JP2020015303A - Image processing device, method for controlling the same and program - Google Patents

Abstract

To suppress deviation of the number of discharge dots for each recording element.SOLUTION: An image processing device that includes a recording head in which a plurality of recording elements are arrayed in a first direction, and generates image data forming an image on a recording medium conveyed in a second direction vertical to the first direction includes: an acquisition part which acquires multi-value input image data; a correction processing part which subjects the input image data to concentration unevenness correction processing according to characteristics for each recording element; and a conversion part which converts input image data subjected to concentration unevenness correction processing by the correction processing part using a dither matrix to half-tone image data indicating presence/absence of dots. The dither matrix is a dispersion type dither matrix, and has characteristics such that in a gradation that is an object of the concentration unevenness correction processing among gradations that can be taken by the input image data, the numbers of dots included in a predetermined range in a second direction at each position in a first direction are the same at the time of conversion from the input image data of uniform gradation into binary half-tone image data.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a technique for converting a multi-tone image into an image having a lower number of tones than the image.

  2. Description of the Related Art Conventionally, an ink jet recording apparatus has been known as an output apparatus of a computer. The ink jet recording apparatus has a recording head in which a plurality of ink ejection ports (nozzles) are arranged, and the recording head is relatively moved with respect to a recording medium to eject ink droplets (dots) from the nozzles. A desired image is formed on a recording medium.

  In particular, inkjet printers for commercial printing often require a single-pass drawing method (also called a full-line method or a full multi-method) because of the demand for print productivity. In the single-pass drawing method, drawing is performed in a recording medium width direction (hereinafter, referred to as a “recording medium width direction” or “y direction”) perpendicular to a recording medium conveyance direction (hereinafter, referred to as “conveying direction” or “x direction”). A configuration is adopted in which a long line head having a nozzle array covering the entire area is used, and the line head extends in a direction perpendicular to the recording medium conveyance direction. Since the recording medium needs to be moved only once with respect to the line head, the single-pass drawing method has a feature that the printing speed is faster than the multi-pass method in which an image is completed by a plurality of scans.

  When printing is performed by such a printer, image processing in which the tone value of the original image data to be printed is represented by on / off of dots, that is, halftone processing is performed. Then, the printer executes a printing process according to the image data after the halftone process.

  Various methods have been proposed for halftone processing, and a dither method is one of the typical methods. The dither method uses a dither matrix of a predetermined size in which different threshold values are arranged. Specifically, this dither matrix is repeatedly developed in tiles on the image data, and the magnitude of the tone value (pixel value) of the input image is compared with the corresponding threshold value. Then, if the gradation value is larger than the threshold value, the dot is turned on, and if the gradation value is equal to or less than the threshold value, the dot is turned off to perform the gradation expression.

  In such a dither method, in the dot pattern generated by the dither matrix, the number of ejection dots (ejection frequency) for each nozzle may be biased, and the life of the nozzle that frequently ejects ink is shortened. There is a problem that the life of the device itself is shortened.

  Patent Document 1 proposes, as an image recording apparatus that solves this problem, a method in which a dither matrix is developed by shifting in the width direction of a recording medium, instead of being developed in a tile shape as in the related art. Accordingly, it is possible to suppress a deviation in the number of ejection dots for each nozzle.

JP 2005-161817 A

  However, in the case of using a so-called dot dispersion type dither matrix in which the spatial frequency distribution is biased to a certain frequency or more that is difficult for humans to perceive to achieve high image quality, the method of Patent Document 1 requires a sufficient amount of ejection dots for each nozzle. In some cases, it is not possible to suppress the number bias.

  Examples of the dot dispersion type dither matrix include a blue noise dither matrix having blue noise characteristics and a green noise dither matrix having green noise characteristics. The blue noise dither matrix is a dither matrix in which storage positions of thresholds are adjusted so that dots are dispersed in consideration of human visual characteristics. As a result, the spatial frequency of the dot arrangement is biased toward a high frequency, and the granularity can be suppressed. The green noise dither matrix is a dither matrix in which the storage positions of the threshold values are adjusted so that the clusters of dots are dispersed as a whole while dots are formed adjacently in units of several dots. In a printer in which it is difficult to stably form fine dots of about one pixel, the occurrence of isolated dots can be suppressed by judging the presence or absence of dot formation with reference to such a green noise matrix.

  In such a dot dispersion type dither matrix, a dither matrix having a relatively large size is used. For example, when a dither matrix having a size of 256 pixels in the transport direction of the recording medium and 256 pixels in a direction orthogonal to the transport direction of the recording medium is used, in order to completely equalize the nozzle use rate, 65536 pixels in the transport direction are required. (Developing a matrix having a width of 256 pixels in the transport direction by shifting it 256 times in the direction of the recording medium). On the other hand, when the image object of the original image data to be drawn is, for example, 20 mm (1200 dpi) in the transport direction, the size of the image object is about 945 pixels. Therefore, a dot is generated at a threshold value of a part of the developed dither matrix, and it is not possible to suppress the bias of the nozzle usage rate. That is, when the image object of the original image data is relatively small with respect to the dither matrix expanded and shifted in the width direction of the recording medium, there is a problem that the number of ejection dots for each nozzle cannot be made sufficiently uniform. Was.

  The present invention has been made in view of the above problems, and has as its object to provide a technique for suppressing a deviation in the number of ejection dots for each recording element such as a nozzle in halftone processing using a dither matrix.

In order to solve this problem, for example, an image processing apparatus of the present invention has the following configuration. That is,
A recording head having a plurality of recording elements arranged in a first direction, the recording head being directed toward a recording medium conveyed relatively along a second direction perpendicular to the first direction; An image processing apparatus that generates image data for an image forming apparatus that forms an image on the recording medium by discharging ink from a recording element that has
Acquiring means for acquiring multi-valued input image data;
Correction processing means for executing, on the input image data, density unevenness correction processing in accordance with the characteristics of each of the recording elements;
Using a dither matrix, the input image data that has been subjected to density unevenness correction processing by the correction processing means, converting means for converting to halftone image data indicating the presence or absence of dots,
The dither matrix is
A distributed dither matrix,
The first direction when converting uniform input image data into binary halftone image data from grayscale to be subjected to the density unevenness correction processing among grayscales that can be obtained by the input image data. The characteristic is that the number of dots included in the predetermined range in the second direction at each position is the same.

  According to the present invention, it is possible to suppress the deviation of the number of ejection dots for each recording element such as a nozzle.

FIG. 1 is a block diagram illustrating a configuration of an image processing apparatus and an image forming apparatus according to a first embodiment. FIG. 2 is a block diagram illustrating a halftone processing unit according to the first embodiment. FIG. 2 is a diagram illustrating a recording head according to the first embodiment. FIG. 3 is a diagram illustrating a halftone process according to the first embodiment. 9 is a flowchart illustrating a procedure for creating dither metrics applied to the first embodiment. 9 is a flowchart illustrating a procedure of an initial pattern in creating dither metrics. FIG. 4 is a diagram illustrating characteristics of a dot pattern generated by dithering applied to the first embodiment. FIG. 9 is a block diagram illustrating a configuration of an image processing apparatus and an image forming apparatus according to a second embodiment. FIG. 8 is a diagram illustrating an example of a test chart for density measurement according to the second embodiment. 9 is a flowchart illustrating a procedure for creating density unevenness correction parameters according to the second embodiment.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. Note that the configurations in the embodiments described below are merely examples, and the present invention is not limited to the illustrated configurations.

[First Embodiment]
FIG. 1 is a block diagram illustrating a configuration of an image processing apparatus and an image forming apparatus applicable to the first embodiment. In FIG. 1, an image processing apparatus 10 and an image forming apparatus 20 are connected by an interface or a circuit. The image processing apparatus 10 is, for example, a general personal computer in which a printer driver is installed. In that case, each unit in the image processing apparatus 10 described below is realized by a computer executing a predetermined program. However, the image forming apparatus 20 may include the image processing apparatus 10.

  The image processing apparatus 10 stores the output target image data input from the image data input unit 101 in the input image buffer 102. Although the format of the image data input to the image data input unit 101 is not particularly limited, here, for the sake of simplicity, the same color type, number of colors, and resolution as the ink colors used in the image forming apparatus 20 are used. It is assumed that the image is a gradation image. For example, when the image forming apparatus 20 is an inkjet printing system that achieves an output resolution of 1200 dpi using four color inks of cyan (C), magenta (M), yellow (Y), and black (K), the image data is CMYK. Is multi-valued image data having 8 bits (256 gradations) for each color. Note that there are various types of image data formats of the image to be printed. When printing image data specified in the form of a combination or resolution different from the type and resolution of the ink colors used in the image forming apparatus 20, a pre-processing unit (not shown) After performing processing such as color conversion and resolution conversion, and converting the image data into image data of the ink color and resolution used in the stamp image forming apparatus 20, the image data may be input from the image data input unit 101.

  The halftone processing unit 103 uses a dither matrix to convert the image data stored in the input image buffer 102 into the number of tones that can be directly expressed by the image forming apparatus 20 (halftone processing) and a nozzle group. Is determined, and halftone image data is created. In the dither matrix, threshold groups are arranged according to the number of gradations that can be represented in a pseudo manner. The number of tones that can be represented in a pseudo manner is determined by the number of values used as thresholds that exist in the dither matrix. A detailed method of generating the dither matrix will be described later. The halftone processing unit 103 outputs the created halftone image data to the halftone image buffer 104. The stored halftone image data is output from the output terminal 105 to the image forming apparatus 20.

  The image forming apparatus 20 moves the recording medium such as recording paper relative to the recording head 203 based on the halftone image data received from the image processing apparatus 10 via the input terminal 201, and An image is formed on a recording medium by generating dots thereon. Here, the recording head 203 is of an inkjet recording type, and includes a recording element array in which a plurality of recording elements capable of discharging ink are arranged. FIG. 3 is a diagram illustrating a configuration example of the recording head 203. Note that the recording head typically has nozzles for four types of ink, cyan (C), magenta (M), yellow (Y), and black (K). Only one is shown. The nozzle array is a long line head that covers the entire range of the drawing area in the recording medium width direction (x direction) perpendicular to the recording medium conveyance direction (y direction). Is ejected to form a print image. The head driving unit 202 generates a driving signal for controlling the recording head 203 based on the halftone image data. The recording head 203 actually records each ink dot on the recording medium based on the drive signal. It is assumed that the recording elements of the cyan (C), magenta (M), and yellow (Y) recording heads are also arranged in parallel with the illustrated and black heads.

  Hereinafter, the halftone processing unit 103 in the present embodiment will be described in detail.

  FIG. 2 is a block diagram showing a detailed configuration of the halftone processing unit 103. The halftone processing unit 103 converts the image data stored in the input image buffer 102 into binary (1 bit) data for each nozzle group and outputs the data. For this processing, the halftone processing unit 103 has a comparator 1031, a dither matrix 1032 (comprising a ROM or the like), and an output data generator 1033.

  Here, the input image data stored in the input buffer 102 is defined as Iin, and the number of pixels in the x direction (here, the conveyance direction of the recording medium) is defined as W and y directions (with respect to the conveyance direction of the recording medium). The number of pixels in the orthogonal direction (hereinafter simply referred to as the width direction) is assumed to be H. The coordinates (x, y) of the input image Iin (0 ≦ x ≦ W−1, 0 ≦ y ≦ H−1) Is represented as Iin (x, y). The dither matrix 1032 has Sx rows (size in the x direction) and Sy columns (size in the y direction), and coordinates in the dither matrix. The threshold value of (i, j) is represented as M (i, j), and the output image data (binary image data) generated by the output data generator 1033 is defined as Iout, and the coordinates (x, y) of Let the output value be Iout (x, y), and replace the integer "a" with the integer "b". In the remainder upon dividing expressed as "a% b".

  The comparator 1031 determines the magnitude of the pixel value Iin (x, y) of the coordinates (x, y) of the input image data Iin from the input image buffer 102 and the threshold M (x% Sx, y% Sy) of the dither matrix. And outputs the determination result (comparison result) to the output data generator 1033. When Iin (x, y) is larger than threshold value M (x% Sx, y% Sy), output data generator 1033 generates “BLACK” indicating that a black dot is to be formed as output value Iout (x, y). I do. When Iin (x, y) is equal to or smaller than the threshold value M (x% Sx, y% Sy), the output data generator 1033 outputs “WHITE” indicating that no dot is to be output to the output value Iout (x, y). ). Note that in the following description, whether or not a dot is hit is also expressed as "turn on / off a dot".

The processing of the halftone processing unit 103 is summarized and shown as follows.
Iout (x, y) = BLACK if Iin (x, y)> M (x% Sx, y% Sy)
Iout (x, y) = WHITE if Iin (x, y) ≤ M (x% Sx, y% Sy)
Actually, the output data generator 1033 generates a binary image in which the output value “BLACK” is, for example, 1-bit “1” and “WHITE” is “0”.

  Here, an example of the above halftone processing will be described with reference to FIG. Reference numeral 401 in FIG. 4 indicates a part of the input image data, reference numeral 402 indicates a part of the dither matrix M, and reference numeral 403 indicates a part of the output data. In the input image data 401, one pixel is represented by, for example, 8 bits, and the possible range of each pixel is a value from 0 to 255.

  The dither matrix 402 has a total of 65,536 thresholds of 256 in the x direction and 256 in the y direction, and these thresholds are assumed to be thresholds uniformly selected from the range of values 0 to 255. .

  Here, if the pixel value of the input image data exceeds the corresponding threshold, BLACK (stroke a black dot) is determined as the output value, and if the pixel value is less than the corresponding threshold, WHITE (do not strike a dot) ) Is determined as the output value. Therefore, when the illustrated dither matrix 402 is used, a dot arrangement of 256 gradations can be obtained. That is, when halftone processing is performed using the dither matrix 402, 256 levels of gradation from 0 to 255 can be simulated as output data in a range of 256 pixels × 256 pixels. In the figure, the input image 401 is an image having a single pixel value “33”, and when a dither matrix 402 is used, the illustrated output data 403 is obtained.

  As can be seen from the above, it can be said that at a pixel position where the threshold value of the dither matrix is small, the probability of turning on the dot is high, and conversely, at a pixel position where the threshold value is high, the probability of turning off the dot is high. Therefore, when a relatively small threshold value of the dither matrix is biased toward a specific nozzle row, the nozzle frequently ejects ink, and the life of the nozzle is shortened.

  In order to make the recording head usable as long as possible, it is preferable to suppress the occurrence of nozzles having an extremely large number of ink ejections by eliminating and uniforming the number of dots of ink ejected from each nozzle. To this end, it can be said that it is effective to adjust the threshold arrangement of the dither matrix so that the generation frequency of dots formed by each nozzle is uniform for the same input tone value.

  Therefore, hereinafter, a method of creating a dot-dispersion type dither matrix adjusted so that the generation frequency of dots formed by each nozzle is uniform will be described. In the following description, the generation process or the generated dither matrix is denoted by M. The dither matrix M is a two-dimensional array in which the number of thresholds (size) arranged in the x direction (conveying direction of the recording medium) is Sx, and the number of thresholds arranged in the y direction (width direction of the recording medium) is Sy. , Sy are natural numbers. Although the size (Sx, Sy) of the dither matrix is arbitrary, it is typically a rectangle having the length of a power of 2 and each side is preferably 256 or more (for example, 256 × 256 or 512 × 512). is there. In the present embodiment, Sx is 256 and Sy is 256. In the dither matrix M created in the present embodiment, threshold values of 0 to 65535 are stored. Thus, when halftone processing is performed using the dither matrix M, 256 gradations can be reproduced in a pseudo manner in the Sx × Sy region.

  The Void & Cluster method is known as a method capable of generating a dot dispersion type dither matrix. The Void & Cluster method obtains a smoothed density image by applying a low-pass filter, and determines an arrangement where dots should be added / deleted so as to suppress local density fluctuation. Also in the present embodiment, a dither matrix having a dot dispersion type blue noise characteristic is generated by a similar method.

  The dot pattern generated in this process is d (x, y). d (x, y) is a two-dimensional array and the size is the same as M (x, y). The value of each pixel of d (x, y) is represented by a binary expression of “1” when a dot exists, and “0” when a dot does not exist.

  The dot pattern d (x, y) changes during the repetition process. According to the repetition process, Sx × Sy + 1 types of dot patterns are generated from a dot pattern having 0 dots to Sx × Sy dots. Therefore, when the number of dots in the dot pattern d (x, y) is g, this g can be used to identify a certain point in the repetition process. In the following description, the number g of dots is referred to as a tone value g. In the following description, the dot pattern d (x, y) when the tone value is g is also referred to as d (g), omitting d (g, x, y) or x, y. . As described above, in the present embodiment, in order to create a dither matrix capable of reproducing 256 gradations in a pseudo manner, in the following processing, values of all natural numbers from 0 to Sx × Sy−1 are set for the gradation g. You.

  Hereinafter, the details of the present embodiment will be described with reference to FIG. FIG. 3 is a flowchart showing the entire flow of the dither matrix generation method, and shows a dither matrix generation process used by the CPU (not shown) of the image processing apparatus 10 in FIG. If the dither matrix is to be stored in a non-volatile memory (not shown) so that it can be reused in the next printing process, the dither matrix need only be executed at the first time. When the image processing apparatus 10 is an information processing apparatus such as a personal computer, the program according to the illustrated flowchart indicates an application or a part of a printer driver.

  In S100, the CPU generates an initial dot pattern d (g0) having a gradation value of a predetermined gradation g0. In this embodiment, the gradation number g0 of the initial dot pattern is set to Sx × Sx × 0.5 = 256 × 256 × 0.5 = 32768. A method for generating the initial dot pattern will be described later. Note that the initial dot pattern d (g0) may be an image with the number of gradations g0 = 0 where no dot exists. In that case, S100 is not performed.

  Steps S101 to S107 are processes for repeating dot addition starting from the initial dot pattern. Steps S201 to S207 are processes for repeating dot deletion starting from the initial dot pattern. In addition, when repeating dot addition, dot deletion is not performed. Conversely, when repeating dot deletion, dot addition is not performed.

  S101 and S107 indicate loop ends, and indicate that the processing from S102 to S106 surrounded by the loop ends is repeated until the tone value g reaches gMAX. The processes from S102 to S106 are processes for generating a continuous tone value, that is, a dot pattern having a tone value of g + 1 by adding one dot to the dot pattern having the tone value of g. Further, gMAX for determining the condition for terminating the processing is gMAX ≧ g0, and in the present embodiment, gMAX = Sx × Sy−1 = 65535.

  Similarly, S201 and S207 indicate loop ends, and indicate that the processing from S202 to S206 surrounded by the loop ends is repeated until the tone value g reaches gMIN. The processing from S202 to S206 is processing for deleting one dot from the dot pattern with the gradation value g and generating a continuous gradation value, that is, a dot pattern with the gradation value g-1. gMIN ≦ g0, and gMIN = 0 in this embodiment. In this way, by repeating the addition of dots and the deletion of dots, a dot pattern of all gradations is generated.

  Since the processing contents of S102 and S202 are the same, S102 will be described below. In S102, the CPU applies a low-pass filter to the dot pattern d (g) corresponding to the tone value g, and calculates a density variation map n (g). In the following description, the density fluctuation map n (g) is abbreviated as density fluctuation n (g).

  The density fluctuation n (g) is used for evaluating dot density in a dot pattern d (g) having a gradation value of g. If the value of the density variation n (g) is smaller, the smoothed density is lower and the dots are evaluated as sparse. Conversely, if the value of the density variation n (g) is larger, the smoothed density is higher and the dots are denser. Be evaluated. In S104 described later, a dot is added at a position where the value of the density fluctuation n (g) is small and the dot is sparse. In S204, dots are deleted from positions where the value of the density fluctuation n (g) is large and the dots are dense. Thereby, low granularity is realized by suppressing local density fluctuation.

  The density fluctuation n (g) is a two-dimensional array having the same size as the dot pattern d (g), and the array value changes according to the tone value g, similarly to the dot pattern d (g). In the following description, n (g) is also described as n (g, x, y). The dither matrix is assumed to be applied periodically to the input image. For this reason, the CPU generates the density fluctuation n (g) by the cyclic convolution of the dot pattern d (g) and the filter coefficient in the filter processing of S103. The cyclic convolution operation is an operation in which a normal convolution operation is performed between a dot pattern d (g) in which a periodic boundary condition is set and a filter coefficient. The details of the filter coefficients used in S103 will be described later.

  Since the processing contents of S103 and S203 are the same, S103 will be described below. In S103, the CPU extracts a total dot number map s (g) in the x direction for each position in the y direction for the dot pattern d (g) corresponding to the gradation value g. The total dot number map s (g) is a one-dimensional array, and s (g) is also described as s (g, y). It is used for an arrangement constraint for equalizing the number of dots for each position (nozzle) in the y direction. In S104 described below, a dot is added to the dot pattern d (g, x, y) at the y position where the value of s (g, y) is small and the total number of dots is small. In S204, a dot is deleted from the dot pattern d (g, x, y) at the y position where the value of s (y) is large and the total number of dots is large. This makes it possible to adjust the difference in the dot generation frequency for each nozzle to be small.

  In S104 and S204, the CPU adds or deletes dots based on the density fluctuation n (g) and the total dot number map s (g, y). In S104, the CPU determines the dot pattern at the position (xMIN, yMIN) where the density variation n (g, x, y) is the smallest in the y-position group having the smallest total dot number map s (g, y). Add a dot to d (g, xMIN, yMIN). On the other hand, in S204, the CPU determines the dot at the position (xMAX, yMAX) where the density fluctuation n (g, x, y) is the largest among the y positions where the total dot number map s (g, y) is the largest. The dots of the pattern d (g, xMAX, yMAX) are deleted. Details of the processing will be described later.

  In S105 and S205, the CPU determines the value of the dither matrix M (x, y) based on the position (xMIN, yMIN, or xMAX, yMAX) where the dot has been added or deleted. In S105, the CPU sets the value of M (xMIN, yMIN) to the gradation value g in the dither matrix M (x, y). In S205, the CPU sets the value of M (xMAX, yMAX) to the gradation value g−1 in the dither matrix M (x, y).

  In S106, the CPU increments the value of the gradation value g to g + 1. Similarly, in S206, the CPU decrements the value of the gradation value g to g-1.

  As described above, after exiting the loop of S101 to S107 and the loop of S201 to S207, the CPU adjusts the range of the pixel value of the input image in S208 according to the range of the value of the dither matrix. At the stage before performing S208, the values from gMIN to gMAX are stored in the dither matrix M (x, y). In the case of the present embodiment, values from 0 to 65535 (mMIN to mMAX) are stored. When the input image at the time of performing the dither processing is 8 bits, the range of the input image is 0 to 255 (thMIN to thMAX). Therefore, even if a dither matrix storing values from 0 to 65535 is used, an appropriate halftone processing result cannot be obtained. In S208, the CPU extends the number of bits of the pixel value of the input image so as to match the value range of the dither matrix M (x, y). For example, when the range of the value of the dither matrix is 16 bits from 0 to 65535 and the input image is 8 bits, the lower part of the input image is padded with 0s and compared with 16 bits.

On the other hand, the value of the dither matrix may be adjusted according to the range of the pixel value of the input image. For example, the lower 8 bits of a 16-bit dither matrix in which values from 0 to 65535 are stored are discarded, and the upper 8 bits are used as the value of the dither matrix. When the CPU wants to change from thMIN to thMAX, the CPU obtains the integer part of the following equation as the value of the adjusted dither matrix.
a × M (x, y) + b
However,
a = (thMAX−thMIN) ÷ (mMAX−mMIN),
b = thMIN-a × mMIN
It is.

  According to the processing flow of FIG. 5 described above, it is possible to generate a dither matrix capable of equalizing the frequency of ink ejection for each nozzle while maintaining low granularity. Hereinafter, details of each processing in FIG. 1 will be described.

First, the filter coefficient f used in S102 and S202 will be described. This filter coefficient is used to extract a density variation map n (g). f is a two-dimensional array and is also described as f (x, y). In the present embodiment, the array size of f (x, y) is the same as the dot pattern d (g). That is, when the filter size in the x direction is Sfx and the filter size in the y direction is Sfy, these values are “256”. The value of the following equation (1) is used as the value of f (x, y).
r = {(x−x ₀ ) ² + (y−y ₀ ) ² } ^1/2
f (x, y) = 1 / (r + 1) (1)
In equation (1), x0 and y0 are the center positions of the filter coefficients, and x0 = Sfx ÷ 2 and y0 = Sfy ÷ 2.

  In S104 and S204, the CPU reduces or reduces the density of the dots by adding or deleting dots so as to reduce the density fluctuation n (g), thereby realizing low granularity. In order to realize this suitably, it is necessary to extract the density between dots. To extract the density between dots, the filter f may be a function of the reciprocal of the distance r from the dot, as shown in equation (1). In the present embodiment, “1” is added to the denominator to avoid zero division at the distance r = 0.

  Note that the filter f used in S104 and S204 is not limited to this, and a low-pass filter that can extract a frequency component perceived as a granular feeling may be used.

  Next, the dot arrangement method in S104 and S204 will be described. The CPU adds or deletes dots based on the density fluctuation n (g) and the total dot number map s (g, y). S104 is a process for adding a dot. In S104, the CPU determines that the value of the density fluctuation n (g, x, y) is the minimum in the y position where no dot exists and the total dot number map s (g, y) is the minimum. A dot is added at the position (xMIN, yMIN). Specifically, the pixel in the y-position group having the smallest number of dots in the total dot number map s (g, y) and the pixel value in the dot pattern d (g, x, y) become “0” The position (xMIN, yMIN) where the value of the density fluctuation n (g, x, y) becomes minimum is searched for from the pixels having the above. Then, a dot is added by setting the pixel value of d (g, xMIN, yMIN) to “1”. When there are a plurality of positions where the value of the density variation map n (g, x, y) is the minimum, the CPU randomly selects a position where a dot is to be added from the positions.

  On the other hand, S204 is a process of deleting a dot. In S204, the CPU determines that the value of the density variation map n (g, x, y) is the largest among the y positions where the dots exist and the total dot number map s (g, y) is the largest. The dot is deleted from the position (xMAX, yMAX). Specifically, the pixel in the y-position group where the number of dots is the maximum in the total dot number map s (g, y), and the pixel value is “1” in the dot pattern d (g, x, y) The position (xMAX, yMAX) at which the value of the density variation map n (g, x, y) becomes maximum is searched for from among the pixels. Then, the CPU deletes the dot by setting the pixel value of d (g, xMAX, yMAX) to “0”. If there are a plurality of positions where the value of the density variation map n (g, x, y) is maximum, the CPU randomly selects a position from which dots are to be deleted.

  Next, details of the method of generating the initial dot pattern in S100 will be described with reference to FIG. Briefly, the CPU first generates a random dot pattern. Then, the CPU repeats the movement of the CPU dots starting from the random dot pattern, thereby generating an initial dot pattern d (g0) with low graininess and inconspicuous density unevenness.

  In S301, the CPU generates a random dot pattern d_r having the number of gray levels g0. d_r is a two-dimensional array and is also referred to as d_r (x, y). The size is the same as the dither matrix M (x, y), the size in the x direction is Sx, and the size in the y direction is Sy. As described above, in the present embodiment, the values of Sx and Sy are “256”. As in the case of d (x, y), the pixel value when a dot exists is “1”, and the pixel value when a dot does not exist is “0”.

  In step S <b> 301, the CPU repeats dot addition until the number of tones reaches g <b> 0, starting from an image having no dots, and generates a random dot pattern with the number of tones g <b> 0.

  The CPU increments and determines the position yr in the y direction at which dots are added in order from 1 to Sy. When it reaches the extreme end Sy, it returns to 1 and repeats. On the other hand, the CPU determines a candidate for the position in the x direction to add a dot using a random number. Specifically, the CPU generates a random number from 1 to Sx, and determines the position xr in the x direction. If there is no dot at (xr, yr), the CPU adds a dot at that position. That is, when the pixel value of d_r (xr, yr) is “0”, the CPU sets the pixel value of d_r (xr, yr) to “1”. However, if the pixel value of d_r (xr, yr) is already “1”, the CPU changes the candidate of the position xr in the x direction to add a dot by generating a new random number. Accordingly, the dot pattern generated in S301 is a random dot pattern in which the deviation of the total number of dots in the x direction is suppressed to 1 dot or less.

  S302 and S305 are loop ends, and mean that the processes from S303 to S304 are repeated a predetermined number of times. In the present embodiment, the number of repetitions is set to 10000, but is not limited thereto, and a predetermined convergence condition may be set.

  In S303, the CPU extracts the density fluctuation n from the dot pattern dr by the same method as in S103. The density fluctuation n is a two-dimensional array and is also described as n (x, y).

  In S304, the CPU moves the dots based on the density change map n. First, a position (xMAX, yMAX) where the value of the density fluctuation n (x, y) is maximized is searched for from the pixels where dots exist in d_r (x, y). Next, the CPU selects the value of the density variation n (x, yMAX) from the pixels at the same y position (yMAX) as the position where the value of the density variation n (x, y) becomes maximum and no dot exists. Is searched for an x position (xMIN, yMAX) at which is minimized. Then, the CPU moves the dot at the position (xMAX, yMAX) to (xMIN, yMAX). That is, the pixel value of the dot pattern d_r (xMAX, yMAX) is set to “0”, and the pixel value of the dot pattern d_r (xMIN, yMAX) is set to “1”. A dot at a position where the density fluctuation n is large is moved to a position where the density fluctuation n is low at the same y position. Accordingly, in the dot pattern generated in S304, the total number of dots at each position yr in the y direction does not change before and after the movement of the dots. This makes it possible to generate a dot pattern with good dot dispersibility while suppressing the deviation of the total number of dots in the x direction to 1 dot or less.

  As described above, when the dot pattern is generated using the dither matrix generated by the method described in the present embodiment, the difference in the number of ejection dots for each nozzle of the print head 203 can be set within a preset range (ejection). The number of dots can be made substantially uniform). As a result, it is possible to prevent the life of the specific nozzle from being shortened, and to prolong the life of the recording head.

[Second embodiment]
The equalization of the number of ejection dots for each nozzle also works favorably in density unevenness correction that suppresses image quality degradation due to fluctuations in ink ejection characteristics for each nozzle. In the second embodiment, effects in density unevenness correction and examples of realizing the effects will be described.

  In an actual image forming apparatus, each nozzle arranged in a print head has a variation in ejection characteristics due to a manufacturing process of the print head, constituent materials, and the like. Variations in the ejection characteristics of each nozzle appear as unevenness in the size and ejection direction of ink droplets ejected from each nozzle, which causes density unevenness in a recorded image. In order to solve such a problem, there is a density unevenness correction technique for correcting the input image data based on the ejection characteristics of each nozzle constituting the print head to suppress the density unevenness. Specifically, a density measurement test chart for obtaining a density conversion function for density unevenness correction is output, an image of the output density measurement test chart is read, and the recording density is measured, and the measurement is performed. A density correction parameter of image data for each nozzle is generated based on the density data. Then, when printing the image, the input image data is corrected based on the density correction parameter, and the image recording is controlled based on the corrected image data to thereby control the image in which the occurrence of the density unevenness is suppressed. Form.

  If the dot position of the dot pattern generated from the test chart for density measurement is biased to a specific nozzle, the number of ejected inks differs for each nozzle. As a result, the output density corresponding to a nozzle having a large number of ejections is high, and the output density corresponding to a nozzle having a small number of ejections is low. Therefore, in the output test chart for density measurement, a density difference due to a discharge characteristic of each nozzle and a density difference due to a difference in the number of ejected inks of each nozzle are mixed. It is difficult to separate and measure the density fluctuation caused by the ejection characteristics of each nozzle from the recording density of the test chart, and the problem that the density unevenness correction effect is reduced is caused.

  Therefore, in order to accurately measure the density fluctuation due to the ejection characteristics of the nozzles, the density is adjusted using a dither matrix adjusted so that the number of dots formed by each nozzle to be subjected to density unevenness correction is uniform. It is effective to generate a dot pattern of the measurement test chart.

  Also, if there is a difference in the number of output dots with respect to the input tone for each nozzle position, even if the density fluctuation caused by the ejection characteristics of each nozzle can be correctly corrected, the method of correction (the number of output dots by correction) , The density unevenness correction effect may be reduced. Specifically, for example, it is assumed that there are two nozzles in which the ejection amount for one dot is smaller than the ideal ejection amount. At this time, the pixel values corresponding to the two nozzles to be corrected in the input image are similarly corrected so that the pixel values become large. However, if the number of output dots differs for each nozzle in accordance with the gradation, a different number of output dots will be ejected even if the pixel values before and after correction are the same. As a result, the density unevenness remains even after the density unevenness correction processing.

  Therefore, in order to improve the density unevenness correction effect of the recorded image for the input image that has been subjected to the density unevenness correction for each nozzle, the number of dots corresponding to each gradation formed by each nozzle is adjusted to be uniform It is effective to use the obtained dither matrix.

  FIG. 8 is a block diagram illustrating a configuration of the image processing apparatus 10 and the image forming apparatus 20 applicable to the second embodiment. In the following description, the description of the same parts as those in the first embodiment will be simplified or omitted.

  In FIG. 8, components other than the image reading device 30, the density unevenness correction parameter calculation unit 105, the density unevenness correction parameter storage unit 106, the density unevenness correction processing unit 107, and the test chart generation unit 108 are the same as those in FIG. .

  The image reading device 30 reads an image recorded on a recording medium by the recording head 203 and converts the image into electronic image data (read image data). As the image reading device 30, for example, a CCD line sensor can be used. The image reading device 30 according to the present embodiment is an in-line sensor installed in the middle of the medium transport path, and reads an image recorded by the recording head 203 during transport before paper discharge. The image reading device 30 can read an output result of a density measurement test chart described later.

  The density unevenness correction parameter calculation unit 105 performs density measurement based on the read image of the density measurement test chart acquired from the image reading device 30, and generates density unevenness correction value data (density unevenness correction parameter) for each nozzle. I do. The density unevenness correction parameter generated by the density unevenness correction parameter calculation unit 105 is stored in the density unevenness correction parameter storage unit 106.

  The density unevenness correction processing unit 107 performs image signal correction for suppressing density unevenness of a print image on a print medium that occurs due to a variation in the ejection characteristics of the nozzles in the print head 203 or the like. In the present embodiment, for each nozzle of the print head 203, a density unevenness correction LUT, which is a lookup table for density unevenness correction, which describes a conversion relationship between an input signal value and an output signal value, is prepared. Is used to convert the signal value.

  The test chart generation unit 108 generates density measurement test chart data for obtaining density measurement data necessary for calculating the density unevenness correction parameter, and provides the data to the input image buffer 102. FIG. 9 is an example of a test chart for density measurement, in which predetermined grayscale values D1 to D10 are set in a rectangular area having a width Ty of the print head 203 in the print medium width direction and a height Tx in the transport direction. Is provided in a band shape in the transport direction. As shown in FIG. 9, in this embodiment, there is chart data for the nozzle width so that the ejection characteristics of all noises can be derived. Further, in the present embodiment, all gradations from 0 to 255 are targets of density unevenness correction. For this reason, the density measurement test chart is generated so as to include patches for each gradation corresponding to densities that are discrete from 0 to 255 but are equally spaced from low density to high density.

  In order to obtain the density fluctuation due to the ejection characteristics, it is necessary to generate a dot pattern of a density measurement test chart using a dither matrix adjusted so that the number of dots formed by each nozzle is uniform. It is valid. Therefore, the x-direction size Tx of the density area of each gradation in the density measurement test chart needs to be equal to or greater than the x-direction size Sx pixels of the dither matrix (Sx is 256 in the embodiment). In the second embodiment, Tx = 800 pixels.

  FIG. 10 is a flowchart showing an outline of a procedure for creating density unevenness correction parameters, which is executed by a CPU (not shown) in the image processing apparatus 10 according to the second embodiment. The program according to the figure is stored in a predetermined memory (RAM or the like).

  In step S <b> 401, the CPU sends the image data of the density measurement test chart generated by the test chart generation unit 108 to the input image buffer 102, and thereby the image of the density measurement test chart on the recording medium by the recording head 203. Output. At this time, the CPU causes the density unevenness correction processing unit 107 not to perform the density unevenness correction, and outputs a uniform density gradation patch. Further, the halftone processing unit 103 generates a dot pattern of the test chart for density measurement by using a dither matrix adjusted so that the number of dots formed by each nozzle becomes uniform.

  In S402, the CPU controls the image reading device 30, reads the output result of the test chart for density measurement, and causes the test chart to be measured.

  In step S403, the CPU integrates a signal in the transport direction (x direction) for each band-shaped density patch corresponding to each gradation from the captured image by image processing, and obtains one signal in the recording medium width direction (y direction). Create dimensional measurement data. At this time, it is desirable to integrate signals in the transport direction (x direction) within a range in which the number of dots for each nozzle is uniform, in order to obtain the density fluctuation due to the ejection characteristics of each nozzle. That is, it is desirable that the integration range Mx in the transport direction (x direction) is an integral multiple of the print width (mm) of the dither matrix in the x direction. When the size of the dither matrix in the x direction Sx = 256 (pixel), the print width (mm) is 256 (pixel) / 1200 (pixel / inch) * 25.4 (mm / inch) ≒ 5.4 (mm). . On the other hand, the print width (mm) of the x-direction size Tx of the density area of each gradation in the density measurement test chart is 800 (pixel) / 1200 (pixel / inch) * 25.4 (mm / inch) ≒ 16.9. (Mm). In the present embodiment, the integrated range Mx in the transport direction (x direction) is set to be equal to or smaller than the print width (16.9 mm) of the density area in the x direction and the print width (5.4 mm) of the dither matrix in the x direction size Sx. 16.3 mm, which is an integral multiple (3 times) of.

  The CPU performs interpolation at intervals of the nozzle resolution (recording resolution) of the recording head 203 based on the measured one-dimensional data of the reading resolution of the image reading device 30 (for example, 600 dpi in the recording medium width direction), and Create concentration measurement data.

  Next, in S404, the CPU calculates a density unevenness correction parameter for each nozzle, that is, a density unevenness correction LUT, from the acquired density measurement data for each nozzle position and a target gradation characteristic. By interpolating the density measurement data for discrete tone values, density data for tone values other than the measurement point can be obtained.

  Next, with reference to FIGS. 7A and 7B, the gradation characteristics of the dot pattern generated by the halftone processing using the dither matrix of the present invention will be described.

  In the graphs of FIGS. 7A and 7B, the horizontal axis is the input gradation value, and the vertical axis is the comparison processing between the input image in and the dither matrix M having the same size (Sx, Sy) as the dither matrix. This is the total number of dots xSum (y) (hereinafter, total number of dots) in the x direction in the generated dot pattern. If the size Sy of the dither matrix in the y direction is 256, 256 graphs can be generated. However, note that some of the correspondences at y positions are displayed here for the sake of easy viewing of the graph. I want to be.

  FIG. 7A shows an example in which a dot pattern is generated using the dither matrix created in the present embodiment. FIG. 7B shows an example in which a dot pattern is generated using a general blue noise dither matrix created without considering that the generation frequency of dots for each nozzle becomes uniform for comparison. Show. Here, a general blue noise dither matrix is created by a method different from the matrix creation method using FIG. 5 only in S103, S104, S203, and S204. It is assumed that a general blue noise dither matrix does not have S103 and S203, and is a matrix generated by referring to the density fluctuation n (g, x, y) in S104 and S204.

  FIG. 7A shows the total number of dots with respect to the input tone value at a plurality of positions in the y direction. However, since the graphs overlap each other, they appear as one line. In other words, the total number of dots of the dot pattern generated by using the dither matrix becomes substantially uniform regardless of the gradation with respect to the input gradation value of uniform density, and the number of increased dots between gradations is constant (linear). ).

  On the other hand, as shown in FIG. 7B, when a dot pattern is generated using a general blue noise dither matrix, the correspondence relationship between the input tone value and the total number of output dots differs for each y position. This indicates that the increase in the number of dots between gradations is non-linear.

  The total number of dots for each nozzle shown in FIG. 7A can be completely matched when the total number of dots is divisible by the size Sy of the dither matrix in the y direction (when it is an integral multiple of Sy). . This is true because the number of gray levels (the number of g) that can be artificially represented by the dither matrix is Sy. That is, assuming that the total number of dots of the dot pattern generated using the dither matrix is d, when d% Sy = 0, the total number of dots for each nozzle can be d / Sy. That is, by adjusting the threshold range of the dither matrix M so that d% Sy = 0, the total number of dots for each nozzle can be made uniform. When the density unevenness correction for each nozzle covers all gradations as in this embodiment, it is desirable to make the total number of dots per nozzle uniform for all gradations.

  In some cases, such as when the size of the dither matrix is limited, the total number of dots for each nozzle may not completely match. For example, depending on the gradation, the total number of dots may not be divisible by the size Sy of the dither matrix in the y direction, and the total number of dots for each y position may not be uniform. In such a case, the difference between the total number of dots for each y position is preferably 1 or less.

  The test chart for density measurement output in the present embodiment generates a dot pattern of the test chart for density measurement using a dither matrix adjusted so that the number of dots formed by each nozzle becomes uniform. Therefore, there is no deviation in the number of ink ejections for each nozzle, and it is possible to accurately measure the density fluctuation due to the ejection characteristics for each nozzle from the recording density of the test chart. Thereby, the effect of the density unevenness correction based on the density measurement data can be improved.

  On the other hand, when the density unevenness correction for each nozzle is performed on the input image by referring to the density unevenness correction parameter for each nozzle, a dither matrix adjusted so that the number of dots formed by each nozzle is uniform is effective. It is. When printing is actually performed, the density unevenness correction processing unit 107 performs density unevenness correction for each nozzle on the input image with reference to the density unevenness correction parameter. Further, the halftone processing unit 103 generates a dot pattern by using a dither matrix adjusted so that the number of output dots with respect to the input gradation for each nozzle position becomes uniform. Since the increase in the number of dots between gradations is constant (linear), a dither pattern with an appropriate number of dots is generated for the density unevenness correction amount for each nozzle, thereby improving the density unevenness correction effect of the recorded image. Can be.

  Therefore, the measurement of the density unevenness correction parameter for each nozzle is not limited to the test chart for density measurement described above, but is a case where density unevenness correction for each nozzle is performed using the density unevenness correction parameter created by another measurement method. Also, the effect of correcting the density unevenness of the recorded image can be improved.

[Third embodiment]
In the second embodiment, the target of density unevenness correction is all gradations. The third embodiment will exemplify a case in which the target of density unevenness correction is performed only for some of the gradations. For example, in the case where patches with low density D9 and D10 in FIG. 9 have almost no density unevenness, only density higher than density D9 may be subjected to density unevenness correction. In this case, the density range in which the density unevenness correction effect should be improved is a range of density D8 to D1 (here, density D1 is the maximum density). The density unevenness correction processing unit 107 determines whether or not the pixel value of the pixel in the input image is equal to or higher than the density D8. If the pixel value is equal to or higher than the density D8, the density unevenness correction processing unit 107 performs density unevenness correction with reference to the density unevenness correction parameter. On the other hand, when the pixel value is less than the density D8, the density non-uniformity correction is not performed and the pixel value is output as it is.

  On the other hand, in the low-density portion, the density unevenness may not be conspicuous, but the emphasis may be placed on the dot dispersibility. When a small number of dots are ejected to the recording medium, a plurality of dots are ejected close to each other, so that a dot lump is visually recognized as having a cluster of dots, and the graininess is reduced. Therefore, in the third embodiment, when performing halftone processing on an input image, a dither matrix is switched and applied. When the pixel value of the pixel to be processed is the density D8 to D1, the same dither matrix (M1) as in the first embodiment is used. If the pixel value of the pixel to be processed is a low-density portion with a density of D9 or more, a dither matrix (M2) created without considering that the dot generation frequency for each nozzle becomes uniform.

The halftone processing unit 103 performs the following processing.
Iin (x, y) ≧ D8
Iout (x, y) = BLACK if Iin (x, y)> M1 (x% Sx, y% Sy)
Iout (x, y) = WHITE if Iin (x, y) ≤ M1 (x% Sx, y% Sy)
Iin (x, y) <D8
Iout (x, y) = BLACK if Iin (x, y)> M2 (x% Sx, y% Sy)
Iout (x, y) = WHITE if Iin (x, y) ≤ M2 (x% Sx, y% Sy)
Since the dematrix M2 refers only to the density variation n when adding or deleting dots, the dematrix M2 is more restricted in determining the dot arrangement than the dot creation method shown in FIG. Few. Therefore, the dither matrix M2 is better than the dither matrix M1 only from the viewpoint of dot dispersibility. As described above, in the present embodiment, the density unevenness correction processing unit performs the density unevenness range processing in accordance with the density range of the pixel value. Execute When the pixel value is within the density range in which the density unevenness correction processing is performed, the effect of the density unevenness correction is improved by using a dither matrix created so that the number of dots for each nozzle becomes uniform. On the other hand, in a density range in which the pixel value does not require the density unevenness correction processing, a dither matrix that enhances the dispersibility is used.

  In this embodiment, the dither matrix referred to by the halftone process is switched according to the pixel value. However, there is also a method of realizing the same effect as in the third embodiment with one dither matrix. This is a method of determining whether or not to refer to a total dot number map according to a tone value g when creating a dither matrix. For example, in the flowchart shown in FIG. 5, if the gradation g is equal to or less than a predetermined value (for example, density D9) among S201 to S207, S203 is not executed, and the density fluctuation n (g, x, y) is performed in S204. ) Alone to determine the position from which to delete the dot. Thus, in the dither matrix, when the gradation g is equal to or more than a predetermined value, the total number of dots for each nozzle becomes uniform in a range of an integer multiple of the size Sy in the y direction. The same effect as in the third embodiment can be obtained by performing a halftone process using the dither matrix created in this way.

(Other Examples)
In the above-described embodiment, the case where the number of gray scales that the dither matrix reproduces in a pseudo manner is 256 has been described as an example. However, the number of tones that the dither matrix reproduces in a pseudo manner may be only discrete tones among the number of tones of the input image. For example, of 0 to 255, only the gray scales of 0, 15, 30, 45, 60, 75, 90, 105, 120, 135, 150, 165, 180, 195, 210, 225, 240, 255 are pseudo. In the case of reproducing in the same manner, the restriction on the total number of dots for each nozzle shown in FIG. In this case, the size of the dither matrix in the y direction may be set so as to be a multiple of the number of gradations 18 to be reproduced in a pseudo manner.

  Further, in the above-described embodiment, the method in which the halftone processing unit executes the comparison processing using the dither matrix has been described as an example. However, the dither matrix created in the first embodiment can also be applied to a method of determining the dot arrangement according to the dither matrix. For example, the dither matrix described above may be used in the method described in JP-A-2006-021735.

  The present invention supplies a program for realizing one or more functions of the above-described embodiments to a system or an apparatus via a network or a storage medium, and one or more processors in a computer of the system or the apparatus read and execute the program. This process can be realized. Further, it can also be realized by a circuit (for example, an ASIC) that realizes one or more functions.

Reference Signs List 10: image processing device, 20: image forming device, 102: input image buffer, 103: halftone processing unit, 104: halftone image buffer, 202: head driving unit, 203: recording head

Claims (20)

A recording head having a plurality of recording elements arranged in a first direction, wherein the recording head is directed toward a recording medium conveyed relatively along a second direction perpendicular to the first direction; An image processing apparatus that generates image data for an image forming apparatus that forms an image on the recording medium by discharging ink from a recording element that has
Acquiring means for acquiring multi-valued input image data;
Correction processing means for performing, on the input image data, density unevenness correction processing according to the characteristics of each of the recording elements;
Using a dither matrix, the input image data that has been subjected to density unevenness correction processing by the correction processing means, the conversion means for converting to halftone image data indicating the presence or absence of dots,
The dither matrix is
A distributed dither matrix,
The first direction when converting the uniform gradation input image data into binary halftone image data for the gradation to be subjected to the density unevenness correction processing among the gradations that can be obtained by the input image data. Wherein the number of dots included in the predetermined range in the second direction at each position is the same.   The image processing apparatus according to claim 1, wherein the predetermined range is an integral multiple of the number of gradations that can be pseudo-reproduced in the halftone image data.   The image processing device according to claim 1, wherein the predetermined range is an integer multiple of the number of values used as thresholds in the dither matrix.   The image processing apparatus according to claim 1, wherein an object of the density unevenness correction processing is all gradations.   The image processing according to claim 4, wherein the dither matrix has a size of Sx rows corresponding to the second direction and Sy columns (Sx, Sy are natural numbers) corresponding to the first direction. apparatus.   The image processing apparatus according to claim 5, wherein the predetermined range is an integral multiple of Sx in the second direction. The dither matrix is
When the input image data of uniform gradation is converted into a binary dot pattern, the difference in the number of dots included in a range of an integer multiple of Sx in the second direction at each position in the first direction is calculated. The image processing apparatus according to claim 1, wherein the image processing apparatus has a characteristic that is equal to or less than 1 in all gradations. A storage unit for storing a density correction parameter for each recording element;
2. The image processing apparatus according to claim 1, wherein the correction processing unit corrects the input image data based on the stored density correction parameter. Recording means for generating a halftone image based on a chart image of uniform gradation having a width of Sx pixels or more in the second direction, and recording the image on a recording medium;
Acquisition means for acquiring density measurement data of an image formed on the recording medium,
The image processing apparatus according to claim 1, further comprising:   The image processing apparatus according to claim 1, wherein both Sx and Sy are 256 or more. A recording head having a plurality of recording elements arranged in a first direction, wherein the recording head is directed toward a recording medium conveyed relatively along a second direction perpendicular to the first direction; A method of controlling an image processing apparatus that generates image data for an image forming apparatus that forms an image on the recording medium by ejecting ink from a recording element of the image processing apparatus,
Acquiring means for acquiring multi-valued input image data,
Correction processing means for performing, on the input image data, density unevenness correction processing according to the characteristics of each of the recording elements;
Conversion means, using a dither matrix, has a conversion step of converting the input image data subjected to density unevenness correction processing in the correction processing step to halftone image data indicating the presence or absence of dots,
The dither matrix is
A distributed dither matrix,
The first direction when converting uniform input image data into binary halftone image data from grayscale to be subjected to the density unevenness correction processing among grayscales that can be obtained by the input image data. Wherein the number of dots included in the predetermined range in the second direction at each position is the same. Image processing for generating a dither matrix used when converting multi-valued image data into a binary dot pattern for determining whether or not to eject ink from a plurality of recording elements of a recording head of the image forming apparatus A device,
Here, the arrangement direction of the plurality of recording elements included in the recording head of the image forming apparatus is a first direction, a direction perpendicular to the first direction, and relative to the recording head. When the direction in which the recording medium is conveyed is defined as a second direction, the dither matrix includes Sx rows corresponding to the second direction and Sy columns corresponding to the first direction (Sx and Sy are natural numbers). Having the size of
The image processing device,
A filter for evaluating local density fluctuations in the dot pattern d (g) of the predetermined gradation g,
First calculating means for calculating a density variation map n (g) by applying the filter;
Second calculating means for calculating a total dot number map s (g) in the second direction for the dot pattern d (g) of the gradation g;
Generating means for generating a dot pattern of a tone that is continuous with the tone g by adding or deleting dots to the dot pattern based on the density variation map and the total dot number map;
By repeatedly applying the first calculating means, the second calculating means, and the generating means starting from an initial dot pattern of a predetermined gradation, a dot pattern of all gradations is generated. An image processing apparatus comprising: a determination unit that determines a value of the dither matrix based on a tone dot pattern. The generation means,
When the position in the first direction in the dither matrix is defined as x and the position in the second direction is defined as y,
Among the y positions where the total dot number map s (g, y) is the minimum, the dot pattern d (g, x, y) is located at the position (x, y) where the density variation map n (g, x, y) is the minimum. Add a dot to y)
Among the y positions where the total dot number map s (g, y) is the maximum, the dot pattern d (g, x, y) starts from the coordinates (x, y) where the density variation map n (g, x, y) is the maximum. The image processing apparatus according to claim 12, wherein the dots of y) are deleted.   The image processing apparatus according to claim 12, wherein the dither matrix has a blue noise characteristic.   13. The image processing apparatus according to claim 12, wherein the predetermined tone g is set according to a process executed in the image processing apparatus using the dither matrix.   13. The image processing apparatus according to claim 12, wherein Sy corresponding to the first direction is set according to the number of dots corresponding to the predetermined gradation g.   2. The method according to claim 1, wherein the generation unit generates a dot pattern such that the total number of dots in each of the first directions is the same in all the gradations set as the predetermined gradation g. 13. The image processing apparatus according to claim 12. Image processing for generating a dither matrix used when converting multi-valued image data into a binary dot pattern for determining whether or not to eject ink from a plurality of recording elements of a recording head of the image forming apparatus A method for controlling an apparatus, comprising:
Here, the arrangement direction of the plurality of recording elements included in the recording head of the image forming apparatus is a first direction, a direction perpendicular to the first direction, and relative to the recording head. When the direction in which the recording medium is conveyed is defined as a second direction, the dither matrix includes Sx rows corresponding to the second direction and Sy columns corresponding to the first direction (Sx and Sy are natural numbers). Having the size of
The control method of the image processing apparatus includes:
A first calculation step of applying a filter for evaluating local density fluctuation in the dot pattern d (g) of the predetermined gradation g to calculate a density fluctuation map n (g); ,
A second calculating step of calculating a total dot number map s (g) in the second direction for the dot pattern d (g) of the gradation g,
Generating means for adding or deleting dots to the dot pattern based on the density variation map and the total dot number map, thereby generating a dot pattern having a tone continuous with the tone g. Process and
The determining means repeatedly generates the dot pattern of all gradations by repeatedly applying the first calculation step, the second calculation step, and the generation step with an initial dot pattern of a predetermined gradation as a starting point. And a determining step of determining the value of the dither matrix based on the dot pattern of all gradations.   A program for causing a computer to execute each step of the method according to claim 11 when read and executed by the computer.   A program for causing a computer to execute each step of the method according to claim 18 when read and executed by the computer.