Image forming apparatus, control method thereof and program

Abstract

To highly accurately correct a sensor characteristic of a scanner device on the basis of colorimetric data.SOLUTION: Correction information for correcting a sensor acquisition value of scanning means is generated on the basis of scan data obtained by reading a first chart printed and output from printing means by scanning means and colorimetric data obtained by measurement by colorimetric means. The first chart includes: a patch having uniform concentration extending in a main scanning direction substantially orthogonal to a conveyance direction of a paper sheet and a positioning pattern formed outside the patch for specifying a position in the main scanning direction on the patch. First generation means associates a pixel position at an end of the patch in the scan data with a colorimetric position at an end of the patch in the colorimetric data, determines a correction value of the sensor acquisition value at a specific pixel position, and generates correction information.SELECTED DRAWING: Figure 12

Description

[Technical field]

[0001] The present disclosure relates to a color adjustment technique for a printer. [Background technology]

[0002] Inkjet (IJ) printers that form images by ejecting ink droplets from multiple nozzles are widely used as image forming devices for forming arbitrary images on paper. It is difficult to eliminate deviations in the ink landing position and ejection amount from the target position and ejection amount for all nozzles arranged in the recording head, and this may appear as band-like or streak-like density unevenness (banding) on ​​the printed matter. Therefore, color adjustment (called "head shading correction") is performed to correct the image data to be printed according to printing characteristics such as the ink ejection amount and landing position deviation for each nozzle (or module) so that density unevenness does not occur. In head shading correction, a test chart is scanned to obtain the printing characteristics of the recording head, but if there is variation in the reading characteristics of the sensor, this is taken in as the printing characteristics. In that case, head shading correction will actually cause density unevenness. Therefore, before scanning the test chart, lighting and sensor calibration are generally performed by referring to a reference white board installed inside or outside. However, due to the angle dependency of the sensor or lighting, the surface characteristics of the paper, etc., there may be cases where the variation in the reading characteristics of the sensor remains, especially when the intensity or spectral characteristics of gray or chromatic colors differ from that of a reference white board. In this regard, Patent Document 1 describes a technology that suppresses density unevenness caused by the reading characteristics of the sensor by correcting the scan data of a multi-patch image of multiple gradations based on the colorimetric data of a uniform patch image of each color of CMYK. In addition, Patent Document 2 describes a technology that inserts a mark into a patch to associate a position on the colorimetric data with a position on the image data in order to determine the colorimetric position on the colorimetric data. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2019-220828 A [Patent Document 2] JP 2014-168933 A Summary of the Invention [Problem to be solved by the invention]

[0004] As described above, in head shading correction, a printed matter is read to obtain printing characteristics. In this case, the reading resolution is often double, the same, or half the print resolution of the image forming device. On the other hand, when measuring the color of a printed matter with a colorimeter, it is difficult to measure the color at a resolution equivalent to the print resolution, and generally, colorimetric values ​​can only be obtained at a resolution of about 1 / 10 to 1 / 100 of the print resolution. In addition, when measuring the color by hand, it is difficult to accurately control the colorimetric position. As a result, when the sensor characteristics of a scanner device are corrected based on colorimetric data, the colorimetric data and the scan data are not properly associated on the image, resulting in insufficient correction. In addition, the colorimetric data and the scan data usually have different color spaces due to the difference in the devices used to acquire the data. As a result, if a high-frequency pattern is present on the test chart used to obtain the reading characteristics of the sensor, the correction accuracy may be reduced. That is, as in the method of Patent Document 2 described above, when a mark (blank portion) for associating the colorimetric data with the scan data is inserted into the patch of the chart, the correction accuracy may be reduced. [Means for solving the problem]

[0005] The image forming apparatus according to the present disclosure includes a printing means for performing a printing process while transporting paper based on image data; a first generating means for generating correction information for correcting a sensor acquisition value of the scanning means based on scan data obtained by reading a first chart printed out from the printing means with a scanning means and colorimetric data obtained by measuring with a colorimetric means; a second generating means for correcting the sensor acquisition value of the scanning means using the correction information based on the colorimetric data obtained by measuring a second chart printed out from the printing means with the colorimetric means, and generating color adjustment information according to characteristics of the printing means; and an image processing means for performing color adjustment processing on the image data using the color adjustment information, wherein the first chart includes a patch having a uniform density extending in a main scanning direction approximately perpendicular to a paper transport direction and an alignment pattern formed outside the patch for identifying a position on the patch in the main scanning direction, and the first generating means determines a correction value for the sensor acquisition value at a specific pixel position by associating a pixel position of an end of the patch in the scan data with a colorimetric position of an end of the patch in the colorimetric data, and generates the correction information. Effect of the Invention

[0006] According to the technique of the present disclosure, it is possible to highly accurately correct the sensor characteristics of a scanner device based on color measurement data. [Brief description of the drawings]

[0007] [Figure 1] FIG. 2 is a diagram showing a hardware configuration of an image forming apparatus. [Diagram 2] 1A is a diagram showing an example of the configuration of the printing unit and its surroundings, FIG. 1B is an enlarged view of a recording head, FIG. 1C is an enlarged view of a head module, and FIG. [Diagram 3] FIG. 2 is a diagram showing the internal configuration of an image processing unit. [Figure 4] FIG. 4 is a diagram showing an example of a color adjustment table. [Diagram 5] FIG. 4 is a diagram showing an example of a scan correction table. [Figure 6]5 is a flowchart showing a processing flow in an image processing unit. [Figure 7] 11 is a flowchart showing the flow of a color adjustment table generation process. [Figure 8] FIG. 13 is a diagram showing an example of an HS chart. [Figure 9] 5A to 5C are diagrams for explaining a position alignment process. [Figure 10] 5A to 5C are diagrams illustrating a line profile synthesis process. [Figure 11] FIG. 4A is a diagram showing an example of a measurement curve, and FIG. 4B is a diagram explaining a calculation process of a correction amount. [Figure 12] 1A is a flowchart showing the flow of a process for generating a scan correction table, and FIG. 1B is a flowchart showing the flow of a process for deriving a correction amount. [Figure 13] 1A is a diagram showing the positional relationship between the SS chart and a line sensor, and FIG. 1B is a diagram showing the positional relationship between the SS chart and the color measurement position of a colorimeter 209. FIG. [Figure 14] 5(a) to 5(c) are diagrams for explaining detection of the edge of a measurement area. [Figure 15] FIG. 4 is a diagram showing an example of the correspondence between ruler numbers and pixel positions. [Figure 16] 5A to 5C are diagrams illustrating a process of converting a color measurement position into a pixel position. [Figure 17] 6A and 6B are diagrams for explaining calculation of a correction value. [Figure 18] 1A and 1B are diagrams for explaining the fluctuation of the sensor acquired value with respect to pixel position x. [Figure 19] FIG. 13 is a diagram showing an example of an SS chart. [Figure 20] FIG. 13 is a diagram showing an example of an SS chart. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] Hereinafter, the present invention will be described in detail according to preferred embodiments with reference to the accompanying drawings. Note that the configurations shown in the following embodiments are merely examples, and the present invention is not limited to the illustrated configurations.

[0009] [Embodiment 1] <Hardware configuration of image forming device> FIG. 1 is a diagram showing a hardware configuration of an inkjet printer as an image forming apparatus according to this embodiment. The image forming apparatus according to this embodiment includes a CPU 100, a RAM 101, a ROM 102, an operation unit 103, a display unit 104, an external storage device 105, an image processing unit 106, a printing unit 107, a scanning unit 108, an I / F unit 109, a color measurement unit 110, and a bus 111. The CPU 100 controls the operation of the entire image forming apparatus by expanding input data and computer programs stored in the ROM 102 or an external storage device (described later) in the RAM 101 and executing them. For example, the CPU 100 interprets PDL data included in an input print job and generates image data in a bitmap format for each page. Note that, although a case where the CPU 100 controls the entire image forming apparatus will be described as an example here, the entire image forming apparatus may be controlled by multiple hardware devices sharing the processing. The RAM 101 temporarily stores computer programs and data read from the external storage device 105 and data received from the outside via the I / F unit 109. The RAM 101 is used as a storage area when the CPU 100 executes arithmetic processing and as a storage area when the image processing unit 106 executes image processing. The ROM 102 stores setting parameters and boot programs for setting each unit in the image forming apparatus. The operation unit 103 is an input device such as a keyboard or a mouse, and receives operations (instructions) by an operator. This allows the operator to input various instructions to the CPU 100. The display unit 104 is a display device such as a CRT or a liquid crystal screen, and displays the processing results by the CPU 100 as images, characters, etc. If the display unit 104 is a touch panel that can detect touch operations, the display unit 104 may function as a part of the operation unit 103. The external storage device 105 is a large-capacity storage device represented by a hard disk drive. The external storage device 105 stores computer programs and data for causing the OS and the CPU 100 to execute various processes. In addition, temporary data generated by the processing of each unit (for example, input / output image data and a threshold matrix used in the image processing unit 106) is also stored.The computer programs and data stored in the external storage device 105 are read as appropriate under the control of the CPU 100, expanded in the RAM 101, and processed by the CPU 100. The image processing unit 106 is realized as a processor capable of executing computer programs or a dedicated image processing circuit, and executes various image processes for converting image data input as a print target into image data that can be output by the printing unit 107. Instead of providing a dedicated processor as the image processing unit 106, the CPU 100 may be configured to execute various image processes as the image processing unit 106. The printing unit 107 forms an image on a sheet of paper as a recording medium using ink as a coloring material based on image data received directly from the image processing unit 106 or via the RAM 101 or the external storage device 105. The printing unit 107 will be described in detail later. The scanning unit 108 is an image sensor (line sensor or area sensor) for optically reading an image formed on a sheet of paper by the printing unit 107. The scanning unit 108 will be described in detail later. The I / F unit 109 functions as an interface for connecting the image forming apparatus to an external device. The I / F unit 109 also functions as an interface for exchanging data with a communication device using infrared communication, a wireless LAN (Local Area Network), or the like, and for connecting to the Internet. This allows, for example, image data to be printed to be received from an external PC (not shown). The colorimetric unit 110 is a colorimeter for measuring the color of an image formed on paper by the printing unit 107. Details of the colorimetric unit 110 will be described later. The above-mentioned units are connected to a bus 111, and data can be exchanged via the bus 111.

[0010] Note that the hardware configuration shown in Fig. 1 is an example, and the image forming apparatus may have a hardware configuration different from that shown in Fig. 1. For example, the image forming apparatus may have a configuration in which the printing unit 107 is connected via an I / F unit 109. Also, instead of a configuration in which the colorimetric unit 110 is provided as part of the image forming apparatus, the image forming apparatus may have a configuration in which colorimetric information is obtained from an external colorimeter via the I / F unit 109.

[0011] (Printing details) As shown in FIG. 2(a), the printing unit 107 includes recording heads 201-204 corresponding to black (K), cyan (C), magenta (M), and yellow (Y). Each of the recording heads 201-204 is a so-called full-line type, and a plurality of nozzles for ejecting ink are arranged in a predetermined direction in a range corresponding to the full width of the paper 206. As shown in FIG. 2(b), the recording heads 201-204 are configured by arranging a plurality of head modules alternately in the paper transport direction. FIG. 2(c) is an enlarged view of the head module 201a, and further shows that it is composed of a plurality of chip modules 201a-1 to 201a-5. Each chip module is connected to an independent board. FIG. 2(d) is an enlarged view of the chip module 20-1a, and 16 nozzles are present. In this embodiment, the description will be given assuming that the resolution of the nozzle arrangement in each of the CMYK recording heads 201-204 is 1200 dpi.

[0012] A sheet of paper 206 serving as a recording medium is conveyed in one direction indicated by an arrow 207 in the figure by a conveying roller 205 (and other rollers, not shown) rotating by the driving force of a motor (not shown). Then, while the sheet of paper 206 is being conveyed, ink is ejected from the multiple nozzles of each of the recording heads 201-204 in accordance with print image data, thereby sequentially forming an image for one raster corresponding to the nozzle rows of each of the recording heads. In this manner, by repeating the ink ejection operation from each of the recording heads 201-204 onto the conveyed sheet of paper 206, for example, an image for one page can be formed on the sheet of paper.

[0013] (Details of the scanning and colorimetric sections) The scanning unit 108 optically reads the paper 206 conveyed thereto, and stores the read image data (scanned data) in the external storage device 105. As shown in FIG. 2(a), the scanning unit 108 is composed of line sensors 208a and 208b, which are arranged to cover the entire surface of the paper 206 while partially overlapping each other. The line sensors 208a and 208b sequentially capture the paper 206 conveyed thereto, and store the image data in the external storage device 105 as two-dimensional image data including RGB information and brightness information. At this time, the resolution of the image data is, for example, 600 dpi in both the x direction and the y direction in FIG. 2. Alternatively, the resolution may be different in the x direction and the y direction, such as 1200 dpi in the x direction and 600 dpi in the y direction. In this embodiment, the two line sensors are overlapped to cover the entire width of the paper 206, but the entire width of the paper 206 may be covered by sliding one line sensor, for example. Alternatively, it is not necessary to cover the entire width of the paper 206 .

[0014] The colorimeter 110 is disposed downstream of the scanner 108, and measures the color of a predetermined y position on the conveyed paper 206 while scanning in the x direction, and stores the measured color as spectral reflectance data in the external storage device 105. Alternatively, the color value in a device-independent color space calculated from the spectral reflectance data is stored in the external storage device 105. Specifically, the colorimeter 110 converts the spectral reflectance data in 10 nm increments from 380 to 780 nm, which is the visible light range, or the spectral reflectance data into data in a color space such as CIE XYZ, CIE Lab, sRGB, or AdobeRGB, and stores the converted data. Here, while the colorimeter 209 scans in the sensor direction, the conveyance of the paper 206 (paper feed in the y direction) is stopped. In other words, after the scanning and colorimetering of a certain colorimeter region are completed, the paper 206 is conveyed by a predetermined amount, and the scanning and colorimetering of another colorimeter region are performed. In this way, the conveyance of the paper 206 and the scanning and colorimetering by the colorimeter 209 are repeated, and colorimeter values ​​corresponding to each colorimeter region are obtained. At this time, in order to avoid complicating the paper transport control and the device configuration, it is also possible to measure the color of the paper 206 by retracting it to a transport path different from the transport path for printing / imaging. Alternatively, a separate colorimeter is provided outside the image forming apparatus, the paper 206 is measured by the colorimeter, and the obtained colorimetric data is input via the I / F unit 109. The time interval for colorimetric measurement by the colorimetric unit 110 is generally longer than the reading interval (scanning interval) in the scanning unit 108, and for example, the colorimetric interval in the y direction is 5 times / inch. In addition, the spectral reflectance data, which is the colorimetric result, is obtained as the reflectance averaged within the aperture shape of the colorimeter 209, for example, a circle with a diameter φ=3.5 mm.

[0015] It should be noted that both the scanning unit 108 and the color measurement unit 110 may be located downstream of the printing unit 107 . For example, the color measurement unit 110 may be located upstream of the scanning unit 108 .

[0016] (Details of the image processing unit) Fig. 3 is a diagram showing the internal configuration of the image processing unit 106. Below, the functions of the image processing unit 106 will be described in detail with reference to Fig. 3. The image processing unit 106 has a color conversion unit 301, a color adjustment unit 302, and a halftone process (hereinafter referred to as the "HT processing unit") 305. Furthermore, the image processing unit 106 has a color adjustment information generation unit 303 and a scan correction information generation unit 304.

[0017] The color conversion unit 301 converts the input image data into image data corresponding to the ink colors used in the printing unit 107. For this conversion, known methods such as matrix calculation processing and processing using a three-dimensional LUT (lookup table) can be used. The input image data has 8-bit coordinate values ​​(R, G, B) in a color space such as sRGB, which is the display color of a monitor, and the image data after color conversion has 8-bit color signal values ​​for each of CMYK corresponding to the printing unit 107. That is, RGB data is converted into CMYK data by the color conversion processing. The CMYK data represents the usage amount (ejection amount) of each ink that is discharged onto the paper surface by the printing unit 107 to display an image. Note that the input image data is not limited to RGB data, and may be CMYK data. Even if CMYK data is input from the beginning, it is preferable to perform conversion processing using a four-dimensional LUT that converts the input CMYK data into C'M'Y'K' data due to restrictions on the total amount of ink and color management.

[0018] The color adjustment unit 302 refers to the color adjustment table generated by the color adjustment information generation unit 303, and performs color adjustment processing (head shading correction processing) on ​​the color-converted CMYK data, taking into account the unevenness of ink ejection of each of the printheads 201 to 204. FIG. 4 shows an example of the color adjustment table (head shading correction table). In the color adjustment table shown in FIG. 4, adjusted color signal values ​​(output color signal values) corresponding to each input color signal value (0, 16, 32, ..., 240, 255) are stored for each of the printheads 201 to 204. For example, in the data corresponding to the ink color K among the CMYK data, if the input color signal value of the pixel corresponding to the head module 201a is "32", the adjusted color signal value of the pixel becomes "28" by the color adjustment processing. Note that the color adjustment processing can be performed not for each printhead, but for each head module, chip module, or nozzle. Furthermore, the color adjustment processing can be performed for each nozzle block divided by a certain number of nozzles, for example, for every 8 nozzles. When performing color adjustment processing for each nozzle, the color adjustment information generating unit 303 generates a color adjustment table having a number of columns equal to the number of nozzles. For input color signal values ​​that are not specified in the color adjustment table shown in FIG. 4, nearby input color signal values ​​are identified from among the specified input color signal values, and the output color signal values ​​are calculated using an interpolation process. Of course, it is also possible to store output color signal values ​​for all input color signal values ​​without using the interpolation process. It is also possible to perform color adjustment processing by function conversion or matrix conversion instead of the table method.

[0019] A color adjustment information generating unit 303 receives scan data of a chart (hereinafter referred to as an "HS chart") for generating a color adjustment table (head shading correction table) from the scanning unit 108, and generates the above-mentioned color adjustment table.

[0020] The scan correction information generating unit 304 generates a table (hereinafter referred to as a "scan correction table") to be referenced in the sensor shading correction process. For this generation, the scan data of the HS chart received from the scanning unit 108 and the colorimetric data of a chart for generating the scan correction table (hereinafter referred to as an "SS chart") received from the colorimetric unit 110 are used. FIG. 5 shows an example of the scan correction table. In FIG. 5, the corrected sensor acquisition values ​​corresponding to the sensor acquisition values ​​(0, 16, 32, ..., 240, 255) included in the scan data are stored in association with pixel positions (0, 100, 200, 300, 400, ..., 3500, ...) in the x direction. For example, if the sensor acquisition value at pixel position "100" is "32", the sensor acquisition value (SS correction value) after the sensor shading correction process is "39". Note that the sensor acquisition values ​​not specified in the scan correction table shown in FIG. 5 are calculated by an interpolation process using the SS correction values ​​for the adjacent sensor acquisition values ​​that are specified. Similarly, sensor acquisition values ​​at pixel positions not present in the scan correction table are calculated by interpolation processing using SS correction values ​​for adjacent sensor acquisition values ​​among those specified. Of course, SS correction values ​​for all sensor acquisition values ​​and image positions may be stored without using interpolation processing. Note that, like the color adjustment table, sensor shading correction processing can also be performed in the scan correction table by function conversion or matrix conversion instead of the table method.

[0021] The HT processing unit 305 performs halftone processing on the color-adjusted CMYK data for each color plate, and generates halftone image data (hereinafter referred to as "HT image data") expressed with halftone dots that can be expressed by the printing unit 107. This halftone processing generates binary HT image data for each CMYK color plate, in which each pixel has a value of "0" or "1." A known method such as a dither method or an error diffusion method may be used for the halftone processing.

[0022] (Processing flow of the image processing unit) Next, each process performed by the image processing unit 106 will be described with reference to the flowchart shown in FIG. 6. When a user inputs a print job to the image forming apparatus through the operation unit 103, image data to be printed (bitmap image in page units) and printing conditions are expanded on the RAM 101. Then, a series of processes shown in the flowchart of FIG. 6 are started and executed in page units. Here, the print job is instruction information for the print process, and includes PDL data that specifies the print contents in page units, as well as the number of prints, information on the paper to be printed, information on the print mode, and printing conditions such as single-sided / double-sided and Nin1. Information on the paper includes the paper type such as plain paper or glossy paper, the paper size such as A4 or A3, the manufacturer name, the model number, and the like. Information on the print mode includes designation of a high color mode that slows down the transport speed to increase the amount of ink, and an ink-saving mode that speeds up the transport speed to reduce the amount of ink. In the following description, the symbol "S" means a step.

[0023] In S601, the color conversion unit 301 performs color conversion processing on the RGB data, which is the input image data, to convert it into CMYK data.

[0024] In the next step S602, the color adjustment unit 302 judges whether a color adjustment table that matches the printing conditions specified in the print job is available. Specifically, if a color adjustment table that corresponds to the manufacturer name, model number, paper type, or the contents of the specified print mode of the specified paper exists in the external storage device 105, etc., it is judged that a color adjustment table that matches the printing conditions is available. On the other hand, if a color adjustment table that matches the manufacturer name, model number, etc. of the specified paper does not exist in the external storage device 105, etc., it is judged that there is no available color adjustment table. This is because it is assumed that the correction amount for correcting the nozzle characteristics is not suitable for the specified paper. Therefore, if there is no concern that the correction amount for correcting the nozzle characteristics will be incompatible, it may be judged that the color adjustment table is available even if some of the printing conditions are incompatible. For example, since the basis weight and paper size of the paper do not affect the correction amount very much, it may be judged that a color adjustment table that matches the printing conditions is available even if the basis weight and size of the paper are different from those of the paper when the saved color adjustment table was generated. When a new type of paper is set, which has a different paper quality from the paper when the stored color adjustment table was generated, it is preferable to derive the correction amount for head shading correction for that paper. Also, the time elapsed since generation and the presence or absence of head cleaning processing may be taken into consideration. That is, when a color adjustment table corresponding to the specified paper has been generated and stored and a certain time has passed since generation, it may be determined that there is no usable color adjustment table. Alternatively, when head cleaning processing has been performed after generation, it may be determined that there is no usable color adjustment table. Furthermore, the user may determine whether or not the table can be used, and flag information indicating the result may be stored, and the determination may be made based on the flag information. In that case, the user may set a flag via the operation unit 103 when new paper is set or the head is replaced. Alternatively, the flag may be set by visually checking the result of test printing. If the result of the determination indicates that a usable color adjustment table exists in the external storage device 105 or the like, the process proceeds to S604. On the other hand, if it is determined that there is no usable color adjustment table, the process proceeds to S603.

[0025] In S603, the color adjustment information generating unit 303 generates a color adjustment table that matches the printing conditions specified in the print job. The color adjustment table generating process will be described in detail later.

[0026] In S604, the color adjustment unit 302 performs color adjustment processing on the CMYK data obtained by the color conversion in S601, using a suitable color adjustment table that can be used. Now, assume that the density of an image formed by the head module 201a when the input color signal value is "32" is relatively high compared to the target density and densities formed by other recording heads. In this case, by changing the pixel value of the input image data to a smaller value (for example, "28"), the probability that dots will be formed by the head module 201a when the input color signal value is "32" can be reduced. By such processing, the difference between the target density and other recording heads can be reduced. In this embodiment, color adjustment tables such as those shown in FIG. 4 described above are generated and saved in advance for various types of paper and printing modes. Then, if there is no color adjustment table corresponding to the paper or printing mode specified in the print job, a new one will be generated. In this way, density changes occurring in units of recording heads and recording nozzles are suppressed.

[0027] In S605, the HT processing unit 305 performs halftone processing on the color-adjusted CMYK data. The generated HT image data is sent to the printing unit 107, which executes printing processing based on the HT image data.

[0028] The above is the content of the processing in the image processing unit 106. This processing is executed every time a print job is submitted, and the image specified by the user can be printed the specified number of times. Note that in the determination in S602, if the model number or manufacturer name of the paper is different but the paper type is the same, it may be determined that the color adjustment table is usable. For example, when coated paper is specified, the same manufacturer's coated paper runs out and a different manufacturer's coated paper is replenished. In such a case, if it is empirically known that there is no problem, the color adjustment table used before the replenishment may be applied as is.

[0029] <Color adjustment table generation process> Next, the above-mentioned color adjustment table generation process in S603 will be described in detail with reference to the flowchart shown in Fig. 7. Note that in the following description, an example will be described in which a color adjustment table is generated for each nozzle.

[0030] First, in S701, it is determined whether a scan correction table that matches the printing conditions specified in the print job is usable. The criteria for this determination may be the same as the criteria shown in S602 in the flowchart of FIG. 6 described above. This is because, as with the color adjustment table, the correction amount for correcting the reading characteristics of the sensor may differ for the scan correction table depending on the paper and print mode used. Therefore, when using a new type of paper, it is preferable to derive the correction amount for the sensor shading correction process for that paper. However, since the spectral characteristics have almost no effect on the basis weight and paper size of the paper, these differences may be tolerated and a slightly looser criterion may be used for the determination. Note that, as with S602 described above, the fact that the elapsed time since generation may be taken into consideration. In other words, it may be determined that a usable scan correction table is available only when there is a scan correction table that has been generated within a certain time. This is because the color of the filter inside the sensor may change over time, or the spectral characteristics of the lighting may change, and the scan correction table may no longer be suitable for the sensor after such changes. If there is a usable scan correction table as a result of the determination, the process proceeds to S703. On the other hand, if there is no available scan correction table, the process proceeds to S702, where a process of generating a new scan correction table is performed. The scan correction table generation process in S702 will be described in detail later.

[0031] In S703, the HS chart is printed out. Specifically, the image data of the HS chart stored in the external storage device 105 or ROM 102 is read out, the HT processing unit 305 performs halftone processing, and the printing unit 107 performs printing processing using the generated HT image data. FIG. 8 shows an example of an HS chart. In addition to a measurement area for acquiring density characteristics for each nozzle, the HS chart has a pattern area for aligning the nozzle position and the reading position. In the case of the HS chart 800 in FIG. 8, there are nine patches (measurement areas) 801 to 809 with different gradations, and each patch has a uniform density extending in the main scanning direction approximately perpendicular to the conveying direction of the paper. In addition, alignment patterns 810a to j are arranged outside the patches so as to sandwich each of the measurement areas 801 to 809. The alignment pattern 810 is generally called a "ruler" and is composed of a plurality of thin lines formed at a predetermined interval in the y direction.

[0032] In S704, scan data of the printed out HS chart is acquired. Specifically, the HS chart printed by the printing unit 107 is read by the line sensors 208a and 208b constituting the scanning unit 108, and scan data of the HS chart is generated. That is, two sets of scan data are generated: one by the line sensor 208a and the other by the line sensor 208b.

[0033] In S705, line profiles corresponding to the line sensors 208a and 208b are generated based on the two scan data acquired in S704. Specifically, the measurement areas (patches) of the HS chart are identified from the scan data, and one-dimensional data (line profile) is obtained by averaging the sensor acquisition values ​​in the transport direction (y direction). The line profile is obtained by averaging the readings at different y positions at the same x position in each measurement area. When the HS chart 800 shown in FIG. 8 described above is used, nine line profiles corresponding to the measurement areas 801 to 809 are obtained for each of the line sensors 208a and 208b.

[0034] In S706, a sensor shading correction process based on the pixel position in the x direction is performed on each line profile obtained in S705 using an available scan correction table. Now, consider a case where the sensor acquisition value at pixel position x=50 of the line profile corresponding to the measurement area 808 of the HS chart 800 shown in FIG. 8 is “24”, and the sensor shading correction process is performed using the scan correction table shown in FIG. 5. In this case, the SS correction value for the sensor acquisition value “24” at pixel positions x=0 and x=100 is first obtained by an interpolation calculation. Specifically, for pixel position x=0, SS correction values ​​“29” and “40” corresponding to the sensor acquisition values ​​“16” and “32” are used to obtain the SS correction value of 29+(40-29)×(24-16)÷(32-16)=34.5. Similarly, the SS correction value is obtained for pixel position x=100 as 32.0. Then, from the two calculated SS correction values, "34.5" and "32.0", 32.0 + (34.5 - 32.0) × (100 - 50) ÷ (100 - 0) = 33.25 is obtained as the SS correction value for pixel position x = 50. In this way, by determining the SS correction value at each pixel position in the x direction for each line profile based on the scan correction table, a line profile that has been subjected to sensor shading correction processing can be obtained.

[0035] In S707, based on the two scan data obtained in S704, the identification numbers (nozzle numbers) of the nozzles that passed through each pixel position in the x direction are specified for each measurement area of ​​the HS chart. Specifically, the image areas in the scan data corresponding to each alignment pattern 810 of the HS chart 800 are specified, and the pixel positions in the x direction on the image in each line profile are associated with the nozzle numbers. A specific description will be given with reference to FIG. 9. The table shown in FIG. 9 indicates the center pixel positions in the x direction in the scan data of each line that constitutes the alignment patterns 810a to 810j. It is assumed that the resolution in the x direction of the printing unit 107 (≒ nozzle interval) is 1200 dpi. It is also assumed that the interval between the nozzles that form the lines is 16 nozzles. It is also assumed that the resolution in the x direction of the scanning unit 108 (≒ pixel interval) is 600 dpi. Now, consider the pixel position X in the x direction that corresponds to the nozzle number "016" in the measurement area 801. First, attention is paid to pixel positions X of alignment patterns 810a and 810b located above and below the measurement area 801. From the table in FIG. 9, it can be seen that the coordinate values ​​of pixel position X for nozzle number "016" are 720 and 721, respectively. Therefore, the average value of pixel position X corresponding to nozzle number "016" in the measurement area 801 is 720.5. Note that, as with the color adjustment table, the nozzle numbers of nozzles that do not contribute to the formation of the alignment pattern can be calculated by linearly interpolating pixel positions in the x direction obtained from the nozzle numbers of adjacent nozzles that contribute to formation. In this way, pixel positions in the x direction corresponding to all nozzle numbers are identified for each line profile.

[0036] In S708, a process of synthesizing line profiles obtained from the scan data of each of the line sensors 208a and 208b is performed. Here, the line profile synthesis process will be described with reference to (a) to (c) of FIG. 10. (a) to (c) of FIG. 10 are graphs in which the horizontal axis represents the nozzle number and the vertical axis represents the sensor output value (for example, the output value of the G sensor). In the graph of FIG. 10(a), curves 1001 and 1002 correspond to the line sensors 208a and 208b, respectively. Specifically, line profiles obtained by reading the same measurement area with both sensors are plotted against the corresponding nozzle numbers. Note that the line sensors 208a and 208b are arranged so as to overlap partially in the sensor row direction (x direction) (see FIG. 2(a)). Therefore, the curves 1001 and 1002 are plotted so as to overlap with the nozzle numbers in the overlapping range 1003. At this time, as shown in FIG. 10(a), even though the same measurement area is read, the sensor acquired value may deviate due to the characteristics (individual difference) inherent to the line sensor. Therefore, an offset is applied so that the average values ​​of the overlapping range 1003 are approximately the same. Specifically, a process such as adding / subtracting or multiplying / dividing a value is performed uniformly for each sensor output value. In FIG. 10(b), a dashed curve 1001' indicates the state of the curve 1001 after the offset, and a dashed curve 1002' indicates the state of the curve 1001 after the offset. The two offset curves 1001' and 1002' thus obtained are combined to obtain a combined curve 1004 shown in FIG. 10(c). In this combination, for the overlapping range 1003 in the two offset curves 1001' and 1002', the average value of both curves is adopted, and for the other part (non-overlapping range), the offset value indicated by each curve may be used as it is. However, when calculating the average value of the overlapping ranges, it is desirable to weight the overlapping ranges so that the connection between the overlapping ranges and the non-overlapping ranges is smooth, for example by giving a larger weight to the center. By performing such processing for each of the measurement areas 801 to 809, nine composite line profiles for each measurement area can be obtained.

[0037] In S709, the nozzle number (nozzle of interest id) of a nozzle of interest among all the nozzles arranged in the x direction is initialized. Specifically, the nozzle of interest id=0 is set.

[0038] In S710, the correction amount for the current nozzle id of interest is derived, and the adjusted color signal value for the nozzle of interest is determined. A specific description will be given with reference to the figures. First, in generating the color adjustment table, a measurement curve corresponding to the nozzle of interest is calculated. Here, the measurement curve is a curve showing the relationship between the color signal value of the target measurement area and the sensor acquisition value at the pixel position corresponding to the nozzle of interest on each line profile. FIG. 11(a) shows an example of the measurement curve. The horizontal axis of FIG. 11(a) is the color signal value of the image formed on the paper by the printing unit 107, and the vertical axis is the sensor acquisition value obtained by scanning the paper by the scanning unit 108. The dashed line 1101 in the figure indicates the upper limit of the horizontal axis, which is "255" if the input color signal value is an 8-bit value. The curve 1102 in the figure is a measurement curve obtained by combining the color signal value of the measurement area included in the HS chart, the sensor acquisition value corresponding to each gradation, and further an interpolation calculation. As the interpolation method, a known method such as piecewise linear interpolation or a spline curve may be used. The measurement curve 1102 represents the density characteristic of the nozzle corresponding to the pixel position in the x direction of the scan data. For example, for a nozzle with a small discharge amount, the curve shifts upward (toward the brighter side). A straight line 1103 in the figure indicates the discharge characteristic (target discharge characteristic) common to all nozzles that is the correction target of each nozzle. The target discharge characteristic may be set by, for example, determining each value that is linear with respect to a sensor acquisition value 1104 corresponding to a predetermined maximum color signal value. Alternatively, a head module, chip module, or nozzle may be used as a reference, and the discharge characteristic of the reference module or nozzle may be used as the target discharge characteristic. Alternatively, the average discharge characteristic of a head module, chip module, or nozzle within a predetermined range may be used as the target discharge characteristic. FIG. 11(b) is a diagram for explaining the calculation process of the correction amount. First, the ID of the nozzle of interest and the input color signal value 1105 to be used for calculating the correction amount are obtained. Next, the target color signal value 1106 corresponding to the acquired input color signal value 1105 is obtained from the target discharge characteristic 1103 of the nozzle of interest. Furthermore, a gradation value corresponding to a target color signal value 1106 is acquired as an adjusted color signal value 1107 from the measurement curve 1102 of the nozzle of interest.The acquired adjusted color signal value 1107 is then associated with the input color signal value 1105 and stored in the color adjustment table being generated in association with the nozzle of interest. By performing this process using all values ​​from 0 to 255 as the input color signal value 1105, a table corresponding to all gradation values ​​for the nozzle of interest can be obtained. Alternatively, the gradation values ​​may be thinned out to generate a table corresponding to, for example, nine specific gradation values. In this case, values ​​other than the specific gradation values ​​may be found by known interpolation processing from the nine specific gradation values.

[0039] In S711, it is determined whether all nozzles have been processed as the nozzle of interest. If the nozzle of interest id is equal to or greater than the number of nozzles included in the print heads 201-204, it is determined that the nozzles have been processed. If there are any nozzles that have not been processed, the process proceeds to S712, the nozzle of interest id is updated, and the process returns to S710 to repeat the same process. On the other hand, if it is determined that all nozzles have been processed as the nozzle of interest, the process proceeds to S713.

[0040] In S713, the color adjustment table obtained by the processing up to this point is stored in the external storage device 105. At this time, information about the paper used, such as the manufacturer name, model number, and paper type, printing conditions such as the print mode, and the creation date and time are stored in association with each other.

[0041] The above is the content of the process of generating the color adjustment table for each nozzle. By performing such a process for each ink color (C, M, Y, K), a color adjustment table is completed. In the flowchart of FIG. 7, the sensor shading correction process based on the pixel position in the x direction is performed on the line profile (S706), but this is not limited to this. For example, it may be performed on the scan data of the HS chart read in S704, and the subsequent nozzle position may be detected and converted into a line profile. However, when the transport error in the x direction in the printing unit 107 is low frequency, it is preferable to perform the sensor shading correction process on the line profile as shown in the flowchart of FIG. 7. For example, in each of the measurement areas 801 to 809 in the HS chart 800 in FIG. 8, the transport error in the x direction is expected to be smaller than the reading resolution in the x direction of the scanning unit 108 or smaller than the printing resolution in the x direction of the printing unit 107. In such a case, it is preferable to perform the sensor shading correction process on the line profile. In these cases, the processing time can be reduced by performing the sensor shading correction process on the line profile compared to performing the correction on each pixel of the scan data of the HS chart. In addition, the noise caused by the sensor and the influence of halftone processing can also be reduced by averaging when calculating the line profile. On the other hand, if a transport error of one pixel or more occurs in the x direction in each measurement area, the correspondence between the nozzles and the reading elements formed in the measurement areas 801 to 809 will differ depending on the y position. Therefore, in the head shading correction process, it is preferable to average while sliding diagonally to generate a line profile. On the other hand, it is preferable to perform the sensor shading correction process based on the image element position. Therefore, when there is a deviation of one pixel or more, it is preferable to apply the sensor shading correction according to the pixel position in the x direction in the scan data of the HS chart, and then perform averaging based on the alignment pattern 810 to calculate the line profile.

[0042] <Scan correction table generation process> Next, the process of generating the scan correction table in S702 described above will be described in detail with reference to the flowchart shown in FIG.

[0043] In S1201, image data of the SS chart is read from the external storage device 105 or the ROM 102. In this embodiment, the HS chart 800 shown in FIG. 8 is also used as the SS chart. Note that in the sensor shading correction process, it is possible to use a chart other than the chart used in the head shading correction process. For example, the color signal values ​​that are emphasized in the head shading correction process and the sensor shading correction process may differ. Therefore, a chart on which more color signal values ​​that are emphasized in the sensor shading correction process are arranged may be prepared and used. Alternatively, a dedicated chart on which only the color signal values ​​that are emphasized in each process are arranged may be used.

[0044] In S1202, the color adjustment unit 302 performs head shading correction processing on the image data of the SS chart using color adjustment information (color adjustment table) for each nozzle. The image data of the SS chart that has been subjected to head shading correction is halftone processed by the HT processing unit 305 and converted into a halftone image, and is sent to the printing unit 107.

[0045] In S1203, the printing unit 107 executes printing processing based on the halftone image data of the SS chart. By the processing up to this point, an SS chart is obtained from which high frequency unevenness caused by the recording heads 201 to 204 in particular has been removed.

[0046] In S1204, the scanning unit 108 uses the line sensors 208a and 208b to read the SS chart output from the printing unit 107 and generate scan data of the SS chart. Furthermore, at this time, similar to S705 described above, the measurement area (patch) of the SS chart is identified from the two pieces of acquired scan data, and line profiles corresponding to the line sensors 208a and 208b are acquired. In addition, the alignment pattern (ruler) of the SS chart is identified, and pixel positions (x positions in the main scanning direction on the patch) corresponding to the alignment patterns 810a to 810j are calculated.

[0047] In the next step S1205, the colorimetric unit 110 measures the measurement areas of the SS chart output from the printing unit 107. In this embodiment, colorimetric measurement is performed on each of the colorimetric areas 801 to 809 by repeating scanning in the sensor row direction (x direction) of the colorimeter 209 and paper feeding in the y direction perpendicular to the x direction by the conveying roller 205. As a result, L * a * b * The value is obtained.

[0048] In S1206, the scan correction information generating unit 304 performs processing for deriving a correction amount at a pixel position (x position) on each line sensor based on the line profile of the sensor acquisition value obtained in S1204 and the colorimetric value obtained in S1205 for each line sensor. Details of this correction amount derivation processing will be described later.

[0049] In S1207, it is determined whether the correction amount derivation process for each line sensor is completed. In this embodiment, if the correction amount derivation process for each of the line sensors 208a and 208b is completed, the process proceeds to S1208. On the other hand, if the correction amount derivation process for any of the line sensors has not been executed, the process returns to S1206, and the correction amount derivation process for the line sensor of interest is executed.

[0050] In S1208, a scan correction table corresponding to each line sensor is generated based on the result of the correction amount derivation process performed for each line sensor, and is stored in the external storage device 105.

[0051] The above is the content of the scan correction table generation process. Note that the scan correction table may be generated only for a color (e.g., yellow) for which the sensor acquisition values ​​tend to diverge between the center and edges of a uniform patch image. Of course, the above generation process may be repeated for all colors (C: cyan, M: magenta, Y: yellow, K: black) to generate a scan correction table corresponding to each color. When generating for all colors, the colorimetric values ​​and sensor acquisition values ​​used are changed for each color to be processed (e.g., L for K). * Alternatively, instead of using only one of the sensors or color measurement values, it is also possible to use values ​​calculated from them. For example, the L value of the paper itself may be used as a color measurement value. * a * b * Value or L * = 100, a * =0, b * It is also possible to use a color difference from 0. Furthermore, it is also possible to use a converted value obtained by performing a 3×3 or 3×1 matrix conversion on the color measurement value or the sensor acquired value.

[0052] <Correction amount derivation process> Next, the correction amount derivation process in S1206 described above will be described in detail with reference to the flowchart shown in FIG.

[0053] First, in S1211, pixel positions corresponding to the ends of the measurement area of ​​the SS chart are detected based on the line profiles corresponding to the line sensors 208a and 208b acquired in S1204. FIG. 13(a) is a diagram showing the positional relationship between the SS chart and the line sensors 208a and 208b. In FIG. 13(a), a rectangular area 1301 represents one of the measurement areas 801 to 809 included in the SS chart, and thin lines 1302 that are continuous at equal intervals represent one of the alignment patterns 810a to j. Note that the rectangular area 1301 is a pattern for acquiring the difference in characteristics between sensors for approximately the same color, and it is preferable that a patch image with uniform density is formed. In FIG. 13(a), xa1 and xb1 represent pixel positions corresponding to the ends of the rectangular area 1301, and in this step, positions corresponding to xa1 and xb1 on the line profiles corresponding to the line sensors 208a and 208b are identified. FIG. 14(a) is a graph with the sensor acquisition value on the vertical axis and the pixel position (x) of the line sensor 208a on the horizontal axis, and the solid line 1401 shows the line profile of the sensor output value (the signal value of blue (B) among the RGB values) of the line sensor 208a. In this case, the pixel position xa1 can be estimated from the shape of the solid line 1401. For this estimation, for example, the first derivative of the sensor output value B with respect to the pixel position x of the solid line 1401 is calculated for each pixel position, and the pixel position with the largest absolute value of the derivative value is estimated to be the position of xa1. Instead of the above method, a known edge detection filter may be used. Alternatively, a threshold value given in advance or calculated from the maximum value, minimum value, average value, etc. of the solid line 1401 may be used, and a position that is larger than the value of the pixel position xa0 and has the smallest value among the pixel positions where the sensor output value falls below the threshold value may be estimated to be the position of xa1. 14B is a graph with the vertical axis representing the sensor acquired value and the horizontal axis representing the pixel position (x) of the line sensor 208b, and a solid line 1402 represents the line profile of the sensor acquired value (the signal value of blue (B) among the RGB values) of the line sensor 208b. In this case, the pixel position xb1 can be estimated from the shape of the solid line 1402.

[0054] In the next step S1212, the color measurement positions corresponding to the ends of the measurement area of ​​the SS chart are calculated based on the color measurement data acquired in step S1205. Fig. 13B is a diagram showing the positional relationship between the SS chart and the color measurement positions of the colorimeter 209. In Fig. 13B, marks 1303a to 1303o indicate the color measurement positions of the colorimeter 209. That is, the colorimeter 209 performs color measurement at each position indicated by the marks 1303a to 1303o while scanning so as to pass over the rectangular area 1301, and calculates the L * a * b * In this embodiment, the moving speed and timing of the colorimeter 209 are adjusted to obtain colorimetric values ​​at intervals that are exactly four times the interval of the thin lines 1302, as shown in FIG. 13B. * a * b * Based on the values, the color measurement positions t0 and t1 corresponding to the ends of the measurement area of ​​the SS chart are estimated. * 14(c) is a graph in which the color measurement positions t0 and t1 corresponding to the ends of the measurement area are plotted against each other. A curve passing through each black circle (plot point) like the dashed line 1403 in FIG. 14(c) can be obtained by known interpolation processing such as spline interpolation, and the color measurement positions t0 and t1 corresponding to the ends of the measurement area can be calculated from the curve by edge detection and threshold processing.

[0055] In S1213, pixel positions corresponding to each color measurement position are estimated. As shown in FIG. 13A, patch images of substantially uniform density are formed in the measurement area of ​​the SS chart, and it is difficult to associate the color measurement positions with pixel positions by analyzing the line profile. Therefore, in this embodiment, the color measurement positions are associated with pixel positions using a thin line 1302 as an alignment pattern. At this time, the correspondence between the color measurement positions and pixel positions changes every time scan data is acquired due to factors such as meandering during transport of the paper, paper floating, and skewing. Therefore, even if the sensor length and the ends of the measurement area (pixel positions xa1 and xb1) are known, it is preferable to associate the two every time scan data is acquired. The estimation of pixel positions corresponding to each color measurement position is realized by calculating pixel position xn for an arbitrary color measurement position tn using pixel positions (x positions) corresponding to the ruler acquired in S1204.

[0056] Here, a concrete example will be used for explanation. FIG. 15 is a graph with pixel position on the vertical axis and ruler number on the horizontal axis, and curve 1501 shows the relationship between the number (ruler number) of each thin line and pixel position x. Note that the ruler numbers may be assigned to each thin line from the left as 0, 1, 2, ..., and a continuous curve can be obtained by performing a known interpolation process. It is preferable that the ruler number and the pixel position have a linear relationship, but due to distortion of the optical system such as paper meandering and aberration, a curve such as that shown in FIG. 15 is often obtained. In this case, the ruler number N corresponding to the color measurement position tn is obtained by the following formula (1). Ruler number N = (tn-t0) × D Equation (1)

[0057] In the above formula (1), D is the colorimetry interval ratio to the ruler interval, and here D=4. By applying the "relationship between ruler number and pixel position" given as curve 1501 to the ruler number N obtained in this way, it is possible to obtain a pixel position corresponding to any colorimetry position. Now, by obtaining the colorimetry intervals at equal intervals and at intervals that are an integer multiple (here, four times) of the number of rulers, the correspondence can be easily determined. Note that the estimation becomes easier by adjusting so that the end of the rectangular area 1301 is included in one of the colorimetry positions. For example, when the start position of colorimetry can be specified, it is preferable to adjust so that t0=0.

[0058] By the above process, the sensor pixel position x and the color measurement value (here, L * a * b * Value of b * A relationship between the sensor pixel position and the color measurement value (b value) can be obtained. Also, by performing an interpolation process using the values ​​of each black circle, a curve 1601 in FIG. 16(a) can be obtained. Also, by performing a similar process for each measurement area of ​​the SS chart, a number of curves equal to the number of measurement areas can be obtained. For example, when an SS chart containing five measurement areas is used, as shown in FIG. 16(b), * Five curves 1602 showing the relationship between the

[0059] In S1214, the pixel position xi of interest from which the correction amount is derived is initialized. Specifically, when generating a scan correction table for the line sensor 208a, xi is set to 0. Alternatively, it may be initialized to pixel position xa0 or xa1. Also, when generating a scan correction table for the line sensor 208b, it is initialized to pixel position xb0.

[0060] In the next step S1215, the sensor acquired value (corrected value) after correction for the pixel of interest position xi is determined. Here, a specific example will be used to explain. First, the sensor pixel position x and the color measurement value (b *Five colorimetric values ​​1602a to 1602e at the pixel of interest position xi are obtained from five curves 1602 showing the relationship between the signal value B and the colorimetric value xi. Also, as shown in FIG. 16(c), the line profile 1603 obtained in S1204 is referenced to obtain sensor acquired values ​​(signal value B) 1603a to 1603e at the pixel of interest position xi. Note that values ​​with the same alphabet in the colorimetric values ​​1602a to 1602e and the sensor acquired values ​​1603a to 1603e mean that they are values ​​for the same colorimetric area. FIG. 17(a) shows the sensor acquired values ​​(signal value B) on the vertical axis and the colorimetric values ​​(b * Each black circle 1701a to 1701e represents the color measurement value (b * 17A represents a plot of the sensor acquisition value (signal value B) and the sensor acquisition value (signal value B). A solid curve 1701 in the graph of FIG. 17A is a curve obtained by applying known interpolation or approximation processing to each of the black circles 1701a to 1701e. A dashed curve 1702 represents a target characteristic after the sensor acquisition value is corrected, and each of the black squares 1702a to 1702e corresponds to each of the black circles 1701a to 1701e. The target value may be, for example, a sensor acquisition value corresponding to any one of the color measurement positions x. Alternatively, the average output value of the entire sensor or the center of the sensor may be used as the target value. Alternatively, a characteristic curve of a sensor that serves as a reference may be held in advance and used as the target sensor characteristic. Alternatively, the sensor acquisition value may be a value representing luminance or L * The color measurement value (b) may be determined in advance so as to be linear with respect to the value and optical density. FIG. 17(b) is a diagram for explaining a method for calculating the correction value. First, in the graph of FIG. 17(b), the white circles 1703 indicate the sensor acquired values ​​before correction, which correspond to "0, 16, 32, ... 255" in the example of the scan correction table shown in FIG. 5 described above. The color measurement value (b *A target sensor acquisition value (correction value) 1704 corresponding to the color measurement value 1704 can be obtained from a characteristic curve 1701. Furthermore, a target sensor acquisition value (correction value) 1705 corresponding to the color measurement value 1704 is obtained from a target characteristic curve 1702. The correction value at the pixel position xi of interest is determined by associating the target sensor acquisition value 1705 thus obtained with a pre-correction sensor acquisition value 1703. In this case, if the pre-correction sensor acquisition value is "0" or the maximum output value of the sensor (e.g., 255), "0" or "255" may be forcibly set as the correction value. By repeating this process for the number of rows in the scan correction table, a scan correction table that specifies the correction value for each pixel position xi can be obtained.

[0061] In S1216, it is determined whether all pixel positions to be processed have been processed as the pixel of interest. If there is an unprocessed pixel position, the process proceeds to S1217, where the pixel of interest position xi is updated, and the process returns to S1215, where the same process is repeated. If the pixel positions in the x direction are specified in increments of "100" as in the scan correction table shown in FIG. 5, the pixel of interest position xi is increased by "100" in the update process of S1215. Here, for example, in the case of generating a scan correction table for the line sensor 208a, if the process is completed up to the pixel position xa2, it is determined that all pixel positions have been processed. Also, in the case of generating a scan correction table for the line sensor 208b, if the process is completed up to the pixel position xb1 or xb2, it is determined that all pixel positions have been processed. If the result of such determination is that all pixel positions to be processed have been processed, the process is terminated.

[0062] The above is the content of the correction amount derivation process.

[0063] It is preferable that the rectangular area 1301 and the alignment pattern 1302 of the SS chart are formed with the maximum width that can be formed by the recording heads 201 to 204. In other words, it is preferable to use paper that can be formed by all the nozzles of each of the recording heads 201 to 204, and to form an image using all the nozzles. This makes it possible to directly obtain more values ​​by calculation processing rather than by interpolation processing.

[0064] Moreover, the relative distance from the first color measurement position (the position of the mark 1303a in FIG. 13B) may be used as the color measurement position t. Alternatively, instead of the color measurement position, the correction amount may be derived using the number of times color measurement is performed (1, 2, 3, 4, . . .) or the time of color measurement.

[0065] In addition, in S1215, when calculating the sensor acquisition value at the pixel position xi of interest, a value that takes into account the color measurement position and aperture diameter of the colorimeter may be used instead of directly referring to the line profile. That is, the line profile is averaged at a size equal to the aperture diameter of the colorimeter centered on the "pixel position corresponding to the color measurement position" estimated in S1213. Furthermore, a curve obtained by performing known interpolation processing on these may be used instead of the line profile.

[0066] In the above description, an example of creating a scan correction table for each of the line sensors 208a and 208b independently has been described. However, one scan correction table may be created for one line profile in which the output values ​​of each line sensor are superimposed. However, when the line sensors are arranged apart in the transport direction, the overlapping length on the paper surface changes due to paper transport errors, etc., so the correction amount of the overlapping portion may differ between when it is created and when it is used. In addition, the characteristics of the correction amount for the position become complicated due to the superimposition process, and especially when the correction amount is held only for discrete positions as in the scan correction table exemplified in FIG. 5, the correction may be insufficient. Therefore, it is preferable to derive the correction amount for the output value of each line sensor before superimposition.

[0067] (Effects of this embodiment) A line sensor may have different sensor characteristics for each pixel. The difference in characteristics often depends on the angle of incidence on the sensor pixel or color filter, and in such cases, the reading value for the same incident light often gradually increases or decreases depending on the sensor pixel position. (a) and (b) of FIG. 18 are conceptual diagrams showing the fluctuation of the sensor acquisition value with respect to pixel position x. (a) and (b) of FIG. 16 are graphs in which the horizontal axis represents the position x on the line sensor and the vertical axis represents the sensor acquisition value. Also, each curve on the graph shows the sensor acquisition value for each pixel position when multiple density charts in which the spectral reflectance on the paper surface is approximately the same regardless of position are read. Here, the higher the reflectance of the paper (i.e., closer to white paper), the larger the sensor acquisition value, and the higher the density of the paper surface, the smaller the sensor acquisition value. At this time, as shown in (a) or (b) of FIG. 16, the greater the deviation between the center and the end portions of the curve corresponding to the measurement area in which the sensor acquisition value is relatively small. In other words, the higher the color signal value and the higher the image density on the paper surface, the greater the difference in the sensor characteristics. One of the reasons for this is that the angle of incidence to the sensor is larger at the end, and the optical path on the color filter provided in the sensor pixel is longer. Specifically, the change in the spectral distribution caused by the color filter for the light incident on the sensor is larger at the end than at the center, and as a result, the sensor acquisition value is changed. Another possible cause is that when the angle of incidence to the sensor is large, for example, light that should be incident on the G sensor enters the adjacent B channel as stray light. Such differences in characteristics between the center and the end depending on the pixel position are particularly likely to occur in the B sensor. For this reason, they often cause problems in the head shading correction process of the head 204 corresponding to yellow ink.

[0068] In this embodiment, the problem of different sensor characteristics between the center and the end of the line sensor is reduced by the sensor shading correction process (S706) during the color adjustment table generation process (flow in FIG. 7). That is, based on the output value of each line sensor and the scan correction table, the color signal value is converted into a color signal value that takes into account the characteristics of the line sensor. Then, the head shading correction process is performed based on the converted color signal value, thereby reducing the influence of the difference in sensor characteristics on the head shading correction process. At this time, by using a colorimeter that is independent of the color measurement position and has high repeatability, it is possible to correct the characteristics that depend on the sensor pixel position. For example, when the sensor acquisition value at the center of the line sensor and the color measurement value are used as the target characteristics, the head shading correction process can be performed with the sensor acquisition value that corresponds to the case where the sensor is read by the sensor at the center, regardless of the position of the printed matter.

[0069] <Modification> When measuring the SS chart in the above-mentioned S1204, instead of measuring the color online using a colorimeter inside the device, it is also possible to measure the color offline using an external colorimeter. In this case, the printed SS chart is taken out of the image forming apparatus, the colorimeter is manually slid over each colorimeter area in the SS chart, and the obtained colorimeter value is input through the operation unit 103 or the I / F unit 109. Alternatively, it is also possible to fix the SS chart on the xy stage and measure the colorimeter over each colorimeter area in the SS chart. Note that, when manually sliding the colorimeter, it is difficult to obtain colorimeter values ​​at equal intervals. Therefore, for example, a chart including a separator position in the measurement area as shown in FIG. 19(a) may be used. However, if separator lines are embedded in the colorimeter area, the colorimeter value or signal value may change due to the influence, and the accuracy of the created SS chart may decrease. In particular, the colorimeter can obtain colorimeter values ​​without including separator lines in its aperture, and colorimeter values ​​can be obtained without being affected by separator lines, while the signal value obtained by the line sensor may change depending on the presence or absence of separator lines due to the influence of light wraparound, etc. Therefore, for example, as shown in FIG. 19(b) or (c), a chart may be used that does not include a partition line inside the measurement area but has partition lines at the top and bottom of the paper surface or between the measurement areas. That is, the chart shown in FIG. 19(a) is used for color measurement in S1204 described above, and the chart shown in FIG. 19(b) or FIG. 19(c) is used for reading in S1203. At this time, the partition lines inside the measurement area in the chart in FIG. 19(a) and the partition lines outside the measurement area in the chart in FIG. 19(b) or FIG. 19(c) are formed at the same interval. Note that if a thin line 1302 as shown in FIG. 13(a) is used as the partition line, it may be difficult to detect it with a colorimeter. Therefore, as shown in FIG. 19(a), it is preferable that the partition lines inside the measurement area are not thin lines but are patterns with a certain width. It is also preferable that the partition lines outside the measurement area are approximately the same as the partition lines inside the measurement area, not only in interval but also in width. This allows approximately the same position to be detected when the same process as the partition line is performed inside the measurement area.The positions of such separator lines are detected for each of the scanned image and colorimetric values, and the colorimetric positions are associated with pixel positions on the line profile based on the detected positions, making it possible to create an SS chart. As shown in (a) and (b) of FIG. 20, it is also possible to form measurement areas that include separator lines for colorimetric measurement and measurement areas that do not include separator lines within one sheet of paper. In this case, the measurement areas for colorimetric measurement and the measurement areas for scanning may be arranged alternately as shown in (b) of FIG. 20. In this case, the separator lines for colorimetric measurement may also be read, and the positions of the separator lines within the measurement areas may be used as separator positions outside the measurement areas.

[0070] In addition, the correction amount for each pixel position often changes gradually with respect to the image position and the sensor acquired value. Therefore, in order to reduce noise contained in the colorimetric value or the color signal value, the calculated correction value may be subjected to a known averaging process or filtering process with respect to the image position direction or the sensor output direction. Correction processing may be performed using an approximation curve obtained by a known approximation method.

[0071] In the above embodiment, the color adjustment unit 302 is described as performing correction processing on the input image data (CMYK). However, the same effect can be obtained by performing correction processing on the threshold matrix used in the HT processing unit 305.

[0072] (Other embodiments) The present disclosure can also be realized by a process in which a program for implementing one or more functions of the above-described embodiments is supplied to a system or device via a network or a storage medium, and one or more processors in a computer of the system or device read and execute the program. It can also be realized by a circuit (e.g., ASIC) that implements one or more functions.

[0073] The present disclosure also includes the following configurations and methods. (Configuration 1) a printing means for carrying out a printing process while conveying paper based on image data; a first generating means for generating correction information for correcting a sensor acquisition value of the scanning means based on scan data obtained by reading the first chart printed out by the printing means with the scanning means and colorimetric data obtained by measuring with the colorimetric means; a second generation means for generating color adjustment information according to the characteristics of the printing means by correcting a sensor acquisition value of the scanning means using the correction information based on color measurement data obtained by measuring a second chart printed out by the printing means with the color measurement means; an image processing unit that performs color adjustment processing on the image data using the color adjustment information; Equipped with the first chart includes a patch having a uniform density extending in a main scanning direction substantially perpendicular to a paper transport direction, and a registration pattern formed outside the patch for specifying a position on the patch in the main scanning direction; the first generation means determines a correction value for the sensor acquisition value at a specific pixel position by associating a pixel position of an edge of the patch in the scan data with a colorimetric position of the edge of the patch in the colorimetric data, and generates the correction information. 1. An image forming apparatus comprising:

[0074] (Configuration 2) The first generating means is Detecting pixel positions corresponding to edges of the patch based on the scan data; Calculating color measurement positions corresponding to edges of the patch based on the color measurement data; estimating pixel positions corresponding to each color measurement position of the color measurement data to determine a correction value for the sensor-acquired value at the specific pixel position; 2. The image forming apparatus according to claim 1,

[0075] (Configuration 3) The image forming apparatus according to configuration 2, wherein the first generation means estimates a pixel position corresponding to each of the color measurement positions by calculating it from a pixel position corresponding to the alignment pattern obtained from the scan data.

[0076] (Configuration 4) the alignment patterns are thin lines formed at equal intervals in the main scanning direction, The color measurement means measures color at intervals that are an integral multiple of the equal intervals. 4. The image forming apparatus according to any one of configurations 1 to 3.

[0077] (Configuration 5) 5. The image forming apparatus according to any one of configurations 1 to 4, wherein the correction information is generated for each color of color material used in the printing process.

[0078] (Configuration 6) The image forming apparatus according to any one of configurations 1 to 4, characterized in that the correction information is generated for a specific color, among the color materials used in the printing process, for which the sensor acquisition values ​​of the scanning means are likely to diverge between the center and ends of the patch.

[0079] (Configuration 7) 7. The image forming apparatus according to configuration 6, wherein the specific color is yellow.

[0080] (Configuration 8) 8. The image forming apparatus according to any one of configurations 1 to 7, wherein the first chart and the second chart are the same.

[0081] (Configuration 9) 8. The image forming apparatus according to any one of configurations 1 to 7, wherein the first chart and the second chart are different.

[0082] (Configuration 10) the first chart has more patches of color signal values ​​that are important in the sensor shading correction process arranged thereon; the second chart is arranged with a larger number of patches of color signal values ​​that are to be emphasized in head shading correction processing; 10. The image forming apparatus according to claim 9,

[0083] (Configuration 11) The image forming apparatus according to any one of configurations 1 to 10, wherein the color measurement means acquires, as color measurement data, a spectral reflectance obtained by measuring a color at a predetermined position on the conveyed paper while scanning in the main scanning direction, or a color value in a device-independent color space calculated from the spectral reflectance.

[0084] (Configuration 12) 12. The image forming apparatus according to claim 11, wherein the color values ​​in the device-independent color space are CIE Lab.

[0085] (Configuration 13) The image forming apparatus according to any one of configurations 1 to 12, characterized in that the printing means performs the printing process by transporting paper in one direction and ejecting ink droplets using a recording head that covers the entire width of the paper being transported.

[0086] (Configuration 14) 14. The image forming apparatus according to any one of configurations 1 to 13, wherein the scanning means covers the entire width of the paper by arranging a plurality of line sensors in an overlapping manner.

[0087] (Configuration 15) 15. The image forming apparatus according to configuration 14, wherein the first generating means generates the correction information for each of the plurality of line sensors.

[0088] (Configuration 16) The first generation means generates, for each of the plurality of line sensors, Identifying an image area of ​​the patch from the scan data and generating a line profile; generating the correction information based on the line profile; 16. The image forming apparatus according to claim 15,

[0089] (Configuration 17) 17. The image forming apparatus according to any one of configurations 1 to 16, wherein a conveying means for conveying paper to the printing means, the scanning means, and the color measuring means is common to both.

[0090] (Configuration 18) 18. The image forming apparatus according to any one of configurations 1 to 17, wherein a transport means for transporting paper to the printing means and the scanning means is different from a transport means for transporting paper to the color measurement means.

[0091] (Method 1) a printing step for performing a printing process while conveying paper based on image data; a first generation step of generating correction information for correcting a sensor acquisition value of the scanning means based on scan data obtained by reading the first chart printed out in the printing step with a scanning means and colorimetric data obtained by measuring with a colorimetric means; a second generation step of correcting a sensor acquired value of the scanning means using the correction information based on color measurement data obtained by measuring the second chart printed out in the printing step with the color measurement means, and generating color adjustment information according to characteristics of the printing means; an image processing step of performing color adjustment processing on the image data using the color adjustment information; Including, the first chart includes a patch having a uniform density extending in a main scanning direction substantially perpendicular to a paper transport direction, and a registration pattern formed outside the patch for specifying a position on the patch in the main scanning direction; In the first generation step, a pixel position of an end of the patch in the scan data is associated with a colorimetric position of the end of the patch in the colorimetric data, a correction value of the sensor acquisition value at a specific pixel position is determined, and the correction information is generated. 2 is a control method for an image forming apparatus according to claim 1 .

[0092] (Configuration 19) A program for causing a computer to function as a part of the image forming apparatus according to any one of configurations 1 to 18.

Claims

1. an acquisition means for acquiring scan data obtained by scanning a first chart printed on a printing medium by a printing means, and colorimetric data obtained by measuring a second chart different from the first chart by a colorimetric means; a generating unit that generates correction information for correcting the sensor-acquired values ​​of the scanning unit based on the scan data and the colorimetric data; and the first chart includes patches having a predetermined density extending in a scanning direction perpendicular to a transport direction of the printing medium, and alignment patterns arranged at predetermined intervals in the scanning direction and formed outside the patches, the second chart includes the patches and dividing lines arranged in the scanning direction at intervals at least equal to or greater than the predetermined intervals and formed on or outside the patches; 1. An image processing device comprising:

2. The generating means Detecting pixel positions corresponding to edges of the patch based on the scan data; calculating color measurement positions corresponding to the edges of the patch based on the color measurement data; a pixel position corresponding to each color measurement position of the colorimetric data is estimated, and a correction value for the sensor-acquired value at the specific pixel position is determined; 2. The image processing device according to claim 1, wherein:

3. 3. The image processing apparatus according to claim 2, wherein the generating unit estimates the pixel positions corresponding to the color measurement positions by calculating them from pixel positions corresponding to the alignment pattern obtained from the scan data.

4. the alignment pattern is a thin line formed at equal intervals in the main scanning direction, The color measurement data is data obtained by the color measurement means measuring the color of the patch at intervals that are an integral multiple of the equal intervals.

2. The image processing device according to claim 1, wherein:

5. 2. The image processing apparatus according to claim 1, wherein the correction information is generated for each color of color material used in the print output.

6. 2. The image processing device according to claim 1, wherein the correction information is generated for a specific color, among the color materials used in the print output, for which the sensor-acquired values ​​of the scanning means tend to diverge between the center and end portions of the patch.

7. 7. The image processing device according to claim 6, wherein the specific color is yellow.

8. the first chart has more patches of color signal values ​​that should be emphasized in the sensor shading correction process arranged thereon; the second chart is arranged with a larger number of patches of color signal values ​​that should be emphasized in head shading correction processing; 2. The image processing device according to claim 1, wherein:

9. 2. The image processing device according to claim 1, wherein the color measurement data is spectral reflectance data obtained by measuring the color of a predetermined position on a sheet of paper being conveyed while the color measurement means scans the sheet in the main scanning direction, or color value data in a device-independent color space calculated from the spectral reflectance.

10. 10. The image processing apparatus according to claim 9, wherein the color values ​​in the device-independent color space are CIE Lab.

11. 2. The image processing apparatus according to claim 1, wherein the printing means carries a sheet of paper in one direction and ejects ink droplets from a recording head that covers the entire width of the carried sheet of paper to produce the printed output.

12. 2. The image processing apparatus according to claim 1, wherein said scanning means covers the entire width of the paper by arranging a plurality of line sensors in an overlapping manner.

13. 13. The image processing apparatus according to claim 12, wherein said generating means generates said correction information for each of said plurality of line sensors.

14. The generating means generates, for each of the plurality of line sensors, Identifying an image area of ​​the patch from the scan data and generating a line profile; generating the correction information based on the line profile; 14. The image processing device according to claim 13.

15. 2. The image processing apparatus according to claim 1, wherein a conveying unit that conveys paper to said printing unit, said scanning unit, and said color measurement unit is common to both.

16. 2. The image processing apparatus according to claim 1, wherein a conveying unit that conveys the paper to the printing unit and the scanning unit is different from a conveying unit that conveys the paper to the color measurement unit.

17. A control method for an image processing device, comprising: a step of acquiring, by an acquiring means, scan data obtained by reading, by a scanning means, a first chart printed out on a printing medium by a printing means, and colorimetric data obtained by measuring, by a colorimetric means, a second chart different from the first chart; generating correction information for correcting the sensor-acquired values ​​of the scanning means based on the scan data and the colorimetric data; and the first chart includes patches having a predetermined density extending in a scanning direction perpendicular to a transport direction of the printing medium, and alignment patterns arranged at predetermined intervals in the scanning direction and formed outside the patches, the second chart includes the patches and dividing lines arranged in the scanning direction at intervals at least equal to or greater than the predetermined intervals and formed on or outside the patches; 2. A method for controlling an image processing apparatus comprising:

18. A program for causing a computer to execute the control method according to claim 17.

Images