Compensation coefficient determination method, printing method, printing device, and compensation coefficient determination program

Abstract

The present invention relates to an ink jet printing device which makes it possible to inhibit the occurrence of defects in a printed image when a defective discharge nozzle is present more effectively than in the prior art.　Regions corresponding to the widths of ink discharge heads are divided into K number of comparison regions, where K is an integer of 2 or more, the comparison regions being respectively associated with K number of provisional compensation coefficients differing from one another (S100). A plurality of provisionally defective nozzles are set for each comparison region (S110). K number of sample image data items are extracted from a captured image of a compensation coefficient calculation chart, which is obtained by applying the K number of provisional compensation coefficients to the K number of comparison regions, respectively, and carrying out an ink amount compensation process (S180). Data items affected by defective discharge nozzles are deleted from the K number of sample image data items (S190). A provisional compensation coefficient to be adopted as a compensation coefficient is determined on the basis of K number of comparison data items thusly obtained (S200).

Description

Compensation coefficient determination method, printing method, printing device, and compensation coefficient determination program

[0001] The present invention relates to an inkjet printing device, and more particularly to a technique for controlling the amount of ink ejected so as to prevent defects from occurring in printed images even when there are nozzles that are in a defective ejection state (hereinafter referred to as "defective ejection nozzles").

[0002] Inkjet printing devices are widely known, which perform printing by ejecting ink onto a printing medium such as printing paper. In inkjet printing devices, if the ejection interval becomes long, problems such as drying of the ink due to evaporation of the solvent near the nozzle, inclusion of air bubbles inside the nozzle, and adhesion of dust to the nozzle may occur during the printing period. In other words, ejection problems may occur from the nozzle.

[0003] Nozzle ejection defects can be roughly classified into three types, which will be explained with reference to FIGS. 40 to 42. Note that FIGS. 40 to 42 show portions of a printed image obtained by printing a regular, stepped pattern (test pattern), with the black shaded areas indicating ink coverage. In the dotted line area labeled 91 in FIG. 40, no ink is applied to the area where ink should be applied. Hereinafter, this type of ejection defect will be referred to as "non-ejection." In the dotted line area labeled 92 in FIG. 41, the shape of the ink-applied area differs from the intended shape. Hereinafter, this type of ejection defect will be referred to as "shape defect." Shape defects also include ejection defects that result in faint ink due to insufficient density and ejection defects in which the ink-applied area spreads in the main scanning direction (the direction perpendicular to the transport direction of the print paper). In the dotted line area labeled 93 in FIG. 42, ink is applied to an area that is shifted in the main scanning direction from the area where ink should be applied. Hereinafter, this type of ejection defect will be referred to as "horizontal skip."

[0004] When the above-described ejection defects occur in an inkjet printing device, defects in the printed image occur, such as white streaks and missing dots. To address this, operations (cleaning and flushing) have been performed to restore the functionality of the ejection-defective nozzle, and alternative droplet ejection methods have been used in which ink droplets that should be ejected by the ejection-defective nozzle are ejected by surrounding nozzles.

[0005] Furthermore, print heads contain many nozzles, and even if the above-mentioned ejection defects do not occur, there is variation in the amount of ink ejected among the multiple nozzles. This variation in ink ejection amount causes uneven density in the printed image. Therefore, shading correction processing has traditionally been performed on print data to compensate for such variations in ink ejection amount.

[0006] With regard to the above-described alternative droplet ejection, Japanese Patent Application Laid-Open No. 2021-160200 discloses a technique for determining a distribution ratio (the ratio at which the correction value of a nozzle with a discharge defect is distributed to adjacent nozzles) using a dedicated chart (a distribution ratio acquisition chart) (see embodiment 2). According to this technique, a distribution ratio acquisition chart is printed in which "0" is intentionally assigned to the gradation value of a nozzle at regular intervals, and the gradation values ​​of the nozzles on either side of the nozzle assigned a gradation value of "0" (the intentionally discharge-defective nozzle) are changed at four different distribution ratios. Then, a distribution ratio corresponding to the gradation value is determined based on a scanned image of the distribution ratio acquisition chart. In this process, a distribution ratio that results in the most uniform density when a known spatial filter is applied to the scanned image is selected for each gradation value.

[0007] Japanese Patent Publication No. 2021-160200

[0008] However, the technology disclosed in Japanese Patent Application Laid-Open No. 2021-160200 is based on the premise that when a nozzle with a discharge defect exists, the discharge defect state is resolved by cleaning. Therefore, if a nozzle with a discharge defect that cannot be resolved by cleaning exists, the technology disclosed in Japanese Patent Application Laid-Open No. 2021-160200 cannot be applied. It is also possible to correct the gradation values ​​at positions corresponding to the nozzles on either side of the discharge defect nozzle using a coefficient predetermined according to the gradation value at the position corresponding to the discharge defect nozzle, but the appropriate coefficient varies depending on the type of printing paper and printing conditions. Therefore, defects in the printed image cannot be effectively suppressed.

[0009] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an inkjet printing apparatus that can more effectively suppress the occurrence of defects in printed images when there is a defective ejection nozzle than ever before.

[0010] A first aspect of the present invention is a compensation coefficient determination method for determining a compensation coefficient to be applied to an ink amount compensation process that compensates for the amount of ink ejected onto a printing medium when a nozzle in a defective ejection state is present in a printing device having a plurality of nozzles that eject ink onto the printing medium, the method comprising: an area division step of dividing a predetermined area in a direction perpendicular to a transport direction of the printing medium into a plurality of divided areas including K comparison areas each associated with K different temporary compensation coefficients, where K is an integer of 2 or greater; a temporary defective nozzle setting step of setting a plurality of temporary defective nozzles that are assumed to be in a defective ejection state from among two or more corresponding nozzles for each of the K comparison areas; a compensation coefficient calculation chart data generation step of generating compensation coefficient calculation chart data representing a compensation coefficient calculation chart for determining the compensation coefficients by performing the ink amount compensation process by applying the K temporary compensation coefficients to the K comparison areas for predetermined image data that has a constant density in the direction perpendicular to the transport direction of the printing medium; and a defective ejection detection chart printing step of printing a defective ejection detection chart for detecting defective ejection nozzles by ejecting ink from the plurality of nozzles. a compensation coefficient calculation chart printing step of printing the compensation coefficient calculation chart by ejecting ink from the plurality of nozzles based on the compensation coefficient calculation chart data; a discharge defect detection chart imaging step of imaging the discharge defect detection chart printed in the discharge defect detection chart printing step; a compensation coefficient calculation chart imaging step of imaging the compensation coefficient calculation chart printed in the compensation coefficient calculation chart printing step; a discharge defect nozzle detection step of detecting nozzles in a discharge defect state based on first imaging data which is imaging data obtained in the discharge defect detection chart imaging step; and a sample image data extraction step of extracting K sample image data corresponding to the K comparison regions from second imaging data which is imaging data obtained in the compensation coefficient calculation chart imaging step.The method includes a data deletion step of generating K pieces of comparison data by deleting data in a range corresponding to the position of the true defective nozzle, which is the nozzle detected in the defective nozzle detection step, from the K pieces of sample image data, and a coefficient determination step of determining one of the K provisional compensation coefficients as the compensation coefficient based on the K pieces of comparison data.

[0011] A second aspect of the present invention is the first aspect of the present invention, wherein the plurality of divided areas include, in addition to the K comparison areas, normal areas that are not affected by the provisional defective nozzles; the sample image data extraction step further extracts normal area data corresponding to the normal areas from the second imaging data; and the coefficient determination step determines the compensation coefficient based on the K comparison data and the normal area data.

[0012] A third aspect of the present invention is related to the second aspect of the present invention, wherein in the coefficient determination step, an average value for each of the K pieces of comparison data and an average value for the normal area data are calculated, and the average value for the normal area data is set as a target value, and a provisional compensation coefficient associated with a comparison area corresponding to the comparison data from which an average value closest to the target value is obtained among the K pieces of comparison data is determined as the compensation coefficient.

[0013] A fourth aspect of the present invention is a method according to the third aspect of the present invention, wherein N is an integer greater than or equal to 2, and each of the K comparison areas corresponds to N provisionally faulty nozzles, and each of the K sample image data includes N partial image data corresponding to the N provisionally faulty nozzles, and in the data deletion step, for each of the K sample image data, partial image data corresponding to an area affected by the true faulty nozzle is deleted from the N partial image data.

[0014] A fifth aspect of the present invention is a method of determining a compensation coefficient for a comparison area corresponding to N provisionally faulty nozzles, where N is an integer of 2 or greater, and P is an integer of 1 or greater (N-1) or less, in the data deletion step, data at P positions affected by the true faulty nozzle from among the positions of the N provisionally faulty nozzles is deleted for each of the K sample image data, and in the coefficient determination step, a peak average value, which is the average value of data at (N-P) positions excluding the data at P positions deleted in the data deletion step from among the data at the positions of the N provisionally faulty nozzles, and an average value for the normal area data are determined for each of the K comparison data, and the average value for the normal area data is used as a target value, and a provisional compensation coefficient associated with the comparison area corresponding to the comparison data from which a peak average value closest to the target value has been obtained from among the K comparison data is determined as the compensation coefficient.

[0015] In a sixth aspect of the present invention, based on the first aspect of the present invention, in the coefficient determination step, a provisional compensation coefficient associated with a comparison area corresponding to a comparison data item having a smallest difference between a maximum value and a minimum value among the K comparison data items is determined as the compensation coefficient.

[0016] A seventh aspect of the present invention is related to the first aspect of the present invention, wherein the printing device includes a plurality of print heads, each print head including a portion of the plurality of nozzles, the K comparison areas are provided for each print head, the K provisional compensation coefficients associated with the K comparison areas are common among the plurality of print heads, and in the sample image data extraction step, one piece of sample image data is extracted by extracting data of the plurality of comparison areas associated with the same provisional compensation coefficient among the plurality of print heads from the second imaging data.

[0017] In an eighth aspect of the present invention, based on the first aspect of the present invention, the compensation coefficient calculation chart is configured by a plurality of sheets, one for each ink color.

[0018] In a ninth aspect of the present invention, based on the eighth aspect of the present invention, in the tentative defective nozzle setting step, a different nozzle is set as the tentative defective nozzle for each of the K comparison regions for each sheet.

[0019] In a tenth aspect of the present invention, in the first aspect of the present invention, the compensation coefficient calculation chart is made up of M density regions corresponding to M different densities in a transport direction of the printing medium, where M is an integer of 2 or more, and in the coefficient determination step, the compensation coefficient is determined for each density.

[0020] An eleventh aspect of the present invention is a printing method using a printing device having a plurality of nozzles that eject ink onto a printing medium, the method including: a shading data generating step of generating shading data to be used in shading correction processing that compensates for variations in ink ejection amount among the plurality of nozzles; a shading data correcting step of correcting the shading data by applying the ink amount compensation processing to the shading data using a compensation coefficient determined by the compensation coefficient determination method of any of the first to tenth aspects of the present invention; a print data generating step of generating print data by applying a rasterization processing to input data; a shading correction step of correcting the print data by applying the shading correction processing to the print data using the shading data corrected in the shading data correcting step; a halftone processing step of generating halftone image data by applying halftone processing to the print data corrected in the shading correction step; and an ink ejecting step of ejecting ink from the plurality of nozzles based on the halftone image data.

[0021] A twelfth aspect of the present invention relates to the eleventh aspect of the present invention, wherein a nozzle adjacent to the true faulty nozzle is defined as an adjacent nozzle, the compensation coefficient is represented by C, which is a value between 0 and 1, a position value corresponding to the true faulty nozzle in the shading data before the shading data correction step is performed is represented by Vf, a position value corresponding to the adjacent nozzle in the shading data before the shading data correction step is performed is represented by Vn, and a position value corresponding to the adjacent nozzle in the shading data after the shading data correction step is performed is represented by V, where V is expressed by the following equation: V=Vn+Vf×C

[0022] In a thirteenth aspect of the present invention, based on the twelfth aspect of the present invention, a value of a position in the shading data after the shading data correction step has been executed that corresponds to the true faulty nozzle is 0.

[0023] A fourteenth aspect of the present invention is a printing device comprising a plurality of nozzles that eject ink onto a printing medium, an imaging device that captures a print image, and a control unit that controls the ejection of ink from the plurality of nozzles and the capturing of the print image by the imaging device, and that performs an ink amount compensation process during printing to compensate for the amount of ink ejected onto the printing medium when a nozzle in a defective ejection state is present, wherein the control unit performs: an area division process that divides a predetermined area in a direction perpendicular to a transport direction of the printing medium into a plurality of divided areas including K comparison areas each associated with K different temporary compensation coefficients, where K is an integer of 2 or greater; a temporary defective nozzle setting process that sets a plurality of temporary defective nozzles that are virtually considered to be in a defective ejection state from among the corresponding two or more nozzles for each of the K comparison areas; and a compensation coefficient calculation chart data generation process that generates compensation coefficient calculation chart data representing a compensation coefficient calculation chart for determining the compensation coefficients by performing the ink amount compensation process by applying the K temporary compensation coefficients to the K comparison areas for predetermined image data that has a constant density in the direction perpendicular to the transport direction of the printing medium, a defective discharge detection chart printing process that controls ink ejection from the plurality of nozzles so that a defective discharge detection chart for detecting nozzles in a defective discharge state is printed; a compensation coefficient calculation chart printing process that controls ink ejection from the plurality of nozzles based on the compensation coefficient calculation chart data so that the compensation coefficient calculation chart is printed; a defective discharge detection chart imaging process that causes the imaging device to image the defective discharge detection chart printed in the defective discharge detection chart printing step; a compensation coefficient calculation chart imaging process that causes the imaging device to image the compensation coefficient calculation chart printed in the compensation coefficient calculation chart printing step; a defective discharge nozzle detection process that detects nozzles in a defective discharge state based on first image data that is the image data obtained in the defective discharge detection chart imaging step; and a sample image data extraction process that extracts K sample image data corresponding to the K comparison regions from second image data that is the image data obtained in the compensation coefficient calculation chart imaging step.A data deletion process is executed to generate K pieces of comparison data by deleting data in a range corresponding to the position of the true defective nozzle, which is the nozzle detected in the defective nozzle detection process, from the K pieces of sample image data; and a coefficient determination process is executed to determine one of the K provisional compensation coefficients as the compensation coefficient based on the K pieces of comparison data.

[0024] A fifteenth aspect of the present invention is a compensation coefficient determination program for determining compensation coefficients to be applied to ink amount compensation processing that compensates for the amount of ink ejected onto a printing medium when a nozzle in a defective ejection state is present in a printing device having a plurality of nozzles that eject ink onto a printing medium and an imaging device that images a print image, the program comprising: an area division step of dividing a predetermined area in a direction perpendicular to a transport direction of the printing medium into a plurality of divided areas including K comparison areas each associated with K different temporary compensation coefficients, where K is an integer of 2 or greater; a temporary defective nozzle setting step of setting a plurality of temporary defective nozzles that are assumed to be in a defective ejection state for each of the K comparison areas; and a compensation coefficient calculation chart data generation step of generating compensation coefficient calculation chart data representing a compensation coefficient calculation chart for determining the compensation coefficients by performing the ink amount compensation processing by applying the K temporary compensation coefficients to the K comparison areas for predetermined image data that has a constant density in the direction perpendicular to the transport direction of the printing medium, respectively. a discharge defect detection chart printing step of controlling ink ejection from the plurality of nozzles so that a discharge defect detection chart for detecting nozzles in a discharge defect state is printed; a compensation coefficient calculation chart printing step of controlling ink ejection from the plurality of nozzles based on the compensation coefficient calculation chart data so that the compensation coefficient calculation chart is printed; a discharge defect detection chart imaging step of having the imaging device capture an image of the discharge defect detection chart printed in the discharge defect detection chart printing step; a compensation coefficient calculation chart imaging step of having the imaging device capture an image of the compensation coefficient calculation chart printed in the compensation coefficient calculation chart printing step; a discharge defect nozzle detection step of detecting nozzles in a discharge defect state based on first imaging data which is imaging data obtained in the discharge defect detection chart imaging step; and a sample image data extraction step of extracting K sample image data corresponding to the K comparison regions from second imaging data which is imaging data obtained in the compensation coefficient calculation chart imaging step.The method executes a data deletion step of generating K pieces of comparison data by deleting data in a range corresponding to the position of the true defective nozzle, which is the nozzle detected in the defective nozzle detection step, from the K pieces of sample image data, and a coefficient determination step of determining one of the K provisional compensation coefficients as the compensation coefficient based on the K pieces of comparison data.

[0025] A sixteenth aspect of the present invention is a compensation coefficient method for determining a compensation coefficient to be applied to an ink amount compensation process that compensates for the amount of ink ejected onto a printing medium when a nozzle in a defective ejection state is present in a printing device having a plurality of nozzles that eject ink onto the printing medium, the compensation coefficient method comprising: a defective ejection detection chart printing step of printing a defective ejection detection chart for detecting defective ejection nozzles by ejecting ink from the plurality of nozzles; and a compensation coefficient calculation chart printing step of printing a compensation coefficient calculation chart for determining the compensation coefficient by ejecting ink from the plurality of nozzles based on compensation coefficient calculation chart data obtained by performing the ink amount compensation process by applying the K temporary compensation coefficients to the K comparison regions to predetermined image data that has a constant density in the direction perpendicular to the transport direction of the printing medium, in a state in which a predetermined region is divided into a plurality of divided regions including K comparison regions each associated with K different temporary compensation coefficients, where K is an integer of 2 or greater, in a direction perpendicular to the transport direction of the printing medium; a compensation coefficient calculation chart imaging step of imaging the compensation coefficient calculation chart printed in the compensation coefficient calculation chart printing step; a defective discharge nozzle detection step of detecting a nozzle in a defective discharge state based on first imaging data, which is imaging data obtained in the defective discharge detection chart imaging step; a sample image data extraction step of extracting K pieces of sample image data corresponding to the K comparison areas from second imaging data, which is imaging data obtained in the compensation coefficient calculation chart imaging step; a data deletion step of generating K pieces of comparison data by deleting data in a range corresponding to the position of the true defective nozzle, which is the nozzle detected in the defective discharge nozzle detection step, from the K pieces of sample image data; and a coefficient determination step of determining one of the K provisional compensation coefficients as the compensation coefficient based on the K pieces of comparison data.

[0026] A seventeenth aspect of the present invention is a printing device comprising a plurality of nozzles that eject ink onto a printing medium, an imaging device that captures a print image, and a control unit that controls the ejection of ink from the plurality of nozzles and the capturing of the print image by the imaging device, and that performs, during printing, an ink amount compensation process that compensates for the amount of ink ejected onto the printing medium when a nozzle in a defective ejection state is present, wherein the control unit performs: a defective ejection detection chart printing process that controls the ejection of ink from the plurality of nozzles so that a defective ejection detection chart for detecting defective ejection nozzles is printed; a compensation coefficient calculation chart printing process that controls ink ejection from the plurality of nozzles based on compensation coefficient calculation chart data obtained by performing the ink amount compensation process by applying the K temporary compensation coefficients to the K comparison areas to predetermined image data that has a constant density in the direction orthogonal to the transport direction of the printing medium, in a state where a predetermined area is divided into a plurality of divided areas in a direction orthogonal to the transport direction of the printing medium, the K comparison areas being associated with K different temporary compensation coefficients, where K is an integer of 2 or greater, so that a compensation coefficient calculation chart for determining the compensation coefficients is printed; a discharge failure detection chart imaging process that causes the imaging device to image the discharge failure detection chart printed in the discharge failure detection chart printing process; a compensation coefficient calculation chart imaging process that causes the imaging device to image the compensation coefficient calculation chart printed in the compensation coefficient calculation chart printing process; and a discharge failure nozzle detection process that detects nozzles that are in a discharge failure state based on first imaging data that is imaging data obtained in the discharge failure detection chart imaging process. a sample image data extraction process for extracting K pieces of sample image data corresponding to the K comparison regions from second image data, which is image data obtained in the compensation coefficient calculation chart image capture process; and a data deletion process for generating K pieces of comparison data by deleting data in a range corresponding to the position of the true defective nozzle, which is the nozzle detected in the ejection-failure nozzle detection process, from the K pieces of sample image data.A coefficient determination process is executed to determine one of the K provisional compensation coefficients as the compensation coefficient based on the K pieces of comparison data.

[0027] An eighteenth aspect of the present invention is a compensation coefficient determination program for determining a compensation coefficient to be applied to an ink amount compensation process for compensating for the amount of ink ejected onto a printing medium when a nozzle in a defective ejection state is present in a printing device having a plurality of nozzles that eject ink onto a printing medium and an imaging device that images a print image, the program including: a defective ejection detection chart printing step in a computer included in the printing device, controlling the ejection of ink from the plurality of nozzles so that a defective ejection detection chart for detecting defective ejection is printed; a compensation coefficient calculation chart printing step for controlling ink ejection from the plurality of nozzles based on compensation coefficient calculation chart data obtained by applying the ink amount compensation process to predetermined image data having a constant density in the direction orthogonal to the transport direction of the printing medium, in a state where a predetermined area is divided into a plurality of divided areas including K comparison areas associated with K different temporary compensation coefficients, where K is an integer of 2 or greater, so that a compensation coefficient calculation chart for determining the compensation coefficients is printed; a discharge failure detection chart imaging step for causing the imaging device to image the discharge failure detection chart printed in the discharge failure detection chart printing step; a compensation coefficient calculation chart imaging step for causing the imaging device to image the compensation coefficient calculation chart printed in the compensation coefficient calculation chart printing step; and a discharge failure nozzle detection step for detecting a discharge failure nozzle based on first image data which is image data obtained in the discharge failure detection chart imaging step. a sample image data extraction step of extracting K pieces of sample image data corresponding to the K comparison regions from second image data, which is image data obtained in the compensation coefficient calculation chart imaging step; and a data deletion step of generating K pieces of comparison data by deleting data in a range corresponding to the position of the true defective nozzle, which is the nozzle detected in the ejection defective nozzle detection step, from the K pieces of sample image data.and a coefficient determination step of determining one of the K provisional compensation coefficients as the compensation coefficient based on the K pieces of comparison data.

[0028] According to a first aspect of the present invention, a temporary compensation coefficient to be actually applied to the ink amount compensation process (a process for compensating for the amount of ink ejected onto a printing medium when a non-ejecting nozzle is present) is determined from a plurality of temporary compensation coefficients based on data obtained by applying the plurality of temporary compensation coefficients to the ink amount compensation process. In this manner, the compensation coefficient is determined based on data obtained by actually performing the ink amount compensation process. Furthermore, prior to the final determination of the compensation coefficient, data from the area affected by the non-ejecting nozzle is deleted. As a result, an appropriate compensation coefficient can be determined with high accuracy, thereby more effectively suppressing the occurrence of defects in printed images when a non-ejecting nozzle is present than in the past. Furthermore, wasteful consumption of ink and printing medium is reduced, thereby contributing to the achievement of the SDGs (Sustainable Development Goals).

[0029] According to the second aspect of the present invention, it is possible to determine the compensation coefficient taking into consideration the density obtained in an area where no ejection-failure nozzles exist.

[0030] According to the third aspect of the present invention, even if there is a discharge-failure nozzle, it is possible to output a printed image having a density similar to that when there is no discharge-failure nozzle.

[0031] According to the fourth aspect of the present invention, the same effects as those of the third aspect of the present invention can be obtained.

[0032] According to the fifth aspect of the present invention, the final determination of the compensation coefficient is made based on a relatively small number of data, thereby reducing the processing load.

[0033] According to the sixth aspect of the present invention, the compensation coefficient to be actually applied to the ink amount compensation process is determined without comparing a plurality of comparison data with data in the normal region.

[0034] According to the seventh aspect of the present invention, it is possible to determine the compensation coefficients to be actually applied to the ink amount compensation process so as to eliminate the influence of variations in characteristics between a plurality of print heads.

[0035] According to the eighth aspect of the present invention, the same effects as those of the first aspect of the present invention can be obtained.

[0036] According to the ninth aspect of the present invention, a different nozzle is set as the provisional defective nozzle for each sheet, so that the influence of variations in ink ejection amount among a plurality of nozzles is more effectively suppressed.

[0037] According to the tenth aspect of the present invention, a compensation coefficient is determined for each of a plurality of densities, so that the occurrence of defects in the printed image is more effectively suppressed than in the past regardless of the density of the printed image in the area corresponding to the nozzle with defective ejection.

[0038] According to the eleventh aspect of the present invention, ink volume compensation processing is applied to the shading data, and by correcting the print data using the corrected shading data, it is possible to suppress not only the occurrence of defects in the printed image due to the presence of nozzles with poor ejection performance, but also the occurrence of uneven density due to variations in the amount of ink ejected among multiple nozzles.

[0039] According to the twelfth aspect of the present invention, ink that should be ejected from the ejection-failure nozzle is instead ejected from the nozzle adjacent to the ejection-failure nozzle, making it possible to more reliably suppress the occurrence of defects in the printed image.

[0040] According to the thirteenth aspect of the present invention, the same effects as those of the twelfth aspect of the present invention can be obtained.

[0041] According to the fourteenth aspect of the present invention, the same effects as those of the first aspect of the present invention can be obtained.

[0042] According to the fifteenth aspect of the present invention, the same effects as those of the first aspect of the present invention can be obtained.

[0043] According to the sixteenth aspect of the present invention, the same effects as those of the first aspect of the present invention can be obtained.

[0044] According to the seventeenth aspect of the present invention, the same effects as those of the first aspect of the present invention can be obtained.

[0045] According to the eighteenth aspect of the present invention, the same effects as those of the first aspect of the present invention can be obtained.

[0046] 1 is a diagram illustrating the overall configuration of a printing system according to one embodiment of the present invention. FIG. 2 is a schematic diagram illustrating an example of the configuration of an inkjet printing device according to the embodiment. FIG. 3 is a plan view illustrating an example of the configuration of a recording unit according to the embodiment. FIG. 4 is a diagram for explaining the arrangement of nozzles in an ink ejection head according to the embodiment. FIG. 5 is a block diagram illustrating the hardware configuration of a print control device according to the embodiment. FIG. 6 is a diagram for explaining shading correction processing according to the embodiment. FIG. 7 is a diagram for explaining ink amount compensation processing according to the embodiment. FIG. 8 is a flowchart illustrating the procedure for print output in the printing system according to the embodiment. FIG. 9 is a flowchart illustrating detailed procedures for shading data generation processing according to the embodiment. FIG. 10 is a diagram illustrating an example of a shading chart according to the embodiment. FIG. 11 is a block diagram illustrating the functional configuration of a control unit implemented by execution of a print control program on the print control device according to the embodiment. FIG. 12 is a block diagram illustrating the detailed configuration of a correction coefficient calculation unit according to the embodiment. FIG. 13 is a block diagram illustrating the detailed configuration of a compensation coefficient calculation unit according to the embodiment. FIG. 14 is a diagram for explaining the concept of a method for determining compensation coefficients according to the embodiment. FIG. 15 is a diagram illustrating an example of the distribution (spatial change) of luminance values ​​obtained by ink amount compensation processing for two comparison regions corresponding to two given provisional compensation coefficients according to the embodiment. FIG. 16 is a graph illustrating an example of the relationship between provisional compensation coefficients and average luminance values ​​according to the embodiment. FIG. 17 is a flowchart illustrating the processing procedure for determining compensation coefficients according to the embodiment. FIG. 18 is a diagram for explaining setting of provisional defective nozzles according to the embodiment. FIG. 1 is a schematic diagram showing an entire compensation coefficient calculation chart in the embodiment. FIG. 2 is a schematic diagram showing a compensation coefficient calculation chart for one color and one sheet for an area corresponding to one ink ejection head in the embodiment. FIG. 3 is a schematic diagram showing a portion of the compensation coefficient calculation chart corresponding to one comparison area for one density in the embodiment. FIG. 4 is a diagram for explaining a case in which the same nozzle is set as the provisional defective nozzle for all five sheets in the embodiment. FIG. 5 is a diagram for explaining a case in which a different nozzle is set as the provisional defective nozzle for each sheet in the embodiment.1 is a schematic diagram showing an entire discharge defect detection chart in the embodiment. FIG. 1 is a schematic diagram showing a discharge defect detection chart for one color in the embodiment. FIG. 2 is a schematic diagram showing a portion of the printed compensation coefficient calculation chart corresponding to one ink discharge head 251 and one density in the embodiment. FIG. 3 is a diagram showing an example of an image represented by first captured data when a discharge-failure nozzle is present in the embodiment. FIG. 4 is a diagram for explaining sample image data in the embodiment. FIG. 5 is a diagram for explaining a case in which the position of a true faulty nozzle is included in the area of ​​one partial image data out of four partial image data in the embodiment. FIG. 6 is a schematic diagram showing an example of comparison data obtained when the position of a true faulty nozzle is included in the area of ​​partial image data in the embodiment. FIG. 7 is a diagram for explaining a case in which the position of a discharge-failure nozzle is close to the area of ​​partial image data in the embodiment. FIG. 8 is a diagram for explaining calculation of an average luminance value when no true faulty nozzle exists in the embodiment. FIG. 9 is a diagram for explaining calculation of an average luminance value when a true faulty nozzle exists in the embodiment. FIG. 10 is a diagram for explaining sample image data in the first modified example. FIG. 11 is a diagram for explaining deletion of data from sample image data when a true faulty nozzle exists in the first modified example. FIG. 10 is a diagram for explaining the concept of a method for determining a compensation coefficient in a second modified example. FIG. 11 is a diagram for explaining the concept of a method for determining a compensation coefficient in a third modified example. FIG. 12 is a flowchart showing a processing procedure for determining a compensation coefficient in a fourth modified example. FIG. 13 is a diagram for explaining non-ejection. FIG. 14 is a diagram for explaining shape defects. FIG. 15 is a diagram for explaining lateral jumping.

[0047] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings.

[0048] 1. Overall Configuration of the Printing System FIG. 1 is a diagram illustrating the overall configuration of a printing system according to an embodiment of the present invention. This printing system comprises an inkjet printing device 10 and a print data generating device 30. The inkjet printing device 10 and the print data generating device 30 are connected to each other via a LAN 4. The print data generating device 30 generates print data by performing rasterization processing on input data, such as a PDF file. This print data is not subjected to halftoning; halftoning is performed by a print control device 100 within the inkjet printing device 10, as described below. The print data generated by the print data generating device 30 is transmitted to the inkjet printing device 10 via the LAN 4. The inkjet printing device 10 comprises a printing machine main body 200 and the print control device 100, which controls the operation of the printing machine main body 200. The inkjet printing device 10 outputs a print image on printing paper as a printing medium based on the print data transmitted from the print data generating device 30, without using printing plates. Note that the present invention can also be applied when a printing medium other than printing paper (e.g., film) is used.

[0049] 2. Configuration of the Printer Main Body of the Inkjet Printing Apparatus> Fig. 2 is a schematic diagram showing an example configuration of the inkjet printing apparatus 10. As described above, the inkjet printing apparatus 10 is made up of the print control device 100 and the printer main body 200.

[0050] The printing machine main body 200 includes a paper feed section 202 that supplies printing paper (in this example, rolled printing paper) 5 to the printing mechanism 201, the printing mechanism 201 that prints on the printing paper 5, and a paper winding section 208 that winds up the printing paper 5 into a roll after printing.

[0051] The printing mechanism 201 includes a first drive roller 203 for transporting the printing paper 5 inside, a plurality of support rollers 204 for transporting the printing paper 5 inside the printing mechanism 201, a recording unit 205 for recording a print image on the printing paper 5, a drying mechanism 206 for drying the printing paper 5 on which the print image has been recorded, and a second drive roller 207 for outputting the printing paper 5 from inside the printing mechanism 201. The recording unit 205 is composed of a K head unit 25K that ejects K (black) ink, a C head unit 25C that ejects C (cyan) ink, an M head unit 25M that ejects M (magenta) ink, and a Y head unit 25Y that ejects Y (yellow) ink. The printing mechanism 201 also includes an inline scanner 40 as an imaging device that captures the print image recorded on the printing paper 5 by the recording unit 205. The image data obtained by the inline scanner 40 capturing the print image is sent to the print control device 100. In the following description, when the color of ink ejected from the head unit is not to be distinguished, the head unit will be denoted by the reference numeral 25.

[0052] FIG. 3 is a plan view showing an example of the configuration of the recording unit 205. As shown in FIG. 3, the recording unit 205 is composed of a K-color head unit 25K, a C-color head unit 25C, an M-color head unit 25M, and a Y-color head unit 25Y, which are arranged in a row in the transport direction (sub-scanning direction) of the printing paper 5. Each head unit 25 is composed of a plurality of ink ejection heads (print heads) 251 arranged in a staggered pattern. Each ink ejection head 251 includes a number of nozzles (not shown in FIG. 3) that eject ink. Each nozzle of the ink ejection head 251 included in the K-color head unit 25K ejects K-color ink, each nozzle of the ink ejection head 251 included in the C-color head unit 25C ejects C-color ink, each nozzle of the ink ejection head 251 included in the M-color head unit 25M ejects M-color ink, and each nozzle of the ink ejection head 251 included in the Y-color head unit 25Y ejects Y-color ink.

[0053] FIG. 4 is a diagram illustrating an example of the arrangement of nozzles in the ink ejection head 251. Typically, the ink ejection head 251 includes multiple rows of nozzle groups, each consisting of a plurality of nozzles arranged in the main scanning direction (a direction perpendicular to the transport direction of the printing paper 5). In the example shown in FIG. 4, the ink ejection head 251 includes four rows of nozzle groups. The portion labeled 41 in FIG. 4 schematically shows the landing positions on the printing paper 5 of ink ejected from each nozzle. The multiple nozzles in the ink ejection head 251 are arranged so that the landing positions of ink ejected from the nozzles in the first row of nozzle group, the landing positions of ink ejected from the nozzles in the second row of nozzle group, the landing positions of ink ejected from the nozzles in the third row of nozzle group, and the landing positions of ink ejected from the nozzles in the fourth row of nozzle group are all different from one another. For example, the landing position of ink ejected from each nozzle included in the first nozzle group is a position between the landing position of ink ejected from the nozzles included in the third nozzle group and the landing position of ink ejected from the nozzles included in the fourth nozzle group. In the example shown in Fig. 4, the landing position 42 of ink ejected from the nozzles labeled 252(p) is a position between the landing position 43 of ink ejected from the nozzles labeled 252(q) and the landing position 44 of ink ejected from the nozzles labeled 252(r). Therefore, the adjacent nozzles (nozzles on both sides) of the nozzle labeled 252(p) are the nozzle labeled 252(q) and the nozzle labeled 252(r).

[0054] 3. Hardware Configuration of Print Control Device FIG. 5 is a block diagram showing the hardware configuration of the print control device 100. As shown in FIG. 5, the print control device 100 includes a main body 110, an auxiliary storage device 121, an optical disk drive 122, a display unit 123, a keyboard 124, and a mouse 125. The main body 110 includes a CPU 111, a memory 112, a first disk interface unit 113, a second disk interface unit 114, a display control unit 115, an input interface unit 116, and a communication interface unit 117. The CPU 111, the memory 112, the first disk interface unit 113, the second disk interface unit 114, the display control unit 115, the input interface unit 116, and the communication interface unit 117 are connected to one another via a system bus. The auxiliary storage device 121 is connected to the first disk interface unit 113. The optical disk drive 122 is connected to the second disk interface unit 114. The display unit (display device) 123 is connected to the display control unit 115. A keyboard 124 and a mouse 125 are connected to the input interface unit 116. The printing machine main body 200 is connected to the communication interface unit 117 via a communication cable. The communication interface unit 117 is also connected to the LAN 4. The auxiliary storage device 121 is a magnetic disk device or the like. An optical disk 19, which is a computer-readable recording medium such as a CD-ROM or DVD-ROM, is inserted into the optical disk drive 122. The display unit 123 is a liquid crystal display or the like. The display unit 123 is used to display information desired by the operator. The keyboard 124 and mouse 125 are used by the operator to input instructions to this printing control device 100.

[0055] The auxiliary storage device 121 stores a print control program 13 (a program for controlling the execution of printing processing by the printing machine main body 200). In this embodiment, the print control program 13 includes, as a subprogram, a compensation coefficient determination program that determines a coefficient (hereinafter referred to as a "compensation coefficient") to be applied to an ink volume compensation process that compensates for the amount of ink ejected onto the printing paper 5 when a nozzle is not ejecting properly. The CPU 111 implements various functions of the print control device 100 by reading the print control program 13 stored in the auxiliary storage device 121 into the memory 112 and executing it. The memory 112 includes RAM (Random Access Memory) and ROM (Read Only Memory). The memory 112 functions as a work area for the CPU 111 to execute the print control program 13 stored in the auxiliary storage device 121. The print control program 13 is provided stored in the computer-readable recording medium (non-transitory recording medium). That is, the user purchases, for example, an optical disk 19 as a recording medium for the print control program 13 , inserts it into the optical disk drive 122 , reads the print control program 13 from the optical disk 19 , and installs it in the auxiliary storage device 121 .

[0056] 4. Overview of Correction Process The inkjet printing apparatus 10 according to this embodiment performs correction processes to reduce density variations resulting from variations in ink ejection volume among multiple nozzles and print image defects (e.g., white streaks and missing dots) resulting from the presence of defective nozzles. Specifically, the correction processes include a shading correction process that compensates for variations in ink ejection volume among multiple nozzles, and an ink volume compensation process that compensates for the amount of ink ejected onto the printing paper 5 when a defective nozzle is present. The ink volume compensation process is a process for achieving the replacement droplet ejection described above. The shading correction process and ink volume compensation process are described below with reference to FIGS. 6 and 7. Here, we focus on five pixel areas 7(1) to 7(5) corresponding to five nozzles. It is assumed that monochrome printing is performed in each of the five pixel areas 7(1) to 7(5) using the same color ink ejected from the five nozzles. Note that in FIGS. 6 and 7, the numerical values ​​within the rectangles corresponding to the pixel areas 7(1) to 7(5) represent density values ​​in the data.

[0057] First, the shading correction process will be described with reference to FIG. 6 . Here, it is assumed that if printing is performed at a constant density (50%) without performing shading correction, 5 / 4 times the amount of ink ejected onto pixel portion 7(1) and pixel portion 7(3) as compared to pixel portion 7(2), 5 / 6 times the amount of ink ejected onto pixel portion 7(4), and 5 / 3 times the amount of ink ejected onto pixel portion 7(2) as compared to pixel portion 7(5). In this case, the density value indicated by the portion labeled 71 is corrected to the density value indicated by the portion labeled 72 by the shading correction process. Correcting the density data in this manner reduces the variation in the amount of ink ejected among multiple nozzles, thereby suppressing the occurrence of density unevenness.

[0058] Next, the ink volume compensation process will be described with reference to FIG. 7 . Here, it is assumed that the nozzle corresponding to pixel portion 7(3) is a defective nozzle. In this case, for example, the density value indicated by the portion labeled 73 is corrected to the density value indicated by the portion labeled 74 through the ink volume compensation process. The density value for pixel portion 7(3) is corrected from 50 to 0, and the density values ​​for pixel portions 7(2) and 7(4) are corrected from 50 to 70. By correcting the density data in this way, a larger amount of ink is ejected from the nozzle adjacent to the defective nozzle. This makes defects in the printed image caused by the defective nozzle not ejecting ink properly less noticeable. In other words, the occurrence of defects in the printed image is suppressed.

[0059] 5. Printing Procedure The printing procedure in this printing system will be described with reference to the flowchart shown in Fig. 8. Here, it is assumed that the compensation coefficients used in the ink amount compensation process have already been determined.

[0060] First, in the inkjet printing device 10, a process for generating shading data (shading data generation process) is carried out (step S10), which is data used when performing shading correction processing on print data. The shading data generation process will be described in detail later.

[0061] Next, the shading data generated in step S10 is subjected to ink amount compensation processing using a compensation coefficient determined as described below, thereby correcting the shading data (step S20). If the compensation coefficient used in the ink amount compensation processing is represented by C, a value between 0 and 1, the value of the position corresponding to the ejection-failure nozzle (the true defective nozzle described below) in the shading data before the processing of step S20 is performed is represented by Vf, the value of the position corresponding to the nozzle adjacent to the ejection-failure nozzle in the shading data before the processing of step S20 is performed is represented by Vn, and the value of the position corresponding to the nozzle adjacent to the ejection-failure nozzle in the shading data after the processing of step S20 is performed is represented by V, then V can be expressed by the following equation (1): V=Vn+Vf×C (1)

[0062] After the process of step S20 is executed, the value of the position corresponding to the ejection-failure nozzle in the shading data is 0.

[0063] As described above, in this embodiment, the shading data is corrected by the ink amount compensation process. Note that in step S50, which will be described later, the print data is corrected using the corrected shading data. Therefore, hereinafter, this corrected shading data will also be referred to as the "correction coefficient."

[0064] After the shading data is corrected, the print data generating device 30 generates print data by rasterizing the input data such as a PDF file (step S30).The print data is then transferred from the print data generating device 30 to the print control device 100 in the inkjet printing device 10 (step S40).

[0065] Then, in the print control device 100, the print data is corrected by performing shading correction processing on the print data using the correction coefficient obtained in step S20 (i.e., the shading data after correction in step S20) (step S50).

[0066] After the shading correction process is completed, the print control device 100 performs halftone processing on the print data corrected in step S50, thereby generating halftone image data (step S60).

[0067] Finally, printing is performed by the recording unit 205 based on the halftone image data generated in step S60 (step S70). That is, an image is formed on the printing paper 5 by controlling the ejection of ink from the multiple nozzles included in each ink ejection head 251 that makes up the recording unit 205 based on the halftone image data.

[0068] It should be noted that the processes of steps S10 and S20 do not necessarily have to be performed for each print job. In other words, after the processes of steps S10 and S20 are performed once, the processes of steps S30 to S70 can be repeatedly performed for multiple print jobs.

[0069] In this embodiment, step S10 realizes a shading data generation step, step S20 realizes a shading data correction step, step S30 realizes a print data generation step, step S50 realizes a shading correction step, step S60 realizes a halftone processing step, and step S70 realizes an ink ejection step.

[0070] FIG. 9 is a flowchart showing the detailed procedure of the shading data generation process (the process of step S10 in FIG. 8 ). After the shading data generation process starts, a shading chart such as that shown in FIG. 10 is printed (step S12). Shading chart data representing the shading chart is stored in advance in the auxiliary storage device 121 of the print control device 100. The shading chart is an image whose density changes stepwise in the transport direction (sub-scanning direction) of the printing paper 5. Regarding the shading chart shown in FIG. 10 , for example, the density value of the portion marked with reference numeral 75(1) is 5%, the density value of the portion marked with reference numeral 75(2) is 10%, the density value of the portion marked with reference numeral 75(3) is 30%, the density value of the portion marked with reference numeral 75(4) is 50%, the density value of the portion marked with reference numeral 75(5) is 80%, and the density value of the portion marked with reference numeral 75(6) is 100%.

[0071] After printing the shading chart, the shading chart is imaged by the inline scanner 40 (step S14). Then, based on the image data obtained in step S14, a density correction amount is calculated using a known method (step S16). Data representing this density correction amount is shading data. When the processing of step S16 ends, the shading data generation processing ends.

[0072] 6. Details of Correction Processing The correction processing will be described in detail below.

[0073] 11 is a block diagram showing the functional configuration of the control unit 150 that is realized by executing the print control program 13 in the print control device 100. The control unit 150 includes a transport control unit 151, an ink ejection control unit 152, a drying control unit 153, an imaging control unit 154, a compensation coefficient calculation chart data generation processing unit 155, a correction coefficient calculation unit 156, a print data correction unit 157, and a halftone processing unit 158.

[0074] The conveyance control unit 151 controls the speed (conveyance speed) at which the conveyance mechanism 29 conveys the printing paper 5. The conveyance mechanism 29 is realized by a paper delivery unit 202, a first drive roller 203, a plurality of support rollers 204, a second drive roller 207, and a paper winding unit 208 (see FIG. 2). The drying control unit 153 controls the temperature (drying temperature) at which the drying mechanism 206 dries the printing paper 5 after printing. The imaging control unit 154 controls the timing at which the inline scanner 40 captures the image of the printed image.

[0075] The compensation coefficient calculation chart data generation processing unit 155 generates compensation coefficient calculation chart data 53 that represents a compensation coefficient calculation chart for determining compensation coefficients used in ink amount compensation processing. The generation of the compensation coefficient calculation chart data 53 will be described in detail later.

[0076] The correction coefficient calculation unit 156 calculates a correction coefficient 57 for correcting the print data 50 generated by the print data generation device 30, based on first image data 54 obtained by the inline scanner 40 capturing an image of a discharge defect detection chart for detecting discharge defect nozzles, second image data 55 obtained by the inline scanner 40 capturing an image of a compensation coefficient calculation chart, and third image data 56 obtained by the inline scanner 40 capturing an image of a shading chart such as that shown in Fig. 10. Details of the correction coefficient calculation unit 156 will be described later.

[0077] The print data correction unit 157 corrects the print data 50 generated by the print data generation device 30 using the correction coefficient 57 calculated by the correction coefficient calculation unit 156. In other words, the print data correction unit 157 performs shading correction processing on the print data 50 generated by the print data generation device 30 using the correction coefficient 57 calculated by the correction coefficient calculation unit 156. The print data correction unit 157 outputs corrected print data 58.

[0078] The halftone processing unit 158 ​​performs halftone processing on the data to be printed, thereby generating halftone image data 59 that includes information indicating the ink dot size corresponding to each pixel. Three ink dot sizes (L, M, and S) are available, for example. In this embodiment, the halftone processing is performed on the print data 58 corrected by the print data correction unit 157, the shading chart data 51 representing the shading chart, the ejection defect detection chart data 52 representing the ejection defect detection chart, and the compensation coefficient calculation chart data 53 generated by the compensation coefficient calculation chart data generation processing unit 155. The specific method of halftone processing is not particularly limited, and known methods such as error diffusion and dithering can be used.

[0079] The ink ejection control unit 152 controls the amount of ink ejected from each nozzle included in each ink ejection head 251 that constitutes the recording unit 205 based on the halftone image data 59 generated by the halftone processing unit 158.

[0080] 12 is a block diagram showing a detailed configuration of the correction coefficient calculation unit 156. The correction coefficient calculation unit 156 includes a shading data generation unit 510, a compensation coefficient calculation unit 520, and a shading data correction unit 530.

[0081] The shading data generation unit 510 generates shading data (data representing the amount of density correction) 61 based on the third image data 56 obtained by the inline scanner 40 capturing an image of a shading chart such as that shown in FIG. 10 .

[0082] The compensation coefficient calculation unit 520 calculates a compensation coefficient 62 to be used in the ink amount compensation process based on first image data 54 obtained by the inline scanner 40 capturing an image of the discharge defect detection chart and second image data 55 obtained by the inline scanner 40 capturing an image of the compensation coefficient calculation chart. Details of the compensation coefficient calculation unit 520 will be described later.

[0083] The shading data correction unit 530 performs ink amount compensation processing on the shading data 61 generated by the shading data generation unit 510, using the compensation coefficient 62 calculated by the compensation coefficient calculation unit 520. As a result, a correction coefficient 57 for correcting the print data 50 is calculated.

[0084] 13 is a block diagram showing the detailed configuration of the compensation coefficient calculation unit 520. The compensation coefficient calculation unit 520 includes a discharge-failure nozzle detection unit 521, a sample image data extraction unit 522, a data deletion unit 523, and a coefficient determination unit 524.

[0085] The discharge-failure nozzle detection unit 521 detects the discharge-failure nozzle 63 based on the first imaging data 54. There are no particular limitations on the specific method for detecting the discharge-failure nozzle 63, but for example, the discharge-failure nozzle 63 can be detected by a method using machine learning.

[0086] The sample image data extraction unit 522 extracts a plurality of sample image data 64 corresponding to a plurality of comparison areas set in order to determine the compensation coefficients 62 to be applied to the ink amount compensation process from the second captured data 55. The sample image data extraction unit 522 further extracts normal area data 65 corresponding to the normal area from the second captured data 55. The comparison areas and normal areas will be described later.

[0087] The data deletion unit 523 deletes data in a range corresponding to the position of the nozzle (failure-to-eject nozzle) 63 detected by the discharge-failure nozzle detection unit 521 from the plurality of sample image data 64, thereby generating a plurality of comparison data 66 corresponding to the plurality of sample image data 64. Note that these plurality of comparison data 66 are associated with a plurality of provisional compensation coefficients that are prepared for the purpose of determining the compensation coefficient 62 that is actually applied to the ink amount compensation process.

[0088] The coefficient determination unit 524 determines one of the plurality of provisional compensation coefficients as the compensation coefficient 62 to be actually applied to the ink amount compensation process, based on the results of comparing each of the plurality of comparison data 66 with the normal area data 65. Note that the determination of the compensation coefficient 62 will be described in detail later.

[0089] 14 to 16, the concept behind how the compensation coefficient 62 is determined will be explained. The compensation coefficient 62 is a coefficient used in the ink amount compensation process, and as described above, in the ink amount compensation process, the density data of the pixel portion corresponding to the non-dischargeable nozzle and the pixel portion corresponding to the nozzles adjacent to the non-dischargeable nozzle (nozzles on both sides of the non-dischargeable nozzle) is corrected (see FIG. 7). In this process, the value obtained by multiplying the original density value of the pixel portion corresponding to the non-dischargeable nozzle by the compensation coefficient 62 is added to the original density value of the pixel portion of the nozzle adjacent to the non-dischargeable nozzle. The appropriate compensation coefficient 62 will vary depending on the type of printing paper 5 and the printing conditions.

[0090] Section A of Figure 14 schematically shows the density values ​​for five pixels in a normal case where no non-dischargeable nozzles exist, and the density values ​​for five pixels in four cases where ink volume compensation processing was performed using four compensation coefficients when non-dischargeable nozzles exist. Also, in section B of Figure 14, the target luminance value is indicated by a dotted line labeled 41, the distribution (spatial variation) of luminance values ​​in the five pixel areas for each case is indicated by a solid line labeled 42, and the average luminance value (average luminance value) for each case other than the normal case is indicated by a thick solid line labeled 43. For example, in the case of a "compensation coefficient of 20%," the amount of ink ejected in the corresponding area is small, so the average luminance value in the corresponding area is higher than the target luminance value. Also, in the case of a "compensation coefficient of 60%," the amount of ink ejected in the corresponding area is large, so the average luminance value in the corresponding area is lower than the target luminance value. In the example shown in Figure 14, in the case of a "compensation coefficient of 40%," the average luminance value in the corresponding area is close to the target luminance value.

[0091] In this embodiment, various compensation coefficients are prepared as provisional compensation coefficients, and the provisional compensation coefficient that, when ink volume compensation processing is performed using these provisional compensation coefficients, produces an average brightness value that is closest to the normal case (a case in which there are no nozzles with defective ejection), is adopted as the compensation coefficient 62 to be applied to the ink volume compensation processing when printing is performed based on an actual print job.

[0092] More specifically, in this embodiment, 21 provisional compensation coefficients (0%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, and 100%) are prepared. Furthermore, for each ink ejection head 251, the area in the main scanning direction is divided into 21 comparison areas corresponding to the 21 provisional compensation coefficients and a normal area corresponding to the normal case. Furthermore, for each of the 21 comparison areas, four provisional defective nozzles are set that are hypothetically considered to be in an ejection failure state from among the corresponding multiple nozzles. Then, a compensation coefficient 62 to be actually applied to the ink amount compensation process is determined from the 21 provisional compensation coefficients based on second image data 55 obtained by capturing an image of a compensation coefficient calculation chart that includes images corresponding to the 21 comparison areas and the normal area.

[0093] FIG. 15 shows an example of the distribution (spatial variation) of luminance values ​​obtained by the ink amount compensation process for two comparison regions 76 corresponding to two given provisional compensation coefficients. The positions of the portions marked with reference numeral 77 correspond to the positions of the provisionally defective nozzles. Based on this distribution of luminance values, an average luminance value is calculated for each comparison region. FIG. 16 shows a graph illustrating an example of the relationship between the provisional compensation coefficient and the average luminance value. Note that the average luminance value in normal regions is set to 0. In FIG. 16, the relationship between the provisional compensation coefficient and the average luminance value is represented by the thick dotted line marked with reference numeral 78. The smaller the provisional compensation coefficient, the lower the average luminance value because less ink is ejected into the corresponding region. The larger the provisional compensation coefficient, the lower the average luminance value because more ink is ejected into the corresponding region. From the portion marked with reference numeral 79 in FIG. 16, it can be seen that in this example, a provisional compensation coefficient of 45% results in an average luminance value closest to the average luminance value in the normal region. Therefore, in this example, the compensation coefficient 62 actually applied to the ink amount compensation process is determined to be 45%.

[0094] 6.2.2 Processing Procedure for Determining Compensation Coefficients The processing procedure for determining the compensation coefficients 62 will be described with reference to the flowchart shown in FIG. 17 . First, for each ink ejection head 251, the area in the main scanning direction (the direction perpendicular to the transport direction of the print paper 5) is divided into 21 comparison areas associated with the 21 different temporary compensation coefficients and one normal area (step S100). In this embodiment, the 21 comparison areas and one normal area correspond to multiple divided areas. Note that the number of temporary compensation coefficients prepared is not limited to 21. Also, it is not necessary to provide a normal area for each ink ejection head 251. That is, it is sufficient if the area corresponding to the width of the ink ejection head 251 in the main scanning direction is divided into multiple divided areas including K comparison areas associated with K different temporary compensation coefficients, where K is an integer greater than or equal to 2.

[0095] Incidentally, if five ink ejection heads 251 are provided for one ink color, the areas corresponding to the five ink ejection heads 251 are divided into 21 comparison areas and one normal area. Furthermore, as will be described later, the compensation coefficient calculation chart is composed of five sheets' worth of images for each color in the sub-scanning direction. As a result, there are 25 (= 5 × 5) comparison areas corresponding to each provisional compensation coefficient. In this regard, the 25 comparison areas corresponding to the same provisional compensation coefficient are treated as one comparison area. However, for ease of explanation, the following will focus on one comparison area out of the 25 comparison areas corresponding to one provisional compensation coefficient.

[0096] After step S100 is completed, for each of the 21 comparison regions, a plurality of provisionally defective nozzles that are assumed to be in a state of hypothetical ejection failure are set from among the corresponding plurality of nozzles (step S110). As described above, in this embodiment, four provisionally defective nozzles are set for each comparison region. That is, for each ink ejection head 251, four provisionally defective nozzles 80 are set for each of the 21 comparison regions 76(1) to 76(21), as shown schematically in FIG. 18.

[0097] Next, compensation coefficient calculation chart data 53 representing a compensation coefficient calculation chart for determining the compensation coefficient 62 is generated (step S120). FIG. 19 is a schematic diagram showing the entire compensation coefficient calculation chart 69. As shown in FIG. 19, the compensation coefficient calculation chart 69 is composed of a portion 690 (K) corresponding to the K color, a portion 690 (C) corresponding to the C color, a portion 690 (M) corresponding to the M color, and a portion 690 (Y) corresponding to the Y color. However, the following description focuses on one color. The portion corresponding to each color contains images for multiple sheets. In this embodiment, the portion corresponding to each color contains images for five sheets.

[0098] FIG. 20 is a schematic diagram showing a compensation coefficient calculation chart 69 for one color and one sheet for an area corresponding to one ink ejection head 251. The compensation coefficient calculation chart 69 is an image in which the density changes stepwise in the transport direction (sub-scanning direction) of the printing paper 5. In this embodiment, it is assumed that the density changes in 11 steps. Specifically, in FIG. 20, the density of the portion marked with reference numeral 691(1) is 5%, the density of the portion marked with reference numeral 691(2) is 10%, the density of the portion marked with reference numeral 691(10) is 90%, and the density of the portion marked with reference numeral 691(11) is 100%. The density changes in 10% increments between the portion marked with reference numeral 691(2) and the portion marked with reference numeral 691(10). 20, the compensation coefficient calculation chart 69 includes portions corresponding to the 21 comparison regions 76(1) to 76(21) and a portion corresponding to the normal region 81. As shown in FIG. 21, the portion corresponding to one comparison region 76 for one density includes a portion 694 corresponding to four provisional defective nozzles. Note that the density changes on the compensation coefficient calculation chart 69 are not limited to 11 levels. Where M is an integer greater than or equal to 2, the compensation coefficient calculation chart 69 may simply include M density regions corresponding to M different densities in the transport direction of the printing paper 5.

[0099] In step S120, compensation coefficient calculation chart data 53 representing the compensation coefficient calculation chart 69 described above is generated by applying 21 provisional compensation coefficients to 21 comparison areas 76(1) to 76(21) of image data having a constant density in the main scanning direction, thereby performing ink volume compensation processing.

[0100] For each combination of ink ejection head 251 and density, the same nozzle may be set as the provisionally defective nozzle for all five sheets, or a different nozzle may be set as the provisionally defective nozzle for each sheet. When the same nozzle is set as the provisionally defective nozzle for all five sheets, the region (comparison region) corresponding to a certain provisional compensation coefficient for the maximum density within the entire region of the compensation coefficient calculation chart 69 is shaded as shown in FIG. 22 . On the other hand, when a different nozzle is set as the provisionally defective nozzle for each sheet, the region (comparison region) corresponding to a certain provisional compensation coefficient for the maximum density within the entire region of the compensation coefficient calculation chart 69 is shaded as shown in FIG. 23 . Note that in FIGS. 22 and 23 , the regions labeled 695(1) to 695(5) represent the regions corresponding to the first to fifth sheets, respectively, and the regions labeled 696(1) to 696(5) represent the regions corresponding to the first to fifth ink ejection heads 251, respectively. When a different nozzle is set as the provisional defective nozzle for each sheet (see FIG. 23), the influence of variations in ink ejection amount among a plurality of nozzles can be more effectively suppressed.

[0101] After the compensation coefficient calculation chart data 53 is generated, a discharge failure detection chart for detecting discharge-failed nozzles is printed (step S130). FIG. 24 is a schematic diagram showing the entire discharge failure detection chart 68. As shown in FIG. 24, the discharge failure detection chart 68 is composed of a portion 680 (K) corresponding to the K color, a portion 680 (C) corresponding to the C color, a portion 680 (M) corresponding to the M color, and a portion 680 (Y) corresponding to the Y color. FIG. 25 is a schematic diagram showing the discharge failure detection chart 68 for one color. The discharge failure detection chart 68 is composed of a portion 681 representing a staircase pattern to be formed by ink being ejected from all the nozzles in one head unit 25, and a portion 682 representing a tint pattern of a constant density as a whole to be formed by ink being ejected from all the nozzles in one head unit 25. In step S130, the above-described ejection defect detection chart 68 is printed by controlling the ejection of ink from each nozzle included in each ink ejection head 251 that constitutes the recording unit 205 based on the ejection defect detection chart data 52. The ejection defect detection chart data 52 is stored in advance in the auxiliary storage device 121 in the print control device 100.

[0102] After printing the ejection defect detection chart 68, the ink ejection from each nozzle included in each ink ejection head 251 constituting the recording unit 205 is controlled based on the compensation coefficient calculation chart data 53 generated in step S120, thereby printing a compensation coefficient calculation chart 69 (step S140). A portion of the compensation coefficient calculation chart 69 printed in step S140 corresponding to one ink ejection head 251 and one density is, for example, as shown in Figure 26. The image of the portion shown in Figure 26 includes images corresponding to 21 comparison regions 76(1) to 76(21) and an image corresponding to a normal region 81.

[0103] After printing the compensation coefficient calculation chart 69, the discharge defect detection chart 68 printed on the printing paper 5 in step S130 is imaged by the inline scanner 40 (step S150), and further the compensation coefficient calculation chart 69 printed on the printing paper 5 in step S140 is imaged by the inline scanner 40 (step S160). First image data 54 is obtained in step S150, and second image data 55 is obtained in step S160.

[0104] Thereafter, the non-dischargeable nozzle detection unit 521 detects non-dischargeable nozzles exhibiting non-discharge, shape defects, lateral jumps, etc., based on the first image data 54 obtained in step S150 (step S170). If no non-dischargeable nozzles are present, the image represented by the first image data 54 will be as shown in FIG. 25 . On the other hand, if a non-dischargeable nozzle is present, the image represented by the first image data 54 will be as shown in FIG. 27 . In the example shown in FIG. 27 , the linear pattern is not printed properly in the portion marked with reference numeral 541 (non-discharge has occurred). Furthermore, the tint pattern is not printed properly in the portion marked with reference numeral 542, which corresponds to the non-discharge failure in the portion marked with reference numeral 541. Thus, if a non-dischargeable nozzle is present, the tint pattern is not printed properly in the portion corresponding to the non-dischargeable nozzle. As described above, in step S170, non-dischargeable nozzles are detected based on the first image data 54. Hereinafter, the non-dischargeable nozzles detected in step S170 will be referred to as "true faulty nozzles."

[0105] Next, the sample image data extraction unit 522 extracts, for each ink color, 21 pieces of sample image data 64 corresponding to the 21 comparison regions 76 set for determining the compensation coefficients 62 from the second captured data 55 obtained in step S160 (step S180). In this embodiment, in step S180, the sample image data extraction unit 522 further extracts, for each ink color, normal region data 65 corresponding to the normal region 81 from the second captured data 55.

[0106] In this embodiment, each of the 21 sample image data 64 includes four partial image data 640(1) to 640(4) corresponding to the four provisional defective nozzles 80 (see FIG. 18), as shown in FIG. 28.

[0107] After step S180 is completed, the data deletion unit 523 generates 21 pieces of comparison data 66 by deleting data in a range corresponding to the position of the true defective nozzle (the ejection-failure nozzle detected in step S170) from the 21 pieces of sample image data 64 (step S190). In this regard, in this embodiment, for example, if the position 641 of the true defective nozzle is included in the area of ​​partial image data 640(3) among the four partial image data 640(1) to 640(4) that make up one piece of sample image data 64, as shown in FIG. 29 , the data obtained by deleting partial image data 640(3) from the four partial image data 640(1) to 640(4) is used as the comparison data 66. Therefore, in this example, the comparison data 66 is composed of partial image data 660(1) corresponding to partial image data 640(1), partial image data 660(2) corresponding to partial image data 640(2), and partial image data 660(4) corresponding to partial image data 640(4), as shown in FIG. 30 .

[0108] Note that even if the position of the non-dischargeable nozzle is not included in the region of the partial image data that constitutes the sample image data 64, if the presence of the non-dischargeable nozzle affects the average value of the partial image data, the corresponding partial image data will be deleted when generating the comparison data 66. For example, as shown in Figure 31, if the position 642 of the non-dischargeable nozzle is not included in the region of partial image data 640(3) but is close to the region of partial image data 640(3), the presence of the non-dischargeable nozzle will affect the average value of partial image data 640(3). Therefore, even in such a case, the data obtained by deleting partial image data 640(3) from the four partial image data 640(1) to 640(4) is used as the comparison data 66.

[0109] As described above, in this embodiment, in step S190, for each of the 21 sample image data 64, the partial image data corresponding to the area affected by the true defective nozzle is deleted from the four partial image data 640(1) to 640(4).

[0110] After step S190 is completed, the coefficient determination unit 524 determines one of the 21 provisional compensation coefficients as the compensation coefficient 62 to be actually applied to the ink amount compensation process based on the 21 pieces of comparison data 66 and the normal area data 65 (step S200). The compensation coefficient 62 is determined for each density of the image that makes up the compensation coefficient calculation chart 69 (in this embodiment, for each of 11 levels of density). The compensation coefficient 62 that corresponds to a density between a certain level and the density of the adjacent level can be determined, for example, using linear interpolation. For example, the compensation coefficient 62 for a density of 62% is determined by linear interpolation using the compensation coefficient 62 for a density of 60% and the compensation coefficient 62 for a density of 70%.

[0111] In step S200, first, the average value for each of the 21 pieces of comparison data 66 and the average value for the normal region data 65 are calculated. Note that the comparison data 66 and the normal region data 65 are luminance value data. Therefore, the average luminance value for each of the 21 pieces of comparison data 66 and the average luminance value for the normal region data 65 are calculated.

[0112] Here, we will explain how to determine the average brightness value for each comparison data 66. Note that, again, for the sake of convenience, we will focus on only one ink ejection head 251 and an area corresponding to one sheet of compensation coefficient calculation chart data 53. Furthermore, the average value of each partial image data that constitutes the comparison data 66 (average brightness value in each partial image data area) will be referred to as the "partial average value."

[0113] When there are no truly defective nozzles, the comparison data 66 is made up of four partial image data 660(1) to 660(4), as shown in Figure 32. If the partial average values ​​of the partial image data 660(1) to 660(4) are represented by V1 to V4, respectively, the average brightness value Vave of the comparison data is calculated using the following equation (2): Vave = (V1 + V2 + V3 + V4) / 4 (2)

[0114] 29 and 31, when a true defective nozzle is located in a position that affects the average brightness value of partial image data 640(3), the comparison data 66 is composed of three partial image data 660(1), 660(2), and 660(4), as shown in Fig. 33. If the partial average values ​​of partial image data 660(1), 660(2), and 660(4) are represented by V1, V2, and V4, respectively, the average brightness value Vave of the comparison data is calculated using the following equation (3): Vave = (V1 + V2 + V4) / 3 (3)

[0115] In this way, an average brightness value is calculated for each of the 21 pieces of comparison data 66. Then, the average brightness value for the normal area data 65 is set as a target value, and this target value is compared with the average brightness value of each of the 21 pieces of comparison data 66. As a result, the provisional compensation coefficient associated with the comparison area 76 corresponding to the comparison data for which the average brightness value closest to the target value was obtained among the 21 pieces of comparison data 66 is determined as the compensation coefficient 62 actually applied to the ink amount compensation process.

[0116] In this embodiment, step S100 implements the area division step, step S110 implements the temporary faulty nozzle setting step, step S120 implements the compensation coefficient calculation chart data generation step, step S130 implements the discharge defect detection chart printing step, step S140 implements the compensation coefficient calculation chart printing step, step S150 implements the discharge defect detection chart imaging step, step S160 implements the compensation coefficient calculation chart imaging step, step S170 implements the discharge defect nozzle detection step, step S180 implements the sample image data extraction step, step S190 implements the data deletion step, and step S200 implements the coefficient determination step.

[0117] 7. Effects According to this embodiment, 21 provisional compensation coefficients are prepared as various compensation coefficients, and a provisional compensation coefficient to be actually applied as the compensation coefficient 62 in the ink amount compensation process is determined from the 21 provisional compensation coefficients based on data obtained by using the 21 provisional compensation coefficients in the ink amount compensation process. In this way, the compensation coefficient 62 is determined based on data obtained by actually performing the ink amount compensation process. Furthermore, prior to the final determination of the compensation coefficient 62, data in the area affected by the non-ejecting nozzle is deleted. As described above, because an appropriate compensation coefficient 62 can be determined with high accuracy, the occurrence of defects in printed images when non-ejecting nozzles is present in the inkjet printing device 10 is more effectively suppressed than in the past. Furthermore, wasteful consumption of ink and printing paper 5 is reduced, which can contribute to the achievement of the SDGs (Sustainable Development Goals).

[0118] 8. Modifications Modifications of the above embodiment will now be described.

[0119] 8.1 First Modification In the above embodiment, each sample image data 64 was made up of four partial image data 640(1) to 640(4) (see FIG. 28), and the partial image data 640 affected by the defective nozzle was deleted when generating the comparison data 66. In contrast, in this modification, as shown in FIG. 34, the data for the entire comparison area 76 is used as the sample image data 64. Below, we will explain the differences from the above embodiment regarding the processing procedure for determining the compensation coefficient 62.

[0120] In step S180 of FIG. 17 , data for the entire comparison region 76 is extracted as sample image data 64, as shown in FIG. 34 . In step S190 of FIG. 17 , a predetermined range of data is deleted from each sample image data 64 based on the position of the true defective nozzle. In this regard, if the position indicated by reference numeral 82 in FIG. 35 corresponds to the position of the true defective nozzle, for example, data in the range indicated by the arrow indicated by reference numeral 85 in FIG. 36 is deleted from the sample image data 64. As a result, the comparison data 66 is composed of data in the range indicated by the arrow indicated by reference numeral 83 in FIG. 36 and data in the range indicated by the arrow indicated by reference numeral 84 in FIG. 36 . Then, in the example shown in FIG. 36 , in step S200 of FIG. 17 , an average luminance value is calculated based on the data in the range indicated by the arrow indicated by reference numeral 83 in FIG. 36 and the range indicated by the arrow indicated by reference numeral 84 in FIG. 36 .

[0121] As described above, each sample image data 64 does not have to be made up of a plurality of partial image data 640 .

[0122] 8.2 Second Modification A method for determining the compensation coefficient 62 in a second modification of the above embodiment will be described with reference to FIG. 37 . It is believed that the more favorable the corresponding provisional compensation coefficient for the comparison data 66, the closer the data value (brightness value) corresponding to the provisional defective nozzle position will be to the average value (average brightness value) for the normal area data 65. Therefore, in this modification, the average value (hereinafter referred to as the "peak average value") of the data values ​​corresponding to the provisional defective nozzle position is calculated for each of the 21 pieces of comparison data 66. Furthermore, before calculating the peak average value, in step S190 of FIG. 17 , data for positions affected by the true defective nozzle are deleted from the positions of the four provisional defective nozzles corresponding to each comparison area 76.

[0123] From the above, for example, if the distribution (spatial variation) of data values ​​for the second image data 55 corresponding to a certain comparison region 76 is as shown by the solid line labeled 86 in Fig. 37, there are no true faulty nozzles, and the four positions labeled 87(1) to 87(4) correspond to the four tentative faulty nozzles, then the average of the data values ​​at the four positions labeled 87(1) to 87(4) is obtained as the peak average for the comparison data 66 corresponding to that certain comparison region 76. Also, if the distribution of data values ​​for the second image data 55 corresponding to a certain comparison region 76 is as shown by the solid line labeled 86 in Fig. 37, and there is a true faulty nozzle that affects the data value at the position labeled 87(1), then the average of the data values ​​at the three positions labeled 87(2) to 87(4) is obtained as the peak average for the comparison data 66 corresponding to that certain comparison region 76.

[0124] After the peak average value is calculated for each of the 21 comparison data 66 in the manner described above, the provisional compensation coefficient corresponding to the comparison data 66 for which the peak average value closest to the average value for the normal area data 65 is obtained is determined as the compensation coefficient 62 to be actually applied to the ink volume compensation process.

[0125] As described above, in this modified example, in step S190 of FIG. 17 , data for P positions affected by the true faulty nozzle out of the four provisional faulty nozzle positions is deleted for each of the 21 sample image data, where P is an integer between 1 and 3. This generates 21 pieces of comparison data 66. Then, in step S200 of FIG. 17 , for each of the 21 pieces of comparison data 66, a peak average value, which is the average value of the data for the (4-P) positions excluding the P positions deleted in step S190 from the data for the positions of the four provisional faulty nozzles, and an average value for the normal area data 65 are calculated, and the average value for the normal area data 65 is used as the target value. The provisional compensation coefficient associated with the comparison area 76 corresponding to the comparison data 66 from among the 21 pieces of comparison data 66 for which a peak average value closest to the target value was obtained is determined as the compensation coefficient 62 to be actually applied to the ink amount compensation process.

[0126] 8.3 Third Modification A method for determining the compensation coefficient 62 in a third modification of the above embodiment will be described with reference to FIG. 38 . It is believed that the more favorable the corresponding provisional compensation coefficient value for the comparison data 66, the smaller the variation in data values ​​(brightness values). Therefore, in this modification, the difference between the maximum and minimum data values ​​(brightness values) is calculated for each of the 21 pieces of comparison data 66. Furthermore, before calculating the difference between the maximum and minimum data values, a predetermined range of data is deleted based on the position of the true defective nozzle in step S190 of FIG. 17 .

[0127] For example, if the distribution (spatial variation) of data values ​​for the second image data 55 corresponding to a certain comparison area 76 is as shown by the solid line labeled 88 in Fig. 38 and no truly faulty nozzles are present, then the difference between the maximum and minimum data values ​​for the comparison data 66 corresponding to that certain comparison area 76 is represented by the length of the arrow labeled 89a in Fig. 38. Also, if the distribution of data values ​​for the second image data 55 corresponding to a certain comparison area 76 is as shown by the solid line labeled 88 in Fig. 38, and a truly faulty nozzle is present and the data in the range indicated by the arrow labeled 90 is deleted in step S190 of Fig. 17, then the difference between the maximum and minimum data values ​​for the comparison data 66 corresponding to that certain comparison area 76 is represented by the length of the arrow labeled 89b in Fig. 38.

[0128] After the difference between the maximum and minimum data values ​​for each of the 21 comparison data 66 is calculated, the provisional compensation coefficient corresponding to the comparison data 66 with the smallest difference is determined as the compensation coefficient 62 to be actually applied to the ink volume compensation process.

[0129] As described above, in this modified example, in step S190 of Fig. 17, a predetermined range of data is deleted based on the position of the true faulty nozzle. This generates 21 pieces of comparison data 66. Then, in step S200 of Fig. 17, the provisional compensation coefficient associated with the comparison region 76 corresponding to the comparison data 66 with the smallest difference between the maximum and minimum values ​​among the 21 pieces of comparison data 66 is determined as the compensation coefficient 62 to actually apply to the ink amount compensation process.

[0130] 8.4 Fourth Modification In the above embodiment, it was assumed that the entire series of processes for determining the compensation coefficients 62 (see FIG. 17 ), from the process of dividing the area corresponding to the width of the ink ejection head 251 into a plurality of divided areas to the process of finally determining the compensation coefficients 62, would be performed within a single printing system (i.e., performed by the user of the inkjet printing apparatus 10). However, this is not limited to this. It is also possible to employ an operation in which the manufacturer of the inkjet printing apparatus 10 performs the processes up to the generation of the compensation coefficient calculation chart data 53 (i.e., the processes up to step S120 in FIG. 17 ), and the processes subsequent to the process of printing the ejection defect detection chart 68 (the processes from step S130 onwards in FIG. 17 ) are performed by the user of the inkjet printing apparatus 10.

[0131] FIG. 39 is a flowchart showing the procedure for determining the compensation coefficient 62 in this modified example. First, a discharge defect detection chart 68 is printed (step S300). Next, the ink discharge from each nozzle included in each ink discharge head 251 constituting the recording unit 205 is controlled based on the compensation coefficient calculation chart data 53, thereby printing a compensation coefficient calculation chart 69 (step S310). In this regard, unlike the above embodiment, in this modified example, the compensation coefficient calculation chart data 53 representing the compensation coefficient calculation chart 69 is stored in advance in the auxiliary storage device 121 of the print control device 100. The compensation coefficient calculation chart data 53 is generated by the manufacturer of the inkjet printing device 10 by performing the processes of steps S100 to S120 of FIG. 17. That is, the manufacturer of the inkjet printing device 10 divides the area of ​​each ink ejection head 251 in the main scanning direction (the direction perpendicular to the transport direction of the printing paper 5) into 21 comparison areas 76 (each comparison area 76 corresponds to a plurality of temporary defective nozzles 80) associated with 21 different temporary compensation coefficients, and one normal area 81, and generates the compensation coefficient calculation chart data 53 by applying the 21 temporary compensation coefficients to predetermined image data of a constant density in the main scanning direction, respectively, to the 21 comparison areas 76. After the compensation coefficient calculation chart data 53 is generated, processing similar to the processing of steps S150 to S200 in FIG. 17 (the processing of steps S320 to S370 in FIG. 39) is performed.

[0132] In this embodiment, step S300 realizes the step of printing a defective discharge detection chart, step S310 realizes the step of printing a compensation coefficient calculation chart, step S320 realizes the step of capturing an image of a defective discharge detection chart, step S330 realizes the step of capturing an image of a compensation coefficient calculation chart, step S340 realizes the step of detecting a defective discharge nozzle, step S350 realizes the step of extracting sample image data, step S360 realizes the step of deleting data, and step S370 realizes the step of determining a coefficient.

[0133] 9. Supplementary Information In the above embodiment, the effects of poor ejection are mitigated by correcting the density values ​​of the nozzles on either side of the nozzle with poor ejection (for example, if the nozzle with reference numeral 252(p) in FIG. 4 is the nozzle with poor ejection, the density values ​​of the nozzles with reference numerals 252(q) and 252(r) in the same figure are corrected). However, the nozzles whose density values ​​are corrected need only be nozzles near the nozzle with poor ejection in the main scanning direction, and do not necessarily have to be adjacent to the nozzle with poor ejection. For example, the density value of a nozzle located two to three nozzles away in the main scanning direction from the nozzle with poor ejection may be corrected. Furthermore, the number of nozzles whose density values ​​are corrected does not have to be one nozzle on each side in the main scanning direction, but may be two or more nozzles. Furthermore, when correcting the density values ​​of two or more nozzles on each side in the main scanning direction, a nozzle whose density value is reduced may be included among those nozzles.

[0134] In the above embodiment, the CPU 111 as a processor executes the print control program 13, which includes a compensation coefficient determination program as a subprogram, to realize various functions of the print control device 100. However, the configuration is not limited to using only one CPU 111 as shown in FIG. 5 . A configuration using multiple processors, such as a configuration using multiple CPUs, can also be adopted. In addition to a CPU, processors such as an MPU (Micro Processing Unit), a GPU (Graphics Processing Unit), or a DSP (Digital Signal Processor) can also be used. Multiple types of processors can also be used in combination. For example, with regard to the components shown in FIG. 11 , some components and the remaining components can be implemented by different processors. Furthermore, a configuration including an FPGA (Field-Programmable Gate Array) or an ASIC (Application Specific Integrated Circuit) can also be adopted.

[0135] 10. Others In the above embodiment (including the modified examples), the inkjet printing device 10 performs color printing. However, the present invention is not limited to this, and an inkjet printing device that performs monochrome printing may also be used.

[0136] Furthermore, in the above-described embodiment (including the modified examples), the inkjet printing apparatus 10 uses aqueous ink. However, the present invention is not limited to this, and an inkjet printing apparatus using UV ink (ultraviolet-curable ink), such as an inkjet printing apparatus for label printing, may also be used. In this case, an ultraviolet irradiation mechanism that cures the UV ink on the printing paper 5 by irradiating it with ultraviolet light, instead of the drying mechanism 206, is provided inside the printing mechanism 201 (see FIG. 2 ).

[0137] 10...inkjet printing device 40...inline scanner 64...sample image data 65...normal area data 66...comparison data 68...ejection defect detection chart 69...compensation coefficient calculation chart 76...comparison area 80...temporary defective nozzle 100...printing control device 155...compensation coefficient calculation chart data generation processing unit 156...correction coefficient calculation unit 157...printing data correction unit 200...printing machine main body 201...printing mechanism 205...recording unit 251...ink ejection head 510...shading data generation unit 520...compensation coefficient calculation unit 521...ejection defect nozzle detection unit 522...sample image data extraction unit 523...data deletion unit 524...coefficient determination unit 530...shading data correction unit

Claims

1. A compensation coefficient determination method for determining a compensation coefficient to be applied to an ink amount compensation process that compensates for the amount of ink ejected onto a printing medium when a nozzle in a defective ejection state is present in a printing device having a plurality of nozzles that eject ink onto a printing medium, comprising: a region division step of dividing a predetermined region in a direction perpendicular to a transport direction of the printing medium into a plurality of divided regions including K comparison regions corresponding to K different provisional compensation coefficients, where K is an integer of 2 or more; a provisional defective nozzle setting step of setting a plurality of provisional defective nozzles that are assumed to be in a defective ejection state from among two or more corresponding nozzles for each of the K comparison regions; a compensation coefficient calculation chart data generation step of generating compensation coefficient calculation chart data representing a compensation coefficient calculation chart for determining the compensation coefficient by applying the K provisional compensation coefficients to the K comparison regions for predetermined image data having a constant density in the direction perpendicular to the transport direction of the printing medium, a compensation coefficient calculation chart printing step of printing the compensation coefficient calculation chart by ejecting ink from the plurality of nozzles based on the compensation coefficient calculation chart data; a discharge failure detection chart imaging step of imaging the discharge failure detection chart printed in the discharge failure detection chart printing step; a compensation coefficient calculation chart imaging step of imaging the compensation coefficient calculation chart printed in the compensation coefficient calculation chart printing step; a discharge-fault nozzle detection step of detecting a nozzle in a discharge fault state based on first imaging data which is imaging data obtained in the discharge failure detection chart imaging step; and a sample image data extraction step of extracting K sample image data corresponding to the K comparison regions from second imaging data which is imaging data obtained in the compensation coefficient calculation chart imaging step.a data deleting step of generating K pieces of comparison data by deleting data in a range corresponding to the position of a true faulty nozzle, which is a nozzle detected in the faulty nozzle detection step, from the K pieces of sample image data; and a coefficient determining step of determining one of the K provisional compensation coefficients as the compensation coefficient based on the K pieces of comparison data.

2. A compensation coefficient determination method as described in claim 1, wherein the plurality of divided regions includes, in addition to the K comparison regions, a normal region that is not affected by the provisional defective nozzle, the sample image data extraction step further extracts normal region data corresponding to the normal region from the second imaging data, and the coefficient determination step determines the compensation coefficient based on the K comparison data and the normal region data.

3. A compensation coefficient determination method as described in claim 2, wherein, in the coefficient determination step, an average value for each of the K comparison data and an average value for the normal area data are calculated, and the average value for the normal area data is set as a target value, and a provisional compensation coefficient associated with a comparison area corresponding to the comparison data for which an average value closest to the target value is obtained among the K comparison data is determined as the compensation coefficient.

4. A compensation coefficient determination method as described in claim 3, wherein N is an integer of 2 or greater, each of the K comparison areas corresponds to N tentative faulty nozzles, each of the K sample image data includes N partial image data corresponding to the N tentative faulty nozzles, and in the data deletion step, for each of the K sample image data, partial image data corresponding to an area affected by the true faulty nozzle is deleted from the N partial image data.

5. A compensation coefficient determination method as claimed in claim 2, wherein N is an integer of 2 or more, each of the K comparison areas corresponds to N tentative faulty nozzles, and P is an integer of 1 or more and (N-1) or less, and in the data deletion step, for each of the K sample image data, data for P positions affected by the true faulty nozzle from among the positions of the N tentative faulty nozzles is deleted, and in the coefficient determination step, for each of the K comparison data, a peak average value which is the average value of data for (N-P) positions excluding the data for the P positions deleted in the data deletion step from among the data for the positions of the N tentative faulty nozzles and an average value for the normal area data are obtained, and the average value for the normal area data is set as a target value, and a provisional compensation coefficient associated with a comparison area corresponding to the comparison data for which a peak average value closest to the target value has been obtained from among the K comparison data is determined as the compensation coefficient.

6. A compensation coefficient determination method as described in claim 1, wherein in the coefficient determination step, a provisional compensation coefficient associated with a comparison area corresponding to the comparison data having the smallest difference between the maximum value and the minimum value among the K comparison data is determined as the compensation coefficient.

7. A compensation coefficient determination method as described in claim 1, wherein the printing device comprises a plurality of print heads, each of which includes a portion of the plurality of nozzles, the K comparison areas are provided for each print head, the K provisional compensation coefficients corresponding to the K comparison areas are common among the plurality of print heads, and in the sample image data extraction step, one sample image data is extracted by extracting from the second imaging data data of a plurality of comparison areas corresponding to the same provisional compensation coefficient among the plurality of print heads.

8. The compensation coefficient determination method according to claim 1, wherein the compensation coefficient calculation chart is made up of a plurality of sheets, one for each ink color.

9. The compensation coefficient determination method according to claim 8, wherein in said tentative defective nozzle designation step, a different nozzle is designated as the tentative defective nozzle for each of said K comparison regions for each sheet.

10. A compensation coefficient determination method as described in claim 1, wherein M is an integer equal to or greater than 2, the compensation coefficient calculation chart is made up of M density regions corresponding to M different densities in the transport direction of the printing medium, and in the coefficient determination step, the compensation coefficient is determined for each density.

11. A printing method using a printing device having a plurality of nozzles that eject ink onto a printing medium, comprising: a shading data generation step of generating shading data to be used in a shading correction process that compensates for variations in the amount of ink ejected among the plurality of nozzles; a shading data correction step of correcting the shading data by applying the ink amount compensation process to the shading data using a compensation coefficient determined by a compensation coefficient determination method defined in any one of claims 1 to 10; a print data generation step of generating print data by applying a rasterization process to input data; a shading correction step of correcting the print data by applying the shading correction process to the print data using the shading data corrected in the shading data correction step; a halftone processing step of generating halftone image data by applying a halftone process to the print data corrected in the shading correction step; and an ink ejection step of ejecting ink from the plurality of nozzles based on the halftone image data.

12. The printing method according to claim 11, wherein a nozzle adjacent to the true faulty nozzle is defined as an adjacent nozzle, the compensation coefficient is represented by C, which is a value between 0 and 1, a position value corresponding to the true faulty nozzle in the shading data before the shading data correction step is executed is represented by Vf, a position value corresponding to the adjacent nozzle in the shading data before the shading data correction step is executed is represented by Vn, and a position value corresponding to the adjacent nozzle in the shading data after the shading data correction step is executed is represented by V, where V is represented by the following formula: V=Vn+Vf×C 13. The printing method according to claim 12, wherein the value of a position in the shading data corresponding to the true faulty nozzle after the shading data correction step has been executed is 0.

14. A printing device comprising a plurality of nozzles which eject ink onto a printing medium, an imaging device which captures a print image, and a control unit which controls the ejection of ink from the plurality of nozzles and the capturing of the print image by the imaging device, and which performs, during printing, an ink amount compensation process which compensates for the amount of ink ejected onto the printing medium when a nozzle having a defective ejection state is present, wherein the control unit performs: a region division process which divides a predetermined region in a direction perpendicular to the transport direction of the printing medium into a plurality of divided regions including K comparison regions which correspond to K different provisional compensation coefficients, where K is an integer of 2 or more; a provisional defective nozzle setting process which sets a plurality of provisional defective nozzles which are assumed to have a defective ejection state from among two or more corresponding nozzles for each of the K comparison regions; and a compensation coefficient calculation chart data generation process which generates compensation coefficient calculation chart data representing a compensation coefficient calculation chart for determining the compensation coefficient by performing the ink amount compensation process by applying the K provisional compensation coefficients to the K comparison regions for predetermined image data which has a constant density in the direction perpendicular to the transport direction of the printing medium, respectively. a discharge failure detection chart printing process that controls the ejection of ink from the plurality of nozzles so that a discharge failure detection chart for detecting nozzles in a discharge failure state is printed; a compensation coefficient calculation chart printing process that controls the ejection of ink from the plurality of nozzles based on the compensation coefficient calculation chart data so that the compensation coefficient calculation chart is printed; a discharge failure detection chart imaging process that causes the imaging device to image the discharge failure detection chart printed in the discharge failure detection chart printing step; a compensation coefficient calculation chart imaging process that causes the imaging device to image the compensation coefficient calculation chart printed in the compensation coefficient calculation chart printing step; a discharge failure nozzle detection process that detects nozzles in a discharge failure state based on first imaging data that is the imaging data obtained in the discharge failure detection chart imaging step; and a sample image data extraction process that extracts K sample image data corresponding to the K comparison regions from second imaging data that is the imaging data obtained in the compensation coefficient calculation chart imaging step.A printing device that executes a data deletion process that generates K pieces of comparison data by deleting data in a range corresponding to the position of a true faulty nozzle, which is a nozzle detected in the faulty nozzle detection process, from the K pieces of sample image data, and a coefficient determination process that determines one of the K provisional compensation coefficients as the compensation coefficient based on the K pieces of comparison data.

15. A compensation coefficient determination program for determining a compensation coefficient to be applied to an ink amount compensation process for compensating for the amount of ink ejected onto a printing medium when a nozzle in a defective ejection state is present in a printing device having a plurality of nozzles that eject ink onto a printing medium and an imaging device that captures a print image, the program comprising: a region division step of dividing a predetermined region in a direction perpendicular to the transport direction of the printing medium into a plurality of divided regions including K comparison regions corresponding to K different provisional compensation coefficients, where K is an integer of 2 or more; a provisional defective nozzle setting step of setting a plurality of provisional defective nozzles that are assumed to be in a defective ejection state from among two or more corresponding nozzles for each of the K comparison regions; and a compensation coefficient calculation chart data generation step of generating compensation coefficient calculation chart data representing a compensation coefficient calculation chart for determining the compensation coefficient by performing the ink amount compensation process by applying the K provisional compensation coefficients to the K comparison regions for predetermined image data having a constant density in the direction perpendicular to the transport direction of the printing medium, respectively. a discharge failure detection chart printing step of controlling the discharge of ink from the plurality of nozzles so that a discharge failure detection chart for detecting nozzles in a discharge failure state is printed; a compensation coefficient calculation chart printing step of controlling the discharge of ink from the plurality of nozzles based on the compensation coefficient calculation chart data so that the compensation coefficient calculation chart is printed; a discharge failure detection chart imaging step of causing the imaging device to image the discharge failure detection chart printed in the discharge failure detection chart printing step; a compensation coefficient calculation chart imaging step of causing the imaging device to image the compensation coefficient calculation chart printed in the compensation coefficient calculation chart printing step; a discharge failure nozzle detection step of detecting nozzles in a discharge failure state based on first imaging data which is the imaging data obtained in the discharge failure detection chart imaging step; and a sample image data extraction step of extracting K sample image data corresponding to the K comparison regions from second imaging data which is the imaging data obtained in the compensation coefficient calculation chart imaging step.a data deletion step of generating K pieces of comparison data by deleting data in a range corresponding to the position of a true faulty nozzle, which is a nozzle detected in the faulty nozzle detection step, from the K pieces of sample image data; and a coefficient determination step of determining one of the K provisional compensation coefficients as the compensation coefficient based on the K pieces of comparison data.

16. A compensation coefficient method for determining a compensation coefficient to be applied to an ink amount compensation process for compensating for the amount of ink discharged onto a printing medium when a nozzle in a defective discharge state is present in a printing device having a plurality of nozzles that discharge ink onto a printing medium, comprising: a defective discharge detection chart printing step of printing a defective discharge detection chart for detecting defective discharge nozzles by discharging ink from the plurality of nozzles; and a compensation coefficient calculation chart printing step of printing a compensation coefficient calculation chart for determining the compensation coefficient by discharging ink from the plurality of nozzles based on compensation coefficient calculation chart data obtained by performing the ink amount compensation process by applying the K temporary compensation coefficients to the K comparison regions for predetermined image data having a constant density in the direction perpendicular to the transport direction of the printing medium, in a state in which a predetermined region is divided into a plurality of divided regions including K comparison regions corresponding to K different temporary compensation coefficients in a direction perpendicular to the transport direction of the printing medium, where K is an integer of 2 or more; a discharge failure detection chart imaging step of imaging the discharge failure detection chart printed in the discharge failure detection chart printing step; a compensation coefficient calculation chart imaging step of imaging the compensation coefficient calculation chart printed in the compensation coefficient calculation chart printing step; a discharge failure nozzle detection step of detecting a nozzle in a discharge failure state based on first imaging data which is the imaging data obtained in the discharge failure detection chart imaging step; a sample image data extraction step of extracting K pieces of sample image data corresponding to the K comparison regions from second imaging data which is the imaging data obtained in the compensation coefficient calculation chart imaging step; a data deletion step of generating K pieces of comparison data by deleting data in a range corresponding to the position of a true defective nozzle which is the nozzle detected in the discharge failure nozzle detection step from the K pieces of sample image data; and a coefficient determination step of determining one of the K tentative compensation coefficients as the compensation coefficient based on the K pieces of comparison data.

17. A printing device comprising a plurality of nozzles which eject ink onto a printing medium, an imaging device which captures a print image, and a control unit which controls the ejection of ink from the plurality of nozzles and the imaging of the print image by the imaging device, and which performs an ink amount compensation process during printing to compensate for the amount of ink ejected onto the printing medium when a nozzle in a defective ejection state is present, wherein the control unit performs: a defective ejection detection chart printing process which controls the ejection of ink from the plurality of nozzles so that a defective ejection detection chart for detecting defective ejection nozzles is printed; a compensation coefficient calculation chart printing process that controls the ejection of ink from the plurality of nozzles based on compensation coefficient calculation chart data obtained by applying the ink amount compensation process to predetermined image data having a constant density in the direction orthogonal to the transport direction of the printing medium, in a state where a predetermined area is divided into a plurality of divided areas including K comparison areas associated with K different temporary compensation coefficients, where K is an integer of 2 or greater, so that a compensation coefficient calculation chart for determining the compensation coefficient is printed, the K comparison areas corresponding to a plurality of temporarily defective nozzles that are virtually set to be in an ejection defective state; a discharge failure detection chart imaging process that causes the imaging device to image the discharge failure detection chart printed in the discharge failure detection chart printing process; a compensation coefficient calculation chart imaging process that causes the imaging device to image the compensation coefficient calculation chart printed in the compensation coefficient calculation chart printing process; and a discharge failure nozzle detection process that detects a nozzle in a discharge failure state based on first imaging data that is imaging data obtained in the discharge failure detection chart imaging process. a sample image data extraction process for extracting K pieces of sample image data corresponding to the K comparison regions from second imaging data, which is imaging data obtained in the compensation coefficient calculation chart imaging process; and a data deletion process for generating K pieces of comparison data by deleting data in a range corresponding to the position of a true defective nozzle, which is a nozzle detected in the discharge-fault nozzle detection process, from the K pieces of sample image data.a coefficient determination process for determining one of the K provisional compensation coefficients as the compensation coefficient based on the K pieces of comparison data.

18. A compensation coefficient determination program for determining a compensation coefficient to be applied to an ink amount compensation process for compensating for the amount of ink discharged onto a printing medium when a nozzle in a discharge failure state is present in a printing device having a plurality of nozzles that discharge ink onto a printing medium and an imaging device that captures a print image, the compensation coefficient determination program comprising: a discharge failure detection chart printing step for controlling the discharge of ink from the plurality of nozzles so that a discharge failure detection chart for detecting a discharge failure state is printed on a computer included in the printing device; and a compensation coefficient calculation chart printing step for controlling the discharge of ink from the plurality of nozzles based on compensation coefficient calculation chart data obtained by applying the K provisional compensation coefficients to the K comparison regions for predetermined image data having a constant density in the direction perpendicular to the transport direction of the printing medium, so that a compensation coefficient calculation chart for determining the compensation coefficient is printed, where K is an integer of 2 or more. a discharge failure detection chart imaging step of causing the imaging device to image the discharge failure detection chart printed in the discharge failure detection chart printing step; a compensation coefficient calculation chart imaging step of causing the imaging device to image the compensation coefficient calculation chart printed in the compensation coefficient calculation chart printing step; a discharge failure nozzle detection step of detecting a nozzle in a discharge failure state based on first imaging data which is the imaging data obtained in the discharge failure detection chart imaging step; a sample image data extraction step of extracting K sample image data corresponding to the K comparison regions from second imaging data which is the imaging data obtained in the compensation coefficient calculation chart imaging step; and a data deletion step of generating K comparison data by deleting data in a range corresponding to the position of the true defective nozzle which is the nozzle detected in the discharge failure nozzle detection step from the K sample image data.a coefficient determination step of determining one of the K provisional compensation coefficients as the compensation coefficient based on the K pieces of comparison data.

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