JP2010094875A - Correction method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a correction method which can reduce the number of correction value tables. <P>SOLUTION: The correction method includes: a step of printing a test pattern composed of a plurality of pixel rows each including a plurality of pixels arranged in a predetermined direction and which are arranged in a direction intersecting the predetermined direction on some kind of a medium; a step of reading the test pattern printed on some kind of the medium at a reading section; a step of determining the correction value of the density of every pixel row based on the read-out results of the test pattern and creating a correction value table where each pixel row is made to correspond to each correction value; a step of printing the test pattern to which the correction value table is applied on a print medium of a kind different from the some kind while changing the dot size; a step of reading the test pattern to which the correction value table is applied at the reading section and selecting a dot size having density variation within a predetermined range and a granularity within an allowable range based on the read-out results; and a step of correcting every pixel row on the correction value table by using the selected dot size when printing on the print medium. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for correcting density unevenness.

For example, when an image is formed on a medium (for example, paper) by a printing apparatus such as an ink jet printer, streaky density unevenness may occur in the image. Therefore, the correction pattern is printed for each ink color using the printing device, the correction pattern is read by a scanner or the like, and the correction value is calculated based on the color information obtained as a result, thereby correcting the density. (For example, refer to Patent Document 1).
JP 2005-206991 A

Conventionally, a correction value table is created for each type of medium. Therefore, there is a problem that it takes time and effort to create the correction value table. Further, when a correction value table is created for each medium type, there is a problem that the number of correction value tables increases.
Therefore, an object of the present invention is to reduce the number of correction value tables.

  The main invention for achieving the above object is to print a test pattern configured by a plurality of pixel rows in which a plurality of pixels are arranged in a predetermined direction in a direction intersecting the predetermined direction on a certain type of medium; Reading the test pattern printed on the certain type of medium with a reading unit, obtaining a correction value of density for each pixel column based on the reading result of the test pattern, each pixel column and each correction value, Creating a correction value table in which the correction value table is associated, printing a test pattern to which the correction value table is applied to a type of print target medium different from the certain type, changing the dot size, and A test pattern to which the correction value table is applied is read by a reading unit, and based on the reading result, a density variation is within a predetermined range and a granularity is within an allowable range. Selecting a size, when printing on the print target medium, by using the dot size selected, a correction method with, and performing a correction for each pixel row by the correction value table.

  Other features of the present invention will become apparent from the description of the present specification and the accompanying drawings.

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

Printing a test pattern comprising a plurality of pixel rows in which a plurality of pixels are arranged in a predetermined direction in a direction intersecting the predetermined direction on a certain type of medium, and the test printed on the certain type of medium Reading a pattern with a reading unit, obtaining a correction value of density for each pixel column based on the reading result of the test pattern, and creating a correction value table in which each pixel column is associated with each correction value Printing a test pattern to which the correction value table is applied on a different type of print target medium from the certain type while changing the dot size; and reading the test pattern to which the correction value table is applied And selecting a dot size whose density variation is within a predetermined range and whose granularity is within an allowable range based on the read result; and When printing on the body, using the dot size selected, the correction method with, and performing a correction for each pixel row by the correction value table reveals.
According to such a correction method, printing can be suitably performed by using a correction value table created with a certain type of medium for printing on another medium. Therefore, since it is not necessary to create a correction value table for each medium, the number of correction value tables can be reduced.

In this correction method, it is desirable to change the dot size by changing the voltage amplitude of a drive signal for driving an element that performs an operation for ejecting liquid.
According to such a correction method, the dot size can be adjusted accurately and easily.

In this correction method, when printing the test pattern in which the correction value table is applied to the print target medium, a plurality of dot sizes are mixed with each pattern at a predetermined ratio, and each dot size is changed to the pattern. Based on the reading result of the test pattern to which the correction value table is applied, the density variation is within the predetermined range, and the granularity is the formation of the pattern within the allowable range. A plurality of the dot sizes used may be selected.
According to such a correction method, it is possible to perform printing in consideration of density fluctuation and graininess using a plurality of dot sizes.

In this correction method, the density variation is obtained for each pixel column by obtaining a difference between the average of the read values of the pixel columns of the test pattern and the read value of each pixel column. It is desirable to calculate based on averaging the differences.
According to such a correction method, it is possible to accurately determine the range of density variation.

Further, in this correction method, the granularity is calculated by calculating a Wiener spectrum calculated based on Fourier transform of the test pattern reading result and a spatial frequency characteristic which is a visual characteristic. It is desirable to calculate based on this.
According to such a correction method, the granularity range can be accurately determined.

=== About the printing system ===
In describing the density unevenness of an image and a method for suppressing the density unevenness, first, a printing system 100 for forming an image on a medium will be outlined with reference to FIG. FIG. 1 is a block diagram illustrating a configuration of the printing system 100.

  As shown in FIG. 1, the printing system 100 according to the present embodiment is a system that includes a printer 1, a computer 110, and a scanner 120.

  The printer 1 is a liquid ejecting apparatus that ejects ink as a liquid onto a medium to form (print) an image on the medium. In the present embodiment, the printer 1 is a color ink jet printer. The printer 1 can print images on a plurality of types of media such as paper, cloth, and film sheets. The configuration of the printer 1 will be described later.

  The computer 110 includes an interface 111, a CPU 112, and a memory 113. The interface 111 exchanges data between the printer 1 and the scanner 120. The CPU 112 performs overall control of the computer 110 and executes various programs installed in the computer 110. The memory 113 stores various programs and various data. Among programs installed in the computer 110, there are a printer driver for converting image data output from an application program into print data, and a scanner driver for controlling the scanner 120. Then, the computer 110 outputs the print data generated by the printer driver to the printer 1.

  The scanner 120 includes a scanner controller 125 and a reading carriage 121. The scanner controller 125 includes an interface 122, a CPU 123, and a memory 124. The interface 122 communicates with the computer 110. The CPU 123 performs overall control of the scanner 120. For example, the reading carriage 121 is controlled. The memory 124 stores a computer program and the like. The reading carriage 121 includes three sensors (CCD and the like) (not shown) corresponding to, for example, R (red), G (green), and B (blue).

  With the above configuration, the scanner 120 irradiates light on a document placed on a document table (not shown), detects the reflected light by each sensor of the reading carriage 121, reads the image of the document, and reads the image. Get color information. Then, data indicating the color information of the image (read data) is transmitted to the scanner driver of the computer 110 via the interface 122.

<Configuration of Printer 1>
Next, the configuration of the printer 1 will be described with reference to FIGS. FIG. 2 is a perspective view for explaining the conveyance process and the dot formation process in the printer 1.

  As shown in FIG. 1, the printer 1 includes a head unit 20, a transport unit 30, a detector group 40, a controller 50, and a drive signal generation circuit 70. When the printer 1 receives print data from the computer 110, the controller 50 controls each unit (head unit 20, transport unit 30, and drive signal generation circuit 70) based on the print data, and prints an image on a print medium. The situation in the printer 1 is monitored by the detector group 40, and the detector group 40 outputs a signal corresponding to the detection result to the controller 50.

  The head unit 20 is for ejecting ink onto the paper S. The head unit 20 forms dots on the paper S by ejecting ink onto the paper S being conveyed, and prints an image on the paper S. The printer 1 of this embodiment is a line printer, and the head unit 20 can form dots for the paper width at a time.

  FIG. 3 is an explanatory diagram of an arrangement of a plurality of heads on the lower surface of the head unit 20. As shown in the figure, a plurality of heads 23 are arranged in a staggered pattern along the paper width direction. In the present embodiment, it is assumed that the head includes three heads (first head 23A, second head 23B, and third head 23C) for simplification of description. Although not shown, each head is formed with a black ink nozzle row, a cyan ink nozzle row, a magenta ink nozzle row, and a yellow ink nozzle row. Each nozzle row includes a plurality of nozzles that eject ink. The plurality of nozzles in each nozzle row are arranged at a constant nozzle pitch along the paper width direction.

  FIG. 4 is an explanatory diagram of the head arrangement and dot formation for simplified explanation. For simplification of description, only a nozzle row of a certain color (for example, a yellow ink nozzle row) of each head is shown. In order to further simplify the description, it is assumed that there are 12 nozzles provided in the nozzle row of each head.

  Each of these nozzles forms a row of dots lined up in the direction in which the head and the paper move relative to each other. This row of dots is called a “raster line”. In the case of a line printer as in this embodiment, “raster line” means a row of dots arranged in the paper transport direction. In the case of a serial printer that prints with a head mounted on a carriage, “raster line” means a row of dots arranged in the carriage movement direction. Hereinafter, as shown in the figure, the raster line at the nth position is referred to as an “nth raster line”.

  As shown in the figure, the nozzle row of each head includes a first nozzle group 231 and a second nozzle group 232. Each nozzle group is composed of, for example, six nozzles arranged in the paper width direction at an interval of 1/180 inch. The first nozzle group 411 and the second nozzle group 412 are configured to be shifted by 1/360 inch in the paper width direction. Thereby, the nozzle row of each head is a nozzle row composed of 12 nozzles arranged at intervals of 1/360 inch in the paper width direction. Numbers are assigned to the nozzle rows of each head in order from the top in the figure.

  Then, 36 raster lines are formed on the paper S by intermittently ejecting ink droplets from each nozzle onto the paper S being conveyed. For example, the nozzle # 1A of the first head 23A forms a first raster line on the paper S, and the nozzle # 1B of the second head 23B forms a thirteenth raster line on the paper S. The nozzle # 1C of the third head 23C forms the 25th raster line on the paper S. Each raster line is formed along the transport direction. In the following description, an area (first raster line to twelfth raster line) printed by the first head 23A is also referred to as band 1, and an area printed by the second head 23B (13th raster line to The 24th raster line) is also referred to as band 2, and the region (25th raster line to 36th raster line) printed by the third head 23C is also referred to as band 3.

  The transport unit 30 is for transporting a medium (for example, paper S) in the transport direction. The transport unit 30 includes an upstream roller 32A and a downstream roller 32B, and a belt 34. When a conveyance motor (not shown) rotates, the upstream roller 32A and the downstream roller 32B rotate, and the belt 34 rotates. The fed paper S is conveyed by the belt 34 to a printable area (area facing the head). When the belt 34 transports the paper S, the paper S moves in the transport direction with respect to the head unit 20. The paper S that has passed through the printable area is discharged to the outside by the belt 34. The paper S being conveyed is electrostatically attracted or vacuum attracted to the belt 34.

  The controller 50 controls each unit of the printer 1 through the unit control circuit 54 by the CPU 52. The printer 1 has a memory 53 having a storage element, and the memory 53 stores a density correction value H (see FIG. 11). The density correction value H will be described later.

  The drive signal generation circuit 70 is a circuit that generates a drive signal COM to be applied to a piezo element (described later) in the head in order to eject ink from the nozzles. The drive signal generation circuit 70 generates an analog waveform drive signal COM by performing digital-analog conversion, voltage amplification, current amplification, and the like based on digital data output from the CPU 52 of the controller 50 to the head unit 20. Output.

<Ink ejection mechanism>
Next, the ink ejection mechanism of the printer 1 will be described.
FIG. 5 is a diagram showing in detail an example of the ink ejection mechanism inside the head 23. The ink ejecting mechanism includes a drive unit 62 and a flow path unit 64. The drive unit 62 includes a plurality of piezo elements 621, a fixing plate 623 to which the piezo element group 621 is fixed, and a flexible cable 624 for supplying power to each piezo element 621. Each piezo element 621 is attached to the fixed plate 623 in a so-called cantilever state. The fixed plate 623 is a plate-like member having rigidity capable of receiving a reaction force from the piezo element 621. The flexible cable 624 is a flexible sheet-like wiring board, and is electrically connected to the piezo element 621 on the side surface of the fixed end opposite to the fixed plate 623. On the surface of the flexible cable 624, a head controller (not shown), which is a control IC for controlling the driving of the piezo element 621 and the like, is mounted.

  The flow path unit 64 includes a flow path forming substrate 65, a nozzle plate 66, and an elastic plate 67. The flow path forming substrate 65 is laminated and integrated so that the flow path forming substrate 65 is sandwiched between the nozzle plate 66 and the elastic plate 67. Constructed. The nozzle plate 66 is a thin plate made of stainless steel on which nozzles are formed.

  In the flow path forming substrate 65, a plurality of empty portions serving as pressure chambers 651 and ink supply ports 652 are formed corresponding to the respective nozzles. The reservoir 653 is a liquid storage chamber for supplying the ink stored in the ink cartridge to each pressure chamber 651, and communicates with the other end of the corresponding pressure chamber 651 through the ink supply port 652. Then, ink from the ink cartridge is introduced into the reservoir 653 through an ink supply pipe (not shown). The elastic plate 67 includes an island portion 673. Then, the tip of the free end portion of the piezo element 621 is bonded to the island portion 673.

  When the drive signal COM is supplied to the piezo element 621 via the flexible cable 624, the piezo element 621 expands and contracts to expand and contract the volume of the pressure chamber 651. Due to such a change in the volume of the pressure chamber 651, pressure fluctuation occurs in the ink in the pressure chamber 651. Then, ink can be ejected from the nozzles by utilizing the fluctuation of the ink pressure.

<About drive signal>
Next, a drive signal for driving the piezo element 621 to perform an operation for ejecting ink from the nozzles will be described. FIG. 6 shows a part of an example of the drive signal COM.
The drive signal COM has a drive pulse PS as shown in FIG. This drive pulse PS has an expansion element P1 that increases the potential with a constant gradient from the intermediate potential VM to the maximum potential VH, an expansion hold element P2 that holds the maximum potential VH for a predetermined time, and a steep gradient from the maximum potential VH to the minimum potential VL. The injection element P3 that lowers the electric potential, the contraction hold element P4 that holds the minimum electric potential VL for a predetermined time, and the damping element P5 that raises the electric potential from the minimum electric potential VL to the intermediate electric potential VM.

When such a driving pulse PS is applied to the piezo element 621, a predetermined amount of ink is ejected from the corresponding nozzle.
That is, the piezoelectric element 621 contracts greatly over the period T1 with the supply of the expansion element P1. As a result, the pressure chamber 651 expands from the normal volume corresponding to the intermediate potential VM to the maximum volume corresponding to the maximum potential VH. Along with this expansion, the pressure chamber 651 is depressurized, and the ink in the reservoir 653 flows into the pressure chamber 651 through the ink supply port 652. The expansion state of the pressure chamber 651 is maintained over the supply period T2 of the expansion hold period P2.

  Subsequently, when the ejection element P3 is supplied to the piezo element 621, the piezo element 621 expands greatly over the period T3. The pressure chamber 651 contracts rapidly to the minimum volume. With this contraction, the ink in the pressure chamber 651 is pressurized and a predetermined amount of ink is ejected from the nozzle. When the contraction hold element P4 is supplied to the piezo element 621 following the discharge element P3, the contraction state of the pressure chamber 651 is maintained during the period T4. In the contracted state of the pressure chamber 651, the meniscus (the surface of the ink exposed at the nozzle opening) vibrates greatly due to the influence of ink ejection.

  Thereafter, the damping element P5 is supplied at a timing at which the meniscus vibration can be suppressed, and the pressure chamber 651 expands and returns to the steady volume over the period T5. That is, in order to cancel the ink pressure in the pressure chamber 651, the pressure chamber 651 is expanded to reduce the ink pressure. Thereby, the vibration of the meniscus can be performed in a short time, and the next ink ejection can be stabilized.

  A plurality of such drive pulses are continuously generated to constitute the drive signal COM. As can be seen from the above description, the amount of ink ejected from the nozzles depends on the voltage amplitude of the pulse PS of the drive signal COM. For example, the amount of ink ejected from the nozzle increases as the voltage amplitude increases. Thereby, the size of the dot to be formed is increased. Conversely, the smaller the voltage amplitude, the smaller the amount of ink ejected from the nozzle. Thereby, the size of the formed dots is reduced.

  FIG. 7 is an explanatory diagram of the drive signal COM. The drive signal COM is repeatedly generated every repetition period T. The drive signal COM includes a first section Ta to a fourth section Td. The first section Ta includes the first drive pulse PS1, and the second section Tb includes the second drive pulse PS2. The third section Tc includes a third drive pulse PS3, and the fourth section Td includes a fourth drive pulse PS4.

When the first drive pulse PS1 is applied to the piezo element 621, ink for forming medium dots is ejected on the paper. The first drive pulse PS1 is a pulse corresponding to a dot gradation value [10] of halftone processing described later.
When the second drive pulse PS2 is applied to the piezo element 621, ink that forms a large dot is ejected on the paper. The second drive pulse PS2 is a pulse corresponding to a dot tone value [11] of halftone processing described later.
When the third drive pulse PS3 is applied to the piezo element 621, the piezo element 621 is slightly vibrated, but no ink is ejected. The third drive pulse PS3 is a pulse corresponding to a dot gradation value [00] of halftone processing described later.
Further, when the fourth drive pulse PS4 is applied to the piezo element 621, ink for forming small dots is ejected on the paper. The fourth drive pulse PS4 is a pulse corresponding to a dot tone value [01] of halftone processing described later.
The first drive pulse PS1 to the fourth drive pulse PS4 are selectively applied to each piezo element 621.

<About print processing>
In such a printer 1, when print data is received from the computer 110, the controller 50 first feeds the paper S to be printed onto the belt 34 by rotating a paper feed roller (not shown) by the transport unit 30. The paper S is conveyed on the belt 34 without stopping at a constant speed, and passes under the head unit 20. While the paper S passes under the head unit 20, ink is intermittently ejected from the nozzles of the first head 23A, the second head 23B, and the third head 23C. That is, the dot formation process and the paper S transport process are performed simultaneously. As a result, a dot row composed of a plurality of dots along the transport direction and the paper width direction is formed on the paper S, and an image is printed. Finally, the controller 50 discharges the paper S on which image printing has been completed.

<Outline of processing by printer driver>
As described above, the printing process starts when print data is transmitted from the computer 110 connected to the printer 1. The print data is generated by processing by the printer driver. Hereinafter, processing by the printer driver will be described with reference to FIG. FIG. 8 is an explanatory diagram of processing by the printer driver.

  As shown in FIG. 8, the print data is generated by executing resolution conversion processing (S011), color conversion processing (S012), halftone processing (S013), and rasterization processing (S014) by the printer driver. The

  First, in the resolution conversion process, the resolution of the RGB image data obtained by executing the application program is converted into a print resolution corresponding to the designated image quality. Next, in the color conversion process, RGB image data whose resolution has been converted is converted into CMYK image data. Here, the CMYK image data means image data for each color of cyan (C), magenta (M), yellow (Y), and black (K). A plurality of pieces of pixel data constituting the CMYK image data are each represented by 256 gradation values. This gradation value is determined based on RGB image data, and is hereinafter also referred to as a command gradation value.

  Next, in the halftone process, the multi-stage gradation value indicated by the pixel data constituting the image data is converted into a small-stage dot gradation value that can be expressed by the printer 1. That is, the 256-level gradation value indicated by the pixel data is converted into a 4-level dot gradation value. Specifically, no dot corresponding to the dot gradation value [00], formation of a small dot corresponding to the dot gradation value [01], formation of a medium dot corresponding to the dot gradation value [10], and The four levels of formation of large dots corresponding to the dot gradation value [11] are converted. Thereafter, after the dot generation rate is determined for each dot size, pixel data is created so that the printer 1 forms the dots in a dispersed manner using a dither method, γ correction, error diffusion method, or the like. .

  Next, in the rasterizing process, with respect to the image data obtained by the halftone process, the data of each dot (dot gradation value data) is changed in the order of data to be transferred to the printer 1. The rasterized data is transmitted as part of the print data.

=== Suppression of density unevenness ===
Next, density unevenness occurring in an image printed using the printer 1 and a method for suppressing the density unevenness will be described.
For the following description, “pixel region” and “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” constituting the image data corresponds to one pixel area. Further, the “row area” is an area on the paper S constituted by a plurality of pixel areas arranged in the transport direction. One column region corresponds to a “pixel column” in which pixels are arranged in a direction opposite to the transport direction on the data.

<About density unevenness>
First, uneven density will be described with reference to the drawings. FIG. 9A 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. When each dot is accurately formed in each pixel region, a raster line (a dot row in which dots are arranged in the transport direction) is accurately formed in the row region.

  FIG. 9B is an explanatory diagram when density unevenness occurs. The raster lines formed in the second row region are formed closer to the third row region 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.

  When a printed image composed of raster lines having different shades is viewed macroscopically, striped density unevenness along the transport direction is visually recognized. This uneven density causes a reduction in image quality of the printed image.

<About the method for suppressing uneven density>
As a measure for suppressing the density unevenness as described above, it is conceivable to correct the gradation value (command gradation value) of the pixel data. That is, it is only necessary to correct the gradation value of the pixel data corresponding to the unit area constituting the row area so that the row area is dark (light) and easily visible, so that the row area is formed light (dark). For this reason, the density correction value H for correcting the gradation value of the pixel data is calculated for each raster line. This density correction value H is a value reflecting the density unevenness characteristic of the printer 1.

  If the density correction value H for each raster line has been calculated, the printer driver performs a process for correcting the gradation value of the pixel data for each raster line based on the density correction value H when the halftone process is executed. . When each raster line is formed with the gradation value corrected by this correction processing, the density of the raster line is corrected, and as a result, the occurrence of uneven density in the printed image is suppressed as shown in FIG. 9C. become. FIG. 9C is a diagram illustrating a state in which the occurrence of density unevenness is suppressed.

  For example, in FIG. 9C, the dot generation rate of the second and fifth row regions that are visually recognized lightly increases, and the dot generation rate of the third row region that is visually recognized darkly decreases. The gradation value of the pixel data of the corresponding pixel is corrected. In this manner, 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.

<Calculation of density correction value H>
Next, a process for calculating the density correction value H for each raster line (hereinafter also referred to as a correction value acquisition process) will be outlined. The correction value acquisition process is performed under the correction value calculation system 200 in, for example, the inspection line of the printer 1 manufacturing factory. The correction value calculation system is a system for calculating the density correction value H corresponding to the density unevenness characteristic of the printer 1 and has a configuration substantially similar to that of the printing system 100 described above. That is, the correction value calculation system includes the printer 1, the computer 110, and the scanner 120 (for convenience, the same reference numerals as those in the printing system 100 are used).

  The printer 1 is a target device of the correction value acquisition process. In order to print an image having no density unevenness using the printer 1, the density correction value H for the printer 1 is calculated in the correction value acquisition process. It will be. The configuration of the printer 1 is omitted because it has already been described. The computer 110 placed on the inspection line is installed with a correction value calculation program for the computer 110 to execute correction value acquisition processing.

  Hereinafter, an outline procedure of the correction value acquisition process will be described with reference to FIG. FIG. 10 is a diagram showing the flow of correction value acquisition processing. When the printer 1 capable of multicolor printing is used as in the present embodiment, the correction value acquisition process for each ink color is performed in the same procedure. In the following description, correction value acquisition processing for one ink color (for example, yellow) will be described.

  First, the computer 110 transmits print data to the printer 1, and the printer 1 forms a correction pattern CP on the paper S by the same procedure as the above-described printing operation (S021). As shown in FIG. 11, the correction pattern CP is formed by sub-patterns CSP having five different densities. FIG. 11 is an explanatory diagram of the correction pattern CP.

  Each sub-pattern CSP is a belt-like pattern, and is configured by arranging a plurality of raster lines along the transport direction in the paper width direction. Each sub-pattern CSP is generated from image data having a constant gradation value (command gradation value). As shown in FIG. 11, the density increases in order from the left sub-pattern CSP. Yes. Specifically, 15%, 30%, 45% and 60% from the left. The sub-pattern has a density of 85%. Hereinafter, Sa is a command gradation value of a sub-pattern CSP having a density of 15%, Sb is a command gradation value of a sub-pattern CSP having a density of 30%, Sc is a command gradation value of a sub-pattern CSP having a density of 45%, and The command gradation value of the sub-pattern CSP is represented as Sd, and the command gradation value of the sub-pattern CSP having a density of 85% is represented as Se. For example, the sub-pattern CSP formed with the command gradation value Sa is expressed as CSP (1) as shown in FIG. Similarly, the sub patterns CSP formed by the command gradation values Sb, Sc, Sd, and Se are denoted as CSP (2), CSP (3), CSP (4), and CSP (5), respectively.

  Next, the inspector sets the paper S on which the correction pattern CP is formed on the scanner 120. Then, the computer 110 causes the scanner 120 to read the correction pattern CP and obtains the result (S022). As described above, the scanner 120 has three sensors corresponding to R (red), G (green), and B (blue). The scanner 120 irradiates the correction pattern CP with light, and reflects the reflected light to each sensor. Detect by. Note that the computer 110 has the same number of pixel columns in which pixels are arranged in the direction corresponding to the conveyance direction on the image data obtained by reading the correction pattern and the number of raster lines (number of column areas) constituting the correction pattern. Adjust so that In other words, the pixel rows read by the scanner 120 and the row regions are made to correspond one-to-one. Then, the average value of the read gradation values indicated by each pixel in the pixel column corresponding to a certain row region is set as the read tone value of the row region.

  Next, the computer 110 calculates the density for each raster line (in other words, for each row region) of each sub-pattern CSP based on the read gradation value acquired by the scanner 120 (S023). Hereinafter, the density calculated based on the read gradation value is also referred to as calculated density.

  FIG. 12 is a graph showing the calculated density for each raster line for the sub-pattern CSP having the command gradation values Sa, Sb, and Sc. The horizontal axis in FIG. 12 indicates the position of the raster line, and the vertical axis indicates the magnitude of the calculated density. As shown in FIG. 12, each sub-pattern CSP is formed with the same command gradation value, but has a density for each raster line. This difference in density of raster lines is a cause of uneven density in the printed image.

  Next, the computer 110 calculates a density correction value H for each raster line (S024). The density correction value H is calculated for each command gradation. Hereinafter, the density correction values H calculated for the command gradations Sa, Sb, Sc, Sd, and Se are referred to as Ha, Hb, Hc, Hd, and He, respectively. In order to explain the calculation procedure of the density correction value H, the density correction for correcting the command gradation value Sb so that the calculated density for each raster line of the sub-pattern CSP (2) of the command gradation value Sb is constant. A procedure for calculating the value Hb will be described as an example. In this procedure, for example, the average value Dbt of the calculated densities of all raster lines in the sub-pattern CSP (2) of the command gradation value Sb is determined as the target density of the command gradation value Sb. In FIG. 12, for the i-th raster line whose calculated density is lighter than the target density Dbt, the command gradation value Sb may be corrected to be darker. On the other hand, for the jth raster line whose calculated density is higher than the target density Dbt, the command gradation value Sb may be corrected to be lighter.

  FIG. 13A is an explanatory diagram of a procedure for calculating a density correction value Hb for correcting the command gradation value Sb for the i-th raster line. FIG. 13B is an explanatory diagram of a procedure for calculating a density correction value Hb for correcting the command gradation value Sb for the j-th raster line. 13A and 13B, the horizontal axis indicates the magnitude of the command gradation value, and the vertical axis indicates the calculated density.

The density correction value Hb for the command gradation value Sb of the i-th raster line is the calculated density Db of the i-th raster line and the command gradation value Sc in the sub-pattern CSP (2) of the command gradation value Sb shown in FIG. 13A. Is calculated based on the calculated density Dc of the i-th raster line in the sub-pattern CSP (3). More specifically, in the sub-pattern CSP (2) of the command gradation value Sb, the calculated density Db of the i-th raster line is smaller than the target density Dbt. In other words, the density of the i-th raster line is lighter than the average density. If it is desired to form the i-th raster line so that the calculated density Db of the i-th raster line is equal to the target density Dbt, the gradation value of the pixel data corresponding to the i-th raster line, that is, the command level As shown in FIG. 13A, the tone value Sb is expressed by the following equation (1) using linear approximation from the correspondence relationship (Sb, Db) and (Sc, Dc) between the command gradation value and the calculated density in the i-th raster line. It is sufficient to correct up to the target command gradation value Sbt calculated by
Sbt = Sb + (Sc−Sb) × {(Dbt−Db) / (Dc−Db)} (1)
Then, a density correction value H for correcting the command tone value Sb for the i-th raster line is obtained from the command tone value Sb and the target command tone value Sbt by the following equation (2).
Hb = ΔS / Sb = (Sbt−Sb) / Sb (2)

On the other hand, the density correction value Hb for the command gradation value Sb of the j-th raster line is the calculated density Db of the j-th raster line in the sub-pattern CSP (2) of the command gradation value Sb shown in FIG. It is calculated based on the calculated density Da of the j-th raster line in the sub pattern CSP (1) of the value Sa. Specifically, in the sub-pattern CSP (2) of the command gradation value Sb, the calculated density Db of the jth raster line is larger than the target density Dbt. If it is desired to form the jth raster line so that the calculated density Db of the jth raster line is equal to the target density Dbt, the command gradation value Sb of the jth raster line is set as shown in FIG. 13B. From the correspondence (Sa, Da), (Sb, Db) between the command tone value and the calculated density in the jth raster line, to the target command tone value Sbt calculated by the following equation (3) using linear approximation. It may be corrected.
Sbt = Sb + (Sb−Sa) × {(Dbt−Db) / (Db−Da)} (3)
Then, the density correction value Hb for correcting the command gradation value Sb for the j-th raster line is obtained by the above equation (2).

  As described above, the computer 110 calculates the density correction value Hb for the command gradation value Sb for each raster line. Similarly, density correction values Ha, Hc, Hd, and He for the command gradation values Sa, Sc, Sd, and Se are calculated for each raster line. For other ink colors, density correction values Ha to He are calculated for each of the command gradation values Sa to Se for each raster line.

  Thereafter, the computer 110 transmits the density correction value H data to the printer 1 and stores it in the memory 53 of the printer 1 (S025). As a result, a correction value table in which the density correction values Ha to He for each of the five command gradation values Sa to Se are collected for each raster line is created in the memory 53 of the printer 1. . FIG. 14 is a diagram showing a correction value table stored in the memory 53.

  Also, as shown in FIG. 14, the correction value table is created for each ink color. As a result, a correction value table for CMYK four colors is formed. The correction value table is referred to by the printer driver when the image is printed using the printer 1 in order to correct the gradation value of each raster line constituting the image data of the image.

  After the correction value acquisition process is completed, the printer 1 is packed and shipped after passing through another inspection process. When an image is printed by the purchaser (user) of the printer 1, an image having a density corrected by the density correction value H is printed.

  For example, the printer driver of the computer 110 of the user uses the gradation value of each pixel data (hereinafter, the gradation value before correction is referred to as Sin) based on the density correction value H of the raster line corresponding to the pixel data. Correction is performed (hereinafter, the corrected gradation value is referred to as Sout).

Specifically, if the gradation value Sin of a certain raster line is the same as any one of the command gradation values Sa, Sb, Sc, Sd, and Se, the density correction value H stored in the memory of the computer 110 is used as it is. Can be used. For example, if the gradation value Sin of the pixel data is Sin = Sb, the corrected gradation value Sout is obtained by the following equation.
Sout = Sb × (1 + Hb)

On the other hand, when the gradation value of the pixel data is different from the command gradation values Sa, Sb, Sc, Sd, Se, the correction value is calculated based on the interpolation using the density correction values of the surrounding command gradation values. For example, when the command tone value Sin is between the command tone value Sb and the command tone value Sc, linearity using the density correction value Hb of the command tone value Sb and the density correction value Hc of the command tone value Sc. If the correction value obtained by interpolation is H ′, the gradation value Sout after the correction of the command gradation value Sin is obtained by the following equation.
Sout = Sin × (1 + H ′)
In this way, density correction processing is performed for each raster line.

=== First Embodiment ===
The correction value acquisition process as described above is performed a plurality of times by changing the type of print medium (for example, paper S). This is because it is considered that an appropriate correction value table needs to be prepared for each type of paper S because there is a possibility that the degree of density unevenness of the image differs when the type of paper S is different. . However, it takes time and effort to create a correction value table for every type of paper S. In addition, the capacity for storing the correction value table in the memory 53 of the printer 1 increases.

  Therefore, in the present embodiment, a correction value table created on a paper different from the print target paper is applied when printing on the print target paper. However, it is conceivable that density unevenness occurs by using a correction value table for another sheet. It is known that the density unevenness can be suppressed by increasing the dot size. On the other hand, the granularity, which is a visual index of the image, increases. Therefore, in this embodiment, the dot size is adjusted based on the relationship between density unevenness and granularity, as will be described later. In the first embodiment, a case where printing is performed using a single dot size will be described.

<About the evaluation pattern>
First, the evaluation pattern used in this embodiment will be described.
FIG. 15 is a diagram illustrating an example of an evaluation pattern used in the first embodiment. In the figure, four patterns A to D are formed. Each of these patterns is formed using three heads (first head 23A, second head 23B, and third head 23C). That is, each pattern is formed over three regions of band 1, band 2, and band 3 in the paper width direction. Further, dots are formed in the same arrangement in each pattern. For example, in this embodiment, dots are formed in a checkered pattern as shown in the enlarged view.

  Further, when forming each of these patterns, the controller 60 changes the magnitude of the voltage amplitude (the difference between VH and VL in FIG. 6) of the drive pulse in the drive signal COM for driving the piezo element. Thereby, the dots of each pattern are formed in different sizes. Specifically, when forming each pattern, the controller 60 increases the voltage amplitude of the drive pulse in the order of pattern A, pattern B, pattern C, and pattern D. Therefore, the pattern A is formed with the smallest dot size, and the dot size increases in the order of the pattern B and the pattern C. The pattern D is formed with the largest dot size. Thus, by adjusting the dot size by changing the voltage amplitude of the drive signal COM, the dot size can be adjusted accurately and easily.

  Such an evaluation pattern is printed on a printing target paper, and the image is read by, for example, the scanner 120, whereby density variations and granularity described later are calculated for each pattern (that is, dot size). Hereinafter, density variation and granularity, which are evaluation indexes in the present embodiment, will be described.

<Concentration variation>
In the present embodiment, the color difference formula ΔE94 is used as an index indicating the density variation. ΔE94 is expressed by the following equation.
ΔE94 = √ {(ΔH ^* / Sh) ² + (ΔL ^* / SL) ² + (ΔC ^* / Sc) ² }
Note that L ^* , C ^* , and H ^* are the lightness, saturation, and hue of the L ^* a ^* b ^* display system, respectively. Further, SL = 1, Sc = 1 + 0.045C ^* , and Sh = 1 + 0.015C ^* .

FIG. 16 is a diagram for explaining the concept of the color difference equation ΔE94.
When printing is performed on the paper S using the nozzles of the heads, raster lines corresponding to the nozzles are formed in the row regions of the paper S as described above. By reading this with the scanner 120, an RGB value indicating the density of the pixel column corresponding to each raster line is obtained for each pixel column. In the present embodiment, RGB values are converted into L ^* a ^* b ^* display system components (hereinafter referred to as Lab values). When the average of Lab values of all raster lines is (L ^* _H , a ^* _H , b ^* _H ) and the Lab values of the nth raster line are (L ^* _n , a ^* _n , b ^* _n ), the Lab value The color difference between the average and the Lab value of the nth raster line is represented by the distance between two points in the L ^* a ^* b ^* space. For example, Lab values of the first raster line and _{^{(L * 1, a * 1}} , b * 1), the average value _{^{(L * H, a * H}} , b * H) color difference Delta] E ₁ with the
ΔE ₁ = √ {(L ^* _H− L ^* ₁ ) ² + (a ^* _H− a ^* ₁ ) ² + (b ^* _H− b ^* ₁ ) ² }
It becomes. Similarly, the average value _{^{(L * H, a * H}} , b * H) and a color difference Delta] E _n the Lab values of the n raster lines respectively obtained. A value obtained by further averaging these color differences (ΔE _{1 to} ΔE _{36 in} this embodiment) corresponds to ΔE94.

  Therefore, as can be seen from the above relationship, the larger the variation in density (the greater the variation in Lab value of each raster line), the greater the value of ΔE94. Conversely, the smaller the density variation (the smaller the variation in Lab value of each raster line), the smaller the value of ΔE94. As described above, by using ΔE94 as an evaluation index, it is possible to accurately determine the range of density variation described later.

  The evaluation index for density variation is not limited to that described above. For example, instead of the average value of each raster line, an absolute value (target value) may be determined, and the color difference between the absolute value and the value of each raster line may be obtained.

<About granularity>
The granularity is a visual index that quantitatively represents the degree of variation in image density.
In the present embodiment, the following formula (4) based on the evaluation formula of Dooly and Shaw is used as a calculation formula for granularity.
Granularity = a (L ^* ) ∫ (WS (u)) ^0.5 VTF (u) du (4)
Here, u is a spatial frequency, WS is a Wiener spectrum of an image, VTF (Visual Transfer Function) is a visual spatial frequency characteristic, and a is a brightness correction term.

Wiener spectrum WS converts the image into L ^* a ^* b ^* space using a scan and read image data of the (RGB) 3D-LUT (3-dimensional look-up table) for L ^* component of the image A two-dimensional Fourier transform (FFT) is performed and then converted into a music coordinate system to make it one-dimensional.

The spatial frequency characteristic VTF is a characteristic related to human vision. In this embodiment, the following formula (5) is used as the VTF.
VTF (u) = 5.05exp (-0.138πlu / 180) {1-exp (-0.1πlu / 180)} (5)
In addition, l is a clear vision distance and is set to 300 mm in this embodiment.

  FIG. 17 is a conceptual diagram of the spatial frequency characteristic VTF. The horizontal axis in the figure is the spatial frequency (u), and the vertical axis is VTF. This spatial frequency characteristic VTF can be said to be a filter (so-called low-pass filter) that suppresses high-frequency components that make human visual sensitivity dull.

The brightness correction term a is a coefficient for matching the value obtained based on the Wiener spectrum WS and VTF with the human sense. In the present embodiment, the following formula (6) is used for the brightness correction term a.
a (L ^* ) = ((L ^* + 16) / 116) ^0.8 (6)

  Thus, the granularity was calculated from the read data of the evaluation pattern using the above-described formula. The granularity is preferably small. When the granularity exceeds a certain size, the granularity (noise) is easily visually recognized. Therefore, in the present embodiment, as will be described later, a permissible range (hereinafter, permissible range) for the granularity is provided in advance. In the present embodiment, since the granularity is quantitatively calculated by the equation (4), the allowable range can be accurately determined.

<About processing during printing>
Next, processing during printing according to the present embodiment will be described.
FIG. 18 is a flowchart of the printing process according to the first embodiment. FIG. 18 shows a process performed when, for example, the user prints on a different type of print target paper than the basic paper. Although not shown in the drawing, a correction value table for basic paper is created in the inspection line of the printer 1 manufacturing factory and stored in the memory 53 of the printer 1.

  The printer driver of the user computer 110 creates print data from the image data of the evaluation pattern. Based on the print data, the computer 110 causes the printer 1 to print the evaluation patterns shown in FIG. 15 (patterns A to D having different dot sizes) on the print target paper (S101). The printer driver creates print data in which the image data of the evaluation pattern is corrected for each raster line (for each pixel column) with the density correction value H. Therefore, it can be said that the evaluation pattern printed on the printing target paper is a pattern (test pattern) to which the correction value table is applied.

  Next, the computer 110 causes the scanner 120 to read each of the printed evaluation patterns (S102). Based on the read result, ΔE94 and granularity described above are calculated for each pattern (that is, for each dot size) (S103). If ΔE94 and granularity are calculated, the computer 110 selects a dot size based on the calculation result so that ΔE94 is within a predetermined value and the granularity is within an allowable range (S104).

  FIG. 19 is a diagram for explaining selection of the dot size. In the figure, the horizontal axis indicates the degree of granularity, and the vertical axis indicates the magnitude of ΔE94. Further, α in the figure is the maximum value of the range of color unevenness determined in advance in the present embodiment. That is, in this embodiment, the color unevenness is set within the range of zero to α. Further, β in the figure is an allowable value of granularity. That is, the allowable range of granularity in this embodiment is between zero and β.

Each point of A, B, C, and D in the figure is a plot of ΔE94 and granularity calculation results of Pattern A, Pattern B, Pattern C, and Pattern D, respectively. As described above, the dot size is A <B <C <D.
It can be seen from the figure that ΔE94 (density variation) decreases as the dot size increases. For example, when A and B are compared in the drawing, the value of ΔE94 is smaller for B having a larger dot size than for A.
It can also be seen from the figure that the larger the dot size, the greater the granularity. For example, when C and D are compared in the drawing, D having a larger dot size has a greater granularity value than C.

  Thus, as the dot size is increased, the density variation can be reduced, but on the other hand, the granularity is increased. Conversely, the smaller the dot size, the smaller the granularity, but the density variation increases.

  Therefore, in the present embodiment, a pattern (dot size) in which ΔE94 is in the range of zero to α and the granularity is in the range of zero to β is selected. That is, the density variation and the granularity are made to be the regions within the shaded area in FIG. By doing so, it is possible to perform printing that satisfies both density variation and granularity.

  In the case of the present embodiment, since the points B and C are in the shaded area in the figure, the dot size of the pattern B or the dot size of the pattern C is selected. Note that which one should be selected may be set, for example, by a priority based on ΔE94 (density variation) and granularity. For example, when priority is given to the granularity, the dot size of the pattern B is selected.

  Then, after determining the dot size in step S104, when printing on the print target paper, the computer 110 applies the correction value table created on the basic paper to the printer 1 using the dot size. Correction is performed for each raster line and printing is performed (S105).

  As described above, since the dot size in which the density variation and the granularity are within the determined ranges is selected and used, the correction value table created on the basic paper is preferably applied to the printing of the printing target paper. be able to.

  As described above, in the present embodiment, the correction value table created on the basic paper is used when printing on the printing target paper. At this time, density variation (ΔE94) and granularity are calculated first with an evaluation pattern, and a dot size is used in which the density variation is within a predetermined range and the granularity is within an allowable range. . Since the density variation and the granularity are taken into consideration in this way, the correction value table created on the basic paper can be suitably applied to printing on the printing target paper. That is, it is not necessary to create a correction value table for each medium, and the number of correction value tables can be reduced.

=== Second Embodiment ===
In the first embodiment, a case where printing is performed using a single dot size has been described. In the second embodiment, a case where printing is performed using a plurality of dot sizes will be described. In the second embodiment, three dot sizes (large dot, medium dot, and small dot) are used when printing. The drive signal COM for forming dots of each dot size is as shown in FIG. When the voltage amplitude of the entire drive signal COM is changed, the sizes of the large dot, medium dot, and small dot sizes are uniformly changed.

  20A, 20B, and 20C are explanatory diagrams of changes in the voltage amplitude of the drive signal COM. 20A is a diagram when the voltage amplitude of the drive signal COM is increased, and FIG. 20B is a diagram when the voltage amplitude of the drive signal COM is decreased. FIG. 20C is a modification of FIG. 20A. Further, the solid line in the figure indicates the drive signal COM before the voltage amplitude is changed, and the dotted line in the figure indicates the drive signal COM after the voltage amplitude is changed. Further, “a” in the figure is the voltage amplitude of the entire drive signal COM (that is, the voltage amplitude of the second drive pulse PS2 having the largest voltage amplitude).

  As described above, the first drive pulse PS1 is a pulse for forming a medium dot, the second drive pulse PS2 is a pulse for forming a large dot, and the third drive pulse PS3 slightly vibrates the piezo element 621 (ink is discharged). The fourth drive pulse PS4 is a pulse for forming a small dot. The voltage amplitude of each pulse is PS3 <PS4 <PS1 <PS2.

  Here, as shown in FIG. 20A, when the voltage amplitude a of the second drive pulse PS2 is increased (to a ′), the waveform of the drive signal COM is accordingly as shown by the dotted line in FIG. 20A. That is, the voltage amplitudes of the first drive pulse PS1, the third drive pulse PS3, and the fourth drive pulse PS4 also increase in proportion to the change of the second drive pulse PS2. As a result, the dot sizes of large dots, medium dots, and small dots are increased.

  20B, when the voltage amplitude a of the second drive pulse PS2 is decreased (to be “a ″), the waveform of the drive signal COM is accordingly as shown by the dotted line in FIG. The voltage amplitude of the PS1, the third drive pulse PS3, and the fourth drive pulse PS4 also decreases in proportion to the change of the second drive pulse PS2, and as a result, each dot of large dots, medium dots, and small dots. The size becomes smaller.

  In this way, by changing the voltage amplitude of the entire drive signal COM, the voltage amplitude of each pulse forming a large dot, a medium dot, and a small dot changes uniformly. As a result, the sizes of large dots, medium dots, and small dots change.

  If the third drive pulse PS3 that slightly vibrates the piezo element 621 is increased when the voltage amplitude of the drive signal COM is increased as shown in FIG. 20A, ink may be accidentally ejected by the third drive pulse PS3. is there. Therefore, as shown in FIG. 20C, the magnitude of only the third drive pulse PS3 may not be changed. By doing so, it is possible to reliably prevent ink from being accidentally ejected during a period in which the piezo element 621 is slightly vibrated.

  In the second embodiment, five patterns with different voltage amplitudes of the drive signal COM are printed as evaluation patterns. Specifically, the computer 100 changes the voltage amplitude of the drive signal COM by 0.8 times, 0.9 times, 1.0 times (reference), 1.1 times, and 1.2 times, respectively. The printed pattern is printed on the print target paper by the printer 1. In the second embodiment, when printing each pattern of the evaluation pattern, three dot sizes of a large dot, a medium dot, and a small dot are used.

  In each pattern, the mixing ratio of large dots, medium dots, and small dots is the same, but the dot sizes of large dots, medium dots, and small dots are different for each pattern. As described above, in the first embodiment, each pattern of the evaluation pattern is formed with one dot size, whereas in the second embodiment, each pattern has three dot sizes of large, medium, and small. It is formed in a mixed ratio.

  Then, the computer 110 causes the scanner 120 to read the printed evaluation pattern, and calculates the density variation and granularity of each pattern from the reading result, as in the first embodiment. Then, an optimum pattern is selected from the result.

  FIG. 21 is an explanatory diagram of dot size selection according to the second embodiment. In addition, each point of Va, Vb, Vc, Vd, and Ve in the figure is 0.8 times, 0.9 times, 1.0 times, 1.1 times, and 1.2 times the voltage amplitude of the drive signal COM, respectively. The calculation result of each pattern when it is doubled is plotted.

  In the second embodiment, similarly to the first embodiment, a range (shaded area in FIG. 21) in which the density variation is zero to α and the granularity is zero to β is determined. That is, the density variation and the granularity are determined so as to be values within the hatched portion in FIG. In the second embodiment, a pattern in which the density variation and granularity calculation results are within this range is selected. In other words, the large, medium, and small dot sizes used to form the pattern that falls within the shaded area are selected.

  From the figure, it can be seen that as the voltage amplitude increases (in the order of Va, Vb, Vc, Vd, and Ve), the density unevenness decreases, but on the other hand, the granularity increases. This is because the size of each size (large, medium, small) dot is uniformly increased according to the voltage amplitude.

  Conversely, as the voltage amplitude decreases (in the order of Ve, Vd, Vc, Vb, and Va), the granularity decreases, but on the other hand, the density variation increases. This is because the size of each size (large, medium, small) dot is uniformly reduced according to the voltage amplitude.

  In FIG. 21, of these five patterns, three are Vb, Vc, and Vd that are within the oblique lines. Therefore, these three become selection candidates, and the computer 110 selects one of these three patterns. In addition, what pattern should be selected may be determined in advance. For example, Vb may be selected when priority is given to granularity, and Vd may be selected when priority is given to density variation (ΔE94). Alternatively, Vc may be selected when the granularity and the density variation priority are equal.

  Then, when printing on the printing target paper, the computer 110 uses the large dot, medium dot, and small dot corresponding to the voltage amplitude of the selected pattern to the printer 1 and creates a correction value table created on the basic paper. Apply and print for each raster line.

As described above, in the second embodiment, printing is performed using large dots, medium dots, and small dots so that the density unevenness and the granularity are within the determined ranges. As a result, even when a plurality of dot sizes are used, printing considering density variation and granularity can be performed, and the correction value table created on the basic paper can be suitably applied to printing on the printing target paper. it can. Therefore, the number of correction value tables can be reduced.
In the second embodiment, three dot sizes are used, but the number is not limited to three. For example, two dot sizes may be used, or four or more dot sizes may be used.

=== Third Embodiment ===
In the above-described embodiment, as shown in FIG. 15, dots are formed in the same arrangement in each pattern of the evaluation pattern. That is, the number of dots formed in each pattern was the same. In contrast, in the third embodiment, the number of dots (dot density) is changed for each pattern of the evaluation pattern. Each pattern of one evaluation pattern is formed with the same dot size. In the third embodiment, such an evaluation pattern is printed in a plurality of sheets (two sheets in this embodiment) by changing the dot size.

22A and 22B are examples of the evaluation pattern of the third embodiment. In the evaluation patterns shown in FIGS. 22A and 22B, a plurality of patterns having different gradation values are printed by changing the number of dots. For example, the left side pattern in the figure has a higher gradation value (the number of dots is smaller), and the right side pattern in the figure has a lower gradation value (a larger number of dots).
22A and 22B differ in dot size used for printing. 22A is printed with a single dot size a, and FIG. 22B is printed with a single dot size b (b> a). As a result, the color in FIG. 22B is generally darker (the gradation value is lower) than in FIG. 22A.

Similar to the above-described embodiment, the computer 110 causes the printer 1 to print the evaluation patterns shown in FIGS. 22A and 22B and causes the scanner 120 to read the printed evaluation patterns. The computer 110 calculates the L ^* value of each pattern by converting the RGB value of the reading result of the scanner 120 into the component (Lab value) of the L ^* a ^* b ^* display system. Then, the computer 110 selects patterns having substantially the same L * value from the patterns shown in FIGS. 22A and 22B. In the present embodiment, L ^{* of} each pattern connected by the arrows in FIGS. 22A and 22B has substantially the same value, and these patterns are selected.

  The computer 110 calculates the granularity and the value of ΔE94 for these selected patterns as in the above-described embodiment. Then, a granularity and an average value of ΔE94 are obtained for each evaluation pattern, and a dot size in which the obtained ΔE94 is in the range of zero to α and the granularity is in the range of zero to β is selected. For example, if ΔE94 of the pattern selected in FIG. 22A is in the range of zero to α and the granularity is in the range of zero to β, the dot size a is selected. Then, when printing on the printing target paper, the selected dot size is used.

  Also in the case of the third embodiment, the dot size considering density variation and granularity can be selected as in the first embodiment, so that the correction value table created on the basic paper can be printed on the print target paper. It can be suitably applied to.

=== Fourth Embodiment ===
In the third embodiment, a single size dot is used for each evaluation pattern, but in the fourth embodiment, a plurality of dot sizes are used for each evaluation pattern.

  FIG. 23A and FIG. 23B are examples of the evaluation pattern of the fourth embodiment. A difference from the third embodiment is that a plurality of dot sizes (large dots, medium dots, small dots) are used for forming each pattern of each evaluation pattern. In each of the evaluation patterns shown in FIGS. 23A and 23B, the ratios of large dots, medium dots, and small dots are the same, but the sizes are different.

  For example, in this embodiment, the voltage amplitude of the drive signal COM when printing the evaluation pattern of FIG. 23A is Va ′, and the voltage amplitude of the drive signal COM when printing the evaluation pattern of FIG. 23B is Vb ′ (Vb '> Va'). Therefore, in FIG. 23B, the dot size is larger for large dots, medium dots, and small dots than in the case of FIG. 23A. As a result, the overall color in FIG. 23B is darker (the gradation value is lower) than in FIG. 23A.

Also in the fourth embodiment, the computer 110 causes the printer 1 to print the evaluation patterns shown in FIGS. 23A and 23B and causes the scanner 120 to read the printed evaluation patterns. The computer 110 converts the RGB value of the reading result of the scanner 120 into an L ^* a ^* b ^* display system component (Lab value), and calculates the L ^* value of each pattern. Then, the computer 110 selects a pattern having substantially the same L * value from the patterns shown in FIGS. 23A and 23B. In this embodiment, L ^{* of} each pattern connected by the arrows in FIGS. 23A and 23B has substantially the same value, and these patterns are selected.

  The computer 110 calculates the granularity and the value of ΔE94 for these selected patterns as in the above-described embodiment. Then, a granularity and an average value of ΔE94 are obtained for each evaluation pattern, and a dot size in which the obtained ΔE94 is in the range of zero to α and the granularity is in the range of zero to β is selected. For example, if ΔE94 of the pattern selected in FIG. 23A is in the range of zero to α and the granularity is in the range of zero to β, the voltage amplitude Va ′ is selected. When printing on the printing target paper, a dot size (large dot, medium dot, small dot) corresponding to the selected voltage amplitude Va ′ is used.

  In the fourth embodiment, similarly to the second embodiment, when a plurality of dot sizes are used, printing considering density variation and granularity can be performed, and a correction value table created on basic paper is printed. The present invention can be suitably applied to the printing of the target paper. Therefore, the number of correction value tables can be reduced.

=== Other Embodiments ===
As described above, the correction value calculation apparatus according to the present invention has been mainly described based on the above embodiment. However, in the above description, a color information selection system and a color information selection system for selecting color information are described. The disclosure of a program for causing the computer 110 to execute color information selection processing is also included. The embodiments of the invention described above are for facilitating the understanding of the present invention, and do not limit the present invention. The present invention can be changed and improved without departing from the gist thereof, and the present invention includes the equivalents thereof.

<About Printer 1>
In the above embodiment, a line head printer in which nozzles are arranged in the paper width direction intersecting the medium conveyance direction is described as an example, but the present invention is not limited to this. For example, while moving the head unit in the moving direction intersecting the nozzle row direction, a dot forming operation for forming a dot row along the moving direction and a transport operation (moving operation) for transporting paper in the transport direction that is the nozzle row direction ) May alternately be repeated.

  In the above embodiment, an ink jet printer that ejects ink, which is an example of a liquid, has been described. However, the present invention is not limited to this, and the present invention can also be applied to a liquid ejecting apparatus that ejects liquid other than ink. For example, textile printing devices for patterning fabrics, color filter manufacturing devices, display manufacturing devices such as organic EL displays, DNA chip manufacturing devices that manufacture DNA chips by applying DNA-dissolved solutions to chips, circuit board manufacturing It may be a device or the like.

<About Scanner 120>
In the above-described embodiment, the scanner 120 has R, G, and B sensors (for example, a CCD), and reads the reflected light of the light irradiated on the document with each sensor to obtain R, G, and B color information. Although the sensor-type thing to acquire is used, it is not limited to this. For example, a fluorescent lamp of each color of R, G, and B is sequentially blinked, a reflected light is read by a monochrome image sensor, and color information of R, G, and B is acquired, or between the light source and the sensor. An R, G, and B color filter may be provided, and a filter switching type that acquires R, G, and B color information by sequentially switching the color filters may be used.

<About the head>
In the above-described embodiment, ink is ejected using a piezo element. However, the method of ejecting the liquid is not limited to this. For example, other methods such as a method of generating bubbles in the nozzle by heat may be used.

<About dot size adjustment>
In the above-described embodiment, the dot size is changed by changing the voltage amplitude of the drive signal COM. However, the method of changing the dot size is not limited to this. For example, the dot size may be changed by changing the waveform shape of the drive pulse PS of the drive signal COM (for example, the slope of the expansion element P1 in FIG. 6).

1 is a block diagram illustrating a configuration of a printing system. FIG. 6 is a perspective view for explaining a conveyance process and a dot formation process of a printer. It is explanatory drawing of the arrangement | sequence of the some head in the lower surface of a head unit. It is explanatory drawing of the head arrangement | positioning for simplified description, and the mode of dot formation. FIG. 3 is a diagram illustrating an example of an ink ejection mechanism in detail. A part of an example of the drive signal COM is shown. It is explanatory drawing of the drive signal COM. FIG. 10 is an explanatory diagram of processing by a printer driver. FIG. 9A is an explanatory diagram of a state when an ideal raster line is formed. FIG. 9B is an explanatory diagram when density variation occurs. FIG. 9C is a diagram illustrating a state in which occurrence of density variation is suppressed. It is a figure which shows the flow of a correction value acquisition process. It is explanatory drawing of correction pattern CP. It is a graph which shows the calculation density | concentration for every raster line about sub pattern CSP. FIG. 13A is an explanatory diagram of a procedure for calculating a density correction value Hb for correcting the command gradation value Sb for the i-th raster line. FIG. 13B is an explanatory diagram of a procedure for calculating a density correction value Hb for correcting the command gradation value Sb for the j-th raster line. It is a figure which shows a correction value table. It is a figure which shows an example of the evaluation pattern used in 1st Embodiment. It is a figure for demonstrating the concept of color difference type | formula (DELTA) E94. It is a conceptual diagram of the spatial frequency characteristic VTF. It is a flowchart about the printing process of 1st Embodiment. It is explanatory drawing about selection of the dot size of 1st Embodiment. It is explanatory drawing when changing the voltage amplitude of the whole drive signal COM. FIG. 20A is an explanatory diagram when the voltage amplitude of the entire drive signal COM is increased, and FIG. 20B is an explanatory diagram when the voltage amplitude of the entire drive signal COM is decreased. FIG. 20C is a modification of FIG. 20A. It is explanatory drawing about selection of the dot size of 2nd Embodiment. It is a figure which shows an example of the evaluation pattern used in 3rd Embodiment. It is a figure which shows an example of the evaluation pattern used in 4th Embodiment.

Explanation of symbols

1 printer, 20 head units,
23 heads, 23A first head, 23B second head, 23C third head,
30 transport unit, 32A upstream roller, 32B downstream roller, 34 belt,
40 detector groups, 50 controllers, 51 interfaces, 52 CPUs,
53 memory, 54 unit control circuit, 62 drive unit,
621 Piezo element, 623 fixed plate, 624 flexible cable,
64 channel units, 65 channel forming substrate,
651 pressure chamber, 652 ink supply port, 653 reservoir,
66 nozzle plate, 67 elastic plate, 673 island, 70 drive signal generation circuit,
100 printing system, 110 computer, 111 interface,
112 CPU, 113 memory, 120 scanner,
121 reading carriage, 122 interface,
123 CPU, 124 memory, 125 scanner controller

Claims (5)

Printing a test pattern including a plurality of pixel rows in which a plurality of pixels are arranged in a predetermined direction in a direction intersecting the predetermined direction on a certain type of medium;
Reading the test pattern printed on the certain type of medium with a reading unit;
Obtaining a correction value of density for each pixel column based on the reading result of the test pattern, and creating a correction value table in which each pixel column is associated with each correction value;
Printing a test pattern, to which the correction value table is applied, on a different type of printing target medium from the certain type by changing the dot size;
A test pattern to which the correction value table is applied is read by a reading unit, and based on the read result, density variation is within a predetermined range, and a dot size with a granularity within an allowable range is selected.
When printing on the print target medium, using the selected dot size, performing correction for each pixel column by the correction value table;
A correction method. The correction method according to claim 1,
A correction method for changing the dot size by changing a voltage amplitude of a drive signal for driving an element that performs an operation for ejecting liquid. The correction method according to claim 1 or 2,
When printing the test pattern to which the correction value table is applied to the print target medium, a plurality of dot sizes are mixed in each pattern at a predetermined ratio, and each dot size is changed for each pattern.
Based on the reading result of the test pattern to which the correction value table is applied, the density variation is within the predetermined range, and the granularity is used to form the pattern within the allowable range. Correction method that selects the dot size. The correction method according to any one of claims 1 to 3,
The density variation is obtained by obtaining a difference between an average read value of the pixel rows of the test pattern and a read value of each pixel row for each pixel row, and averaging the difference obtained for each pixel row. A correction method calculated based on this. The correction method according to any one of claims 1 to 4,
The correction method, wherein the granularity is calculated based on a calculation of a Wiener spectrum calculated based on Fourier transform of the test pattern reading result and a spatial frequency characteristic that is a visual characteristic.