Image processing method, image processing device, program, and image forming device

Abstract

To provide an image processing method, an image processing device, a program, and an image formation device which can improve prediction accuracy of a correction value suppressing density irregularity in a high density region.SOLUTION: An image processing method forms a test image for density measurement including a plurality of patches in which uniform concentration is set in a first direction at which a plurality of recording elements of a recording head are arranged; allows a processor to acquire density measurement values for recording elements of each patch; and calculates a correction value correcting output gradation for input gradation for each of the recording elements from the density measurement value. The processor predicts the density measurement value in a region with higher density than a first patch set at the highest density and performs extrapolation processing on the basis of the concentration measurement values of the first patch and a second patch set at the second highest density, when inclination of characteristics for inter-patch gradation between the first patch and the second patch is represented by u, inclination of characteristics between arbitrary two-point gradation in a gradation region of input gradation used for correction on the higher density than the first patch is represented by bu, the processor performs halftone processing satisfying 0≤b≤1.5.SELECTED DRAWING: Figure 14

Description

[Technical Field]

[0001] The present disclosure relates to an image processing method, an image processing device, a program, and an image forming device, and more particularly to an image processing technique applied to digital printing such as inkjet printing. [Background technology]

[0002] Patent Document 1 describes an ink density error correction method that includes a step of adjusting the operating conditions of an inkjet head in an inkjet printing device to individually adjust the ink density, a step of printing a test pattern, a step of capturing an image of the test pattern, a step of calculating correction data based on data of the captured test pattern, and a step of correcting ink density errors in the entire image printed on a substrate by converting the entire image to be printed based on the correction data.

[0003] Patent Document 2 describes a diagnostic device having an acquisition means for acquiring output characteristics that represent the correspondence between gradation values ​​corrected by a correction means according to determined correction characteristics and measured values ​​of the density of a toner image output by an output device using those gradation values; an estimation means for estimating, based on the output characteristics acquired by the acquisition means, the density of a toner image when the output device outputs using a predetermined reference gradation value if that reference gradation value is not included in the range that the corrected gradation value can take; and a diagnostic means for comparing the density estimated by the estimation means with a predetermined threshold value to diagnose whether or not the correction characteristics need to be changed.

[0004] Patent document 3 describes an image recording method that has a multi-recording head arranged in a predetermined direction with multiple image recording elements, multiple quantization means for quantizing an image input signal, a selection means for selecting this quantization means, a printing means for applying a uniform image input signal to the multi-recording head to print a test pattern, a reading means for reading this test pattern and outputting a read signal, and a density unevenness correction means for correcting density unevenness using a calculation means for calculating the amount of density unevenness of the multi-recording head from the value of this read signal, and that switches the calculation processing of the image input signal depending on the quantization processing method of the selected quantization means. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 6416432 [Patent Document 2] Patent No. 6428357 [Patent Document 3] Japanese Patent Application Publication No. 11-165407 Summary of the Invention [Problem to be solved by the invention]

[0006] In Patent Document 1, a test pattern including multiple density levels is printed, and discrete measurement data is supplemented when correcting each nozzle from the measurement results of the test pattern. However, no consideration is given to areas outside the density range measured by the test pattern, and there is a concern that errors may occur when attempting to use higher gradation correction in correcting high-density areas.

[0007] In Patent Document 2, density is estimated by extrapolation based on output characteristics that indicate the correspondence between gradation values ​​and density measurement values, but this is not performed for each recording element, and therefore cannot deal with unevenness that occurs for each recording element.

[0008] In Patent Document 3, the quantization means is switched depending on the characteristics of density unevenness correction and quantization. However, just like Patent Document 1, there is no consideration given to the density range outside the range used to print the test pattern, which makes it easier for errors to occur in correction in high density areas, and switching between quantization processes requires the use of multiple quantization means and the need to determine when to switch, which increases the calculation load.

[0009] The present disclosure has been made in consideration of these circumstances, and aims to provide an image processing method, an image processing device, a program, and an image forming device that can improve the prediction accuracy of correction values ​​that suppress density unevenness in high-density areas. [Means for solving the problem]

[0010] An image processing method according to one aspect of the present disclosure is an image processing method for correcting density unevenness resulting from a colorant amount distribution in a first direction when an image is formed using a print head in which a plurality of print elements are arranged in the first direction, the image processing method including: forming a test image for density measurement using the print head, the test image including a plurality of patches set to a uniform density in the first direction, and acquiring, by a processor, density measurement values ​​for each print element for each patch of the test image; and calculating, by the processor, a correction value for correcting an output gradation relative to an input gradation for each print element from the density measurement values, wherein the processor calculates the correction value. When performing the correction, the method includes: predicting and extrapolating unobtained density measurement values ​​in areas with higher densities than the first patch based on the density measurement values ​​of a first patch that is set to the highest density among the multiple patches and a second patch that is set to the second highest density after the first patch; and performing halftone processing such that, when the slope of the characteristics with respect to the gradation between the first patch and the second patch is u, the slope of the characteristics between any two gradations in the gradation area of ​​the input gradation used for correction on the side with higher densities than the first patch is bu, and the value of b satisfies 0≦b≦1.5.

[0011] According to this aspect, it is possible to improve the prediction accuracy of the correction value in an area with a density higher than the set density of the patch in the final stage.

[0012] In the image processing method according to another aspect of the present disclosure, the characteristic may be an optical characteristic that is correlated with visually perceived density.

[0013] In an image processing method according to another aspect of the present disclosure, the plurality of patches in the test image may be arranged side by side in a second direction perpendicular to the first direction.

[0014] In an image processing method according to another aspect of the present disclosure, the test image may be formed based on image data that has been subjected to a correction process to correct density unevenness using a first correction value predetermined for each recording element, and the processor may be configured to modify the first correction value using a second correction value, which is a correction value calculated from density measurement values ​​for each recording element obtained based on the test image.

[0015] In an image processing method according to another aspect of the present disclosure, the test image may include a first patch and a second patch such that the difference between a linear approximation line obtained from the input gradation values ​​and density measurement values ​​for each recording element for each of the first patch and the second patch and the density measurement values ​​corresponding to the input gradation values ​​between the first patch and the second patch is less than 10% of the difference between the density measurement values ​​of each of the first patch and the second patch.

[0016] In an image processing method according to another aspect of the present disclosure, the processor may be configured to perform a calculation to calculate a correction value by extrapolation processing calculated from the characteristics of the gradation between the first patch and the second patch for each recording element.

[0017] In an image processing method according to another aspect of the present disclosure, the processor may be configured to perform a calculation to calculate a correction value by an extrapolation process calculated using the average value of the density measurement values ​​obtained from each of the first patch and the second patch of all the recording elements of the recording head.

[0018] In an image processing method according to another aspect of the present disclosure, the processor may be configured to perform a calculation to calculate a correction value by an extrapolation process calculated using values ​​obtained by smoothing density measurement values ​​obtained from each of the first patch and second patch of multiple recording elements of the recording head.

[0019] In the image processing method according to another aspect of the present disclosure, the number of recording elements that undergo smoothing processing may be two or more and not more than the total number of recording elements in the recording head.

[0020] An image processing device according to another aspect of the present disclosure is an image processing device that corrects density unevenness caused by a color material amount distribution in a first direction when an image is formed using a recording head in which a plurality of recording elements are arranged in the first direction, and includes a processor and a memory that stores a program executed by the processor, and the processor executes the program to perform a process of acquiring density measurement values ​​for each recording element for each patch of the test image obtained by forming a test image for density measurement including a plurality of patches set to a uniform density in the first direction using the recording head, and calculating an output corresponding to an input gradation for each recording element from the density measurement values. and calculating a correction value for correcting the gradation, and when calculating the correction value, based on the density measurement values ​​of a first patch set to the highest density among the plurality of patches and a second patch set to the second highest density after the first patch, predicting and extrapolating unobtained density measurement values ​​in an area with a density higher than that of the first patch, and performing halftone processing such that, when the slope of the characteristics with respect to the gradation between the first patch and the second patch is u, the slope of the characteristics between any two points in the gradation area of ​​the input gradation used for correction on the side with a density higher than that of the first patch is bu, and the value of b satisfies 0≦b≦1.5. Carry out the process.

[0021] A program according to another aspect of the present disclosure is a program that causes a computer to realize an image processing function for correcting density unevenness resulting from a color material amount distribution in a first direction when forming an image using a recording head in which a plurality of recording elements are arranged in the first direction, the program including: a function for acquiring density measurement values ​​for each recording element for each patch of a test image obtained by forming a test image for density measurement including a plurality of patches set to a uniform density in the first direction using the recording head; and a function for calculating a correction value for correcting an output gradation relative to an input gradation for each recording element from the density measurement values. The present invention is realized by a computer, and when calculating a correction value, the correction value is calculated by predicting and extrapolating unobtained density measurement values ​​in an area with a density higher than that of the first patch based on the density measurement values ​​of a first patch set to the highest density of the multiple patches and a second patch set to the second highest density after the first patch; and when the slope of the characteristic for the gradation between the first patch and the second patch is u, the slope of the characteristic between any two points in the gradation area of ​​the input gradation used for correction on the side with a density higher than that of the first patch is bu, and the value of b is 0≦b≦1.5. It also provides a function for performing halftone processing that satisfies the requirements.

[0022] An image forming apparatus according to another aspect of the present disclosure includes a recording head having a plurality of recording elements arranged in a first direction, a relative movement mechanism for relatively moving the recording medium and the recording head, a density measurement device for measuring the density of an image formed on the recording medium using the recording head, and a processor, wherein the processor performs a process of forming a test image for density measurement using the recording head, the test image including a plurality of patches each having a uniform density set in the first direction, and a process of measuring the density of each patch of the test image formed by the recording head using the density measurement device, and a process of acquiring density measurements for each recording element for each input gradation from the density measurements, a process of calculating a correction value for correcting an output gradation for an input gradation for each recording element from the density measurements, and a correction process of generating corrected image data using the correction value for image data to be formed; when calculating the correction value, based on the density measurements of a first patch set to the highest density of a plurality of patches and a second patch set to the second highest density after the first patch, an extrapolation process is performed to predict unobtained density measurements in an area of ​​higher density than the first patch; when the slope of the characteristics for the gradation between the first patch and the second patch is u, a halftone process is performed in which the slope of the characteristics between any two points in the gradation area of ​​the input gradation used for correction on the side of higher density than the first patch is bu, and the value of b satisfies 0≦b≦1.5; and the recording head is operated after the correction process has been performed using the correction value. Image formation is performed based on the image data.

[0023] In the image forming apparatus according to another aspect of the present disclosure, the recording head may be an inkjet head having nozzles as recording elements. [Effects of the Invention]

[0024] According to the present disclosure, it is possible to improve the prediction accuracy of correction values ​​that suppress density unevenness in high density regions beyond the final patch. [Brief explanation of the drawings]

[0025] [Figure 1] FIG. 1 is a diagram showing the overall configuration of an inkjet printing machine according to an embodiment. [Figure 2] FIG. 2 is a block diagram showing an example of the hardware configuration of a control device that controls an inkjet printing machine. [Figure 3] FIG. 3 is a functional block diagram showing the configuration of a control system in a printing system including an inkjet printing machine and a control device. [Figure 4] FIG. 4 is an example of a density correction chart. [Figure 5] FIG. 5 is a diagram schematically showing an actually printed density correction chart. [Figure 6] FIG. 6 is a graph plotting data points of density readings obtained for a particular nozzle. [Figure 7] FIG. 7 is an explanatory diagram showing an example of setting gradation in a printing system. [Figure 8] FIG. 8 is a conceptual diagram of the density correction process for a certain nozzle. [Figure 9] FIG. 9 is a graph showing an example in which the read density value of the patch in the final stage is lower than the target density value. [Figure 10] FIG. 10 is a graph showing an example of extrapolation of predicted values ​​by linear prediction. [Figure 11] FIG. 11 is a graph showing an example of a slope variation prediction in which a predicted value is extrapolated so that the slope approaches 0 in a gradation region with higher density than the final patch. [Figure 12] FIG. 12 is a graph showing an example in which the slope of the characteristics increases in a gradation region with higher density than the final patch. [Figure 13] FIG. 13 is a graph showing an example in which the slope of the characteristics becomes smaller in a gradation region with higher density than the patch in the final stage. [Figure 14] FIG. 14 is a graph illustrating the conditions for predictable characteristics. [Figure 15] FIG. 15 is a graph showing an example of characteristics in the gradation of the area outside the patch setting. [Figure 16] FIG. 16 is a graph showing an example of optical density characteristics by halftone processing according to a comparative example. [Figure 17] FIG. 17 is a graph showing an example of optical density characteristics due to halftone processing employed in the inkjet printer according to the embodiment. [Figure 18] FIG. 18 is a graph showing an example of selection of patch gradations employed in the density correction chart of the embodiment. [Figure 19] FIG. 19 is a graph showing an example of patch tone selection according to a comparative example. [Figure 20] FIG. 20 is a chart showing Example 1 of a method for calculating the slope used in extrapolation. [Figure 21] FIG. 21 is a chart showing a second example of a method for calculating the slope used in extrapolation. [Figure 22] FIG. 22 is a chart showing Example 3 of the method for calculating the slope used in extrapolation. [Figure 23] FIG. 23 is a functional block diagram showing the functional configuration of an image processing apparatus that performs density unevenness correction according to this embodiment. [Figure 24] FIG. 24 is a flowchart showing an example of the procedure for calculating a correction value applied to correct density unevenness. [Figure 25] FIG. 25 is a flowchart showing an example of a procedure for a printing process when forming an image to be printed. [Figure 26] FIG. 26 is a perspective view showing an example of the configuration of an inkjet head. [Figure 27] FIG. 27 is a partially enlarged view of the inkjet head as seen from the nozzle surface side. [Figure 28] FIG. 28 is a plan view of the nozzle surface of the head module. [Figure 29] FIG. 29 is a vertical cross-sectional view showing the three-dimensional structure of one ejector in the head module. DETAILED DESCRIPTION OF THE INVENTION

[0026] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. In this embodiment, a printing system including an inkjet printer as an example of an image forming apparatus and its control device will be described.

[0027] <<Configuration of an inkjet printer>> FIG. 1 is a diagram showing the overall configuration of an inkjet printer 1 according to an embodiment. The inkjet printer 1 is an inkjet color digital printing device that uses four colors of ink: cyan (C), magenta (M), yellow (Y), and black (K) to print a desired image on a sheet of paper P in a single pass. In this embodiment, an example will be described in which aqueous ink is used as the ink for drawing. Aqueous ink refers to ink in which coloring materials such as pigments and dyes are dissolved or dispersed in water and / or a water-soluble solvent.

[0028] The inkjet printer 1 includes a paper feed unit 10, a treatment liquid deposition unit 20, a treatment liquid drying unit 30, a printing unit 40, an ink drying unit 50, and an accumulation unit 60.

[0029] The paper feed section 10 includes a paper feed device 12, a feeder board 14, and a paper feed drum 16. Paper sheets P are placed on the paper feed tray 12A in a stack of many sheets. There are no particular limitations on the type of paper sheets P, but for example, printing paper mainly made of cellulose, such as high-quality paper, coated paper, or art paper, can be used.

[0030] The paper feeder 12 takes out the sheets P one by one from the stack set on the paper feed tray 12A, starting from the top, and supplies them to the feeder board 14. The feeder board 14 transports the sheets P received from the paper feeder 12 to the paper feed drum 16.

[0031] The paper feed drum 16 receives the paper P fed from the feeder board 14 and transports the received paper P to the treatment liquid deposition unit 20.

[0032] The treatment liquid application unit 20 applies treatment liquid to the paper P. The term "treatment liquid" is synonymous with "pretreatment liquid." The treatment liquid may also be called a "precoat," "preconditioner," "undercoat liquid," or "treatment agent." The treatment liquid is a liquid that has the function of aggregating, insolubilizing, or thickening the colorant components in the ink. The treatment liquid application unit 20 includes a treatment liquid application drum 22 and a treatment liquid application device 24.

[0033] The treatment liquid application drum 22 receives the paper P from the paper feed drum 16 and The paper P is then transported to the treatment liquid drying section 30. The treatment liquid application drum 22 is provided with a gripper 23 on the drum circumferential surface, and by rotating while gripping the leading edge of the paper P with the gripper 23, the paper P is wrapped around the drum circumferential surface and transported.

[0034] The treatment liquid application device 24 includes an application roller, and applies treatment liquid to the paper P transported by the treatment liquid application drum 22. The application roller is supported by a contact / separation mechanism (not shown) that can move between an application position where the application roller comes into contact with the paper P to apply treatment liquid to the paper P, and a retracted position where the application roller is separated from the paper P and does not apply treatment liquid. The method for applying the treatment liquid is not limited to the roller application method, and may be a blade application method, an inkjet method, a spray method, or the like.

[0035] The treatment liquid drying unit 30 dries the paper P on which the treatment liquid has been applied. The treatment liquid drying unit 30 includes a treatment liquid drying drum 32 and a warm air blower 34. The treatment liquid drying drum 32 receives the paper P from the treatment liquid application drum 22 and transports the received paper P to the imaging unit 40. The treatment liquid drying drum 32 includes a gripper 33 on the circumferential surface of the drum. The treatment liquid drying drum 32 transports the paper P by rotating while gripping the leading edge of the paper P with the gripper 33.

[0036] The warm air blower 34 is installed inside the treatment liquid drying drum 32. The warm air blower 34 blows warm air onto the paper P being transported by the treatment liquid drying drum 32, thereby drying the treatment liquid.

[0037] The imaging unit 40 includes an imaging drum 42, a head unit 44, a paper pressure roller 47, and a scanner 48. The imaging drum 42 receives the paper P from the treatment liquid drying drum 32 and transports the received paper P to the ink drying unit 50. The imaging drum 42 includes a gripper 43 on its circumferential surface, and by rotating while gripping the leading edge of the paper P with the gripper 43, the paper P is wound around the drum circumferential surface and transported. The imaging drum 42 includes a suction mechanism (not shown), which adsorbs the paper P wound around the drum circumferential surface and transports it. Negative pressure is used for adsorption. The imaging drum 42 includes a number of suction holes on its circumferential surface, and by drawing suction from inside the imaging drum 42 through these suction holes, the paper P is adsorbed to the circumferential surface of the imaging drum 42.

[0038] The head unit 44 is configured to include inkjet heads 46C, 46M, 46Y, and 46K. The inkjet head 46C is a recording head that ejects droplets of cyan ink. The inkjet head 46M is a recording head that ejects droplets of magenta ink. The inkjet head 46Y is a recording head that ejects droplets of yellow ink. The inkjet head 46K is a recording head that ejects droplets of black ink. Each of the inkjet heads 46C, 46M, 46Y, and 46K is supplied with ink from an ink tank (not shown), which is an ink supply source of the corresponding color, via a piping path (not shown).

[0039] Each of the inkjet heads 46C, 46M, 46Y, and 46K is configured as a line head that can print on the paper P transported by the imaging drum 42 in a single scan, i.e., by a single pass method. The inkjet heads 46C, 46M, 46Y, and 46K are arranged so that their nozzle surfaces face the circumferential surface of the imaging drum 42. The inkjet heads 46C, 46M, 46Y, and 46K are arranged at regular intervals along the transport path of the paper P by the imaging drum 42.

[0040] Although not shown in Fig. 1, a plurality of nozzles, which are ink ejection ports, are two-dimensionally arranged on the nozzle surface of each of the inkjet heads 46C, 46M, 46Y, and 46K. The term "nozzle surface" refers to the ejection surface on which the nozzles are formed, and is also referred to as "ink ejection surface" or "nozzle formation surface." A nozzle array of multiple nozzles arranged two-dimensionally is called a "two-dimensional nozzle array."

[0041] Each of the inkjet heads 46C, 46M, 46Y, and 46K can be configured by connecting multiple head modules in the paper width direction. The paper width here refers to the paper width in the direction perpendicular to the transport direction of the paper P. The transport direction of the paper P is referred to as the Y direction. The paper width direction perpendicular to the Y direction is referred to as the X direction. Each of the inkjet heads 46C, 46M, 46Y, and 46K is a line-type recording head having a nozzle array that can record an image at a specified printing resolution over the entire recording area of ​​the paper P in the X direction with a single scan. This type of recording head is also called a "full-line type recording head" or "page-wide head."

[0042] The specified print resolution may be a print resolution predetermined by the inkjet printer 1, or may be a print resolution selected by the user or automatically selected by a program according to the print mode. The print resolution may be, for example, 1200 dpi (dots per inch) in the X direction and 1200 dpi in the Y direction.

[0043] The paper width direction (X direction) perpendicular to the transport direction of the paper P is sometimes called the nozzle row direction of the line head, and the transport direction of the paper P (Y direction) is sometimes called the nozzle row perpendicular direction.

[0044] In the case of an inkjet head with a two-dimensional nozzle array, the projected nozzle array, in which each nozzle in the two-dimensional nozzle array is projected (orthogonally projected) along the nozzle array direction, can be considered equivalent to a single nozzle array in which the nozzles are arranged at approximately equal intervals in the nozzle array direction at a nozzle density that achieves maximum printing resolution. "Approximately equal intervals" means that the nozzles are substantially evenly spaced as droplet ejection points that can be recorded by an inkjet printing device. For example, the concept of "equally spaced" also includes slight variations in spacing to account for droplet movement on the medium due to manufacturing errors and / or landing interference. The projected nozzle array corresponds to a substantial nozzle array. When considering the projected nozzle array, a nozzle number representing the nozzle position can be assigned to each nozzle in the order of the projected nozzles arranged along the nozzle array direction. The nozzle array direction is synonymous with the nozzle arrangement direction.

[0045] The nozzle arrangement in each of the inkjet heads 46C, 46M, 46Y, and 46K is not limited, and various nozzle arrangements can be adopted. For example, instead of a two-dimensional matrix arrangement, a linear nozzle arrangement, a V-shaped nozzle arrangement, or a polygonal nozzle arrangement such as a W-shape in which V-shaped arrangements are repeated, are also possible.

[0046] Ink droplets are ejected from at least one of the inkjet heads 46C, 46M, 46Y, and 46K toward the paper P being transported by the drawing drum 42, and an image is formed on the paper P by the ejected droplets adhering to the paper P.

[0047] The imaging drum 42 functions as a relative movement mechanism that moves the inkjet heads 46C, 46M, 46Y, and 46K relative to the paper P. The imaging drum 42 is one form of a mechanism that moves the paper P relative to the inkjet heads 46C, 46M, 46Y, and 46K. The ejection timing of each of the inkjet heads 46C, 46M, 46Y, and 46K is synchronized with a rotary encoder signal obtained from a rotary encoder (not shown) that is installed on the imaging drum 42. The ejection timing is the timing at which ink droplets are ejected, and is synonymous with droplet ejection timing.

[0048] In this example, a configuration using four colors of ink, CMYK, is illustrated, but the combination of ink colors and the number of colors is not limited to this embodiment. Light ink, dark ink, A special color ink may be added. For example, it is possible to add an inkjet head that ejects light-colored ink such as light cyan or light magenta, and / or to add an inkjet head that ejects special color ink such as green, orange, or white. There is also no particular limitation on the order in which the inkjet heads of each color are arranged.

[0049] Scanner 48 is an image reading device that optically reads an image recorded on paper P by inkjet heads 46C, 46M, 46Y, and 46K and generates electronic image data representing the read image. Scanner 48 includes an imaging device that captures an image recorded on paper P and converts it into an electrical signal representing image information. In addition to the imaging device, scanner 48 may also include an illumination optical system that illuminates the object to be read and a signal processing circuit that processes a signal obtained from the imaging device to generate digital image data.

[0050] The scanner 48 is preferably configured to be capable of reading color images. The scanner 48 in this example has, for example, a color CCD (Charge-Coupled Device) image pickup device. A color CCD linear image sensor is used. A color CCD linear image sensor is an image sensor in which light receiving elements equipped with color filters of R (red), G (green), and B (blue) are arranged in a line. Note that a color CMOS (Complementary Metal Oxide Semiconductor) linear image sensor can also be used instead of a color CCD linear image sensor. The scanner 48 reads the image on the paper P while the paper P is being transported by the imaging drum 42. A scanner installed in the paper transport path in this way is sometimes called an "inline scanner." The scanner 48 may also be a camera.

[0051] When the paper P, on which an image has been recorded using at least one of the inkjet heads 46C, 46M, 46Y, and 46K, passes through the reading area of ​​the scanner 48, the image on the paper P is read. The image recorded on the paper P may include a user image to be printed as specified in the print job, as well as a defective nozzle detection pattern for inspecting the ejection status of each nozzle, a test pattern for correcting uneven density, and various other test images.

[0052] The printed image is inspected based on the data of the read image read by scanner 48, and the presence or absence of image quality abnormalities is determined. Furthermore, information such as the density of the image and the ejection status of each nozzle of inkjet heads 46K, 46C, 46M, and 46Y is obtained based on the data of the read image read by scanner 48. In this example, scanner 48 is disposed on the paper transport path between head unit 44 and ink drying section 50, and is configured to read the image before the ink dries, but it is also possible to dispose a scanner that reads the image after the ink dries instead of or in combination with scanner 48.

[0053] The ink drying unit 50 dries the paper P on which the image has been formed by the drawing unit 40. The ink drying unit 50 includes a chain gripper 70, a paper guide 80, and a heat drying processing unit 90.

[0054] The chain gripper 70 receives the paper sheet P from the imaging drum 42 and transports the received paper sheet P to the accumulation unit 60. The chain gripper 70 has a pair of endless chains 72 that travel along a specified travel path, and transports the paper sheet P along the specified transport path while gripping the leading edge of the paper sheet P with grippers 74 provided on the pair of chains 72. A plurality of grippers 74 are provided on the chains 72 at regular intervals.

[0055] The chain gripper 70 of this example is configured to include a first sprocket 71A, a second sprocket 71B, a chain 72, and a plurality of grippers 74, and has a structure in which a pair of endless chains 72 are wound around a pair of the first sprocket 71A and the second sprocket 71B. B and only one of the pair of chains 72 is shown.

[0056] The chain gripper 70 has a structure in which a plurality of grippers 74 are arranged at a plurality of positions in the feed direction (length direction) of the chain 72. The chain gripper 70 also has a structure in which a plurality of grippers 74 are arranged along the paper width direction between the pair of chains 72. Of the plurality of grippers 74 arranged between the pair of chains 72, only one gripper 74 is shown in Figure 1.

[0057] The transport path of the paper P by the chain gripper 70 includes a horizontal transport area where the paper P is transported horizontally, and an inclined transport area where the paper P is transported diagonally upward from the end of the horizontal transport area. The horizontal transport area is called the first transport section, and the inclined transport area is called the second transport section.

[0058] The paper guide 80 is a mechanism that guides the transport of paper P by the chain gripper 70. The paper guide 80 is composed of a first paper guide 82 and a second paper guide 84. The first paper guide 82 guides paper P transported through the first transport section of the chain gripper 70. The second paper guide 84 guides paper transported through the second transport section subsequent to the first transport section. Although the detailed structure of the first paper guide 82 is not shown, a suction transport device using a suction-type suction belt is applied as the first paper guide 82.

[0059] The heating and drying processing unit 90 applies heat to the paper P on which an image has been formed by the drawing unit 40 to evaporate the ink solvent and dry the paper P. The heating and drying processing unit 90 is, for example, a hot air blowing unit, and is disposed opposite the first paper guide 82, and blows hot air onto the paper P being transported by the chain gripper 70.

[0060] The collecting section 60 includes a collecting device 62 that receives and collects the paper sheets P transported from the ink drying section 50 by a chain gripper 70. The chain gripper 70 releases the paper sheets P at a predetermined collecting position. The collecting device 62 includes a collecting tray 62A, receives the paper sheets P released from the chain gripper 70, and collects them in a bundle on the collecting tray 62A. The collecting section 60 corresponds to a paper discharge section.

[0061] <<Example of Hardware Configuration of Control Device 100>> FIG. 2 is a block diagram showing an example of the hardware configuration of a control device 100 that controls the inkjet printer 1. The control device 100 is configured, for example, by a combination of computer hardware and software. The control device 100 includes a processor 102, a computer-readable medium 104 that is a non-transitory tangible entity, a communication interface 106, and an input / output interface 108. The processor 102 includes a CPU (Central Processing Unit). The processor 102 includes a GPU (Graphics Processing Unit). The processor 102 is connected to the computer-readable medium 104, the communication interface 106, and the input / output interface 108 via a bus 110.

[0062] The computer-readable medium 104 includes, for example, a random access memory (RAM) 112, a read-only memory (ROM) 114, and storage 116. The RAM 112 is a memory that functions as a main storage device. The storage 116 is an auxiliary storage device. The storage 116 may be, for example, a hard disk drive (HDD) device, a solid state drive (SSD) device, or a combination of two or more of these. A part or all of the storage area of ​​the computer-readable medium 104 may be included in the processor 102.

[0063] The computer-readable medium 104 contains a program that causes the control device 100 to function. The term "program" includes the concept of a program module. Some of the processing functions of the control device 100 may be realized using integrated circuits such as a DSP (Digital Signal Processor) or an FPGA (Field Programmable Gate Array). good.

[0064] The control device 100 is connected to an input device 122 and a display device 124. The input device 122 is, for example, a keyboard, a mouse, a multi-touch panel, or other pointing devices, or a voice input device, or an appropriate combination thereof. The display device 124 is, for example, a liquid crystal display, an organic electro-luminescence (OEL) display, a projector, or an appropriate combination thereof. The input device 122 and the display device 124 may be integrated into one unit, such as a touch panel. The input device 122 and the display device 124 may be included in the control device 100, or the control device 100, the input device 122, and the display device 124 may be integrated into one unit.

[0065] <<Outline of the control system of inkjet printing machine 1>> 3 is a functional block diagram showing the schematic configuration of a control system in a printing system 101 including the inkjet printing machine 1 and the control device 100. The processor 102 functions as a processing unit and / or a control unit that performs various processes by executing instructions from a program stored in a computer-readable medium 104. The processor 102 functions as a system control unit 310, an image processing unit 311, a conveyance control unit 312, a paper feed control unit 313, a treatment liquid application control unit 314, a treatment liquid drying control unit 316, a drawing control unit 318, an ink drying control unit 320, and a paper discharge control unit 324. The processing functions of each of these units may be realized using multiple processors.

[0066] The control device 100 includes a communication unit 304, an image memory 332, a parameter storage unit 334, and a program storage unit 336. The communication unit 304 includes a communication interface 106 (see FIG. 2). The control device 100 is connected to a host computer 350 via the communication unit 304, and can send and receive data to and from the host computer 350. Note that "connection" includes a wired connection, a wireless connection, or a combination thereof. The communication unit 304 may be equipped with a buffer memory to speed up communication processing. The communication unit 304 serves as an image input interface unit for acquiring image data representing an image to be printed.

[0067] Image data received from the host computer 350 via the communication unit 304 is stored in the image memory 332. The image memory 332 functions as a temporary storage unit for various data including image data.

[0068] The parameter storage unit 334 stores various parameters used in the inkjet printing machine 1. The various parameters stored in the parameter storage unit 334 are read out via the processor 102 and set in each unit of the device.

[0069] The program storage unit 336 stores programs used by each unit of the inkjet printing machine 1. The various programs stored in the program storage unit 336 are read out via the processor 102 and executed in each unit of the device. The computer-readable medium 104 functions as the image memory 332, the parameter storage unit 334, and the program storage unit 336.

[0070] The system control unit 310 functions as an overall control unit that comprehensively controls each unit of the inkjet printing machine 1. The system control unit 310 also functions as a calculation unit that performs various calculation processes. Furthermore, the system control unit 310 controls the reading and writing of data in the storage device 302.

[0071] The image processing unit 311 performs various conversion processes, correction processes, and halftone processes on the image data to be printed. Conversion processes include pixel count conversion, tone conversion, color conversion, and the like. Correction processes include density correction, including unevenness correction to suppress density unevenness, and non-discharge correction to reduce the visibility of image defects caused by faulty nozzles. Halftone processing generally involves quantizing m-level (m is an integer of 3 or greater) multi-level image data and converting it into n-level (n is an integer of 2 or greater and less than m) data. For example, multi-level image data with 8 bits or more per color (256 levels) is essentially considered to be continuous-tone image data. The image processing unit 311 converts continuous-tone image data for each color, for example, CMYK, into dot data representing a multi-level dot arrangement of ternary or greater values, on a pixel-by-pixel basis.

[0072] In this example, the inkjet heads 46C, 46M, 46Y, and 46K can eject three different droplet sizes (dot sizes): small, medium, and large. In this case, the image processing unit 311 converts the continuous-tone, color-separated image data of each color into four-level (n=4) signals: "eject large ink droplets," "eject medium ink droplets," "eject small ink droplets," and "no ink ejection (no droplets)." Such halftone processing can be performed using, for example, a dithering method or an error diffusion method. The image processing unit 311 also performs correction processing based on the scanned image obtained from the scanner 48. The combination of the processor 102 and the computer-readable medium 104, which realize the processing functions of the image processing unit 311, functions as an image processing device.

[0073] The transport control unit 312 controls the operation of the transport mechanism 11. The transport mechanism 11 includes the mechanical elements related to the transport of the paper P from the paper feed unit 10 to the accumulation unit 60 described in FIG. 1. The transport mechanism 11 includes the paper feed drum 16, the treatment liquid application drum 22, the treatment liquid drying drum 32, the imaging drum 42, and the chain gripper 70 shown in FIG. 1. The transport mechanism 11 also includes a motor (not shown) as a power source and a drive unit such as a motor drive circuit (not shown). The transport control unit 312 controls the transport speed of the paper P by the transport mechanism 11 in response to commands from the system control unit 310, and also controls the transport of the paper P from the paper feed unit 10 to the accumulation unit 60.

[0074] The paper feed control unit 313 operates the paper feed unit 10 in response to a command from the system control unit 310. The paper feed control unit 313 controls the operation of starting and stopping the supply of paper P.

[0075] The treatment liquid application control unit 314 operates the treatment liquid application unit 20 in response to commands from the system control unit 310. The treatment liquid application control unit 314 controls the application operation of the treatment liquid application device 24, such as turning on / off the pre-coating function, the amount of treatment liquid to be applied, and the application timing.

[0076] The processing liquid drying control unit 316 operates the processing liquid drying unit 30 in response to commands from the system control unit 310. The processing liquid drying control unit 316 controls the drying temperature, the flow rate of the drying gas, and the timing of spraying the drying gas.

[0077] The drawing control unit 318 operates the drawing unit 40 in response to commands from the system control unit 310. The drawing control unit 318 includes a waveform storage unit, a waveform generation unit, and a drive circuit (not shown). The waveform storage unit stores the waveforms of the drive voltages to be applied to the ejection energy generating elements of the inkjet heads 46C, 46M, 46Y, and 46K. The waveform generation unit generates the waveforms of the drive voltages. The drive circuit generates drive voltages having drive waveforms corresponding to the dot data.

[0078] The drawing control unit 318 controls each ink generated through the halftone process of the image processing unit 311. Based on the color dot data, the ejection operation of each of the inkjet heads 46C, 46M, 46Y, and 46K is controlled so as to record an image on the paper P transported by the imaging drum 42. That is, based on the dot data generated through processing by the image processing unit 311, the ejection timing and ink ejection amount for each pixel position are determined, and a control signal is generated that determines the ejection timing for each pixel position, a drive voltage according to the ink ejection amount, and the ejection timing for each pixel. This drive voltage is supplied to the inkjet heads 46C, 46M, 46Y, and 46K, and dots are recorded on the paper P by the ink ejected from the inkjet heads 46C, 46M, 46Y, and 46K.

[0079] The ink drying control unit 320 operates the ink drying unit 50 in response to a command from the system control unit 310. The ink drying control unit 320 controls the temperature of the drying gas, the flow rate of the drying gas, the ejection timing of the drying gas, and the like.

[0080] The paper discharge control unit 324 operates the stacking unit 60 in response to a command from the system control unit 310. When the stacking device 62 shown in Fig. 1 includes a lifting mechanism, the paper discharge control unit 324 controls the operation of the lifting mechanism in response to an increase or decrease in the number of sheets P.

[0081] Information input via the input device 122 is sent to the system control unit 310. The system control unit 310 executes various processes in accordance with the information input from the input device 122.

[0082] The display device 124 can display various information such as various setting information of the device or abnormality information in response to commands from the system control unit 310. A user (operator) can use the input device 122 to set various parameters and input and edit various information while viewing the content displayed on the display device 124.

[0083] <<Outline of density correction method>> In the printing system 101 according to this embodiment, each of the inkjet heads 46C, 46M, 46Y, and 46K has an input / output gradation table that defines the output gradation for each input gradation for each nozzle, and density correction is performed by modifying this input / output gradation table based on the density measurement values ​​for each nozzle obtained by measuring the print results of a density measurement test image.

[0084] Density correction is a correction that suppresses density unevenness (non-uniformity of density) caused by the ink amount distribution in the nozzle alignment direction, and is sometimes called "density unevenness correction" or simply "unevenness correction." Ink is an example of a "colorant" in this disclosure, and ink amount distribution is an example of a "colorant amount distribution" in this disclosure. To correct density unevenness, a correction value must be calculated for each nozzle. The calculation of the correction value for density unevenness correction is performed by reading the printout of a density measurement test image with the scanner 48. The density measurement test image, which is printed to understand the state of density unevenness when calculating the correction value for density correction, is called a "density correction chart."

[0085] FIG. 4 is an example of a density correction chart 400. The horizontal direction in FIG. 4 is the nozzle arrangement direction (X direction), and the vertical direction is the paper transport direction (Y direction). The X direction is an example of the "first direction" in this disclosure, and the Y direction is an example of the "second direction" in this disclosure. Here, a chart drawn by one of the four color heads is shown. Similar charts are drawn by inkjet heads 46C, 46M, 46Y, and 46K of each color. Here, an example of drawing using inkjet head 46K is described.

[0086] The density correction chart 400 includes a plurality of patches PT, the gradation of which is set so as to provide a uniform density in the nozzle arrangement direction. In FIG. 4, an example of the density correction chart 400 including eight patches PT, the density of which is changed in stages from low to high, is shown. The arrangement order is not limited to this example.

[0087] The length in the X direction of each patch PT is the maximum drawing width of the inkjet head 46K, and the length in the Y direction is set to a predetermined length required for measuring density using the scanner 48. Such patches PT for density measurement are sometimes called "density patches." The density correction chart 400 may be printed on a single sheet of paper P, or may be printed separately on multiple sheets of paper P.

[0088] By applying an initial input / output gradation table predetermined for each nozzle to the image data of the density correction chart 400 shown in FIG. 4 to perform density correction and then printing the chart, the printed result is, for example, a printed matter of the density correction chart 402 as shown in FIG. 5.

[0089] Figure 5 is a diagram that schematically illustrates an actually printed density correction chart 402. In the density correction chart 402 shown in Figure 5, the density of the area corresponding to the nozzle range NR, which is a portion of all the nozzles from nozzle numbers NZ1 to NZn, is relatively low. Figure 5 schematically illustrates the density difference (density unevenness) in the nozzle arrangement direction, but in actual print results, there may be variations in print density for each nozzle.

[0090] The density correction chart 402 shown in FIG. 5 is read by the scanner 48, and density measurement values ​​of the patches PT at each position corresponding to the nozzle number NZi of each nozzle are obtained from the read image, thereby obtaining density measurement data corresponding to eight discrete input gradations for each nozzle. The density measurement values ​​may be, for example, read density values ​​obtained from the scanner 48. The read density values ​​of the scanner 48 are calculated from the RGB brightness of the reflected light. Note that the subscript i in the notation of the nozzle number NZi is an index number and represents an integer from 1 to n.

[0091] When the density of the printed image is high, the amount of light incident on the image sensor of the scanner 48 decreases, and the output signal of the image sensor indicates a small value. On the other hand, when the density of the image is low, the amount of light incident on the image sensor increases, and the output signal of the image sensor indicates a large value. Therefore, when performing measurements using the scanner 48, the magnitude relationship of the output signal of the image sensor is reversed to obtain the density measurement value. The scanner 48 is an example of a "density measuring device" in this disclosure. An offline scanner may be used instead of the scanner 48.

[0092] Figure 6 is a graph plotting data points of read density values ​​obtained for a certain nozzle. The horizontal axis of Figure 6 represents input gradation, and the vertical axis represents read density values. The black circles in the figure represent data points. Based on the measurement results for each nozzle as shown in Figure 6, processor 102 corrects the input / output gradation table for each nozzle so as to suppress density differences (density unevenness) between different nozzles within the same patch PT.

[0093] <<Example of gradation settings>> FIG. 7 is an explanatory diagram showing an example of gradation settings in the printing system 101. In this system, not all of the gradation range that can be used (outputtable) by the inkjet printer 1 is used during printing; there is a gradation range that is used during normal printing and a gradation range that is used during corrective printing. Normal printing means printing under conditions within the standard range assumed in the specifications (design). The gradation range that is used during normal printing is called the "normally used gradation range." Corrective printing is printing in which a correction process for uneven density is applied. The gradation range that is used during corrective printing is called the "correction-used gradation range." The correction-used gradation range includes a gradation range that is even higher in density than the normal-used gradation range, so that corrections can be made to even higher densities than the normal-used gradation range. Note that the horizontal axis shown in FIG. 7 indicates input gradations of 1500, 1800, and 2000. The numbers shown are just an example.

[0094] For example, the inkjet printer 1 can use 2048 gradations, and the gradation that corresponds to the output of 100% density for each color is determined depending on the color of ink. For example, if sufficient density can be achieved by using up to 1500 gradations for normal printing, then the normal use gradation range is set to 1500 gradations. Then, for correction, for example, up to 1800 gradations can be used to make corrections for insufficient density and suppress uneven density.

[0095] The set gradation of the final patch PT, which is set to the highest density in the density correction charts 400 and 402, may be set within the normal use gradation range or within the correction use gradation range, as long as a gradation with a density higher than that of the final patch PT can be used for correction.

[0096] <<Outline of corrections processed for each nozzle>> FIG. 8 is a conceptual diagram of the density correction process for a certain nozzle. The horizontal axis of FIG. 8 represents input gradation, and the vertical axis represents density values ​​read by scanner 48. Data point DP(k) shown in FIG. 8 represents the read density value of patch PT(k) in the kth row in density correction chart 402. Data point DP(k+1) represents the read density value of patch PT(k+1) in the (k+1)th row. If the number of rows (total number of rows) of patches PT with different densities in density correction chart 402 is L, k is an integer that satisfies 1≦k≦L−1.

[0097] Data point DP(k) is associated with the input gradation value GV(k) of patch PTk and the corresponding read density value RD(k), and is expressed as DP(k) = [GV(k), RD(k)]. Similarly, data point DP(k+1) is associated with the input gradation value GV(k+1) and the corresponding read density value RD(k+1), and is expressed as DP(k+1) = [GV(k+1), RD(k+1)].

[0098] When the read density value RD(k) corresponding to the input gradation value GV(k) is lower than the target density value TD(k), as in the case of data point DP(k) in Figure 8, the input gradation value (corrected input gradation value) required to achieve the target density value TD(k) is calculated by an interpolation method that applies the linear function indicated by the straight line connecting the two data points DP(k) and DP(k+1).

[0099] On the other hand, as shown in Figure 9, when the read density value RD(L) of the final data point DP(L) is lower than the target density value TD(L), the input gradation value (correction value) required to achieve the target density value TD(L) for the input gradation value GV(L) cannot be calculated by interpolation.

[0100] In this embodiment, based on the data point DP(L) in the final stage and the data point DP(L-1) in the previous (L-1) stage, the unobtained read density value beyond the final stage (higher density side than the final stage) is predicted, and a corrected input gradation value is calculated by extrapolation processing.

[0101] <<Final density correction method>> When performing a correction to increase the density in the final row (Lth row) of the patches PT of the density correction chart 402, which has the highest density set, the processor 102 extrapolates a density profile of an even higher gradation region from the acquired read density value, and calculates the gradation (corrected input gradation value) that outputs the corrected density (target density value) from the extrapolated density profile.

[0102] The extrapolated density profile is calculated from the read density values ​​of the final stage and the stage before that. For example, processor 102 may calculate the slope of the characteristics between patches from the average density of each nozzle in the final stage and the stage before that, and use this to create a linear density profile. This slope indicates the rate of change in density relative to the gradation.

[0103] Alternatively, the processor 102 may set the density profile so that the slope converges to zero when the increase in density attenuates as the gradation increases in the high density portion.

[0104] Fig. 10 is a graph showing an example of extrapolation of predicted values ​​by linear prediction. For example, as shown in Fig. 10, it is possible to obtain predicted values ​​for a density region higher than the read density value of the final patch PT by extrapolation based on linear prediction.

[0105] 11 is a graph showing an example of a slope variation prediction in which a predicted value is extrapolated so that the slope approaches 0 in a gradation region with higher density than the final patch PT. When the increase in density attenuates in the high-density portion, for example, as shown in FIG. 11, it is conceivable to extrapolate a predicted value so that the slope approaches 0 in a density region higher than the read density value of the final stage.

[0106] In this way, the unobtained read density value beyond the final stage can be predicted from the read density values ​​of the final stage and the patch immediately before it, and the result can be extrapolated to determine the input gradation value that will yield the target density value.

[0107] The patch PT in the final row is an example of a "first patch" in the present disclosure, and the patch just before the final row (the L-1th row) is an example of a "second patch" in the present disclosure.

[0108] <<Output for input tone>> The patch PT is generally designed to be easy to correct and predict up to the final density level using linear methods, etc. If the density of the final patch PT is insufficient (a density lower than the target density is output), a gradation in a high gradation area outside the patch setting range (hereinafter referred to as "gradation outside the patch setting range") will be used when performing correction.

[0109] In this case, it is important to make it possible to easily predict the characteristics of the gradations outside the patch setting range from the characteristics of the gradations used in the patch PT. In other words, if the output is such that the difference between the characteristics obtained by the correction value calculated by extrapolation processing and the characteristics of the actual gradations is small enough to fall within an acceptable range, accurate correction will be possible even when a gradation outside the patch setting range is used as a correction value.

[0110] In other words, for the extrapolation process to work, the actual characteristics of the extrapolation region must be close to those predicted. In other words, the actual characteristics of the extrapolation region must be predictable. The extrapolation region is a gradation region outside the patch setting range, and is a gradation region with a higher gradation than the gradation of the final patch.

[0111] The characteristics referred to here are factors that affect density, such as the reflective brightness used when reading, optical characteristics such as optical density, or ink droplet volume (ink volume) that correlate with the density actually perceived by humans. Optical characteristics are, for example, CIE Lab values ​​defined by the International Commission on Illumination (CIE). It is desirable that the value correlates with the human visual density characteristics. The read density value of the scanner 48 in this embodiment may be, for example, a luminance value calculated from the RGB values ​​of the read image. The read density value is an example of a value obtained by measuring reflected luminance.

[0112] The scanner 48 receives the reflected light of the illumination reflected from a substrate such as paper P as an input as brightness information. However, in high density areas, there is not enough reflected light, and it is possible that the resolution of the density measurement value will not be sufficient.

[0113] On the other hand, optical density, which can be measured offline, such as by measuring color using a densitometer after printing, or drop volume, which can be calculated theoretically, correlates with the density measurements obtained by the scanner 48, so by making the characteristics of these values ​​into a predictable function shape (linear or smooth curve, etc.), it is possible to make the density measurements by the scanner 48 predictable as well. When performing such density unevenness correction, the elements that correlate with the density measurements by the scanner 48 used and that correlate with the density as perceived by the human eye are referred to as "characteristics."

[0114] About predictability of characteristics As an example of a case where the characteristics of the extrapolation region can be predicted, for example, if the slope of the line connecting the data point DP(L) in the final stage and the data point DP(L-1) in the previous stage (hereinafter referred to as the "previous slope") is the same as the slope of the actual characteristics in the extrapolation region beyond the final stage, then prediction is possible using a linear function.

[0115] It is also possible to predict, for example, that the slope of the actual characteristics in the extrapolated region beyond the final stage will initially be the same as the slope of the previous stage, but as the gradation increases, the slope will gradually decrease (approaching 0), indicating a smooth curve.

[0116] On the other hand, if the slope of the actual characteristics in the extrapolated region beyond the final stage increases suddenly, or if the characteristics show a completely different trend from the previous slope, it is difficult to say that the characteristics are predictable.

[0117] In this embodiment, the following two conditions are considered as conditions for the characteristics of the extrapolation region to be predictable from the immediately preceding slope.

[0118] [Condition 1] It must be possible to calculate it using simple calculations (such as linear prediction) from the previous gradient. [Condition 2] If there is a change in the slope, the slope must approach 0 as the gradation increases. These two conditions can be expressed as the conditions for the previous slope u and the slope at the gradation outside the patch setting range, as follows: In other words, in order to make it possible to easily predict the gradation outside the patch setting range, if the slope of the characteristics between the gradation of the final stage and the stage immediately before it is u, and the slope of the characteristics between any two gradations of the gradation outside the patch setting range is bu, then it is preferable that the slope bu matches u at any two points. In other words, it is preferable that the value of the coefficient b is b=1 at any two points. When b=1, linear prediction is possible.

[0119] On the other hand, because the gradations outside the patch range are high density, there is insufficient reflected light for gradations higher than the final patch PT, and the scanner 48 may lose density and not be able to accurately read the density difference. If the value of coefficient b is large, if there is an error in the prediction, the density difference on the high-density side cannot be accurately recognized, resulting in insufficient correction and density unevenness. That is, for example, as shown in Figure 12, if the value of coefficient b is large, the error in the correction value based on the prediction becomes large, and if an error occurs on the high-density side, the density after correction will be too high and the density difference will not be read, making correction impossible.

[0120] On the other hand, as shown in Figure 13, by decreasing the value of coefficient b, the error is reduced and the possibility of reading the density difference is increased. Regarding the value of coefficient b, considering the allowable error range, the preferable condition for coefficient b is 0≦b≦1.5. More preferably, 0≦b≦1.3, even more preferably, 0≦b≦1.1, and most preferably, 0≦b≦1. When the condition 0≦b≦1 is satisfied, the slope attenuates in one direction.

[0121] In other words, the condition for the characteristics of the extrapolation area to be predictable from the previous slope is that, as shown in Figure 14, when the slope of the characteristics between any two gradations outside the patch range is represented by the product bu of the previous slope u and the coefficient b, it satisfies 0≦b≦1.5, that is, the slope of the line connecting any two points on the characteristic curve is bu (0≦b≦1.5). Characteristics that satisfy this condition are realized by halftone processing. In other words, the characteristics can be adjusted by halftone processing.

[0122] 12 to 14, the vertical axis represents the read density value, but as already mentioned, the vertical axis may represent other characteristics such as optical density.

[0123] Fig. 15 is an example of a graph of optical density characteristics, which shows the relationship between input gradation and optical density. Each of the four black circles in Fig. 12 represents a data point of a density patch. In the example shown in Fig. 14, to simplify the illustration, only four data points are shown, and the data point shown on the far right corresponds to the patch in the final row. Gradations higher than the gradation of this final patch are outside the patch setting range.

[0124] When looking at the optical density characteristics illustrated in Figure 15, if the slope showing the ratio (rate of change) of the amount of change in optical density to the amount of change in input gradation, as in graph Ga, is larger than the slope of the characteristics between patches, the error when the prediction is off will be larger and the accuracy of correction will be reduced.

[0125] Furthermore, if there is a slope of the characteristic that is less than 0 in the gradation region outside the patch setting range as in graph Gc, even if correction is performed to increase the density, the density will decrease, and the accuracy of the correction will deteriorate.

[0126] In this embodiment, in order to improve the accuracy of correction, halftone processing is performed such that the characteristic slope u in the gradation area used in the patch (hereinafter referred to as the "patch gradation area") is 0≦b≦1.5, as shown in graph Gb1 or graph Gb2, and the characteristic slope bu between any two points in the gradation area outside the patch setting range.

[0127] The halftone process in this example is a process for converting an input grayscale image into an arrangement of three types of dots: small, medium, and large. This halftone process can include various processing elements and techniques, such as a lookup table for selecting the drop ratio and drop type, a threshold matrix, a dot pattern, grayscale conversion, noise addition and / or noise removal, or blurring. Desired characteristics can be achieved by adjusting at least one parameter of these elements that define the content of the halftone process.

[0128] Halftoning, which is performed to match the characteristics within and outside the patch tone range, may be performed by selecting a lookup table that records the ratio of each drop to be used for the input tone, or it may be performed with a dot pattern.

[0129] The halftone processing and extrapolation calculation processing for gradations outside the patch setting range can be determined based on characteristics measured in advance, and it is desirable that the slope of the characteristics change linearly or smoothly.

[0130] <<About adjusting halftone processing>> The halftone processing adjustment to achieve predictable characteristics for gradations outside the patch setting range is performed using the following procedure. Here, we will explain the adjustment method using inkjet printing machine 1, but the halftone processing adjustment does not need to be performed for each individual printing machine, and can be performed for each model with the same specifications.

[0131] [Step 1] Set the operating mode of the inkjet printing machine 1 to "development mode" and output multiple patches for halftone adjustment using the entire gradation range of gradations that can be output by the inkjet printing machine 1. The development mode is a mode used when operating the inkjet printing machine 1 in the development process for the model of the inkjet printing machine 1 or in the pre-shipment adjustment work process, etc.

[0132] [Step 2] Next, the color of each patch output in the development mode is measured. This color measurement may be performed offline using a colorimeter separate from the scanner 48. This color measurement provides the optical density of each patch.

[0133] [Step 3] From the color measurement results, halftone processing is designed and adjusted so that the optical density, including the maximum gradation that the inkjet printer 1 can output, can be predicted. Designing halftone processing includes designing the halftone processing algorithm. Adjustments may be made to any element of halftone processing that is related to gradation characteristics. For example, adjustments may be made to the dot pattern (dot arrangement), or to the droplet type selection and droplet ratio, or to the gradation conversion, or to a combination of adjustments to multiple elements.

[0134] For example, tone conversion adjustments are made by modifying a lookup table, such as changing the output tone to 1850 when the input tone is 1800. Because the droplet types and their ratios output at each tone are determined by the design of the halftone process, tone conversion can essentially be said to be a selection of droplet types and droplet type ratios.

[0135] By repeating steps 1 to 3, halftone processing with desired characteristics can be achieved.

[0136] FIG. 16 is a graph showing an example of optical density characteristics obtained by halftoning according to a comparative example. FIG. 16 shows the characteristics obtained without the halftoning adjustment described above. Without the halftoning adjustment, as shown in FIG. 16, the slope of the characteristics can suddenly decrease or increase in the high gradation range exceeding the gradation used in normal printing, resulting in unpredictable characteristics. The area indicated by arrow A in FIG. 16 indicates, for example, a state in which the substrate surface is filled with dots, making it difficult for the optical density to increase as the gradation increases. Furthermore, the sudden increase in the slope, as indicated by arrow B, is due to, for example, the use of large droplets, which makes it easier for the density to increase.

[0137] Fig. 17 is a graph showing an example of optical density characteristics due to halftone processing employed in the inkjet printer 1 according to the embodiment. Fig. 17 shows an example of characteristics after adjustment of the halftone processing described above. By adjusting the halftone processing, for example, linear characteristics can be achieved.

[0138] <<How to select patch gradation>> When determining the level of gradation to which the density of each of the multiple patches PT in the density correction chart 400 is set, it is preferable to take into account the characteristics of halftone processing so as to ensure the accuracy of the correction value calculated by interpolation or extrapolation based on the density measurement values ​​from two adjacent patches PT.

[0139] In this embodiment, a linear approximation line is calculated based on the input tone values ​​and read density values ​​for each nozzle of the patch PT(L) in the final row with the highest density and the patch PT(L-1) in the row immediately preceding the final row with the second highest density, and the actual value of the input tone values ​​between these two patches. The gradation of each patch is selected so that the difference between the actual density value and the actual density value is less than 10% of the difference between the read density values ​​of these two patches.

[0140] Fig. 18 is a graph showing an example of patch tone selection used in the density correction chart of this embodiment. In the example shown in Fig. 18, the difference between the linear approximation line calculated from the input tone values ​​and read density values ​​for each nozzle of the final patch PT(L) and the previous patch PT(L-1) and the actual density value for the input tone values ​​between these two patches is less than 10% of the difference in read density values ​​for these two patches. As shown in Fig. 19, when the difference between the linear approximation line and the actual density value is less than 10% of the difference in read density values ​​for each patch PT(L) and P(L-1), the error in the extrapolated prediction is small.

[0141] FIG. 19 is a graph showing an example of patch tone selection according to a comparative example. In the example shown in FIG. 19, the difference between the linear approximation line calculated from the input tone values ​​and read density values ​​for each nozzle of the final patch PT(L) and the previous patch PT(L-1) and the actual density value relative to the input tone values ​​between these two patches is 10% or more of the difference in read density values ​​between these two patches. The symbol SL in the figure represents the upper limit of the tone that can be read by the scanner 48, and tone areas higher than SL are tone areas that cannot be read by the scanner 48. As shown in FIG. 19, when the difference between the linear approximation line and the actual density value is 10% or more of the difference in read density values ​​between the patches PT(L) and P(L-1), the error in the extrapolated prediction becomes large. In other words, if the accuracy of the slope immediately before the patch PT(L) that serves as the basis for the extrapolated prediction is poor, a large error will occur in the prediction result.

[0142] Therefore, as explained in FIG. 18, it is desirable to select patch gradations so that the slope calculated from the readability values ​​of the patches PT(L) and PT(L-1) in the final and previous rows is close to the actual characteristics. In other words, it is desirable to determine patch gradations so that the difference between the linear approximation line and the actual density value is less than 10% of the difference in the read density values ​​of each patch PT(L) and PT(L-1). This improves the accuracy of corrected gradations obtained by prediction for gradations that cannot be read by the scanner 48. It is also desirable to set patch gradations for the other patches PT(1) to PT(L-2) at points of change in gradation and density, print state, or predicted density value based on the characteristics of the halftone processing, so that similar conditions are met.

[0143] <<Example 1: Calculation method for slope u used for extrapolation>> FIG. 20 is a chart showing Example 1 of a method for calculating the slope u used for extrapolation. FIG. 20 shows an example of calculating the slope u for each nozzle. Here, ten nozzles are shown as an example. The processor 102 may calculate the slope u used for extrapolation for each nozzle. FIG. 20 shows the read density values ​​for each nozzle when the input gradation value GV(L) of the final patch PT(L) is 1600 and the input gradation value GV(L-1) of the patch PT(L-1) one step before the final one is 1500. The processor 102 can calculate the slope u for each nozzle based on this data.

[0144] <<Example 2: Calculation method for slope u used for extrapolation>> FIG. 21 is a chart showing a second example of a method for calculating the slope u used in extrapolation. FIG. 21 shows an example in which the average value of all nozzles is used to calculate the slope u. Processor 102 may calculate the average value of the read densities of all nozzles from the read density values ​​of each nozzle, and calculate the slope u using the average value of the read density values ​​of all nozzles for patch PT(L) in the final row and the average value of the read density values ​​of all nozzles for patch PT(L-1) in the previous row. In this case, a common slope u is applied to each nozzle.

[0145] <<Example 3 of how to calculate the slope u used for extrapolation>> FIG. 22 is a chart showing Example 3 of a method for calculating the slope u used for extrapolation. FIG. 22 shows an example of calculating the slope u using values ​​obtained by smoothing the data for each nozzle. FIG. 22 shows an example of smoothing using a moving average of data from three nozzles. Note that the range to be smoothed is not limited to three nozzles and can be set appropriately. The number of nozzles to be smoothed may be two or more and less than the total number of nozzles in the head. The total number of nozzles is an example of the "total number of printing elements" in this disclosure.

[0146] The processor 102 may calculate the moving average value of the three nozzles from the read density values ​​of each nozzle, and calculate the slope u for each nozzle using the moving average value for each nozzle for the patch PT(L) in the final row and the moving average value for each nozzle for the patch PT(L-1) in the previous row.

[0147] <<Functional configuration of image processing device>> Fig. 23 is a functional block diagram showing the functional configuration of an image processing device that corrects density unevenness according to this embodiment. The control device 100 includes the functions of an image processing device 360. In Fig. 23, the same elements as those shown in Fig. 4 are given the same reference numerals, and duplicated explanations will be omitted.

[0148] The image processing device 360 ​​includes a test image data storage unit 362, a density correction chart generation unit 363, an image data acquisition unit 364, a dot data generation unit 365, and a correction value generation unit 366. The test image data storage unit 362 stores test image data including image data of a test image for density measurement, such as the density correction chart 400 illustrated in FIG.

[0149] The density correction chart generation unit 363 generates chart printing data necessary for printing the density correction chart 400 by the inkjet printer 1. The image data acquisition unit 364 includes an interface for acquiring image data IMG to be printed. The image data IMG is an example of "image data to be used for image formation" in this disclosure. The image data acquisition unit 364 may include the communication interface 106, or may include a media interface for reading data from an external storage device such as a memory card (not shown).

[0150] The dot data generation unit 365 includes a density correction processing unit 370 and a halftone processing unit 372, and generates dot data based on the image data IMG. The dot data generation unit 365 may also include a color conversion processing unit and a color separation (color separation) processing unit, both not shown. The color conversion processing unit performs processing to convert image data IMG represented by red (R), green (G), and blue (B) color signals, for example, into image data represented by C, M, Y, and K color signals corresponding to the ink colors. The color separation processing unit performs processing to separate (color separate) multi-tone image data of C, M, Y, and K, for example, into multi-tone image data for each ink color.

[0151] The density correction processing unit 370 performs a correction process (unevenness correction process) on the multi-tone image data for each ink color using a correction value (unevenness correction value) for correcting density unevenness. The halftone processing unit 372 performs halftone processing on the multi-tone image data for each color after the correction process by the density correction processing unit 370.

[0152] The correction value generation unit 366 performs processing to calculate a correction value from the scanned image read by the scanner 48. The correction value generation unit 366 includes a measurement value acquisition unit 380, a measurement value storage unit 382, ​​an arithmetic processing unit 384, and a correction value storage unit 386.

[0153] The measurement value acquisition unit 380 acquires data of the density measurement value (read density value) for each nozzle read by the scanner 48. The density measurement values ​​are stored in a measurement value storage unit 382. An arithmetic processing unit 384 calculates correction values ​​and the like based on the density measurement values ​​for each nozzle stored in the measurement value storage unit 382. The correction values ​​calculated by the arithmetic processing unit 384 are stored in a correction value storage unit 386. For example, the correction value storage unit 386 may store an input / output gradation table including correction values ​​that can output a target density value for each gradation. The density correction processing unit 370 performs correction processing using the correction values ​​stored in the correction value storage unit 386.

[0154] The test image data storage unit 362, the measurement value storage unit 382, ​​and the correction value storage unit 386 are configured using the storage areas of the computer-readable medium 104. The functions of each processing unit, such as the density correction chart generation unit 363, the dot data generation unit 365, and the calculation processing unit 384, are realized by the processor 102.

[0155] <<Correction value calculation process flow>> 24 is a flowchart showing an example of the procedure for calculating correction values ​​applied to correct density unevenness. Here, a case where the currently set initial correction value is updated to a new correction value is described. Note that although a correction value is calculated for each of the inkjet heads 46C, 46M, 46Y, and 46K of each color, the calculation process for the correction value for one inkjet head 46K is described here.

[0156] In step S11, the processor 102 acquires image data of a test image for density measurement from the test image data storage unit 362 of the computer-readable medium 104. The processor 102 acquires, for example, image data of a density correction chart 400 as shown in FIG.

[0157] Next, in step S12, processor 102 applies correction processing to the image data of the density measurement test image using a predetermined correction value (initial correction value). The correction value applied here may include a correction value component for non-ejection correction that compensates for a lack of density due to a non-ejecting nozzle. The correction value applied in step S12 is an example of a "first correction value" in the present disclosure.

[0158] Next, in step S13, the processor 102 ejects ink from the inkjet head 46K based on the image data of the density measurement test image that has been subjected to the correction process, and forms the density measurement test image on the paper P.

[0159] Thereafter, in step S14, the scanner 48 is used to read the test image for density measurement on the paper P. Then, in step S15, the processor 102 obtains the density measurement value for each set gradation of each patch PT for each nozzle based on the read image obtained from the scanner 48.

[0160] Next, in step S16, the processor 102 calculates a correction value that will obtain a target density value for the set gradation of each patch PT based on the density measurement value of each patch PT for each nozzle. At this time, if the density measurement value of the final patch PT(L) is lower than the target density value TD(L), the correction value is calculated by extrapolation based on the previous slope u. The correction value calculated in step S12 is an example of a "second correction value" in this disclosure. Then, in step S17, the processor 102 stores the correction value calculated in step S16 as the latest correction value and updates the correction value. After step S17, the processor 102 ends the flowchart in FIG. 23. Thereafter, the latest correction value is applied and printing is performed.

[0161] <<Printing process of image data>> FIG. 25 is a flowchart showing an example of a printing process procedure when forming an image to be printed. For example, when forming an image to be printed that is specified in a print job, the flowchart shown in Fig. 25 is executed.

[0162] In step S21, the processor 102 obtains image data to be printed.

[0163] Next, in step S22, the processor 102 performs a correction process on the acquired image data to correct the density using the latest correction value.

[0164] Next, in step S23, the processor 102 performs halftone processing on the corrected image data to generate dot data for each ink color.

[0165] Then, in step S24, the processor 102 controls the ejection operations of the inkjet heads 46C, 46M, 46Y, and 46K of each color based on the dot data for each ink color, and forms the image to be printed on the paper P.

[0166] This makes it possible to suppress uneven density caused by the ink amount distribution in the nozzle arrangement direction, resulting in a printed matter with good print quality. The image processing method including the processing executed by image processing device 360 ​​is an example of the "image processing method" in this disclosure.

[0167] <<Inkjet head configuration example>> Since the inkjet heads 46K, 46C, 46M, and 46Y used in the inkjet printer 1 have a common structure, an example of a specific configuration of the inkjet head 46 will be described here.

[0168] Figure 26 is a perspective view showing an example configuration of the inkjet head 46. Figure 26 shows the nozzle surface of the inkjet head 46 viewed from diagonally below. The inkjet head 46 is a full-line type line head in which multiple head modules 212 are arranged in the paper width direction to form an elongated length.

[0169] 26 shows an example in which 17 head modules 212 are connected together, but the structure of the head modules 212 and the number and arrangement of the head modules 212 are not limited to the example shown in the figure. Reference numeral 214 in the figure denotes a base frame that serves as a framework for connecting and fixing multiple head modules 212 in a bar shape. Reference numeral 216 denotes a flexible substrate connected to each head module 212. Multiple head modules 212 are attached to the base frame 214 and integrated to form a single bar-shaped inkjet head 46.

[0170] Figure 27 is a partially enlarged view of the inkjet head 46 as viewed from the nozzle surface side. The head module 212 is supported by module support members 218B from both sides in the vertical direction in Figure 27, which is the short direction of the inkjet head 46, and is attached to the base frame 214 via the module support members 218B. Furthermore, both ends of the inkjet head 46 in the longitudinal direction are supported by head protection members 218D.

[0171] Although individual nozzles are not shown in FIG. 27, the diagonal solid line shown and labeled with reference numeral 219 represents a nozzle row in which a plurality of nozzles are aligned in a row.

[0172] Figure 28 is a plan view of the nozzle surface 212A of the head module 212. For convenience of illustration, the number of nozzles is reduced in Figure 28, but for example, 32 x 64 nozzles 220 are arranged two-dimensionally on the nozzle surface 212A of one head module 212. In addition, a liquid-repellent film is formed on the nozzle surface 212A.

[0173] 28 is the paper transport direction, and the X direction perpendicular to the Y direction is the paper width direction. The head module 212 has a long side end face along the V direction inclined at an angle γ with respect to the X direction, and a short side end face along the W direction inclined at an angle α with respect to the Y direction, and has a parallelogram shape in plan view.

[0174] By connecting a plurality of such head modules 212 in the X direction, a nozzle row that covers the entire drawing range of the paper P in the X direction is formed, and a line head that can record an image at a specified recording resolution in a single drawing scan is configured. Note that the full-line type line head applied to the single-pass method is not limited to cases where the entire surface of the paper P as the recording medium is the printing range, and in cases where only a part of the recording medium is the printing area (for example, when a margin is provided around the recording medium), it is sufficient that the nozzle row required for printing is formed.

[0175] 29 is a vertical cross-sectional view showing the three-dimensional structure of one ejector 222 in the head module 212. The ejector 222 includes a nozzle 220, a pressure chamber 250 communicating with the nozzle 220, and a piezoelectric element 252. The nozzle 220 communicates with the pressure chamber 250 via a nozzle flow path 254. The pressure chamber 250 communicates with a supply-side common flow path 226 via an individual supply path 224. The nozzle 220 is an example of a "recording element" in this disclosure.

[0176] The diaphragm 256 that forms the top surface of the pressure chamber 250 has a conductive layer (not shown) that functions as a common electrode corresponding to the lower electrode of the piezoelectric element 252. The walls of the pressure chamber 250 and other flow path portions, the diaphragm 256, etc. can be made of silicon. The material of the diaphragm 256 is not limited to silicon, and it can also be made of a non-conductive material such as resin. A conductive layer made of a conductive material is formed on the surface of the diaphragm member. Note that the diaphragm 256 itself may be made of a metal material such as stainless steel, and may also serve as a diaphragm that also serves as a common electrode.

[0177] A piezoelectric unimorph actuator is formed by a structure in which piezoelectric elements 252 are stacked on a vibration plate 256. Applying a drive voltage to individual electrodes 258, which are the upper electrodes of the piezoelectric elements 252, deforms the piezoelectric body 260, causing the vibration plate 256 to bend, thereby changing the volume of the pressure chamber 250. The pressure change that accompanies this volume change causes ink to be ejected from the nozzle 220. When the piezoelectric element 252 returns to its original state after ejecting ink, new ink is filled into the pressure chamber 250 from the supply-side common flow path 226 through the individual supply path 224. The action of filling the pressure chamber 250 with ink is called "refill." This example illustrates a configuration in which the vibration plate 256 is bent using the distortion deformation of the piezoelectric body 260 in the d31 mode, but ejection using the d33 mode or shear mode (shear deformation) is also possible.

[0178] The shape of the pressure chamber 250 in plan view is not particularly limited, and may be any of a variety of shapes, such as a rectangle or other polygon, a circle, or an ellipse.

[0179] Furthermore, the head module 212 of this example includes a recovery-side common flow path 280, and individual recovery paths 282 are connected to the nozzle flow paths 254 of each ejector 222. The individual recovery paths 282 are connected to the recovery-side common flow path 280.

[0180] Reference numeral 266 in FIG. 29 denotes a cover plate. The cover plate 266 is a member that maintains a movable space 268 for the piezoelectric element 252 and seals the periphery of the piezoelectric element 252. A supply-side ink chamber and a recovery-side ink chamber (not shown) are formed above the cover plate 266. The supply-side ink chamber is connected to the supply-side common flow path 226 via a communication path (not shown). The recovery-side ink chamber is connected to the recovery-side common flow path 280 via a communication path (not shown). Ink supplied to the pressure chamber 250 from the supply-side common flow path 226 via the individual supply path 224 passes through the nozzle flow path 254 and is ejected from the nozzle 220. In addition, ink not used for ejection is The ink is collected from the nozzle flow paths 254 through the individual collection paths 282 into the collection-side common flow path 280 .

[0181] When there is a pressure difference between the pressure in the supply side common flow path 226 and the pressure in the recovery side common flow path 280 and ink is not being ejected from the ejector 222, ink flows from the individual supply path 224 through the pressure chamber 250 and the individual recovery path 282 to the recovery side common flow path 280.

[0182] By employing such an ink circulation structure, it is possible to prevent the ink in the pressure chamber 250 from thickening, thereby improving ejection stability. During refilling when ink is ejected from the ejector 222, ink is supplied from the supply-side common flow path 226 to the pressure chamber 250 via the individual supply path 224, and ink is supplied from the recovery-side common flow path 280 to the pressure chamber 250 via the individual recovery path 282. In other words, the recovery-side common flow path 280 not only serves to recover ink from the ejector 222, but also to supply ink to the ejector 222 during refilling.

[0183] <Inkjet head ejection method> Although FIG. 29 illustrates an inkjet head 46 equipped with a piezoelectric element 252, inkjet heads using other ejection methods may also be used. Generally, an ejector of an inkjet head includes a nozzle for ejecting ink, a pressure chamber connected to the nozzle, and an ejection energy generating element for applying ejection energy to the liquid in the pressure chamber. Regarding the ejection method for ejecting droplets from the nozzle of the ejector, the means for generating ejection energy is not limited to a piezoelectric element; various ejection energy generating elements, such as a heating element or an electrostatic actuator, may also be used. For example, a method can be adopted in which droplets are ejected by utilizing the pressure of film boiling caused by heating the liquid with a heating element. Depending on the ejection method of the inkjet head, a corresponding ejection energy generating element is provided in the flow path structure.

[0184] About the programs that run computers A program that causes a computer to realize some or all of the processing functions of the control device 100, including the image processing device 360, can be recorded on a computer-readable medium such as an optical disk, a magnetic disk, a semiconductor memory, or other tangible, non-transitory information storage medium, and the program can be provided through this information storage medium.

[0185] In addition, instead of providing the program by storing it on such a non-transitory computer-readable medium, it is also possible to provide the program signal as a download service using a telecommunications line such as the Internet.

[0186] Some or all of the processing functions of the control device 100 may be realized by cloud computing, or may be provided as a SasS (Software as a Service) service. It is also possible to

[0187] <<Hardware configuration of each processing unit>> The hardware structure of the processing units that execute various processes, such as the density correction chart generation unit 363, image data acquisition unit 364, dot data generation unit 365, density correction processing unit 370, halftone processing unit 372, correction value generation unit 366, measurement value acquisition unit 380, and calculation processing unit 384 in the image processing device 360, and the system control unit 310, image processing unit 311, conveyance control unit 312, paper feed control unit 313, treatment liquid application control unit 314, treatment liquid drying control unit 316, drawing control unit 318, ink drying control unit 320, and paper discharge control unit 324 in the control device 100, is, for example, various as shown below. It is a processor.

[0188] There are various types of processors, including CPUs, which are general-purpose processors that execute programs and function as various processing units, GPUs, which are processors specialized for image processing, and FPGAs (Field Programmable Gate Arrays), which are processors whose circuit configuration can be changed after manufacturing. This includes dedicated electrical circuits such as programmable logic devices (PLDs) and application specific integrated circuits (ASICs), which are processors with circuit configurations designed specifically to perform specific processes.

[0189] A single processing unit may be configured with one of these various processors, or may be configured with two or more processors of the same or different types. For example, a single processing unit may be configured with multiple FPGAs, or a combination of a CPU and an FPGA, or a combination of a CPU and a GPU. Alternatively, multiple processing units may be configured with a single processor. A first example of multiple processing units configured with a single processor is a configuration in which one or more CPUs and software are combined to form a single processor, as typified by client or server computers, and this processor functions as multiple processing units. A second example is a configuration in which a processor is used to realize the functions of an entire system including multiple processing units on a single IC (Integrated Circuit) chip, as typified by a system-on-chip (SoC). In this way, the various processing units are configured with one or more of the above-mentioned various processors as a hardware structure.

[0190] Furthermore, the hardware structure of these various processors is, more specifically, an electric circuit made up of a combination of circuit elements such as semiconductor elements.

[0191] Effects of this embodiment According to this embodiment, the following effects can be obtained.

[0192] [1] It is possible to improve the prediction accuracy of the correction value in the high density area where the density resolution of the scanner 48 is reduced while reducing the ink amount of the final patch PT(L).

[0193] [2] Furthermore, by selecting patch gradations that satisfy the conditions explained in FIG. 18, the prediction accuracy of the correction value can be further improved.

[0194] <<About recording media>> In the above embodiment, an example was described in which sheet-like paper P was used as the recording medium. However, the recording medium used for image formation is not limited to sheet-like media, but may also be continuous media such as roll paper. Furthermore, sheet-like media are not limited to cut sheets of paper cut to a predetermined size in advance, but may also be obtained by cutting continuous media to a predetermined size as needed. The term "recording medium" includes various terms such as paper, recording paper, printing paper, printing medium, print medium, print-receiving medium, image-forming medium, image-forming medium, image-receiving medium, image-receiving medium, print substrate, or substrate. The material and shape of the recording medium are not particularly limited, and it can take various forms, regardless of material or shape, such as sticker paper, resin sheet, film, cloth, nonwoven fabric, and others.

[0195] <<Regarding the recording medium transport mechanism>> In the above embodiment, a printing system 101 using a drum conveyance system is exemplified, but the conveyance system for the recording medium is not limited to the drum conveyance system. The conveyance mechanism for conveying the recording medium can adopt various types, such as a belt conveyance system, a nip conveyance system, a chain conveyance system, a pallet conveyance system, or a roll-to-roll conveyance system, and these systems can be combined as appropriate.

[0196] Variation 1 In the above embodiment, an example in which water-based ink is used has been described. However, instead of water-based ink, purple ink may be used. If ultraviolet-curable ink is used, the inkjet printer 1 is provided with an ultraviolet irradiation device instead of or in addition to the heating and drying treatment unit 90.

[0197] Variation 2 In the above embodiment, an inkjet printer 1 using a page-wide full-line head has been described, but the scope of application of the present disclosure is not limited to this, and the technology of the present disclosure can also be applied to inkjet printers that form images by moving a short recording head, such as a serial head, and performing multiple head scans. Furthermore, the method of forming images is not limited to the inkjet method.

[0198] <<Description of Recording Element>> A "recording element" is the smallest structural unit in a recording head that records an image. It is arranged at a position corresponding to a recording point on the recording medium and forms dots on the recording medium. For example, in the case of an inkjet recording head, the recording element may be understood to be a nozzle. The recording element may be a heating element in a thermal transfer recording method or an LED (Light Emitting Diode) element in an electrophotographic recording method.

[0199] <<Using recording heads other than inkjet type>> In the above embodiment, an inkjet printer 1 is shown as an example of an image forming apparatus, but the scope of application of the present invention is not limited to this. In addition to the inkjet method, the present invention can also be applied to various types of image forming apparatuses that perform dot recording, such as a thermal transfer recording device equipped with a recording head that uses thermal elements as recording elements, an LED electrophotographic printer equipped with a recording head that uses LED elements as recording elements, and a silver halide photographic printer equipped with an LED line exposure head. The "color material" used in image formation is not limited to ink, and may be, for example, toner.

[0200] <<Terminology>> The term "printing machine" is synonymous with terms such as image forming apparatus, printing apparatus, printer, printing device, image recording device, image output device, or drawing device. The term "image formation" includes concepts of terms such as printing, image recording, printing, drawing, and printing. The term "drawing" includes the concept of the term digital printing based on digital data. Image forming apparatuses include inkjet devices that draw various shapes and patterns using liquid functional materials, such as wiring drawing devices that draw wiring patterns of electronic circuits, resist printing devices that use resin liquid as the functional liquid to be ejected, color filter manufacturing devices, and microstructure forming devices that form microstructures using materials for material deposition.

[0201] "Image" is to be interpreted broadly and includes color images, black and white images, single color images, gradation images, uniform density (solid) images, etc. "Image" is not limited to photographic images, but is used as a comprehensive term that includes designs, characters, symbols, line drawings, mosaic patterns, color-coded patterns, other various patterns, or appropriate combinations of these.

[0202] <Combinations of embodiments and modifications> The configurations described in the above embodiments and the features described in the modified examples can be used in appropriate combinations, and some features can also be replaced.

[0203] "others" The present disclosure is not limited to the above-described embodiments, and various modifications are possible within the scope of the gist of the technical idea of ​​the present disclosure. [Explanation of symbols]

[0204] 1. Inkjet printer 10 Paper feed section 11. Transport mechanism 12 Paper feeder 12A paper feed tray 14 Feeder Board 16 Paper feed drum 20 Processing liquid application section 22 Processing liquid application drum 23 Gripper 24 Processing liquid application device 30 Processing liquid drying section 32 Processing liquid drying drum 33 Gripper 34 Warm air blower 40 Drawing section 42 Drawing Drum 43 Gripper 44 Head Unit 46, 46C, 46M, 46Y, 46K inkjet heads 47 Paper pressure roller 48 Scanner 50 Ink drying unit 60 Accumulation section 62 Accumulation Device 62A Accumulation Tray 70 Chain Gripper 71A 1st sprocket 71B 2nd sprocket 72 Chain 74 Gripper 80 Paper guide 82 First Paper Guide 84 Second Paper Guide 90 Heating and drying processing section 100 control device 101 Printing System 102 processors 104 Computer-readable medium 106 Communication Interface 108 Input / Output Interface 110 Bus 112 RAM 114 ROM 116 Storage 122 Input Device 124 Display device 212 Head Module 212A Nozzle surface 214 base frame 218B Module support member 218D Head protection material 219 nozzle row 220 nozzle 222 Ejector 224 Individual supply route 226 Supply side common flow path 250 Pressure Chamber 252 Piezoelectric element 254 Nozzle flow path 256 Diaphragm 258 individual electrodes 260 Piezoelectric 266 Cover Plate 268 Movable space 280 Recovery side common flow path 282 Individual Collection Route 302 Storage device 304 Communications Department 310 System Control Unit 311 Image Processing Unit 312 Transport control unit 313 Paper feed control unit 314 Processing liquid application control unit 316 Processing liquid drying control unit 318 Drawing control unit 320 Ink drying control unit 324 Paper ejection control unit 332 Image Memory 334 Parameter storage unit 336 Program Memory Unit 350 host computer 360 Image Processing Device 362 Test image data storage unit 363 Density correction chart generation unit 364 Image data acquisition unit 365 Dot Data Generation Unit 366 Correction value generation unit 370 Density correction processing unit 372 Halftone Processing Unit 380 Measurement acquisition unit 382 Measurement value memory unit 384 Processing Unit 386 Correction value memory unit 400 Density Correction Chart 402 Density Correction Chart Ga graph Gb1 graph Gb2 graph Gc graph GV Input tone value IMG image data NR Nozzle Range NZ1, NZi, NZn nozzle number P Paper PT Patch DP(k), DP(k+1), DP(L-1), DP(L) data points RD(k), RD(L) reading density value TD(k), TD(L) target concentration values GV(k), GV(L) Input tone value SL Upper limit of the gradation that the scanner can read S11 to S17: Steps for calculating correction values S21~S24 Printing process steps

Claims

1. 1. An image processing method for correcting density unevenness caused by a color material amount distribution in a first direction when forming an image using a recording head in which a plurality of recording elements are arranged in the first direction, comprising: a processor acquires density measurement values ​​for each of the patches of a test image for density measurement, the test image including a plurality of patches set to a uniform density in the first direction, by the recording head; and the processor calculates a correction value for correcting an output gradation relative to an input gradation for each of the recording elements from the density measurement values; Including, When calculating the correction value, the processor predicts and extrapolates unobtained density measurement values ​​in an area with a density higher than that of the first patch based on the density measurement values ​​of a first patch set to the highest density among the plurality of patches and a second patch set to the second highest density after the first patch; When a slope of a characteristic with respect to a gradation between the first patch and the second patch is u, a slope of the characteristic between any two gradations in a gradation region of an input gradation used for correction on a higher density side than the first patch is bu, and halftone processing is performed in which the value of b satisfies 0≦b≦1.5; An image processing method comprising:

2. The image processing method according to claim 1 , wherein the characteristic is an optical characteristic that is correlated with visually perceived density.

3. The image processing method according to claim 1 , wherein the plurality of patches in the test image are arranged side by side in a second direction perpendicular to the first direction.

4. the test image is formed based on image data that has been subjected to a correction process for correcting the density unevenness using a first correction value that is predetermined for each of the recording elements; the processor corrects the first correction value using a second correction value as the correction value calculated from the density measurement value for each of the recording elements acquired based on the test image; The image processing method according to any one of claims 1 to 3.

5. The test image is a linear approximation line obtained from the input tone value and the density measurement value for each of the first patch and the second patch; the first patch and the second patch, wherein a difference between density measurement values ​​corresponding to input gradation values ​​between the first patch and the second patch is less than 10% of a difference between density measurement values ​​of the first patch and the second patch, The image processing method according to any one of claims 1 to 4.

6. the processor: performing a calculation to calculate the correction value by the extrapolation process calculated from the characteristics of the gradation between the first patch and the second patch for each of the recording elements; The image processing method according to any one of claims 1 to 5.

7. the processor: performing a calculation to calculate the correction value by the extrapolation process calculated using an average value of the density measurement values ​​obtained from the first patch and the second patch of all the recording elements provided in the recording head; The image processing method according to any one of claims 1 to 5.

8. the processor: performing a calculation to calculate the correction value by the extrapolation process calculated using values ​​obtained by smoothing the density measurement values ​​obtained from the first patch and the second patch of the plurality of recording elements provided in the recording head; The image processing method according to any one of claims 1 to 5.

9. the number of the recording elements to be subjected to the smoothing process is equal to or greater than two and equal to or less than the total number of the recording elements of the recording head; The image processing method according to claim 8.

10. The processor, performing the extrapolation process so that the slope approaches 0 as the gradation increases in an area with a higher density than the first patch; The image processing method according to any one of claims 1 to 9.

11. 1. An image processing apparatus that corrects density unevenness caused by a color material amount distribution in a first direction when forming an image using a recording head in which a plurality of recording elements are arranged in the first direction, comprising: a processor; a memory for storing a program executed by the processor; Equipped with The processor executes the program, a process of acquiring density measurement values ​​for each of the patches of a test image obtained by forming, using the recording head, a test image for density measurement, the test image including a plurality of patches set to a uniform density in the first direction; and calculating a correction value for correcting an output gradation relative to an input gradation for each of the recording elements from the density measurement values; When calculating the correction value, based on the density measurement values ​​of a first patch set to the highest density among the plurality of patches and a second patch set to the second highest density after the first patch, unobtained density measurement values ​​in an area with a higher density than the first patch are predicted and extrapolated; When a slope of a characteristic with respect to a gradation between the first patch and the second patch is u, a slope of the characteristic between any two gradations in a gradation region of an input gradation used for correction on a higher density side than the first patch is bu, and halftone processing is performed such that the value of b satisfies 0≦b≦1.

5. Image processing device.

12. A program that causes a computer to realize an image processing function of correcting density unevenness caused by a color material amount distribution in a first direction when forming an image using a recording head in which a plurality of recording elements are arranged in the first direction, the program comprising: a function of acquiring density measurement values ​​for each of the patches of a test image obtained by forming, by the recording head, a test image for density measurement including a plurality of patches set to a uniform density in the first direction; and a function of calculating a correction value for correcting an output gradation for an input gradation for each of the recording elements from the density measurement values, a function of performing an extrapolation process by predicting unobtained density measurement values ​​in an area with a higher density than a first patch based on the density measurement values ​​of a first patch set to the highest density among the plurality of patches and a second patch set to the second highest density after the first patch when calculating the correction value; A program that realizes a function of performing halftone processing in which, when the slope of the characteristics for gradation between the first patch and the second patch is u, the slope of the characteristics between any two gradations in the gradation range of the input gradation used for correction on the higher density side than the first patch is bu, and the value of b satisfies 0≦b≦1.

5.

13. a print head in which a plurality of print elements are arranged in a first direction; a relative movement mechanism for moving the recording medium and the recording head relative to each other; a density measuring device for measuring the density of an image formed on the recording medium using the recording head; a processor, The processor: a process of forming a test image for density measurement, which includes a plurality of patches set to a uniform density in the first direction, by the recording head; a process of measuring the density of each of the patches of the test image formed by the recording head with the density measuring device, and acquiring density measurement values ​​for each of the patches for each of the recording elements; a process of calculating a correction value for correcting an output gradation relative to an input gradation for each of the recording elements from the density measurement values; performing a correction process for generating corrected image data using the correction value for the image data to be formed; When calculating the correction value, based on the density measurement values ​​of a first patch set to the highest density among the plurality of patches and a second patch set to the second highest density after the first patch, unobtained density measurement values ​​in an area with a higher density than the first patch are predicted and extrapolated; When a slope of a characteristic with respect to a gradation between the first patch and the second patch is u, a slope of the characteristic between any two gradations in a gradation region of an input gradation used for correction on a higher density side than the first patch is bu, and halftone processing is performed such that the value of b satisfies 0≦b≦1.5; the recording head forms an image based on the image data that has been subjected to the correction process using the correction value; Image forming device.

14. the recording head is an inkjet head having nozzles as the recording elements; The image forming apparatus according to claim 13.

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