JP5990072B2 - Threshold matrix generation device, image recording device, and threshold matrix generation method - Google Patents

Description

  The present invention relates to a technique for generating a threshold matrix used for halftoning a multi-tone original image in an image recording apparatus.

  Conventionally, a multi-tone (that is, continuous tone) original image is halftoned using a threshold matrix and recorded on a recording medium. As a method for generating a threshold matrix, as shown in Patent Document 1, a method of sequentially calculating a threshold value of each pixel based on a distance or an angle from a pixel that is lighted first, or as shown in Patent Document 2, A method is known in which a halftone image (dot ON / OFF profile) at each density level is temporarily created, and a profile that minimizes a predetermined function value is obtained by optimization. Patent Document 3 discloses a method for giving a blue noise characteristic and a green noise characteristic to a threshold value matrix, and a method for reflecting human visual characteristics.

JP-A-7-170400 JP 2006-165930 A JP 2008-6591 A

  By the way, in an image recording apparatus that records an image by ejecting fine ink droplets from a plurality of nozzles toward a recording medium, tension acts between the droplets that have landed on the recording medium, and the ink liquid that has landed later A phenomenon occurs in which the droplet is attracted to the droplet that has landed first. For this reason, there is a possibility that the colored region (hereinafter simply referred to as “landing position”) due to the landing of the liquid droplets varies depending on whether or not there is a liquid droplet that has already landed in the vicinity of the liquid droplet landing position. is there.

  Further, the force with which the ink droplet is attracted to the previously landed droplet varies depending on the type of ink, and also varies depending on the type of recording medium, even if the ink has the same color component. That is, the force with which the ink droplets are attracted differs depending on the contact angle of the ink with respect to the recording medium. When the contact angle is large, the ink droplets are attracted by a large force, and the landing position is largely shifted. As described above, in the image recording apparatus, if the contact angles of the ink with respect to the recording medium are different, the images recorded on the recording medium may be different. Further, when the landing positions of a large number of ink droplets are shifted in the moving direction of the recording medium, streak unevenness extending in the moving direction of the recording medium occurs, particularly in an intermediate area between the highlight area and the shadow area. there is a possibility.

  The present invention has been made in view of the above problems, and an object of the present invention is to suppress the occurrence of stripe unevenness when an image is recorded at each contact angle with respect to a plurality of contact angles of ink with respect to a recording medium. .

  According to the first aspect of the present invention, an image is recorded on the recording medium by ejecting ink droplets from the ejection unit toward a recording medium that moves relative to the ejection unit in a predetermined movement direction. In the image recording apparatus, a threshold matrix generating apparatus that generates a threshold matrix used for halftoning a multi-tone original image, a storage unit that stores a plurality of threshold matrices, and a plurality of constituents constituting each threshold matrix A threshold value determination unit that determines a threshold value assigned to each threshold value matrix element, and a matrix region in which each threshold value matrix is generated is defined in a column direction corresponding to the moving direction and a row direction perpendicular to the column direction. The threshold value determination unit a) assigning an initial threshold value to one threshold value matrix element included in each threshold value matrix; and b) assigning a threshold value. Selecting one unassigned element that is not a threshold matrix element, and c) assigning the one unassigned element and the threshold while considering repetitive application of each of the threshold matrices at the time of halftoning the original image An evaluation value indicating the distribution bias of assigned elements when a threshold value is assigned to the one unassigned element to give an assigned element based on a distance between each assigned element that is a given threshold matrix element D) with respect to all unassigned elements, b) and c) steps are repeated to obtain an evaluation value, and e) the evaluation values obtained in step d) are referred to. A step of assigning the next threshold value to the one unassigned element having the smallest bias indicated by the evaluation value, and f) step b) to step e until the threshold value is assigned to all of the plurality of threshold value matrix elements. ) Repeating the process And c) step c1) for each given element, the first distance between the one unassigned element and the given element in the row direction, and the one in the column direction. A step of obtaining an evaluation value element based on an unassigned element and a second distance between each of the assigned elements; and c2) a step of obtaining the evaluation value by summing the evaluation value elements of each of the assigned elements. A plurality of threshold matrixes, a first threshold matrix corresponding to a first contact angle of ink with respect to a recording medium, and a second threshold matrix corresponding to a second contact angle larger than the first contact angle. And at least the ratio of the number of assigned elements to the number of all threshold matrix elements is not less than a first ratio that is greater than 0% and less than 20%, and greater than the first ratio and less than 100%. When the ratio is less than or equal to the second ratio, in the step c1), a weighting factor that makes the influence of the first distance on the bias indicated by the evaluation value larger than the influence of the second distance is the first distance or the The evaluation value element is obtained by multiplying by the second distance, and when the threshold value of the second threshold value matrix is determined rather than the weighting factor that is a weighting factor in the step c1) when the threshold value of the first threshold value matrix is determined The second weighting factor, which is the weighting factor in step c1), increases the influence of the first distance on the bias indicated by the evaluation value.

  The invention according to claim 2 is the threshold value matrix generation device according to claim 1, wherein in the step c), the ratio of the number of assigned elements to the number of all threshold matrix elements is less than the first ratio. In some cases, in the step c1), the influence of the first distance and the influence of the second distance on the bias indicated by the evaluation value are equal.

  The invention according to claim 3 is the threshold value matrix generation device according to claim 1 or 2, wherein the resolution in the moving direction in the image recording apparatus is higher than the resolution in the width direction perpendicular to the moving direction. In the calculation of the evaluation value element in step c1), another weighting factor that increases the influence of the first distance on the bias indicated by the evaluation value compared to the influence of the second distance is the first distance. Alternatively, when the second distance is multiplied and the resolution in the moving direction in the image recording apparatus is lower than the resolution in the width direction, the bias indicated by the evaluation value when calculating the evaluation value element in the step c1) The first distance or the second distance is multiplied by another weighting factor that reduces the influence of the first distance on the relative to the influence of the second distance.

  According to a fourth aspect of the present invention, an image is recorded on the recording medium by ejecting ink droplets from the ejection unit toward a recording medium that moves relative to the ejection unit in a predetermined movement direction. A threshold value matrix generation device that generates a threshold value matrix used for halftoning a multi-tone original image in an image recording device, the storage unit storing a plurality of threshold value matrices, and a plurality of values constituting each threshold value matrix A threshold value determination unit that determines a threshold value assigned to a threshold value matrix element, and a matrix region in which each threshold value matrix is generated is defined in a column direction corresponding to the moving direction and a row direction perpendicular to the column direction. The threshold determination unit a) provisionally assigns a threshold value to one threshold value matrix element group to which no threshold value is assigned in each threshold value matrix; ) A step of obtaining a first evaluation value indicating a bias in distribution of threshold matrix elements to which a threshold value is given or provisionally given; c) one threshold value matrix element is removed from the one threshold matrix element group, and a threshold value is given and provisionally given. A second threshold matrix element including another threshold matrix element that has not been assigned is included in the one threshold matrix element group, and the step a) is executed to show a bias in the distribution of the threshold matrix elements to which the threshold is given or provisionally given A step of obtaining an evaluation value; and d) when the first evaluation value shows a bias smaller than the second evaluation value, the one threshold matrix element group is returned to the state of the step a), and the first evaluation is performed. A step of changing the first evaluation value to a value equal to the second evaluation value when the value indicates a bias greater than the second evaluation value; and e) the one threshold matrix element group in the step c) Included in The step c) and the step d) are repeated until a predetermined end condition is satisfied, and the threshold value is added to the one threshold matrix element group after the step e) while changing the threshold matrix element to be changed. G) changing the one threshold matrix element group to the next one threshold matrix element group until the threshold values are assigned to all the threshold matrix elements included in the threshold matrix, and a) And a step of repeating the step f), and a visual characteristic filter is applied to the power spectrum obtained by Fourier transforming the distribution of threshold matrix elements to which threshold values are given or provisionally given in the step b). The sum of the differences in power spectrum before and after application of the visual characteristic filter is set as the first evaluation value, and a threshold value is assigned or subtracted in the step c). Applies the visual characteristic filter to a power spectrum obtained by Fourier-transforming the distribution of the temporarily assigned threshold value matrix elements, and the total sum of the power spectrum differences before and after application of the visual characteristic filter is the second evaluation value. The plurality of threshold matrices includes a first threshold matrix corresponding to a first contact angle of ink with respect to a recording medium, and a second threshold matrix corresponding to a second contact angle larger than the first contact angle; At least a ratio of the number of threshold matrix elements to which a threshold is given or provisionally given to the total number of threshold matrix elements is not less than a first ratio that is greater than 0% and less than 20% and greater than the first ratio. Step b) and step c) when determining the threshold value of the first threshold value matrix when the second ratio is less than or equal to 100%. The second filter, which is the visual characteristic filter in the steps b) and c), when determining the threshold value of the second threshold value matrix is more deflectable in the column direction than the first filter, which is the visual characteristic filter in the above. strong.

The invention according to claim 5 is the threshold value matrix generation device according to claim 4, wherein in the steps b) and c), threshold values for the number of all threshold matrix elements are given or provisionally given. When the ratio of the number of matrix elements is less than the first ratio, the first filter and the second filter do not have deflectability in the column direction and the row direction.

  The invention according to claim 6 is an image recording apparatus, wherein the discharge unit discharges micro droplets of ink toward the recording medium, and the recording medium is relatively moved in the moving direction with respect to the discharge unit. Based on a contact angle of the ink to the recording medium from the moving mechanism that moves, the threshold value matrix generation device according to any one of claims 1 to 5, and the plurality of threshold value matrices generated by the threshold value matrix generation device A control unit that generates a halftone image by halftoning a multi-tone original image using the one threshold matrix selected in this way, and controls ejection of ink from the ejection unit based on the halftone image With.

  A seventh aspect of the present invention is the image recording apparatus according to the sixth aspect, wherein the recording medium is moved relative to the ejection unit only once in the moving direction to the recording medium. Recording of the image is completed.

  According to an eighth aspect of the present invention, an image is recorded on the recording medium by ejecting ink droplets from the ejection unit toward a recording medium that moves relative to the ejection unit in a predetermined movement direction. A threshold matrix generation method for generating a threshold matrix used for halftoning a multi-tone original image in an image recording apparatus, the method comprising: storing a plurality of threshold matrices; and a plurality of threshold values constituting each threshold matrix Determining a threshold value assigned to a threshold value matrix element, and a matrix region in which each threshold value matrix is generated is defined in a column direction corresponding to the movement direction and a row direction perpendicular to the column direction, The step of determining the threshold value includes a) a step of assigning an initial threshold value to one threshold value matrix element included in each threshold value matrix, and b) a threshold value is assigned. Selecting one unassigned element that is not a threshold matrix element, and c) assigning the one unassigned element and the threshold while considering repetitive application of each of the threshold matrices at the time of halftoning the original image An evaluation value indicating the distribution bias of assigned elements when a threshold value is assigned to the one unassigned element to give an assigned element based on a distance between each assigned element that is a given threshold matrix element D) with respect to all unassigned elements, b) and c) steps are repeated to obtain an evaluation value, and e) the evaluation values obtained in step d) are referred to. A step of assigning the next threshold value to the one unassigned element having the smallest bias indicated by the evaluation value, and f) step b) to step e until the threshold value is assigned to all of the plurality of threshold value matrix elements. ) Repeating the process And c) c1) for each given element, the first distance between the one unassigned element in the row direction and each given element and the one unassigned in the column direction. Obtaining an evaluation value element based on a given element and a second distance between each given element; c2) obtaining the evaluation value by summing the evaluation value elements of each given element; The plurality of threshold matrixes include a first threshold matrix corresponding to a first contact angle of ink with respect to a recording medium, and a second threshold matrix corresponding to a second contact angle larger than the first contact angle. At least the ratio of the number of assigned elements to the number of all threshold matrix elements is not less than a first ratio that is greater than 0% and less than 20%, and is greater than the first ratio and less than 100%. A weighting factor that increases the influence of the first distance on the bias indicated by the evaluation value compared to the influence of the second distance in the step c1) when the second ratio is equal to or less than the second ratio. The evaluation value element is obtained by multiplying by the second distance, and when the threshold value of the second threshold value matrix is determined rather than the weighting factor that is a weighting factor in the step c1) when the threshold value of the first threshold value matrix is determined The second weighting factor, which is the weighting factor in step c1), increases the influence of the first distance on the bias indicated by the evaluation value.

  According to a ninth aspect of the invention, an image is recorded on the recording medium by ejecting ink droplets from the ejection unit toward a recording medium that moves relative to the ejection unit in a predetermined movement direction. A threshold matrix generation method for generating a threshold matrix used for halftoning a multi-tone original image in an image recording apparatus, the method comprising: storing a plurality of threshold matrices; and a plurality of threshold values constituting each threshold matrix Determining a threshold value assigned to a threshold value matrix element, and a matrix region in which each threshold value matrix is generated is defined in a column direction corresponding to the movement direction and a row direction perpendicular to the column direction, The step of determining the threshold value includes: a) provisionally assigning a threshold value to one threshold value matrix element group to which no threshold value is given in each threshold value matrix; b) obtaining a first evaluation value indicating a bias in distribution of threshold matrix elements to which a threshold value is given or provisionally given; c) removing one threshold value matrix element from the one threshold value matrix element group; Another threshold matrix element that is not provisionally assigned is included in the one threshold matrix element group, the step a) is executed, and the distribution of threshold matrix elements to which the threshold is assigned or provisionally given is shown. 2) obtaining the evaluation value; and d) returning the one threshold matrix element group to the state of the step a) when the first evaluation value is less biased than the second evaluation value, A step of changing the first evaluation value to a value equal to the second evaluation value when the evaluation value is larger than the second evaluation value; and e) the one threshold matrix element in the step c) In groups The step c) and the step d) are repeated until a predetermined end condition is satisfied, and f) the threshold matrix element group after the step e) is changed. G) changing the one threshold matrix element group to the next one threshold matrix element group until the threshold values are given to all the threshold matrix elements included in the threshold matrix; ) Step or step f), and in step b), a visual characteristic filter is applied to the power spectrum obtained by Fourier transforming the distribution of threshold matrix elements to which threshold values are given or provisionally given. The sum of the differences in power spectrum before and after application of the visual characteristic filter is set as the first evaluation value, and a threshold value is assigned or subtracted in the step c). Applies the visual characteristic filter to a power spectrum obtained by Fourier-transforming the distribution of the temporarily assigned threshold value matrix elements, and the total sum of the power spectrum differences before and after application of the visual characteristic filter is the second evaluation value. The plurality of threshold matrices includes a first threshold matrix corresponding to a first contact angle of ink with respect to a recording medium, and a second threshold matrix corresponding to a second contact angle larger than the first contact angle; At least a ratio of the number of threshold matrix elements to which a threshold is given or provisionally given to the total number of threshold matrix elements is not less than a first ratio that is greater than 0% and less than 20% and greater than the first ratio. Step b) and step c) when determining the threshold value of the first threshold value matrix when the second ratio is less than or equal to 100%. The second filter, which is the visual characteristic filter in the steps b) and c), when determining the threshold value of the second threshold value matrix is more deflectable in the column direction than the first filter, which is the visual characteristic filter in the above. strong.

  In the present invention, it is possible to suppress the occurrence of streak unevenness when an image is recorded at each contact angle with respect to a plurality of contact angles of the ink with respect to the recording medium.

1 is a diagram illustrating a configuration of an image recording apparatus according to a first embodiment. It is a bottom view which shows a discharge unit. It is a block diagram which shows the function of a control unit. It is a figure which shows an original image and a threshold value matrix. It is a figure which shows the flow of the production | generation of a threshold value matrix. It is a figure which shows the flow of the production | generation of a threshold value matrix. It is a figure which shows a threshold value matrix. It is a figure which shows a halftone image. It is a figure which shows the spatial frequency characteristic of the dot in a halftone image. It is a figure which shows a halftone image. It is a figure which shows the spatial frequency characteristic of the dot in a halftone image. It is a figure which shows a halftone image. It is a figure which shows the spatial frequency characteristic of the dot in a halftone image. It is a figure which shows the flow of the production | generation of a threshold value matrix. It is a figure which shows the flow of the production | generation of a threshold value matrix. It is a figure which shows the flow of the production | generation of a threshold value matrix. It is a figure which shows a threshold value matrix. It is a figure which shows a blue noise filter. It is a figure which shows a green noise filter. It is a figure which shows a blue noise filter. It is a figure which shows a green noise filter. It is a figure which shows a blue noise filter. It is a figure which shows a green noise filter.

  FIG. 1 is a diagram showing a configuration of an image recording apparatus 1 according to the first embodiment of the present invention. The image recording apparatus 1 is a sheet-fed printing apparatus (a so-called inkjet printer) that sequentially records color images on a plurality of recording media 9 by ejecting fine ink droplets onto a recording medium 9 that is printing paper. ).

  As shown in FIG. 1, the image recording apparatus 1 moves a plurality of recording media 9 in a moving direction that is the (+ Y) direction in FIG. A discharge unit 3 for discharging minute droplets of ink, a supply unit 51 for supplying the recording medium 9 to the moving mechanism 2, a discharge unit 52 for receiving the recording medium 9 after printing from the moving mechanism 2, and these mechanisms. A control unit 4 for controlling is provided. The discharge unit 3 is arranged above the moving mechanism 2 (on the (+ Z) side) and is fixed to a frame (not shown).

  The moving mechanism 2 includes a plurality of stages 21, an annular guide 22, and a belt driving mechanism 23. Each of the plurality of stages 21 sucks and holds one sheet-like recording medium 9. The guide 22 internally includes a belt to which a plurality of stages 21 are connected, and guides the plurality of stages 21. The belt drive mechanism 23 moves the belt in the guide 22 counterclockwise in FIG. 1 to move the stage 21 holding the recording medium 9 below the discharge unit 3 (that is, on the (−Z) side) ( Move in the + Y) direction.

  FIG. 2 is a bottom view showing the discharge unit 3. The ejection unit 3 includes a plurality of heads 31 (four in the present embodiment) that eject different colors of ink toward the recording medium 9, and these heads 31 have the same structure. Have The plurality of heads 31 are arranged in the Y direction (that is, the moving direction) and attached to the attachment portion 30 of the discharge unit 3. Each head 31 has a plurality of ejection openings arranged in the X direction (hereinafter referred to as “width direction”) perpendicular to the Y direction, which is the moving direction of the recording medium 9. The plurality of discharge ports are not necessarily arranged in the X direction, and may be arranged in a direction intersecting the Y direction.

  The most (−Y) side head 31 in FIG. 2 ejects K (black) color ink, and the (+ Y) side head 31 of the K head 31 ejects C (cyan) color ink. The (+ Y) side head 31 of the C heads 31 ejects M (magenta) color ink, and the (+ Y) side head 31 ejects Y (yellow) color ink. The discharge unit 3 may be provided with heads for other colors such as light cyan, light magenta, and white.

  In the image recording apparatus 1, each head 31 is provided over the entire recording area on the recording medium 9 in the X direction that is the width direction (in the present embodiment, over the entire X direction of the recording medium 9). It is done. Then, the main body control unit 41 (see FIG. 3) controls the discharge unit 3 and the moving mechanism 2, and the position where the recording medium 9 faces the plurality of heads 31 of the discharge unit 3 is determined with respect to the discharge unit 3. By relatively moving in the direction only once (that is, passing once), the image recording on the recording medium 9 is completed. In other words, the image recording apparatus 1 performs single pass printing on the recording medium 9.

  The control unit 4 has a general computer system configuration in which a CPU that performs various arithmetic processes, a ROM that stores basic programs, and a RAM that stores various information are connected to a bus line. FIG. 3 is a block diagram showing functions of the control unit 4. FIG. 3 also shows a part of the configuration of the image recording apparatus 1 connected to the control unit 4. The control unit 4 includes a control unit 40 and a threshold matrix generation unit 43 that generates a threshold matrix. The control unit 40 includes the main body control unit 41 and a calculation unit 42 that performs various calculations. The control unit 40 is connected to an input unit 44 for inputting various types of information.

  The threshold value matrix generation unit 43 includes a storage unit 431 that stores a plurality of threshold value matrices and the like, and a threshold value determination unit 432 that determines threshold values assigned to a plurality of threshold value matrix elements that constitute each threshold value matrix. The threshold matrix generation unit 43 generates a plurality of threshold matrices used for halftoning a multi-tone original image. A method for generating the threshold matrix will be described later.

  The calculation unit 42 includes an image memory 421, a plurality of matrix storage units 422 (also referred to as SPM (Screen Pattern Memory)), and a comparator 423 (halftoning circuit) as a comparison unit. The image memory 421 stores color original image data (hereinafter referred to as “original image data”) input from the outside. The plurality of matrix storage units 422 are memories that store the threshold matrixes generated by the threshold matrix generation unit 43 for a plurality of color components, respectively. The comparator 423 is a comparison unit that compares the original image data and the threshold matrix for each color component. Note that the comparison unit may be realized by software.

  The main body control unit 41 includes a discharge control unit 411 and a movement control unit 412. The movement control unit 412 controls the relative movement of the recording medium 9 with respect to the ejection unit 3 by the movement mechanism 2 based on the output from the calculation unit 42. The ejection control unit 411 controls the ejection of ink from the plurality of heads 31 of the ejection unit 3 in synchronization with the relative movement of the recording medium 9 based on the output from the calculation unit 42.

  Next, details of the operation of the image recording apparatus 1 for recording an image will be described. In the image recording apparatus 1, color original image data is input from an external computer to the image memory 421 of the calculation unit 42 via the input unit 44 and stored. For each color component ink, the ink contact angle with respect to the recording medium 9 is input via the input unit 44. The contact angle varies depending on the type of the recording medium 9 and the type of ink. For example, when the surface tension of the ink is low or the surface of the recording medium 9 is made of a material that easily gets wet with ink, the contact angle becomes small. On the other hand, when the surface tension of the ink is high or the surface of the recording medium 9 is made of a material that can easily repel ink, the contact angle becomes large. Then, for each color component, one threshold matrix is selected based on the contact angle from a plurality of threshold matrices stored in the storage unit 431 (that is, a plurality of threshold matrices generated by the threshold matrix generation unit 43). .

  In the image recording apparatus 1, for example, only one type of recording medium 9 is used, or the materials of the plurality of types of recording media 9 used are approximately the same. When the contact angle of the ink is almost determined, the surface tension of the ink is input from the input unit 44, and is used for image recording from a plurality of threshold matrixes stored in the storage unit 431 based on the surface tension. One threshold matrix may be selected. Further, since the contact angle is determined by the type of the recording medium 9 and the type of ink, the type of the recording medium 9 and the type of ink are input from the input unit 44, and based on the recording medium 9 and the type of ink. One threshold value matrix used for image recording may be selected. In any case, the selection of the one threshold value matrix is substantially performed based on the contact angle of the ink with respect to the recording medium 9.

  FIG. 4 is a diagram abstractly showing the original image 70 and the threshold matrix 81. In each of the original image 70 and the threshold value matrix 81, a column direction corresponding to the moving direction of the recording medium 9 (shown as a y direction in FIG. 4) and a row direction perpendicular to the column direction (in FIG. 4). A plurality of pixels or a plurality of elements are arranged in the x direction). In the following description, it is assumed that the original image 70 is expressed in the gradation range from 0 to 255.

  The original image data representing the original image 70 is compared with a threshold matrix 81 selected for each color component (that is, using the selected threshold matrix 81) to represent a multi-tone image represented by the original image data. Color halftone image data (actually, an image represented by an FM screen when recorded on the recording medium 9) is generated.

  When halftoning the original image, as shown in FIG. 4, the entire area indicated by the original image 70 is divided into a plurality of areas of the same size, and a repetitive area 71 serving as a halftoning unit is set. Is done. Each matrix storage unit 422 has a storage area corresponding to one repetitive area 71, and a threshold value is set for each address (coordinate) of this storage area to store the selected threshold value matrix 81. Yes. Conceptually, each repetition area 71 of the original image 70 and the threshold matrix 81 of each color component are superimposed, and the gradation value (pixel value) of the color component of each pixel in the repetition area 71 corresponds to the threshold matrix 81. By comparing with the threshold value to be recorded, it is determined whether or not to perform recording (formation of a dot of the color) at the position of the pixel on the recording medium 9.

  Actually, the gradation value of one pixel of the original image data is read out for each color component from the image memory 421 based on the address signal from the address generator included in the comparator 423 of FIG. On the other hand, the address generator also generates an address signal indicating the position in the repetitive area 71 corresponding to the pixel in the original image 70, specifies one threshold value in the threshold value matrix 81 for each color component, and reads it from the matrix storage unit 422. It is. Then, the gradation value from the image memory 421 and the threshold value from the matrix storage unit 422 are compared for each color component by the comparator 423, whereby the position of the pixel in the binary output image data of each color component ( Address) gradation value is determined. Therefore, when focusing on one color component, for example, the gradation value “1” is located at a position where the gradation value is larger than the corresponding threshold value of the threshold value matrix 81 in the multi-tone original image 70 shown in FIG. "(That is, a dot is placed), and the gradation value“ 0 ”is assigned to the remaining pixels (that is, no dot is placed), and the binary output image data is a halftone image of the color component. Generated as data.

  As described above, the calculation unit 42 of the control unit 40 illustrated in FIG. 3 uses the comparator 423 to correspond the plurality of gradation values included in the input original image data and the plurality of threshold values included in the threshold matrix 81 to each other. The original image data is halftoned by comparing the pixel positions to be performed (element positions in the threshold matrix 81) to generate halftone image data. In other words, the calculation unit 42 is a halftone image generation device that uses the threshold value matrix generated by the threshold value matrix generation unit 43 and generates a halftone image by halftoning the original image.

  In the image recording apparatus 1, the ejection control unit 411 of the main body control unit 41 controls the ejection of ink from each head 31 of the ejection unit 3 based on the halftone image data. In the main body control unit 41, the movement control unit 412 also controls the movement speed, movement amount, and the like of the recording medium 9 by the movement mechanism 2.

  In the image recording apparatus 1, when halftone image data (parts) of a part (for example, a plurality of (−y) side repeating regions 71) most first recorded in the original image 70 is generated for each color. When the movement control unit 412 drives the moving mechanism 2, the movement of the recording medium 9 is started. In parallel with the halftoning process (halftone image data generation process), ink from a plurality of ejection openings in each head 31 is synchronized with the relative movement of the recording medium 9 with respect to the ejection unit 3 in the scanning direction. Is controlled by the discharge controller 411.

  Here, since the halftone image data is data for recording an image on the recording medium 9, it can be considered that a plurality of pixels of the halftone image data are set and arranged on the recording medium 9. . In the ejection control unit 411, a halftone corresponding to the ejection position (that is, the dot formation position) of each ejection port on the recording medium 9 in synchronization with the relative movement of the ejection unit 3 serving as a dot formation unit with respect to the recording medium 9. When the gradation value of the image data is “1”, a dot is formed at the ejection position. When the gradation value of the halftone image data is “0”, no dot is formed at the ejection position. .

  In this way, for each of black, cyan, magenta, and yellow, a plurality of ejection positions on the recording medium 9 corresponding to the plurality of ejection ports are moved relative to the recording medium 9 while moving a plurality of ejection positions. Based on halftone image data that is a comparison result between the gradation value of the original image 70 at the position and the corresponding threshold value of the threshold value matrix 81 for halftoning the original image 70, a plurality of ejection openings of each head 31. Is controlled (that is, the amount of ink discharged from each discharge port is determined).

  In the image recording apparatus 1, for black, cyan, magenta, and yellow, an operation of generating halftone image data and recording an image according to the halftone image data is performed in parallel, and a color original image is recorded on the recording medium 9. A color halftone image representing the above is recorded.

  Next, FIG. A and FIG. With reference to B, generation of a threshold matrix by the threshold matrix generation unit 43, that is, determination of threshold values of a plurality of threshold matrix elements in the threshold matrix will be described. In the image recording apparatus 1, for each color component, a plurality of threshold matrixes corresponding to a plurality of ink contact angles with respect to the recording medium are generated. In the following description, generation of a first threshold matrix and a second threshold matrix included in a plurality of threshold matrices related to one color component will be described. The first threshold value matrix corresponds to the first contact angle of ink with respect to the recording medium, and the second threshold value matrix corresponds to a second contact angle larger than the first contact angle.

  As shown in FIG. 6, the matrix area where the first threshold value matrix 81 is generated is in the column direction (y direction) corresponding to the moving direction of the recording medium 9 and the row direction (x direction) perpendicular to the column direction. It is defined (the same applies to the second threshold matrix). In other words, the plurality of threshold value matrix elements 811 included in the first threshold value matrix 81 are arranged in the row direction and the column direction. The first threshold value matrix 81 and each threshold value matrix element 811 are assumed to be square. In FIG. 6, only threshold matrix elements 811 near the corners of the first threshold matrix 81 are depicted.

  In the threshold value matrix generation unit 43, first, the first threshold value is given to one threshold value matrix element 811 among the plurality of threshold value matrix elements 811 included in the first threshold value matrix 81 (step S11). The threshold matrix element 811 to which the threshold has been assigned is hereinafter referred to as “assigned element”. Subsequently, one threshold value matrix element 811 to which no threshold value is assigned (hereinafter referred to as “unassigned element”) is selected (step S12).

  Next, based on the distance between the selected unassigned element that is one selected unassigned element and each assigned element, a grant is given when a threshold is assigned to the selected unassigned element An evaluation value indicating an uneven distribution of elements is obtained (step S13). Actually, since the repeated area 71 (see FIG. 4) is repeated vertically and horizontally in the original image 70, when the evaluation value is calculated, the first threshold matrix 81 is repeatedly applied when the original image 70 is halftone. In consideration, after arranging the nine first threshold value matrices 81 in a square shape, the selected unassigned elements of the central first threshold value matrix 81 and the assigned values located at the same position in each of the nine first threshold value matrices 81 An evaluation value is obtained based on each distance between the elements.

  FIG. B is a diagram illustrating in detail a flow of calculation of an evaluation value in step S13. In the calculation of the evaluation value, first, an application ratio that is the ratio of the number of elements already applied to the number of all threshold matrix elements 811 of the first threshold value matrix 81 is obtained. When the grant ratio is less than the predetermined first ratio, a weight coefficient R1 described later is 1 (steps S131 and S132). The first ratio is greater than 0% and less than 20%. When the grant ratio is greater than or equal to the first ratio and less than or equal to the second ratio, the weighting factor R1 is a predetermined value that is greater than 0 and less than 1 (steps S131 and S133). The second rate is greater than the first rate and less than 100%. When the grant ratio is greater than the second ratio, the weighting factor R1 is 1 (steps S131 and S132). In the present embodiment, the first ratio is 10% and the second ratio is 70%.

  When the weighting factor R1 is determined, for each given element, the first distance dxi, which is the distance between the selected unassigned element in the row direction and each given element, the unselected element in the column direction, and each given Based on the second distance dyi, which is the distance between the already-processed elements, and the weighting factor R1, the evaluation value element E1 is obtained using Equation 1 (step S134).

  The subscript i in Equation 1 is an integer from 0 to 8, and indicates the nine first threshold value matrices 81 described above. As the first distance dxi increases, the evaluation value element E1 decreases. The evaluation value element E1 also decreases when the second distance dyi increases. The weight coefficient R1 is a coefficient that changes the influence of the first distance dxi and the influence of the second distance dyi on the evaluation value element E1. When the weight coefficient R1 is less than 1, the influence of the first distance dxi on the evaluation value element E1 is larger than the influence of the second distance dyi on the evaluation value element E1. When the weight coefficient R1 is 1, the influence of the first distance dxi and the influence of the second distance dyi on the evaluation value element E1 are equal.

  When the evaluation value elements E1 related to all the assigned elements are obtained, the evaluation value elements E1 selected in step S12 are obtained by summing up the evaluation value elements E1 (step S135). . As described above, the evaluation value indicates a bias in the distribution of the assigned elements when it is assumed that a threshold is assigned to the unselected elements. When the evaluation value is large, the distribution of the assigned elements is large, and the assigned elements exist in a certain amount in the matrix area. When the evaluation value is small, the distribution of the assigned elements is small, and the assigned elements are distributed approximately uniformly in the matrix region.

  When the weighting factor R1 used in step S134 is greater than 0 and less than 1, that is, when the grant ratio is greater than or equal to the first ratio and less than or equal to the second ratio, the evaluation relating to the unassigned element The influence of the first distance dxi on the deviation indicated by the value is larger than the influence of the second distance dyi on the deviation. In Equation 1, the square of the first distance dxi is multiplied by the reciprocal of the weighting factor R1, and the square of the second distance dyi is multiplied by the weighting factor R1, but in step S134, the second against the bias indicated by the evaluation value is obtained. The evaluation value element E1 only needs to be obtained by substantially multiplying the first distance dxi or the second distance dyi by a weighting factor that makes the influence of the first distance dxi larger than the influence of the second distance dyi.

  When the weighting factor R1 used in step S134 is 1, that is, when the assignment ratio is less than the first ratio or greater than the second ratio, the first distance with respect to the bias indicated by the evaluation value related to the selection unassigned element The influence of dxi is equal to the influence of the second distance dyi. In the present embodiment, the weighting factor R1 in Equation 1 is 1. However, if the influence of the first distance dxi is equal to the influence of the second distance dyi, for example, the weighting factor R1 from Equation 1 is used. May be omitted, and the evaluation value element E1 may be obtained.

  When there is an unassigned element for which an evaluation value is not found (step S14), the process returns to step S12, selection of one unassigned element, calculation of the evaluation value element E1 related to each assigned element, and evaluation value An evaluation value that is the sum of the elements E1 is calculated (steps S12 and S13). In the image recording apparatus 1, steps S12 and S13 are repeated for all unassigned elements to obtain evaluation values (step S14).

  Subsequently, one unassigned element having the smallest evaluation value is specified with reference to the evaluation values obtained for all the unassigned elements. In other words, among all the unassigned elements, one unassigned element having the smallest distribution bias of the given elements when the threshold value is assumed is specified. Then, the next unassigned element is given the next threshold value (that is, the next largest threshold value after the most recently given threshold value) (step S15). In the present embodiment, as the next threshold value, a threshold value that is larger by one than the most recently assigned threshold value is assigned. Note that the threshold increment may be changed as appropriate.

  If there is an unassigned element that is a threshold value matrix element 811 to which no threshold value is assigned (step S16), the process returns to step S12, and evaluation values are obtained for all unassigned elements (steps S12 to S14). The next threshold value is assigned to the smallest unassigned element (step S15). In the image recording apparatus 1, steps S <b> 12 to S <b> 16 are repeated until threshold values are assigned to all of the plurality of threshold value matrix elements 811 constituting the first threshold value matrix 81. Thereby, all the threshold values assigned to the plurality of threshold value matrix elements 811 constituting the first threshold value matrix 81 are determined.

  Actually, the threshold values assigned to the threshold value matrix elements 811 in steps S11 to S16 are temporary threshold values before normalization, and these temporary threshold values are based on the number of gradations of an image recorded on the recording medium 9. The final threshold value is acquired by normalization, and the threshold value is assigned to each threshold value matrix element 811. The first threshold value matrix 81 is stored in the storage unit 431 (see FIG. 4).

  When the determination of the threshold value assigned to the plurality of threshold value matrix elements 811 constituting the first threshold value matrix 81 is completed, the threshold value assigned to the plurality of threshold value matrix elements constituting the second threshold value matrix is determined. Determination of threshold values in the second threshold matrix is shown in FIG. A and FIG. This is performed in substantially the same manner as the determination of threshold values in the first threshold value matrix shown in B. First, the threshold value matrix generating unit 43 assigns the first threshold value to one threshold value matrix element among a plurality of threshold value matrix elements included in the second threshold value matrix (step S11). Subsequently, one unassigned element is selected (step S12).

  Next, based on the distance between each selected element and the selected unassigned element, which is one selected unassigned element, while taking into account the repeated application of the second threshold matrix at the time of halftoning the original image Then, an evaluation value indicating the bias of the distribution of the assigned elements when a threshold value is assigned to the selected unassigned element to obtain the assigned element is obtained (step S13).

  Specifically, a grant ratio that is a ratio of the number of elements already granted to the number of all threshold matrix elements of the second threshold matrix is obtained, and when the grant ratio is less than the first ratio, or the second If it is larger than the ratio, the weighting factor R1 is 1 (steps S131 and S132). When the grant ratio is equal to or higher than the first ratio and equal to or lower than the second ratio, the weighting factor R1 is a predetermined value that is greater than 0 and less than 1 (steps S131 and S133).

  Here, the weighting factor R1 in step S134 when determining the threshold value of the first threshold matrix is referred to as “first weighting factor”, and the weighting factor R1 in step S134 when determining the threshold value of the second threshold matrix is “second weighting factor”. The second weighting factor is smaller than the first weighting factor. In the present embodiment, the first weighting factor is 0.8 and the second weighting factor is 0.6.

  When the weighting factor R1 is determined, the evaluation value element E1 is obtained by the above-described equation 1 based on the first distance dxi, the second distance dyi, and the second weighting factor for each given element (step S134). . When the evaluation value elements E1 related to all the assigned elements are obtained, the evaluation value elements E1 selected in step S12 are obtained by summing up the evaluation value elements E1 (step S135). .

  As described above, since the second weighting coefficient is smaller than the first weighting coefficient, using the second weighting coefficient in the calculation of the evaluation value in step S135 makes the evaluation value smaller than when using the first weighting coefficient. The influence of the first distance dxi on the bias of the given element shown increases. More specifically, the influence of the first distance dxi on the bias indicated by the evaluation value is even greater than the influence of the second distance dyi on the bias indicated by the evaluation value. In other words, by using the second weighting coefficient in step S135, the ratio of the influence of the first distance dxi to the influence of the second distance dyi on the bias indicated by the evaluation value is larger than when the first weighting coefficient is used. growing.

  When there is an unassigned element for which an evaluation value is not found (step S14), the process returns to step S12, selection of one unassigned element, calculation of the evaluation value element E1 related to each assigned element, and evaluation value An evaluation value that is the sum of the elements E1 is calculated (steps S12 and S13). In the image recording apparatus 1, steps S12 and S13 are repeated for all unassigned elements to obtain evaluation values (step S14).

  Subsequently, with reference to the evaluation values obtained for all unassigned elements, the next threshold value is assigned to the one unassigned element having the smallest evaluation value (step S15). If there is an unassigned element (step S16), the process returns to step S12, evaluation values are obtained for all unassigned elements (steps S12 to S14), and the next threshold value is assigned to the unassigned element having the smallest evaluation value. Is given (step S15). In the image recording apparatus 1, steps S12 to S16 are repeated until threshold values are assigned to all of the plurality of threshold value matrix elements constituting the second threshold value matrix. As a result, all the threshold values assigned to the plurality of threshold value matrix elements constituting the second threshold value matrix are determined. The second threshold value matrix is stored in the storage unit 431.

  When the image recording is performed in the image recording apparatus 1, as described above, one threshold value matrix is generated based on the contact angle of the ink with respect to the recording medium 9 from the plurality of threshold value matrices generated by the threshold value matrix generating unit 43. Is selected. When the contact angle is relatively small (for example, when the contact angle is less than 45 °), the control unit 4 selects the first threshold matrix and generates a halftone image using the first threshold matrix.

  FIG. A is a diagram showing a halftone image generated by halftoning a 25% density tint image using a first threshold matrix. As described above, in the generation of the first threshold value matrix, the evaluation value is shown when the application ratio is 10% or more and 70% or less (that is, the intermediate area of the image density is 10% density or more and 70% density or less). A first weighting factor (= 0.8) is used that makes the influence of the first distance dxi on the bias larger than the influence of the second distance dyi.

  As a result, FIG. As shown in A, in the x direction, which is the row direction of the halftone image, a plurality of dots (that is, pixels to which ink droplets are to be applied when an image is recorded on the recording medium 9) are in the column direction. It becomes easier to continue compared to the y direction. When a plurality of continuous dots and one dot that is not continuous with other dots are referred to as one “dot group”, the number of dot groups per unit length in the x direction is the unit in the y direction. Less than the number of dots per length.

  FIG. B is shown in FIG. It is a figure which shows the spatial frequency characteristic of the dot distributed in the halftone image shown to A. FIG. FIG. In B, there is a colored ring-shaped region centered on the origin with the center of the figure as the origin. The diameters in the x and y directions of the ring-shaped regions are shown in FIG. This corresponds to the frequency of appearance of dot groups per unit length in the x and y directions in A. The smaller the diameter, the lower the frequency of appearance of dot groups, and the smaller the number of dot groups per unit length. In other words, the smaller the diameter of the ring-shaped region, the more likely the dots are continuous with other dots (the same applies to FIGS. 8.B and 9.B). FIG. B also shows that a plurality of dots are more likely to be continuous in the row direction of the halftone image than in the column direction.

  In the image recording apparatus 1, FIG. Ink droplets are ejected onto the recording medium 9 based on the halftone image shown in FIG. On the recording medium 9, as described above, in the X direction (that is, the width direction) corresponding to the row direction, the pixel to which the ink droplet is to be applied is in the Y direction (that is, the recording direction) corresponding to the column direction. The relative movement direction of the medium 9 is likely to be continuous. For this reason, the ink droplets that have landed on the recording medium 9 are more likely to be attracted in the X direction than in the Y direction by the ink that has landed first, and are prevented from being attracted in the Y direction. As a result, when an image is recorded on the recording medium 9 at the first contact angle, streak unevenness extending in the relative movement direction of the recording medium 9 (that is, an area to which no continuous ink is attached in the relative movement direction) ) Can be suppressed. Even if the streak unevenness extending in the X direction perpendicular to the relative movement direction of the recording medium 9 is generated, it is difficult for the person viewing the image to recognize it as unevenness.

  On the other hand, in the image recording apparatus 1, when the ink contact angle with respect to the recording medium 9 is relatively large (for example, the contact angle is 45), such as when the ink used for image recording is switched to an ink with a large surface tension. When the angle is greater than or equal to (°), the control unit 4 selects the second threshold matrix and generates a halftone image using the second threshold matrix.

  FIG. A is a diagram showing a halftone image generated by halftoning a 25% density tint image using a second threshold matrix. As described above, in the generation of the second threshold value matrix, the evaluation value is shown when the application ratio is 10% or more and 70% or less (that is, the intermediate region where the image density is 10% or more and 70% or less). A second weighting factor (= 0.6) is used that makes the influence of the first distance dxi on the bias larger than the influence of the second distance dyi.

  As a result, FIG. As shown in A, in the x direction that is the row direction of the halftone image, a plurality of dots are more likely to be continuous than in the y direction that is the column direction. FIG. B is shown in FIG. It is a figure which shows the spatial frequency characteristic of the dot distributed in the halftone image shown to A. FIG. FIG. B also shows that a plurality of dots are more likely to be continuous in the row direction of the halftone image than in the column direction.

  In the image recording apparatus 1, FIG. Ink droplets are ejected onto the recording medium 9 based on the halftone image shown in FIG. On the recording medium 9, as described above, in the X direction corresponding to the row direction, pixels to which ink droplets are to be applied are more likely to be continuous than in the Y direction corresponding to the column direction. For this reason, the ink droplets that have landed on the recording medium 9 are more likely to be attracted in the X direction than in the Y direction by the ink that has landed first, and are prevented from being attracted in the Y direction. As a result, when an image is recorded on the recording medium 9 at the second contact angle, it is possible to suppress the occurrence of stripe unevenness extending in the relative movement direction of the recording medium 9.

  Here, assuming an image recording apparatus that generates the threshold value matrix by obtaining the evaluation value element E1 without using the weighting factor R1 in step S134, the image recording apparatus (hereinafter referred to as “comparative example image recording apparatus”) is assumed. ), The halftone image of the 25% density tint image is shown in FIG. Like A. FIG. B is shown in FIG. It is a figure which shows the spatial frequency characteristic of the dot distributed in the halftone image shown to A. FIG. FIG. A and FIG. As shown in B, in the x direction and the y direction of the halftone image, the degree to which a plurality of dots are continuous is approximately the same. In other words, the number of dot groups per unit length in the x direction is approximately equal to the number of dot groups per unit length in the y direction. Therefore, in the image recording apparatus of the comparative example, the ink droplets that have landed on the recording medium are attracted approximately equally in the X direction and the Y direction by the ink that has landed first. For this reason, it is not possible to suppress the occurrence of streak irregularities that tend to occur in the relative movement direction of the recording medium.

  In contrast, in the image recording apparatus 1 according to the present embodiment, as described above, the first threshold value matrix and the second threshold value matrix generated using the first weight coefficient and the second weight coefficient, respectively, are used. An image is recorded. As a result, even when the image is recorded at the first contact angle in the intermediate region where the image density is 10% or more and 70% or less, the image is recorded at the second contact angle. Even at that time, it is possible to suppress the occurrence of uneven stripes extending in the relative movement direction of the recording medium 9. For this reason, the image recording apparatus 1 is particularly suitable for an image recording apparatus that performs single-pass printing in which streak unevenness extending in the relative movement direction of the recording medium 9 is relatively likely to occur.

  In addition, when an image is recorded at a second contact angle larger than the first contact angle, a second weighting factor that increases the influence of the first distance dxi on the bias indicated by the evaluation value is greater than the first weighting factor. The second threshold value matrix generated by use is used. As a result, FIG. A, FIG. B, FIG. A and FIG. As shown in B, the ratio of the number of dot groups in the row direction to the number of dot groups in the column direction of the halftone image generated using the second threshold matrix is generated using the first threshold matrix. Further, the ratio of the number of dot groups in the row direction to the number of dot groups in the column direction of the halftone image becomes smaller. In other words, in the halftone image generated using the second threshold matrix, the dots in the row direction are more likely to be continuous compared to the halftone image generated using the first threshold matrix. As a result, when the image recording is performed at the second contact angle at which the ink droplet is more likely to be attracted than the first contact angle, the ink droplet landed on the recording medium 9 is attracted in the Y direction. Can be suppressed to approximately the same level as when the image is recorded at the first contact angle. As described above, the image recording apparatus 1 can appropriately suppress the occurrence of stripe unevenness when an image is recorded at each contact angle with respect to a plurality of contact angles of ink with respect to the recording medium.

  In the image recording apparatus 1, as described above, in step S13, when the application ratio is less than the first ratio (in this embodiment, the highlight area where the image density is less than 10% density), the weighting factor R1. Becomes 1, and the influence of the first distance dxi and the influence of the second distance dyi on the bias indicated by the evaluation value are equal. This prevents the distance in the X direction between the ink droplets that have landed on the recording medium 9 from becoming excessively small in the highlight region in which the stripe unevenness extending in the relative movement direction of the recording medium 9 is difficult to recognize. It is possible to suppress the landed droplets from being drawn in the X direction.

  As described above, in step S13, when the application ratio is larger than the second ratio (in the present embodiment, the shadow area in which the image density is greater than 70% density), the weighting factor R1 is 1. The influence of the first distance dxi and the influence of the second distance dyi on the bias indicated by the evaluation value are equal. This prevents the distance in the X direction between the ink droplets that have landed on the recording medium 9 from becoming excessively small in the shadow region where the stripe unevenness extending in the relative movement direction of the recording medium 9 is difficult to be recognized. It is possible to suppress the drawn droplets from being drawn in the X direction.

  In step S134 described above, the first distance dxi, which is the distance between the selected unassigned element in the row direction and each assigned element, and the distance between the selected unassigned element in the column direction and each assigned element. Based on the second distance dyi and the weighting factor R1, the evaluation value element E1 is obtained by Equation 1. In the image recording apparatus 1, when the resolution Ry in the Y direction of the image recorded on the recording medium 9 and the resolution Rx in the X direction are different, the evaluation value element E1 is determined using another weighting factor R2 = Rx / Ry. Instead of the above equation 1, the equation 2 may be obtained.

  In Equation 2, the square of the first distance dxi is multiplied by the reciprocal of the weighting factor R1, and further, the reciprocal of the other weighting factor R2 is multiplied. Also, the square of the second distance dyi is multiplied by the weighting factor R1, and further, another weighting factor R2 is multiplied. When the resolution Ry is higher than the resolution Rx, the influence of the second distance dyi on the bias indicated by the evaluation value is excessively increased by multiplication of the reciprocal of R2 and R2, as compared with the case where the multiplication is not performed. Can be suppressed. This prevents the distance in the Y direction between the ink droplets that have landed on the recording medium 9 from becoming excessively smaller than the distance in the X direction, and streak unevenness due to the difference in resolution between the X direction and the Y direction. Can be suppressed.

  On the other hand, when the resolution Ry is lower than the resolution Rx, the influence of the first distance dxi on the bias indicated by the evaluation value is excessively increased by multiplication of the reciprocal of R2 and R2, as compared with the case where the multiplication is not performed. This can be suppressed. This prevents the distance in the Y direction between the ink droplets that have landed on the recording medium 9 from becoming excessively large compared to the distance in the X direction, and streak unevenness due to a difference in resolution between the X direction and the Y direction. Can be suppressed.

  In Equation 2, the square of the first distance dxi is multiplied by the reciprocal of the weighting factor R2, and the square of the second distance dyi is multiplied by the weighting factor R2, but in the calculation of the evaluation value element E1 in step S134, When the resolution Ry is higher than the resolution Rx, a weighting factor that makes the influence of the first distance dxi on the bias indicated by the evaluation value larger than the influence of the second distance dyi is substantially the first distance dxi or the second distance dyi. The evaluation value element E1 only has to be obtained by multiplying by. Further, when the resolution Ry is lower than the resolution Rx, a weighting factor that makes the influence of the first distance dxi on the bias indicated by the evaluation value smaller than the influence of the second distance dyi is the first distance dxi or the second distance dyi. It is only necessary that the evaluation value element E1 is obtained by substantially multiplying. This prevents the Y-direction distance between the ink droplets that have landed on the recording medium 9 from becoming excessively larger than the X-direction distance in the same manner as described above. The generation of muscle unevenness due to the difference between the two can be suppressed.

  The evaluation value element E1 calculated in step S134 does not necessarily need to gradually decrease as the first distance dxi and the second distance dyi increase. For example, the evaluation value element E1 is constant when the first distance dxi is less than the predetermined distance dxi0, and gradually decreases as the first distance dxi increases when the first distance dxi is greater than or equal to the distance dxi0. May be. The evaluation value element E1 is constant when the second distance dyi is less than the predetermined distance dyi0, and gradually decreases as the second distance dyi increases when the second distance dyi is greater than or equal to the distance dyi0. May be.

  The evaluation value element E1 gradually increases as the first distance dxi increases when the first distance dxi is less than the predetermined distance dxi0, and the first distance dxi increases when the first distance dxi is greater than or equal to the distance dxi0. You may decrease gradually as it becomes. The evaluation value element E1 gradually increases as the second distance dyi increases when the second distance dyi is less than the predetermined distance dyi0, and the second distance dyi when the second distance dyi is greater than or equal to the distance dyi0. As the value increases, it may gradually decrease. Thereby, in the highlight area where the density of the dots is low, the dots are arranged so as to be separated from each other. As the density of the dots increases, the dots can be appropriately arranged in the vicinity of the already arranged dots.

  Next, FIG. A to FIG. With reference to C, generation of a threshold matrix in the image recording apparatus according to the second embodiment of the present invention, that is, determination of threshold values of a plurality of threshold matrix elements in the threshold matrix will be described. The configuration of the image recording apparatus according to the second embodiment is the same as that of the image recording apparatus 1 shown in FIGS. 1 to 3, and the same reference numerals are given to the corresponding configurations in the following description.

  In the image recording apparatus according to the second embodiment, a plurality of threshold matrixes corresponding to a plurality of ink contact angles with respect to the recording medium are generated for each color component. In the following description, generation of a first threshold matrix and a second threshold matrix included in a plurality of threshold matrices related to one color component will be described. Similar to the first embodiment, the first threshold value matrix corresponds to the first contact angle of the ink with respect to the recording medium, and the second threshold value matrix corresponds to the second contact angle larger than the first contact angle.

  As shown in FIG. 11, the matrix area in which the first threshold value matrix 81 is generated is in the column direction (y direction) corresponding to the moving direction of the recording medium 9 and the row direction (x direction) perpendicular to the column direction. It is defined (the same applies to the second threshold matrix). In other words, the plurality of threshold value matrix elements 811 included in the first threshold value matrix 81 are arranged in the row direction and the column direction. The first threshold value matrix 81 and each threshold value matrix element 811 are assumed to be square. In FIG. 11, only a part of the threshold matrix elements 811 of the first threshold matrix 81 is drawn.

  In the image recording apparatus according to the second embodiment, first, among the plurality of threshold value matrix elements 811 included in the first threshold value matrix 81, a threshold value is temporarily assigned to one threshold value matrix element group to which no threshold value is assigned. (Step S21). In FIG. 11, parallel diagonal lines are added to a plurality of threshold value matrix elements 811 included in a threshold value matrix element group to which threshold values are provisionally assigned. The threshold value matrix element 811 to which the threshold value is provisionally assigned is hereinafter referred to as “temporary provision element”.

  The number of threshold value matrix elements 811 included in the threshold value matrix element group is obtained by dividing the number of all threshold value matrix elements 811 included in the first threshold value matrix 81 by the number of gradations of the image recorded on the recording medium 9. . In the present embodiment, the threshold matrix element group includes a plurality of threshold matrix elements 811, and the plurality of threshold matrix elements 811 use random numbers from all the threshold matrix elements 811 included in the first threshold matrix 81. Chosen. Note that the threshold matrix element group may include only one threshold matrix element 811. Further, the threshold matrix element group may be determined by various methods other than using random numbers.

  Next, in the first threshold value matrix 81, a first evaluation value indicating the distribution bias of the threshold value matrix elements 811 to which threshold values are given or provisionally given is obtained (step S22). In the state where the threshold value is provisionally assigned to the threshold value matrix element group for the first time, there is no assigned element that is the threshold value matrix element 811 to which the threshold value has already been assigned. Therefore, the first evaluation value corresponds to the threshold value matrix element group. The deviation of the distribution of only the pixels to be shown is shown.

  FIG. C is a diagram illustrating in detail a flow of calculation of a first evaluation value in step S22. In the calculation of the first evaluation value, first, a grant ratio, which is a ratio of the number of granted elements and provisional grant elements to the number of all threshold matrix elements 811 in the first threshold matrix 81, is obtained.

  When the application ratio is less than the above-described first ratio or greater than the above-described second ratio, the visual characteristic filter used for calculating the evaluation value described later has a deflectability in the row direction and the column direction. (Step S221, S222). The visual characteristic filter is a filter that extracts frequency components so as to match human visual characteristics. Also in the present embodiment, as in the first embodiment, the first ratio is 10% and the second ratio is 70%.

  FIG. A and FIG. Each B shows an example of a visual characteristic filter having no deflection. FIG. The filter shown in A is a blue noise filter that cuts low frequency components. FIG. A circular blank area 61 in the center in A corresponds to a frequency component cut by the filter, and an area 62 with parallel diagonal lines (that is, an area excluding the blank area 61) is a frequency component passing through the filter. Correspond. FIG. The filter shown in B is a green noise filter that cuts low frequency components and high frequency components. FIG. A ring-shaped region 63 with parallel diagonal lines in B corresponds to a frequency component passing through the filter, and a circular blank region 64 inside the region 63 and a blank region 65 outside the region 63 are: Corresponds to the frequency component cut by the filter. The ring-shaped region 63 has an annular shape in which the inner peripheral edge and the outer peripheral edge are substantially perfect circles.

  When the application ratio is equal to or higher than the first ratio and equal to or lower than the second ratio, the visual characteristic filter used for calculating the evaluation value corresponds to the y direction that is the column direction (that is, the relative movement direction of the recording medium 9). The visual characteristic filter has a deflection property in the direction (steps S221 and S223).

  FIG. A and FIG. Each B shows an example of a visual characteristic filter having a deflection property in the column direction. FIG. The filter shown in A is a blue noise filter that cuts low frequency components. FIG. A blank region 61 of an ellipse that is long in the y direction at the center in A (that is, an ellipse having a major axis parallel to the y direction and a minor axis parallel to the x direction) corresponds to the frequency component cut by the filter. The region 62 with parallel diagonal lines (that is, the region excluding the blank region 61) corresponds to the frequency component passing through the filter (the same applies to FIG. 14A). FIG. The filter shown in B is a green noise filter that cuts low frequency components and high frequency components. FIG. A ring-shaped region 63 with parallel diagonal lines in B corresponds to a frequency component passing through the filter, and blank regions 64 and 65 inside and outside the region 63 correspond to frequency components cut by the filter. To do. The ring-shaped region 63 has an elliptical ring shape in which the inner peripheral edge and the outer peripheral edge are ellipses long in the y direction (the same applies to FIG. 14.B).

  When the visual characteristic filter is determined, in the first threshold value matrix 81, the power spectrum is calculated by Fourier-transforming the distribution of the added elements and the provisional elements. Subsequently, the above-described visual characteristic filter determined in steps S221 to S223 is applied to the power spectrum. Then, the sum of the differences between the power spectrum before application of the visual characteristic filter and the power spectrum after application of the visual characteristic filter is obtained as a first evaluation value (step S224). As described above, the first evaluation value indicates a bias in the distribution of the granted element and the provisional grant element. When the first evaluation value is large, the distribution of the provisional elements and provisional provision elements is largely biased, and the provisional elements and provisional provision elements exist in a certain amount in the matrix region. If the evaluation value is small, the distribution of the provisional elements and the provisional provision elements is small, and the imparted elements are distributed almost uniformly in the matrix region.

  When the visual characteristic filter used in step S224 is a filter having a deflectability in the column direction, that is, when the application ratio is equal to or higher than the first ratio and equal to or lower than the second ratio, the granted element and the provisional grant element The influence of the distance in the x direction (hereinafter referred to as “first distance group”) on the bias indicated by the first evaluation value is the y direction between the given element and the provisionally given element. (Hereinafter referred to as “second distance group”) is greater than the influence of the first evaluation value on the bias. When the visual characteristic filter used in step S224 does not have a deflectability in the row direction and the column direction, that is, when the application ratio is less than the first ratio or greater than the second ratio, the first evaluation value is The influence of the first distance group and the influence of the second distance group on the shown bias are equal.

  When the first evaluation value is obtained, one threshold value matrix element 811 is removed from the threshold value matrix element group, and another threshold value matrix element 811 that is not given a threshold value and is not provisionally given is It is included in the threshold matrix element group. In other words, one threshold value matrix element 811 in the threshold value matrix element group is replaced with another threshold value matrix element 811 to which a threshold value is not given or provisionally given (step S23). Then, similarly to steps S21 and S22, a threshold value is provisionally assigned to the threshold matrix element group, and a second evaluation value indicating the distribution deviation of the provisioned element and the provisional provision element is obtained (steps S24 and S25). The calculation of the second evaluation value is the same as the calculation of the first evaluation value. Specifically, the above-described visual characteristic filter is applied to the power spectrum obtained by Fourier transforming the distribution of the provisioned element and the provisional provision element, and the total sum of the power spectrum differences before and after the application of the visual characteristic filter is It is obtained as the second evaluation value.

  When the second evaluation value is obtained, the first evaluation value and the second evaluation value are compared (step S26). If the first evaluation value is smaller than the second evaluation value, that is, the first evaluation value is the second value. If the biased elements and the provisional provision elements are smaller than the evaluation values, the two threshold matrix elements 811 replaced in step S23 are replaced again, and the threshold matrix elements are returned to the state in step S21 (step S21). S27). Further, when the first evaluation value is larger than the second evaluation value, that is, when the first evaluation value indicates a larger bias between the given element and the provisional grant element than the second evaluation value, the first evaluation value is the second The value is changed to a value equal to the evaluation value (step S28).

  Thereafter, it is confirmed whether or not a predetermined end condition is satisfied (step S29). If the end condition is not satisfied, the process returns to step S23. Then, while changing the threshold matrix element 811 included in the threshold matrix element group, steps S23 to S28 are repeated until a predetermined end condition is satisfied (step S29). The termination condition is, for example, that the number of repetitions of steps S23 to S28 reaches a predetermined number. Alternatively, the end condition may be that the first evaluation value is equal to or less than a predetermined value, that is, the bias of the assigned element and the provisional grant element indicated by the first evaluation value is equal to or less than the degree of the predetermined bias. .

  When the end condition is satisfied, a threshold is assigned to the threshold matrix element group at the time when the end condition is satisfied, that is, the plurality of threshold matrix elements 811 of the threshold matrix element group after step S29 (step S30). Subsequently, one threshold value matrix element group is changed to the next one threshold value matrix element group (steps S31 and S32), and the process returns to step S21 to temporarily assign a threshold value to the threshold value matrix element group (step S21). . Thereafter, the above-described steps S22 to S30 are performed, and a threshold value is given to the threshold matrix element group. In the image recording apparatus, steps S <b> 21 to S <b> 32 are repeated until threshold values are assigned to all threshold value matrix elements 811 included in the first threshold value matrix 81. Thereby, all the threshold values assigned to the plurality of threshold value matrix elements 811 constituting the first threshold value matrix 81 are determined. The first threshold value matrix 81 is stored in the storage unit 431.

  When the determination of the threshold value assigned to the plurality of threshold value matrix elements 811 constituting the first threshold value matrix 81 is completed, the threshold value assigned to the plurality of threshold value matrix elements constituting the second threshold value matrix is determined. Determination of threshold values in the second threshold matrix is shown in FIG. A to FIG. The determination is performed in substantially the same manner as the determination of threshold values in the first threshold value matrix shown in FIG. First, the threshold value matrix generation unit 43 provisionally assigns a threshold value to one threshold value matrix element group to which no threshold value is assigned among a plurality of threshold value matrix elements included in the second threshold value matrix (step S21). Subsequently, in the second threshold value matrix, a first evaluation value indicating the distribution bias of the threshold value matrix element 811 to which the threshold value is given or provisionally given is obtained (step S22).

  Specifically, a grant ratio that is a ratio of the number of elements already granted to the number of all threshold matrix elements in the second threshold matrix is obtained, and the grant ratio is less than the first ratio, or the second ratio Is larger than the visual characteristic filter, the visual characteristic filter does not have a deflectability in the row direction and the column direction (steps S221 and S222). Further, when the application ratio is equal to or higher than the first ratio and equal to or lower than the second ratio, the visual characteristic filter becomes a visual characteristic filter having a deflection property in the column direction (steps S221 and S223).

  FIG. A and FIG. Each B shows an example of a visual characteristic filter having a deflection property in the column direction. FIG. A is a blue noise filter, and FIG. B is a green noise filter.

  Here, the visual characteristic filter in step S22 and step S25 when the threshold value of the first threshold matrix is determined is referred to as “first filter”, and the visual characteristic filter in step S22 and step S25 when the threshold value of the second threshold matrix is determined is “ When referred to as a “second filter”, the deflectability of the second filter in the column direction is stronger than the deflectability of the first filter in the column direction. FIG. A first filter of A and FIG. When compared with the second filter of A, FIG. A blank area 61 of the second filter of A is shown in FIG. The width in the x direction, which is the row direction, is smaller than the blank area 61 of the first filter A, and the length in the y direction, which is the column direction, is longer. FIG. B first filter and FIG. Compared with the second filter of B, FIG. The ring-shaped region 63 and the blank region 64 of the second filter B are shown in FIG. The width in the x direction that is the row direction is smaller and the length in the y direction that is the column direction is longer than the ring-shaped region 63 and the blank region 64 of the first filter B.

  When the visual characteristic filter is determined, the power spectrum is calculated by performing Fourier transform on the distribution of the added elements and the provisional elements in the second threshold value matrix. Subsequently, the second filter that is the above-described visual characteristic filter determined in steps S221 to S223 is applied to the power spectrum. And the sum total of the difference of the power spectrum before and behind application of a 2nd filter is calculated | required as a 1st evaluation value (step S224). As described above, since the deflectability in the column direction of the second filter is stronger than the deflectability in the column direction of the first filter, by using the second filter in the calculation of the first evaluation value in step S22, Compared to the case where the first filter is used, the influence of the first distance group on the bias indicated by the first evaluation value is further greater than the influence of the second distance group on the bias indicated by the first evaluation value. growing. In other words, by using the second filter in step S22, the ratio of the influence of the first distance group to the influence of the second distance group on the bias indicated by the first evaluation value is larger than when the first filter is used. growing.

  When the first evaluation value is obtained, one threshold value matrix element is removed from the threshold value matrix element group, and another threshold value matrix element that is not given a threshold value and is not provisionally assigned is a threshold value matrix. It is included in the element group (step S23). Then, similarly to steps S21 and S22, a threshold value is provisionally assigned to the threshold matrix element group, and a second evaluation value indicating the distribution deviation of the provisioned element and the provisional provision element is obtained (steps S24 and S25). The calculation of the second evaluation value is the same as the calculation of the first evaluation value. Specifically, the above-described second filter is applied to the power spectrum obtained by Fourier transforming the distribution of the provisioned element and the provisional provision element, and the total sum of the power spectrum differences before and after the application of the second filter is: It is obtained as the second evaluation value.

  When the second evaluation value is obtained, the first evaluation value and the second evaluation value are compared (step S26). If the first evaluation value shows a bias smaller than the second evaluation value, the second evaluation value is replaced in step S23. The two threshold value matrix elements 811 are replaced again, and the threshold value matrix element group is returned to the state of step S21 (step S27). Further, when the first evaluation value shows a bias larger than the second evaluation value, the first evaluation value is changed to a value equal to the second evaluation value (step S28). And while changing the threshold value matrix element included in the threshold value matrix element group, steps S23 to S28 are repeated until the above-described end condition is satisfied (step S29).

  When the end condition is satisfied, a threshold value is assigned to the plurality of threshold value matrix elements 811 of the threshold value matrix element group after step S29 (step S30). Subsequently, one threshold matrix element group is changed to the next threshold matrix element group (steps S31 and S32), and the process returns to step S21. Then, steps S21 to S32 are repeated until threshold values are assigned to all threshold value matrix elements included in the second threshold value matrix. As a result, all the threshold values assigned to the plurality of threshold value matrix elements constituting the second threshold value matrix are determined. The second threshold value matrix is stored in the storage unit 431.

  When an image is recorded in the image recording apparatus, as described above, one threshold value matrix is generated from a plurality of threshold value matrices generated by the threshold value matrix generating unit 43 based on the contact angle of ink with respect to the recording medium 9. Selected. When the contact angle is relatively small (for example, when the contact angle is less than 45 °), the control unit 4 selects the first threshold matrix and generates a halftone image using the first threshold matrix.

  As described above, in the generation of the first threshold value matrix, when the application ratio is 10% or more and 70% or less (that is, the intermediate area where the image density is 10% density or more and 70% density or less), A first filter that is a visual characteristic filter having a deflectability is used. As a result, FIG. Similarly to A, in the x direction, which is the row direction of the halftone image, a plurality of dots (that is, pixels to which ink droplets are to be applied when an image is recorded on the recording medium 9) are in the column direction y It becomes easier to continue compared to the direction.

  Then, ink droplets are ejected onto the recording medium 9 based on the halftone image. On the recording medium 9, as described above, in the X direction (that is, the width direction) corresponding to the row direction, the pixel to which the ink droplet is to be applied is in the Y direction (that is, the recording direction) corresponding to the column direction. The relative movement direction of the medium 9 is likely to be continuous. For this reason, the ink droplets that have landed on the recording medium 9 are more likely to be attracted in the X direction than in the Y direction by the ink that has landed first, and are prevented from being attracted in the Y direction. As a result, when an image is recorded on the recording medium 9 at the first contact angle, it is possible to suppress the occurrence of stripe unevenness extending in the relative movement direction of the recording medium 9.

  On the other hand, in the image recording apparatus, when the ink used for image recording is switched to an ink having a large surface tension, the contact angle of the ink with respect to the recording medium 9 is relatively large (for example, the contact angle is 45 °). In this case, the control unit 4 selects the second threshold value matrix, and generates a halftone image using the second threshold value matrix.

  As described above, in the generation of the second threshold value matrix, when the application ratio is 10% or more and 70% or less (that is, the intermediate area where the image density is 10% density or more and 70% density or less), A second filter that is a visual characteristic filter having a deflectability is used. As a result, FIG. Similar to A, in the x direction that is the row direction of the halftone image, a plurality of dots are more likely to be continuous than in the y direction that is the column direction.

  Then, ink droplets are ejected onto the recording medium 9 based on the halftone image. On the recording medium 9, as described above, in the X direction corresponding to the row direction, pixels to which ink droplets are to be applied are more likely to be continuous than in the Y direction corresponding to the column direction. For this reason, the ink droplets that have landed on the recording medium 9 are more likely to be attracted in the X direction than in the Y direction by the ink that has landed first, and are prevented from being attracted in the Y direction. As a result, when an image is recorded on the recording medium 9 at the second contact angle, it is possible to suppress the occurrence of stripe unevenness extending in the relative movement direction of the recording medium 9.

  As described above, the image recording apparatus according to the second embodiment records an image using the first threshold value matrix and the second threshold value matrix generated using the first filter and the second filter, respectively. Thus, even when the image is recorded at the first contact angle in the intermediate region where the image density is 10% or more and 70% or less, the image is recorded at the second contact angle. Even at that time, it is possible to suppress the occurrence of uneven stripes extending in the relative movement direction of the recording medium 9.

  In addition, when an image is recorded at a second contact angle larger than the first contact angle, a second threshold value generated using a second filter that is more deflectable in the column direction than the first filter. A matrix is used. Thus, the ratio of the number of dot groups in the row direction to the number of dot groups in the column direction of the halftone image generated using the second threshold matrix is the halftone generated using the first threshold matrix. The ratio of the number of dot groups in the row direction to the number of dot groups in the column direction of the image becomes smaller. In other words, in the halftone image generated using the second threshold matrix, the dots in the row direction are more likely to be continuous compared to the halftone image generated using the first threshold matrix. As a result, when the image recording is performed at the second contact angle at which the ink droplet is more likely to be attracted than the first contact angle, the ink droplet landed on the recording medium 9 is attracted in the Y direction. Can be suppressed to approximately the same level as when the image is recorded at the first contact angle.

  As described above, the image recording apparatus can appropriately suppress the occurrence of uneven stripes when an image is recorded at each contact angle with respect to a plurality of contact angles of ink with respect to the recording medium. For this reason, the image recording apparatus according to the second embodiment is also particularly suitable for an image recording apparatus that performs single-pass printing in which streak unevenness extending in the relative movement direction of the recording medium 9 is relatively likely to occur.

  In the image recording apparatus 1, as described above, in step S22, when the application ratio is less than the first ratio (in this embodiment, the highlight area where the image density is less than 10% density), the visual characteristic filter However, it becomes a visual characteristic filter having no deflection in the row direction and the column direction. Thus, as in the first embodiment, the distance in the X direction between the ink droplets that have landed on the recording medium 9 in the highlight area where the stripe unevenness extending in the relative movement direction of the recording medium 9 is difficult to be recognized. Can be prevented from becoming excessively small, and landing droplets can be prevented from being drawn in the X direction.

  Further, as described above, in step S22, when the application ratio is larger than the second ratio (in the present embodiment, the shadow area in which the image density is higher than 70% density), the visual characteristic filter is changed in the row direction. In addition, the visual characteristic filter has no deflection in the column direction. As a result, as in the first embodiment, the distance in the X direction between the ink droplets that have landed on the recording medium 9 is reduced in the shadow area where the stripe unevenness extending in the relative movement direction of the recording medium 9 is difficult to be recognized. It is possible to prevent the droplets from becoming excessively small and to suppress the landing droplets from being drawn in the X direction.

  As mentioned above, although embodiment of this invention has been described, this invention is not limited to the said embodiment, A various change is possible.

  In the image recording apparatus 1 according to the first embodiment, even in step S13, the application ratio is less than the first ratio (in this embodiment, the highlight area where the image density is less than 10% density). The weight coefficient R1 may be a predetermined value greater than 0 and less than 1. Also, when the value of the application ratio is larger than the second ratio (in this embodiment, the shadow area where the image density is higher than 70% density), the weighting factor R1 is greater than 0 and less than 1. May be a predetermined value. In other words, in the image recording apparatus 1, the influence of the first distance dxi on the bias indicated by the evaluation value is compared with the influence of the second distance dyi at least when the application ratio is not less than the first ratio and not more than the second ratio. The evaluation value element E1 only needs to be obtained by substantially multiplying the first distance dxi or the second distance dyi by the weighting factor to be increased.

  In the image recording apparatus according to the second embodiment, in steps S22 and S25, when the application ratio is less than the first ratio (in this embodiment, the highlight area where the image density is less than 10% density). Alternatively, the first evaluation value and the second evaluation value may be obtained using a visual characteristic filter having a deflection property in the column direction. Further, even when the value of the application ratio is larger than the second ratio (in this embodiment, a shadow area in which the image density is higher than 70% density), the visual characteristic filter having the deflection in the column direction is used. Then, the first evaluation value and the second evaluation value may be obtained. In other words, in the image recording apparatus 1, at least when the application ratio is not less than the first ratio and not more than the second ratio, the first evaluation value and the visual characteristic filter having a deflection property in the column direction are used. It is sufficient that the second evaluation value is obtained.

  In the image recording apparatus according to the second embodiment, a method such as greedy method, simulated annealing, tabu search, etc., is performed at the time of selection of threshold matrix elements in steps S21 and S32 and replacement of threshold matrix elements in step S23. May be applied. In addition, as a visual characteristic filter, Campbell's CSF (Contrast Sensitivity Function), Manno's CSF, Nasanen's CSF, Dary's CSF, Barten's CSF, or the like may be used.

  In the above-described image recording apparatus, if the recording medium 9 moves relative to the ejection unit 3 in the Y direction, for example, the ejection unit 3 moves above the stopped recording medium 9. May move in the Y direction. The structure of the image recording apparatus may be applied to, for example, an image recording apparatus that performs interlaced printing, or may be applied to an image recording apparatus that records an image on a long roll paper.

  A computer including the above-described threshold value matrix generation unit 43 may be used independently as a threshold value matrix generation device.

  The configurations in the above-described embodiments and modifications may be combined as appropriate as long as they do not contradict each other.

DESCRIPTION OF SYMBOLS 1 Image recording apparatus 2 Moving mechanism 9 Recording medium 31 Head 40 Control part 43 Threshold matrix production | generation part 70 Original image 81 Threshold matrix 431 Storage part 432 Threshold determination part 811 Threshold matrix element S11-S16, S21-S32, S131-S135, S221 ~ S224 step

Claims (9)

An image recording apparatus for recording an image on a recording medium by ejecting ink droplets from the ejecting unit toward a recording medium that moves relative to the ejecting unit in a predetermined moving direction. A threshold value matrix generation device for generating a threshold value matrix used for halftoning an original image,
A storage unit for storing a plurality of threshold matrixes;
A threshold value determination unit for determining threshold values assigned to a plurality of threshold value matrix elements constituting each threshold value matrix;
With
A matrix region in which each threshold value matrix is generated is defined in a column direction corresponding to the moving direction and a row direction perpendicular to the column direction;
The threshold value determination unit
a) assigning an initial threshold value to one threshold value matrix element included in each threshold value matrix;
b) selecting one unassigned element that is a threshold matrix element to which no threshold is assigned;
c) Based on the distance between the one unassigned element and each assigned element which is a threshold matrix element to which a threshold value has been assigned, considering repeated application of each threshold matrix at the time of halftoning the original image A step of obtaining an evaluation value indicating a distribution deviation of the assigned element when a threshold value is given to the one unassigned element to be an assigned element;
d) For all unassigned elements, the step of obtaining the evaluation value by repeating the step b) and the step c);
e) With reference to the evaluation value obtained in the step d), the step of assigning the next threshold value to one unassigned element with the smallest bias indicated by the evaluation value;
f) repeating the steps b) to e) until thresholds are assigned to all of the plurality of threshold matrix elements;
Run
Step c)
c1) For each given element, a first distance between the one unassigned element in the row direction and each given element, and the one unassigned element and each given element in the column direction Obtaining an evaluation value element based on the second distance between and
c2) calculating the evaluation value by summing the evaluation value elements of the given elements;
With
The plurality of threshold matrixes include a first threshold matrix corresponding to a first contact angle of ink with respect to a recording medium, and a second threshold matrix corresponding to a second contact angle larger than the first contact angle,
At least a ratio of the number of assigned elements to the number of all threshold matrix elements is a first ratio that is greater than 0% and less than 20%, and a second ratio that is greater than the first ratio and less than 100%. If
In the step c1), the evaluation value is obtained by multiplying the first distance or the second distance by a weighting factor that increases the influence of the first distance on the bias indicated by the evaluation value compared to the influence of the second distance. Elements are required,
The second weighting factor that is the weighting factor in the step c1) at the time of determining the threshold value of the second threshold value matrix is larger than the weighting factor that is the weighting factor in the step of c1) at the time of determining the threshold value of the first thresholding matrix. A threshold value matrix generating apparatus characterized by increasing the influence of the first distance on the bias indicated by the evaluation value. The threshold value matrix generation device according to claim 1,
In the step c), when the ratio of the number of assigned elements to the number of all threshold matrix elements is less than the first ratio, the influence of the first distance on the bias indicated by the evaluation value in the step c1) The threshold value matrix generation device characterized in that the influence of the second distance is equal. The threshold value matrix generation device according to claim 1 or 2,
When the resolution in the moving direction in the image recording apparatus is higher than the resolution in the width direction perpendicular to the moving direction, the first with respect to the bias indicated by the evaluation value when calculating the evaluation value element in the step c1). Another weighting factor that increases the influence of the distance compared to the influence of the second distance is multiplied by the first distance or the second distance;
When the resolution in the moving direction in the image recording apparatus is lower than the resolution in the width direction, the influence of the first distance on the bias indicated by the evaluation value is calculated when calculating the evaluation value element in the step c1). The threshold value matrix generation device, wherein the first distance or the second distance is multiplied by another weighting factor that is smaller than the influence of the second distance. In an image recording apparatus that records an image on a recording medium by ejecting ink droplets from the ejection unit toward a recording medium that moves relative to the ejection unit in a predetermined movement direction, A threshold matrix generation device for generating a threshold matrix used for halftoning an image,
A storage unit for storing a plurality of threshold matrixes;
A threshold value determination unit for determining threshold values assigned to a plurality of threshold value matrix elements constituting each threshold value matrix;
With
A matrix region in which each threshold value matrix is generated is defined in a column direction corresponding to the moving direction and a row direction perpendicular to the column direction;
The threshold value determination unit
a) provisionally assigning a threshold value to one threshold value matrix element group to which no threshold value is given in each threshold value matrix;
b) obtaining a first evaluation value indicating a bias in the distribution of threshold matrix elements to which threshold values are given or provisionally given;
c) removing one threshold value matrix element from the one threshold value matrix element group and including another threshold value matrix element to which no threshold value is assigned or provisionally assigned in the one threshold value matrix element group, and the step a) And calculating a second evaluation value indicating a bias in the distribution of threshold matrix elements to which threshold values are given or provisionally given, and
d) When the first evaluation value shows a bias smaller than the second evaluation value, the one threshold matrix element group is returned to the state of the step a), and the first evaluation value is the second evaluation value. Changing the first evaluation value to a value equal to the second evaluation value when exhibiting a greater bias than
e) a step of repeating the c) step and the d) step while changing a threshold matrix element included in the one threshold matrix element group in the c) step until a predetermined end condition is satisfied;
f) providing a threshold to the one threshold matrix element group after the e);
g) The one threshold matrix element group is changed to the next one threshold matrix element group until threshold values are given to all threshold matrix elements included in the threshold matrix, and the steps a) to f) Repeating the process,
Run
In the step b), a visual characteristic filter is applied to a power spectrum obtained by performing Fourier transform on the distribution of threshold matrix elements to which threshold values are given or provisionally given, and the difference in power spectrum before and after application of the visual characteristic filter is calculated. The sum is taken as the first evaluation value,
In the step c), the visual characteristic filter is applied to a power spectrum obtained by performing Fourier transform on the distribution of threshold matrix elements to which threshold values are given or provisionally given, and the difference in power spectrum before and after the visual characteristic filter is applied. Is the second evaluation value,
The plurality of threshold matrixes include a first threshold matrix corresponding to a first contact angle of ink with respect to a recording medium, and a second threshold matrix corresponding to a second contact angle larger than the first contact angle,
At least a ratio of the number of threshold matrix elements to which a threshold is given or provisionally given to the total number of threshold matrix elements is not less than a first ratio that is greater than 0% and less than 20% and greater than the first ratio. When the ratio is equal to or less than a second ratio smaller than 100%, the second threshold value is more than the first filter which is a visual characteristic filter in the steps b) and c) when determining the threshold value of the first threshold value matrix. A threshold value matrix generating device characterized in that the second filter, which is a visual characteristic filter in the steps b) and c) when determining the matrix threshold value, has a strong deflection in the column direction. The threshold value matrix generation device according to claim 4,
In the step b) and the step c), when the ratio of the number of threshold matrix elements to which the threshold value is given or provisionally given to the total number of threshold matrix elements is less than the first ratio, the first filter and the The threshold value matrix generation device, wherein the second filter has no deflection in the column direction and the row direction. An image recording device,
An ejection unit that ejects micro droplets of ink toward the recording medium;
A movement mechanism for moving the recording medium relative to the ejection unit in the movement direction;
A threshold value matrix generation device according to any one of claims 1 to 5;
Using a single threshold matrix selected from the plurality of threshold matrices generated by the threshold matrix generation device based on the contact angle of the ink with respect to the recording medium, halftoning a multi-tone original image A control unit that generates a halftone image and controls ejection of ink from the ejection unit based on the halftone image;
An image recording apparatus comprising: The image recording apparatus according to claim 6,
The image recording apparatus, wherein the recording of the image on the recording medium is completed by the relative movement of the recording medium only once in the movement direction with respect to the ejection unit. An image recording apparatus for recording an image on a recording medium by ejecting ink droplets from the ejecting unit toward a recording medium that moves relative to the ejecting unit in a predetermined moving direction. A threshold matrix generation method for generating a threshold matrix used for halftoning an original image,
Storing a plurality of threshold matrices;
Determining a threshold value assigned to a plurality of threshold value matrix elements constituting each threshold value matrix;
With
A matrix region in which each threshold value matrix is generated is defined in a column direction corresponding to the moving direction and a row direction perpendicular to the column direction;
Determining the threshold comprises:
a) assigning an initial threshold value to one threshold value matrix element included in each threshold value matrix;
b) selecting one unassigned element that is a threshold matrix element to which no threshold is assigned;
c) Based on the distance between the one unassigned element and each assigned element which is a threshold matrix element to which a threshold value has been assigned, considering repeated application of each threshold matrix at the time of halftoning the original image A step of obtaining an evaluation value indicating a distribution deviation of the assigned element when a threshold value is given to the one unassigned element to be an assigned element;
d) For all unassigned elements, the step of obtaining the evaluation value by repeating the step b) and the step c);
e) With reference to the evaluation value obtained in the step d), the step of assigning the next threshold value to one unassigned element with the smallest bias indicated by the evaluation value;
f) repeating the steps b) to e) until thresholds are assigned to all of the plurality of threshold matrix elements;
With
Step c)
c1) For each given element, a first distance between the one unassigned element in the row direction and each given element, and the one unassigned element and each given element in the column direction Obtaining an evaluation value element based on the second distance between and
c2) calculating the evaluation value by summing the evaluation value elements of the given elements;
With
The plurality of threshold matrixes include a first threshold matrix corresponding to a first contact angle of ink with respect to a recording medium, and a second threshold matrix corresponding to a second contact angle larger than the first contact angle,
At least a ratio of the number of assigned elements to the number of all threshold matrix elements is a first ratio that is greater than 0% and less than 20%, and a second ratio that is greater than the first ratio and less than 100%. If
In the step c1), the evaluation value is obtained by multiplying the first distance or the second distance by a weighting factor that increases the influence of the first distance on the bias indicated by the evaluation value compared to the influence of the second distance. Elements are required,
The second weighting factor that is the weighting factor in the step c1) at the time of determining the threshold value of the second threshold value matrix is larger than the weighting factor that is the weighting factor in the step of c1) at the time of determining the threshold value of the first thresholding matrix. A threshold matrix generation method characterized by increasing the influence of the first distance on the bias indicated by the evaluation value. An image recording apparatus for recording an image on a recording medium by ejecting ink droplets from the ejecting unit toward a recording medium that moves relative to the ejecting unit in a predetermined moving direction. A threshold matrix generation method for generating a threshold matrix used for halftoning an original image,
Storing a plurality of threshold matrices;
Determining a threshold value assigned to a plurality of threshold value matrix elements constituting each threshold value matrix;
With
A matrix region in which each threshold value matrix is generated is defined in a column direction corresponding to the moving direction and a row direction perpendicular to the column direction;
Determining the threshold comprises:
a) provisionally assigning a threshold value to one threshold value matrix element group to which no threshold value is given in each threshold value matrix;
b) obtaining a first evaluation value indicating a bias in the distribution of threshold matrix elements to which threshold values are given or provisionally given;
c) removing one threshold value matrix element from the one threshold value matrix element group and including another threshold value matrix element to which no threshold value is assigned or provisionally assigned in the one threshold value matrix element group, and the step a) And calculating a second evaluation value indicating a bias in the distribution of threshold matrix elements to which threshold values are given or provisionally given, and
d) When the first evaluation value shows a bias smaller than the second evaluation value, the one threshold matrix element group is returned to the state of the step a), and the first evaluation value is the second evaluation value. Changing the first evaluation value to a value equal to the second evaluation value when exhibiting a greater bias than
e) a step of repeating the c) step and the d) step while changing a threshold matrix element included in the one threshold matrix element group in the c) step until a predetermined end condition is satisfied;
f) providing a threshold to the one threshold matrix element group after the e);
g) The one threshold matrix element group is changed to the next one threshold matrix element group until threshold values are given to all threshold matrix elements included in the threshold matrix, and the steps a) to f) Repeating the process,
With
In the step b), a visual characteristic filter is applied to a power spectrum obtained by performing Fourier transform on the distribution of threshold matrix elements to which threshold values are given or provisionally given, and the difference in power spectrum before and after application of the visual characteristic filter is calculated. The sum is taken as the first evaluation value,
In the step c), the visual characteristic filter is applied to a power spectrum obtained by performing Fourier transform on the distribution of threshold matrix elements to which threshold values are given or provisionally given, and the difference in power spectrum before and after the visual characteristic filter is applied. Is the second evaluation value,
The plurality of threshold matrixes include a first threshold matrix corresponding to a first contact angle of ink with respect to a recording medium, and a second threshold matrix corresponding to a second contact angle larger than the first contact angle,
At least a ratio of the number of threshold matrix elements to which a threshold is given or provisionally given to the total number of threshold matrix elements is not less than a first ratio that is greater than 0% and less than 20% and greater than the first ratio. When the ratio is equal to or less than a second ratio smaller than 100%, the second threshold value is more than the first filter which is a visual characteristic filter in the steps b) and c) when determining the threshold value of the first threshold value matrix. A threshold value matrix generation method characterized in that the second filter, which is a visual characteristic filter in the step b) and the step c) at the time of determining a matrix threshold value, has strong deflection in the column direction.