JP5471795B2 - Threshold matrix generation method, threshold matrix generation apparatus, threshold matrix, quantization apparatus, and image forming apparatus - Google Patents

Description

  The present invention relates to a threshold matrix generation method, a threshold matrix generation apparatus, a threshold matrix, a quantization apparatus, and an image forming apparatus.

  When an image based on a multi-tone image is output and formed on a print recording medium such as paper, it is widely performed that an output image is obtained by performing a quantization process on the multi-tone image. The term “quantization processing” used here represents an image similar to the multi-gradation image with gradations smaller than the gradation of the input image, so that each pixel of the output image is represented according to each pixel of the input image. This is a process for determining the dot formation state.

As a quantization processing method, an error diffusion method and a dither method are known. Among these, the error diffusion method has a problem that the calculation process is complicated and processing time is required. On the other hand, the dither method is a method of performing quantization based on the comparison result between the pixel value of the pixel of the input image and the threshold value corresponding to the pixel, and the calculation time is very short compared to the error diffusion method. There are advantages.
The threshold value used for comparison with the pixel value of each pixel in the dither processing is prepared in advance as matrix data corresponding to a predetermined number of pixels. Hereinafter, matrix data corresponding to the predetermined number of pixels is referred to as a “threshold matrix”.

The threshold matrix is, for example, matrix data corresponding to a pixel region in which a plurality of pixels are arranged in a square shape, and the size thereof can be represented by “the number of vertical pixels × the number of horizontal pixels” (for example, Patent Document 1). .
The threshold value matrix generally corresponds to the number of pixels smaller than the number of vertical and horizontal pixels of a multi-tone image subjected to dither processing. In the dither processing, a multi-tone image having a larger image size than the threshold matrix is quantized by repeatedly applying a threshold matrix to the multi-tone image subjected to dither processing in a tile shape.

As a method for generating a threshold matrix used for dither processing, for example, a threshold matrix generating method under a stacking constraint condition is known (for example, Patent Document 2).
A threshold matrix generation method under the Stacking Constraint condition will be described with reference to an image diagram of the threshold matrix generation process illustrated in FIG. 23 and a flowchart illustrated in FIG. Here, a case where a threshold value matrix is generated by software processing by a computer having a CPU, RAM, ROM, and the like is illustrated.

First, for the input dot pattern 101 shown in FIG. 23, the CPU determines the number of dots necessary to realize the dot pattern of the next gradation value (the output dot pattern 102 in FIG. 23), and the input dot pattern 101. (Step S101). The required number of dots is determined according to the difference in dot rate of the next gradation value pattern with respect to the dot rate of the input dot pattern 101. The dot rate is a ratio of pixels in which dots are formed with respect to all pixels of the pattern.
Next, the CPU applies a spatial filter prepared in advance to the input dot pattern to which the necessary number of dots is added (step S102). Next, the CPU refers to the dot pattern to which the spatial filter is applied, and rearranges the dot positions added to the input dot pattern 101 (step S103). Then, the CPU calculates the evaluation value of each pattern before and after the rearrangement of the additional dots in step S103, compares the two calculated evaluation values (step S104), and the evaluation value of the dot pattern after the rearrangement is It is determined whether or not the evaluation value of the dot pattern before rearrangement is equal to or higher (step 105). That is, the CPU determines whether the evaluation value calculated in step S104 has not decreased due to the rearrangement of dots. If the evaluation value of the dot pattern after the rearrangement is equal to or higher than the evaluation value of the dot pattern before the rearrangement (step S105: YES), based on the output dot pattern 103 immediately before performing the final rearrangement in step S103. A dot pattern 102 having the next gradation value after the input dot pattern 101 is created (step S106), and the process is terminated. In step S105, if the evaluation value of the dot pattern after rearrangement is less than the evaluation value of the dot pattern before rearrangement (step S105: NO), the process returns to step S102. By repeating this process, a dot pattern at each dot rate is created to obtain a threshold matrix.

The above description using FIG. 23 and FIG. 24 shows a case where the output dot pattern 102 having a large dot rate is created while maintaining the dot position of the input dot pattern 101, that is, the number of dots increases. When generating the pattern further dot rate is greater than the output dot pattern 102, the same processing an output dot pattern 102 as an input dot pattern 101. Dot rate of the output dot pattern emissions 102 to the input dot pattern 101 is small, that is, when the dot decreases, CPU is similarly processed for the pixel to remove the dots from the dot positions that are already arranged in input dot pattern 101 . By using a dot pattern creation method for creating a dot pattern at each dot rate obtained under such a Stacking Constraint condition, threshold matrixes with various dot rate patterns can be obtained based on one input dot pattern. A threshold value matrix is created using the dot patterns of the dot rates created in this way.
Hereinafter, the threshold value matrix generation method under the Stacking Constraint condition described with reference to FIGS. 23 and 24 is referred to as a “spatial filter method”.

  A dot pattern based on a threshold matrix generated by a conventional threshold matrix generation method based on the spatial filter method has a problem that the probability of dot occurrence varies greatly in a predetermined direction.

FIG. 25 shows a dot pattern with a dot rate of 50% according to the conventional spatial filter method. The dot pattern in FIG. 25 is a 256 × 256 [pixel] pattern having a width of 256 [pixel] in two directions (x direction and y direction) orthogonal to each other.
FIG. 26 is a graph showing the number of dots generated in a predetermined direction (y direction) of the dot pattern shown in FIG. 25 along the other direction (x direction) orthogonal to the predetermined one direction.
FIG. 27 shows a two-dimensional spatial frequency distribution (two-dimensional Wiener spectrum) of the dot pattern of FIG. The center o in FIG. 27 corresponds to the DC component, the u direction corresponds to the frequency component in the x direction in FIG. 25, the v direction corresponds to the frequency component in the y direction in FIG. 25, and black in the figure has a component in that frequency band. It means no, and white means that it contains a lot of components in that frequency band.

  The dot pattern shown in FIG. 25 appears to be a dot pattern in which dots are dispersed uniformly at first glance. However, as shown in FIG. 26, the number of dots generated in a predetermined direction (y direction) is different in each other direction (x direction). There is a large variation between pixel columns. For example, in the graph shown in FIG. 26, there is a difference of about 1.5 times in the dot generation probability between the pixel row with the lowest number of dots and the pixel row with the highest number of dots. Such variation in the occurrence probability of dots causes a problem that the consumption of the image forming apparatus is accelerated.

FIG. 28 shows an example of the configuration in the vicinity of the print head H of the line head printer. FIG. 29 shows an example of the configuration when the print head H for one row is viewed from below. The line head printer shown in FIG. 28 is an image forming apparatus that forms dots on a recording medium (paper P) transported along a predetermined direction (y direction) by the transport belt B. Each print head H of the line head printer shown in FIG. 28 has a plurality of recording elements (nozzles N) along another direction (x direction) substantially orthogonal to a predetermined direction (y direction), as shown in FIG. Have
As shown in FIGS. 28 and 29, when a dot pattern is formed by the conventional spatial filter method using a line head printer, the dot generation probability varies greatly in the y direction. The frequency of use of the recording element varies greatly between the elements. For this reason, the degree of consumption of the recording elements varies greatly among the recording elements, and some of the recording elements that are frequently used are consumed at an early stage, which may cause a failure of the image forming apparatus.

Even if the failure does not occur, if the degree of consumption of the printing elements varies among the printing elements, the deterioration of the image forming quality may become remarkable.
Each recording element of the line head printer as shown in FIG. 28 and FIG. 29 causes a dot landing position shift due to a shift in its mounting position. The shift in the mounting position of each recording element of the line head printer is caused in the manufacturing process of the line head printer, and it is difficult to eliminate the shift completely.
Because of the characteristics of such a line head printer, for example, when printing a single density area on one side, if a recording element with a large dot landing position deviation corresponds to a frequently used pixel position, Straight lines due to landing position deviations are likely to appear in the image. Unevenness deteriorates the appearance of the image and reduces the quality of image formation.

In order to suppress the failure of the image forming apparatus and the deterioration of image formation quality due to the variation in the probability of occurrence of dots in one predetermined direction as described above, instead of suppressing the variation in the probability of occurrence of dots, the use of each recording element calculating a distribution of frequency, an image corresponding to the recording element there is a technique of shifting the recording element array direction according to the calculation result (for example, Patent Document 3).

Japanese Patent No. 4168033 Japanese Patent No. 2622429 JP 2006-188003 A

  However, the technique of Patent Document 3 requires a circuit that shifts the image in the direction of the recording element array, which increases the number of parts of the image forming apparatus and increases the cost. Further, since the process of shifting the image takes time, performing the process of shifting the image decreases the processing performance (for example, printing speed) of the image forming apparatus. In particular, when the printing resolution of the image to be shifted is high, the processing content for shifting the image becomes enormous, and a great deal of time is wasted to shift the image, which greatly reduces the processing performance of the image forming apparatus. As described above, in the solution method other than the method of suppressing the variation in the probability of occurrence of dots as in the technique of Patent Document 3, other problems may be caused and the variation in the probability of occurrence of dots is suppressed. No fundamental solution has been reached.

  An object of the present invention is to provide a recording material from the recording element onto the recording medium when the recording medium is transported relative to the recording element in a direction substantially orthogonal to the direction in which the recording elements are arranged. In providing a threshold matrix used for an image forming apparatus that forms an image by adhering, the dot generation frequency of the small sections divided in the direction along the transport direction is made uniform with respect to the recording element arrangement direction. In other words, this extends the life of the recording element and reduces the occurrence of unevenness due to landing deviation.

The invention according to claim 1, the recording medium immediately interlinked conveying direction to the recording element array direction in which the recording elements are arranged, when the relative conveyance to the recording element, the recording from the recording element A threshold value matrix generating method for generating a threshold value matrix used for quantizing an input image in an image forming apparatus for forming an image by attaching a material on the recording medium. Based on the first dot pattern having the number of dots, the first dot ratio is within a range from 0 to 1 indicating a ratio of the number of pixels in which dots are formed with respect to all the pixels included in the pixel region . A new dot pattern generation step for generating a second dot pattern in which the number of dots of the dot pattern is increased or decreased, and the second dot pattern includes A rearrangement step for obtaining a third dot pattern in which a dot is rearranged, and the new dot pattern generation step, wherein the third dot pattern is used as the first dot pattern until a dot pattern having a desired number of dots is obtained. Repeating the rearrangement step, and the rearrangement step performs a predetermined filtering process on the second dot pattern and the second dot pattern in order to realize a desired spatial frequency. A dot pattern for calculating a dot pattern evaluation value based on a difference from the dot pattern by associating an error matrix ERR (x, y, g) represented by the following formula (1) with each position in the threshold value matrix For the plurality of small sections obtained by dividing the evaluation value calculating step and the pixel area of the predetermined size along the transport direction, the number of dots in each small section is averaged. Matrix AVE representing the excess or deficiency of the number of dots the small sections each with respect to the number of dots per a small section to be generated when dots formed uniformity evaluation value indicating the degree for each of the small section are all uniform (A, b) (where a is an integer value of the quotient when x is divided by the number of pixels in the X direction of the small section, and b is the quotient of y when divided by the number of pixels in the Y direction of the small section. and uniformity evaluation value calculating step of calculating, based on the corresponding) to an integer value, in the re-arrangement of the dot and not dot is formed, and a predetermined coefficient to the AVE (a, b) A pixel having a minimum value obtained by adding the multiplied value to the ERR (x, y, g) is determined as a position where a new dot is to be arranged, a dot is formed, and the AVE (a, a The value obtained by multiplying b) by a predetermined coefficient is added to the ERR (x, y, g). A dot rearrangement position determination step for determining a pixel having the maximum calculated value as a position to delete a dot, and a dot is disposed at a position for newly arranging a dot determined by the dot rearrangement position determination step. A rearrangement pattern generation step of rearranging the dots of the third dot pattern by deleting dots from the position of deleting the dot, and a dot dispersibility evaluation value for the rearranged third dot pattern by the following formula Based on the dot dispersibility evaluation value calculation step calculated based on the evaluation value MSE (n) represented by (2) and the dot dispersibility evaluation value of the rearranged third dot pattern , MSE (n) is smaller than before rearrangement of MSE (n-1), the dot pattern evaluation value calculating step, the uniformity evaluation value calculating step, the dot relocation position Determining step, the repeated re-arrangement pattern generation step and the dot dispersion evaluation value calculating step, if after relocation of MSE (n) is before rearrangement of MSE (n-1) or more, so as not repeated Te, and whether relocation repetition determining step of determining is repeated, the case where it is determined that repeat in relocation repetition determining step, the step of the rearranged third dot pattern and the second dot pattern When,
It is characterized by having.
Here, g is a gradation value, x is a pixel coordinate in the recording element array direction, y is a pixel coordinate in the recording medium conveyance direction, and p ′ (x, y, g) is the second dot pattern. It is a real space pattern calculated by applying inverse Fourier transform to a pattern subjected to a predetermined filter process.
Here, n is the number of repetitions of dot rearrangement, and p ′ (x, y, g) in ERR (x, y, g) is a pattern obtained by applying a predetermined filter process to the third dot pattern. It is a real space pattern calculated by applying an inverse Fourier transform .

The invention according to claim 2 is the threshold value matrix generation method according to claim 1, wherein in the calculation of the MSE (n), a spatial filter applied to the third dot pattern is at least low. It is a spatial frequency filter that allows a frequency component to pass .

A third aspect of the present invention is the threshold value matrix generation method according to the first aspect, wherein in the calculation of the MSE (n), the spatial filter applied to the third dot pattern is a low frequency. A spatial frequency filter that allows components to pass through, wherein the spatial frequency filter is applied to the third dot pattern and integrated over all frequency components .

The invention according to claim 4 is the threshold value matrix generation method according to any one of claims 1 to 3 , wherein the predetermined coefficient is different depending on the number of dots included in the dot pattern. To do.

The invention according to claim 5 is the threshold value matrix generation method according to claim 4 , wherein the predetermined coefficient is obtained when the number of dots included in the dot pattern is half the number of pixels of the dot pattern. It is characterized by being the maximum.

The invention according to claim 6 is the threshold value matrix generation method according to any one of claims 1 to 3 , wherein the predetermined coefficient is a manifestation of uneven stripes in an image formed by the dot pattern. It is set so as to increase as the possibility increases , and is different depending on the possibility of the occurrence of stripes .

The invention according to claim 7, The threshold matrix generation method according to any one of claims 1 to 3, wherein the pixel width along the other direction perpendicular to the direction of the cubicle, the The dot pattern has a dot rate of 0.5, which is set to increase as it approaches 0 or 1 at the minimum, and varies depending on the dot rate of the dot pattern .

The invention described in claim 8 is the threshold matrix generation method according to claim 7, wherein the pixel width along the other direction perpendicular to the direction of the small compartments, the number of dots included in the dot pattern Is minimum when the number of pixels is half the number of pixels of the dot pattern.

The invention according to claim 9, a threshold matrix generation method according to any one of claims 1 to 3, wherein the pixel width along said other direction perpendicular to the direction of the cubicle is the dot The image is formed such that the higher the possibility of the occurrence of uneven stripes in the image formed by the pattern, the smaller the possibility, and the difference depending on the possibility of the occurrence of uneven stripes .

The invention described in claim 10 is the threshold matrix generation method according to any one of claims 1 to 9, wherein the pixel width along the other direction perpendicular to the direction of the small compartment 1 or It is a divisor of the width of the other term of the dot pattern.

The invention according to claim 11, the recording medium immediately interlinked conveying direction to the recording element array direction in which the recording elements are arranged, when the relative conveyance to the recording element, the recording from the recording element A threshold value matrix generating device for generating a threshold value matrix used for quantizing an input image in an image forming device for forming an image by attaching a material on the recording medium, wherein a predetermined size pixel region has a predetermined size Based on the first dot pattern having the number of dots, the first dot ratio is within a range from 0 to 1 indicating a ratio of the number of pixels in which dots are formed with respect to all the pixels included in the pixel region . A second dot pattern in which the number of dots in the dot pattern is increased or decreased is generated, and a third dot in which the dots included in the second dot pattern are rearranged is generated. Control that repeats the generation of the second dot pattern and the process of obtaining the third dot pattern using the third dot pattern as the first dot pattern until a dot pattern having a desired number of dots is obtained. In the process of obtaining the third dot pattern, the control unit performs a predetermined filtering process on the second dot pattern in order to realize the second dot pattern and a desired spatial frequency. The dot pattern evaluation value based on the difference between the applied dot pattern and the error matrix ERR (x, y.g) represented by the following formula (1) is calculated corresponding to each position in the threshold matrix. Uniformity evaluation value indicating the uniformity of the number of dots in each small section for a plurality of small sections obtained by dividing the pixel region of the predetermined size along the transport direction Wherein said matrix AVE representing the excess or deficiency of the number of dots cubicle each for the number of dots the small sections each to be generated when a dot is to be formed for each small section is all equal (a, b) (where , A corresponds to the integer value of the quotient when x is divided by the number of pixels in the X direction of the small section, and b corresponds to the integer value of the quotient when y is divided by the number of pixels in the Y direction of the small section) calculated, in the re-arrangement of the dot and not dot is formed, and the AVE (a, b) the value obtained by multiplying a predetermined coefficient with respect ERR (x, y, g) to The pixel having the smallest added value is determined as a position where a new dot is to be arranged, and a value obtained by multiplying the AVE (a, b) by a predetermined coefficient when the dot is formed. Position at which the dot is deleted from the pixel having the maximum value added to (x, y, g) The dot is arranged at the position where the newly determined dot is arranged and the dot is deleted from the position where the dot is deleted, the dot of the third dot pattern is rearranged, and the rearranged The dot dispersibility evaluation value for the third dot pattern is calculated based on the evaluation value MSE (n) represented by the following formula (2), and the dot dispersibility evaluation value of the rearranged third dot pattern is calculated. If the MSE (n) after rearrangement is smaller than the MSE (n-1) before rearrangement, the dot pattern evaluation value is calculated, the uniformity evaluation value is calculated, and a new dot is placed. position and determination of the position to remove the dots, the third repeat calculation of relocation and the dot dispersion evaluation value of the dot of the dot pattern, after relocation of MSE (n) is pre-relocation MSE (n If it is 1) or more, so as not repeated, it is judged whether or not the, that said second dot pattern the third dot pattern when it is determined rearranged and repeated repeated Features.
Here, g is a gradation value, x is a pixel coordinate in the recording element array direction, y is a pixel coordinate in the recording medium conveyance direction, and p ′ (x, y, g) is the second dot pattern. It is a real space pattern calculated by applying inverse Fourier transform to a pattern subjected to a predetermined filter process.
Here, n is the number of repetitions of dot rearrangement, and p ′ (x, y, g) in ERR (x, y, g) is a pattern obtained by applying a predetermined filter process to the third dot pattern. It is a real space pattern calculated by applying an inverse Fourier transform.

The invention according to claim 12, the to the recording element array direction in which the recording elements are arranged recording medium immediately intersects the transport direction, when the relative conveyance to the recording element, from the recording element A threshold value matrix used for quantizing an input image in an image forming apparatus that forms an image by attaching a recording material onto the recording medium, wherein the threshold value matrix includes a plurality of dot patterns for each dot. A threshold value matrix generated by adding together for each pixel constituting the pattern, wherein the plurality of dot patterns are applied to all the pixels included in the pixel region in a predetermined pixel region each composed of a plurality of pixels. A dot pattern having a number of dots corresponding to the dot rate indicating the ratio of the number of pixels on which dots are formed, and , When increasing the dot rate than the initial dot pattern having a predetermined dot rate, the dot arrangement in the initial dot pattern adds additional dot without changing, to reduce the dot rate than the initial dot pattern The arrangement of the pixels in which the dots in the initial dot pattern are not formed is not changed, and the dot ratio from 0 to a predetermined value created under the condition that the dots are deleted and the number of pixels in which the dots are not formed is increased. Each of the corresponding dot patterns, and the predetermined pixel region is a pixel region having a plurality of pixels arranged along the recording element array direction and the transport direction, and at least a dot of each of the dot patterns The dot pattern corresponding to the rate of 0.5 is the same as the dot pattern in the pixel area of the dot pattern. When dividing into a plurality of first small sections having a predetermined pixel width smaller than the total number of pixels along the recording element array direction and having the total pixel width along the transport direction of the pixel area The difference in the number of dots contained in each first sub-compartment has the entire pixel width along the recording element arrangement direction of the pixel area of the dot pattern, and the conveyance of the pixel area When divided into a plurality of second small sections having a predetermined pixel width smaller than the total number of pixels along the direction, the difference is smaller than the difference in the number of dots included in each small section.

The invention according to claim 13, the to the recording element array direction in which the recording elements are arranged recording medium immediately intersects the transport direction, when the relative conveyance to the recording element, from the recording element A quantization device that quantizes an input image in an image forming apparatus that forms an image by attaching a recording material onto the recording medium, the storage unit storing a threshold matrix, and the pixel value of the input image data It has a comparison unit for comparing the threshold of the threshold matrix stored in the front term memory unit, and an output section which generates and outputs the output image data based on the comparison result of the comparison unit, the threshold matrix A threshold value matrix generated by adding a plurality of dot patterns for each pixel constituting each dot pattern, wherein the plurality of dot patterns are A dot pattern having a number of dots corresponding to a dot rate indicating a ratio of the number of pixels in which dots are formed with respect to all pixels included in the pixel area in a predetermined pixel area composed of a plurality of pixels. If the dot ratio is larger than the initial dot pattern having a predetermined dot ratio, additional dots are added without changing the dot arrangement in the initial dot pattern, and the dot ratio is higher than the initial dot pattern. In the initial dot pattern, each pixel from 0 to a predetermined value created under the condition that the dots are not formed and the dots are not formed and the number of pixels in which the dots are not formed is increased is not changed . Each of the dot patterns corresponding to the dot rate, and the predetermined pixel area in the recording element array direction and the transport direction A dot region corresponding to at least a dot ratio of 0.5 among the dot patterns, the dot pattern being the recording element of the pixel region of the dot pattern. Each divided into a plurality of first subdivisions having a predetermined pixel width smaller than the total number of pixels along the arrangement direction and having a total pixel width along the transport direction of the pixel area. The difference in the number of dots included in the first small section is that the dot pattern has a total pixel width along the recording element arrangement direction of the pixel area of the dot pattern, and in the transport direction of the pixel area. When divided into a plurality of second small sections having a predetermined pixel width smaller than the total number of pixels along, the difference is smaller than the difference in the number of dots included in each small section.

The invention according to claim 14, the to the recording element array direction in which the recording elements are arranged recording medium immediately intersects the transport direction, when the relative conveyance to the recording element, from the recording element an image forming apparatus for forming an image by depositing the recording material on the recording medium, threshold matrix storage unit, stored in the pixel value before Symbol storage unit of the input image data for storing the threshold matrix A quantization unit for comparing an input image with the comparison unit, an output unit for generating and outputting the output image data based on a comparison result of the comparison unit, and the quantization device And an image forming unit that forms an image on a recording medium based on the output image data output by the threshold value matrix, and the threshold value matrix forms a plurality of dot patterns. A threshold value matrix generated by adding elements together, wherein the plurality of dot patterns have dots for all the pixels included in the pixel area in a predetermined pixel area each composed of a plurality of pixels. When the dot pattern has a number of dots corresponding to the dot rate indicating the ratio of the number of pixels to be formed and the dot rate is larger than the initial dot pattern having a predetermined dot rate, the dot pattern is in the initial dot pattern When adding additional dots without changing the dot arrangement and making the dot rate smaller than the initial dot pattern, delete the dots without changing the arrangement of the pixels where the dots in the initial dot pattern are not formed. the dot rate of each of the 0 that was created under conditions to increase the pixel in which no dot is formed to a predetermined value Each of the corresponding dot patterns, and the predetermined pixel region is a pixel region having a plurality of pixels arranged along the recording element array direction and the transport direction, and at least a dot of the dot patterns. The dot pattern corresponding to the rate of 0.5 has a predetermined pixel width smaller than the total number of pixels along the recording element arrangement direction of the pixel area of the dot pattern, and the pixel area The difference in the number of dots contained in each of the first small sections when divided into a plurality of first small sections having a total pixel width along the transport direction of And a plurality of second pixels having a predetermined pixel width smaller than the total number of pixels along the transport direction of the pixel region. It is characterized by being smaller than the difference in the number of dots contained in each of the small sections when divided into small sections.

  According to the present invention, when the recording medium is transported relative to the recording element in a direction substantially perpendicular to the direction in which the recording elements are arranged, the recording material is transferred from the recording element onto the recording medium. In providing a threshold matrix used for an image forming apparatus that forms an image by adhering, the dot generation frequency of the small sections divided in the direction along the transport direction is made uniform with respect to the recording element arrangement direction. As a result, the life of the recording element can be extended, and the occurrence of unevenness due to landing deviation can be reduced.

It is a block diagram which shows the structure of the threshold value matrix production | generation apparatus 1 which performs the process using the threshold value matrix production | generation method by 1st embodiment. It is a flowchart which shows the flow of the production | generation process of an initial dot pattern. It is an image figure which shows an example of the threshold value matrix divided | segmented by the small division. It is a subflow which shows the flow of the optimization process of the pattern based on the filtering process and the average value of a small division. It is a graph which shows an example of the frequency of a blue noise filter. It is a graph which shows an example of the frequency of a green noise filter. It is a subflow which shows the flow of a dot arrangement change process. It is an image figure which applied an example of the value of matrix AVE (a, b) to each subdivision. It is a flowchart which shows the flow of the production | generation process of the threshold value matrix based on the initial dot pattern produced | generated by the production | generation process of the initial dot pattern. It is a figure which shows an example of a part of threshold value matrix Th (x, y). It is explanatory drawing which illustrates the production | generation mechanism of the threshold value matrix by the addition of 0/1 according to the dot pattern of each gradation value. It is a figure which shows an example of the dot pattern of the 50% dot rate produced | generated by the threshold value matrix production | generation method by 1st embodiment. 13 is a graph showing the probability of dot generation in a predetermined direction (y direction) of the dot pattern shown in FIG. 12 along another direction (x direction) orthogonal to the predetermined direction. 13 is a graph showing the probability of dot occurrence in a predetermined other direction (x direction) of the dot pattern shown in FIG. 12 along a predetermined one direction (y direction). It is a figure which shows the two-dimensional spatial frequency distribution in the dot pattern shown in FIG. 13 is a graph showing RAPSD (Radial Average Power Spectral Density) corresponding to the dot pattern shown in FIG. It is a figure which shows an example of the relationship between the gradation value g and coefficient (alpha) in 2nd embodiment. FIG. 17A is a graph showing an example of the relationship between the gradation value g and the coefficient α in the second embodiment. FIG. 17B is a table showing dot dispersibility evaluation results when g = 0.1, g = 0.3, and g = 0.5 when α is changed from 0.00 to 0.05. It is a graph which shows an example of the relationship between the gradation value g in 3rd embodiment, and the pixel width m of the X direction of each small division. It is a graph which shows an example of the relationship between the gradation value g and coefficient (alpha) in 4th embodiment. FIG. 2 is a block diagram showing a configuration of a quantization device 200. 2 is a block diagram illustrating a configuration of an image forming apparatus 300. FIG. A graph showing the probability of dot occurrence in a predetermined direction (y direction) of a dot pattern having a 50% dot rate when the coefficient α is 0.01 along the other direction (x direction) orthogonal to the predetermined direction It is. It is an image figure of the threshold value matrix production | generation process on the conventional Stacking Constraint conditions. It is a flowchart which shows the flow of the threshold value matrix production | generation process on the conventional Stacking Constraint conditions. It is a figure which shows the dot pattern of the dot rate 50% by the conventional spatial filter method. FIG. 26 is a graph showing the number of dots generated in a predetermined direction (y direction) of the dot pattern shown in FIG. 25 along another direction (x direction) orthogonal to the predetermined direction. It is a figure which shows the two-dimensional spatial frequency distribution of the dot pattern of FIG. It is a figure which shows an example of a structure of the print head vicinity of a line head printer. It is a figure which shows an example of a structure at the time of seeing the print head for one row from the downward direction.

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

(First embodiment)
First, a first embodiment of the present invention will be described.
FIG. 1 shows a configuration of a threshold matrix generation apparatus 1 that performs processing using the threshold matrix generation method according to the first embodiment. The threshold value matrix generation device 1 transfers a recording material from a recording element to a recording medium when conveying the recording medium in a conveyance direction substantially orthogonal to the recording element arrangement direction in which the recording elements are arranged. A threshold value matrix used to quantize an input image is generated in an image forming apparatus that forms an image by being deposited on the threshold value matrix.
The threshold matrix generation device 1 includes a CPU 11, a RAM 12, a ROM 13, and a storage device 14. The CPU 11, RAM 12, ROM 13 and storage device 14 are connected by a bus 15.

The CPU 11 reads out and executes a program, data, and the like from a storage device such as the ROM 13 and the storage device 14, and performs predetermined processing.
The RAM 12 is a storage device that stores programs and data read by the CPU 11, temporary parameters generated by processing performed by the CPU 11, and the like.
The ROM 13 is a storage device that stores programs and data read by the CPU 11 in a non-rewritable state.
The storage device 14 is a storage device that stores a program and data read by the CPU 11 and data generated by processing performed by the CPU 11 in a rewritable state. The storage device 14 may be replaced with a medium that can be rewritten a plurality of times or a predetermined number of times, a device that writes data to the medium, and the like, or may be connected to an external storage device or medium via a connection such as a network. You may make it transmit / receive data between.

  The threshold value matrix generation device 1 is a computer having the CPU 11, the RAM 12, the ROM 13, and the like as described above, and generates a threshold value matrix by so-called software processing. In the first embodiment, the ROM 13 stores a threshold matrix generation program 16, and the threshold matrix is generated when the CPU 11 reads and executes the threshold matrix generation program 16. Note that various data read out along with the execution of the threshold matrix generation program 16 such as a low-pass filter described later is stored in the ROM 13 or the storage device 14.

  The threshold matrix generation includes an initial dot pattern generation process and a threshold matrix generation process based on the initial dot pattern generated by the initial dot pattern generation process.

First, the initial dot pattern generation process will be described with reference to the flowchart of FIG.
First, the CPU 11 determines the size of the threshold matrix (step S1). In the first embodiment, for two orthogonal directions (X direction and Y direction), 256 × 256 [pixels], which is a rectangular matrix size in which 256 pixels in the X direction and 256 pixels in the Y direction are arranged, is determined. This is used as the size of the threshold matrix. The X direction corresponds to the recording element arrangement direction, and the Y direction corresponds to the transport direction. The size of the threshold matrix is not limited to 256 × 256 [pixels], and any M × N [pixels] (M and N are natural numbers) can be set. M and N are preferably 64 or more. M and N may be the same natural number or different natural numbers.

Next, the CPU 11 determines the size of the small section (step S2). In the first embodiment, 256 × 256 [pixels], which is the matrix size of the threshold matrix, is divided into a plurality of small sections.
FIG. 3 is an image diagram showing an example of a threshold matrix divided by small sections. As shown in FIG. 3, the 256 × 256 [pixel] threshold matrix is divided into a plurality of small sections along the Y direction. In the first embodiment, the pixel width m in the X direction of one small section is 1, and 256 small sections are provided.

  Next, the CPU 11 selects and determines a pixel on which dot formation is performed within the threshold value matrix. In the first embodiment, the gradation value g indicates the ratio of the pixels selected and determined as pixels for dot formation to all the pixels in the threshold matrix. The gradation value g = 1 when all the pixels in the threshold matrix are selected and determined as pixels that perform dot formation, and the gradation value when any pixel in the threshold value matrix is not selected and determined as a pixel that performs dot formation. Let g = 0. For example, when the gradation value g = 0.5, half of all the pixels in the threshold value matrix are selected and determined as pixels for dot formation. In the first embodiment, the gradation value g of the initial dot pattern is set to 0.5, and the CPU 11 selects and determines a pixel based on the gradation value g = 0.5.

  Then, the CPU 11 generates a matrix pattern p (x, y, g) representing 1 as a pixel selected and determined as a pixel for performing dot formation and 0 as a pixel not selected and determined as a pixel for performing dot formation (step) S3). p (x, y, g) is a matrix indicating the matrix pattern p of the gradation value g, where x is a coordinate in the X direction and y is a coordinate in the Y direction.

Next, the CPU 11 sets all initial values of the matrix BAN _init (x, y) to 0 (step S4). The value of the matrix BAN _init (x, y) is used as the initial value of the exchange prohibition matrix BAN (x, y). The exchange prohibition matrix BAN (x, y) is used in a dot arrangement changing process described later.

Next, the CPU 11 performs filter processing and pattern optimization processing based on the average value of the small sections (step S5).
The filter processing and the pattern optimization processing based on the average value of the small sections will be described with reference to the subflow of FIG.

First, the CPU 11 sets a variable n with an initial value 0 (step S11).
Next, the CPU 11 calculates a pattern P (u, v, g) obtained by applying Fourier transform to the pattern p (x, y, g) (step S12). u and v represent frequency spaces in the X direction and the Y direction, respectively.

Next, the CPU 11 calculates a pattern P ′ (u, v, g) obtained by filtering the pattern P (u, v, g) (step S13). Examples of the filter used for the filter process in step S13 include a low-pass filter such as a blue noise filter and a green noise filter. The blue noise filter and the green noise filter are frequency filters that pass at least a low frequency component.
FIG. 5 shows an example of a blue noise filter, and FIG. 6 shows an example of a green noise filter.
A low-pass filter such as a blue noise filter or a green noise filter is applied as an isotropic filter independent of direction. 5 and 6, the unit of the frequency in the radial direction is [cycle / pixel].

  Next, the CPU 11 calculates a pattern p ′ (x, y, g) by applying inverse Fourier transform to the pattern P ′ (u, v, g) (step S14).

Next, the CPU 11 calculates an error matrix ERR (x, y, g) indicating a deviation from the gradation value g by the following equation (1) (step S15).
ERR (x, y, g) functions as a dot pattern evaluation value based on the difference between the pattern p ′ (x, y, g) and the dot pattern of the gradation value g. Each value included in the matrix of the error matrix ERR (x, y, g) corresponds to each pixel position of the pattern p ′ (x, y, g) that is a dot pattern for generating a threshold matrix. That is, the CPU 11 calculates the error matrix ERR (x, y, g) corresponding to each position in the threshold matrix.

Next, the CPU 11 calculates an evaluation value MSE (n) (Mean Square Error) by the following equation (2) (step S16).
MSE (n) is a pattern deviation value of the pattern p ′ (x, y, g) and functions as a dot dispersibility evaluation value.

  Next, the CPU 11 determines whether n is not 0 (step S17). When n is not 0 (step S17: YES), the CPU 11 determines whether MSE (n) is smaller than MSE (n-1) (step S18).

When n is 0 in step S1 7 (Step S17: NO) or if in step S1 8 MSE (n) is smaller than the MSE (n-1) (Step S18: YES), CPU11 is a dot arrangement conversion process Perform (step S19).
The dot arrangement changing process will be described with reference to the subflow of FIG.

First, the CPU 11 sets a variable SWAPNUM for managing the number of exchanged pixels with a predetermined initial value (step S31). The predetermined initial value is an integer of 1 or more.
Next, the CPU 11 sets the exchange prohibition matrix BAN (x, y) and copies the value of BAN _init (x, y) as the value (step S32).

Next, the CPU 11 calculates a matrix AVE (a, b) indicating the uniformity of dots in each small section (step S33). Here, a corresponds to the integer value of the quotient when x is divided by the number of pixels in the X direction of the small section, and b corresponds to the integer value of the quotient when y is divided by the number of pixels in the Y direction of the small section.
FIG. 8 shows an image diagram in which an example of the value of the matrix AVE (a, b) is applied to each small section.
The value of the matrix AVE (a, b) indicates the excess or deficiency of the number of dots in each small section with respect to the number of dots in each small section to be generated when all the dots formed in each small section are equal.
For example, in the case of a matrix pattern with a gradation value g = 0.5, the number of dots in each small section to be generated when all dots formed in each small section are equal is 128 (1 × 256 × 0.5). = 128). The value of the matrix AVE (a, b) indicates the excess or deficiency of the number of dots in each small section, for example, as shown in FIG. Thus, the value of the matrix AVE (a, b) functions as a uniformity evaluation value indicating the uniformity of the number of dots in each small section.

  Next, the CPU 11 determines whether or not the value of the variable SWAPNUM exceeds 0 (step S34). When the value of the variable SWAPNUM exceeds 0 (step S34: YES), the CPU 11 performs a process of determining a position where a dot is newly arranged and a position where a dot is deleted.

  Specifically, the CPU 11 satisfies BAN (x, y) = 0 with p (x, y, g) = 0, and ERR (x, y, g) + αAVE (a, b) is included therein. The minimum pixel coordinate (x1, y1) and p (x, y, g) = 1 satisfy BAN (x, y) = 0, and ERR (x, y, g) + αAVE ( The coordinates (x2, y2) of the pixel where a, b) are maximized are detected (step S35).

Here, p (x, y, g) = 0 indicates that no dot is formed in the pixel. On the other hand, p (x, y, g) = 1 indicates that a dot is formed in the pixel.
Further, satisfying BAN (x, y) = 0 indicates that the pixel is permitted to perform dot rearrangement. On the other hand, when BAN (x, y) = 1, it indicates that the pixel is not permitted to perform dot rearrangement.
In addition, the fact that ERR (x, y, g) is the minimum indicates that there is a gap (void) where dots are not formed at the peripheral position including the pixel. On the other hand, the fact that ERR (x, y, g) is maximum indicates that there is a cluster of dots (Cluster) at the peripheral position including the pixel. In the first embodiment, in addition to the magnitude of ERR (x, y, g), a predetermined coefficient α (α> 0) is added to the value AVE (a, b) indicating the excess or deficiency of the number of dots in each small section. Correction is performed by adding the multiplied values. A value AVE (a, b) obtained by multiplying a value AVE (a, b) indicating the excess or deficiency of the number of dots in each small section by a predetermined coefficient α (α> 0) is the average number of dots in the small section. When exceeding (AVE (a, b)> 0), it works to increase ERR (x, y, g) + αAVE (a, b). On the other hand, when the number of dots in the small section is below the average (AVE (a, b) <0), α AVE (a, b) is set so as to reduce ERR (x, y, g) + α AVE (a, b). To work.
In the first embodiment, α = 0.05, but the present invention is not limited to this.

That is, the coordinates of the pixel that satisfies BAN (x, y) = 0 when p (x, y, g) = 0 and ERR (x, y, g) + αAVE (a, b) is the smallest among them. (X1, y1) means that a dot is not formed in the pixel, the dot rearrangement is permitted for the pixel, and a dot is not formed in a peripheral position including the pixel. (Void) is present. Here, since ERR (x, y, g) + αAVE (a, b) is the smallest, there is a relatively high possibility that the small section including the pixel has a dot below the average.
On the other hand, the coordinates of the pixel that satisfies BAN (x, y) = 0 with p (x, y, g) = 1, and in which ERR (x, y, g) + αAVE (a, b) is maximum. (X2, y2) means that a dot is formed at the pixel, and it is permitted to rearrange the dots for the pixel, and a set of dots (Cluster) at a peripheral position including the pixel. Indicates that there is. Here, since ERR (x, y, g) + αAVE (a, b) is the maximum, there is a relatively high possibility that the small section including the pixel has a dot exceeding the average.
As described above, by performing the evaluation using ERR (x, y, g) + αAVE (a, b), based on the evaluation that takes into consideration both the uniformity of the local dot arrangement and the uniformity of the dot arrangement in the entire mask. Pixels for dot rearrangement can be determined.

  Then, for the pixel (x1, y1) and the pixel (x2, y2) specified as described above, the CPU 11 determines that p (x1, y1, g) = 1 and p (x2, y2, g) = 0. Thus, the dot arrangement is exchanged (step S36). In other words, the dot is not formed in the pixel before the rearrangement, the dot rearrangement is permitted for the pixel, and there is a gap (void) where the dot is not formed in the peripheral position including the pixel. A dot is placed at the pixel (x1, y1), a dot is formed at that pixel before the rearrangement, and it is permitted to rearrange the dots for that pixel, and at a peripheral position including that pixel. A dot is deleted from the pixel (x2, y2) where the dot cluster (Cluster) was present.

  After the processing in step S36, the CPU 11 changes the value of AVE (a, b) of the small section including the pixel on which the dot rearrangement has been performed. Specifically, the CPU 11 adds 1 to the value AVE (c, d) indicating the dot excess / deficiency of the small section including the pixel (x1, y1), and the dot of the small section including the pixel (x2, y2). 1 is subtracted from the value AVE (e, f) indicating the excess / deficiency (step S37). Here, c and e are integer values of quotients obtained by dividing x1 and x2 by the number of pixels in the X direction of the small section (for example, 1) by the value of x when the number of pixels in the X direction of the small section is 1. D and f can be obtained from integer values of quotients obtained by dividing y1 and y2 by the number of pixels in the Y direction of the small section (for example, 256).

  Next, the CPU 11 performs processing for prohibiting further rearrangement of the pixels on which the dot rearrangement has been performed. Specifically, the CPU 11 sets the values of BAN (x1, y1) and BAN (x2, y2) to 1 (step S38).

Next, the CPU 11 subtracts 1 from the value of the variable SWAPNUM (step S39). After the process of step S39, the process returns to the determination of step S34.
In step S34, when the value of the variable SWAPNUM does not exceed 0 (step S34: NO), the CPU 11 ends the dot arrangement conversion process shown by step S19 in FIG. 4 and the subflow in FIG.

  In other words, in the dot arrangement conversion process, the CPU 11 is permitted to perform dot rearrangement for the pixel, with no dots formed for that pixel, the number of times set as the initial value of the variable SWAPNUM, and A pixel having a void (void) where a dot is not formed at a peripheral position including the pixel, a dot formed with the pixel, and dot rearrangement for the pixel being permitted, and including the pixel The dot arrangement is exchanged with a pixel having a cluster of dots at the peripheral position. In determining the dots to be replaced, correction is performed based on the excess or deficiency of the number of dots in each small section. Further, further rearrangement is prohibited for the dots that have been rearranged.

  After completion of the dot arrangement changing process shown in step S19 of the subflow of FIG. 7 and the subflow of FIG. 4, the CPU 11 adds 1 to the value of the variable n (step S20). Thereafter, the process returns to step S12.

  By returning to step S12 after the process of step S20, the application of the Fourier transform of step S12, the filtering process of step S13, the application of the inverse Fourier transform of step S14 to the matrix pattern after the dot arrangement conversion process, Error matrix calculation and MSE calculation in step S16 are performed. When the dot arrangement conversion process is performed once or more, the variable n becomes 1 or more, so this corresponds to the case where n is not 0 in the determination in step S17 (step S17: YES), and the determination in step S18, that is, MSE ( A determination is made whether n) is less than MSE (n-1). Since the initial value of n is 0, the dot arrangement conversion process is performed at least once.

In step S18, while MSE (n) is smaller than MSE (n-1), the processing from step S12 is repeated (step S18: YES).
In step S18, when MSE (n) is equal to or greater than MSE (n-1) (step S18: NO), the CPU 11 is based on the filtering process shown in step S5 of FIG. 2 and the subflow of FIG. The pattern optimization process ends, and the initial dot pattern creation process shown in FIG. 2 ends. The initial dot pattern is generated as p _init (x, y) and stored in the storage device 14.

Next, threshold value matrix generation processing based on the initial dot pattern generated by the initial dot pattern generation processing will be described.
The threshold value matrix generation process is performed under a stacking constraint condition. Specifically, based on the gradation value g of the initial dot pattern, the number of dots is increased or decreased according to the gradation value change amount δg to be changed. At this time, when the gradation value becomes large due to the gradation value change amount δg, the dot arrangement in the original initial dot pattern is not changed. That is, when increasing the number of dots, the original initial dot pattern is maintained and additional dots are added. Further, when the gradation value is decreased by the gradation value change amount δg, the arrangement of the pixels where the dots in the original initial dot pattern are not formed is not changed. That is, when the number of dots is reduced, the pixels where dots are not formed in the original initial dot pattern are left without being formed, and the dots are deleted to increase the number of pixels where dots are not formed. In this way, the threshold value matrix generation device 1 generates each dot pattern composed of 256 × 256 [pixels] corresponding to each dot rate.

Further, after generating a dot pattern of gradation value g + δg in which dots are increased or decreased according to the gradation value change amount δg to be changed based on the gradation value g of the initial dot pattern, the dot pattern of gradation value g + δg is generated. When the number of dots is increased or decreased according to the gradation value change amount δg based on the above, the dot pattern of the gradation value g + δg generated immediately before is treated as the initial dot pattern.
For example, when the change amount δg of the gradation value to be changed is 0.01, the dot pattern having the gradation value g + δg = 0.51 based on the dot pattern having the gradation value g = 0.5, and the gradation value g− A dot pattern of δg = 0.49 is generated. In the next processing, a dot pattern having a gradation value of 0.52 and a gradation value of 0.48 is generated using a dot pattern having a gradation value of 0.51 and a dot pattern having a gradation value of 0.49 as an initial dot pattern. Is done. Thereafter, the same processing is repeated, and a dot pattern corresponding to each gradation value is generated to generate a threshold matrix.
The threshold matrix is generated as a matrix Th (x, y) and stored in the storage device 14.

  The threshold matrix generation process based on the initial dot pattern generated by the initial dot pattern generation process will be described below with reference to the flowchart of FIG.

First, the CPU 11 reads the initial dot pattern p _init (x, y) (step S41). The gradation value g _init of the initial dot pattern p _init (x, y) in the first embodiment is 0.5. Here, the initial value of the gradation value g to be used hereinafter is g _init .
In the first embodiment, the gradation value g _init of the initial dot pattern p _init (x, y) is 0.5, but may be any value from 0 to 1. In the initial dot pattern generation process, an initial dot pattern having an arbitrary gradation value g between 0 and 1 can be generated and used for the threshold matrix generation process.

Next, the CPU 11 sets a matrix q (x, y, g) indicating a matrix pattern for generating a threshold matrix, and sets the value of the initial dot pattern p _init (x, y) to q (x, y, g). Is copied (step S42).

  Next, the CPU 11 sets the exchange prohibition matrix ban (x, y) and copies the value of q (x, y, g) to ban (x, y) (step S43). Here, the value of the exchange prohibition matrix ban functions similarly to the exchange prohibition matrix BAN. That is, when ban (x, y) = 1, it indicates that dot rearrangement is not permitted. Here, since the value of q (x, y, g) is copied to the exchange prohibition matrix ban (x, y), the exchange prohibition matrix ban (x) corresponding to the pixel (value = 1) where the dot is formed. , Y) is set to 1, and the rearrangement of dots is prohibited for pixels that already have dots in the initial dot pattern, and dots are not deleted from the pixels.

  Next, the CPU 11 adds the gradation value δg to be changed to the gradation value g, and adds a number of dots corresponding to the gradation value δg to be changed to q (x, y, g) (step). S44). The dot pattern after the dot addition is stored in the RAM 12 or the storage device 14 as a new q (x, y, g). The addition of dots is performed based on predetermined random number processing from positions where dots are not formed in q (x, y, g) before adding dots.

Next, the CPU 11 performs a filter process and a pattern optimization process based on the average value of the small sections (step S45). In the filtering process in step S45 and the pattern optimization process based on the average value of the small blocks, the pattern p (x, y, g) in the subflow shown in FIG. 4 is changed to q (x, y, g), and the exchange prohibition matrix BAN. Except for replacing (x, y) with an exchange prohibition matrix ban (x, y), this is the same as step S5 in the initial dot pattern generation process.

That is, the CPU 11 applies the Fourier transform of Step S12, the filtering process of Step S13, the application of the inverse Fourier transform of Step S14, and the error matrix that functions as the dot pattern evaluation value to the pattern q (x, y, g). Various processes such as the process of step S15 for calculating ERR (x, y, g), the process of step S16 for calculating MSE functioning as a dot dispersibility evaluation value, and the dot arrangement changing process shown in FIG. 7 are performed.
In the dot arrangement changing process, the CPU 11 calculates the matrix AVE (a, b) that functions as an evaluation value indicating the excess or deficiency of the dots in each small section, that is, the uniformity, and the error matrix ERR (x , Y, g) and the matrix AVE (a, b), a new dot pattern is generated by performing the process of step S35 for determining a position where a dot is newly arranged and a position where a dot is deleted, and changing the dot arrangement. The process of step S36 is performed.
In step S18, while the dot dispersibility evaluation value satisfies a predetermined condition, that is, as long as MSE (n) is smaller than MSE (n-1), the CPU 11 performs filtering processing and small sections including dot arrangement conversion processing. The pattern optimization process based on the average value is repeated.

After the filtering process shown in step S45 and the pattern optimization process based on the average value of the small sections, the CPU 11 sets q (x, y, g) after the optimization process to the matrix Th (x, y) indicating the threshold matrix. Add (step S46).
Next, the CPU 11 determines whether g is 1 or more (step S47). If g is not 1 or more (step S47: NO), the process returns to step S43.

In step S47, the case g is 1 or more (step S47: YES), substitutes the gradation value _{g init} of the initial dot pattern _p init (x, y) to the gray scale value g (step S48).

Next, the CPU 11 copies the value of the initial dot pattern p _init (x, y) to the pattern q (x, y, g) (step S49).

  Next, the CPU 11 copies the value of (1-q (x, y, g)) to the exchange prohibition matrix ban (x, y) (step S50). Here, since the value of (1-q (x, y, g)) is copied to the replacement prohibition matrix ban (x, y), the replacement prohibition matrix corresponding to the pixel (value = 0) in which no dot is formed. The value of ban (x, y) is set to 1, and the rearrangement of dots is prohibited for a pixel having no dot in the initial dot pattern, and no new dot is arranged for that pixel.

  Next, the CPU 11 subtracts the tone value change amount δg to be changed from the tone value g, and deletes the number of dots corresponding to the tone value change amount δg to be changed from q (x, y, g) (step). S51). The dot pattern after dot deletion is stored in the RAM 12 or the storage device 14 as a new q (x, y, g). The deletion of dots is performed based on a predetermined random number process from the positions where dots are formed in q (x, y, g) before deleting the dots.

  Next, the CPU 11 performs a filter process and a pattern optimization process based on the average value of the small sections (step S52). The filter process in step S52 and the pattern optimization process based on the average value of the small sections are the same as the process in step S45.

After the filter process shown in step S52 and the pattern optimization process based on the average value of the small sections, the CPU 11 adds q (x, y, g) after the optimization process to the threshold matrix Th (x, y) (step S52). S53).
Next, the CPU 11 determines whether or not g is less than 0 (step S54). If g is not less than 0 (step S54: NO), the process returns to step S50.

  In step S54, when g is less than 0 (step S54: YES), the CPU 11 ends the threshold matrix generation process based on the initial dot pattern generated by the initial dot pattern generation process.

  The addition of dots in step S44 and the deletion of dots in step S51 may be performed based on a method other than the predetermined random number process. Another method includes, for example, a method using error diffusion. At this time, as the distribution of the dots in the dot pattern after addition or deletion is closer to the desired pattern, the repetition of the filter optimization and the pattern optimization process based on the average value of the small sections and the dot arrangement change process may be completed earlier. it can.

Thus, the threshold matrix creation process of the first embodiment includes
A second dot pattern (q (x, y,) in which the number of dots of the first dot pattern is increased or decreased based on a first dot pattern (initial dot pattern) having a predetermined number of dots in an image of a predetermined size. g)) a new dot pattern generation process (step S44, step S51);
A rearrangement step (step S45, step S52) for obtaining a third dot pattern in which dots included in the second dot pattern are rearranged;
Repeat step (steps S43 to S47, steps S50 to S54) of repeating the new dot pattern generation step and the rearrangement step using the third dot pattern as the first dot pattern until a dot pattern having the number of dots based on a desired dot rate is obtained. And having
The relocation process
An error matrix ERR (x that functions as a dot pattern evaluation value based on a difference between the second dot pattern and a dot pattern obtained by subjecting the second dot pattern to a predetermined filter process in order to realize a desired spatial frequency , Y, g) corresponding to each position in the threshold matrix, a dot pattern evaluation value calculating step (steps S12 to S15);
Uniformity for calculating a matrix AVE (a, b) that functions as a uniformity evaluation value indicating the uniformity of the number of dots in each small partition for a plurality of small partitions obtained by dividing the predetermined size along the Y direction. An evaluation value calculating step (step S33);
Based on the dot pattern evaluation value and the first uniformity evaluation value, a dot rearrangement position determination step (step S35) for determining a position for newly arranging a dot and a position for deleting a dot in the rearrangement of dots;
A rearrangement pattern generation step (step S36) for rearranging the third dot pattern based on a position for newly arranging a dot and a position for deleting a dot determined by the dot rearrangement position determination step;
A dot dispersibility evaluation value calculating step (step S16) for calculating MSE (n) that functions as a dot dispersibility evaluation value for the rearranged third dot pattern;
Dot pattern evaluation value calculation step, first uniformity evaluation value calculation step, dot rearrangement position determination step, rearrangement pattern generation until the dot dispersibility evaluation value of the rearranged third dot pattern satisfies a predetermined condition A rearrangement repetition determination step (step S18) for determining whether to repeat the step and the dot dispersibility evaluation value calculation step;
If it is determined to repeat in the rearrangement repetition determination step, the step of setting the rearranged third dot pattern as the second dot pattern (q (x, y, g)) (step S12 after steps S19 and S20) And).

In the first embodiment, the spatial frequency distribution of the dot pattern to be realized is a space in P (u, v, g) (step S12) obtained by performing Fourier transform on the input dot pattern p (x, y, g). This can be realized by removing the frequency component P ′ (u, v, g) after the filtering process (step S13).
Here, the process of removing P ′ (u, v, g) is obtained by obtaining the pattern p ′ (x, y, g) by inverse Fourier transform (step S14), and then calculating the difference from the gradation value g by ERR (x , Y, g) (step S15), and the variation is realized as a process of removing the variation by the dot arrangement conversion process (step S19).
Therefore, assuming that “desired pattern” = “pattern from which low-frequency components have been removed”, the error calculation for realizing the dot pattern corresponding to the desired spatial frequency distribution is calculated using the input dot pattern p (x, y, g) Filter processing for extracting unnecessary low-frequency components (steps S12 to S14) and calculation of error matrix ERR (x, y, g) for removing the extracted unnecessary low-frequency components (step S15) )

FIG. 10 shows an example of a part of the threshold matrix Th (x, y). FIG. 11 illustrates a threshold matrix generation mechanism by adding 0/1 according to the dot pattern of each gradation value.
The threshold value matrix Th (x, y) of the first embodiment is generated as a matrix corresponding to 256 × 256 [pixels], and the threshold value of each pixel is set as the value of the threshold value matrix Th (x, y). The value of the threshold matrix Th (x, y) set as the value of each pixel is obtained by inverting the matrix value determined by the integration of 0/1 according to the dot pattern of each gradation value.
In other words, the threshold matrix includes each dot pattern composed of M × N pixels (for example, 256 × 256 [pixel] in the present embodiment) corresponding to each dot rate created under the stacking constraint condition. It is generated by adding each pixel constituting the dot pattern and then inverting it.

  As shown in FIG. 11, when a matrix value of 0/1 indicating a dot pattern corresponding to a certain second gradation value is added, a pixel “1” in which dots are formed in both of the two matrices is added. As a result, it becomes “2”. Similarly, a pixel in which a dot is formed in only one of the two matrices is “1”. Further, “0” pixels in which no dot is formed in both of the two matrices are also “0” after addition. As described above, the threshold value of a pixel in which dots are formed at a large number of gradation values is relatively increased by integration. Conversely, the threshold value of a pixel in which dots are not formed at a large number of gradation values is relatively large. Get smaller. Thus, the threshold value corresponding to each pixel of the threshold value matrix Th (x, y) is determined by the integration of 0/1 according to the dot pattern of each gradation value.

  In the first embodiment, an example in which the gradation value δg to be changed is set to 0.01 is described. The size of the matrix pattern is 256 × 256 = 65,536 [pixel]. When the gradation value is 0, the dot formation pixel is 0 [pixel], and when the gradation value is 1, the dot formation pixel is 65,536 [pixel]. Become. Therefore, the difference in the number of dots between the gradation values at the gradation value δg = 0.01 increments to be changed is an average of 655.36 [pixels]. Here, since the pixel needs to be an integer value, the difference in the number of dots between the gradation values is 655 [pixel] or 656 [pixel]. Whether the difference in the number of dots between the gradation values is 655 [pixels] or 656 [pixels] can be determined arbitrarily, but when the gradation value is 0, the number of dots formed is 0 [pixels]. When the gradation value is 1, the pixel in which dots are formed is adjusted to 65,536 [pixels]. Even if the range of the gradation value g, the value of the gradation value δg to be changed, and the size of the matrix pattern are different from those of the first embodiment, the same processing is performed.

FIG. 12 shows an example of a dot pattern with a 50% dot rate generated by the threshold matrix generation method according to the first embodiment.
FIG. 13 is a graph showing the dot generation probability in the predetermined one direction (y direction) of the dot pattern shown in FIG. 12 along the other direction (x direction) orthogonal to the predetermined one direction. The x direction corresponds to the recording element arrangement direction, and the y direction corresponds to the transport direction.
FIG. 14 is a graph showing the probability of dot occurrence in a predetermined other direction (x direction) of the dot pattern shown in FIG. 12 along a predetermined one direction (y direction).
As shown in FIG. 13, the dot occurrence probability of 50% dot rate generated by the threshold matrix generation method according to the first embodiment has substantially the same dot generation probability in one predetermined direction (y direction). This is because the dot generation probability in each region when the dot pattern of x direction × y direction = 256 × 256 [pixel] shown in FIG. 12 is divided into 1 × 256 [pixel] regions is almost equal. Indicates. The fact that the occurrence probability of dots in each region is almost equal indicates that the difference in the number of dots between the regions is extremely small. A 50% dot pattern obtained using a threshold matrix obtained by integrating values based on the dot pattern shown in FIG. 12 shows the same tendency as in FIG. Therefore, when a dot pattern generated by the threshold matrix generation method generated by the threshold matrix generation method according to the first embodiment is formed on a recording medium by an image forming apparatus such as a line head printer, the x direction coincides with the arrangement direction of the recording elements. By quantizing using the threshold value matrix applied as described above, the frequency of use of the printing elements becomes almost uniform among the printing elements. As a result, it is possible to suppress a large variation in the degree of consumption of the printing elements that has occurred due to the dot pattern by the threshold value matrix generated by the conventional spatial filter method. That is, it is possible to solve a conventional problem that may cause a failure of the image forming apparatus due to early consumption of some of the frequently used recording elements. In addition, it is possible to suppress deterioration in image formation quality such as uneven stripes, which has occurred due to the degree of consumption of the recording elements varying between the recording elements.

  In addition, the dot generation probability in the other direction (x direction) shows variation as shown in FIG. This shows variation in the dot generation probability in each region when the dot pattern of x direction × y direction = 256 × 256 [pixel] shown in FIG. 12 is divided into 256 × 1 [pixel] regions. That the occurrence probability of dots in each region shows variation indicates that the difference in the number of dots between the regions is large. That is, the graphs of FIGS. 13 and 14 show the number of dots in each region when the dot pattern of x direction × y direction = 256 × 256 [pixel] shown in FIG. 12 is divided into 1 × 256 [pixel] regions. This difference is smaller than the difference in the number of dots in each area when divided into 256 × 1 [pixel] areas.

  13 and 14 exemplify the probability of occurrence of dots in each direction in a dot pattern with a 50% dot rate. However, dot patterns having a dot rate of 50% or more of the threshold matrix in this embodiment have the same tendency. Indicates. 13 and 14 exemplify a case where a 256 × 256 [pixel] dot pattern is divided into 1 × 256 [pixel] areas and a case where it is divided into 256 × 1 [pixel] areas. However, when the pixel area constituting the dot pattern is expressed by M × N [pixel], the dot pattern is divided into h × N [pixel] small sections, and the dot pattern is divided into M × h small sections. The same holds true for the case where h <M and h <N are satisfied. In other words, among dot patterns corresponding to each dot rate for generating a threshold matrix, a dot pattern of M × N [pixel] corresponding to at least a dot rate of 0.5 is represented by h × N [pixel]. The difference in the number of dots between the small sections when divided into small sections is smaller than the difference in the number of dots between the small sections when the dot pattern is partitioned into M × h small sections.

FIG. 15 shows a two-dimensional spatial frequency distribution in the dot pattern shown in FIG. The center o in FIG. 13 corresponds to the DC component, the u direction corresponds to the frequency component in the x direction in FIG. 12, and the v direction corresponds to the frequency component in the y direction in FIG.
As shown in FIG. 15, a dot pattern with a 50% dot rate by the threshold value matrix generated by the threshold value matrix generation method according to the first embodiment is a straight line along the u direction and passing through the center o in the spatial frequency distribution. The components in the overlapping peripheral area (the area surrounded by the dashed ellipse shown in FIG. 15) are suppressed as compared with the pattern in FIG. 27 (that is, the display is black). The component of v = 0 in the spatial frequency distribution represents the distribution of the average value of the dot occurrence rate in the y direction at each x value in the corresponding real space. It shows that the number of dots generated in one direction (Y direction) is almost uniform.
In this way, by eliminating the component of v = 0 in the rearrangement step, it is possible to make the dot generation probability in a predetermined one direction (y direction) of the dot pattern substantially equal. For this reason, when the dots are formed, the frequency of use of the recording elements can be made almost uniform among the recording elements, and the variation in the degree of consumption of the recording elements can be suppressed well, and the life of the print head H can be greatly prolonged. can do. In addition, since the occurrence probability of dots in the other direction (x direction) shows variation, it is possible to ensure the apparent dot dispersion in the dot pattern. That is, the dot generation probability in one direction (y direction) can be made substantially equal without substantially changing the appearance of the conventional dot pattern.

FIG. 16 shows a graph representing RAPSD (Radial Average Power Spectral Density) corresponding to the dot pattern shown in FIG. In FIG. 16, as a comparative example, a graph representing RAPSD corresponding to a dot pattern with a 50% dot rate generated by the conventional spatial filter method is indicated by a broken line.
As shown in FIG. 16, RA PSD corresponding to a dot pattern having a 50% dot rate by the threshold matrix generated by the threshold matrix generating method according to the first embodiment is a 50% dot rate generated by the conventional spatial filter method. The waveform and frequency peak positions are almost equivalent to RA PSD corresponding to the dot pattern. This indicates that the noise sensation of the dot pattern by the threshold value matrix generated by the threshold value matrix generation method according to the first embodiment is almost equivalent to the conventional one.
Also, from the two-dimensional spatial frequency distributions shown in FIGS. 15 and 27, the frequency distribution of the dot pattern by the threshold matrix generated by the threshold matrix generation method according to the first embodiment is a straight line along the u direction in the spatial frequency distribution. It is shown that it is almost the same as the prior art except for the peripheral area that overlaps the straight line passing through the center o.

As described above, according to the first embodiment, the matrix AVE (a, b) indicating the evaluation value based on the error matrix ERR (x, y, g) and the uniformity of the number of dots in each small section partitioned along the Y direction. ) Is determined based on the correction value based on (2), and the dot arrangement exchange is performed, so that a dot pattern having a substantially uniform dot arrangement is generated between the small sections provided along the Y direction. can do. For this reason, for each gradation value, a threshold value matrix Th (x, y) can be generated with a dot pattern having a substantially uniform dot arrangement among the small sections provided along the Y direction. That is, regardless of the gradation value g, the dot arrangement is almost uniform between the small sections provided along the Y direction. Thus, according to the first embodiment, it is possible to provide a threshold matrix that can suppress variation in the probability of occurrence of dots in a predetermined one direction (Y direction).
The dot pattern generated by the threshold value matrix generated by the threshold value matrix generation method of the first embodiment has a substantially uniform dot arrangement among the small sections provided along the Y direction. It is possible to solve the conventional problems that may cause a failure of the image forming apparatus due to the early consumption of the recording elements. In addition, it is possible to suppress deterioration in image formation quality such as uneven stripes, which has occurred due to the degree of consumption of the recording elements varying between the recording elements. In addition, by using the threshold value matrix generated by the threshold value matrix generation method of the first embodiment, even if the image forming apparatus is not an image forming apparatus provided with a circuit that shifts the image in the direction of the printing element array as in the technique of Patent Document 3, it is frequently used. It is possible to solve the conventional problems that may cause a failure of the image forming apparatus due to the early consumption of the recording elements. In addition, it is possible to suppress deterioration in image formation quality such as uneven stripes, which has occurred due to the degree of consumption of the recording elements varying between the recording elements. Furthermore, it is possible to provide a threshold value matrix that can eliminate the waste of time for the process of shifting the image and can greatly improve the processing performance of the image forming apparatus.

(Second embodiment)
Next, a second embodiment of the present invention will be described. The same components as those in the first embodiment are denoted by the same reference numerals and description thereof is omitted.
In the second embodiment, the coefficient α (α used in the calculation of ERR (x, y, g) + αAVE (a, b) in the process of determining the position where a new dot is to be placed and the position where the dot is to be deleted (step S35). > 0) is set to a different value depending on the gradation value g.

An example of the relationship between the gradation value g and the coefficient α in the second embodiment will be described with reference to FIGS. FIG. 17A is a graph showing an example of the relationship between the gradation value g and the coefficient α in the second embodiment. FIG. 17B is a table showing dot dispersibility evaluation results when g = 0.1, g = 0.3, and g = 0.5 when α is changed from 0.00 to 0.05.
As shown in FIG. 17A, in the second embodiment, the value of the coefficient α increases as the gradation value g is closer to g = 0.5, and the coefficient is closer to g = 0 or g = 1. The value of α becomes smaller. When the gradation value g = 0.5, the value of the coefficient α is maximized (for example, α = 0.05), and the correction value for equalizing the occurrence probability of dots in each small section at a dot rate of 50% is maximized. . On the other hand, when the gradation value g is 0 or 1, the value of the coefficient α is minimized (for example, α = 0), and the correction value for making the occurrence probability of dots in each small section uniform is minimized. The coefficient α is obtained by plotting the maximum value that can ensure the dispersibility of dots at each dot rate. There are two evaluation criteria: dot dispersibility and uniformity of usage frequency of each nozzle, and the sum of a predetermined ratio of the two evaluation criteria is used for the determination. In FIG. 17B, ◯ means that the dispersibility is sufficiently secured and the usage frequency of each nozzle is uniform, and x means that either or both of the evaluation criteria are not satisfied. The graph of FIG. 17A is derived by discretely changing α in such an evaluation at a dot rate from g = 0.1 to 0.9.

  The MSE (n) functioning as the dot dispersibility evaluation value functions as an evaluation value for evaluating how evenly the dot arrangement in the dot pattern subjected to the nth dot arrangement change is distributed. Then, ERR (x, y, g) used in determining two pixels for replacing the dot arrangement in the dot arrangement conversion functions as an index for performing an evaluation based on the presence of a set of dots or a gap in the dot pattern, and αAVE (a , B) function as an index for performing evaluation based on the uniformity of dots between the small sections.

  Here, if the value of the coefficient α is too large, the influence of the evaluation value by AVE (a, b), which is an evaluation based on the uniformity of dots between the small sections, becomes too large, and ERR (x, The influence of evaluation based on the presence or absence of dots in the dot pattern based on the value of y, g) is handled relatively small. In this case, a coefficient α that is too large will cause a set of dots or gaps in the resulting dot pattern. The degree of influence due to the magnitude of the coefficient α increases as the gradation value g approaches 0 or 1.

This is because both the “pixels that already have dots” and the “pixels that do not have dots” are required when changing the arrangement of dots.
For example, when the gradation value is g (for example, gradation value = 0.5), that is, when the number of dots is halved with respect to all pixels of the dot pattern, “pixels that already have dots” and “pixels that do not have dots” Are even. At this time, the degree of freedom in changing the arrangement of dots is maximized, and it becomes easy to ensure uniformity by changing the arrangement of dots.
On the other hand, in a dot pattern having a gradation value g (for example, gradation value g = 0 or gradation value g = 1) having a large difference between “a pixel already having dots” and “a pixel having no dots”, dot rearrangement is performed. The degree of freedom is low, and it is difficult to ensure uniformity compared to the case where “pixels with dots” and “pixels without dots” are equal. That is, the degree of freedom of dot rearrangement is affected by the number of dots included in the dot pattern.

  Therefore, in the second embodiment, as shown in FIG. 17, the coefficient α is increased in the gradation value g having a high degree of freedom of dot rearrangement and the gradation values in the vicinity thereof, so that the dots are uniformly distributed between the small sections. The influence of AVE (a, b), which is an evaluation value based on sex, is relatively increased. This eliminates dot collection and gaps in the dot pattern and realizes dot rearrangement that makes the dots uniform between the small sections. On the other hand, AVE (a, b), which is an evaluation value based on the uniformity of the dots between the small sections by reducing the coefficient α in the gradation value g having a low degree of freedom of dot rearrangement and the gradation values in the vicinity thereof. The influence of the is made relatively small. This makes it possible to reduce the possibility of dot collection and gaps in the dot pattern when priority is given to ensuring the uniformity of the dots between each sub-compartment, and to reduce uneven dot placement. To realize.

  As mentioned above, except the content described about 2nd embodiment, the structure and process content of 2nd embodiment are the same as that of 1st embodiment.

  According to the second embodiment, the value of the coefficient α is changed according to the value of the gradation value g, and the influence degree of the index for performing the evaluation based on the uniformity of the dots between the small sections in the dot rearrangement is determined. Adjust. Accordingly, a uniform dot pattern with reduced dot arrangement unevenness can be generated regardless of the gradation value g.

(Third embodiment)
Next, a third embodiment of the present invention will be described. The same components as those in the first embodiment are denoted by the same reference numerals and description thereof is omitted.

In the third embodiment, the pixel width m in the X direction of each small section is made different according to the value of the gradation value g.
FIG. 18 shows an example of the relationship between the gradation value g and the pixel width m in the X direction of each small section in the third embodiment.
As shown in FIG. 18, in the third embodiment, as the gradation value g is closer to g = 0.5, the pixel width m in the X direction of each small section becomes smaller, and g = 0 or g = 1. The closer it is, the larger the pixel width m in the X direction of each small section. When the gradation value g = 0.5, the pixel width m in the X direction of each small section is the minimum (for example, m = 1). On the other hand, when the gradation value g is 0 or 1, pixel width m in the X direction of each small section is maximum (e.g. m = 256).

  As described in the second embodiment, the set of dots and the gaps in the dot pattern are determined according to the value of the gradation value g, that is, the balance between “pixels that already have dots” and “pixels that do not have dots”. It is necessary to balance the rearrangement of the dots to be reduced and the uniformity of the dot arrangement between the small sections. Therefore, in the third embodiment, the ease of dot rearrangement between the small sections is adjusted by varying the pixel width m in the X direction of each small section in accordance with the gradation value g. The greater the pixel width m in the X direction of each small section, the greater the degree of freedom of dot arrangement within each small section. For this reason, the larger the pixel width m in the X direction of each small section, the easier it is to ensure the uniformity of dots between the small sections.

  As described above, according to the third embodiment, by varying the value of the pixel width m in the X direction of each small section in accordance with the gradation value g, the dot pattern in the dot pattern is independent of the value of the gradation value g. It is possible to realize a dot rearrangement that eliminates a set of dots and gaps and makes the dots between the small sections uniform.

  In addition, the pixel width m in the X direction of each small section is either 1 or a divisor of the pixel width in the X direction of the matrix pattern. This is because dot placement is performed in units of pixels, and each small section for ensuring uniformity of dot placement needs to have an integer pixel width. For this reason, for example, in the case of a 256 × 256 [pixel] matrix pattern, the possible values of the pixel width m in the X direction of each subdivision are 1, 2, 4, 8, 16, 32, 64, 128, or 256. Either.

  As described above, except for the contents described in the third embodiment, the configuration and processing contents of the third embodiment are the same as those in the first embodiment.

(Fourth embodiment)
Next, a fourth embodiment of the present invention will be described. The same components as those in the first embodiment are denoted by the same reference numerals and description thereof is omitted.
In the fourth embodiment, the coefficient α (α used in the calculation of ERR (x, y, g) + αAVE (a, b) in the process of determining the position where a dot is newly placed and the position where a dot is deleted (step S35). > 0) is set to a different value depending on the gradation value g. In the fourth embodiment, unlike the second embodiment, the gradation at which the coefficient α takes a relatively large value is a gradation that easily causes unevenness when a dot pattern is formed on a recording medium by the image forming apparatus. That is, the coefficient α varies depending on the possibility of the occurrence of uneven stripes in the image formed by the dot pattern.

FIG. 19 shows an example of the relationship between the gradation value g and the coefficient α in the fourth embodiment.
Ink jet printers such as line head printers generally form dots larger than the pixel size corresponding to the resolution. For example, when the resolution is 1200 × 1200 [dpi], the size of each pixel is about 20 × 29 [μm]. On the other hand, the dot size by the ink jet printer is φ = 33 to 36 [μm]. The ink jet printer takes a dot size larger than the pixel size with respect to the resolution of the image, so that the formed dots surely contain each pixel.

  Such a tendency of the dot size of the ink-jet printer affects the correlation between the above-described unevenness due to landing deviation and the tone value g of the dot pattern. The gradation value g that is likely to cause unevenness due to landing deviation when a dot pattern is formed differs depending on various factors such as the mounting condition of the recording element of each image forming apparatus. Here, ensuring the uniformity of the dot pattern between the small sections is caused by equalizing the frequency of use between the recording elements and increasing the frequency of use of some of the recording elements that cause landing deviation. There is an effect of reducing the uneven streaks. Therefore, in the fourth embodiment, priority is given to ensuring the uniformity of dots between the small sections by increasing the coefficient α when the gradation value g is likely to cause unevenness. FIG. 19 illustrates the coefficient α for threshold matrix generation processing corresponding to an image forming apparatus that is most likely to cause unevenness when the gradation value g = 0.4. The apparatus responds to a gradation value g that is likely to cause uneven stripes.

  As mentioned above, except the content described about 4th embodiment, the structure and process content of 4th embodiment are the same as that of 1st embodiment.

  According to the fourth embodiment, the index α is changed based on the uniformity of the dots between the small sections in the dot rearrangement by changing the value of the coefficient α in accordance with the gradation value g that is likely to cause unevenness. Adjust the degree of influence. This makes it possible to generate a dot pattern with high uniformity of dot arrangement between the small sections when generating the dot pattern corresponding to the gradation value g that is likely to cause unevenness, and to suitably suppress the occurrence of unevenness. it can.

  In order to suppress the occurrence of uneven stripes, the dot arrangement between the small sections is made different by changing the value of the pixel width m in the X direction of each small section in accordance with the gradation value g shown in the third embodiment. The intensity of uniformity may be adjusted. In other words, when the gradation value g is likely to cause uneven stripes, the value of the pixel width m in the X direction of each small section is reduced to form a dot pattern with high uniformity of dot arrangement between the small sections corresponding to each recording element. By generating, generation | occurrence | production of a stripe unevenness can be suppressed suitably.

(Quantizer)
Next, the quantization apparatus 200 that performs quantization using the threshold matrix generated by the threshold matrix generation apparatus and / or the threshold matrix generation method described above will be described. The quantizing device 200 is a quantizing device that is used when quantizing m-value (m ≧ 3) gradation data in each pixel constituting an original image into n values (an integer satisfying m> n). . The quantization device 200 records a recording material from the recording element when the recording medium is conveyed relative to the recording element in the conveying direction orthogonal to the recording element arrangement direction in which the recording elements are arranged. This is a quantizing device that quantizes an input image in an image forming apparatus that forms an image by being attached to a medium.
FIG. 20 is a block diagram showing the configuration of the quantization apparatus 200.
The quantization device 200 includes an input unit 201, a storage unit 202, a comparison unit 203, and an output unit 204.

The input unit 201 inputs image data to be subjected to quantization processing to the comparison unit 203.
The storage unit 202 stores a threshold matrix for forming a dot pattern as a quantized image. The threshold value matrix stored in the storage unit 202 is a threshold value matrix generated using the threshold value matrix generation method described above. For example, as the threshold value matrix stored in the storage unit 202, the threshold value matrix generated according to the first to fourth embodiments described above can be used.
The comparison unit 203 compares the pixel value of the image data input via the input unit 201 and the threshold value matrix threshold value stored in the storage unit 202.
The output unit 204 generates and outputs quantized image data based on the comparison result of the comparison unit.

  According to the quantization apparatus 200, it is possible to generate a dot pattern having a substantially uniform dot arrangement among the small sections provided along the y direction. For this reason, it is possible to solve a conventional problem that may cause a failure of the image forming apparatus due to early consumption of some frequently used recording elements. In addition, it is possible to suppress deterioration in image formation quality such as uneven stripes, which has occurred due to the degree of consumption of the recording elements varying between the recording elements. Furthermore, time is not wasted for the process of shifting the image, and the processing performance of the image forming apparatus can be greatly improved.

(Image forming device)
Next, an image forming apparatus 300 having the above-described quantization apparatus 200 will be described. The image forming apparatus 300 uses, as input image data, m (m ≧ 3) gradation data in each pixel constituting the original image, and has n values (an integer satisfying m> n) based on the input image data. An image forming apparatus that generates output image data having gradation and forms an image on a recording medium based on the output image data. The output image data of the image forming apparatus 300 of this embodiment is quantized image data output from the quantization apparatus 200.
FIG. 21 shows the configuration of the image forming apparatus 300.
As illustrated in FIG. 21, the image forming apparatus 300 includes a quantization device 200, an image forming unit 301, a transport unit 302, and an operation control unit 303.
The image forming unit 301 forms an image on a recording medium based on the quantized image data output from the quantizing device 200. The image forming unit 301 includes a print head H of a line head printer as shown in FIG. 28, for example.
The conveyance unit 302 has a conveyance belt B as shown in FIG. 28, for example, and a recording medium is arranged in a conveyance direction orthogonal to the recording element arrangement direction in which the recording elements of the print head H included in the image forming unit 301 are arranged. The sheet is conveyed relative to the recording element.
The operation control unit 303 controls the operation of each unit of the image forming apparatus 300.
The image forming apparatus 300 forms an image by attaching a recording material from a recording element of the print head H of the image forming unit 301 onto a recording medium. In the quantized image data output by the quantization device 200, a dot pattern of x direction (printing element array direction) × y direction (transport direction) = M × N [pixel] corresponding to at least a dot rate of 0.5. Is the difference in the number of dots between each small section when the dot pattern is partitioned into h × N [pixel] small sections, and the dot between the small sections when the dot pattern is partitioned into M × h small sections. Small compared to the number difference.

  According to the image forming apparatus 300 of the present embodiment, it is possible to perform image formation with a dot pattern having a substantially uniform dot arrangement among the small sections provided along the y direction. For this reason, it is possible to solve a conventional problem that may cause a failure of the image forming apparatus due to early consumption of some frequently used recording elements. In addition, it is possible to suppress deterioration in image formation quality such as uneven stripes, which has occurred due to the degree of consumption of the recording elements varying between the recording elements. Furthermore, time is not wasted for the process of shifting the image, and the processing performance of the image forming apparatus can be greatly improved.

  The embodiments of the present invention should be considered that the embodiments disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

For example, in the threshold matrix generation process, it is not always necessary to stack dot patterns from g = 0 to g = 1. For example, if the output limit in the quantizer is g = Q (Q <1) based on the physical restrictions of the image forming apparatus that forms the dot pattern, the threshold stored in the memory is also 0 <g ≦ Q. Sufficient processing can be performed with only the threshold value composed of the numerical values up to.
In this case, when g = 0 to g = Q are created for η gradations, it is sufficient to create a threshold matrix by stacking dot patterns from g = 0 to g = Q−1 / η.
For example, in the threshold value matrix generation device of this embodiment, the tone value g = 0.5, and each dot pattern corresponding to the tone value of 0 to 1 is added or subtracted by the tone value change amount δg. Although generated, the gradation value of each generated dot pattern can be arbitrarily set in a range from 0 to a predetermined value Q (0.5 ≦ Q ≦ 1).

  In the description of each embodiment described above, a set of dots in the dot pattern is obtained by multiplying AVE (a, b), which is an evaluation value based on the uniformity of dots between the small sections, by a predetermined coefficient α. And the elimination of the gap and the ensuring of the uniformity of the dots between the small sections, but ERR (x, y, g) may be multiplied by a coefficient to balance the collection of dots and gaps in the dot pattern in the dot arrangement change and ensuring the uniformity of the dots between the small sections. A coefficient may be applied to both ERR (x, y, g) and AVE (a, b). In other words, it is sufficient that a balance between elimination of a set of dots and gaps in the dot pattern and securing of uniformity of dots between the small sections can be achieved by a coefficient for at least one.

In each embodiment in which the coefficient α is fixed, a value other than α = 0.05 may be taken.
FIG. 22 shows the dot generation probability in a predetermined direction (y direction) of a dot pattern having a 50% dot rate when the coefficient α = 0.01 along another direction (x direction) orthogonal to the predetermined direction. It is the graph shown.
As shown in FIG. 22, the present invention is not limited to the case of α = 0.05, and the present invention has an effect of increasing the uniformity of dots between the small sections. Even if the coefficient α is not correlated with the gradation value g or the occurrence of uneven stripes, by appropriately adjusting the value of the coefficient α, a set of dots and gaps in the dot pattern can be obtained. It is possible to balance the elimination and ensuring the uniformity of the dots between the small sections.

In each of the above-described embodiments, the threshold matrix generation processing is performed by software processing by a computer, but may be performed by a dedicated device.
Further, a computer that performs threshold matrix generation processing can also perform processing other than threshold matrix generation processing. For example, a threshold matrix may be generated by a general-purpose computer such as a personal computer, or threshold matrix generation processing may be performed as one processing performed by a CPU or the like provided in a device such as an MFP (Multifunction Peripheral). .
Each configuration of the quantization device 200 may be realized by software processing by a computer.

  Moreover, you may use other than MSE for a dot dispersibility evaluation value. For example, by multiplying RAPSD by a visual sensitivity curve and calculating a noise value integrated by frequency, the dot arrangement exchange is terminated when the noise value satisfies a predetermined condition (for example, less than a predetermined value). Also good.

11 CPU
12 RAM
13 ROM
14 Storage devices

Claims (14)

The recording element to the recording element array direction arranged recording medium immediately intersects the transport direction, when the relative conveyance to the recording element, to attach the recording material onto the recording medium from the recording element A threshold value matrix generation method for generating a threshold value matrix used for quantizing an input image in an image forming apparatus that forms an image by:
Based on a first dot pattern having a predetermined number of dots in a pixel area of a predetermined size, the dot rate indicating the ratio of the number of pixels in which dots are formed with respect to all the pixels included in the pixel area is from 0 to 1 A new dot pattern generation step of generating a second dot pattern in which the number of dots of the first dot pattern is increased or decreased within the range of
A rearrangement step of obtaining a third dot pattern in which dots included in the second dot pattern are rearranged;
And repeating the new dot pattern generation step and the rearrangement step with the third dot pattern as the first dot pattern until a dot pattern with a desired number of dots is obtained, and
The rearrangement step includes
A dot pattern evaluation value based on a difference between the second dot pattern and a dot pattern obtained by applying a predetermined filter process to the second dot pattern to realize a desired spatial frequency is expressed by the following formula ( A dot pattern evaluation value calculating step for calculating the error matrix ERR (x, y, g) represented by 1) corresponding to each position in the threshold value matrix;
For a plurality of small sections obtained by dividing the pixel area of the predetermined size along the transport direction, dots formed for each of the small sections have a uniformity evaluation value indicating the uniformity of the number of dots in each small section. A matrix AVE (a, b) representing the excess / deficiency of the number of dots per sub-compartment with respect to the number of dots per sub-compartment to be generated when all are uniform (where a is x in the X direction of the sub-compartment) A uniformity evaluation value calculating step of calculating based on an integer value of a quotient when dividing by the number of pixels of b, and b corresponding to an integer value of a quotient when y is divided by the number of pixels in the Y direction of the small section ,
In rearrangement of dot and not dot is formed, and, by adding the AVE (a, b) the value obtained by multiplying a predetermined coefficient to the ERR (x, y, g) with respect to A pixel having a minimum value is determined as a position where a new dot is to be arranged, and a value obtained by multiplying the AVE (a, b) by a predetermined coefficient when the dot is formed and the ERR (x, a dot rearrangement position determination step of determining a pixel having a maximum value added to y, g) as a position to delete dots;
Rearrangement that rearranges dots in the third dot pattern by arranging dots at positions where new dots are to be arranged determined by the dot rearrangement position determination step and deleting dots from positions where dots are deleted A pattern generation process;
A dot dispersibility evaluation value calculating step of calculating a dot dispersibility evaluation value for the rearranged third dot pattern based on an evaluation value MSE (n) represented by the following mathematical formula (2) ;
Based on the dot dispersibility evaluation value of the rearranged third dot pattern, when the MSE (n) after the rearrangement is smaller than the MSE (n−1) before the rearrangement, the dot pattern evaluation value The calculation step, the uniformity evaluation value calculation step, the dot rearrangement position determination step, the rearrangement pattern generation step, and the dot dispersibility evaluation value calculation step are repeated, and the MSE (n) after rearrangement is before the rearrangement. If it is greater than or equal to MSE (n-1), a rearrangement repetition determination step for determining whether or not to repeat without repeating,
If the is determined repeatedly in relocation repetition determining step, and the rearranged third dot pattern step of said second dot pattern,
A threshold value matrix generation method characterized by comprising:
Here, g is a gradation value, x is a pixel coordinate in the recording element array direction, y is a pixel coordinate in the recording medium conveyance direction, and p ′ (x, y, g) is the second dot pattern. It is a real space pattern calculated by applying inverse Fourier transform to a pattern subjected to a predetermined filter process.
Here, n is the number of repetitions of dot rearrangement, and p ′ (x, y, g) in ERR (x, y, g) is a pattern obtained by applying a predetermined filter process to the third dot pattern. It is a real space pattern calculated by applying an inverse Fourier transform. 2. The threshold matrix according to claim 1 , wherein in the calculation of MSE (n), the spatial filter applied to the third dot pattern is a spatial frequency filter that passes at least a low-frequency component. Generation method. In the calculation of MSE (n), the spatial filter applied to the third dot pattern is a spatial frequency filter that passes a low-frequency component, and the spatial frequency filter is applied to the third dot pattern. The threshold value matrix generating method according to claim 1, wherein the threshold value matrix is integrated for all frequency components . 4. The threshold value matrix generation method according to claim 1, wherein the predetermined coefficient differs depending on the number of dots included in the dot pattern. 5. 5. The threshold value matrix generation method according to claim 4, wherein the predetermined coefficient is maximized when the number of dots included in the dot pattern is half of the number of pixels of the dot pattern. The predetermined coefficient is set so as to increase as the possibility of the occurrence of stripe unevenness in the image formed by the dot pattern increases, and varies depending on the possibility of the occurrence of stripe unevenness. The threshold value matrix production | generation method as described in any one of Claim 1 to 3. The pixel width along the other direction orthogonal to the one direction of the small section is set so as to increase as the dot rate of the dot pattern is 0.5 and as close to 0 or 1 as possible. The threshold value matrix generation method according to claim 1, wherein the threshold value matrix generation method varies depending on a rate. The pixel width along another direction orthogonal to the one direction of the small section is minimum when the number of dots included in the dot pattern is half of the number of pixels of the dot pattern. 8. The threshold value matrix generation method according to 7. The pixel width along the other direction orthogonal to the one direction of the small section is set so as to be smaller as the possibility of the occurrence of the stripe unevenness in the image formed by the dot pattern is increased, and the occurrence of the stripe unevenness is possible. The threshold value matrix generation method according to claim 1, wherein the threshold value matrix generation method varies depending on the magnitude of the sex. The pixel width along the other direction orthogonal to the one direction of the small section is 1 or a divisor of the width of the dot pattern in the other direction, according to any one of claims 1 to 9. Threshold matrix generation method. When a recording medium is transported relative to the recording element in a transport direction orthogonal to the recording element array direction in which the recording elements are arrayed, a recording material is attached to the recording medium from the recording element. A threshold value matrix generating device for generating a threshold value matrix used for quantizing an input image in an image forming device that forms an image by:
Based on a first dot pattern having a predetermined number of dots in a pixel area of a predetermined size, the dot rate indicating the ratio of the number of pixels in which dots are formed with respect to all the pixels included in the pixel area is from 0 to 1 Generating a second dot pattern in which the number of dots of the first dot pattern is increased or decreased within the range, obtaining a third dot pattern in which the dots included in the second dot pattern are rearranged, A controller that repeats the generation of the second dot pattern and the process of obtaining the third dot pattern using the third dot pattern as the first dot pattern until a dot pattern having a desired number of dots is obtained;
In the process of obtaining the third dot pattern, the control unit includes the second dot pattern, a dot pattern obtained by performing a predetermined filter process on the second dot pattern to achieve a desired spatial frequency, The dot pattern evaluation value based on the difference between the two is calculated by associating the error matrix ERR (x, y.g) represented by the following formula (1) with each position in the threshold matrix, When the dots formed for each of the small sections are even for a plurality of small sections obtained by dividing the pixel area along the transport direction, the uniformity evaluation value indicating the uniformity of the number of dots in each of the small sections Matrix AVE (a, b) representing the excess or deficiency of the number of dots for each small section with respect to the number of dots for each small section to be generated in (where a is x divided by the number of pixels in the X direction of the small section When An integer value of the quotient, b is calculated based on the integer value of the quotient when y is divided by the number of pixels in the Y direction of the small section), and dots are not formed in the rearrangement of dots, and Then, a pixel obtained by adding a value obtained by multiplying the AVE (a, b) by a predetermined coefficient to the ERR (x, y, g) is determined as a position where a new dot is to be arranged. A pixel having a maximum value obtained by adding a value obtained by multiplying the AVE (a, b) by a predetermined coefficient to the ERR (x, y, g) in which a dot is formed. Is determined at the position to delete, the dot is arranged at the determined position to newly arrange the dot, the dot is deleted from the position to delete the dot, the dots of the third dot pattern are rearranged, Dosage for the arranged third dot pattern The dispersibility evaluation value is calculated based on the evaluation value MSE (n) represented by the following formula (2), and after the rearrangement based on the dot dispersibility evaluation value of the rearranged third dot pattern If the MSE (n) of the image is smaller than the MSE (n−1) before the rearrangement, the dot pattern evaluation value is calculated, the uniformity evaluation value is calculated, and the position and dot at which the dot is newly arranged are deleted. When position determination, dot rearrangement of the third dot pattern, and calculation of the dot dispersibility evaluation value are repeated, and MSE (n) after rearrangement is greater than or equal to MSE (n-1) before rearrangement Determines whether or not to repeat without repeating, and if it is determined to repeat, the rearranged third dot pattern is used as the second dot pattern. apparatus.
Here, g is a gradation value, x is a pixel coordinate in the recording element array direction, y is a pixel coordinate in the recording medium conveyance direction, and p ′ (x, y, g) is the second dot pattern. It is a real space pattern calculated by applying inverse Fourier transform to a pattern subjected to a predetermined filter process.
Here, n is the number of repetitions of dot rearrangement, and p ′ (x, y, g) in ERR (x, y, g) is a pattern obtained by applying a predetermined filter process to the third dot pattern. It is a real space pattern calculated by applying an inverse Fourier transform. When a recording medium is transported relative to the recording element in a transport direction orthogonal to the recording element array direction in which the recording elements are arrayed, a recording material is attached to the recording medium from the recording element. A threshold value matrix used to quantize an input image in an image forming apparatus that forms an image,
The threshold matrix is a threshold matrix generated by adding a plurality of dot patterns for each pixel constituting each dot pattern,
  The plurality of dot patterns is a number corresponding to a dot rate indicating a ratio of the number of pixels in which dots are formed with respect to all pixels included in the pixel region in a predetermined pixel region each composed of a plurality of pixels. When the dot pattern is a dot pattern and the dot ratio is larger than the initial dot pattern having a predetermined dot ratio, additional dots are added without changing the dot arrangement in the initial dot pattern. When the dot rate is made smaller than the dot pattern, the pixel arrangement in which the dots in the initial dot pattern are not formed is not changed, and the dot is deleted, and the predetermined value is set from 0 created under the condition that the number of pixels in which dots are not formed is increased. Each dot pattern corresponding to each dot rate up to a value of
The predetermined pixel region is a pixel region having a plurality of pixels arranged along the recording element array direction and the transport direction,
A dot pattern corresponding to at least a dot rate of 0.5 among the respective dot patterns has a predetermined pixel width that is smaller than the total number of pixels along the recording element arrangement direction in the pixel area of the dot pattern. And the difference in the number of dots included in each of the first subsections when divided into a plurality of first subsections having the entire pixel width along the transport direction of the pixel area, A plurality of dot patterns having a total pixel width along the recording element arrangement direction of the pixel area of the dot pattern and a predetermined pixel width smaller than the total number of pixels along the transport direction of the pixel area The threshold value matrix characterized by being small compared with the difference of the number of dots contained in each small division when it divides | segments into this 2nd small division. When a recording medium is transported relative to the recording element in a transport direction orthogonal to the recording element array direction in which the recording elements are arrayed, a recording material is attached to the recording medium from the recording element. A quantization apparatus that quantizes an input image in an image forming apparatus that forms an image,
A storage unit for storing a threshold matrix;
A comparison unit that compares a pixel value of the input image data with a threshold value of a threshold value matrix stored in the storage unit;
An output unit that generates and outputs the output image data based on a comparison result of the comparison unit;
The threshold matrix is a threshold matrix generated by adding a plurality of dot patterns for each pixel constituting each dot pattern,
  The plurality of dot patterns is a number corresponding to a dot rate indicating a ratio of the number of pixels in which dots are formed with respect to all pixels included in the pixel region in a predetermined pixel region each composed of a plurality of pixels. When the dot pattern is a dot pattern and the dot ratio is larger than the initial dot pattern having a predetermined dot ratio, additional dots are added without changing the dot arrangement in the initial dot pattern. When the dot rate is made smaller than the dot pattern, the pixel arrangement in which the dots in the initial dot pattern are not formed is not changed, and the dot is deleted, and the predetermined value is set from 0 created under the condition that the number of pixels in which dots are not formed is increased. Each dot pattern corresponding to each dot rate up to a value of
The predetermined pixel region is a pixel region having a plurality of pixels arranged along the recording element array direction and the transport direction,
A dot pattern corresponding to at least a dot rate of 0.5 among the respective dot patterns has a predetermined pixel width that is smaller than the total number of pixels along the recording element arrangement direction in the pixel area of the dot pattern. And the difference in the number of dots included in each of the first subsections when divided into a plurality of first subsections having the entire pixel width along the transport direction of the pixel area, A plurality of dot patterns having a total pixel width along the recording element arrangement direction of the pixel area of the dot pattern and a predetermined pixel width smaller than the total number of pixels along the transport direction of the pixel area A quantization apparatus characterized by being smaller than the difference in the number of dots contained in each small section when divided into the second small sections. When a recording medium is transported relative to the recording element in a transport direction orthogonal to the recording element array direction in which the recording elements are arrayed, a recording material is attached to the recording medium from the recording element. An image forming apparatus for forming an image by
A storage unit for storing a threshold matrix;
A comparison unit that compares a pixel value of the input image data with a threshold value of a threshold value matrix stored in the storage unit;
A quantizing device that quantizes an input image having an output unit that generates and outputs the output image data based on a comparison result of the comparing unit;
An image forming unit that forms an image on a recording medium based on output image data output by the quantization device;
The threshold matrix is a threshold matrix generated by adding a plurality of dot patterns for each pixel constituting each dot pattern,
The plurality of dot patterns is a number corresponding to a dot rate indicating a ratio of the number of pixels in which dots are formed with respect to all pixels included in the pixel region in a predetermined pixel region each composed of a plurality of pixels. When the dot pattern is a dot pattern and the dot ratio is larger than the initial dot pattern having a predetermined dot ratio, additional dots are added without changing the dot arrangement in the initial dot pattern. When the dot rate is made smaller than the dot pattern, the pixel arrangement in which the dots in the initial dot pattern are not formed is not changed, and the dot is deleted, and the predetermined value is set from 0 created under the condition that the number of pixels in which dots are not formed is increased. Each dot pattern corresponding to each dot rate up to a value of
The predetermined pixel region is a pixel region having a plurality of pixels arranged along the recording element array direction and the transport direction,
A dot pattern corresponding to at least a dot rate of 0.5 among the respective dot patterns has a predetermined pixel width that is smaller than the total number of pixels along the recording element arrangement direction in the pixel area of the dot pattern. And the difference in the number of dots included in each of the first subsections when divided into a plurality of first subsections having the entire pixel width along the transport direction of the pixel area, A plurality of dot patterns having a total pixel width along the recording element arrangement direction of the pixel area of the dot pattern and a predetermined pixel width smaller than the total number of pixels along the transport direction of the pixel area An image forming apparatus characterized by being smaller than the difference in the number of dots contained in each of the small sections when divided into the second small sections.