Printing apparatus and printing method

Abstract

To effectively suppress the occurrence of spatially periodic density unevenness caused by differences in dynamic surface tension between ink that first lands on a print medium and ink that lands later on the print medium in an ink jet printing apparatus in which a large number of nozzles are arranged in a two-dimensional pattern.SOLUTION: In an ink jet printing apparatus including a printing unit in which nozzle sets composed of multiple nozzles arranged in a regular two-dimensional pattern are continuously arranged in a main scanning direction, a print data processing part 42 in a control unit includes a density value reduction processing section 424 that reduces a density value of a pixel corresponding to a correction target nozzle. A nozzle that discharges ink to any of first to third ones from the last one among the multiple nozzles that constitute the nozzle set is defined for the correction target nozzle.SELECTED DRAWING: Figure 11

Description

[Technical field]

[0001] The present invention relates to a technique for suppressing the occurrence of unevenness in density in an inkjet printing device that performs printing by ejecting ink onto a printing medium. [Background technology]

[0002] In recent years, inkjet printing devices that perform printing by ejecting ink onto a printing medium (substrate) such as printing paper or film using heat or pressure have become widespread. Inkjet printing devices that perform high-speed printing using a one-pass method (single-pass method), from the viewpoint of ease of processing the head and ease of replacing the head when the head breaks down, for example, a configuration in which multiple heads are arranged in a main scanning direction (width direction) perpendicular to the transport direction of the printing medium for each ink color, or a configuration in which multiple heads are arranged in a staggered pattern in the main scanning direction for each ink color is adopted. In addition, regarding the head configuration, a configuration in which a large number of nozzles are arranged two-dimensionally (for example, a configuration as shown in FIG. 41) is generally adopted in order to increase the resolution in the main scanning direction.

[0003] However, when a configuration is adopted in which a large number of nozzles are arranged two-dimensionally, uneven shading may appear in the printed image. The main causes of uneven shading include differences in the amount of ink ejected between the multiple nozzles and the flow of ink on the printing medium.

[0004] Proposals to address the occurrence of uneven shading due to differences in the amount of ink ejected between multiple nozzles include arranging the nozzles so that the timing of ink ejection from two or more nozzles belonging to the same flow path does not overlap, and shifting the image so that the part of the image where uneven shading is likely to occur is located at the boundary between two adjacent heads.

[0005] An example of a measure to prevent uneven shading caused by the flow of ink on a printing medium is disclosed in JP 2017-81104 A. JP 2017-81104 A states the following. When three nozzle rows (nozzle row 1, nozzle row 2, nozzle row 3) are present and ink is ejected in the order of "nozzles included in nozzle row 1, nozzles included in nozzle row 2, nozzles included in nozzle row 3", the ink ejected from the nozzles included in nozzle row 1 and the ink ejected from the nozzles included in nozzle row 2 attract each other due to surface tension. The ink ejected from the nozzles included in nozzle row 3 subsequently lands in the approximate center of a relatively large blank area, and does not come into contact with either the ink ejected from the nozzles included in nozzle row 1 or the ink ejected from the nozzles included in nozzle row 2. As a result, white stripes are generated on both sides of the line formed by the ink ejected from the nozzles included in nozzle row 3 so as to extend in the transport direction of the printing medium. Therefore, in the inkjet recording system disclosed in JP 2017-81104 A, when image data of a uniform density is input, ejection data is generated so that the number of ink ejections from nozzles that eject ink at a relatively late timing (in the above example, the nozzles included in nozzle row 3) is greater than the number of ink ejections from other nozzles (in the above example, the nozzles included in nozzle row 1 and the nozzles included in nozzle row 2). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2017-81104 A Summary of the Invention [Problem to be solved by the invention]

[0007] In the inkjet recording system disclosed in JP2017-81104A, the size of the ink droplets ejected from each nozzle is considered to be relatively small. When ink is ejected from two nozzles at adjacent positions on a print medium, if the ink droplets are large enough to overlap with each other, uneven shading occurs due to a mechanism different from that described in JP2017-81104A. This will be explained below.

[0008] When multiple nozzles are arranged two-dimensionally as shown in FIG. 41, two nozzles that eject ink to adjacent positions on the printing medium are located at different positions on the head in the transport direction of the printing medium. For example, a nozzle labeled with reference numeral 91 in FIG. 41 and a nozzle labeled with reference numeral 92 in FIG. 41 eject ink to adjacent positions on the printing medium, but are located at different positions on the head in the transport direction of the printing medium. For such two nozzles, a time difference occurs between the ink ejection timing from one nozzle and the ink ejection timing from the other nozzle. This causes a time difference between the landing timing of the ink ejected from the one nozzle on the printing medium and the landing timing of the ink ejected from the other nozzle on the printing medium. Then, a pull occurs between the ink that lands on the printing medium first and the ink that lands on the printing medium later, depending on the time difference between the landing timing of the two inks on the printing medium. In this regard, when there is a large difference in dynamic surface tension between the ink that lands on the print medium first and the ink that lands on the print medium later, the ink that lands on the print medium first tends to be attracted to the ink that lands on the print medium later. In contrast, when the ink has high fixability to the print medium (when the force of attraction between the print medium and the ink is stronger than the force of attraction between the inks themselves), the ink that lands on the print medium later tends to be attracted to the ink that lands on the print medium first.

[0009] The movement of ink when ink that lands on the printing medium first is attracted to ink that lands on the printing medium later will be further described with reference to FIG. 42. Note that here, the nozzle that ejects ink onto the printing medium first is called the "first nozzle," and the nozzle that ejects ink onto the printing medium later is called the "second nozzle." At time t91, ink is ejected from the first nozzle. At time t92, the ink ejected from the first nozzle lands on the printing medium, and ink is ejected from the second nozzle. At time t93, the ink ejected from the second nozzle lands on the printing medium. At this time, the dynamic surface tension of the ink ejected from the second nozzle is greater than that of the ink ejected from the first nozzle. Therefore, at time t94, the ink ejected from the first nozzle is attracted to the ink ejected from the second nozzle, as shown by the arrow with the reference numeral 96.

[0010] Here, attention is paid to the configuration shown in FIG. 41. In the example shown in FIG. 41, eight nozzles form one nozzle set, and multiple nozzle sets are arranged in the main scanning direction. Focusing on the eight nozzles constituting the nozzle set located within the dotted line labeled with reference numeral 93 in FIG. 41, ink is ejected from the eight nozzles in sequence at a predetermined time interval. At that time, the ink that lands on the printing medium later is attracted toward the ink that landed on the printing medium earlier. As a result, the cross section of the ink on the printing medium after the ink is ejected from the eight nozzles is typically as shown in FIG. 43. That is, ink ejected from other nozzles is attracted to the position corresponding to the nozzle that ejected the ink last. Furthermore, focusing on three nozzle sets located within the dotted line labeled with reference numeral 94 in FIG. 41, the cross section of the ink on the printing medium after the ink is ejected from the 24 nozzles constituting the three nozzle sets is typically as shown in FIG. 44. FIG. 45 is a graph showing spatial changes in the density of a printed image obtained when an image of horizontal lines spaced at equal intervals is printed at a constant density in a nozzle set consisting of 32 nozzles. From FIG. 45, it can be seen that a difference in density corresponding to the length of the arrow marked with the reference numeral 97 occurs. Thus, even if the amount of ink ejected from all nozzles is the same, there are areas on the printing medium where the amount of ink is greater than other areas (i.e., areas where the density is higher than other areas) at a certain spatial period. In this way, regular unevenness in density appears in the printed image, and print quality is reduced. Furthermore, the above description focuses on the case where only one color of ink is ejected onto the printing medium, but unevenness in density may be more pronounced in the case where multiple colors of ink are ejected onto the printing medium.

[0011] In view of the above circumstances, the present invention aims to effectively suppress the occurrence of spatial periodic unevenness in shading caused by the difference in dynamic surface tension between ink that lands on a printing medium first and ink that lands on the printing medium later in an inkjet printing device in which a large number of nozzles are arranged in a two-dimensional array. [Means for solving the problem]

[0012] A first aspect of the present invention is an inkjet printing device that has a plurality of nozzles and performs printing by ejecting ink from the plurality of nozzles onto a printing medium, a movement mechanism that relatively moves the positional relationship between the printing medium and the plurality of nozzles in a predetermined direction; a printing unit including a plurality of nozzles, each of which is a nozzle set including N nozzles arranged regularly in a two-dimensional manner, N being a natural number, and which is arranged continuously in a width direction perpendicular to the predetermined direction; a control unit that controls the amount of ink ejected from the plurality of nozzles; Equipped with when ink is ejected onto the printing medium from the N nozzles included in the nozzle set to the same position in the predetermined direction, ink is ejected first from a nozzle that is arranged on one end side of the N nozzles in the width direction, and ink is ejected last from a nozzle that is arranged on the other end side of the N nozzles in the width direction, Among the N nozzles, a nozzle arranged on the other end side in the width direction is determined as a correction target nozzle, The control unit is characterized in that it controls the amount of ink ejected from the nozzle to be corrected so that the average amount of ink ejected from the nozzle to be corrected is smaller than the average amount of ink that should originally be ejected from the nozzle to be corrected.

[0013] The second invention is the first invention, The nozzle that is disposed second or third from the other end side in the width direction among the N nozzles is further determined as the nozzle to be corrected.

[0014] A third invention is an inkjet printing device having a plurality of nozzles and performing printing by ejecting ink from the plurality of nozzles onto a printing medium, a movement mechanism that relatively moves the positional relationship between the printing medium and the plurality of nozzles in a predetermined direction; a printing unit including a plurality of nozzles, each of which is a nozzle set including N nozzles arranged regularly in a two-dimensional manner, N being a natural number, and which is arranged continuously in a width direction perpendicular to the predetermined direction; a control unit that controls the amount of ink ejected from the plurality of nozzles; Equipped with when ink is ejected onto the printing medium from the N nozzles included in the nozzle set to the same position in the predetermined direction, ink is ejected first from any one of the N nozzles that is arranged in the first to third positions based on one end side of the width direction, and ink is ejected last from any one of the N nozzles that is arranged in the first to third positions based on the other end side of the width direction, Among the N nozzles, the nozzle that is arranged second or third based on the other end side in the width direction is determined as a correction target nozzle, The control unit is characterized in that it controls the amount of ink ejected from the nozzle to be corrected so that the average amount of ink ejected from the nozzle to be corrected is smaller than the average amount of ink that should originally be ejected from the nozzle to be corrected.

[0015] The fourth invention is any one of the first to third inventions, The control unit is a shading correction process for generating second image data including density values ​​of a plurality of pixels by correcting first image data including density values ​​of the plurality of pixels so as to compensate for variations in ink ejection amount among the plurality of nozzles; a density value reduction process for generating third image data including density values ​​of the plurality of pixels by reducing density values ​​of pixels corresponding to the nozzles to be corrected among the density values ​​of the plurality of pixels included in the second image data; a halftone process for generating halftone image data for controlling an amount of ink ejected from each of the plurality of nozzles by halftoning the third image data; The present invention is characterized by carrying out the following steps.

[0016] The fifth invention is any one of the first to third inventions, The control unit is a shading correction process for generating second image data including density values ​​of a plurality of pixels by correcting first image data including density values ​​of the plurality of pixels so as to compensate for variations in ink ejection amount among the plurality of nozzles; a halftone process for generating halftone image data that controls an amount of ink ejected from each of the plurality of nozzles by halftoning the second image data; Run The halftone processing is characterized in that the halftone image data is generated so that the average amount of ink ejected from the nozzle to be corrected is smaller than the average amount of ink that should originally be ejected from the nozzle to be corrected.

[0017] The sixth aspect of the present invention relates to the fifth aspect of the present invention, the control unit performs the halftone process by using a threshold matrix including a plurality of thresholds arranged two-dimensionally to be compared with density values ​​of the plurality of pixels included in the second image data, The multiple thresholds included in the threshold matrix are set so that the average amount of ink ejected from the nozzles to be corrected is smaller than the average amount of ink ejected from nozzles other than the nozzles to be corrected.

[0018] The seventh invention is any one of the first to third inventions, the printing unit includes only one ink ejection head for each ink color in the predetermined direction; the ink ejection head includes an upstream nozzle group located on the upstream side in the predetermined direction, and a downstream nozzle group located on the downstream side in the predetermined direction, P nozzles out of the N nozzles included in the nozzle set are included in the upstream nozzle group, where P is an integer less than N, when ink is ejected onto the printing medium from the P nozzles included in the upstream nozzle group at the same position in the predetermined direction, ink is ejected last from a nozzle among the P nozzles that is disposed on the other end side in the width direction, The nozzles arranged on the other end side in the width direction out of the P nozzles are further determined as the nozzles to be corrected.

[0019] The eighth aspect of the present invention is any one of the first to third aspects of the present invention, the printing unit includes, for each ink color, an upstream ink ejection head disposed on the upstream side in the predetermined direction, and a downstream ink ejection head disposed on the downstream side in the predetermined direction; P nozzles out of the N nozzles are included in the upstream ink ejection head, where P is an integer less than N, when ink is ejected from the P nozzles included in the upstream ink ejection head to the same position in the predetermined direction onto the printing medium, ink is ejected last from a nozzle that is disposed on the other end side of the P nozzles in the width direction, The nozzles arranged on the other end side in the width direction out of the P nozzles are further determined as the nozzles to be corrected.

[0020] The ninth aspect of the present invention is any one of the first to third aspects of the present invention, With regard to printing of an image portion in which two or more ink colors should be of the same density, the control unit is characterized in that it sets a different average reduction value for each ink color, which is the difference between the average amount of ink ejected from the nozzle to be corrected and the average amount of ink that should originally be ejected from the nozzle to be corrected.

[0021] A tenth aspect of the present invention relates to the ninth aspect of the present invention, the two or more ink colors being cyan, magenta, yellow, and black; The control unit is characterized in that it makes the average reduction value for magenta greater than the average reduction value for yellow, makes the average reduction value for black greater than the average reduction value for magenta, and makes the average reduction value for cyan greater than the average reduction value for black.

[0022] An eleventh aspect of the present invention is an inkjet printing apparatus having a plurality of nozzles, which performs printing by ejecting ink from the plurality of nozzles onto a printing medium, a movement mechanism that relatively moves the positional relationship between the printing medium and the plurality of nozzles in a predetermined direction; a printing unit including a plurality of nozzles, each of which is a nozzle set including N nozzles arranged regularly in a two-dimensional manner, N being a natural number, and which is arranged continuously in a width direction perpendicular to the predetermined direction; a control unit that controls the amount of ink ejected from the plurality of nozzles; Equipped with when ink is ejected onto the printing medium from the N nozzles included in the nozzle set to the same position in the predetermined direction, ink is ejected first from any one of the N nozzles that is arranged in the first to third positions based on one end side of the width direction, and ink is ejected last from any one of the N nozzles that is arranged in the first to third positions based on the other end side of the width direction, Among the N nozzles, the first to third nozzles arranged based on the other end side in the width direction are selectively determined as correction target nozzles; the control unit controls the amount of ink ejected from the correction target nozzles so that an average amount of ink ejected from the correction target nozzles is smaller than an average amount of ink that should originally be ejected from the correction target nozzles; If the difference between the average amount of ink ejected from the nozzle to be corrected and the average amount of ink that should originally be ejected from the nozzle to be corrected is defined as the reduced ink amount, the reduced ink amount when the second nozzle based on the other end side of the width direction is determined as the nozzle to be corrected is greater than the reduced ink amount when the first nozzle based on the other end side of the width direction is determined as the nozzle to be corrected, and the reduced ink amount when the third nozzle based on the other end side of the width direction is determined as the nozzle to be corrected is greater than the reduced ink amount when the second nozzle based on the other end side of the width direction is determined as the nozzle to be corrected.

[0023] A twelfth aspect of the present invention is an inkjet printing apparatus having a plurality of nozzles, which performs printing by ejecting ink from the plurality of nozzles onto a printing medium, a movement mechanism that relatively moves the positional relationship between the printing medium and the plurality of nozzles in a predetermined direction; a printing unit including a plurality of nozzles, each of which is a nozzle set including N nozzles arranged regularly in a two-dimensional manner, N being a natural number, and which is arranged continuously in a width direction perpendicular to the predetermined direction; a control unit that controls the amount of ink ejected from the plurality of nozzles; Equipped with when ink is ejected onto the printing medium from the N nozzles included in the nozzle set to the same position in the predetermined direction, ink is ejected first from any one of the N nozzles that is arranged in the first to third positions based on one end side of the width direction, and ink is ejected last from any one of the N nozzles that is arranged in the first to third positions based on the other end side of the width direction, Among the N nozzles, the nozzle that is arranged second or third based on the other end side in the width direction is determined as a correction target nozzle, The control unit is characterized in that it controls the amount of ink ejected from the nozzle to be corrected so that the average amount of ink ejected from the nozzle to be corrected is greater than the average amount of ink that should originally be ejected from the nozzle to be corrected.

[0024] A thirteenth aspect of the present invention is a printing method using an inkjet printing device having a plurality of nozzles that eject ink onto a printing medium, comprising: The positional relationship between the printing medium and the plurality of nozzles is such that they move relatively in a predetermined direction, the printing device comprises a printing unit including a plurality of nozzles, each of which is a nozzle set including N nozzles arranged regularly in a two-dimensional manner, N being a natural number, and which is arranged continuously in a width direction perpendicular to the predetermined direction; when ink is ejected onto the printing medium from the N nozzles included in the nozzle set to the same position in the predetermined direction, ink is ejected first from a nozzle that is arranged on one end side of the N nozzles in the width direction, and ink is ejected last from a nozzle that is arranged on the other end side of the N nozzles in the width direction, Among the N nozzles, a nozzle arranged on the other end side in the width direction is determined as a correction target nozzle, The printing method includes: a rasterization step of generating first image data including density values ​​of a plurality of pixels by performing a rasterization process on the input data; a shading correction step of generating second image data including density values ​​of the plurality of pixels by correcting the first image data so as to compensate for variations in ink ejection amount among the plurality of nozzles; a density value reducing step of generating third image data including density values ​​of the plurality of pixels by reducing density values ​​of pixels corresponding to the nozzles to be corrected among the density values ​​of the plurality of pixels included in the second image data; a halftone image generating step of generating halftone image data for controlling an amount of ink ejected from each of the plurality of nozzles by halftoning the third image data; an ink ejection step of ejecting ink from the plurality of nozzles based on the halftone image data; The present invention is characterized by comprising:

[0025] A fourteenth aspect of the present invention relates to the thirteenth aspect of the present invention, The nozzle that is disposed second or third from the other end side in the width direction among the N nozzles is further determined as the nozzle to be corrected.

[0026] A fifteenth aspect of the present invention is a printing method using an inkjet printing device having a plurality of nozzles that eject ink onto a printing medium, comprising: The positional relationship between the printing medium and the plurality of nozzles is such that they move relatively in a predetermined direction, the printing device comprises a printing unit including a plurality of nozzles, each of which is a nozzle set including N nozzles arranged regularly in a two-dimensional manner, N being a natural number, and which is arranged continuously in a width direction perpendicular to the predetermined direction; when ink is ejected onto the printing medium from the N nozzles included in the nozzle set to the same position in the predetermined direction, ink is ejected first from any one of the N nozzles that is arranged in the first to third positions based on one end side of the width direction, and ink is ejected last from any one of the N nozzles that is arranged in the first to third positions based on the other end side of the width direction, Among the N nozzles, the nozzle that is arranged second or third based on the other end side in the width direction is determined as the correction target nozzle, The printing method includes: a rasterization step of generating first image data including density values ​​of a plurality of pixels by performing a rasterization process on the input data; a shading correction step of generating second image data including density values ​​of the plurality of pixels by correcting the first image data so as to compensate for variations in ink ejection amount among the plurality of nozzles; a density value reducing step of generating third image data including density values ​​of the plurality of pixels by reducing density values ​​of pixels corresponding to the nozzles to be corrected among the density values ​​of the plurality of pixels included in the second image data; a halftone image generating step of generating halftone image data for controlling an amount of ink ejected from each of the plurality of nozzles by halftoning the third image data; an ink ejection step of ejecting ink from the plurality of nozzles based on the halftone image data; The present invention is characterized by comprising:

[0027] A sixteenth aspect of the present invention is any one of the thirteenth to fifteenth aspects of the present invention, The printing method includes: a test chart printing step of printing a test chart for determining an amount of reduction in density value when reducing density values ​​of pixels corresponding to the nozzles to be corrected in the density value reducing step, the test chart including a plurality of pattern images with different amounts of reduction in density value; a pattern image selection step of selecting one of the plurality of pattern images based on the test chart printed in the test chart printing step; Further comprising: In the density value reducing step, the third image data is generated by reducing the density value of the pixel corresponding to the nozzle to be corrected in accordance with the pattern image selected in the pattern image selecting step.

[0028] A seventeenth aspect of the present invention relates to the sixteenth aspect of the present invention, The range of possible pixel density values ​​from the minimum to the maximum is divided into a number of stages, In the test chart printing step, the test chart is printed in each step, In the pattern image selection step, one of the plurality of pattern images is selected for each stage.

[0029] An eighteenth aspect of the present invention is a printing method using an inkjet printing device having a plurality of nozzles that eject ink onto a printing medium, comprising: The positional relationship between the printing medium and the plurality of nozzles is such that they move relatively in a predetermined direction, the printing device comprises a printing unit including a plurality of nozzles, each of which is a nozzle set including N nozzles arranged regularly in a two-dimensional manner, N being a natural number, and which is arranged continuously in a width direction perpendicular to the predetermined direction; when ink is ejected onto the printing medium from the N nozzles included in the nozzle set to the same position in the predetermined direction, ink is ejected first from a nozzle that is arranged on one end side of the N nozzles in the width direction, and ink is ejected last from a nozzle that is arranged on the other end side of the N nozzles in the width direction, Among the N nozzles, a nozzle arranged on the other end side in the width direction is determined as a correction target nozzle, The printing method includes: a rasterization step of generating first image data including density values ​​of a plurality of pixels by performing a rasterization process on the input data; a shading correction step of generating second image data including density values ​​of the plurality of pixels by correcting the first image data so as to compensate for variations in ink ejection amount among the plurality of nozzles; a halftone image generating step of generating halftone image data for controlling an amount of ink ejected from each of the plurality of nozzles by halftoning the second image data; an ink ejection step of ejecting ink from the plurality of nozzles based on the halftone image data; Including, The halftone image generation step is characterized in that the halftone image data is generated so that the average amount of ink ejected from the nozzle to be corrected is smaller than the average amount of ink that should originally be ejected from the nozzle to be corrected.

[0030] A nineteenth aspect of the present invention relates to the eighteenth aspect of the present invention, The nozzle that is disposed second or third from the other end side in the width direction among the N nozzles is further determined as the nozzle to be corrected.

[0031] A twentieth aspect of the present invention is a printing method using an inkjet printing device having a plurality of nozzles that eject ink onto a printing medium, comprising: The positional relationship between the printing medium and the plurality of nozzles is such that they move relatively in a predetermined direction, the printing device comprises a printing unit including a plurality of nozzles, each of which is a nozzle set including N nozzles arranged regularly in a two-dimensional manner, N being a natural number, and which is arranged continuously in a width direction perpendicular to the predetermined direction; when ink is ejected onto the printing medium from the N nozzles included in the nozzle set to the same position in the predetermined direction, ink is ejected first from any one of the N nozzles that is arranged in the first to third positions based on one end side of the width direction, and ink is ejected last from any one of the N nozzles that is arranged in the first to third positions based on the other end side of the width direction, Among the N nozzles, the nozzle that is arranged second or third based on the other end side in the width direction is determined as a correction target nozzle, The printing method includes: a rasterization step of generating first image data including density values ​​of a plurality of pixels by performing a rasterization process on the input data; a shading correction step of generating second image data including density values ​​of the plurality of pixels by correcting the first image data so as to compensate for variations in ink ejection amount among the plurality of nozzles; a halftone image generating step of generating halftone image data for controlling an amount of ink ejected from each of the plurality of nozzles by halftoning the second image data; an ink ejection step of ejecting ink from the plurality of nozzles based on the halftone image data; Including, The halftone image generation step is characterized in that the halftone image data is generated so that the average amount of ink ejected from the nozzle to be corrected is smaller than the average amount of ink that should originally be ejected from the nozzle to be corrected. Effect of the Invention

[0032] According to the first aspect of the present invention, the printing unit of the printing device has nozzle sets consisting of N nozzles regularly arranged in a two-dimensional manner, which are arranged continuously in the width direction (the direction perpendicular to the relative movement direction of the positional relationship between the printing medium and the multiple nozzles included in the printing unit), and when ink is ejected from the N nozzles constituting each nozzle set to the same position in the relative movement direction, ink is ejected first from the nozzles arranged at one end side of the N nozzles in the width direction, and ink is ejected last from the nozzles arranged at the other end side of the N nozzles in the width direction. In this configuration, the nozzle that ejects ink last among the N nozzles constituting each nozzle set is determined as the nozzle to be corrected, and the ink ejection amount is controlled so that the average amount of ink ejected from the nozzle to be corrected is smaller than the average amount of ink that should originally be ejected from the nozzle to be corrected. Therefore, after the ink ejected from the nozzle to be corrected lands on the printing medium, the ink that has already landed on the printing medium is prevented from flowing toward the ink that was ejected from the nozzle to be corrected and landed on the printing medium, compared to the conventional method. This makes it difficult for the density of the area near the position corresponding to the nozzle that ejects ink last among the N nozzles that make up each nozzle set to become higher than other areas. In other words, the occurrence of shading unevenness is suppressed. As described above, in a printing device in which a large number of nozzles are arranged two-dimensionally, the occurrence of spatial periodic shading unevenness caused by the difference in dynamic surface tension between the ink that lands on the printing medium first and the ink that lands on the printing medium later is effectively suppressed. In addition, since the occurrence of shading unevenness is suppressed, there is less need to reprint compared to the conventional method, and it is possible to reduce the consumption of printing media and ink. In this way, it is possible to contribute to the achievement of the SDGs (Sustainable Development Goals).

[0033] According to the second aspect of the present invention, it is possible to more effectively suppress the occurrence of spatially periodic unevenness in shading.

[0034] According to the third aspect of the invention, the printing unit of the printing device has nozzle sets each consisting of N nozzles regularly arranged in a two-dimensional manner, which are arranged continuously in the width direction (the direction perpendicular to the relative movement direction of the positional relationship between the printing medium and the multiple nozzles included in the printing unit), and when ink is ejected from the N nozzles constituting each nozzle set to the same position in the relative movement direction, ink is ejected first from any of the N nozzles arranged first to third based on one end side of the width direction, and ink is ejected last from any of the N nozzles arranged first to third based on the other end side of the width direction. In this configuration, the nozzle that ejects ink one before the slowest ink ejection nozzle (the nozzle that ejects ink last among the N nozzles constituting each nozzle set) among the N nozzles constituting each nozzle set or the nozzle that ejects ink two before the slowest ink ejection nozzle is determined as the correction target nozzle, and the ink ejection amount is controlled so that the average amount of ink ejected from the correction target nozzle is smaller than the average amount of ink that should originally be ejected from the correction target nozzle. Therefore, after the ink ejected from the nozzle to be corrected lands on the printing medium, the ink that has already landed on the printing medium is prevented from flowing toward the ink that has been ejected from the nozzle to be corrected and landed on the printing medium, compared to the conventional method. This also prevents the ink that has already landed on the printing medium from flowing toward the ink that has been ejected from the slowest ink ejection nozzle and landed on the printing medium, compared to the conventional method. As a result, the phenomenon in which the density of the area near the position corresponding to the slowest ink ejection nozzle becomes higher than other areas is less likely to occur. In other words, the occurrence of uneven shading is suppressed. As described above, in a printing device in which a large number of nozzles are arranged two-dimensionally, the occurrence of spatial periodic uneven shading caused by the difference in dynamic surface tension between the ink that has landed on the printing medium first and the ink that has landed on the printing medium later is effectively suppressed. In addition, as in the first invention, it is possible to reduce the consumption of printing media and ink, which can contribute to the achievement of the SDGs.

[0035] According to the fourth aspect of the invention, the same effects as those of the first to third aspects of the invention can be obtained.

[0036] According to the fifth aspect of the present invention, there is no need to apply correction to the image data before halftoning (second image data) to suppress the occurrence of spatial periodic unevenness in shading. In other words, no additional processing is required in addition to conventional processing when performing print output. As a result, it is possible to effectively suppress the occurrence of spatial periodic unevenness in shading caused by the difference in dynamic surface tension between the ink that lands first on the print medium and the ink that lands later on the print medium, without requiring any additional processing in addition to conventional processing for print output.

[0037] According to the sixth aspect of the invention, the same effects as those of the fifth aspect of the invention can be obtained.

[0038] According to the seventh aspect of the present invention, when ink is ejected from the P nozzles included in the upstream nozzle group, the last of the P nozzles to eject ink ejects a smaller amount of ink than would otherwise be ejected. Furthermore, the last nozzle of the downstream nozzle group to eject ink ejects a smaller amount of ink than would otherwise be ejected. This effectively prevents ink that has landed on the print medium first from flowing toward ink that has landed on the print medium later, more effectively preventing uneven shading.

[0039] According to the eighth aspect of the invention, when ink is ejected from the P nozzles included in the upstream ink ejection head, the last of the P nozzles to eject ink ejects a smaller amount of ink than would otherwise be ejected. Furthermore, the last nozzle of the downstream ink ejection head to eject ink also ejects a smaller amount of ink than would otherwise be ejected. This effectively prevents ink that has landed on the printing medium first from flowing toward ink that has landed on the printing medium later, more effectively preventing uneven shading.

[0040] According to the ninth aspect of the present invention, the occurrence of spatially periodic unevenness in shading in an image portion printed using a plurality of colors of ink is more effectively suppressed.

[0041] According to the tenth aspect of the present invention, the occurrence of spatially periodic unevenness in shading in image portions printed using cyan, magenta, yellow, and black inks is more effectively suppressed.

[0042] According to the eleventh aspect of the present invention, in a configuration that allows three nozzles to be selectively determined as correction target nozzles from among the N nozzles that constitute each nozzle set, the occurrence of spatial periodic unevenness in shading is effectively suppressed regardless of the nozzles determined as correction target nozzles.

[0043] According to the twelfth aspect of the present invention, in a case where ink that lands later on the printing medium is attracted toward ink that lands earlier on the printing medium, the occurrence of spatial periodic unevenness in shading is effectively suppressed.

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

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

[0046] According to the fifteenth aspect of the present invention, the same effects as those of the third aspect of the present invention can be obtained.

[0047] According to the sixteenth aspect of the present invention, a user of the printing device can print a test chart and vary the amount of reduction in density value of pixels corresponding to nozzles to be corrected depending on the print result. This makes it possible to effectively suppress the occurrence of spatial periodic unevenness in shading by taking into account the state of the printing device at the time of printing and the printing medium used for printing.

[0048] According to the seventeenth aspect of the present invention, the occurrence of spatial periodic unevenness in shading is effectively suppressed regardless of the density of the printed image.

[0049] According to the eighteenth aspect of the present invention, the same effects as those of the first and fifth aspects of the present invention can be obtained.

[0050] According to the nineteenth aspect of the present invention, the same effects as those of the second aspect of the present invention can be obtained.

[0051] According to the twentieth aspect of the present invention, the same effects as those of the third and fifth aspects of the present invention can be obtained. [Brief description of the drawings]

[0052] [Figure 1] FIG. 1 is a diagram for explaining the basic concept of the present invention. [Diagram 2] 1 is an overall configuration diagram of a printing system according to a first embodiment of the present invention. [Diagram 3] FIG. 2 is a schematic diagram showing a configuration example of an inkjet printing apparatus in the first embodiment. [Figure 4] FIG. 2 is a plan view showing a schematic configuration of a printing unit in the first embodiment. [Diagram 5] FIG. 4 is a plan view showing another example of the schematic configuration of the printing unit in the first embodiment. [Figure 6] FIG. 4 is a plan view showing the configuration of one head in the first embodiment. [Figure 7] FIG. 2 is a diagram showing a schematic arrangement of eight nozzles that constitute one nozzle set on a head in the first embodiment. [Figure 8] FIG. 11 is a diagram for explaining the order in which ink is ejected from eight nozzles that constitute one nozzle set in the first embodiment. [Figure 9] FIG. 2 is a block diagram showing a hardware configuration of a print control device in the first embodiment. [Figure 10] 3 is a block diagram showing a schematic functional configuration of a control unit realized by execution of a print control program in a print control device in the first embodiment. FIG. [Figure 11] 4 is a block diagram showing a detailed functional configuration of a print data processing unit in the first embodiment. FIG. [Figure 12] FIG. 4 is a diagram for explaining a shading correction process in the first embodiment. [Figure 13] 5 is a flowchart showing a procedure for print output in the first embodiment. [Figure 14] FIG. 2 is a diagram illustrating a test chart (correction amount adjustment chart) used for determining a correction amount in the first embodiment. [Figure 15] FIG. 2 is a diagram illustrating a test chart (correction width adjustment chart) used for determining a correction width in the first embodiment. [Figure 16] FIG. 4 is a diagram for explaining the flow of ink in relation to the first embodiment. [Figure 17] FIG. 4 is a diagram for explaining the flow of ink in relation to the first embodiment. [Figure 18] FIG. 4 is a diagram for explaining the flow of ink in relation to the first embodiment. [Figure 19] 5A to 5C are diagrams showing the difference in appearance of shading when density value reduction processing is not performed and when density value reduction processing is performed in the first embodiment. [Figure 20] FIG. 10 is a diagram illustrating spatial variations in density of a printed image obtained when an image of equally spaced horizontal lines is printed at a constant density, in accordance with the second embodiment of the present invention. [Figure 21] FIG. 11 is a diagram for explaining why uneven shading becomes less noticeable in the second embodiment even when a nozzle located close to the slowest ink ejection nozzle is determined as the nozzle to be corrected. [Figure 22] FIG. 11 is a diagram for explaining the flow of ink in relation to the second embodiment. [Diagram 23] FIG. 11 is a block diagram showing a detailed functional configuration of a print data processing unit in a first modified example. [Figure 24] 13 is a flowchart showing a procedure for print output in the first modified example. [Diagram 25] FIG. 11 is a diagram for explaining a method for suppressing the flow of ink in the main scanning direction in relation to the first modified example. [Figure 26] FIG. 11 is a block diagram showing a detailed functional configuration of a print data processing unit in a second modified example. [Figure 27] 13 is a flowchart showing a procedure for determining a correction amount to be applied to the density value reduction process in the second modified example. [Figure 28] FIG. 13 is a diagram illustrating a correction amount adjustment chart in the second modified example. [Figure 29] FIG. 13 is a diagram showing an example of a pattern image selection screen in the second modified example. [Diagram 30] FIG. 13 is a diagram for explaining the effect of the third modified example. [Diagram 31] FIG. 13 is a diagram for explaining the effect of the fourth modified example. [Diagram 32] FIG. 13 is a diagram for explaining the movement of ink in a case where ink that lands later on the print medium is attracted toward ink that lands earlier on the print medium, relating to a fifth modified example. [Diagram 33] FIG. 13 is a diagram for explaining the flow of ink in the fifth modified example. [Diagram 34] FIG. 13 is a diagram for explaining the flow of ink in the fifth modified example. [Diagram 35] FIG. 13 is a schematic diagram showing the configuration of a head in a sixth modified example. [Diagram 36] FIG. 37 is an enlarged view of a portion indicated by reference numeral 84 in FIG. 35 in relation to the sixth modified example. [Figure 37] FIG. 13 is a schematic diagram showing a schematic configuration of a head unit for each ink color in a seventh modified example. [Figure 38] FIG. 13 is a diagram for explaining one nozzle set in the seventh modified example. [Figure 39] FIG. 23 is a schematic diagram showing the configuration of a portion of a head unit for each ink color in an eighth modified example. [Diagram 40] FIG. 13 is a diagram for explaining one nozzle set in a ninth modified example. [Diagram 41] FIG. 11 is a diagram showing an example of a configuration in which a large number of nozzles are arranged two-dimensionally, relating to a conventional example. [Diagram 42]11A and 11B are diagrams for explaining the movement of ink when ink that lands first on a print medium is attracted toward ink that lands later on the print medium. [Diagram 43] 1 is a schematic diagram illustrating a cross section of ink on a printing medium after ink has been ejected from eight nozzles that make up a nozzle set. FIG. [Diagram 44] 1A is a schematic diagram showing a cross section of ink on a print medium after ink has been ejected from 24 nozzles that make up three nozzle sets. FIG. [Diagram 45] 13 is a graph showing spatial variations in density of a printed image obtained when an image of equally spaced horizontal lines is printed at a constant density in a nozzle set having 32 nozzles. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0053] <0. Introduction> Before describing the embodiments of the present invention, the basic concept of the present invention will be described. When an inkjet printing device employs a head (see FIG. 41) in which a plurality of nozzles are arranged two-dimensionally, even if the amount of ink ejected from all the nozzles is the same, periodic shading unevenness appears according to the nozzle arrangement pattern, as shown in FIG. 44. In FIG. 44, when focusing on the area corresponding to each nozzle set composed of a plurality of nozzles corresponding to the spatial period of the nozzle arrangement pattern, the density of the area near the position corresponding to the nozzle that ejects ink last is higher than other areas. This phenomenon occurs because, as described above, if the difference in dynamic surface tension between the ink that lands first on the printing medium and the ink that lands second on the printing medium is large, the ink that lands first on the printing medium is attracted to the ink that lands second on the printing medium.

[0054] Here, it is considered that by reducing the amount of ink ejected from the nozzle that ejects ink last, the ink that has already landed on the printing medium is prevented from flowing toward the ink that landed last on the printing medium. Therefore, in the present invention, in order to suppress the "flow of ink" that causes periodic unevenness in density according to the nozzle arrangement pattern, typically, the amount of ink ejected from the nozzle that ejects ink last among the multiple nozzles that make up each nozzle set (hereinafter, for convenience, referred to as the "latest ink ejection nozzle") or the amount of ink ejected from a nozzle located relatively close to the latest ink ejection nozzle is reduced from the original amount. In this way, the ink ejection amount is adjusted in the present invention. If the ink ejection amount is not adjusted, as shown in part A of FIG. 1, an area with a higher density than other areas (see the dotted line portion with reference number 30) appears. As shown in part B of FIG. 1, if the ink ejection amount from the latest ink ejection nozzle is reduced from the original amount (see the dotted line portion with reference number 31), unevenness in density is suppressed. 1, uneven shading can be suppressed even if the amount of ink ejected from the nozzle adjacent to the slowest ink ejection nozzle is reduced (see the dotted line area marked with reference numeral 32). It is also considered that uneven shading can be suppressed by increasing the amount of ink ejected from the nozzle that ejects ink first or a nozzle relatively close to that nozzle.

[0055] In cases where the ink has high adhesion to the printing medium and the ink that lands on the printing medium later is attracted to the ink that landed on the printing medium earlier, the amount of ink ejected from nozzles that are relatively close to the slowest ink ejection nozzle among the multiple nozzles that make up each nozzle set is made larger than normal (see the fifth variant).

[0056] From the above, in the inkjet printing device according to the present invention, nozzles for which the amount of ink ejected is less than normal or nozzles for which the amount of ink ejected is more than normal are determined. Hereinafter, such nozzles will be referred to as "nozzles to be corrected."

[0057] Incidentally, each nozzle ejects ink of a predetermined droplet size. Therefore, in a case where the amount of ink ejected from a nozzle to be corrected is made smaller than the original amount, the ink ejection amount is actually controlled so that the average amount of ink ejected from the nozzle to be corrected is smaller than the average amount of ink that should have been ejected from the nozzle to be corrected. Similarly, in a case where the amount of ink ejected from a nozzle to be corrected is made larger than the original amount, the ink ejection amount is actually controlled so that the average amount of ink ejected from the nozzle to be corrected is larger than the average amount of ink that should have been ejected from the nozzle to be corrected.

[0058] Based on the above, an embodiment of the present invention will be described with reference to the accompanying drawings.

[0059] <1. First embodiment> <1.1 Overall configuration of the printing system> FIG. 2 is an overall configuration diagram of a printing system according to a first embodiment of the present invention. This printing system is composed of an inkjet printing device 10 and a print data generating device 30. The inkjet printing device 10 and the print data generating device 30 are connected to each other via a LAN 4. The print data generating device 30 generates image data for printing by performing a rasterization process on input data such as a PDF file. The image data generated by the print data generating device 30 is transmitted to the inkjet printing device 10 via the LAN 4. The inkjet printing device 10 is composed of a printing machine main body 200 and a print control device 100 that controls the operation of the printing machine main body 200. The inkjet printing device 10 outputs a print image on a printing medium such as printing paper or film based on the image data transmitted from the print data generating device 30 without using a printing plate. Note that the inkjet printing device 10 in this embodiment performs printing using water-based ink.

[0060] <1.2 Structure of the main body of the inkjet printing device> 3 is a schematic diagram showing an example of the configuration of the inkjet printing apparatus 10. As described above, the inkjet printing apparatus 10 is made up of the print control device 100 and the printing machine main body 200.

[0061] The printer body 200 includes a feed section 21 that supplies the print medium 5 (in this example, a roll-shaped print medium), a print mechanism 20 that prints on the print medium 5, and a take-up section 28 that winds up the print medium 5 into a roll after printing. The print mechanism 20 includes a first drive roller 22 for transporting the print medium 5 inside, a plurality of support rollers 23 for transporting the print medium 5 inside the print mechanism 20, a print section 24 that ejects ink onto the print medium 5, a drying mechanism 25 for drying the print medium 5 after printing, an in-line scanner 26 that captures the print image recorded on the print medium 5 by the print section 24, and a second drive roller 27 for outputting the print medium 5 from inside the print mechanism 20.

[0062] The print control device 100 controls the operation of the printing press main body 200 configured as described above. When a print output instruction command is given to the print control device 100, the print control device 100 controls the operation of the printing press main body 200 so that the print medium 5 is transported from the delivery unit 21 to the winding unit 28. Then, first, printing is performed on the print medium 5 by ejecting ink from the printing unit 24 onto the print medium 5, then the print medium 5 after printing is dried by the drying mechanism 25, and finally, the print image is captured by the inline scanner 26 as necessary. Note that the image data Dc obtained by capturing the print image by the inline scanner 26 is sent to the print control device 100.

[0063] 4 is a plan view showing a schematic configuration of the printing unit 24 in this embodiment. As shown in FIG. 4, the printing unit 24 is composed of a K (black) head unit 240K, a C (cyan) head unit 240C, an M (magenta) head unit 240M, and a Y (yellow) head unit 240Y. The head units 240K, 240C, 240M, and 240Y are aligned in the transport direction of the print medium 5. Each head unit 240 is composed of a plurality of heads (ink ejection heads) 241 (five in the example shown in FIG. 4) arranged in the main scanning direction. Each head 241 includes a number of nozzles that eject ink. Each nozzle of the head 241 included in the head unit 240K ejects K ink, each nozzle of the head 241 included in the head unit 240C ejects C ink, each nozzle of the head 241 included in the head unit 240M ejects M ink, and each nozzle of the head 241 included in the head unit 240Y ejects Y ink. Note that a head unit for W (white) and a head unit for a special color may also be provided. Also, in each head unit 240, a plurality of heads 241 may be arranged in a so-called staggered pattern as shown in FIG. 5.

[0064] FIG. 6 is a plan view showing the configuration of one head 241 in this embodiment. As shown in FIG. 6, a large number of nozzles 2 are arranged two-dimensionally in the head 241. Furthermore, eight nozzles 2 form one nozzle set, and multiple nozzle sets are arranged in the main scanning direction. FIG. 7 is a diagram showing the arrangement of eight nozzles 2(1) to 2(8) constituting one nozzle set on the head 241. As shown in FIG. 7, on the head 241, the eight nozzles 2(1) to 2(8) are arranged from the upstream side to the downstream side in the transport direction of the print medium 5 while being shifted slightly in the main scanning direction. Therefore, the positions of the eight nozzles 2(1) to 2(8) in the main scanning direction are different from each other, and the positions in the sub-scanning direction (the transport direction of the print medium) are also different from each other.

[0065] FIG. 8 is a diagram for explaining the order of ink ejection from eight nozzles 2(1) to 2(8) constituting one nozzle set. As shown in FIG. 8, ink is ejected onto the print medium 5 in the order of "nozzle 2(1), nozzle 2(2), nozzle 2(3), nozzle 2(4), nozzle 2(5), nozzle 2(6), nozzle 2(7), nozzle 2(8)". In other words, the nozzle 2 located on the upstream side of the transport direction of the print medium 5 ejects ink at an earlier timing, and the nozzle 2 located on the downstream side of the transport direction of the print medium 5 ejects ink at a later timing. From the above, nozzle 2(8) corresponds to the slowest ink ejection nozzle described above.

[0066] 7 and 8, it can be seen that, of the eight nozzles 2(1) to 2(8), nozzle 2(1) is arranged at one end in the main scanning direction, and, of the eight nozzles 2(1) to 2(8), nozzle 2(8) is arranged at the other end in the main scanning direction. Therefore, in this embodiment, when ink is ejected from the eight nozzles 2(1) to 2(8) included in each nozzle set to the same position in the transport direction of the printing medium 5 (for example, when a line extending in the main scanning direction is printed), ink is ejected first from nozzle 2(1) of the eight nozzles 2(1) to 2(8), which is arranged at one end in the main scanning direction, and ink is ejected last from nozzle 2(8), which is arranged at the other end in the main scanning direction, of the eight nozzles 2(1) to 2(8).

[0067] In this embodiment, the nozzle 2(8) that is the slowest ink ejection nozzle among the eight nozzles 2(1)-2(8) that make up each nozzle set (i.e., the nozzle located at the other end in the main scanning direction) is determined as the nozzle to be corrected. Then, the amount of ink ejected from nozzle 2(8) that has been determined as the nozzle to be corrected is adjusted so that the amount of ink ejected from the nozzle is less than it should be.

[0068] As described above, in this embodiment, one nozzle set is composed of eight nozzles 2(1) to 2(8), but the present invention is not limited to this. The present invention can be applied as long as one nozzle set is composed of N nozzles, where N is a natural number (typically an integer of 4 or more). More specifically, the present invention can be applied as long as a printing unit 24 is employed in which nozzle sets made of N nozzles 2 regularly arranged in a two-dimensional manner, where N is a natural number, are arranged continuously in the main scanning direction (width direction).

[0069] <1.3 Print control device> <1.3.1 Hardware configuration> FIG. 9 is a block diagram showing a hardware configuration of the print control device 100. As shown in FIG. 9, the print control device 100 includes a main body 110, an auxiliary storage device 121, an optical disk drive 122, a display unit 123, a keyboard 124, and a mouse 125. The main body 110 includes a CPU 111, a memory 112, a first disk interface unit 113, a second disk interface unit 114, a display control unit 115, an input interface unit 116, and a communication interface unit 117. The CPU 111, the memory 112, the first disk interface unit 113, the second disk interface unit 114, the display control unit 115, the input interface unit 116, and the communication interface unit 117 are connected to each other via a system bus. The auxiliary storage device 121 is connected to the first disk interface unit 113. The optical disk drive 122 is connected to the second disk interface unit 114. The display unit (display device) 123 is connected to the display control unit 115. A keyboard 124 and a mouse 125 are connected to the input interface unit 116. The printer main body 200 is connected to the communication interface unit 117 via a communication cable. The communication interface unit 117 is also connected to the LAN 4. The auxiliary storage device 121 is a magnetic disk device or the like. An optical disk 19 serving as a computer-readable recording medium such as a CD-ROM or DVD-ROM is inserted into the optical disk drive 122. The display unit 123 is a liquid crystal display or the like. The display unit 123 is used to display information desired by the operator. The keyboard 124 and the mouse 125 are used by the operator to input instructions to this print control device 100.

[0070] The auxiliary storage device 121 stores a print control program (program for controlling the execution of print processing by the printer body 200) 13. The CPU 111 reads the print control program 13 stored in the auxiliary storage device 121 into the memory 112 and executes it to realize various functions of the print control device 100. The memory 112 includes a RAM and a ROM. The memory 112 functions as a work area for the CPU 111 to execute the print control program 13 stored in the auxiliary storage device 121. The print control program 13 is stored in the computer-readable recording medium (non-transient recording medium) and provided. That is, the user, for example, purchases an optical disk 19 as a recording medium for the print control program 13, inserts it into the optical disk drive 122, reads out the print control program 13 from the optical disk 19, and installs it in the auxiliary storage device 121.

[0071] 9, the print control device 100 is provided with only one CPU 111 as a processor, but is not limited to this. A configuration using multiple processors, such as a configuration using multiple CPUs, can also be adopted. In addition to the CPU 111, a micro processing unit (MPU), a graphics processing unit (GPU), a digital signal processor (DSP), etc. can also be adopted as the processor.

[0072] <1.3.2 Configuration of the control unit> FIG. 10 is a block diagram showing a schematic functional configuration of the control unit 40 realized by executing the print control program 13 in the print control device 100. The control unit 40 generally includes a transport control unit 41, a print data processing unit 42, an ink ejection control unit 43, a drying control unit 44, and an image capture control unit 45. The transport control unit 41 controls the speed (transport speed) at which the transport mechanism 29 transports the print medium 5. The delivery unit 21, the first driving roller 22, the multiple support rollers 23, the second driving roller 27, and the winding unit 28 constitute the transport mechanism 29 (see FIG. 3), and in this embodiment, the transport mechanism 29 realizes the moving mechanism. The print data processing unit 42 performs various processes, which will be described later, on the image data 51 transmitted from the print data generating device 30, and finally generates halftone image data 59 including information indicating whether ink ejection to each pixel is on or off, or information on the dot size of the ink ejected to each pixel. The print data processing unit 42 will be described in detail later. The ink ejection control unit 43 controls the ejection of ink from each nozzle 2 of each head 241 included in each head unit 240 constituting the printing unit 24 based on halftone image data 59. For example, the ink ejection timing and ink dot size (amount of ink ejected) are controlled. The drying control unit 44 controls the temperature (drying temperature) when the drying mechanism 25 dries the print medium 5 after printing. The imaging control unit 45 controls the timing of imaging the print image by the in-line scanner 26. From the above, it can be understood that the control unit 40 has a function of controlling the amount of ink ejected from the multiple nozzles 2 included in the printing unit 24.

[0073] Fig. 11 is a block diagram showing a detailed functional configuration of the print data processing unit 42. As shown in Fig. 11, the print data processing unit 42 includes a color conversion processing unit 421, an image shift correction unit 422, a shading correction processing unit 423, a density value reduction processing unit 424, a threshold matrix storage unit 425, and a halftone processing unit 426. In the print data processing unit 42, corrections are made to the image data by a plurality of components. The image data output from the plurality of components will be distinguished below by reference symbols.

[0074] The color conversion processing unit 421 performs color conversion processing on the image data 51 sent from the print data generating device 30. The image data 51 sent from the print data generating device 30 is CMYK data or RGB data, and the ink jet printing device 10 in this embodiment ejects ink for each of the C, M, Y, and K ink colors. In order to perform color matching taking into account the characteristics of the input device and the characteristics of the printing machine body, which is the output device, the color conversion processing unit 421 performs color conversion processing based on the input ICC profile and the output ICC profile, and the input data is converted into CMYK data consisting of density value data for each ink color. The color conversion processing unit 421 outputs image data 52, which is the CMYK data obtained by the color conversion processing.

[0075] The image shift correction unit 422 performs an image shift process to shift the position of the image for each ink color so as to suppress misregistration between the multiple ink colors. Through this image shift process, the image data 52 is converted into image data 53. The image data 53 includes density values ​​of multiple pixels for each ink color. In this embodiment, the first image data is realized by this image data 53.

[0076] The shading correction processing unit 423 performs a shading correction process to compensate for the variation in the amount of ink ejected among the multiple nozzles. This shading correction process will be described with reference to FIG. 12. Here, attention is focused on five pixel portions 7(1) to 7(5) corresponding to five nozzles. It is assumed that the five pixel portions 7(1) to 7(5) are printed in a single color using ink of the same color ejected from the five nozzles. In FIG. 12, the numerical values ​​inside the rectangles corresponding to the pixel portions 7(1) to 7(5) respectively represent density values ​​in the image data. It is assumed that when printing is performed at a constant density (50%) without executing the shading correction process, the amount of ink ejected to the pixel portion 7(1) and the pixel portion 7(3) is (5 / 4) times that of the pixel portion 7(2), the amount of ink ejected to the pixel portion 7(4) is (5 / 6) times that of the pixel portion 7(2), and the amount of ink ejected to the pixel portion 7(5) is (5 / 3) times that of the pixel portion 7(2). At this time, the density value indicated by the portion marked with the reference numeral 71 is corrected to the density value indicated by the portion marked with the reference numeral 72 by the shading correction process. By correcting the density value data in this manner, the variation in the amount of ink ejected among the multiple nozzles is reduced, and the occurrence of uneven shading is suppressed. In this embodiment, the shading correction process is performed on the image data 53 output from the image shift correction unit 422. The image data 54 obtained in this manner is output from the shading correction processing unit 423. The image data 54 contains density values ​​of multiple pixels for each ink color, similar to the image data 53. In this embodiment, the second image data is realized by this image data 54.

[0077] The density value reduction processing unit 424 reduces the density values ​​of pixels corresponding to the nozzles to be corrected among the density values ​​of the pixels included in the image data 54 output from the shading correction processing unit 423 so as to suppress the occurrence of spatially periodic unevenness in shading, thereby generating image data 55 including the density values ​​of the plurality of pixels. Hereinafter, the processing performed by this density value reduction processing unit 424 is referred to as "density value reduction processing." The amount of reduction in density value (correction amount) by the density value reduction processing will be described later. In this embodiment, the third image data is realized by the image data 55 generated by the density value reduction processing.

[0078] The threshold matrix storage unit 425 holds a threshold matrix 56 in which a large number of thresholds are arranged in a matrix for comparison with the density value of each pixel included in the image data 55 during halftone processing (screening processing). This threshold matrix 56 is also called a "screen pattern." In this embodiment, the threshold matrix storage unit 425 holds a threshold matrix 56 similar to a conventional one. However, in a first modified example described later, a threshold matrix 56 different from the conventional one is held in the threshold matrix storage unit 425.

[0079] The halftone processing unit 426 performs halftone processing on the image data 55. More specifically, the halftone processing unit 426 generates halftone image data 59 by halftoning the image data 55 obtained by the density value reduction processing using a threshold matrix 56. This halftone image data 59 is data for controlling the ejection of ink from each nozzle 2 included in the multiple heads 241 that make up each head unit 240. The halftone image data 59 is provided to the ink ejection control unit 43, which controls the ejection of ink from each nozzle 2 based on the halftone image data 59 as described above.

[0080] <1.4 Print output procedure> The procedure for print output (printing method) in this printing system will be described with reference to the flowchart shown in Fig. 13. Here, it is assumed that the correction amount (amount of reduction in density value) when reducing the density value of a pixel corresponding to a nozzle to be corrected by the density value reduction process has already been determined.

[0081] First, a PDF file, for example, including data to be printed is provided as input data to the print data generating device 30 (step S10). Next, the print data generating device 30 performs a rasterization process on the input data to generate image data 51 for printing (step S20). Then, the image data 51 generated by the print data generating device 30 is transferred to the print control device 100 in the inkjet printing device 10 (step S30).

[0082] Next, the color conversion processing unit 421 performs the color conversion processing described above (step S40). After the color conversion processing is completed, the image shift correction unit 422 performs the image shift processing described above (step S50). After the image shift processing is completed, the shading correction processing unit 423 performs the shading correction processing described above (step S60).

[0083] Thereafter, the density value reduction processing unit 424 performs the density value reduction processing described above (step S70). Then, halftone processing is performed on the image data 55 generated in step S70, and halftone image data 59 is generated (step S80).

[0084] Finally, printing is performed by the printing unit 24 based on the halftone image data 59 generated in step S80 (step S90). That is, an image is formed on the printing medium 5 by controlling the ejection of ink from the multiple nozzles 2 included in each head 241 constituting the printing unit 24 based on the halftone image data 59.

[0085] By the way, both the shading correction process and the density value reduction process are processes for correcting the density value of image data, but the reason why the density value reduction process for suppressing the occurrence of spatial periodic unevenness in shading is performed separately from the shading correction process will be described below. Since the spatial periodic unevenness in shading is caused by the flow of ink between a plurality of nozzles, the difference in shading in the spatial periodic unevenness in shading is greater than the difference in shading in the nozzle unit that is to be suppressed by the shading correction process. In general, the shading correction process uses a moving average to correct the density value. Even if the method is used to suppress the occurrence of spatial periodic unevenness in shading, a good correction effect cannot be obtained because the spatial change in shading is dulled by the moving average. In addition, when a method of correcting the density value without using the moving average is adopted, the correction amount of the density value of the pixel corresponding to each nozzle becomes large. As a result, the image after correction does not have a smooth spatial change in density, and the spatial change in density is visually recognized as streaks. Taking the above into consideration, a configuration is adopted in which a density value reduction process for suppressing the occurrence of spatially periodic shading unevenness resulting from the nozzle arrangement is performed separately from the shading correction process.

[0086] In this embodiment, the rasterization step is realized by step S20, the shading correction step is realized by step S60, the density value reduction step is realized by step S70, the halftone image generation step is realized by step S80, and the ink ejection step is realized by step S90.

[0087] <1.5 Density value reduction process> In the density value reduction process, the image data is corrected so as to reduce the density value of the pixel corresponding to the nozzle to be corrected. In order to enhance the effect of suppressing the occurrence of uneven shading, it is preferable to determine in advance a suitable correction amount (amount of reduction in density value) in the density value reduction process. In this regard, the suitable correction amount differs depending on the density of the image to be printed. Therefore, it is preferable to determine in advance a suitable correction amount according to the density of the image to be printed.

[0088] Therefore, in this embodiment, a suitable correction amount according to the density of the image to be printed is determined in advance by the manufacturer of this inkjet printing device 10, and the correction amount data is stored in the auxiliary storage device 121 of the print control device 100. In other words, when print output is executed in this printing system, a density value reduction process is performed based on the correction amount data stored in the auxiliary storage device 121.

[0089] In order to obtain a suitable correction amount according to the density of the image to be printed, in this embodiment, the range from the minimum value to the maximum value that the pixel density value can take is divided into a plurality of stages, and a test chart (hereinafter referred to as a "correction amount adjustment chart") 61 as shown in FIG. 14 is prepared for each stage. For example, the range from the minimum value to the maximum value that the pixel density value can take is divided into 10 stages, and a correction amount adjustment chart 61 is prepared for each of the 10 stages. The correction amount adjustment chart 61 includes a plurality of pattern images 611 with different correction amounts (amount of reduction in density value). In the example shown in FIG. 14, the correction amount of the pattern image 611 arranged at the bottom of the figure is smaller. For each stage, the pattern image 611 with the least noticeable unevenness in shading is selected from the plurality of pattern images 611 based on the printed image obtained by actually printing the correction amount adjustment chart 61 as described above with the target inkjet printing device 10. The correction amount corresponding to the selected pattern image 611 becomes the correction amount (amount of reduction in density value) actually used in the density value reduction process. The selection of the pattern image 611 with the least noticeable shading unevenness does not necessarily have to be based on the visual confirmation of the printed image. For example, the control unit 40 may select the pattern image 611 based on the analysis result of the imaging data Dc obtained by the in-line scanner 26 imaging the printed image of the correction amount adjustment chart 61 (for example, based on the uniformity of the luminance value).

[0090] In addition, in this embodiment, the ink ejection amount is adjusted so that only the amount of ink ejected from the nozzles to be corrected among the eight nozzles 2(1) to 2(8) constituting each nozzle set is reduced from the original amount, but this is not limited to this. The ink ejection amount may be adjusted so that the amount of ink ejected from each of the multiple nozzles among the eight nozzles 2(1) to 2(8) constituting each nozzle set is reduced from the original amount. Therefore, a test chart (hereinafter referred to as a "correction width adjustment chart") 62 as shown in FIG. 15, which includes multiple pattern images 621 with different correction widths (the number of nozzles for which the ink ejection amount is reduced), may be prepared for each of the above stages, and the correction width may be determined based on the printed image of the correction width adjustment chart 62. Regarding the pattern image 621 included in the correction width adjustment chart 62, for the portion marked with the reference numeral 622 in FIG. 15, the correction amount (amount of reduction in density value) of the pixel corresponding to the nozzle located at the center of the portion is made relatively large, and the correction amount (amount of reduction in density value) of the pixel corresponding to the nozzles on both sides of the nozzle is made relatively small. In this way, a plurality of nozzles to be corrected may be determined for each nozzle set, and the amount of correction may be made different between these plurality of nozzles to be corrected.

[0091] In this embodiment, the density value reduction process is performed using data on the correction amount determined based on the printed image of the correction amount adjustment chart 61 as shown in Fig. 14. As described above, in this density value reduction process, since ink droplets of a predetermined size are ejected from each nozzle 2, the density value of the pixel corresponding to the nozzle to be corrected among the density values ​​of the pixels included in the image data 54 is reduced so that the average amount of ink ejected from the nozzle to be corrected is smaller than the average amount of ink that should be ejected from the nozzle to be corrected. As a result, in a case where the nozzle to be corrected ejects ink at nine ejection times out of ten ejection times, for example, the density value reduction process causes the nozzle to eject ink at seven ejection times out of ten ejection times.

[0092] As described above, the control unit 40 in this embodiment selects nozzle 2(8) located at the other end in the main scanning direction (width direction) of the eight nozzles 2(1) to 2(8) that make up the nozzle set (see Figures 7 and 8) as the nozzle to be corrected, and controls the amount of ink ejected from the nozzle to be corrected so that the average amount of ink ejected from the nozzle to be corrected is smaller than the average amount of ink that should originally be ejected from the nozzle to be corrected.

[0093] Incidentally, the correction amount and correction width in the density value reduction process may be different for each color. That is, in printing an image portion where two or more ink colors should have the same density, the control unit 40 may make the difference between the average amount of ink ejected from the nozzles to be corrected and the average amount of ink that should originally be ejected from the nozzles to be corrected (hereinafter referred to as the "average reduction value") different for each ink color. As an example, taking into consideration the ink characteristics and human visual characteristics, the average reduction value for M color may be made larger than the average reduction value for Y color, the average reduction value for K color may be made larger than the average reduction value for M color, and the average reduction value for C color may be made larger than the average reduction value for K color. In this way, the occurrence of spatial periodic unevenness in shading in an image portion printed using inks of multiple colors is more effectively suppressed.

[0094] <1.6 Ink flow> The flow of ink will be described with reference to Figures 16 to 18. Here, we focus on two nozzle sets consisting of five nozzles 2(1) to 2(5). One of the two nozzle sets is called the "first nozzle set" and the other is called the "second nozzle set." In Figures 16 to 18, ink ejected from the nozzles and landing on the printing medium 5 is represented by a rectangle (the same applies to Figures 22, 33, and 34). Of times t1 to t10, times t1, t3, t5, t7, and t9 are the times when ink lands on the printing medium 5. More specifically, ink ejected from nozzle 2(1) lands on the printing medium 5 at time t1, ink ejected from nozzle 2(2) lands on the printing medium 5 at time t3, ink ejected from nozzle 2(3) lands on the printing medium 5 at time t5, ink ejected from nozzle 2(4) lands on the printing medium 5 at time t7, and ink ejected from nozzle 2(5) lands on the printing medium 5 at time t9. It is assumed that the same amount of ink is ejected from all nozzles 2.

[0095] 16 shows the flow of ink in a case where the pulling of ink due to differences in dynamic surface tension can be ignored. In this case, since there is no pulling of ink due to differences in dynamic surface tension, ink discharged from each nozzle 2 and landing on the printing medium 5 does not flow toward ink discharged from other nozzles 2 and landing on the printing medium 5. Therefore, at time t10, no unevenness in shading appears at all.

[0096] FIG. 17 shows the flow of ink in a case where the ink that has landed on the printing medium 5 first is attracted to the ink that has landed on the printing medium 5 later and where the density value reduction process described above is not performed (i.e., a typical conventional case). After the ink ejected from nozzle 2(2) lands on the printing medium 5 at time t3, at time t4, the ink that has landed on the printing medium 5 at time t1 flows toward the ink that has landed on the printing medium 5 at time t3. In this way, the ink that has landed on the printing medium 5 first flows toward the ink that has landed on the printing medium 5 later, so that at time t10, the density at the position corresponding to nozzle 2(5) is significantly higher than the density at the positions corresponding to the other nozzles 2(1) to 2(4). Note that the ink ejected from nozzle 2(1) included in the first nozzle set hardly flows toward the ink ejected from nozzle 2(5) included in the second nozzle set. This is because there is a large time difference between the timing when ink is ejected from nozzle 2(1) and the timing when ink is ejected from nozzle 2(5), and the ink ejected from nozzle 2(1) has dried to a certain extent by the time the ink ejected from nozzle 2(5) lands on printing medium 5.

[0097] FIG. 18 shows the flow of ink in a case where the ink that landed on the printing medium 5 first is attracted to the ink that landed on the printing medium 5 later and the density value reduction process described above is performed (i.e., the case of this embodiment). The flow of ink up to time t8 is the same as the case shown in FIG. 17. In this case, a smaller amount of ink is ejected from the nozzle 2 (5) than the amount that should be ejected. Therefore, as shown by the dotted line portion labeled with reference numeral 64 in FIG. 18, the amount of ink that landed on the printing medium 5 at time t9 is smaller than the original amount. Therefore, the magnitude of the dynamic surface tension of the ink that landed on the printing medium 5 at time t9 is smaller than the original amount, and the ink that has already landed on the printing medium 5 is prevented from flowing toward the ink that landed on the printing medium 5 at time t9. As a result, unlike the case shown in FIG. 17, there is almost no unevenness in shading at time t10.

[0098] <1.7 Effects> According to this embodiment, a large number of nozzles 2 are arranged two-dimensionally in the head 241 constituting the printing unit 24 of the inkjet printing device 10, and when ink is ejected from the eight nozzles 2(1) to 2(8) constituting each nozzle set to the same position in the transport direction of the print medium 5, ink is ejected first from the nozzle 2(1) arranged at one end of the eight nozzles 2(1) to 2(8) in the main scanning direction (width direction), and ink is ejected last from the nozzle 2(8) arranged at the other end of the eight nozzles 2(1) to 2(8) in the main scanning direction (width direction). In this configuration, the nozzle 2(8) that ejects ink last is determined as the nozzle to be corrected, and the amount of ink ejected is controlled so that the average amount of ink ejected from the nozzle to be corrected is smaller than the average amount of ink that should originally be ejected from the nozzle to be corrected. Therefore, after ink ejected from the nozzle to be corrected lands on the printing medium 5, the ink that has already landed on the printing medium 5 is less likely to flow toward the ink that was ejected from the nozzle to be corrected and landed on the printing medium 5 than in the past. This makes it less likely that the density of the area near the position corresponding to the nozzle 2 that last ejected ink becomes higher than other areas. In other words, the occurrence of uneven shading is suppressed.

[0099] Fig. 19 shows the difference in shading between when the density value reduction process is not performed and when it is performed for an inkjet printing device 10 employing a head 241 in which a nozzle set is made up of 32 nozzles 2. The dotted line labeled with reference numeral 71 represents the spatial change in luminance value when the density value reduction process is not performed, and the solid line labeled with reference numeral 72 represents the spatial change in luminance value when the density value reduction process is performed. It can be seen from Fig. 19 that the spatial change in luminance value is smaller when the density value reduction process is performed than when the density value reduction process is not performed. In other words, it can be seen that the occurrence of uneven shading is suppressed by performing the density value reduction process.

[0100] As described above, according to this embodiment, in an inkjet printing device 10 in which a large number of nozzles 2 are arranged two-dimensionally, the occurrence of periodic spatial unevenness in shading caused by the difference in dynamic surface tension between ink that lands first on the printing medium 5 and ink that lands later on the printing medium 5 is effectively suppressed. In addition, because the occurrence of uneven shading is suppressed, there is less need to reprint compared to the past, making it possible to reduce consumption of the printing medium 5 and ink. In this way, it is possible to contribute to the achievement of the SDGs (Sustainable Development Goals).

[0101] <2. Second embodiment> <2.1 Overview and composition> In the first embodiment described above, of the eight nozzles 2(1)-2(8) (see FIG. 7) that make up the nozzle set, the nozzle 2(8) that is located at the other end in the main scanning direction and that ejects ink last (the slowest ink ejection nozzle) is designated as the nozzle to be corrected (note that the nozzle that ejects ink first is located at one end in the main scanning direction), and the ink ejection amount from this nozzle to be corrected is controlled so that it is less than the actual amount. In this regard, it is believed that the occurrence of uneven shading can be suppressed even if the ink ejection amount from a nozzle relatively close to the slowest ink ejection nozzle is made less than the actual amount.

[0102] Therefore, in the case of a nozzle set consisting of 32 nozzles, the applicant determined that the nozzle relatively close to the slowest ink ejection nozzle was the nozzle to be corrected, adjusted the ink ejection amount, and checked the unevenness in shading of the printed image obtained by adjusting the amount of ink ejection. This will be explained below with reference to Figures 20 and 21.

[0103] 20, the graphs labeled with reference numerals 651 to 654 show spatial changes in density of four printed images obtained when an image of equally spaced horizontal lines was printed at a constant density under four different conditions. In more detail, the dotted line graph labeled with reference numeral 651 represents the results obtained with a conventional configuration (i.e., a configuration in which the ink ejection amount is not adjusted), the solid line graph labeled with reference numeral 652 represents the results obtained by adjusting the ink ejection amount by setting the slowest ink ejection nozzle as the nozzle to be corrected, the thick dotted line graph labeled with reference numeral 653 represents the results obtained by setting the nozzle that ejects ink one nozzle before the slowest ink ejection nozzle as the nozzle to be corrected and adjusting the ink ejection amount, and the thick solid line graph labeled with reference numeral 654 represents the results obtained by setting the nozzle that ejects ink two nozzles before the slowest ink ejection nozzle as the nozzle to be corrected and adjusting the ink ejection amount. The difference in shading in the conventional configuration corresponds to the length of the arrow labeled 661, the difference in shading when the slowest ink-ejecting nozzle is set as the nozzle to be corrected corresponds to the length of the arrow labeled 662, the difference in shading when the nozzle that ejects ink just before the slowest ink-ejecting nozzle is set as the nozzle to be corrected corresponds to the length of the arrow labeled 663, and the difference in shading when the nozzle that ejects ink two nozzles before the slowest ink-ejecting nozzle is set as the nozzle to be corrected corresponds to the length of the arrow labeled 664. It can be seen from Fig. 20 that the difference in shading is significantly smaller compared to the conventional configuration even when the nozzle that ejects ink just before the slowest ink-ejecting nozzle or the nozzle that ejects ink two nozzles before the slowest ink-ejecting nozzle is set as the nozzle to be corrected.

[0104] The arrows labeled with reference numbers 661 to 664 in Figure 20 are transcribed in Figure 21. The length of the arrow labeled with reference number 669 in Figure 21 corresponds to the shading difference at which spatial periodic unevenness in shading starts to become noticeable. It can be seen from Figure 21 that when the ink ejection amount is adjusted by determining as the nozzle to be corrected the slowest ink ejection nozzle, the nozzle that ejects ink one nozzle before the slowest ink ejection nozzle, or the nozzle that ejects ink two nozzles before the slowest ink ejection nozzle.

[0105] In view of the above, in this embodiment, the nozzle that ejects ink one nozzle before the slowest ink ejection nozzle or the nozzle that ejects ink two nozzles before the slowest ink ejection nozzle is determined as the nozzle to be corrected, and the ink ejection amount is adjusted.

[0106] The only difference from the first embodiment is the nozzle 2 determined as the nozzle to be corrected, so the overall configuration of the printing system (see FIG. 2), the configuration of the main body of the inkjet printing device (see FIGS. 3 to 7), and the configuration of the print control device 100 (see FIGS. 9 to 11) are the same as those of the first embodiment. The print output procedure (see FIG. 13) is also the same as that of the first embodiment.

[0107] <2.2 Ink flow> The flow of ink in this embodiment will be described with reference to Fig. 22. Note that, again, attention will be focused on two nozzle sets each consisting of five nozzles 2(1) to 2(5), and a case will be described in which the nozzle that ejects ink immediately before the slowest ink ejection nozzle is determined to be the nozzle to be corrected.

[0108] The flow of ink up to time t6 is the same as in the first embodiment (the case shown in FIG. 18). In this case, a smaller amount of ink is ejected from nozzle 2(4) than the amount that should have been ejected. Therefore, as shown by the dotted line portion with reference numeral 69 in FIG. 22, the amount of ink that lands on the printing medium 5 at time t7 is smaller than the original amount. Therefore, the magnitude of the dynamic surface tension of the ink that lands on the printing medium 5 at time t7 is smaller than the original amount, and the ink that has already landed on the printing medium 5 is prevented from flowing toward the ink that has landed on the printing medium 5 at time t7. The amount of ink that should have been ejected from nozzle 2(5) is ejected. At this time, the ink that has landed on the printing medium 5 before time t5 is prevented from flowing toward the ink that has landed on the printing medium 5 at time t7, so that the ink that has already landed on the printing medium 5 is also prevented from flowing toward the ink that has landed on the printing medium 5 at time t9, compared to the conventional case. As a result, there is almost no unevenness in shading at time t10.

[0109] <2.3 Effects> According to this embodiment, among the multiple nozzles 2 constituting each nozzle set, a nozzle that ejects ink one before the slowest ink ejection nozzle or a nozzle that ejects ink two before the slowest ink ejection nozzle is determined as a correction target nozzle, and the ink ejection amount is controlled so that the average amount of ink ejected from the correction target nozzle is smaller than the average amount of ink that should be ejected from the correction target nozzle. Therefore, after the ink ejected from the correction target nozzle lands on the printing medium 5, the ink that has already landed on the printing medium 5 is prevented from flowing toward the ink ejected from the correction target nozzle and landed on the printing medium 5, compared to the conventional method. This also prevents the ink that has already landed on the printing medium 5 from flowing toward the ink ejected from the slowest ink ejection nozzle and landed on the printing medium 5, compared to the conventional method. As a result, the phenomenon that the density of the area near the position corresponding to the slowest ink ejection nozzle is higher than other areas is less likely to occur. In other words, the occurrence of uneven shading is suppressed. As described above, in an inkjet printing device 10 in which a large number of nozzles 2 are arranged in a two-dimensional manner, as in the first embodiment described above, the occurrence of periodic spatial unevenness in shading caused by the difference in dynamic surface tension between the ink that lands first on the printing medium 5 and the ink that lands later on the printing medium 5 is effectively suppressed.

[0110] <3. Modifications> The following describes the modified example, focusing mainly on the differences from the above-described embodiments.

[0111] <3.1 First modified example> In each of the above embodiments, in order to reduce the amount of ink ejected from the nozzle to be corrected from the original amount, a density value reduction process was performed to reduce the density value of a pixel corresponding to the nozzle to be corrected among the density values ​​of a plurality of pixels contained in the post-shading correction image data 54. However, the present invention is not limited to this, and it is also possible to reduce the amount of ink ejected from the nozzle to be corrected from the original amount by a process other than the density value reduction process, as in this modified example.

[0112] FIG. 23 is a block diagram showing a detailed functional configuration of the print data processing unit 42 in this modified example. Unlike the first embodiment (see FIG. 11), the print data processing unit 42 does not include a density value reduction processing unit 424. In this modified example, a threshold matrix 56 different from the conventional one is held in a threshold matrix storage unit 425. The multiple thresholds included in the threshold matrix 56 are set so that the average amount of ink ejected from the nozzles to be corrected is smaller than the average amount of ink ejected from the nozzles other than the nozzles to be corrected. By using such a threshold matrix 56 during halftone processing, the amount of ink ejected from the nozzles to be corrected is reduced from the original amount. Note that the halftone processing can also be performed using an error diffusion method.

[0113] In this modified example, since the density value reduction processing unit 424 is not provided, the image data 54 generated by the shading correction processing is provided to the halftone processing unit 426. The halftone processing unit 426 then halftones the image data 54 using a threshold matrix 56 to generate halftone image data 59.

[0114] FIG. 24 is a flowchart showing the procedure (printing method) of print output in this modified example. The processes of steps S110 to S160 are similar to the processes of steps S10 to S60 in the first embodiment (see FIG. 13). In this modified example, unlike the first embodiment, halftone processing is performed after shading correction processing (step S170). In this halftone processing, as described above, a threshold matrix 56 different from the conventional one is used. After the halftone processing is completed, printing is performed by the printing unit 24 based on the halftone image data 59 generated in step S170 (step S180).

[0115] Typically, the halftone image data 59 is data that holds, for each pixel, a value "1" that indicates the ejection of ink or a value "0" that indicates no ejection of ink. In this regard, if the density value of a certain pixel in the image data 54 is greater than the corresponding threshold value in the threshold matrix 56, the value of the certain pixel in the halftone image data 59 is "1", and if the density value of the certain pixel in the image data 54 is equal to or less than the corresponding threshold value in the threshold matrix 56, the value of the certain pixel in the halftone image data 59 is "0". In this modified example, for example, the threshold value in the threshold matrix 56 that corresponds to a pixel from which ink is ejected from the nozzle to be corrected is set to a value higher than normal, thereby reducing the amount of ink ejected from the nozzle to be corrected. Note that, in this embodiment, it is assumed that there is one type of ink dot size (droplet size), but the present invention can also be applied to a case where multiple types of dot sizes (for example, three sizes of L, M, and S) are prepared.

[0116] According to this modification, the amount of ink ejected from the nozzles to be corrected is reduced from the original amount by using a threshold matrix 56 different from the conventional one during halftone processing, so there is no need to perform correction for suppressing the occurrence of spatial periodic unevenness in shading on image data before halftoning. In other words, unlike the first and second embodiments, no additional processing is required in addition to the conventional processing when executing print output. As described above, according to this modification, the occurrence of spatial periodic unevenness in shading caused by the difference in dynamic surface tension between the ink that lands first on the print medium 5 and the ink that lands later on the print medium 5 is effectively suppressed without requiring any additional processing in addition to the conventional processing for print output.

[0117] In addition, by performing halftone processing using a threshold matrix 56 in which a large number of thresholds are arranged in a matrix so that the value "1" in the halftone image data 59 is more likely to continue in the transport direction of the printing medium 5 and less likely to continue in the main scanning direction than usual, the ink flow in the main scanning direction is suppressed, and the occurrence of the spatial periodic shading unevenness described above can be suppressed. In this regard, when halftone processing is performed on data of a solid image of a certain predetermined density using the conventional threshold matrix 56, halftone image data 59 as shown in part A of FIG. 25 is obtained. In addition, in FIG. 25, black parts represent values ​​"1" and white parts represent values ​​"0". In contrast, when halftone processing is performed on data of a solid image of a certain predetermined density using a threshold matrix 56 in which thresholds are arranged so that the value "1" is more likely to continue in the transport direction of the printing medium 5 and less likely to continue in the main scanning direction than usual, halftone image data 59 as shown in part B of FIG. 25 is obtained. The shortest distance in the main scanning direction between two pixels with a value of "1" is one pixel in the example shown in part A of FIG. 25 (see the arrow with reference number 731), but is two pixels in the example shown in part B of FIG. 25 (see the arrow with reference number 732). In this way, by using a threshold matrix 56 that makes it difficult for the value "1" to be continuous in the main scanning direction, the occurrence of overlapping of ink ejected from two different nozzles 2 in the main scanning direction is suppressed. As a result, the flow of ink in the main scanning direction is suppressed, and the occurrence of the above-mentioned spatial periodic unevenness in shading is suppressed.

[0118] <3.2 Second modified example> In each of the above embodiments, the correction amount (amount of reduction in density value) applied to the density value reduction process is determined in advance by the manufacturer of the inkjet printing device 10, and the data on the correction amount is stored in advance in the auxiliary storage device 121 of the print control device 100 so as to be referenced during the density value reduction process. However, the present invention is not limited to this, and the correction amount to be applied to the density value reduction process may be determined by the user based on the printed image of the correction amount adjustment chart 61. Such an embodiment will be described below as a second modified example.

[0119] Fig. 26 is a block diagram showing a detailed functional configuration of the print data processing unit 42 in this modified example. In addition to the components in the first embodiment (see Fig. 11), the print data processing unit 42 is provided with a correction amount determination unit 427. The correction amount determination unit 427 determines a correction amount 58 to be applied to the density value reduction process based on selection result data 57, which will be described later. The density value reduction processing unit 424 reduces the density value of a pixel corresponding to the nozzle to be corrected, among the density values ​​of a plurality of pixels included in the image data 54 output from the shading correction processing unit 423, based on the correction amount 58 determined by the correction amount determination unit 427, thereby generating image data 55 including the density values ​​of the plurality of pixels.

[0120] Next, a procedure for determining the correction amount 58 to be applied to the density value reduction process will be described with reference to the flowchart shown in FIG. 27. First, a correction amount adjustment chart 61 is printed using the inkjet printing device 10 provided in this printing system (step S210). FIG. 28 is a diagram showing a schematic diagram of the correction amount adjustment chart 61 in this modified example. As shown in the dotted line portion indicated by reference numeral 619 in FIG. 28, the correction amount adjustment chart 61 in this modified example includes an image showing a number for identifying a plurality of pattern images 611. In this example, the correction amount adjustment chart 61 including six pattern images 611 identified by numbers A1 to A6 is printed.

[0121] Next, the user checks the printed image of the correction amount adjustment chart 61 (Step S220). In detail, the degree of shading unevenness of each of the six pattern images 611 included in the correction amount adjustment chart 61 is checked by the user.

[0122] After that, the user selects the pattern image 611 with the least noticeable shading unevenness among the six pattern images 611 (step S230). In this step S230, the display unit 123 of the print control device 100 displays a pattern image selection screen 80 as shown in FIG. 29, for example. The pattern image selection screen 80 includes a selection area 810 configured to allow one pattern image to be selected for each of the above-mentioned stages (density levels), an OK button 820, and a cancel button 830. After the user selects the number of the pattern image 611 with the least noticeable shading unevenness for each density level, he or she presses the OK button 820. As a result, the selection result data 57 indicating the content selected by the user is provided to the correction amount determination unit 427 (see FIG. 26). Note that when the cancel button 830 is pressed, the pattern image selection screen 80 is hidden.

[0123] After the user has finished selecting the pattern image 611, the correction amount determination unit 427 determines the correction amount 58 to be applied to the density value reduction process based on the selection result data 57 (step S240).

[0124] In this modified example, step S210 realizes a test chart printing step, and step S230 realizes a pattern image selection step.

[0125] According to this modification, the user of the inkjet printing device 10 can print the correction amount adjustment chart 61 and vary the amount of decrease in density value of the pixel corresponding to the nozzle to be corrected depending on the print result. This makes it possible to effectively suppress the occurrence of spatial periodic unevenness in shading by taking into consideration the state of the inkjet printing device 10 at the time of printing and the printing medium 5 used for printing.

[0126] Similarly, the correction width may also be determined by the user based on the printed image of the correction width adjustment chart 62 .

[0127] <3.3 Third modified example> In the first embodiment, the slowest ink-ejecting nozzle is determined as the nozzle to be corrected, and in the second embodiment, the nozzle that ejects ink one nozzle before the slowest ink-ejecting nozzle or the nozzle that ejects ink two nozzles before the slowest ink-ejecting nozzle is determined as the nozzle to be corrected. In this way, in the first and second embodiments, the ink ejection amount is always adjusted with the same nozzle 2 as the nozzle to be corrected in one inkjet printing device 10. However, the present invention is not limited to this, and it is also possible to selectively determine a plurality of nozzles 2 as the nozzle to be corrected in one inkjet printing device 10. Such an embodiment will be described below as a third modified example.

[0128] In this modified example, the first to third nozzles of the multiple nozzles constituting the nozzle set are selectively determined as the nozzles to be corrected based on the other end side in the main scanning direction (width direction). That is, the slowest ink-ejecting nozzle, the nozzle that ejects ink one nozzle before the slowest ink-ejecting nozzle, and the nozzle that ejects ink two nozzles before the slowest ink-ejecting nozzle are selectively determined as the nozzles to be corrected. Note that when ink is ejected from the multiple nozzles included in the nozzle set to the same position in the transport direction of the printing medium 5, ink is ejected first from any of the multiple nozzles that are first to third based on one end side in the main scanning direction, and ink is ejected last from any of the multiple nozzles that are first to third based on the other end side in the main scanning direction.

[0129] 20 and 21, the difference in shading (see the arrow labeled 663) when the nozzle that ejects ink just before the slowest ink-ejecting nozzle is set as the nozzle to be corrected is larger than the difference in shading (see the arrow labeled 662) when the slowest ink-ejecting nozzle is set as the nozzle to be corrected, and the difference in shading (see the arrow labeled 664) when the nozzle that ejects ink just before the slowest ink-ejecting nozzle is set as the nozzle to be corrected is larger than the difference in shading (see the arrow labeled 663) when the nozzle that ejects ink just before the slowest ink-ejecting nozzle is set as the nozzle to be corrected. In other words, the difference in shading becomes larger the farther the position of the nozzle set as the nozzle to be corrected is from the position of the slowest ink-ejecting nozzle in the main scanning direction.

[0130] Therefore, in this modified example, when a nozzle that ejects ink one nozzle before the slowest ink-ejecting nozzle or a nozzle that ejects ink two nozzles before the slowest ink-ejecting nozzle is set as the nozzle to be corrected, a different correction amount (amount of reduction in density value) is applied in the density value reduction process according to the nozzle set as the nozzle to be corrected, so that the difference in density can be reduced to the same extent as when the slowest ink-ejecting nozzle is set as the nozzle to be corrected. In detail, when a nozzle that ejects ink one nozzle before the slowest ink-ejecting nozzle is set as the nozzle to be corrected, a larger correction amount is applied in the density value reduction process than when the slowest ink-ejecting nozzle is set as the nozzle to be corrected, and when a nozzle that ejects ink two nozzles before the slowest ink-ejecting nozzle is set as the nozzle to be corrected, a larger correction amount is applied in the density value reduction process than when the nozzle that ejects ink one nozzle before the slowest ink-ejecting nozzle is set as the nozzle to be corrected. In other words, if the difference between the average amount of ink ejected from the nozzle to be corrected and the average amount of ink that should originally be ejected from the nozzle to be corrected is defined as the reduced ink amount, in this modified example, the reduced ink amount when the second nozzle positioned based on the other end side of the main scanning direction is determined as the nozzle to be corrected is greater than the reduced ink amount when the first nozzle positioned based on the other end side of the main scanning direction (slowest ink ejecting nozzle) is determined as the nozzle to be corrected, and the reduced ink amount when the third nozzle positioned based on the other end side of the main scanning direction is determined as the nozzle to be corrected is greater than the reduced ink amount when the second nozzle positioned based on the other end side of the main scanning direction is determined as the nozzle to be corrected.

[0131] According to this modified example as described above, a graph showing the same content as the graph shown in Fig. 20 (a graph showing spatial changes in density of a printed image obtained when an image of equally spaced horizontal lines is printed at a constant density) is as shown in Fig. 30. From Fig. 30, it can be seen that the same degree of difference in shading appears when the slowest ink-ejecting nozzle is set as the nozzle to be corrected, when the nozzle that ejects ink one nozzle before the slowest ink-ejecting nozzle is set as the nozzle to be corrected, and when the nozzle that ejects ink two nozzles before the slowest ink-ejecting nozzle is set as the nozzle to be corrected. In this way, the occurrence of spatial periodic unevenness in shading is effectively suppressed regardless of the nozzle set as the nozzle to be corrected.

[0132] <3.4 Fourth modified example> In each of the above embodiments, only one nozzle is determined as the nozzle to be corrected at a time. However, the present invention is not limited to this, and two nozzles can be determined as the nozzle to be corrected at a time. Such an embodiment will be described below as a fourth modified example.

[0133] In this modified example, the slowest ink-ejecting nozzle and the nozzle that ejects ink one nozzle before the slowest ink-ejecting nozzle or the nozzle that ejects ink two nozzles before the slowest ink-ejecting nozzle are simultaneously determined as nozzles to be corrected. That is, two nozzles are simultaneously determined as nozzles to be corrected. Then, from among the multiple nozzles constituting the nozzle set, the two nozzles determined as nozzles to be corrected eject an amount of ink that is less than the amount that should have been ejected.

[0134] The effect of this modified example will be described with reference to FIG. 31. The graphs labeled with reference numbers 651, 658, and 659 in FIG. 31 show spatial changes in the density of three printed images obtained when an image of equally spaced horizontal lines is printed at a constant density under three different conditions. In detail, the dotted line graph labeled with reference number 651 shows the results obtained with a conventional configuration (i.e., a configuration in which the ink ejection amount is not adjusted), the solid line graph labeled with reference number 658 shows the results obtained by adjusting the ink ejection amount by setting only the slowest ink ejection nozzle as the nozzle to be corrected, and the thick dotted line graph labeled with reference number 659 shows the results obtained by setting the slowest ink ejection nozzle and the nozzle that ejects ink immediately before the slowest ink ejection nozzle as the nozzle to be corrected and adjusting the ink ejection amount. It can be seen from FIG. 31 that the difference in shading is significantly reduced by setting two nozzles for each nozzle set as the nozzles to be corrected and adjusting the ink ejection amount. In this way, according to this modified example, it is possible to extremely effectively suppress the occurrence of spatial periodic unevenness in shading caused by the difference in dynamic surface tension between ink that lands on the printing medium first and ink that lands on the printing medium later.

[0135] <3.5 Fifth modified example> In the above embodiments, it was assumed that the difference in dynamic surface tension between the ink that lands first on the printing medium 5 and the ink that lands second on the printing medium 5 is large, and the ink that lands first on the printing medium 5 is attracted to the ink that lands second on the printing medium 5. However, for example, when the fixability of the ink to the printing medium 5 is high (when the force of attraction between the printing medium 5 and the ink is stronger than the force of attraction between the inks), the ink that lands second on the printing medium 5 may be attracted to the ink that lands first on the printing medium 5. The movement of the ink in this case is shown in FIG. 32. Note that here too, the nozzle that ejects ink first onto the printing medium 5 is called the "first nozzle," and the nozzle that ejects ink second onto the printing medium 5 is called the "second nozzle." At time t91, ink is ejected from the first nozzle. At time t92, the ink ejected from the first nozzle lands on the printing medium 5, and ink is ejected from the second nozzle. At time t93, the ink ejected from the second nozzle lands on the printing medium 5. At this time, the contact area between the ink ejected from the second nozzle and the printing medium 5 is smaller than the contact area between the ink ejected from the first nozzle and the printing medium 5. Therefore, the force of attraction between the ink ejected from the second nozzle and the printing medium 5 is weaker. On the other hand, the ink ejected from the first nozzle is fixed on the printing medium 5 with a relatively strong force. As a result, at time t94, the ink ejected from the second nozzle is attracted toward the ink ejected from the first nozzle, as shown by the arrow with the reference numeral 98. An embodiment of the case where the ink that lands later on the printing medium 5 is attracted toward the ink that landed earlier on the printing medium 5 will be described below as a fifth modified example.

[0136] In this modification, the nozzle that ejects ink one nozzle before the slowest ink ejection nozzle or the nozzle that ejects ink two nozzles before the slowest ink ejection nozzle is determined as the nozzle to be corrected, and the amount of ink ejected from the nozzle determined as the nozzle to be corrected is adjusted so that the amount of ink ejected from the nozzle to be corrected is greater than the original amount. That is, the nozzle that is the second or third nozzle based on the other end side of the main scanning direction among the multiple nozzles is determined as the nozzle to be corrected, and the amount of ink ejected from the nozzle to be corrected is controlled so that the average amount of ink ejected from the nozzle to be corrected is greater than the average amount of ink that should be ejected from the nozzle to be corrected. When the head 241 having the configuration shown in FIG. 6 and FIG. 7 is used, the nozzle 2(7) or the nozzle 2(6) of the eight nozzles 2(1) to 2(8) that constitute the nozzle set is determined as the nozzle to be corrected. In addition, when ink is ejected from multiple nozzles included in a nozzle set onto the printing medium 5 at the same position in the transport direction of the printing medium 5, ink is ejected first from any of the multiple nozzles that are arranged in the first to third positions based on one end side of the multiple nozzles in the main scanning direction, and ink is ejected last from any of the multiple nozzles that are arranged in the first to third positions based on the other end side of the multiple nozzles in the main scanning direction.

[0137] Here, the flow of ink will be explained by focusing on two nozzle sets each consisting of five nozzles 2(1)-2(5). In a case where the pulling of ink due to differences in dynamic surface tension can be ignored, ink discharged from each nozzle and landing on the printing medium 5 will not flow toward ink discharged from other nozzles and landing on the printing medium 5, as shown in FIG.

[0138] 33 shows the flow of ink in a case where ink that lands later on the printing medium 5 is attracted to ink that landed earlier on the printing medium 5 and where the density value reduction process described above is not performed. After the ink ejected from nozzle 2(2) lands on the printing medium 5 at time t3, at time t4, the ink that landed on the printing medium 5 at time t3 flows toward the ink that landed on the printing medium 5 at time t1. In this way, the ink that lands later on the printing medium 5 flows toward the ink that landed earlier on the printing medium 5, so that at time t10, the density at the position corresponding to nozzle 2(1) is significantly higher than the density at the positions corresponding to the other nozzles 2(2)-2(5).

[0139] FIG. 34 shows the flow of ink in a case where the ink that lands on the printing medium 5 later is attracted to the ink that lands on the printing medium 5 earlier and the above-mentioned density value reduction process is performed (i.e., the case of this modified example). It is assumed that nozzle 2(4) is determined as the nozzle to be corrected. In this case, a larger amount of ink is ejected from nozzle 2(4) than should be ejected. Therefore, as shown by the dotted line portion labeled with reference numeral 75 in FIG. 34, the amount of ink that lands on the printing medium 5 at time t7 is larger than should be. At this time, the contact area between the ink ejected from nozzle 2(4) and the printing medium 5 becomes relatively large, so the dynamic surface tension of the ink ejected from nozzle 2(4) becomes larger than usual. Therefore, the ink that is ejected from nozzle 2(4) and lands on the printing medium 5 is prevented from flowing toward the ink that has already landed on the printing medium 5, compared to the conventional case. The ink that is ejected from nozzle 2 (5) and lands on the printing medium 5 at time t9 flows toward the ink that is ejected from nozzle 2 (4) and lands on the printing medium 5 at time t7, but the overall difference in shade at time t10 is smaller than in the conventional method (see Figure 33).

[0140] As described above, according to this modified example, in cases where ink that lands later on the printing medium 5 is attracted toward ink that landed earlier on the printing medium 5, the occurrence of spatial periodic unevenness in shading is effectively suppressed.

[0141] <3.6 Modifications regarding the arrangement of head units and nozzles> Below, modifications regarding the arrangement of the head unit 240 and the plurality of nozzles 2 will be described as sixth to ninth modifications.

[0142] <3.6.1 Sixth Modification> Fig. 35 is a schematic diagram showing the configuration of a head 241 in this modified example. As shown in Fig. 35, the head 241 in this modified example includes an upstream nozzle group 241a located on the upstream side in the transport direction of the print medium 5, and a downstream nozzle group 241b located on the downstream side in the transport direction of the print medium 5. In this way, the multiple nozzles 2 that make up each head 241 are divided into two groups. Note that only one head 241 is provided for each ink color in the transport direction of the print medium 5.

[0143] Figure 36 is an enlarged view of the portion indicated by reference numeral 84 in Figure 35. One nozzle set is made up of the six nozzles shown in Figure 36. Of these six nozzles, three nozzles located on the upstream side in the transport direction of the print medium 5 are included in an upstream nozzle group 241a, and three nozzles located on the downstream side in the transport direction of the print medium 5 are included in a downstream nozzle group 241b. When ink is ejected from the six nozzles onto the print medium 5 to the same position in the transport direction of the print medium 5, ink is ejected sequentially from the nozzles located upstream in the transport direction of the print medium 5.

[0144] In this modified example, in addition to the nozzle (nozzle marked with reference number 842 in FIG. 36) that corresponds to the slowest ink ejection nozzle among the six nozzles constituting the nozzle set, the nozzle (nozzle marked with reference number 841 in FIG. 36) that ejects ink last among the three nozzles included in the upstream nozzle group 241a is determined as the nozzle to be corrected. That is, the nozzles included in the downstream nozzle group 241b that are located on the other end side in the main scanning direction (the left end side in the figure) and the nozzles included in the upstream nozzle group 241a that are located on the other end side in the main scanning direction are determined as the nozzles to be corrected. For each nozzle set, the two nozzles thus determined as the nozzles to be corrected eject an amount of ink that is less than the amount of ink that should be ejected.

[0145] In this modified example, the number of nozzles included in the upstream nozzle group 241a is the same as the number of nozzles included in the downstream nozzle group 241b, but the number of nozzles included in the upstream nozzle group 241a may be different from the number of nozzles included in the downstream nozzle group 241b. If the number of nozzles included in each nozzle set is N and the number of nozzles included in the upstream nozzle group 241a is P, when ink is ejected from P nozzles included in the upstream nozzle group 241a to the same position in the transport direction of the printing medium 5, and ink is ejected last from a nozzle located at the other end of the main scanning direction (width direction) among the P nozzles, in addition to the slowest ink ejection nozzle included in the downstream nozzle group 241b, the nozzle located at the other end of the P nozzles in the main scanning direction can be determined as a correction target nozzle.

[0146] According to this modification, when ink is ejected from the multiple nozzles included in the upstream nozzle group 241a onto the printing medium 5, the last of the multiple nozzles to eject ink ejects a smaller amount of ink than would normally be ejected. Furthermore, when ink is ejected from the multiple nozzles included in the downstream nozzle group 241b onto the printing medium 5, the last of the multiple nozzles to eject ink ejects a smaller amount of ink than would normally be ejected. This effectively prevents ink that has landed earlier on the printing medium 5 from flowing toward ink that has landed later on the printing medium 5, more effectively preventing uneven shading.

[0147] <3.6.2 Seventh Modification> Fig. 37 is a schematic diagram showing a schematic configuration of a head unit 240 for each ink color in this modified example. As shown in Fig. 37, the head unit 240 in this modified example includes five heads (hereinafter referred to as "upstream ink ejection heads") 241U arranged on the upstream side in the transport direction of the print medium 5, and five heads (hereinafter referred to as "downstream ink ejection heads") 241D arranged on the downstream side in the transport direction of the print medium. As shown in the portion marked with reference numeral 85 in Fig. 38, one nozzle set is composed of three nozzles included in the upstream ink ejection head 241U and three nozzles included in the downstream ink ejection head 241D.

[0148] In the above configuration, in addition to the nozzle (nozzle marked with reference numeral 852 in FIG. 38) that corresponds to the slowest ink-ejecting nozzle among the six nozzles that make up the nozzle set, the nozzle (nozzle marked with reference numeral 851 in FIG. 38) that ejects ink last among the three nozzles included in the upstream ink-ejecting head 241U is determined as the nozzle to be corrected. That is, the nozzles included in the downstream ink-ejecting head 241D that are located on the other end side in the main scanning direction (the left end side in the figure) and the nozzles included in the upstream ink-ejecting head 241U that are located on the other end side in the main scanning direction are determined as the nozzles to be corrected. For each nozzle set, the two nozzles thus determined as the nozzles to be corrected eject an amount of ink that is less than the amount of ink that should have been ejected.

[0149] In this modified example, the number of nozzles included in the upstream ink ejection head 241U is the same as the number of nozzles included in the downstream ink ejection head 241D, but the number of nozzles included in the upstream ink ejection head 241U may be different from the number of nozzles included in the downstream ink ejection head 241D. If the number of nozzles included in each nozzle set is N and the number of nozzles included in the upstream ink ejection head 241U is P, when ink is ejected onto the printing medium 5 from the P nozzles included in the upstream ink ejection head 241U to the same position in the transport direction of the printing medium 5, and ink is ejected last from the nozzles arranged on the other end side of the main scanning direction (width direction) among the P nozzles, in addition to the slowest ink ejection nozzle included in the downstream ink ejection head 241D, the nozzles arranged on the other end side of the main scanning direction among the P nozzles can be determined as nozzles to be corrected.

[0150] According to this modification, when ink is ejected from the multiple nozzles included in the upstream ink ejection head 241U onto the printing medium 5, the last of the multiple nozzles to eject ink ejects a smaller amount of ink than would normally be ejected. Furthermore, when ink is ejected from the multiple nozzles included in the downstream ink ejection head 241D onto the printing medium 5, the last of the multiple nozzles to eject ink ejects a smaller amount of ink than would normally be ejected. This effectively prevents ink that has landed earlier on the printing medium 5 from flowing toward ink that has landed later on the printing medium 5, more effectively preventing the occurrence of uneven shading.

[0151] <3.6.3 Eighth modified example> FIG. 39 is a schematic diagram showing a part of the configuration of the head unit 240 for each ink color in this modification. As in the seventh modification, the head unit 240 in this modification includes an upstream ink ejection head 241U and a downstream ink ejection head 241D. As in the sixth modification, each head 241 in this modification includes an upstream nozzle group and a downstream nozzle group. In FIG. 39, the upstream nozzle group included in the upstream ink ejection head 241U is given the reference symbol 241Ua, the downstream nozzle group included in the upstream ink ejection head 241U is given the reference symbol 241Ub, the upstream nozzle group included in the downstream ink ejection head 241D is given the reference symbol 241Da, and the downstream nozzle group included in the downstream ink ejection head 241D is given the reference symbol 241Db. As shown in the part given the reference symbol 86 in FIG. 39, one nozzle set is composed of 12 nozzles.

[0152] In the above configuration, with regard to the part marked with symbol 86 in FIG. 39, only the nozzle corresponding to the slowest ink ejection nozzle (nozzle marked with symbol 864) may be defined as the nozzle to be corrected, or the nozzle marked with symbol 864 and the nozzle that ejects ink last among the multiple nozzles included in the upstream ink ejection head 241U (nozzle marked with symbol 862) may be defined as the nozzle to be corrected, or the nozzle marked with symbol 864 and the nozzle that ejects ink last among the multiple nozzles included in the upstream nozzle group 241Da (nozzle marked with symbol 863) may be defined as the nozzles to be corrected, or the nozzle marked with symbol 864, the nozzle marked with symbol 862, the nozzle marked with symbol 863, and the nozzle that ejects ink last among the multiple nozzles included in the upstream nozzle group 241Ua (nozzle marked with symbol 861) may be defined as the nozzles to be corrected.

[0153] <3.6.4 Ninth modified example> The schematic configuration of the head unit 240 in this modified example is the same as that of the head unit 240 in the seventh modified example (see FIG. 37). As shown in the portion indicated by reference numeral 87 in FIG. 40, one nozzle set is made up of three nozzles included in the upstream ink ejection head 241U and three nozzles included in the downstream ink ejection head 241D. Focusing on this nozzle set, the nozzle located furthest to the other end in the main scanning direction among the six nozzles is not the nozzle corresponding to the slowest ink ejection nozzle described above (nozzle indicated by reference numeral 872), but the nozzle that ejects ink last among the multiple nozzles included in the upstream ink ejection head 241U (nozzle indicated by reference numeral 871).

[0154] In the above configuration, for the portion marked with reference numeral 87 in Fig. 40, the nozzle marked with reference numeral 872 and the nozzle marked with reference numeral 871 are determined as nozzles to be corrected. That is, of the multiple nozzles included in the downstream ink ejection head 241D, the nozzle located at the other end in the main scanning direction (the left end in the figure) and of the multiple nozzles included in the upstream ink ejection head 241U, the nozzle located at the other end in the main scanning direction are determined as nozzles to be corrected. For each nozzle set, the two nozzles thus determined as nozzles to be corrected eject an amount of ink less than the amount of ink that should have been ejected.

[0155] <4.Other> The present invention is not limited to the above-mentioned embodiments (including modified examples), and can be implemented in various modifications without departing from the spirit of the present invention. For example, in the above-mentioned embodiments, the configuration of an inkjet printing device that performs color printing is exemplified, but the present invention can also be applied to the case where an inkjet printing device that performs monochrome printing is adopted. In that case, image shift processing (processing of step S50 in FIG. 13) is not required when printing output. Also, in the above-mentioned embodiments, the position of the printing unit 24 is fixed (the positions of the multiple nozzles 2 included in the printing unit 24 are fixed) and the printing medium 5 is transported by the transport mechanism 29 during printing, but this is not limited. It is sufficient that the positional relationship between the printing medium 5 and the multiple nozzles 2 moves relatively in a predetermined direction during printing, and the printing unit 24 including the multiple nozzles 2 may move in a predetermined direction during printing. [Explanation of symbols]

[0156] 2,2(1)~2(8)…Nozzle 5…Print media 10...Inkjet printing device 24…Printing section 40...Control unit 42...print data processing unit 43...Ink discharge control unit 61...Correction amount adjustment chart 62...Correction range adjustment chart 100...Print control device 200…Printing machine body 240…Head unit 241…Head 424...Density value reduction processing unit 425...Threshold matrix storage unit 426...Halftone processing section 427...Correction amount determination unit

Claims

1. An inkjet printing device having a plurality of nozzles and performing printing by ejecting ink from the plurality of nozzles onto a printing medium, a movement mechanism that relatively moves the positional relationship between the printing medium and the plurality of nozzles in a predetermined direction; a printing unit including a plurality of nozzles, each of which is a nozzle set including N nozzles arranged regularly in a two-dimensional manner, N being a natural number, and which are arranged continuously in a width direction perpendicular to the predetermined direction; a control unit that controls the amount of ink ejected from the plurality of nozzles; Equipped with when ink is ejected onto the printing medium from the N nozzles included in the nozzle set to the same position in the predetermined direction, ink is ejected first from a nozzle that is arranged on one end side of the width direction among the N nozzles, and ink is ejected last from a nozzle that is arranged on the other end side of the width direction among the N nozzles, Among the N nozzles, a nozzle arranged on the other end side in the width direction is determined as a correction target nozzle, A printing device characterized in that the control unit controls the amount of ink ejected from the nozzle to be corrected so that the average amount of ink ejected from the nozzle to be corrected is smaller than the average amount of ink that should originally be ejected from the nozzle to be corrected.

2. 2. The printing device according to claim 1, wherein the nozzle that is arranged second or third from the other end side in the width direction among the N nozzles is further determined as the nozzle to be corrected.

3. An inkjet printing device having a plurality of nozzles and performing printing by ejecting ink from the plurality of nozzles onto a printing medium, a movement mechanism that relatively moves the positional relationship between the printing medium and the plurality of nozzles in a predetermined direction; a printing unit including a plurality of nozzles, each of which is a nozzle set including N nozzles arranged regularly in a two-dimensional manner, N being a natural number, and which are arranged continuously in a width direction perpendicular to the predetermined direction; a control unit that controls the amount of ink ejected from the plurality of nozzles; Equipped with when ink is ejected onto the printing medium from the N nozzles included in the nozzle set to the same position in the predetermined direction, ink is ejected first from any one of the N nozzles that is arranged in the first to third positions based on one end side of the width direction, and ink is ejected last from any one of the N nozzles that is arranged in the first to third positions based on the other end side of the width direction, Among the N nozzles, the nozzle located second or third based on the other end side in the width direction is determined as a correction target nozzle; A printing device characterized in that the control unit controls the amount of ink ejected from the nozzle to be corrected so that the average amount of ink ejected from the nozzle to be corrected is smaller than the average amount of ink that should originally be ejected from the nozzle to be corrected.

4. The control unit is a shading correction process for generating second image data including density values ​​of a plurality of pixels by correcting first image data including density values ​​of the plurality of pixels so as to compensate for variations in ink ejection amount among the plurality of nozzles; a density value reduction process for generating third image data including density values ​​of the plurality of pixels by reducing density values ​​of pixels corresponding to the nozzles to be corrected among the density values ​​of the plurality of pixels included in the second image data; a halftone process for generating halftone image data that controls an amount of ink ejected from each of the plurality of nozzles by halftoning the third image data; 4. The printing device according to claim 1, further comprising:

5. The control unit is a shading correction process for generating second image data including density values ​​of a plurality of pixels by correcting first image data including density values ​​of the plurality of pixels so as to compensate for variations in ink ejection amount among the plurality of nozzles; a halftone process for generating halftone image data that controls an amount of ink ejected from each of the plurality of nozzles by halftoning the second image data; Run 4. A printing device as claimed in claim 1, characterized in that in the halftone processing, the halftone image data is generated so that the average amount of ink ejected from the nozzle to be corrected is smaller than the average amount of ink that should originally be ejected from the nozzle to be corrected.

6. the control unit performs the halftone process by using a threshold matrix including a plurality of thresholds arranged two-dimensionally to be compared with density values ​​of the plurality of pixels included in the second image data; 6. The printing device according to claim 5, wherein the plurality of threshold values ​​included in the threshold matrix are set so that the average amount of ink ejected from the nozzles to be corrected is smaller than the average amount of ink ejected from nozzles other than the nozzles to be corrected.

7. the printing unit includes only one ink ejection head for each ink color in the predetermined direction; the ink ejection head includes an upstream nozzle group located on the upstream side in the predetermined direction, and a downstream nozzle group located on the downstream side in the predetermined direction, P nozzles out of the N nozzles included in the nozzle set are included in the upstream nozzle group, where P is an integer less than N, when ink is ejected from the P nozzles included in the upstream nozzle group to the same position in the predetermined direction onto the printing medium, ink is ejected last from a nozzle that is disposed on the other end side of the P nozzles in the width direction, 4. The printing device according to claim 1, wherein a nozzle disposed on the other end side in the width direction among the P nozzles is further determined as the nozzle to be corrected.

8. the printing unit includes, for each ink color, an upstream ink ejection head disposed on the upstream side in the predetermined direction, and a downstream ink ejection head disposed on the downstream side in the predetermined direction; P nozzles out of the N nozzles are included in the upstream ink ejection head, where P is an integer less than N, when ink is ejected from the P nozzles included in the upstream ink ejection head to the same position in the predetermined direction onto the printing medium, ink is ejected last from a nozzle that is disposed on the other end side of the width direction among the P nozzles, 4. The printing device according to claim 1, wherein a nozzle disposed on the other end side in the width direction among the P nozzles is further determined as the nozzle to be corrected.

9. A printing device as described in any one of claims 1 to 3, characterized in that, with regard to printing of an image portion in which two or more ink colors should be the same density, the control unit varies an average reduction value, which is the difference between the average amount of ink ejected from the nozzle to be corrected and the average amount of ink that should originally be ejected from the nozzle to be corrected, for each ink color.

10. the two or more ink colors being cyan, magenta, yellow, and black; 10. The printing device according to claim 9, wherein the control unit sets the average reduction value for magenta to be greater than the average reduction value for yellow, sets the average reduction value for black to be greater than the average reduction value for magenta, and sets the average reduction value for cyan to be greater than the average reduction value for black.

11. An inkjet printing device having a plurality of nozzles and performing printing by ejecting ink from the plurality of nozzles onto a printing medium, a movement mechanism that relatively moves the positional relationship between the printing medium and the plurality of nozzles in a predetermined direction; a printing unit including a plurality of nozzles, each of which is a nozzle set including N nozzles arranged regularly in a two-dimensional manner, N being a natural number, and which are arranged continuously in a width direction perpendicular to the predetermined direction; a control unit that controls the amount of ink ejected from the plurality of nozzles; Equipped with when ink is ejected onto the printing medium from the N nozzles included in the nozzle set to the same position in the predetermined direction, ink is ejected first from any one of the N nozzles that is arranged in the first to third positions based on one end side of the width direction, and ink is ejected last from any one of the N nozzles that is arranged in the first to third positions based on the other end side of the width direction, Among the N nozzles, the first to third nozzles arranged based on the other end side in the width direction are selectively determined as correction target nozzles; the control unit controls the amount of ink ejected from the correction target nozzles so that an average amount of ink ejected from the correction target nozzles is smaller than an average amount of ink that should originally be ejected from the correction target nozzles; A printing device characterized in that, when the difference between the average amount of ink ejected from the nozzle to be corrected and the average amount of ink that should originally be ejected from the nozzle to be corrected is defined as the reduced ink amount, the reduced ink amount when the second nozzle based on the other end side of the width direction is determined as the nozzle to be corrected is greater than the reduced ink amount when the first nozzle based on the other end side of the width direction is determined as the nozzle to be corrected, and the reduced ink amount when the third nozzle based on the other end side of the width direction is determined as the nozzle to be corrected is greater than the reduced ink amount when the second nozzle based on the other end side of the width direction is determined as the nozzle to be corrected.

12. An inkjet printing device having a plurality of nozzles and performing printing by ejecting ink from the plurality of nozzles onto a printing medium, a movement mechanism that relatively moves the positional relationship between the printing medium and the plurality of nozzles in a predetermined direction; a printing unit including a plurality of nozzles, each of which is a nozzle set including N nozzles arranged regularly in a two-dimensional manner, N being a natural number, and which are arranged continuously in a width direction perpendicular to the predetermined direction; a control unit that controls the amount of ink ejected from the plurality of nozzles; Equipped with when ink is ejected onto the printing medium from the N nozzles included in the nozzle set to the same position in the predetermined direction, ink is ejected first from any one of the N nozzles that is arranged in the first to third positions based on one end side of the width direction, and ink is ejected last from any one of the N nozzles that is arranged in the first to third positions based on the other end side of the width direction, Among the N nozzles, the nozzle located second or third based on the other end side in the width direction is determined as a correction target nozzle; A printing device characterized in that the control unit controls the amount of ink ejected from the nozzle to be corrected so that the average amount of ink ejected from the nozzle to be corrected is greater than the average amount of ink that should originally be ejected from the nozzle to be corrected.

13. A printing method using an inkjet printing device having a plurality of nozzles that eject ink onto a printing medium, comprising: The positional relationship between the printing medium and the plurality of nozzles is such that they move relatively in a predetermined direction, the printing device comprises a printing unit including a plurality of nozzles, each of which is a nozzle set including N nozzles arranged regularly in a two-dimensional manner, N being a natural number, and which is arranged continuously in a width direction perpendicular to the predetermined direction; when ink is ejected onto the printing medium from the N nozzles included in the nozzle set to the same position in the predetermined direction, ink is ejected first from a nozzle that is arranged on one end side of the width direction among the N nozzles, and ink is ejected last from a nozzle that is arranged on the other end side of the width direction among the N nozzles, Among the N nozzles, a nozzle arranged on the other end side in the width direction is determined as a correction target nozzle, The printing method includes: a rasterization step of generating first image data including density values ​​of a plurality of pixels by performing a rasterization process on the input data; a shading correction step of generating second image data including density values ​​of the plurality of pixels by correcting the first image data so as to compensate for variations in ink ejection amount among the plurality of nozzles; a density value reducing step of generating third image data including density values ​​of the plurality of pixels by reducing density values ​​of pixels corresponding to the nozzles to be corrected among the density values ​​of the plurality of pixels included in the second image data; a halftone image generating step of generating halftone image data for controlling an amount of ink ejected from each of the plurality of nozzles by halftoning the third image data; an ink ejection step of ejecting ink from the plurality of nozzles based on the halftone image data; A printing method comprising:

14. 14. The printing method according to claim 13, wherein the second or third nozzle, based on the other end side in the width direction, out of the N nozzles is further determined as the nozzle to be corrected.

15. A printing method using an inkjet printing device having a plurality of nozzles that eject ink onto a printing medium, comprising: The positional relationship between the printing medium and the plurality of nozzles is such that they move relatively in a predetermined direction, the printing device comprises a printing unit including a plurality of nozzles, each of which is a nozzle set including N nozzles arranged regularly in a two-dimensional manner, N being a natural number, and which is arranged continuously in a width direction perpendicular to the predetermined direction; when ink is ejected onto the printing medium from the N nozzles included in the nozzle set to the same position in the predetermined direction, ink is ejected first from any one of the N nozzles that is arranged in the first to third positions based on one end side of the width direction, and ink is ejected last from any one of the N nozzles that is arranged in the first to third positions based on the other end side of the width direction, Among the N nozzles, the second or third nozzle based on the other end side in the width direction is determined as the correction target nozzle, The printing method includes: a rasterization step of generating first image data including density values ​​of a plurality of pixels by performing a rasterization process on the input data; a shading correction step of generating second image data including density values ​​of the plurality of pixels by correcting the first image data so as to compensate for variations in ink ejection amount among the plurality of nozzles; a density value reducing step of generating third image data including density values ​​of the plurality of pixels by reducing density values ​​of pixels corresponding to the nozzles to be corrected among the density values ​​of the plurality of pixels included in the second image data; a halftone image generating step of generating halftone image data for controlling an amount of ink ejected from each of the plurality of nozzles by halftoning the third image data; an ink ejection step of ejecting ink from the plurality of nozzles based on the halftone image data; A printing method comprising:

16. a test chart printing step of printing a test chart for determining an amount of reduction in density value when reducing density values ​​of pixels corresponding to the nozzles to be corrected in the density value reducing step, the test chart including a plurality of pattern images with different amounts of reduction in density value; a pattern image selection step of selecting one of the plurality of pattern images based on the test chart printed in the test chart printing step; Further comprising: A printing method according to any one of claims 13 to 15, characterized in that in the density value reduction step, the third image data is generated by reducing the density value of the pixel corresponding to the nozzle to be corrected in accordance with the pattern image selected in the pattern image selection step.

17. The range of possible pixel density values ​​from the minimum to the maximum is divided into a number of stages, In the test chart printing step, the test chart is printed in each step, 17. The printing method according to claim 16, wherein in the pattern image selection step, one of the plurality of pattern images is selected for each stage.

18. A printing method using an inkjet printing device having a plurality of nozzles that eject ink onto a printing medium, comprising: The positional relationship between the printing medium and the plurality of nozzles is such that they move relatively in a predetermined direction, the printing device comprises a printing unit including a plurality of nozzles, each of which is a nozzle set including N nozzles arranged regularly in a two-dimensional manner, N being a natural number, and which is arranged continuously in a width direction perpendicular to the predetermined direction; when ink is ejected onto the printing medium from the N nozzles included in the nozzle set to the same position in the predetermined direction, ink is ejected first from a nozzle that is arranged on one end side of the width direction among the N nozzles, and ink is ejected last from a nozzle that is arranged on the other end side of the width direction among the N nozzles, Among the N nozzles, a nozzle arranged on the other end side in the width direction is determined as a correction target nozzle, The printing method includes: a rasterization step of generating first image data including density values ​​of a plurality of pixels by performing a rasterization process on the input data; a shading correction step of generating second image data including density values ​​of the plurality of pixels by correcting the first image data so as to compensate for variations in ink ejection amount among the plurality of nozzles; a halftone image generating step of generating halftone image data for controlling an amount of ink ejected from each of the plurality of nozzles by halftoning the second image data; an ink ejection step of ejecting ink from the plurality of nozzles based on the halftone image data; Including, A printing method characterized in that in the halftone image generation step, the halftone image data is generated so that the average amount of ink ejected from the nozzles to be corrected is smaller than the average amount of ink that should originally be ejected from the nozzles to be corrected.

19. 20. The printing method according to claim 18, wherein the second or third nozzle, based on the other end side in the width direction, out of the N nozzles is further determined as the nozzle to be corrected.

20. A printing method using an inkjet printing device having a plurality of nozzles that eject ink onto a printing medium, comprising: The positional relationship between the printing medium and the plurality of nozzles is such that they move relatively in a predetermined direction, the printing device comprises a printing unit including a plurality of nozzles, each of which is a nozzle set including N nozzles arranged regularly in a two-dimensional manner, N being a natural number, and which is arranged continuously in a width direction perpendicular to the predetermined direction; when ink is ejected onto the printing medium from the N nozzles included in the nozzle set to the same position in the predetermined direction, ink is ejected first from any one of the N nozzles that is arranged in the first to third positions based on one end side of the width direction, and ink is ejected last from any one of the N nozzles that is arranged in the first to third positions based on the other end side of the width direction, Among the N nozzles, the nozzle located second or third based on the other end side in the width direction is determined as a correction target nozzle; The printing method includes: a rasterization step of generating first image data including density values ​​of a plurality of pixels by performing a rasterization process on the input data; a shading correction step of generating second image data including density values ​​of the plurality of pixels by correcting the first image data so as to compensate for variations in ink ejection amount among the plurality of nozzles; a halftone image generating step of generating halftone image data for controlling an amount of ink ejected from each of the plurality of nozzles by halftoning the second image data; an ink ejection step of ejecting ink from the plurality of nozzles based on the halftone image data; Including, A printing method characterized in that in the halftone image generation step, the halftone image data is generated so that the average amount of ink ejected from the nozzles to be corrected is smaller than the average amount of ink that should originally be ejected from the nozzles to be corrected.

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