JP2008093906A - Printing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To calculate an accurate correction value to density irregularities even if a scanner does not work normally. <P>SOLUTION: A pattern is formed when a printer repeats the action of forming a band comprised of a plurality of raster lines along a predetermined direction and the action of conveying a medium by a first conveyance amount in a direction that intersects the predetermined direction. At the time of calculating the correction value by making the scanner read the pattern and correcting a read tone value on the basis of an approximate straight line calculated with a part of a plurality of the read tone values acquired for every pixel row being made a calculation range, the band is formed by conveying the medium in the intersection direction by a second conveyance amount smaller than the first conveyance amount. The pattern has a plurality of the bands arranged in the intersection direction. The pixel row corresponding to a boundary of the bands is made a boundary pixel row. An interval between a downstream pixel row as the pixel row of the most downstream side in the calculation range and a first boundary pixel row located on the upstream side more than the downstream pixel row is equal to an interval between an upstream pixel row as the pixel row of the most upstream side in the calculation range and a second boundary pixel row located on the downstream side more than the upstream pixel row. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a printing method.

2. Related Art There is known an ink jet printer that completes a print image by moving a head in a moving direction and discharging ink from nozzles during the movement.
In such a printer, ink droplets may not land at the correct position on the medium due to problems such as nozzle processing accuracy. As a result, shading occurs in the vicinity of the region where the ink droplets should have landed, and striped density unevenness occurs in the printed image.

In view of this, a method has been proposed in which an image is sampled by a CCD sensor and data output by an ink jet printer is corrected based on the characteristics of uneven gain of the CCD sensor to improve uneven density. (See Patent Document 1)
In addition, a method of printing a density unevenness test pattern and correcting density unevenness based on density data of the density unevenness test pattern has been proposed. (See Patent Document 2)
Japanese Patent Laid-Open No. 2-54676 JP-A-6-166247

  If the scanner does not operate normally, accurate measurement values (read gradation values) cannot be obtained, and density unevenness may not be improved.

  Therefore, an object of the present invention is to calculate an accurate correction value for density unevenness even when the scanner does not operate normally.

A main invention for solving the problems is a band forming operation for forming a band composed of a plurality of raster lines along a predetermined direction, and a medium is transported by a first transport amount in a crossing direction that is a direction crossing the predetermined direction. A step of forming a pattern by the printing apparatus repeating the first transporting operation, and reading the pattern for each pixel column composed of a plurality of pixels arranged in a direction corresponding to the predetermined direction. A step of obtaining a value, a step of calculating an approximate line using at least a part of the obtained plurality of read tone values as a calculation range, and correcting the read tone value based on the approximate line A printing method comprising: a step; calculating a correction value based on the corrected read gradation value; and printing based on the correction value, In the band forming operation, the raster line formed based on the pixel row is formed on the medium while moving the nozzle in the predetermined direction, and the medium is transported in the intersecting direction to the first transport amount. The printing apparatus repeats a second transport operation for transporting with a smaller second transport amount, and the pattern is formed by arranging a plurality of the bands in the intersecting direction, and corresponds to the boundary of the bands. The pixel column is a boundary pixel column, and the interval between the downstream pixel column that is the pixel column on the most downstream side in the calculation range and the first boundary pixel column located on the upstream side of the downstream pixel column is, It is characterized by being equal to the interval between the upstream pixel column that is the pixel column on the most upstream side in the calculation range and the second boundary pixel column that is located on the downstream side of the upstream pixel column.
Other features of the present invention will become apparent from the description of this specification and the accompanying drawings.

=== Summary of disclosure ===
At least the following will become apparent from the description of the present specification and the accompanying drawings.

That is, a band forming operation for forming a band composed of a plurality of raster lines along a predetermined direction, and a first transport operation for transporting the medium by a first transport amount in a crossing direction that is a direction crossing the predetermined direction, A step of forming a pattern by the printing apparatus repeating, a step of causing the scanner to read the pattern, and acquiring a read gradation value for each pixel row composed of a plurality of pixels arranged in a direction corresponding to the predetermined direction; Calculating an approximate line using at least a part of the plurality of read gradation values as a calculation range, correcting the read gradation value based on the approximate line, and correcting the read A printing method comprising: calculating a correction value based on a gradation value; and printing based on the correction value, wherein in the band forming operation, A forming operation for forming the raster lines formed based on the pixel columns on the medium while moving the slip in the predetermined direction, and a second transport in which the medium is smaller than the first transport amount in the intersecting direction. The printing apparatus repeats a second transport operation for transporting by a quantity, and the pattern is formed by arranging a plurality of bands in the intersecting direction, and the pixel columns corresponding to the boundaries of the bands are defined as boundary pixel columns. And the interval between the downstream pixel column, which is the most downstream pixel column in the calculation range, and the first boundary pixel column located upstream of the downstream pixel column is the most in the calculation range. A printing method, characterized by being equal to an interval between an upstream pixel column, which is the upstream pixel column, and a second boundary pixel column located downstream of the upstream pixel column.
According to such a printing method, an approximate straight line can be calculated without being affected by the read gradation value near the boundary pixel column. Therefore, the inclination of the graph indicating the read gradation value can be corrected accurately without inclining upstream or downstream. As a result, an accurate correction value for density unevenness can be calculated.

In this printing method, when the pattern is composed of the two bands, the first boundary pixel column is equal to the second boundary pixel column.
According to such a printing method, the inclination of the graph indicating the read gradation value can be corrected accurately.

In this printing method, when the pattern is composed of three or more bands, the first boundary pixel column is the boundary pixel column on the most downstream side of the pattern, and the second boundary The pixel column is the boundary pixel column on the most upstream side of the pattern.
According to such a printing method, the inclination of the graph indicating the read gradation value can be corrected accurately. In addition, since the calculation range for calculating the approximate straight line is set wide, a more accurate approximate straight line can be calculated.

In this printing method, when a plurality of the nozzles are arranged in the intersecting direction to form a nozzle row, the first boundary pixel row is a raster line formed by the most upstream nozzle in the nozzle row. The second boundary pixel column is a pixel column corresponding to a raster line formed by the nozzle on the most downstream side of the nozzle column.
According to such a printing method, the inclination of the graph indicating the read gradation value can be corrected accurately. A calculation range for calculating an approximate straight line can be set in advance based on a pixel row that is likely to correspond to a band boundary.

In this printing method, when a plurality of the nozzles are arranged in the intersecting direction to form a nozzle row, the first boundary pixel row and the second boundary pixel row are located on the most upstream side of the nozzle row. It is a pixel row corresponding to a raster line formed by a nozzle, or the first boundary pixel row and the second boundary pixel row correspond to a raster line formed by the nozzle on the most downstream side of the nozzle row. It must be a pixel column.
According to such a printing method, the inclination of the graph indicating the read gradation value can be corrected accurately. A calculation range for calculating an approximate straight line can be set in advance based on a pixel row that is likely to correspond to a band boundary.

In this printing method, the correction value is calculated for each of the pixel columns corresponding to the plurality of raster lines constituting the band, and the correction value is repeatedly used when printing the plurality of bands. thing.
According to such a printing method, the inclination of the graph indicating the read gradation value can be corrected accurately. If it does so, the graph which shows a correction value will also not incline, and there will be no tone difference of a band boundary.

In this printing method, the downstream raster line formed based on the downstream pixel row is located on the downstream side of the raster line at the center of the band to which the downstream raster line belongs, and is based on the upstream pixel row. The upstream raster line formed in this manner is positioned upstream of the raster line at the center of the band to which the upstream raster line belongs.
According to such a printing method, the calculation range is set wide with the pixel column far from the first boundary pixel column as the downstream pixel column and the pixel column away from the second boundary pixel column as the upstream pixel column. A more accurate approximate straight line can be calculated without being affected by the read gradation value of the boundary pixel column.

=== System Configuration of this Embodiment ===
FIG. 1 is a system configuration diagram of this embodiment. The printer 1 and the scanner 70 are connected to the computer 60. In this embodiment, in order to improve the density unevenness of the printer 1, the printer 1 completed in a manufacturing factory or the like prints a test pattern. The test pattern is read by the scanner 70. The read image data is transmitted to the computer 60, and the computer 60 calculates a correction value H for improving density unevenness based on the image data. The correction value H is stored in the memory 53 of the printer 1. In the present embodiment, the printer 1 will be described as an inkjet printer.

<Inkjet printer configuration>
FIG. 2 is a block diagram of the overall configuration of the printer 1 according to the present embodiment. FIG. 3A is a schematic diagram of the overall configuration of the printer 1. FIG. 3B is a cross-sectional view of the overall configuration of the printer 1. The printer 1 that has received print data from the computer 60 that is an external device controls each unit (conveyance unit 10, carriage unit 20, head unit 30) by the controller 50, and uses the medium S (hereinafter referred to as paper S). Form an image. Further, the detector group 40 monitors the situation in the printer 1, and the controller 50 controls each unit based on the detection result.

  The controller 50 is a control unit for controlling the printer 1, and includes an interface unit 51, a CPU 52, a memory 53, and a unit control circuit 54. The interface unit 51 is for transmitting and receiving data between the computer 60 as an external device and the printer 1. The CPU 52 is an arithmetic processing unit for controlling the entire printer 1. The memory 53 is for securing an area for storing a program of the CPU 52, a work area, and the like, and has storage means such as a RAM and an EEPROM. The CPU 52 has a unit control circuit and controls each unit in accordance with a program stored in the memory 53.

  The transport unit 10 feeds the paper S to a printable position, and transports the paper S by a predetermined transport amount in the transport direction during printing. The paper feed roller 11, the transport motor 12, and the transport roller 13, a platen 14, and a paper discharge roller 15.

  The head unit 30 is for ejecting ink onto the paper S and has a head 31. The head 31 has a plurality of nozzles that are ink ejection portions. Each nozzle is provided with a piezo element PZT, which is a drive element for driving each nozzle to eject ink, and a pressure chamber (not shown) containing ink.

  The carriage unit 20 is for moving the head 31 in the movement direction (predetermined direction), and includes a carriage 21 and a carriage motor 22.

  The detector group 40 includes a linear encoder 41, a rotary encoder 42, a paper detection sensor 43, an optical sensor 44, and the like.

  FIG. 4 is an explanatory diagram showing the arrangement of nozzles on the lower surface (nozzle surface) of the head 31. On the lower surface of the head 31, a yellow ink nozzle row Y, a black ink nozzle row K, a cyan ink nozzle row C, and a magenta ink nozzle row M are formed. Each nozzle row includes 180 nozzles that are ejection openings for ejecting ink of each color. Out of the 180 nozzles, the lower nozzles are assigned with lower numbers (# i = # 1 to # 180). The nozzles in each nozzle row are aligned at a constant interval k · D (nozzle pitch) along the transport direction (cross direction).

<Printing procedure>
First, the controller 50 receives a print command and print data from the computer 60 (print command reception). The controller 50 analyzes the contents of various commands included in the print data, and performs the following processing using each unit.

  Next, the controller 50 rotates the paper feed roller 11 to send the paper S to be printed to the transport roller 13 (paper feed process). When the paper detection sensor 43 detects the position of the leading edge of the paper S sent from the paper supply roller 11, the controller 50 rotates the transport roller 13 to position the paper S at the print start position (indexing position). When the paper S is positioned at the print start position, at least some of the nozzles of the head 31 are opposed to the paper S.

  Then, the controller 50 drives the carriage motor 22 to move the carriage 21 in the movement direction. Since the head 31 is provided on the carriage 21, the head 31 also moves in the movement direction together with the carriage 21. One movement of the carriage 21 in the movement direction is called a pass. Then, the controller 50 ejects ink from the nozzles based on the print data while the carriage 21 is moving. When the ink droplets ejected from the nozzles land on the paper S, dots are formed on the paper S (dot formation processing). Since ink is intermittently ejected from the moving head 31, a dot row (raster line) along the moving direction is formed on the paper S. The linear encoder 41 detects the position of the carriage 21 in the moving direction, and the optical sensor 44 attached to the carriage 21 (head 31) detects the position of the end of the paper S.

  Thereafter, the controller 50 drives the transport motor 12 and rotates the transport roller 13 to transport the paper S by a predetermined transport amount in the transport direction (transport process). As a result, the head 31 can form dots at positions different from the positions of the dots formed by the previous dot formation process. The conveyance amount of the paper S is determined according to the rotation amount of the conveyance roller 13, and the rotation amount of the conveyance roller 13 is detected by the rotary encoder 42. The paper S being printed is supported by the platen 14.

  Finally, the controller 50 determines whether or not to discharge the paper S being printed (paper discharge process). If data to be printed remains on the paper S being printed, the paper is not discharged, and the dot formation process and the conveyance process are alternately repeated until there is no more data to be printed, thereby completing the image. When there is no more data to be printed on the paper S being printed, the paper S is discharged by the rotation of the paper discharge roller 15.

<About print data>
FIG. 5 is a flowchart of the print data creation process. Print data transmitted from the computer 60 to the printer 1 is created according to a printer driver stored in the memory of the computer 60. That is, the printer driver is a program for causing the computer 60 to create print data and sending the print data to the printer 1.

  The resolution conversion process (S001) is a process for converting the image data output from the application program into a resolution for printing on the paper S. When the resolution for printing on the paper S is specified as 720 × 720 dpi, the image data received from the application program is converted into image data having a resolution of 720 × 720 dpi. Note that the image data after the resolution conversion process is 256-gradation data (RGB data) represented by an RGB color space.

  Here, the image data is a collection of pixel data. A pixel is a unit element constituting an image. An image is formed by two-dimensionally arranging the pixels. The image data having 256 gradations means that one pixel is expressed with 256 gradations, and one pixel data is 8-bit data (2 8 = 256).

  The color conversion process (S002) is a process for converting RGB data into CMYK data represented by a CMYK color space corresponding to the ink of the printer 1. This color conversion process is performed by the printer driver referring to a table (not shown) in which the gradation values of RGB data and the gradation values of CMYK data are associated with each other.

  The density correction process (S003) is a process for correcting the gradation value of each pixel data based on the correction value corresponding to the column region to which the pixel data belongs. Details will be described later.

  The halftone process (S004) is a process for converting high gradation number data (256 gradations) into data of gradation numbers (4 gradations) that can be formed by the printer. In the present embodiment, the printer 1 forms an image by forming “no dot”, “small dot”, “medium dot”, or “large dot” on the pixel. That is, the high gradation number data is converted into four gradation data that can be expressed by the printer 1 of the present embodiment.

  The rasterization process (S005) is a process in which matrix-like image data is rearranged for each pixel data in the order of data to be transferred to the printer 1. The print data generated through these processes is transmitted to the printer 1 by the printer driver together with command data (conveyance amount, etc.) corresponding to the printing method.

<Scanner configuration>
FIG. 6A is a longitudinal sectional view of the scanner 70. FIG. 6B is a top view of the scanner 70 with the upper lid 71 removed. The scanner 70 includes an upper lid 71, a document table glass 73 on which the document 72 is placed, a reading carriage 74 that moves in the sub-scanning direction while facing the document 72 through the document table glass 73, and the scanning carriage 74 in the sub-scanning direction. A guide unit 75 for guiding in the direction, a moving mechanism 76 for moving the reading carriage 74, and a scanner controller (not shown) for controlling each unit in the scanner 70 are provided. In the reading carriage 74, an exposure lamp 77 that irradiates light to the original 72, a line sensor 78 that detects an image of a line in the main scanning direction that is perpendicular to the sub-scanning direction, and reflected light from the original 72 are lined. An optical system 79 for guiding to the sensor 78 is provided. A broken line inside the reading carriage 74 in the drawing indicates a locus of light.

  When reading the image of the document 72, the operator opens the upper cover 71, places the document 72 on the document table glass 73, and closes the upper cover 71. Then, the scanner controller moves the reading carriage 74 along the sub-scanning direction with the exposure lamp 77 emitted, and reads the image on the surface of the document 72 by the line sensor 78. The scanner controller transmits the read image data to the scanner driver of the computer 60, whereby the computer 60 acquires the image data of the document 72.

=== About microfeed printing ===
FIG. 7 is an explanatory diagram of microfeed printing. However, for simplification of explanation, only the cyan ink nozzle row C among the nozzle rows of the head 31 is shown, and the number of nozzles in the nozzle row is also reduced to eight. FIG. 7 shows the position of the cyan ink nozzle row C and the state of dot formation in pass 1 to pass 12. Here, “pass” means that the carriage 21 moves once in the movement direction. The number after the path indicates the order in which the path is performed. In FIG. 7, for convenience of explanation, the cyan ink nozzle row C is illustrated as moving in the transport direction with respect to the paper S. However, the paper S actually moves in the transport direction.

  Hereinafter, the flow of microfeed printing will be described. First, in pass 1, ink is ejected from each nozzle while the carriage 21 is moving in the moving direction. By doing so, a dot row corresponding to each nozzle is formed along the moving direction. A dot row along this moving direction is defined as a raster line. An operation in which raster lines are formed by one pass is defined as a forming operation. FIG. 7 shows a state in which ink is ejected from all nozzles at predetermined intervals. Therefore, in pass 1, eight raster lines are formed side by side in the transport direction at every nozzle interval (k · D). In addition, since the nozzle row of this embodiment includes 180 nozzles arranged in the transport direction, when ink is ejected from all the nozzles at predetermined intervals, 180 raster lines are formed in one pass. They are formed side by side in the transport direction at nozzle intervals. One raster line is formed by one nozzle.

  After pass 1, the paper S is transported by the transport unit 10 by the second transport amount F2, and this transport operation is referred to as a second transport operation. The second transport amount F2 is a minute transport amount. After the second transport operation, in pass 2, a raster line is formed adjacent to each raster line formed in pass 1 on the upstream side of each raster line formed in pass 1. Then, again, the paper S is transported by the second transport amount F2.

  In the present embodiment, after the raster line is formed in one pass, the second transport operation and the forming operation are repeated three times. That is, after the raster line forming operation in pass 1, the paper S is finely fed (= second transport operation), and after the raster line is formed in pass 2, the paper S is finely fed (= second transport operation). Then, after the forming operation in pass 3, the paper S is finely fed (= second transport operation), and a raster line is formed in pass 4. In this way, the operation of repeating the forming operation for forming the raster line while moving the head 31 in the moving direction and the second conveying operation for conveying the paper S by the second conveying amount F2 in the conveying direction is the band forming operation. To do.

  An image composed of raster lines formed by one band forming operation (between pass 1 and pass 4) is defined as a “band”. In FIG. 7, a band-like shape in which 32 raster lines arranged in the transport direction (= “number of raster lines formed by one nozzle between pass 1 to pass 4 × number of nozzles N” = “4 × 8”) are combined. The image becomes a “band”. In this embodiment, an image obtained by combining 720 (= “4 × 180”) raster lines is a “band”. In actual printing, ink is not ejected from every nozzle at a predetermined interval, but ink is ejected from each nozzle in accordance with print data. For this reason, the interval between raster lines arranged in the transport direction and the interval between dots arranged in the movement direction are not always constant.

  Further, the interval k · D between adjacent raster lines formed in pass 1 is complemented by the forming operation of pass 2 to pass 4. For example, the raster formed by nozzle # 1 in pass 2 through pass 4 between the first raster line formed by nozzle # 1 in pass 1 and the fifth raster line formed by nozzle # 2 in pass 1 A line is formed.

  The interval between the raster lines formed between the raster lines formed in pass 1 is the minimum dot interval in the transport direction, and is set as the interval D. Since the raster line is formed at every interval D, the second transport amount F2 = D. In the present embodiment, since three raster lines are printed during the interval k · D between adjacent raster lines formed in pass 1, k = 4. The nozzle interval of each nozzle row is also 4 · D.

  After the band forming operation, the paper S is transported by the first transport amount F1, which is a transport amount greater than the second transport amount F2. This operation is referred to as a first transfer operation. At this time, it is formed by the raster line formed by the nozzle on the most upstream side in pass 4 (nozzle # 8 in FIG. 7, nozzle # 180 in this embodiment) and nozzle # 1 in pass 5 after the first transport operation. The interval between the raster lines is the same length as the second transport amount F2 (= minimum dot interval D). Therefore, the first transport amount F1 is a length obtained by adding the second transport amount F2 to the length in the transport direction of a plurality of raster lines formed in one pass (pass 4) before transport. That is, the first transport amount F1 = (N−1) × (k · D) + F2. N is the number of nozzles that can be used in one pass, and in FIG. 7, the first transport amount F1 = (8-1) × (4 · D) + D = 29 · D. Since the number of usable nozzles is 180, the first carry amount F1 of the present embodiment is 717D (F2 = (180-1) × (4 · D) + D).

  Then, after the first transport operation, the band forming operation is performed again. That is, the raster line forming operation in pass 5 to pass 8 and the second transporting operation, which is a fine feed of the paper S, are repeated to form a band. That is, in microfeed printing, a band forming operation for forming a band by printing a raster line while finely feeding the paper S and a first transporting operation for transporting the paper S by the first transport amount F1 should be printed. Continue until there is no more data. As a result, a plurality of bands are arranged in the transport direction, thereby completing a print image.

=== About density unevenness ===
It is considered that density unevenness occurs due to two causes, nozzle failure and transport error. In order to explain the density unevenness, first, a “pixel region” and a “column region” are set. The pixel area refers to a rectangular area virtually defined on the paper S, and the size and shape are determined according to the printing resolution. One pixel area corresponds to one pixel constituting image data. For example, when the print resolution is 720 dpi (movement direction) × 720 dpi (conveyance direction), the pixel area has a square shape with a size of about 35.28 μm × 35.28 μm (≈ 1/720 inch × 1/720 inch). Become an area. The “row region” refers to a region on the paper S corresponding to each pixel row (a plurality of pixel regions arranged in a direction corresponding to the moving direction).

<Uneven density due to defective nozzle>
FIG. 8A is an explanatory diagram of a state when dots are ideally formed. The ideal formation of a dot means that an ink droplet has landed at the center position of the pixel region, the ink droplet spread on the paper S, and a dot is formed in the pixel region. The fact that each dot is accurately formed in each pixel area means that the raster line is accurately formed in the row area.

  FIG. 8B is an explanatory diagram when density unevenness occurs due to defective nozzles. The raster lines formed in the second row region are formed closer to the third row region (upstream in the transport direction) due to variations in the flight direction of the ink droplets ejected from the nozzles. As a result, the second row region is light and the third row region is dark. In addition, the ink amount of the ink droplets ejected to the fifth row region is smaller than the prescribed ink amount, and the dots formed in the fifth row region are small. As a result, the fifth row region becomes lighter.

  In this way, when a printed image composed of raster lines having different shades is viewed macroscopically, stripe-like density unevenness along the moving direction of the carriage is visually recognized. That is, density unevenness occurs due to ink droplets not being ejected from the nozzles in the vertical direction or a prescribed amount of ink not being ejected from the nozzles, thereby degrading the image quality of the printed image.

<Uneven density due to transport error>
Next, the occurrence of density unevenness due to a transport error will be described. Due to a manufacturing error of the transport roller 13, the paper S may not be transported a predetermined transport amount, and a transport error may occur. In this case, dots are not formed at the correct positions on the paper S, causing density unevenness. Hereinafter, a case where a conveyance error occurs in microfeed printing will be described.

  FIG. 9A is a diagram illustrating how dots are formed when the transport amount is larger than the first transport amount F1. For simplicity, the number of nozzles is reduced in FIG. 9A and FIG. 10A described later. In addition, it is assumed that ink is ejected from all nozzles at predetermined intervals.

  In pass 1 to pass 4, a dot is ideally formed at the center of the 1st to 32nd row regions. Thereafter, in FIG. 9A, D is transported more than the first transport amount F1 = 29 · D. Therefore, nozzle # 1 in pass 5 originally forms a raster line in the 33rd row region, but forms a raster line in the 34th row region.

  FIG. 9B is a diagram showing the density (measured value) of each row region in FIG. 9A. The density of the row region other than the 33rd is a value in the vicinity of the density X. On the other hand, the density of the 33rd row area is significantly smaller than the density of other row areas. This is because the paper S has been transported more than the first transport amount F1, so that no raster line is formed in the 33rd row area, which is a blank space.

  Then, after nozzle # 1 in pass 5 forms a raster line in the 34th row area, paper S is carried by the second carry amount F2, and nozzle # 1 in pass 6 places a raster line in the 35th row area. Form. In the second transport operation, the second transport amount F2 is smaller than the first transport amount F1, so that a large transport error such as the interval D does not occur as shown in FIG. 9A. As a result, the density of the 34th and subsequent row regions is a value near the density X.

  That is, in the microfeed printing, when the paper S is transported the first transport amount F1 after the band forming operation, a transport error is likely to occur. When the paper S is transported more than the first transport amount F1, a margin is formed at the boundary between the bands, streaks occur in the moving direction, and uneven density is caused. Further, a row region having a lighter density than other row regions is generated. This low density row region is a row region corresponding to the boundary between a band formed in a certain pass and a band formed in the next pass. In FIG. 9A, the 33rd row region is a row region corresponding to the boundary between the band formed from pass 1 to pass 4 and the band formed from pass 5 to pass 8.

  FIG. 10A is a diagram illustrating how dots are formed when transported less than the first transport amount F1. FIG. 10B is a diagram showing the density (measured value) of each row region in FIG. 10A.

  After the band forming operation, in FIG. 10A, the first transport amount F1 is transported by D less than 29 · D. For this reason, nozzle # 1 in pass 5 again forms a raster line in the 32nd row region where a raster line has already been formed by nozzle # 8 in pass 4.

  Therefore, as shown in FIG. 10B, only the density of the 32nd row region is not a value near the density X but a value significantly larger than the density X. That is, in the microfeed printing, when the paper S is transported less than the first transport amount F1 in the first transport operation in which the possibility of transport error is greater than that in the second transport operation, The edges overlap and dark streaks appear in the direction of movement, causing density unevenness. Further, a row region having a higher density than other row regions is generated. The dark row region is a row region corresponding to a joint between adjacent bands in the transport direction. In FIG. 10A, the thirty-second column region is a column region corresponding to a band joint.

<Density unevenness correction>
FIG. 8C is an explanatory diagram showing a state when dots are formed by the printing method of the present embodiment. In the present embodiment, the gradation value of the pixel data of the pixel corresponding to the row region is corrected so that a dark image piece is formed in the row region that is dark and easily visible. Further, the gradation value of the pixel data of the pixel corresponding to the row region is corrected so that a dark image piece is formed in a row region that is easily viewed.

  For example, in the figure, the dot generation rate of the second and fifth row regions that are visually recognized as light is high, and the dot generation rate of the third row region that is visually recognized as low is low, corresponding to each row region. The gradation value of the pixel data of the pixel to be corrected is corrected. As a result, the dot generation rate of the raster line in each row area is changed, the density of the image pieces in the row area is corrected, and the density unevenness of the entire print image is suppressed.

  By the way, in FIG. 8B, the reason why the density of the image piece formed in the third row region is high is not due to the influence of the nozzle that forms the raster line in the third row region, but the adjacent second row. This is due to the influence of nozzles that form raster lines in the region. For this reason, when a nozzle that forms a raster line in the third row region forms a raster line in another row region, the image piece formed in that row region is not always dark. That is, even if the image pieces are formed by the same nozzle, the density may be different if the nozzles that form adjacent image pieces are different. In such a case, the density unevenness cannot be suppressed by simply using the correction value associated with the nozzle. Therefore, in this embodiment, the gradation value of the pixel data is corrected based on the correction value set for each row area.

<Uneven density correction processing at printer manufacturing plant>
FIG. 11 is a flowchart of correction value acquisition processing performed in the inspection process after manufacturing the printer. For inspection, as shown in FIG. 1, the printer 1 to be inspected for density unevenness and the scanner 70 are connected to a computer 60. The computer 60 includes, in advance, a printer driver for causing the printer 1 to print a test pattern, a scanner driver for controlling the scanner 70, image processing, analysis, and the like on the test pattern image data read from the scanner 70. A correction value acquisition program for performing the above is installed.

=== S101: Printing Test Pattern ===
First, the printer driver of the computer 60 causes the printer 1 to print a test pattern by the above-described microfeed printing. FIG. 12A is an explanatory diagram of a test pattern. FIG. 12B is an explanatory diagram of a correction pattern.

  The test pattern is composed of four correction patterns formed for every four nozzle rows. Each correction pattern is composed of a strip pattern having three types of density, an upper ruled line, a lower ruled line, a left ruled line, and a right ruled line. The belt-like patterns are generated from image data having a constant gradation value, and the gradation values 76 (density 30%), 128 (density 50%), 179 (density 70%) are sequentially arranged from the left belt-like pattern. ), And the pattern is a band pattern having a higher density in order. These three types of gradation values are referred to as “command gradation values” and are represented by symbols Sa (= 76), Sb (= 128), and Sc (= 179). The test pattern is printed with a resolution of 720 dpi (movement direction) × 720 dpi (conveyance direction).

  The correction pattern is composed of three bands. In the microfeed printing of the present embodiment, one band is formed in four passes while the paper S is finely fed using a nozzle row having 180 nozzles. That is, one band is composed of 720 raster lines. The correction pattern includes 720 raster lines formed from pass 1 to pass 4, 720 raster lines formed from pass 5 to pass 8, and 720 raster lines formed from pass 9 to pass 12. It consists of lines and is composed of a total of 2160 raster lines.

=== S102: Reading Correction Pattern ===
Next, the printed test pattern is read by the scanner 70. When the original on which the test pattern is printed is set in the scanner 70, the raster line is set in the main scanning direction of the scanner 70, and the plurality of raster lines are arranged in the sub-scanning direction of the scanner 70. . The setting direction of the scanner 70 is shown in parentheses indicated by arrows in FIG. 12A.

  In this embodiment, the test pattern is read at a resolution of 720 dpi in the main scanning direction and is read at a resolution of 2880 dpi in the sub-scanning direction. The reason why the direction in which the plurality of raster lines are arranged (sub-scanning direction) is read at a resolution four times the print resolution (720 dpi) is to facilitate the specification of the range of the row area. Conversely, the reason why the reading resolution is lowered in the main scanning direction relative to the sub-scanning direction is to reduce the amount of data to be read and increase the reading speed.

  Further, the reading range is specified with reference to the upper left scanning origin of the read test pattern image. As shown in FIG. 12A, the range of the alternate long and short dash line surrounding the yellow correction pattern is set as the reading range of the yellow correction pattern. Note that the parameters SX1, SY1, SW1, and SH1 for specifying the reading range are set in advance in the scanner driver by the correction value acquisition program. Further, since the range larger than the correction pattern is set as the reading range, the entire yellow correction pattern can be read even if the document is set on the scanner 70 with a slight deviation. Similarly, the reading range of the correction pattern formed by another nozzle row is specified.

=== Detection of Correction Pattern Inclination (S103) and Rotation Process (S104) ===
Next, the correction value acquisition program detects the inclination θ of the image of the correction pattern included in the read image data of each nozzle row (dotted line reading range: SW1 × SH1 rectangular image), and outputs the detected image data to the image data. On the other hand, a rotation process corresponding to the inclination θ is performed.

  FIG. 13A is an explanatory diagram of image data at the time of tilt detection. Hereinafter, description will be made using a coordinate system (x direction, y direction) in the computer 60. The upper left corner of the image data is the origin. Actually, since four correction patterns are arranged in the x direction, the upper and lower ruled lines of other correction patterns are included in the reading range, but are omitted in FIG. 13A. FIG. 13B is an explanatory diagram of detection of the position of the upper ruled line. FIG. 13C is an explanatory diagram of the image data after the rotation process. In practice, the amount of data in the y direction (the direction in which the raster lines are arranged) is four times the amount of data in the x direction, so the image of the correction pattern is an image that has been stretched four times in the y direction. Yes. However, here, for ease of explanation, the y direction of the correction pattern is compressed to ¼ so that the appearance looks the same as the correction pattern at the time of printing.

In order to calculate the inclination θ, the correction value acquisition program reads from the read image data the pixel data of KX1 from the left and KH pixels from the top, and the pixel data of KX2 from the left and the pixels from the left. To extract pixel data of KH pixels. The parameters KX1, KX2, and KH are determined so that the upper ruled line is included in the pixels extracted at this time, and the right ruled line and the left ruled line are not included. Then, the correction value acquisition program obtains the gravity center positions KY1 and KY2 of the gradation values of the extracted KH pieces of pixel data in order to detect the position of the upper ruled line. Then, the correction value acquisition program calculates the inclination θ of the image of the correction pattern from the following equation based on the parameters KX1 and KX2 and the gravity center positions KY1 and KY2.
θ = tan ⁻¹ {(KY2-KY1) / (KX2-KX1)}
After that, based on the calculated inclination θ, the correction pattern image is rotated.

=== S105: Trimming ===
Next, the correction value acquisition program of the computer 60 trims unnecessary pixels from the image data. FIG. 14A is an explanatory diagram of image data at the time of trimming. FIG. 14B is an explanatory diagram of a trimming position on the upper ruled line. Here, as in FIG. 14A, the correction pattern in the y direction is shown to be compressed to ¼.

  The correction value acquisition program extracts pixel data of KX1 pixels from the left and KH pixels from the top and pixel data of KX2 pixels from the left and KH pixels from the top from the image data. . Then, in order to detect the position of the upper ruled line, the correction value acquisition program obtains the barycentric positions of the gradation values of the extracted KH pixel data, and calculates the average value of the two barycentric positions. Then, the nearest pixel boundary is determined as a trimming position at a position that is ½ the width of the row region (for four pixels) from the average barycentric position. Then, the correction value acquisition program cuts out pixels above the determined trimming position and performs trimming.

  FIG. 14C is an explanatory diagram of the trimming position at the lower ruled line. Similarly to the upper ruled line side, the center of gravity position of the lower ruled line is calculated. Then, a pixel lower than the boundary of the nearest pixel at a position lower by ½ of the width of the column region from the center of gravity is cut out and trimmed.

=== S106: Resolution Conversion ===
Next, the correction value acquisition program converts the resolution of the trimmed image data so that the number of pixels in the y direction is the same as the number of raster lines (number of column regions) of the correction pattern. That is, the pixel columns and the column regions along the x direction have a one-to-one correspondence. For example, the uppermost pixel column corresponds to the first column region, and the lower pixel column corresponds to the second column region.

  Since 2160 raster lines printed at 720 dpi are read at a resolution of 2880 dpi, the number of pixels in the y direction of the trimmed image data is 8640 (= 2160 × 4). That is, in order to make the number of raster lines equal to the number of pixel columns, resolution conversion (reduction processing) is performed at a magnification of 1/4. Here, the bicubic method is used for resolution conversion. Since the data in the x direction is read at 720 dpi, it is not necessary to perform resolution conversion.

  However, in practice, the number of pixels in the y direction of the image data may not be 8640 due to an error in printing the correction pattern and an influence of a reading error by the scanner 70. In this case, for example, if the number of pixels in the y direction is 8650, resolution conversion (reduction processing) is performed at a magnification of 2160/8650 (= [number of raster lines] / [number of pixels in the y direction]). The number of pixels in the y direction of the image data after resolution conversion is 2160.

=== S107: Measure the density of the row region ===
Next, the correction value acquisition program calculates the measurement value of each row area of the three types of belt-like patterns in each row area. Hereinafter, the measured value of the first row region of the left belt-like pattern having a density of 30% of the correction pattern formed by the dark ink nozzle row will be described. It should be noted that the measurement of the density of other row regions and other belt-like patterns is similarly performed.

  FIG. 15A is an explanatory diagram of image data when the left ruled line is detected. The correction value acquisition program extracts pixel data of KX pixels from the left, which are H2 pixels from the top, from the resolution-converted image data. The parameter KX is determined in advance so that the left ruled line is included in the pixels extracted at this time. Then, the correction value acquisition program obtains the barycentric position of the left ruled line from the pixel data of the extracted KX pixels.

  FIG. 15B is an explanatory diagram of the measurement range of the density of the band-like pattern having a density of 30% in the first row region. It is known from the shape of the correction pattern that a strip-shaped pattern having a width W3 of 30% density exists on the right side by X2 from the center of gravity of the left ruled line. Therefore, the correction value acquisition program extracts the pixel data in the dotted line range excluding the left and right W4 ranges in the band-like pattern having a density of 30% for each row region. The average value of the gradation values of the extracted pixel data becomes a measured value of the density of 30% in each row region. In this way, the correction value acquisition program measures the density of the three types of belt-like patterns for each row region.

  FIG. 16 is a measurement value table summarizing the measurement results of the density of the three types of yellow belt-like patterns. In this way, the correction value acquisition program creates a measurement value table by associating the measurement values of the density of the three types of belt-like patterns for each row region. Note that the nth measurement value for the yellow command gradation value Sa (= 76) is the measurement value Ya_n, the nth measurement value for the command gradation value Sb (= 128) is the measurement value Yb_n, and the command gradation value is FIG. 16 shows the n-th measurement value for Sc (= 179) as the measurement value Yc_n.

  FIG. 17 is a graph of measured values of the belt-like pattern of yellow command gradation values Sa, Sb, and Sc. The horizontal axis is the row region number, and the vertical axis is the measured value. Although each strip pattern is uniformly formed with each command gradation value, the measured value varies. This variation is a shading difference for each row region, and causes uneven density in the printed image.

  In order to eliminate unevenness in density, it is necessary to eliminate variations in measurement values for each row region at the same gradation value. That is, the density unevenness is improved by bringing the measurement value of each row region close to a constant value. Therefore, in this embodiment, for the same gradation value, the average value of the measurement values of all the column regions is set as the target value, and the command gradation value is corrected so that the measurement value of each column region approaches the target value. For example, the target value of the yellow ink nozzle row having the command gradation value Sb (density 50%) is Ybt, and in the row region i where the measured value is lower than the target value Ybt, printing is darker than the setting of the command gradation value Sb. The gradation value is corrected so that On the other hand, in the row region j where the measured value is higher than the target value Ybt, the gradation value is corrected so that it is printed lighter than the setting of the command gradation value Sb.

=== S108: Calculation of Correction Value ===
By the way, the correction pattern of this embodiment is composed of three bands by microfeed printing. The 2160 raster lines constituting the correction pattern have regularity for every 720 raster lines. For example, the first raster line is formed by nozzle # 1 in pass 1, and the adjacent second raster line is formed by nozzle # 1 in pass 2. The 721st raster line is also formed by nozzle # 1 in pass 5, and the adjacent 722nd raster line is formed by nozzle # 1 in pass 6.

  That is, the nth raster line from the downstream side of the 720 raster lines constituting a certain band and the nth raster line from the downstream side of the 720 raster lines constituting the other band are the same. The same applies to nozzles that are formed by nozzles and affect density unevenness. Therefore, in this embodiment, a correction value is calculated for each regular row region.

  First, 720 average values of the measured values of every three 720 row regions are calculated. Then, 720 correction values calculated from the average value of the measured values are used based on regularity. For example, in the first row area corresponding to the first (downstream) raster line of the raster lines constituting the band, the first, 721, and 1441th three row areas of the correction pattern are included. The correction value calculated from the average value of each measured value is used.

  Next, the correction value calculation method will be described in detail. For example, the average value of the measured values of the fifth, 725th, and 1445th row regions of the band-like pattern of 50% density (command gradation value Sb = 128) formed by yellow is the measured value Y of the fifth row region. It is assumed that 'b_5. In addition, the average value of the measurement values of the sixth, 726th, and 1446th row regions is defined as the measurement value Y′b_6 of the sixth row region. The measured value Y′b_5 in the fifth row region is lower than the target value Ybt, and the measured value Y′b_6 in the sixth row region is higher than the target value Ybt.

FIG. 18A is an explanatory diagram of the target command tone value Sbt with respect to the command tone value Sb in the fifth row region. In the fifth row region, in order to print a density pattern of the target value Ybt, the printer driver may instruct based on the target command gradation value Sbt calculated by the following equation (linear interpolation based on the straight line BC). Good.
Sbt = Sb + (Sc−Sb) ×
{(Ybt-Y'b_5) / (Y'c_5-Y'b_5)}

FIG. 18B is an explanatory diagram of the target command tone value Sbt with respect to the command tone value Sb in the sixth row region. In the sixth row area, in order to print a density pattern of the target value Ybt, the printer driver may command based on the target command tone value Sbt calculated by the following equation (linear interpolation based on the straight line AB). Good.
Sbt = Sb− (Sb−Sa) ×
{(Ybt-Y'b_6) / (Y'a_6-Y'b_6)}

After calculating the target command tone value Sbt in this way, the correction value acquisition program calculates a correction value Hb for the command tone value Sb in the row region by the following equation.
Hb = (Sbt−Sb) / Sb
Then, the correction value acquisition program calculates a correction value Hb for the gradation value Sb for every 720 row regions.

  Further, the correction value acquisition program sets the measured value for the lowest gradation value (= 0) to 0 (point D) and the measured value for the highest gradation value 255 to 255 (point E). And Sc) are calculated as correction values (Ha and Hc). Based on the point D (0, 0), the point A, and the point B (linear interpolation based on the straight line DA or the straight line AB), the correction value Ha for the command gradation value Sa is calculated for each row region. Then, based on the points B, C, and E (255, 255) (linear interpolation based on the straight line BC or the straight line CE), the correction value Hc for the command gradation value Sc is calculated. For all nozzle rows, three correction values (Ha, Hb, Hc) are calculated for each row region.

=== S109: Storage of Correction Value ===
FIG. 19 is an explanatory diagram of a yellow correction value table. Next, the correction value acquisition program stores the correction value in the memory 53 of the printer 1. In the correction value table for each nozzle row, three correction values (Ha, Hb, Hc) are associated with each row region. For example, three correction values (Ha_n, Hb_n, Hc_n) are associated with the nth row region. The memory 53 stores a correction value table for each nozzle row.

  After the correction value is stored in the memory 53 of the printer 1, the correction value acquisition process ends. Then, the CD-ROM storing the printer driver is bundled with the printer 1, and the printer 1 is shipped from the factory.

=== Processing under the user ===
A user who has purchased the printer 1 connects the printer 1 to a computer 60 owned by the user (a computer different from the computer at the printer manufacturing factory).

  Next, the user sets the enclosed CD-ROM in the recording / reproducing apparatus 80 and installs the printer driver. The printer driver installed in the computer 60 requests the printer 1 to transmit the correction value stored in the memory 53 to the computer 60. The printer 1 transmits a correction value table to the computer 60 in response to the request. The printer driver stores the correction value sent from the printer 1 in a memory in the computer 60. As a result, the image data created by the computer 60 can be printed by the printer 1.

  When the printer driver receives a print command from the user, the printer driver generates print data and transmits the print data to the printer 1. The printer 1 performs a printing process according to the print data. The print data creation method is as described above (FIG. 5).

=== About Density Correction Processing ===
Hereinafter, the density correction process will be described in detail. In the density correction process, the gradation value for each pixel data (gradation value S_in before correction) is corrected based on the correction value H corresponding to the column region to which the pixel data belongs (after correction). (Gradation value S_out) processing. Hereinafter, the density correction process of the yellow ink nozzle row will be described.

If the gradation value S_in before correction is the same as one of the command gradation values (Sa, Sb, Sc) of the yellow ink nozzle row, the gradation value S_in is stored in the memory of the computer 60 and the correction value Ha. , Hb, Hc can be used as they are. For example, if the gradation value S_in before correction is S_in = Sc, the gradation value S_out after correction is obtained by the following equation.
S_out = Sc × (1 + Hc)

FIG. 20 is an explanatory diagram of density correction processing for the nth row region of the yellow ink nozzle row. The horizontal axis is the gradation value S_in before correction, and the vertical axis is the gradation value S_out after correction. This figure is a diagram showing a correction method when the gradation value S_in before correction is different from the command gradation values (Sa, Sb, Sc). The dotted line in the figure is a graph when there is no need to correct the gradation value, and is a graph when the correction value H is 0. The corrected gradation value S_out for the gradation value S_in is calculated by the following equation by linear interpolation based on the correction value Ha of the command gradation value Sa and the correction value Hb of the command gradation value Sb.
S_out = Sat + (Sbt−Sat) × {(S_in−Sa) / (Sb−Sa)}

In addition, a correction value H_out corresponding to the gradation value S_in is calculated by linear interpolation between the correction values (Ha, Hb, Hc) corresponding to each command gradation value, and based on the calculated correction value H_out. Then, the corrected gradation value S_out may be calculated by the following equation.
S_out = S_in × (1 + H_out)

  Since microfeed printing has regularity for every 720 row regions, the printer driver repeatedly uses density correction processing by repeatedly using 720 correction values H in order for every 720 row regions. I do. Thereby, the data amount of the correction value H to be stored can be reduced. Then, the printer driver similarly performs density correction processing not only on the yellow ink nozzle row but also on the gradation values of the pixel data of other nozzle rows.

  Through the above-described density correction processing, correction is performed so that the gradation value of the pixel data of the pixel corresponding to the row area is low for the row area that is easily visually recognized. On the other hand, for a column region that is faint and easily visible, correction is performed so that the gradation value of the pixel data of the pixel corresponding to the column region is high. In other words, as shown in FIG. 8C, in a row region that is easily visible, it is corrected so that the gradation value of the pixel data of the row region is low, so that the dots that make up the raster line of the row region The dot generation rate becomes lower. On the other hand, the dot generation rate is high in the row region that is easily recognized visually. And the density unevenness of the whole printed image is improved.

  Hereinafter, the characteristic part of this embodiment is demonstrated.

=== About slope of measured value ===
FIG. 21 is a graph of measured values (reading gradation values) of the density of each row region of the belt-like pattern when the scanner 70 is normal. It is the same graph as FIG. 17, a horizontal axis shows a row | line | column area | region number, and a vertical axis | shaft shows the measured value of each row | line | column area | region. For simplification of explanation, FIG. 21 shows only measured values in each row region of a band-like pattern with a yellow density of 50%. When the scanner 70 is normal, the measurement values of each row region are distributed in the vicinity of the average value Ybt of the measurement values of all the row regions.

  FIG. 22 is a graph of measured values of the density of each row region when the scanner 70 is abnormal. The scanner 70 is abnormal because, for example, the guide portion 75 of the scanner 70 is attached obliquely or the upper cover 71 is not sufficiently closed and the document 72 is floating, and the reading carriage 74 is in the sub-scanning direction. That is, the optical distance between the document 72 and the line sensor 78 varies according to the position. Then, the output result of the line sensor 78 changes according to the position of the reading carriage 74 in the sub-scanning direction. That is, when there is an abnormality in the scanner 70, the measured value changes according to the position of the row region, and the measured value is inclined overall. In FIG. 22, as an example of a reading result when there is an abnormality in the scanner 70, a state in which the graph of the measurement value is inclined downward as a whole is shown. In this case, the measurement values in each row region are not distributed in the vicinity of the average value Ybt of the measurement values in all row regions. The measured value of the smaller row area number (hereinafter referred to as the downstream side) is a value that is significantly larger than the average value Ybt. On the other hand, the measured value of the larger row area number (hereinafter referred to as the upstream side) is significantly smaller than the average value Ybt.

  FIG. 23A is a reference diagram when the correction value H is calculated without correcting the slope of the graph of measured values. When the measurement value is tilted according to the position of the row area, the correction value H calculated based on the measurement value is also tilted according to the position of the row area. FIG. 23B is a reference diagram showing how a correction pattern is formed based on a correction value H calculated without correcting the slope of the measurement value.

  In the present embodiment, the correction value H is calculated from the average value of three measurement values for every 720 row regions. In the graph of FIG. 22, for example, the first row region having a relatively high measurement value in the first to 720th row regions and the relatively high measurement value 721 in the 721 to 1440th row regions. The correction value Hb_1 of the first column region is calculated from the average value of the respective measured values of the 1441st column region having a relatively high measurement value in the 1441st to 2160th column regions. . Conversely, the 720th row region having a relatively low measurement value in the 1st to 720th row regions, and the 1440th row region having a relatively low measurement value in the 721st to 1440th row regions, The correction value Hb_720 of the 2160th row region is calculated from the average value of the measured values of the row regions having relatively low measurement values in the 1441st to 2160th row regions.

  That is, in FIG. 22, the larger the row area number, the more erroneously read by the scanner 70 that the printed density is lighter than the actually printed density. As the row area number increases, the correction value H increases so as to print darker. Therefore, if the correction value H is calculated without correcting the inclination of the graph when the measured value graph is inclined downwardly as shown in FIG. 22, the graph showing the correction value H of each row region is as follows. It becomes a graph of rising right like FIG. 23A. That is, the slope of the graph of the measured value and the slope of the graph of the correction value H are reversed.

  As described above, density unevenness is improved even if the correction value H is calculated from the reading result when the scanner 70 is abnormal (FIG. 22) instead of the reading result when the scanner 70 is normal (FIG. 21). Not. If the correction value H increases as the row area number increases due to an abnormality in the scanner 70 (FIG. 23A), even if an attempt is made to print a band of the same density, the band is gradually darkened from upstream to downstream in one band. (FIG. 23B). That is, when printing a belt-like pattern having the same density, the raster line printed by the nozzle # 180 on the upstream side is printed darker than the raster line printed by the nozzle # 1 on the downstream side. In addition, since the difference in shade between adjacent row regions is small, deterioration in image quality is not noticeable, but when viewed macroscopically, the shade appears to change step by step for each band.

  FIG. 23C is a reference diagram in which a part of the figure showing the state of forming the belt-like pattern of FIG. 23B is enlarged. In microfeed printing, a raster line printed relatively darkly by nozzle # 180 and a raster line printed relatively lightly by nozzle # 1 are adjacent to each other. That is, a portion where the difference in density is conspicuous occurs for each band, and the density unevenness becomes conspicuous.

  As described above, when the correction value H is calculated without correcting the inclination even though the graph of the measurement value as the reading result is inclined due to the abnormality of the scanner 70, the band is stepwise from the upstream side to the downstream side. As a result, the shading changes and a portion where the shading difference is conspicuous occurs at every joint of the bands. Therefore, in this embodiment, after measuring the density of the row region in S107, the inclination of the graph of the measured value is corrected.

=== About Correction of Inclination (Continued in S107) ===
First, the correction value acquisition program takes out the measurement value of the row region within the preset approximate straight line calculation range. Then, the correction value acquisition program approximates the measured value of each row region within the approximate straight line calculation range with a straight line, and corrects the slope of the graph of the measured value based on the approximate straight line.

  Hereinafter, a case where the graph showing the measurement result of the belt-like pattern having a yellow density of 50% that is printed by being transported more than the first transport amount F <b> 1 is inclined due to an abnormality of the scanner 70 will be described as an example.

  FIG. 24A is a diagram showing an approximate straight line calculation range of the present embodiment. FIG. 24B is a diagram after correcting the slope of the graph of measured values. FIG. 24A and FIG. 24B are graphs in which the horizontal axis indicates the row region number and the vertical axis indicates the measured value of each row region. The approximate straight line calculation range of this embodiment is the 20th to 2142th row region (details will be described later). Then, the correction value acquisition program calculates an approximate line by the least square method from the measured values in the row area within the approximate line calculation range. Note that the thick solid line in FIG. 24A is an approximate line.

  Thereafter, the correction value acquisition program calculates a correction value, which is a difference between the value indicated by the approximate straight line in each column region and the average value Ybt, for each column region. Then, the correction value corresponding to each column region is subtracted from the measured value of each column region, and the inclination of the graph is corrected as shown in FIG. 24B. For example, the value indicated by the approximate straight line in the first row region in FIG. 24A is A_1. Although the first row region is outside the approximate straight line calculation range, the correction value is calculated by extending the approximate straight line. The difference between A_1 and the average value Ybt indicated by the approximate straight line is the correction value R_1 of the first row region. Then, the correction value R_1 is subtracted from the measurement value Yb_1 of the first row region. By performing this correction operation on all the column regions, the inclination of the graph is corrected.

  By the way, in the case of microfeed printing, when the sheet is transported more than the first transport amount F1, as shown in FIG. 9, the measured value of the row region corresponding to the joint of the bands is significantly smaller than the average value X. Value. On the other hand, as shown in FIG. 10, when transported less than the first transport amount F1, the measured value of the row region corresponding to the portion where the band ends overlap is significantly larger than the average value X. .

  FIG. 24A is a graph showing a measurement result when the belt-shaped pattern is printed while being transported more than the first transport amount F1. For this reason, the measurement values of the 721st and 1441st row regions corresponding to the joints of the bands are not located in the vicinity of the average value Ybt, and are significantly smaller than the measurement values of the other row regions.

  Thus, if the measured value of the row region corresponding to the joint of the bands is a value far from the average value Ybt, the approximate straight line may not be accurately calculated depending on how the approximate straight line calculation range is set. Hereinafter, a method for obtaining the approximate straight line calculation range when the approximate straight line cannot be accurately calculated will be described with reference to a comparative example.

<Approximate straight line calculation range of first comparative example>
FIG. 25 is a diagram showing an approximate straight line calculation range of the first comparative example. 24A and 25 are the same graphs except for the approximate straight line calculation range (the same applies to FIGS. 26 and 27). In the first comparative example, the 620th to 2142th column regions are set as approximate straight line calculation ranges. That is, the approximate straight line of the first comparative example is calculated based on the measurement values of the 620th to 2142th row regions. In FIG. 25, the approximate straight line of the first comparative example is indicated by a thick solid line. In FIG. 25, an ideal approximate line calculated from the approximate line calculation range of the present embodiment is also indicated by a dotted line.

  Now, in the approximate straight line calculation range (hereinafter referred to as calculation range) of the first comparative example, the 721st and 1441th column regions corresponding to the joints of the bands are not evenly arranged. More specifically, 700 column regions are included between the 1441th column region and the 2142th column region, which is the largest column region in the calculation range, whereas (1441st and 2142th are Not included), only 100 column regions are included between the 721st column region and the 620th column region which is the smallest column region in the calculation range. That is, the row regions (721 and 1441) corresponding to the joints of the bands are located closer to the downstream side in the calculation range.

  Therefore, when calculating the approximate straight line of the first comparative example, the approximate straight line on the downstream side within the calculation range is easily affected by the result of the measurement value of the 721st row region. That is, since the measured value of the 721st row region is significantly smaller than the average value Ybt, the value indicated by the approximate straight line of the first comparative example also becomes small. If it demonstrates on a graph, compared with the approximate straight line (dotted line) of this embodiment, the approximate straight line (solid line) of a 1st comparative example is located in the lower part, so that it becomes downstream. The approximate straight line calculated under the influence of the row region corresponding to the joint of such a band cannot correct the inclination of the graph of the measured value accurately. As a result, the graph indicating the correction value H with respect to the density unevenness is also inclined, and the density of the band changes gradually from the upstream side to the downstream side.

  The reason why the slope of the measured value graph is not corrected is that the correction value calculated from the approximate line of the present embodiment is different from the correction value calculated from the approximate line of the first comparative example. For example, in the first row region of FIG. 25, the value indicated by the approximate line of the first comparative example is smaller than the value indicated by the approximate line of the present embodiment. Therefore, the correction value R′_1 of the first comparative example is smaller than the correction value R_1 of the present embodiment for the first row region. Then, even if the correction value R′_1 of the first comparative example is subtracted from the measurement value Yb_1 of the first row region, the measurement value of the first row region after correction remains a value that is still larger than the average value Ybt. is there. That is, because the correction value of the present embodiment is different from the correction value of the first comparative example, the corrected measurement value is not positioned in the vicinity of the average value Ybt depending on the row region.

  In addition, when correcting the measurement value of each column region, the correction direction (vertical direction on the graph) is different between the first comparative example and this embodiment depending on the column region, because the inclination of the graph is not corrected. is there. That is, the first comparative example is different from the present embodiment in that the correction is made so that the measured value is increased or the measurement value is corrected.

  In the present embodiment, the value indicated by the approximate straight line in the 1080th row region is the average value Ybt. The measured value of the row region downstream of the 1080th row region (1-1079th) tends to be larger than the average value Ybt, and is upstream of the 1080th row region (1081st to 2160th). The measured value in the row region of the above tends to be smaller than the average value Ybt. Therefore, in order to bring the measurement value closer to the average value Ybt, the measurement value in the row region downstream from the 1080th is corrected so that the value becomes smaller. On the graph, the measured value in the row region downstream from the 1080th is corrected downward. Conversely, the measured values in the column region upstream from the 1080th are corrected so as to increase.

  However, in the first comparative example, the value indicated by the approximate line in the 800th row region is the average value Ybt. That is, in the first comparative example, the measurement value in the row region downstream from the 800th is corrected so that the value becomes smaller, and the measurement value in the row region upstream from the 800th becomes larger. To be corrected. As a result, the measured values in the 721st to 800th row regions tend to be larger than the average value Ybt, but are corrected so as to be larger. That is, since the correction value direction of the present embodiment is different from the correction direction of the first comparative example, the measurement value is corrected so as to move away from the average value Ybt depending on the row region.

  As described above, in the method of setting the approximate straight line calculation range of the first comparative example, an approximate straight line having a slope different from the approximate straight line of the present embodiment is calculated, and the slope of the graph of the measured value is not corrected. Therefore, in the first comparative example, it becomes a problem to prevent the approximate straight line from being inclined to the upstream side or the downstream side due to the influence of the row region corresponding to the joint of the bands.

<Approximate straight line calculation range of second comparative example>
FIG. 26 is a diagram illustrating an approximate straight line calculation range of the second comparative example. In the second comparative example, the approximate straight line is calculated using the 20th to 1542th row regions as the approximate straight line calculation range. In FIG. 26, the approximate straight line of the second comparative example is indicated by a thick solid line, and the approximate straight line of the present embodiment is indicated by a dotted line.

  In the second comparative example as well, in the approximate straight line calculation range (hereinafter referred to as the calculation range), the 721st and 1441th column regions corresponding to the joints of the bands are equally distributed in the second comparative example. Not placed. However, contrary to the first comparative example, the row regions (721 and 1441) corresponding to the joints of the bands are located closer to the upstream side in the calculation range.

  When calculating the approximate straight line of the second comparative example, the upstream approximate straight line within the calculation range is easily affected by the result of the measurement value of the 721st row region. As a result, the value indicated by the approximate line of the second comparative example is smaller than the value indicated by the approximate line of the present embodiment.

  Then, since the correction value of the second comparative example is different from the correction value of the present embodiment, the inclination of the graph is not corrected. In particular, in the upstream row region, the difference between the correction values of the second comparative example and this embodiment is large, and the inclination of the graph is not corrected. In this embodiment, the measured value of the approximate line in the 1080th row region indicates the average value Ybt, whereas in the second comparative example, the measured value of the approximate line in the i-th row region indicates the average value Ybt. . For this reason, in the i-th to 1080th row regions, the direction in which the measured value is corrected (whether the measured value is increased or decreased) differs between this embodiment and the second comparative example. As a result, depending on the row region, the measured value is corrected so as to move away from the average value Ybt, and the inclination of the graph is not corrected. As a result, the slope of the graph showing the correction value H for density unevenness is not completely corrected, and the band changes in gradation from the upstream side to the downstream side.

  Therefore, in the second comparative example, it is a problem to prevent the approximate straight line from being inclined to the upstream side or the downstream side due to the influence of the row region corresponding to the joint of the bands.

<Approximate straight line calculation range of this embodiment>
Therefore, in the present embodiment (FIG. 24A), it is affected by the column region corresponding to the joint of the bands (the boundary pixel column / the column region and the pixel column have a one-to-one correspondence relationship due to the resolution conversion). The approximate straight line calculation range (hereinafter referred to as the calculation range) is set so that the row regions corresponding to the joints of the bands are evenly arranged so as not to be present.

  First, the column region on the most upstream side (second boundary pixel column) and the column region on the most downstream side (first boundary pixel column) among the column regions likely to correspond to the joints of the bands are used as a reference. In this embodiment, since the correction pattern is composed of three bands, there are two row regions that are likely to correspond to the joints of the bands. In this embodiment, the downstream reference is the 721st column region (first boundary pixel column), and the upstream reference is the 1441th column region (second boundary pixel column).

  Then, the number of column regions included between the downstream reference column region and the most downstream column region (downstream pixel column) in the calculation range, the upstream reference column region, and the maximum in the calculation range. The most downstream row region and the most upstream row region are determined so that the number of row regions included between the upstream row regions (upstream pixel rows) becomes equal. That is, the interval between the downstream pixel column and the first boundary pixel column is equal to the interval between the upstream pixel column and the second boundary pixel column.

  In addition, about 20 row regions upstream of the most downstream row region (first row region) of all row regions and the most upstream row region (2160th row region) of all row regions. ) About 20 row regions downstream are removed from the approximate straight line calculation range. This is because the 20 row areas from the first row area and the 20 row areas from the 2160th row area are close to the margin of the paper S on which the correction pattern is printed, and therefore are read by the scanner 70. This is because the density may have been measured to be lighter than actual due to the influence of the margin. In other words, the most downstream column region (20th column region) in the calculation range is more than the center column region (360th column region) of the band (1st to 720th column region) to which the column region belongs. The most upstream row region (2142nd row region) located downstream is the center row region (1801st row region) of the band (1441 to 2160th row region) to which the row region belongs. ) Located upstream of).

  Summarizing the above, in the present embodiment, the 701st row region counting from the downstream side reference 721st row region to the downstream side is regarded as the most downstream row region (21st row region) within the calculation range. ). Then, the 701st row region counting from the upstream side reference 1441 to the upstream side is set as the most upstream row region (2142st row region) in the calculation range. That is, the approximate straight line calculation range of the present embodiment is the 20th to 2142th row region.

  In the approximate straight line calculation range of the present embodiment, since the row regions corresponding to the joints of the bands are arranged uniformly, the approximate straight line is inclined to either the upstream side or the downstream side on the graph indicating the measured value. There is no end to it. In addition, an approximate straight line is calculated from a wide calculation range using a row region other than the row region that may be printed lightly under the influence of the margin as the approximate straight line calculation range of the present embodiment. For this reason, the value indicated by the approximate straight line is neither too large nor too small without being affected by the row region corresponding to the joint of the bands. That is, the inclination of the graph of the measured value is accurately corrected by the approximate straight line of the present embodiment that is not affected by the row region corresponding to the band joint. As a result, the graph indicating the correction value H for density unevenness does not tilt, and the density of the band does not change stepwise from the upstream side to the downstream side. In addition, there is no difference in shading between the bands.

  By the way, in the calculation range, the column regions corresponding to the joints of the bands (hereinafter referred to as boundary column regions) are arranged uniformly, in other words, the column region on the most downstream side in the calculation range and the calculation range The distance from the central row region (hereinafter referred to as the central row region) is equal to the distance between the uppermost row region and the central row region in the calculation range. For example, in the present embodiment (FIG. 24A), the 1081st row region is the central row region, and is included between the most downstream row region (20th row region) and the 1081st row region in the calculation range. The number of column regions is equal to the number of column regions included between the upstreammost column region (the 2142th column region) and the 1081st column region in the calculation range.

  If the calculation range includes three or more boundary row regions and the number of boundary row regions other than the most downstream and uppermost boundary row regions is an even number, the boundary row closest to the upstream side from the central row region The interval between the region and the central row region is equal to the interval between the boundary row region and the central row region that are closest to the downstream side from the central row region. In the calculation range, if the number of boundary row areas other than the most downstream and uppermost boundary row areas is an odd number, the middle boundary row region of the odd number of boundary row regions is the central row region. It becomes.

  In addition, a row region that is likely to correspond to a joint between bands can be predicted in advance depending on what correction pattern is printed. Therefore, an approximate straight line calculation range is set in advance in the correction value acquisition program in accordance with the correction pattern to be printed.

  In the present embodiment, the case where a belt-shaped pattern is printed by being transported more than the first transport amount F1 is taken as an example, and the 721st and 1441th row regions correspond to the band joints. Not exclusively. For example, if it is transported less than the first transport amount F1, the 720th and 1440th row regions may correspond to the joint of the bands. In addition, there may be a large amount of conveyance error and a plurality of row regions corresponding to the joints of the bands. Even in such a case, since the approximate straight line calculation range is set in advance, the row regions corresponding to the joints of the bands are not necessarily arranged evenly. For example, when the 720th and 1440th column regions correspond to the band joint, the number of column regions (699) between the 720th column region and the 20th column region, the 1441th column region, and 2142 The number of row regions (701) between the first row regions is different. However, even if the position of the row area corresponding to the band joint in the approximate straight line calculation range is tilted upstream by two of the hundreds of row areas, the slope of the approximate straight line It does not affect. For this reason, there is no problem even if the approximate straight line calculation range is set in advance with reference to the row region that is likely to correspond to the joint of the bands.

=== Other Embodiments ===
Each of the above embodiments is mainly described for a printing system having an inkjet printer, but includes disclosure of a method for setting an approximate straight line calculation range when calculating an approximate straight line for correcting the inclination of a graph. Yes. The above-described embodiments are for facilitating understanding of the present invention, and are not intended to limit the present invention. The present invention can be changed and improved without departing from the gist thereof, and it is needless to say that the present invention includes equivalents thereof. In particular, the embodiments described below are also included in the present invention.

<Criteria for setting approximate straight line calculation range>
In the above-described embodiment, it is described that the approximate straight line calculation range is set on the basis of the row region that is likely to correspond to the joint of the bands. However, the present invention is not limited to this. For example, as in microfeed printing, the most upstream raster line of a certain band is formed by the nozzle (# 180) at the most upstream side of the nozzle array, and the most downstream raster line of the next band is formed as the most upstream raster line of the nozzle array. In the case of a printing method in which the downstream nozzle (# 1) is formed, a raster line formed by nozzle # 180 (the most upstream nozzle in the nozzle row) or nozzle # 1 (the most downstream nozzle in the nozzle row). In other words, the approximate straight line calculation range is set on the basis of the row region corresponding to.
In addition, the row region corresponding to the raster line formed by the nozzle # 180 on the most upstream side in the nozzle row may be set as a row region that is likely to correspond to a band joint. Alternatively, the row region corresponding to the raster line formed by the nozzle # 1 on the most downstream side of the nozzle row may be a row region that is likely to correspond to the band joint.

<About band joints>
In the above-described embodiment, the correction pattern composed of three bands is taken as an example, so that there are two band joints. However, depending on the size of the pattern, the number of band joints is not always two.
For example, when the correction pattern is composed of two bands and there is one column region that is likely to correspond to the joint between the bands, n is counted from the column region (first boundary pixel column = second boundary pixel column) on the downstream side. The number of column regions is defined as the most downstream column region within the approximate straight line calculation range, and the nth column region counted upstream from the column region is defined as the most upstream column region within the approximate line calculation range.
In addition, when there are three or more column regions that are likely to correspond to the joint of the band, the most downstream column region (first boundary pixel column) of the three or more column regions is used as the downstream reference, The approximate straight line calculation range is set with the upstreammost row region (second boundary pixel row) as the upstream reference.

<About approximate straight line calculation range>
In the above-described embodiment, the row area other than the row area where the density may be measured lightly under the influence of the margin is set as the approximate straight line calculation range. However, the present invention is not limited to this. For example, if the approximate straight line calculation range is set so that the column areas corresponding to the joints of the bands are evenly arranged, the approximate straight line calculation range is determined so that the column area not affected by the margin is also outside the approximate straight line calculation range. May be set narrower (first range example).
FIG. 27 is a diagram showing a narrow approximate straight line calculation range of the first range example. In the first range example, the approximate straight line is calculated using the 620th to 1542th column regions as the approximate straight line calculation range. In FIG. 27, the approximate straight line of the first range example is indicated by a thick solid line, and the approximate straight line of the present embodiment is indicated by a dotted line.
Within the approximate straight line calculation range (hereinafter referred to as the calculation range) of the first range example, the two column regions 721 and 1441 corresponding to the joint of the bands are equally arranged. The number of column regions between the 721st column region and the 620th column region, which is the smallest column region in the calculation range (100, excluding 620 and 721), the 1441th column region and the calculation range The number (100) of the row areas between the 1542st row area, which is the largest row area, is equal. Therefore, unlike the first comparative example and the second comparative example, the approximate straight line in the first range example does not tilt upstream or downstream with respect to the approximate straight line of the present embodiment. As a result, although it is not perfect, the slope of the graph indicating the measured value is corrected, and the density of the band does not change stepwise from the upstream side to the downstream side. In addition, there is no difference in shading between the bands.
However, as shown in FIG. 27, the value indicated by the approximate line of the first range example is smaller than the value indicated by the approximate line of the present embodiment. That is, the approximate straight line (dotted line) in the first range example is positioned below the approximate straight line (solid line) of the present embodiment.
This is because, in the first range example, the method of taking the approximate straight line calculation range is narrow, so that the influence of the column regions (721st and 1441th) corresponding to the joint of the bands becomes large. That is, if the approximate straight line calculation range is narrow, the number of measurement values for calculating the approximate straight line is small, and therefore the measurement values of the 721st and 1441th column regions, which are extremely smaller than the average value Ybt, are measured. Will affect the position of the approximate straight line.
In other words, even if the approximate straight line calculation range is set so that the column areas corresponding to the band joints are evenly arranged, if the approximate straight line calculation range is set narrow, the influence of the column areas corresponding to the band joints is reduced. Accordingly, there is a possibility that the value indicated by the approximate straight line is too large or too small than necessary.

It is a system configuration figure of this embodiment. 1 is an overall configuration block diagram of a printer according to an embodiment. 3A is a schematic diagram of the overall configuration of the printer, and FIG. 3B is a cross-sectional view of the overall configuration of the printer. It is explanatory drawing which shows the arrangement | sequence of the nozzle in the lower surface (nozzle surface) of a head. It is a flowchart of a print data creation process. 6A is a longitudinal sectional view of the scanner, and FIG. 6B is a top view of the scanner with the upper lid removed. It is explanatory drawing of microfeed printing. FIG. 8A is an explanatory diagram of a state when dots are ideally formed, FIG. 8B is an explanatory diagram when density unevenness occurs, and FIG. 8C is a diagram in which dots are formed by the printing method of the present embodiment. It is explanatory drawing of the mode. FIG. 9A is a diagram illustrating how dots are formed when transported more than the first transport amount F1, and FIG. 9B is a diagram illustrating the density (measured value) of each row region in FIG. 9A. FIG. 10A is a diagram illustrating how dots are formed when transported less than the first transport amount F1, and FIG. 10B is a diagram illustrating the density (measured value) of each row region in FIG. 10A. It is a flowchart of the correction value acquisition process performed at the inspection process after printer manufacture. It is explanatory drawing of a test pattern. It is explanatory drawing of the pattern for a correction | amendment. 13A is an explanatory diagram of image data at the time of tilt detection, FIG. 13B is an explanatory diagram of detection of the position of the upper ruled line, and FIG. 13C is an explanatory diagram of image data after the rotation processing. 14A is an explanatory diagram of image data at the time of trimming, FIG. 14B is an explanatory diagram of a trimming position at an upper ruled line, and FIG. 14C is an explanatory diagram of a trimming position at a lower ruled line. FIG. 15A is an explanatory diagram of image data at the time of detection of the left ruled line, and FIG. 15B is an explanatory diagram of a density measurement range of a band-like pattern having a density of 30% in the first row region. It is the measurement value table which put together the measurement result of the density | concentration of three types of strip | belt-shaped patterns of yellow. It is a graph of the measured value of the strip | belt-shaped pattern of yellow command gradation value Sa, Sb, Sc. 18A is an explanatory diagram of the target command tone value Sbt with respect to the command tone value Sb in the fifth row region, and FIG. 18B is an explanation of the target command tone value Sbt with respect to the command tone value Sb in the sixth row region. FIG. It is explanatory drawing of the correction value table of yellow. It is explanatory drawing of the density correction process of the nth row | line area | region of a yellow ink nozzle row. It is a graph of the measured value of the density | concentration of each row area | region of a strip | belt-shaped pattern when a scanner is normal. It is a graph of the measured value of the density | concentration of each row | line area | region when a scanner is abnormal. FIG. 23A is a reference diagram when the correction value is calculated without correcting the slope of the measured value graph, and FIG. 23B shows the correction pattern based on the correction value calculated without correcting the slope of the measured value. FIG. 23C is a reference diagram showing an enlarged view of a part of the figure showing a state of forming the belt-like pattern of FIG. 23B. FIG. 24A is a diagram showing the approximate straight line calculation range of the present embodiment, and FIG. 24B is a diagram after correcting the slope of the graph of measured values. It is a figure which shows the approximate line calculation range of a 1st comparative example. It is a figure which shows the approximate line calculation range of a 2nd comparative example. It is a figure which shows the narrow approximate line calculation range of the 1st range example.

Explanation of symbols

1 printer,
10 transport unit, 11 paper feed roller, 12 transport motor, 13 transport roller,
14 platen, 15 paper discharge roller,
20 Carriage unit, 21 Carriage, 22 Carriage motor,
30 head units, 31 heads,
40 detector groups, 41 linear encoder, 42 rotary encoder,
43 Paper detection sensor, 44 Optical sensor,
50 controller, 51 interface unit, 52 CPU, 53 memory,
60 computers,
70 Scanner, 71 Top cover, 72 Document, 73 Platen glass,
74 reading carriage, 75 guide section, 76 moving mechanism,
77 exposure lamp, 78 line sensor, 79 optical system,
80 Recording / playback device

Claims (7)

A printing apparatus comprising: a band forming operation for forming a band composed of a plurality of raster lines along a predetermined direction; and a first transport operation for transporting a medium by a first transport amount in a crossing direction that is a direction crossing the predetermined direction. A step of forming a pattern by repeating,
Causing the scanner to read the pattern, and obtaining a read gradation value for each pixel column composed of a plurality of pixels arranged in a direction corresponding to the predetermined direction;
Calculating an approximate straight line using at least a part of the acquired reading gradation values as a calculation range;
Modifying the read tone value based on the approximate straight line;
Calculating a correction value based on the corrected read gradation value;
Printing based on the correction value;
A printing method comprising:
In the band forming operation, the raster line formed based on the pixel column is formed on the medium while moving the nozzle in the predetermined direction, and the medium is transported in the intersecting direction to the first transport amount. The printing apparatus repeats the second transport operation for transporting with a smaller second transport amount,
The pattern is formed by arranging a plurality of bands in the intersecting direction, and the pixel column corresponding to the boundary of the band is a boundary pixel column,
An interval between a downstream pixel column that is the pixel column on the most downstream side in the calculation range and a first boundary pixel column that is located upstream of the downstream pixel column;
The interval between the upstream pixel column that is the pixel column on the most upstream side in the calculation range and the second boundary pixel column located on the downstream side of the upstream pixel column is equal,
A printing method characterized by the above. The printing method according to claim 1, comprising:
When the pattern is composed of two bands,
The first boundary pixel column and the second boundary pixel column are equal;
Printing method. The printing method according to claim 1, comprising:
When the pattern is composed of three or more bands,
The first boundary pixel column is the boundary pixel column on the most downstream side of the pattern,
The second boundary pixel column is the boundary pixel column on the most upstream side of the pattern.
Printing method. The printing method according to claim 3, wherein
When a plurality of the nozzles are arranged in the intersecting direction to form a nozzle row,
The first boundary pixel column is a pixel column corresponding to a raster line formed by the nozzle on the most upstream side of the nozzle column,
The second boundary pixel row is a pixel row corresponding to a raster line formed by the nozzle on the most downstream side of the nozzle row,
Printing method. The printing method according to claim 3, wherein
When a plurality of the nozzles are arranged in the intersecting direction to form a nozzle row,
The first boundary pixel column and the second boundary pixel column are pixel columns corresponding to a raster line formed by a nozzle on the most upstream side of the nozzle columns, or
The first boundary pixel column and the second boundary pixel column are pixel columns corresponding to raster lines formed by the nozzles on the most downstream side of the nozzle columns.
Printing method. A printing method according to any one of claims 1 to 5,
The correction value is calculated for each pixel row corresponding to the plurality of raster lines constituting the band,
The correction value is repeatedly used when printing the plurality of bands.
Printing method. A printing method according to any one of claims 1 to 6,
The downstream raster line formed based on the downstream pixel row is located on the downstream side of the raster line at the center of the band to which the downstream raster line belongs,
The upstream raster line formed based on the upstream pixel column is located upstream of the raster line at the center of the band to which the upstream raster line belongs.
Printing method.