as described above can be implemented by a combination of step S20 and step S25 in the flowchart of FIG. In this case, the combination of step S20 and step S25 corresponds to an example of a step of dividing the first additional constraint condition into a plurality of different threshold sections.

The process of setting a plurality of threshold sections with different priority order rules can be implemented by a combination of step S20 and step S26 in the flowchart of FIG. In this case, the combination of step S20 and step S26 corresponds to an example of a step of dividing the priority order rule into a plurality of threshold sections.

[About exception handling]
When there is no pixel having the perimeter length limit value or less among the pixels for which the perimeter is not set, the pixel having the shortest perimeter may be set as the threshold candidate pixel.

<Example 3 of additional constraint conditions>
Next, an embodiment in which the threshold candidate pixels are narrowed down based on the surrounding dot arrangement will be described. It has already been described that the threshold candidate pixels may be narrowed down based on the perimeter of each pixel or the priority order may be set based on the perimeter of each pixel in order to improve printability. In addition to these additional constraints, the threshold candidates may be narrowed down based on the surrounding dot arrangement. A specific example will be described. Here, an example will be described in which, in addition to setting the priority order based on the perimeter of each pixel as an additional constraint condition, the threshold candidate pixels are further narrowed down based on the surrounding dot arrangement.

For example, as a priority order setting, a pixel with a perimeter of 0 is first priority, a pixel with a perimeter of 1 is second priority, a pixel with a perimeter of 2 and a pixel of perimeter 3 is third. When the priority order is set to a certain degree, first, a pixel with a perimeter of 0 is searched, and if a pixel with a perimeter of 0 exists, a pixel with a perimeter of 0 is set as a threshold candidate. If there is no pixel with a perimeter of 0, a pixel with a perimeter of 1 is searched next, and if a pixel with a perimeter of 1 exists, the pixel with a perimeter of 1 is used as a threshold candidate pixel. If there is no pixel with a perimeter of 1, a pixel with a perimeter of 2 or 3 is searched for as a threshold candidate pixel.

In the constraint condition according to such a priority order, the pixel having the peripheral length of 2 is set as the threshold candidate pixel only when the peripheral dot arrangement is the arrangement form shown in either FIG. 25 or FIG.

In each of FIGS. 25 and 26, a thin black pixel displayed by filling in gray indicates a pixel having a perimeter of 2. Further, the dark black pixels displayed by filling black around the pixels having the perimeter of 2 indicate the surrounding pixels for checking the presence or absence of dots. Pixels around which the presence or absence of dots is checked are called “check pixels”. The surrounding dot arrangement including the check pixels is called "surrounding dot arrangement".

25 and 26, for each pixel having a perimeter of 2, it is checked whether or not the surrounding check pixels have already been set to the threshold, and only when the thresholds have been set to all the check pixels, the pixels having the perimeter of 2 are set to the threshold. This indicates that the pixel is a candidate pixel.

In the case of FIG. 25, there are four types of threshold value setting check patterns around the candidate pixel, and if any one of the patterns is satisfied, it is regarded as a candidate. If none of the four types, remove it from the candidates. Note that in each of FIGS. 25 and 26, it does not matter whether or not the threshold value has already been set for the white pixel among the pixels around the candidate pixel.

Similarly, a pixel having a perimeter of 3 is set as a threshold candidate pixel only when the surrounding dot arrangement corresponds to any of the arrangements shown in FIG.

As described above, the printability can be further improved by providing the priority order of the threshold value candidate pixels based on the perimeter of the pixels and further narrowing down the threshold value candidate pixels based on the surrounding dot arrangement. That is, the threshold candidate pixels having the perimeter of 0 or 1 are prioritized, and when the perimeter is 2, only the pixels corresponding to the peripheral dot arrangement shown in FIG. When the peripheral length is 2, the threshold value can be set preferentially to the missing pixels such as in the case of concave, and only the pixels corresponding to the peripheral dot arrangement shown in FIG. Even if the halftone dots are enlarged in a convex shape by setting only the pixels corresponding to the surrounding dot arrangement shown in (1) as the threshold candidate pixels, the size is increased while ensuring the minimum dot size of 2×2 pixels in the portion where the convex pixels are in contact. be able to. The convex pixel refers to a black pixel at a position projecting in a convex shape among the pixels forming the halftone dot.

When it is desired to secure the dot size of a minimum of 3×3 pixels, instead of the check pixels of 2×2 pixels shown in FIG. 26 or FIG. 27, it is determined whether the threshold value around the candidate pixel has been set. The check pixels to be checked may be 3×3 pixels.

Even if it is not necessary to secure the dot size of at least 2×2 pixels, it is desirable that the shape of the halftone dots is not as distorted as possible. You may check. The check of the surrounding dot arrangement means checking the arrangement of surrounding threshold value set pixels.

For example, in the case of a pixel having a perimeter of 2, a threshold candidate pixel may be used only in the case where the arrangement shown in FIG. Further, in the case of a pixel having a perimeter of 3, a threshold candidate pixel may be used instead of the case of FIG. 27 only when the arrangement form shown in FIG. 29 is applicable. As described above, by narrowing down the threshold value candidate pixels based on the surrounding dot arrangement condition, it is possible to prevent the halftone dot width from being one pixel and extending in one direction. The surrounding dot arrangement constraints shown in FIGS. 25 to 29 are examples of second additional constraints. The parameter setting unit 32 of the threshold matrix creation device 30 may include a second additional constraint condition setting unit (not shown) which is a form of the additional constraint condition setting unit. The second additional constraint condition setting unit sets a second additional constraint condition based on the surrounding dot arrangement.

FIG. 30 is a flow chart showing a fifth example of parameter setting processing in the threshold matrix creation device 30.

In the flowchart of FIG. 30, steps that are the same as or similar to the steps described in the flowcharts of FIGS. 11 and 24 are given the same step numbers, and description thereof is omitted. The flowchart of FIG. 30 may be adopted instead of the flowchart of FIG.

The flowchart of FIG. 30 includes a step (step S27) of setting a second additional constraint condition based on the surrounding dot arrangement. Although FIG. 30 illustrates an example in which the second additional constraint condition setting step of step S27 is performed after the priority order setting step (step S26), the order of performing the second additional constraint condition setting step is not particularly limited. ..

In step S27, the parameter setting unit 32 sets the second additional constraint condition based on the surrounding dot arrangement. The user can input information for specifying the second additional constraint condition from the input device 40. The parameter setting unit 32 sets the second additional constraint condition according to the information input from the input device 40. The flowchart of FIG. 30 may include the AM line number setting step (step S23) described with reference to FIG.

When the flowchart of FIG. 30 is executed and the second additional constraint condition is set in the threshold matrix creation device 30, the threshold candidate pixels are limited in the threshold setting process according to the second additional constraint condition.

For example, between step S48 and step S50 in the flowchart of FIG. 13, a step of setting a threshold candidate pixel based on the perimeter of each pixel according to the priority order definition, and a surrounding dot arrangement according to the second additional constraint condition. A step of limiting the candidates for the arrangement position of the threshold based on the above. Based on the perimeter limit value, the priority order, and the second additional constraint condition, the threshold value candidate pixel determination unit 72 selects the pixels satisfying the second additional constraint condition from the pixels having the perimeter limit value or less in the priority order. It is determined as a threshold candidate pixel. The step of setting the threshold candidate pixels according to the priority order and the step of limiting the threshold candidate pixels according to the second additional constraint condition may be included in the process of step S48.

In the above-described example, the priority order is set based on the perimeter of each pixel, and the threshold candidate pixels are narrowed down based on the surrounding dot arrangement.

In implementing the invention, the threshold candidate pixels may be narrowed based on the surrounding dot arrangement without narrowing the threshold candidate pixels based on the perimeter or setting the priority order. That is, the perimeter of the threshold candidate pixel is not checked at all, only the dot arrangement around each threshold candidate pixel (arrangement of threshold-set pixels) is checked, and the dot arrangement of 2×2 pixels shown in FIG. 27 is checked. Alternatively, the threshold value candidate pixel may be set only when it comes into contact with a dot arrangement of 3×3 pixels or another dot arrangement (for example, the arrangement form shown in FIG. 29).

<Example 4 of additional constraint conditions>
Next, an embodiment in which the threshold candidate pixels are narrowed down based on the perimeter of the periphery will be described. The threshold value matrix creating device 30 according to the above-described embodiment has a margin of 4 or more in the perimeter length limit value, and when a pixel with a perimeter length of 4 is not excluded from the threshold candidate pixels, the new isolated dot of 1×1 pixel is used. Can occur. When a new isolated dot of 1×1 pixel is generated, if only pixels having the surrounding dot arrangements shown in FIGS. 27 and 29 are used as threshold candidate pixels, a new dot in contact with the isolated dot of 1×1 pixel is generated. Instead, an isolated dot of 1×1 pixel remains.

Specifically, when a new isolated dot of 1×1 pixel occurs, the next dot comes into contact with the isolated dot of 1×1 pixel to generate a larger dot. It is necessary that the dots come into contact with the previously generated 1×1 pixel isolated dot. However, if the restrictions on the surrounding dot arrangement described in FIGS. 27 and 29 are imposed, pixels with a perimeter of 3 that come into contact with an isolated dot of 1×1 pixel are excluded from the threshold candidate pixels.

In order to prevent such a situation, the threshold candidate pixels may be narrowed down based on the perimeter of the surrounding dot arrangement, together with the above-described second constraint condition of the surrounding dot arrangement.

Specifically, when the perimeter is 3, as shown in FIG. 31, the perimeter of the contacting dots is checked, and when the perimeter of the contact dots is 4, the pixels with the perimeter 3 contacting the contact dots are selected. The pixel is a threshold candidate pixel. A contact dot is a pixel for which a threshold value has been set.

Further, in the case of applying the constraint of the surrounding dot arrangement described in FIG. 27, the presence or absence of the adjacent dot of the contacting dot is checked as shown in FIG. 32, and when there is the adjacent dot, the contact dot and the adjacent dot are detected. The perimeter of each dot is checked, and if the perimeters of both the contact dot and the adjacent dot are both 3, the pixel having the perimeter of 3 in contact with the contact dot is set as the threshold candidate pixel.

The constraint condition based on the peripheral length of the surrounding dots shown in FIGS. 31 and 32 is an example of the third additional constraint condition. The parameter setting unit 32 of the threshold matrix creation device 30 may include a third additional constraint condition setting unit (not shown), which is one form of the additional constraint condition setting unit. The third additional constraint condition setting unit sets a third additional constraint condition based on the perimeter length of surrounding dots.

FIG. 33 is a flowchart showing a sixth example of the parameter setting process in the threshold matrix creating device 30.

In the flowchart of FIG. 33, steps that are the same as or similar to the steps described in the flowcharts of FIGS. 11 and 30 are given the same step numbers, and description thereof is omitted. The flow chart of FIG. 33 can be adopted instead of the flow chart of FIG.

The flowchart of FIG. 33 includes a step (step S28) of setting a third additional constraint condition based on the perimeter length of surrounding dots. Although FIG. 33 shows an example in which the third additional constraint condition setting step of step S28 is performed after the second additional constraint condition step (step S27), the order of performing the third additional constraint condition setting step is It is not particularly limited.

In step S28, the parameter setting unit 32 sets the third additional constraint condition based on the perimeter length of the surrounding dots. The user can input information for specifying the third additional constraint condition from the input device 40. The parameter setting unit 32 sets the third additional constraint condition according to the information input from the input device 40. Note that the flowchart of FIG. 33 may include the AM line number setting step (step S23) described in FIG.

When the flowchart of FIG. 33 is executed and the third additional constraint condition is set in the threshold matrix creation device 30, in the threshold setting process, the threshold candidate pixel is included in the second additional constraint condition and the third additional constraint condition. Limited.

For example, between step S48 and step S50 in the flowchart of FIG. 13, a step of setting a threshold candidate pixel based on the perimeter of each pixel according to the priority order definition, and a surrounding dot arrangement according to the second additional constraint condition. A step of limiting the threshold placement position candidates based on the above, and a step of limiting the threshold placement position candidates based on the perimeter length of the surrounding dots according to the third additional constraint condition are added.

The threshold value candidate pixel determination unit 72 follows the peripheral length limit value, the priority order, the second additional constraint condition, and the third additional constraint condition, and selects the surrounding dots according to the priority order from the pixels having the peripheral length limit value or less. Threshold candidate pixels are determined based on the arrangement and the perimeter of surrounding dots.

The threshold value candidate pixel determination unit 72 may determine a pixel satisfying the second additional constraint condition and the third additional constraint condition as a threshold value candidate pixel in accordance with the priority order from the pixels having the perimeter length limit value or less.

Alternatively, the threshold value candidate pixel determination unit 72 may select a pixel satisfying at least one of the second additional constraint condition and the third additional constraint condition in accordance with the priority order from the pixels having the perimeter length limit value or less. May be determined as the threshold candidate pixel.

The step of setting the threshold candidate pixels according to the priority order, the step of limiting the threshold candidate pixels according to the second additional constraint condition, and the step of limiting the threshold candidate pixels according to the third additional constraint condition are the same as those in the process of step S48. It may be included therein.

<Regarding the combination of additional constraints>
One or more of the first additional constraint condition, the priority constraint condition, the second additional constraint condition, and the various additional constraint conditions exemplified as the third additional constraint condition described above. The conditions can be used in combination as appropriate.

For example, among all the candidate pixels that are equal to or less than the perimeter limit value, pixels that satisfy the second additional constraint condition according to the priority constraint condition, or pixels that do not satisfy the second additional constraint condition but the third additional constraint condition There may be a form in which the threshold candidate pixels are limited to pixels that satisfy the above condition.

For example, as an embodiment of <Additional constraint example 4> which has already been described, for pixels having a perimeter of 3, the surrounding dot arrangement may be one of the arrangement forms shown in FIG. 27 as the second additional constraint condition. Only when applicable, the threshold candidate pixel is used, and the arrangement form of FIG. 27, which is the second additional constraint condition, is not applicable, but the peripheral length of the surrounding dot arrangement is the third additional constraint condition as shown in FIG. 31 or FIG. There may be an embodiment in which the threshold candidate pixel is used only when any of the forms shown in 32 is applicable.

[Example of hardware configuration of threshold matrix creating device]
FIG. 34 is a block diagram showing an example of the hardware configuration of the threshold matrix creation device 30. The threshold matrix creation device 30 can be realized by using a computer. The computer includes various types of computers such as a desktop type, a notebook type, and a tablet type. The computer may be a server computer or a microcomputer.

The threshold matrix creation device 30 includes a computer main body 100, an input device 40, and a display device 42. The computer main body 100 includes a CPU (Central Processing Unit) 101, a memory 102, a hard disk drive (HDD; Hard Disk Drive) 103, an input interface unit 104, a communication interface unit 105 for network connection, and a display control unit 106. And a peripheral device interface 107 and a bus 108. In FIG. 34, the notation “IF” represents “interface”.

The hard disk device 103 stores various programs and data necessary for the threshold matrix creation processing. A program stored in the hard disk device 103 is loaded into the memory 102, and the CPU 101 executes the program, whereby the computer functions as a unit that performs various processes defined by the program.

The memory 102 is used as a work storage area for storing data necessary for each processing of the parameter setting unit 32 and the threshold setting processing unit 34 described in FIG. The memory 102 can also function as the threshold matrix storage unit 36.

The input device 40 is connected to the input interface unit 104. The display device 42 is connected to the display control unit 106.

The threshold matrix creation device 30 may be connected to the telecommunication line 110 via the communication interface unit 105. Telecommunications line 110 may be a local area network, a wide area network, or a combination thereof.

A media interface may be provided as the peripheral device interface unit 107. Each of the peripheral device interface unit 107 and the communication interface unit 105 can function as an information output unit that outputs the data of the threshold matrix created by the threshold matrix creating device 30 to the outside.

[Configuration Example of Printing System Including Threshold Matrix Creating Device]
FIG. 35 shows an example of a printing system 200 including the threshold matrix creation device 30. The printing system 200 is a system that performs halftone processing using the threshold matrix created by the threshold matrix creating device 30 to perform plate making and printing.

In the printing system 200, image data captured by a digital camera 202 as an imaging device or image data captured by a plate making input device 204 as an image reading device is supplied to a RIP (raster image processor) 206. The image data captured by the digital camera 202 is, for example, RGB image data. R represents red, G represents green, and B represents blue, and the RGB image data is digital image data in which the signal values of the R, G, and B signal components are pixel values. The image data captured by the plate-making input device 204 is RGB image data or CMYK image data. C represents cyan, M represents magenta, Y represents yellow, and K represents black. A CMYK image is digital image data having pixel values of signal values of C, M, Y, and K signal components. The RGB image data is converted into CMYK image data by the RIP 206.

In the RIP 206, threshold matrix data created by the threshold matrix creating device 30 is stored in advance in the storage device of the RIP 206 through the optical disc 208 or by communication. The RIP 206 compares the image data of each color component of CMYK with the threshold value of the corresponding threshold matrix of CMYK, and converts it into dot pattern data of each color of CMYK.

The dot pattern data of each of CMYK is sent to the DDCP system 210, and the print proof 212 is created on the paper. DDCP system 210 is also referred to as a "real network proofer." The DDCP system 210 confirms whether or not a noise component is mixed and the print quality before being applied to the printing machine 220. In this case, the printing paper itself may be used as the paper.

In addition, the dot pattern data of each color of CMYK is sent from the RIP 206 to the color inkjet printer 230 or the color electrophotographic printer 240, and the print proofs 232 and 242 can be easily created on the paper.

Further, the dot pattern data of each color of CMYK is sent to the exposure unit 252 which is a film setter or a plate setter which constitutes the output system 250. When the exposure unit 252 is a film setter, a film 264 is created through the automatic developing machine 254, the film 264 is superposed on the printing plate material 260 for the printing plate, and exposed by a surface exposure device (not shown). As a result, the printing plate 266 is created.

When the exposure unit 252 is a platesetter, the printing plate 266 is directly output through the automatic developing machine 254. The printing plate material 260 is supplied from the photosensitive material magazine 256 to the exposure unit 252.

The plate 266 for each color of CMYK is attached to a plate cylinder (not shown) of the K plate printing unit 214K, the C plate printing unit 214C, the M plate printing unit 214M, and the Y plate printing unit 214Y that configure the printing machine 220. .. A color image is reproduced by being overprinted on the printing paper supplied from the printing paper supply unit 216 by the K plate printing unit 214K, the C plate printing unit 214C, the M plate printing unit 214M, and the Y plate printing unit 214Y. A printed matter 270 is obtained. When the printing machine 220 has a CTC device configuration, the dot pattern data of each color of CMYK is directly supplied from the RIP 206, and the printing plate material wound on the plate cylinder is exposed and recorded, and directly developed. , A printing plate 266.

<Modification>
The function of the threshold value matrix creating device 30 may be realized by one computer, or may be realized by combining a plurality of computers. For example, the function of receiving the input of the parameter and the function of executing the threshold setting process may be realized by different computers. Further, the function of the threshold matrix generation device 30 may be incorporated in the RIP 206.

The items described in the configurations and the modified examples described in the above-described embodiment can be appropriately combined and used, and some items can be replaced.

<Regarding a program that causes a computer to function as a threshold matrix creation device>
A non-transitory information storage medium, which is a tangible object such as a CD-ROM (Compact Disc read-only memory), a magnetic disk, or the like, stores a program that causes a computer to implement the threshold matrix generation processing function of the threshold matrix generation device 30 described in the above embodiment. It is possible to record the program on a computer-readable medium and provide the program through this information storage medium. Instead of storing the program in a non-temporary information storage medium, which is a tangible object, and providing the program, it is also possible to provide the program signal as a download service by using a communication network such as the Internet.

Further, it is also possible to provide a part or all of the processing functions of the threshold value matrix creating device 30 as an application server and provide a service that provides the processing functions through a communication network.

[Advantages of Embodiment]
According to the above-described embodiment, it is possible to freely adjust the balance between the image quality and the printability, which are contradictory quality items, and create the threshold matrix that obtains the dot pattern that achieves the target balance.

In the embodiment of the present invention described above, it is possible to appropriately change, add, or delete the constituent features without departing from the spirit of the present invention. The present invention is not limited to the embodiments described above, and many modifications can be made by a person having ordinary knowledge in the field within the technical idea of the present invention. Further, the specific numerical values exemplified in the embodiments can be changed to other numerical values without departing from the spirit of the invention.

10A dot pattern 10B dot pattern 14 1×1 pixel dot 18 2×2 pixel dot 30 Threshold matrix creation device 32 Parameter setting unit 34 Threshold setting processing unit 36 Threshold matrix storage unit 40 Input device 42 Display device 50 Matrix size setting unit 52 Evaluation Function setting unit 54 Minimum dot size setting unit 56 Initial placement setting unit 58 Threshold section setting unit 60 Minimum dot size new dot number condition setting unit 62 Perimeter length limit value setting unit 70 Threshold setting order control unit 72 Threshold candidate pixel determination unit 74 Image quality Evaluation value calculation unit 76 Threshold placement determination unit 80 Halftone dot 100 Computer body 101 CPU
102 memory 103 hard disk device (HDD)
104 input interface unit 105 communication interface unit 106 display control unit 107 peripheral device interface unit 108 bus 110 telecommunication line 200 printing system 201 threshold 202 digital camera 204 plate making input device 206 RIP
208 optical disk 210 DDCP system 212 printing proof 214C C plate printing unit 214K K plate printing unit 214M M plate printing unit 214Y Y plate printing unit 216 printing paper supply unit 220 printing machine 230 color inkjet printer 232 printing proof 240 color electrophotographic printer 242 printing Proof 250 Output system 252 Exposure unit 254 Automatic developing machine 256 Magazine 260 Printing plate material 264 Film 266 Printing plate 270 Printed materials S1 to S2 Steps S12 to S28 of the threshold value matrix creating method Steps S42 to S60 Steps of ascending threshold value setting processing Steps S71 to S79 Steps of threshold setting processing

Claims (17)

A method of creating a threshold matrix for converting a continuous tone image into a dot pattern which is a binary image,
Setting an evaluation function for calculating an image quality evaluation value of a dot pattern corresponding to the arrangement of thresholds in the threshold matrix,
A step of setting a perimeter limit value of the dot pattern for each threshold value according to a halftone dot value corresponding to the threshold value for at least some threshold values;
A step of setting a pixel having the perimeter limit value or less for each of the at least some threshold values as a candidate for a threshold position,
Determining a threshold position based on the image quality evaluation value from among threshold candidate pixels that are candidates for the threshold position.
A method of creating a threshold matrix including. The step of setting the perimeter length limit value sets the perimeter length limit value for each of the threshold values belonging to at least a part of the threshold sections of the entire threshold value range from the minimum threshold value to the maximum threshold value. How to create a threshold matrix. A step of setting a minimum dot size, which is the number of pixels constituting the minimum size dot,
A highlight section that is a threshold range that is greater than or equal to the minimum threshold and is less than or equal to the first threshold, and a threshold range that is greater than or equal to the second threshold and greater than or equal to the maximum threshold than the first threshold, of the entire threshold range from the minimum threshold to the maximum threshold. In at least one of the shadow sections, the constraint of the perimeter limitation based on the perimeter limitation value is not applied, and the threshold placement position is determined so that the dots of the minimum dot size are placed,
The method of creating a threshold matrix according to claim 1 or 2, further comprising: A perimeter limit based on the perimeter limit value for each of the thresholds included in a halftone interval that is a threshold range that is equal to or greater than a third threshold that is greater than the first threshold and equal to or less than a fourth threshold that is less than the second threshold. The method of creating a threshold value matrix according to claim 3, wherein the threshold value is arranged in pixels that are equal to or less than the perimeter length limit value by applying the constraint of. In at least one transition section of a highlight transition section which is a threshold range between the first threshold value and the third threshold value and a shadow transition section which is a threshold value range between the fourth threshold value and the second threshold value, The perimeter length non-limitation process of determining the arrangement position of the threshold so as to arrange the dots of the minimum dot size without applying the constraint of the perimeter limit, and the image quality from the threshold value candidate pixels that are equal to or less than the perimeter length limit value. The method of creating a threshold value matrix according to claim 4, which is implemented in combination with a perimeter length limiting process for determining the arrangement position of the threshold value based on the evaluation value. A step of setting a plurality of threshold sections which is a threshold range in which a unit threshold is a continuous threshold of a threshold number that is a multiple of the minimum dot size;
For each of the plurality of threshold sections, a step of setting a threshold number that is a multiple of the minimum dot size by the perimeter length non-limiting process,
The method of creating a threshold value matrix according to claim 5, further comprising: 7. An evaluation function applied to a part of the halftone percentages in at least one of the highlight section and the shadow section and an evaluation function applied to another halftone percent other than the part of the halftone percents are different. A method of creating a threshold matrix according to any one of 1. The method for creating a threshold matrix according to claim 1, wherein when there is no threshold candidate pixel that is equal to or less than the perimeter length limit value, the pixel having the smallest perimeter is set as the threshold candidate pixel. Setting the number of lines of the amplitude modulation screen,
Setting the perimeter limit value for each threshold value from the number of lines,
9. The method of creating a threshold matrix according to claim 1, further comprising: Setting a first additional constraint condition based on the perimeter of each pixel in addition to the perimeter limit based on the perimeter limit value;
Limiting candidates for threshold position positions based on the perimeter of each pixel according to the first additional constraint condition;
The method of creating a threshold matrix according to claim 1, further comprising: In addition to the perimeter limit based on the perimeter limit value, setting a priority order based on the perimeter of each pixel,
According to the priority order, based on the perimeter of each pixel, setting a threshold candidate pixel,
The method for creating a threshold matrix according to claim 1, further comprising: A plurality of different first addition constraint conditions based on the perimeter of each pixel in a halftone section that is a threshold range that is equal to or greater than a third threshold and equal to or less than a fourth threshold in the entire threshold range from the minimum threshold to the maximum threshold. The method of creating a threshold matrix according to claim 10, further comprising the step of dividing the threshold matrix into the threshold sections. A plurality of thresholds in which the rule of the priority order based on the perimeter of each pixel is different in the halftone section which is the threshold range of the third threshold value to the fourth threshold value in the entire threshold value range from the minimum threshold value to the maximum threshold value. The method of creating a threshold value matrix according to claim 11, including a step of dividing into sections. Setting a second additional constraint based on the placement of thresholded pixels around non-thresholded pixels;
Limiting the candidates for threshold placement based on the placement of threshold-set pixels around the threshold-unset pixels according to the second additional constraint condition;
The method of creating a threshold matrix according to claim 1, further comprising: Setting a third additional constraint based on the perimeter of the perimeter thresholded pixels;
According to the second additional constraint condition and the third additional constraint condition, threshold placement candidates are selected based on the placement of threshold-set pixels around the threshold-unset pixels and the perimeter of the surrounding threshold-set pixels. Limited steps,
15. The method of creating a threshold matrix according to claim 14, wherein the method includes: A threshold matrix creating device for creating a threshold matrix for converting a continuous tone image into a dot pattern that is a binary image,
An evaluation function setting unit that sets an evaluation function for calculating an image quality evaluation value of a dot pattern corresponding to the arrangement of thresholds in the threshold matrix,
A perimeter length limit value setting unit that sets the perimeter length limit value of the dot pattern for each threshold value according to the dot percentage corresponding to the threshold value for at least some threshold values,
A threshold value candidate pixel determination unit that makes a pixel having the perimeter limit value or less for each of the at least some threshold values a candidate for the arrangement position of the threshold value;
From threshold candidate pixels that are candidates for the threshold position, a threshold position determining unit that determines the threshold position based on the image quality evaluation value,
An apparatus for creating a threshold matrix. A program for causing a computer to realize a function of creating a threshold matrix for converting a continuous tone image into a dot pattern which is a binary image,
On the computer,
A function of setting an evaluation function for calculating an image quality evaluation value of a dot pattern corresponding to the arrangement of thresholds in the threshold matrix;
A function of setting the perimeter limit value of the dot pattern for each threshold value according to the halftone dot value corresponding to the threshold value for at least some threshold values,
A function of setting a pixel having the perimeter limit value or less for each of the at least some threshold values as a candidate for a threshold position,
A function of determining a threshold position based on the image quality evaluation value from among threshold candidate pixels that are candidates for the threshold position,
A program that realizes.