Image processing device and image processing method

Abstract

To provide a technique for achieving color stabilization more accurately.SOLUTION: An image processing device creates correction information for reducing a density change in image formation using recording materials in plural colors, and creates correction information corresponding to the recording material for which the correction information has not been created yet out of the recording materials used for a region in the order from the region with the smaller recording materials in a read image obtained by reading an image formed by using the recording materials in plural colors in the creation.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to image processing technology. [Background technology]

[0002] Inkjet printers that form images by ejecting ink from multiple nozzles are widely used as image forming devices for forming any image on a paper surface, as well as electrophotographic printers that form images using a laser photoconductor and charged toner.

[0003] Incidentally, in the electrophotographic method, it is known that the color of the formed image changes depending on the remaining amount of toner in the device and the environment such as the surrounding temperature and humidity. It is also known that in the inkjet method, the color changes depending on the adhesion of ink around the nozzle, the aging of the piezoelectric element and heater that control the ink discharge, and the surrounding environment such as temperature and humidity. A technology is known that continues to suppress the change in color by performing a stabilization process at regular intervals in response to such changes in the surrounding environment and over time.

[0004] In the above stabilization process, a dedicated chart is often output to measure the characteristics of recording materials such as toner or ink of each color. However, the recording materials, paper, and time that would normally be used by the user to output the desired image are used to output the dedicated chart, leading to increased costs. In addition, if the chart output interval is increased to avoid increased costs, the accuracy of stabilization may decrease. Patent Document 1 discloses a technology that maintains the accuracy of stabilization while avoiding increased costs by performing stabilization process based on the output user image. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2015-160352 A Summary of the Invention [Problem to be solved by the invention]

[0006] In the color stabilization process in the above-mentioned Patent Document 1, the apparent recording volume of each toner is estimated by searching from the results of measuring the color of a multi-color toner image, and the color of the output image is stabilized by changing the image-making conditions of each toner.

[0007] In this case, in Patent Document 1, the same process is performed regardless of the type of color. However, in a mixed color reproduced by overlapping multiple recording materials, the relationship between the amount of each recording material and the color development degree of the mixed color is generally nonlinear. Therefore, when estimating the amount of each recording material from the mixed color, the estimation accuracy deteriorates, and color stabilization may not be performed sufficiently. The present invention provides a technology for achieving color stabilization with higher accuracy. [Means for solving the problem]

[0008] One aspect of the present invention includes a creation means for creating correction information for reducing density changes in image formation using recording materials of multiple colors, and the creation means is characterized in that in a read image obtained by reading an image formed using recording materials of multiple colors, the creation means creates correction information corresponding to the recording materials used in the read image, starting from areas using the least amount of recording material, for which correction information has not yet been created. Effect of the Invention

[0009] According to the configuration of the present invention, color stabilization can be achieved with higher accuracy. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a block diagram showing an example of a hardware configuration of an image forming system. [Diagram 2] FIG. 1 is a diagram illustrating a printer applied to an image forming unit 107. [Diagram 3] 2 is a block diagram showing an example of the functional configuration of an image processing unit 106. FIG. [Figure 4] 11 is a flowchart for printing a user image and creating a correction table. [Diagram 5] FIG. 4 is a diagram showing an example of a correction table. [Figure 6] 10 is a flowchart showing details of the process in step S405. [Figure 7] FIG. 13 is a diagram showing an example of a uniform pattern. [Figure 8] 16A and 16B are diagrams showing the result of identification performed by a identification unit 1605 on a CMYK image. [Figure 9] 10 is a flowchart showing details of the process in step S604. [Figure 10] FIG. 13 is a diagram showing an example of an image formed of pixels with RGB values ​​obtained in step S408. [Figure 11] A plot of signal value versus directional reflectance. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, the embodiments will be described in detail with reference to the attached drawings. Note that the following embodiments do not limit the invention according to the claims. Although the embodiments describe a number of features, not all of these features are essential to the invention, and the features may be combined in any manner. Furthermore, in the attached drawings, the same reference numbers are used for the same or similar configurations, and duplicated descriptions are omitted.

[0012] [First embodiment] <Example of hardware configuration of image forming system> First, a hardware configuration example of an image forming system (image processing device) according to this embodiment will be described with reference to the block diagram of Fig. 1. Fig. 1 shows the main configuration related to each process described below, but does not show all the configurations of the image forming system.

[0013] A central processing unit (CPU) 100 executes various processes using computer programs and data stored in a random access memory (RAM) 101 and a read only memory (ROM) 102. As a result, the CPU 100 controls the operation of the entire image forming system, and executes or controls various processes that will be described as being performed by the image forming system. Note that, although this embodiment will be described as the CPU 100 controlling the entire image forming system, the entire image forming system may be controlled by multiple hardware devices sharing the processing.

[0014] The RAM 101 has an area for storing computer programs and data loaded from the ROM 102 or the external storage device 105, and an area for storing images read by the image acquisition unit 108. The RAM 101 also has an area for storing data received from the outside via the I / F unit 109, and a work area used when the CPU 100 or the image processing unit 106 executes various processes. In this way, the RAM 101 can provide various areas as appropriate.

[0015] The ROM 102 stores setting data for the image forming system, computer programs and data related to the startup of the image forming system, computer programs and data related to the basic operations of the image forming system, and the like.

[0016] The operation unit 103 is a user interface including a keyboard, a mouse, a touch panel, a numeric keypad, and various buttons, and the user can input various instructions and information to the CPU 100 by operating it.

[0017] The display unit 104 is a display device having a screen such as a liquid crystal screen or a touch panel screen, and can display the results of processing by the CPU 100 and the image processing unit 106 as images, characters, and the like.

[0018] The external storage device 105 is a large-capacity information storage device such as a hard disk drive. The external storage device 105 stores an OS (operating system), computer programs and data for causing the CPU 100 and image processing unit 106 to execute various processes. The data stored in the external storage device 105 includes, for example, various matrices used in the processes described below. The computer programs and data stored in the external storage device 105 are loaded into the RAM 101 under the control of the CPU 100, and are processed by the CPU 100 and image processing unit 106.

[0019] Image processing unit 106 is realized by a processor capable of executing a computer program or a dedicated image processing circuit, and executes various types of image processing for converting an image input as a print target into an image that can be output by image forming unit 107. In this embodiment, image processing unit 106 also executes color stabilization processing based on the results of reading a user image (an image that a user desires to print). Note that instead of providing a dedicated processor as image processing unit 106, CPU 100 may operate as image processing unit 106 and perform various types of processing.

[0020] Image forming unit 107 forms an image using a recording material on a recording medium such as paper, based on an image acquired from image processing unit 106, RAM 101, external storage device 105, I / F unit 109, etc. In this embodiment, image forming unit 107 uses at least four types of recording materials, such as cyan (C), magenta (M), yellow (Y), and black (K).

[0021] The image acquisition unit 108 is an image sensor (a line sensor or an area sensor) for capturing an image formed on a recording medium by the image formation unit 107. In this embodiment, the image acquisition unit 108 (image sensor) is an image sensor capable of acquiring three output values ​​(an R (red) output value, a G (green) output value, and a B (blue) output value) each having a peak at a different wavelength. The output values ​​are color information such as a luminance value.

[0022] The I / F unit 109 functions as an interface for connecting the image forming system to an external device. For example, the I / F unit 109 functions as an interface for exchanging data with a communication device using infrared communication, a wireless LAN (Local Area Network), or the like, or as an interface for connecting to the Internet.

[0023] Each of the above-mentioned units is connected to a bus 110, and data can be exchanged via the bus 110. However, the image forming system may be configured such that each of the above-mentioned units (e.g., image forming unit 107) is connected via an I / F unit 109.

[0024] <Image Forming Unit 107 and Image Acquiring Unit 108> First, a description will be given of the image forming unit 107. In this embodiment, the image forming unit 107 is an inkjet printer that forms an image on a recording medium by ejecting ink of cyan (C), magenta (M), yellow (Y), and black (K) from nozzles onto the recording medium.

[0025] 2(a), the image forming unit 107 has recording heads 201 to 204 on a frame that constitutes the structural material of the printer. The recording heads 201 to 204 are of the so-called full line type, in which a plurality of nozzles for ejecting K, C, M, and Y inks are arranged in a predetermined direction within a range corresponding to the width of a recording paper 206 as a recording medium.

[0026] 2B, the recording heads 201 to 204 are configured by combining a plurality of head modules (recording units). For example, the head modules 201a, 201b, and 201c of the recording head 201 are arranged alternately in the conveying direction of the recording medium. The same is true for the head modules 202a, 202b, and 202c of the recording head 202, the head modules 203a, 203b, and 203c of the recording head 203, and the head modules 204a, 204b, and 204c of the recording head 204.

[0027] 2(c), the head module 201a further includes chip modules 201a-1 to 201a-5 (recording units). In this case, it is assumed that the chip modules are connected to independent boards. This also applies to the other head modules.

[0028] Fig. 2(d) is a diagram of one of the chip modules as viewed from the recording medium side, showing that the chip module has multiple nozzles (recording units). In the example shown in Fig. 2(d), the chip module has 16 nozzles. The nozzle arrangement resolution of the nozzle rows for each ink color is, for example, 1200 dpi.

[0029] A recording paper 206 as a recording medium is conveyed in a direction (conveying 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 recording paper 206 is being conveyed, ink is ejected from the multiple nozzles of each of the recording heads 201 to 204 in accordance with recording data (images converted by the image processing unit 106), thereby sequentially forming an image for one raster corresponding to each nozzle row of the recording heads 201 to 204. By repeating such ink ejection operation from the recording heads 201 to 204 onto the conveyed recording paper 206, for example, an image for one page can be recorded on the recording paper 206.

[0030] As shown in FIG. 2A, the image acquisition unit 108 is disposed downstream of the recording heads 201 to 204 in the transport direction indicated by the arrow 207, and is a line sensor that covers the entire surface of the recording paper 206 in order to read the entire surface of the recording paper 206.

[0031] The image acquisition unit 108 generates an image (two-dimensional image) in which each pixel has a pixel value of three channels, RGB, by sequentially capturing images of the conveyed recording paper 206 in parallel with image formation on the recording paper 206 by the recording heads 201 to 204. Then, the CPU 100 stores the generated image in the external storage device 105 as a two-dimensional read image.

[0032] It should be noted that the image forming unit 107 is not limited to the full-line type device shown in Fig. 2. For example, the image forming unit 107 may be a so-called serial type recording device that performs recording by scanning a recording head in a direction intersecting the conveyance direction of the recording paper. In addition, the image forming method by the image forming unit 107 may be an electrophotographic method that forms an image using a laser photosensitive body and charged toner, or a thermal transfer method that vaporizes solid ink by heat and transfers it to the printing paper.

[0033] Furthermore, the image acquisition unit 108 is not limited to a line sensor as shown in Fig. 2. For example, the image acquisition unit 108 may be a device having a carriage for moving in a direction intersecting the conveyance direction of the recording paper and configured to acquire an arbitrary area with a width smaller than that of the recording paper 206.

[0034] <Overview of color stabilization processing> Next, the color stabilization process performed by the image processing unit 106 will be described. It is known that the density of an image formed on a recording medium changes even when the same image is formed due to ink adhesion around the nozzles, aging of the piezoelectric elements and heaters that control ink discharge, and surrounding environments such as temperature and humidity. Such changes in image density are visually recognized as uneven density and color shifts, and may impair the quality of the image (the quality of the printed matter on which the image is printed), so it is preferable to suppress them as much as possible.

[0035] 2, in the image forming system according to this embodiment, the image formed by the recording heads 201 to 204 can be read by the image acquisition unit 108 (line sensor) installed downstream in the transport direction from the recording heads 201 to 204. That is, the density change can be suppressed by estimating the above-mentioned density change from the image read by the image acquisition unit 108 and performing color stabilization processing.

[0036] 2, the image forming system according to this embodiment forms any image by mixing four colors of ink: C, M, Y, and K. Therefore, density changes over time occur independently for each of the colors C, M, Y, and K. Therefore, it is possible to effectively reduce density changes by performing independent gamma correction processing for each color on the recording amount of ink corresponding to each color.

[0037] Incidentally, the density change of each recording material often changes on a head module or chip module basis as described above. Moreover, the amount of change often differs for each nozzle even within the same module. Therefore, it is desirable to estimate the density change on a nozzle basis and perform gamma correction processing. However, as described below, depending on the processing load, processing may be performed on a chip module or head module basis rather than on a nozzle basis.

[0038] For example, in the example shown in Fig. 2, if each of the print heads 201 to 204 is corrected independently, four processes (C, M, Y, and K) are required to estimate the density change. Alternatively, if it is done for each head module, as shown in Fig. 2(b), 12 estimation processes are required for 4 colors x 3 head modules, since three head modules for each color are further corrected independently. Similarly, if it is done in chip module units as shown in Fig. 2(c), 60 estimation processes are required because there are five more chip modules, and if it is done in nozzle units, 960 estimation processes are required because there are 16 nozzles per chip module.

[0039] In this way, the finer the unit of correction, the more times the estimation process is performed. Therefore, in order to perform correction in fine units while maintaining the same printing speed, a high-speed estimation process is required.

[0040] Therefore, in this embodiment, a matrix that associates the sensor output values ​​(RGB values ​​(sensor output value corresponding to R, sensor output value corresponding to G, sensor output value corresponding to B)) output by the image acquisition unit 108 with the reflectance on the recording medium is created and stored in advance. Then, each time a user image is printed, the image acquisition unit 108 reads the user image, converts the sensor output values ​​(RGB values) output by the image acquisition unit 108 using the matrix, and creates a correction table based on the values ​​obtained by the conversion, thereby enabling high-speed estimation.

[0041] Furthermore, if a dedicated chart is used to detect density changes in the color stabilization process, it is possible to efficiently obtain the desired density information required for the color stabilization process. However, from the user's point of view, costs are incurred for paper, recording material, time, etc. for a dedicated chart that is not the user's image that is actually desired to be output. For this reason, it is preferable to use a user's image rather than a dedicated chart.

[0042] When a user image is used for color stabilization processing, the user image is not necessarily an image advantageous for obtaining the desired density information required for color stabilization processing. In particular, when estimating the density change of each recording material from the sensor output value of a multi-color region where multiple colors are mixed, the estimation accuracy is likely to deteriorate due to the nonlinearity of the relationship between the amount of each recording material and the color development degree of the mixed color. For this reason, there is a possibility that color stabilization cannot be performed sufficiently.

[0043] Therefore, in this embodiment, the input user image is referenced, and density information is obtained by using areas (single-color areas) where only a single recording material is used preferentially to perform the above estimation, and the result is used to correct the density change of the remaining recording materials. In this way, it is possible to improve the estimation accuracy of the density change of each recording material without using a dedicated chart.

[0044] The configuration of the image processing unit 106 that executes the above-mentioned processing and the color stabilization processing will be described below with reference to Figures 3 and 4. Figure 3 is a block diagram showing an example of the functional configuration of the image processing unit 106.

[0045] The conversion unit 1601 converts an input image read from the external storage device 105 into an image corresponding to the color reproduction range of the image forming unit 107. The input image is described, for example, according to color coordinates (R, G, B) in a color space coordinate system such as sRGB, which is the display color of a monitor. The conversion unit 1601 performs a process of converting the input image into color signals corresponding to a plurality of inks used in the image forming unit 107. For example, if the image forming unit 107 uses inks of black (K), cyan (C), magenta (M), and yellow (Y), the input image is converted by the conversion unit 1601 into an image (CMYK image) consisting of 8-bit color signals for each of K, C, M, and Y. The CMYK image output by the conversion unit 1601 represents the usage amount (ejection amount) of each ink discharged by the image forming unit 107 onto a recording medium to display an image. The above conversion by the conversion unit 1601 can be performed using a known method such as a matrix calculation process or a process using a three-dimensional LUT (lookup table).

[0046] The input image is not limited to an RGB image in which each pixel has pixel values ​​of R, G, and B (i.e., each pixel has an RGB value), but may be an image (CMYK image) in which each pixel has pixel values ​​of C, M, Y, and K. However, even in this case, due to limitations on the total amount of ink and color management, it is preferable that the conversion unit 1601 performs processing using a four-dimensional LUT that converts C, M, Y, and K in the input image (CMYK image) into different C', M', Y', and K', respectively.

[0047] The correction unit 1602 performs correction processing to stabilize colors against changes over time. More specifically, the correction unit 1602 uses a correction table in which converted signal values ​​(pixel values) for each module or nozzle are registered, and performs gamma correction processing on the CMYK image output from the conversion unit 1601. An example of the correction table according to this embodiment is shown in Fig. 5. The correction table in Fig. 5 is a correction table for performing conversion on a head module basis.

[0048] 5 stores corrected color signal values ​​corresponding to input color signal values ​​(0, 16, 32, ..., 240, 255) for head modules 201a, 201b, and 201c in printhead 201, head modules 202a, 202b, and 202c in printhead 202, and head modules 203a, 203b, and 203c in printhead 203. For example, if the pixel value (input color signal value) of the K pixel corresponding to head module 201a of printhead 201 is 32, correction unit 1602 changes the pixel value of the pixel to 28.

[0049] In this embodiment, the correction process is performed by referencing the correction table for each of C, M, Y, and K. In this way, by correcting the CMYK image that represents the ejection amount, it is possible to cancel out density changes. Note that while FIG. 5 shows one correction table that combines conversions for each of C, M, Y, and K, it may be interpreted as a collection of a C ink correction table, an M ink correction table, a Y ink correction table, and a K ink correction table.

[0050] When correction processing is performed not in units of head modules but in units of chip modules or nozzles, the correction table has a number of columns equal to the number of chip modules or nozzles.

[0051] In the correction table shown in Fig. 5, input color signal values ​​that are not present in the LUT are calculated using an interpolation process from nearby input color signal values ​​stored in the LUT. Of course, it is also possible to store converted color signal values ​​for all input color signal values ​​without using the interpolation process. Alternatively, it is also possible to perform correction processing using other correction information such as function conversion or matrix conversion instead of the correction table.

[0052] Returning to FIG. 3, the HT (halftone) processing unit 1603 performs conversion (HT processing) of the CMYK image corrected by the correction unit 1602 into a CMYK image with a number of gradations that can be directly expressed by the image forming unit 107, thereby generating a halftone image. In this embodiment, the "number of gradations that can be directly expressed by the image forming unit 107" refers to a binary value of on or off for ink dots. That is, specifically, an 8-bit image per pixel is converted into a 1-bit binary halftone image having a value of either 0 or 1 for each pixel. In this embodiment, a known method such as dither processing is used for the HT processing. A threshold matrix required for dither processing is created in advance and stored in the external storage device 105, and is read out from the external storage device 105 and used as necessary. Note that instead of dither processing, a method such as an error diffusion method can also be applied.

[0053] Then, the HT processing unit 1603 outputs the halftone image generated from the CMYK image as described above to the image forming unit 107, and causes the image forming unit 107 to print the halftone image on a recording medium.

[0054] The image acquisition unit 108 acquires the RGB values ​​for each pixel by reading the formed image 400 printed on the recording medium by the image forming unit 107 (an image printed (formed) on the recording medium in accordance with the halftone image generated by the HT processing unit 1603) as described above.

[0055] The color conversion unit 1604 reads out the color conversion matrix 1607 (the above matrix) stored in the external storage device 105. Then, using the read out color conversion matrix 1607, the color conversion unit 1604 calculates the "CMY ink reflectance (ρ c , ρ m , ρ y ) is calculated. The color conversion unit 1604 calculates the reflectance (ρ c , ρ m , ρ y ) is acquired as a reflectance image.

[0056] A specification unit 1605 specifies the ink color to be used for each region of the reflectance image obtained by the color conversion unit 1604, based on the CMYK image obtained by the conversion unit 1601. A creation unit 1606 references the reflectance image obtained by the color conversion unit 1604 and the specified region, and creates a correction table used by the correction unit 1602. Target density characteristics 1608 referenced by the creation unit 1606 will be described later.

[0057] When the resolution of the read image acquired by the image acquisition unit 108 differs from the resolution of the CMYK image, it is preferable to perform resolution conversion on the read image to match the two. For the resolution conversion, a known nearest neighbor method, bilinear interpolation, bicubic interpolation, etc. can be used.

[0058] Furthermore, when the recording medium is skewed or the spectroscopic sensor has a large aberration, it is preferable to perform geometric correction on the sensor output value. Well-known affine transformation or projective transformation can be used for the geometric correction. In that case, the image processing unit 106 needs to have a processing unit that performs resolution conversion and geometric correction. Alternatively, the image acquisition unit 108 may perform resolution conversion and geometric correction in units of a predetermined number of lines when acquiring a raster image, and then output the sensor output value to the color conversion unit 1604. In addition, at this time, the image forming unit 107 may form an image on the recording medium by adding a marker that facilitates the above conversion.

[0059] <Printing user images and creating correction tables> Next, printing of a user image and creation of a correction table by the image forming system according to this embodiment will be described with reference to the flowchart in Fig. 4. The user operates the operation unit 103 to specify the file name (path name) of the image (user image) to be printed among the images stored in the external storage device 105, and the number of copies to be printed (number of copies). Thus, in step S401, the CPU 100 obtains the print contents including the file name and number of copies of the user image specified by the user.

[0060] In step S402, the conversion unit 1601 acquires an image (user image) with a file name included in the print content acquired by the CPU 100 as an input image from the external storage device 105. In the following, the user image is assumed to be an "RGB 8-bit format image" in which the R, G, and B pixel values ​​of each pixel are represented by 8 bits. In step S403, the conversion unit 1601 converts the user image acquired in step S402 into a CMYK image as described above.

[0061] In step S404, CPU 100 determines whether or not to perform correction processing for density conversion. If the result of this determination is that correction processing for density conversion is to be performed, the process proceeds to step S405, and if correction processing for density conversion is not to be performed, the process proceeds to step S406.

[0062] In this embodiment, it is determined that correction processing for density conversion will not be performed when printing the first sheet, but that correction processing for density conversion will be performed when printing the second sheet and thereafter (printing in which an image is formed on a recording medium).

[0063] In step S406, the HT processing unit 1603 takes the CMYK image converted in step S403 or the CMYK image corrected in step S405 as the target image, and performs the above-mentioned HT processing on the target image to generate a 1-bit binary halftone image, with each pixel having a value of either 0 or 1.

[0064] In step S407, the HT processing unit 1603 outputs the halftone image generated in step S406 to the image forming unit 107, and causes the image forming unit 107 to print (form an image) the halftone image as a formed image 400 on a recording medium.

[0065] In step S408, the image acquisition unit 108 acquires the RGB value for each pixel by reading the formed image 400 printed on the recording medium by the image forming unit 107 in step S407.

[0066] In step S409, CPU 100 determines whether or not the user's images have been printed for the number of prints included in the above-mentioned print content (whether or not printing designated by the user has been completed).

[0067] If it is determined that the user image has been printed on the number of sheets included in the print content, the process according to the flowchart in Fig. 4 ends. On the other hand, if the user image has not been printed on the number of sheets included in the print content, the process proceeds to step S404. Note that the first sheet on which no correction process is performed may not be included in the number of sheets printed as a test print. Also, if the user operates the operation unit 103 to input an instruction to cancel printing, the process according to the flowchart in Fig. 4 may end.

[0068] In step S405, a correction process for stabilizing color to stabilize density changes is performed. Details of the process in step S405 will be described with reference to the flowchart in FIG.

[0069] In step S601, the color conversion unit 1604 reads out a color conversion matrix 1607 that has been created before printing from the external storage device 105. As described above, the color conversion matrix 1607 is a color conversion matrix (matrix) that converts the sensor output values ​​(RGB values ​​((R output value, G output value, B output value)) output by the image acquisition unit 108 in step S408 into a dimension (color space) in which correction processing for color stabilization is performed. The color space in which correction processing is performed can be any color space related to the density on the formed image 400. In this embodiment, the description will be given using the reflectance at a certain wavelength corresponding to CMY on the recording medium. Other examples of color spaces include optical density, luminance, and brightness.

[0070] Next, the color transformation matrix 1607 will be described. Various arithmetic processes for obtaining the color transformation matrix 1607 may be performed by the CPU 100, the image processing unit 106, or a device or system separate from the image forming system according to this embodiment. The color transformation matrix 1607 obtained (generated) by various arithmetic processes described below is stored in the external storage device 105. The storage destination of the color transformation matrix 1607 is not limited to the external storage device 105, and the color transformation matrix 1607 may be stored in another device capable of communicating with the image forming system, such as an external device capable of communicating via the I / F unit 109.

[0071] In this embodiment, before printing a user image, a dedicated chart including a uniform pattern of each ink color is output and read. An example of this uniform pattern is shown in Fig. 7. Fig. 7 shows a patch image 701 as a uniform pattern.

[0072] The first patch row in the patch image 701 shows patches printed with only K ink, each having a different signal value (i.e., amount of ink applied). In the first row, from the left end to the right end, there are arranged a K ink patch with a signal value of 0, a K ink patch with a signal value of 32, ..., a K ink patch with a signal value of 255.

[0073] The second patch row in patch image 701 shows a row of patches printed with only C ink, each with a different signal value (i.e., amount of ink applied). On the second row, from the left end to the right end, there is a patch of C ink with a signal value of 0, a patch of C ink with a signal value of 32, ..., a patch of C ink with a signal value of 255.

[0074] The third patch row in patch image 701 shows a row of patches printed with only M ink, each with a different signal value (i.e., amount of ink applied). On the third row, from the left end to the right end, there is an M ink patch with a signal value of 0, an M ink patch with a signal value of 32, ..., an M ink patch with a signal value of 255.

[0075] The fourth patch row in the patch image 701 shows a row of patches printed with only Y ink, each patch having a different signal value (i.e., amount of ink applied). On the fourth row, from the left end to the right end, there are arranged a Y ink patch with a signal value of 0, a Y ink patch with a signal value of 32, ..., a Y ink patch with a signal value of 255.

[0076] 7 shows an example of nine patch patterns for each color, but the number is not limited to this.Furthermore, the arrangement of the patch patterns for each ink color is not limited to the arrangement shown in FIG.

[0077] In this embodiment, such a uniform pattern is printed on a recording medium by the image forming unit 107, and the printed recording medium (uniform pattern) is read by the image acquisition unit 108 to obtain RGB values ​​for each impact amount (RGB values ​​read by the image acquisition unit 108).

[0078] Furthermore, the uniform pattern is read again or simultaneously (with the above reading) with a known colorimeter capable of obtaining spectral information to obtain the spectral reflectance characteristics ρ for the usage amount (ejection amount) of each ink.

[0079] Next, a color conversion matrix is ​​calculated to convert the RGB values ​​into the reflectance of each color of CMY ink. Specifically, the wavelength for each color of ink is first determined from the spectral reflectance characteristics of each color of CMY ink. For this wavelength, a representative wavelength (representative wavelength) that characterizes the color of each ink is selected. For example, a wavelength of 650 nm is selected as the representative wavelength for C ink (ink of color C), a wavelength of 550 nm is selected as the representative wavelength for M ink (ink of color M), and a wavelength of 450 nm is selected as the representative wavelength for Y ink (ink of color Y).

[0080] Next, the change in reflectance for each color ink amount at the representative wavelength of each of C, M, and Y is calculated from the output of the colorimeter mentioned above as the spectral reflectance characteristic (reflectance) ρ c (k c ), ρ m (k m ), ρ y (k y ) is obtained.

[0081] where k x (where x=c, m, y) represents the amount of ink applied to the ink color x, and ρ x (k x ) is the amount of ink color x applied k x The reflectance at the representative wavelength defined for each ink color x is expressed in [%], and indicates the ratio of dots ejected to each pixel of the nozzle resolution of 1200 dpi. For example, the ejection amount k c = 100[%] indicates that C ink is ejected onto all pixels of the nozzle resolution, and ρ c (100) represents the reflectance of ink color C at the representative wavelength of 650 nm.

[0082] The resulting reflectance ρ x (k x) and the RGB values ​​read from the uniform pattern by the image acquisition unit 108, the following formula is established. x The RGB value for Rx(k x ), Gx(k x ), Bx(k x First, we focus on the representative wavelength of ink color C in a uniform pattern of ink color C (i.e., x=c), and establish the following (Formula 1).

[0083]

number

[0084] Each row in (Equation 1) is a linear combination of RGB values ​​for a patch with a certain amount of impact using unknown coefficients a, b, and c. Here, the coefficients a, b, and c are common to all rows. Here, (Equation 1) is expressed as the amount of impact with variable k c and abbreviated as (Equation 2) below.

[0085]

number

[0086] Equation 1 and Equation 2 represent the same equation. Next, similarly, attention is paid to the representative wavelength of ink color C in the uniform pattern of ink color M, and the following Equation 3 is established.

[0087]

number

[0088] The difference with (Formula 1) is that all reflectance values ​​on the left side are set to 1 instead of the actual measured values, and the RGB values ​​on the right side are set to R m , G m , B m The unknown coefficients a, b, and c represent the same coefficients as in (Equation 1). This is also abbreviated as (Equation 4) below, just like (Equation 2).

[0089]

number

[0090] Furthermore, an equation similar to (Equation 3) is established by focusing on the wavelength of ink color C in the uniform pattern of ink color Y. Since the procedure is the same, only the abbreviated version is written as (Equation 5) below.

[0091]

number

[0092] Since common unknown coefficients a, b, and c appear in (Equation 2), (Equation 4), and (Equation 5), if we write these together in a matrix, we can write them as shown below in (Equation 6).

[0093]

number

[0094] Here, we will explain the meaning of the formulas that have appeared so far. Formula 2 is a formula that linearly combines the RGB values ​​at each ejection amount of single color C ink (single color recording material) to obtain a value equal to the measured reflectance of the representative wavelength of ink color C at that time. This represents an assumption regarding reproducibility that reflectance can be calculated from the RGB values ​​read by the image acquisition unit 108. The coefficients for the RGB values ​​at that time are a, b, and c.

[0095] On the other hand, (Formula 4) and (Formula 5) show that the linear combination of RGB values ​​using the above coefficients a, b, and c for single color C ink has a reflectance that is identical to 1 when M ink and Y ink are the target. In other words, the linear combination of the RGB values ​​read by the image acquisition unit 108 and the coefficients a, b, and c represents the representative wavelength of C ink, and does not react even if RGB values ​​for M ink or Y ink are input, indicating that it does not have an absorption (non-reflective) component for the representative wavelength of C ink. While (Formula 2) has an actual measured value on the left side, the 1 on the left side of (Formula 4) and (Formula 5) is an assumption, and functions as a conceptual constraint.

[0096] In (Formula 6), which includes the reproducibility of (Formula 2) and the constraints of (Formula 4) and (Formula 5), ​​an approximate solution that minimizes the error for vector (a, b, c) can be found. Then, using the coefficients a, b, and c thus obtained, a new linear combination can be created for the RGB values.

[0097]

number

[0098] The left side ρ' obtained by the following (Equation 7) c (k c ) is an approximate solution (Equation 6), which contains not only the actual measured values ​​but also the constraints, so the pure original ρ c (k c), it has characteristics that are more directional and selective for the representative wavelength of C ink. And converting to a reflectance with such characteristics works effectively when dealing with user images that can be any color, not just a single color of ink, unlike a dedicated chart. In fact, similar equations can be established not only for the representative wavelength of C ink described above, but also for the representative wavelength of M ink and the representative wavelength of Y ink. The coefficient matrix is ​​set to a 3x3 matrix X that includes a coefficient set representing the representative wavelength of M ink and a coefficient set representing the representative wavelength of Y ink, instead of the coefficient set (a, b, c) for the representative wavelength of C ink, and the matrix X obtained by approximately solving the following (Equation 8) is the color conversion matrix 1607.

[0099]

number

[0100] The color space, ρ′, which is a directional reflectance, is corrected by multiplying the RGB values ​​read by the image acquisition unit 108 by the color conversion matrix 1607 obtained by performing the calculation process according to each of the above formulas. c (k c ), ρ' m (k m ), ρ' y (k y ) can be obtained. Hereafter, these are called directional reflectances.

[0101] The color space for which the correction is performed is not limited to the above-mentioned (directional) reflectance. It is also possible to perform matrix conversion on R'G'B' obtained by applying 1DLUT or logarithmic conversion to RGB values. For example, the density d of the ink x obtained by logarithmic conversion of the reflectance ratio can be x When performing estimation processing using (x=c, m, y), the color transformation matrix 1607 is calculated by the following (Formula 9) to obtain the RGB value and the density d x We can use a matrix X that associates

[0102]

number

[0103] Note that in (Formula 9), the value at a wavelength different from the ink color on the left side of the constraint condition is set to density 0 instead of the reflectance of 1 in (Formula 8). Also, the matrix side may include quadratic or cubic terms of RGB values, and for example, it is possible to use matrix X according to the following (Formula 10).

[0104]

number

[0105] In the case of (Formula 10), the matrix X has a size of 6 rows and 3 columns. By multiplying the RGB values ​​by this matrix X, ρ' c (k c ), ρ' m (k m ), ρ' y (k y ) is obtained.

[0106] Basically, the color transformation matrix 1607 only needs to be generated at least once in advance for each condition of the recording medium, etc., and there is no need to read using a spectroscopic sensor or obtain the above-mentioned approximate solution each time printing is performed. Each time printing is performed, the generated color transformation matrix 1607 is read, and the obtained RGB values ​​are multiplied using the color transformation matrix 1607. Therefore, in cases where reading using a spectroscopic sensor or calculation processing is a cost or speed constraint, it is possible to achieve color stabilization at higher speed or lower cost.

[0107] Returning to FIG. 6, in step S602, the user image, which is the read image (RGB sensor output value) read from the recording medium by the image acquisition unit 108 in the previous step S408, is converted into a directional reflectance using the color conversion matrix 1607 acquired in step S601.

[0108] In this embodiment, as described above, the directional reflectance that has directional characteristics for a specific wavelength is set as the color space to be corrected, thereby improving the accuracy of the correction. However, since the conversion coefficient calculated based on (Equation 8) is based on an approximate solution, it is difficult to completely eliminate the influence of other wavelengths in the target wavelength. This is especially true for mixed colors. Generally or qualitatively speaking, various factors are involved in the color development due to the mixture of multiple colors on a recording medium, and it is more difficult to accurately calculate the density change of a single ink from that.

[0109] Therefore, in this embodiment, in order to further improve the correction accuracy, the following process is performed. That is, if there is an area in the user image where an image is formed with a single color ink, the density change of that ink is obtained from that area. For the remaining ink colors used in the mixed color, the characteristics are calculated using the density change obtained from the single color area.

[0110] Specifically, in step S603, the identifying unit 1605 identifies which ink was used to form each pixel in the user image on the recording medium. For example, the identifying unit 1605 can make this determination by referring to the pixel values ​​(signal values ​​of C, M, Y, and K) of each pixel in the CMYK image converted from the user image. If the pixel value of C among the C, M, Y, and K pixel values ​​of a pixel of interest in the CMYK image is not 0 (or is equal to or greater than a specified value) and the pixel values ​​of M, Y, and K are 0 (or are less than a specified value), it can be determined that the pixel of interest is formed with only C ink (single color). Therefore, in this case, it can be determined that the corresponding pixel in the user image that corresponds to the pixel of interest is a pixel formed with only C ink.

[0111] The result of identification by the identification unit 1605 for the CMYK image is shown in FIG. 8. In the CMYK image 800, an area 801 is a pixel area made up of C pixels identified as pixels formed only with C ink. In the CMYK image 800, an area 802 is a pixel area made up of M pixels identified as pixels formed only with M ink. In the CMYK image 800, an area 803 is a pixel area made up of white pixels identified as pixels for which no ink is used. In the CMYK image 800, an area 804 is a pixel area made up of multi-color pixels identified as pixels formed with ink of multiple colors (two or more colors). The CMYK image 800 in FIG. 8 is formed on a recording medium in the direction indicated by the arrow 207.

[0112] In step S604, the creation unit 1606 creates the above-mentioned correction table to be used by the correction unit 1602. Details of the process in step S604 will be described with reference to the flowchart in FIG.

[0113] 9, the processes of steps S901 to S914 are performed for the entire image in units of correction processing. In the present embodiment, the correction processing is performed on a nozzle-by-nozzle basis, and the processes of steps S901 to S914 are performed for each nozzle (each column).

[0114] Furthermore, the processes in steps S902 to S907 are repeated for the colors of all the single-color areas in the column identified in step S603. In steps S902 to S907, the density characteristics of the primary colors are estimated and a correction table is created.

[0115] In this embodiment, it is assumed that the image composed of pixels with RGB values ​​obtained in step S408 is acquired with the same resolution and size as the CMYK image. An example of an image composed of pixels with RGB values ​​obtained in step S408 is shown in Fig. 10. The CMYK image of image 1000 in Fig. 10 is CMYK image 800 in Fig. 8. Areas 801, 802, 803, and 804 in Fig. 8 correspond to areas 1001, 1002, 1003, and 1004 in Fig. 10, respectively.

[0116] Here, it is assumed that the nozzle column of interest to be processed is column 1005. In this case, in column 1005, an area 1006 overlaps with area 1001 on image 1000 corresponding to single-color area 801 identified in step S603.

[0117] At this time, in step S903, the creation unit 1606 acquires, for each pixel in the region 1006, the pixel value (signal value of the ink used) of the corresponding pixel in the CMYK image corresponding to that pixel and the corresponding directional reflectance acquired in step S602 in association with each other. FIG. 11 shows a diagram plotting the relationship between the acquired signal value and directional reflectance. The horizontal axis of FIG. 11 is the signal value of a single ink in the CMYK image, in this case the signal value of C. Meanwhile, the vertical axis is the reflectance of the color corresponding to the signal value among the estimated reflectances of each ink, in this case the reflectance for the wavelength of C. One point on the plot 1101 in FIG. 11(a) corresponds to one pixel in the region 1006.

[0118] Next, in step S904, the creation unit 1606 estimates density characteristics for signal values ​​based on the signal values ​​and directional reflectance of each pixel in the region 1006. The density characteristics here are characteristics that represent the density on the recording medium, and in this embodiment, are described by reflectance. A curve 1102 in Fig. 11(a) is a curve that represents the density characteristics calculated based on each of the plotted points.

[0119] Curve 1102 can be derived, for example, by fitting polynomial coefficients obtained by the least squares method to plot 1101 in a polynomial function of a predetermined degree. A power equation (in the form y=x^a, where ^ is the power symbol) may be used instead of a polynomial. Alternatively, it can be obtained by averaging plots within intervals separated by constant signal values ​​(for example, 16 or 32) to obtain representative values ​​of reflectance and signal value for each interval, and then performing an interpolation operation on the representative values ​​for each interval. In this case, for example, piecewise linear interpolation or a spline curve can be used as the interpolation method.

[0120] The relationship between this signal value and directional reflectance can also be plotted in the same way by keeping the signal value on the horizontal axis at C and setting the directional reflectance on the vertical axis to M wavelength (wavelength of M) and Y wavelength (wavelength of Y). For later steps, the relationship regarding M wavelength and Y wavelength is also obtained (not shown).

[0121] Next, in step S905, the creation unit 1606 acquires target density characteristics 1608 that are the target of the density characteristics. The target density characteristics 1608 are described in a color space (reflectance in this case) in which correction processing is performed, in which the color density that the image forming unit 107 should output on a recording medium for the CMYK signal value is performed, and are design values ​​that are determined and stored in advance. In FIG. 11B, the target density characteristics 1608 are shown as a curve 1103 together with a curve 1102 that indicates the density characteristics obtained in step S904. The difference between the two means that the image forming unit 107 should originally output an image with density characteristics like the curve 1103, but is reproduced with a deviation like the curve 1102 due to aging of the piezo element or heater of the nozzle, surrounding environments such as temperature and humidity, and the like.

[0122] In step S906, the correction table creation unit 1601 refers to the curve 1103 indicating the target density characteristic 1608 and the curve 1102 indicating the actual density characteristic, and creates a correction table (FIG. 5) for correcting the curve 1102 to the characteristic of the curve 1103. To create the correction table, first, as shown in FIG. 11B, a set of an input color signal value In and a target reflectance ρ_t corresponding to the input color signal value In is calculated from the curve 1103. Next, the output color signal value Out corresponding to the target reflectance ρ_t is obtained from the actual curve 1102. In other words, the target reflectance ρ_t is realized for the input color signal value In according to the design value, but the target reflectance ρ_t is not realized for the input signal value In according to the actual curve 1102. Since it is the signal value out that actually realizes the target reflectance ρ_t, if the input color signal value In is corrected to the signal value out, the target reflectance ρ_t according to the design value is realized. In this way, a table storing the acquired input color signal values ​​In and output color signal values ​​Out in correspondence with each other is created as a correction table. The table in Fig. 5 stores the input color signal values ​​In = 0, 16, ..., 240, 255 in correspondence with the output color signal values ​​Out (correction values) acquired for the input color signal values. Alternatively, correction values ​​may be calculated for all input color signal values ​​0 to 255 and stored as a table for the nozzle of interest. In other words, the intervals between the input color signal values ​​registered in the correction table are not limited to specific intervals.

[0123] In the above example, the primary color estimation process was described with focus on column 1005 in Fig. 10. For other columns in Fig. 10, such as column 1007, the same process is performed on the overlapping area within column 1002 where M is a monochromatic area instead of C. Also, when there are monochromatic areas for multiple colors, such as column 1008, the same process is performed independently for the monochromatic areas of each color to create a correction table.

[0124] Next, when the loop of steps S902 to S907 ends, the process proceeds to step S908. Steps S908 to S913 are a process loop for creating density characteristics and correction tables for other colors based on the density characteristics of a single ink color determined in steps S902 to S907.

[0125] The processing in steps S908 to S913 refers to the inks specified in step S603 in column 1005, and targets areas in ascending order of the number of inks used that are of the same type.

[0126] In the following explanation, an example of processing for a target area will be described, with the assumption that an area using only ink C and ink M exists in the overlapping area between column 1005 and area 1004. In this case, the inks used are ink C and ink M, and the number of inks used is 2.

[0127] In step S909, similar to step S903 described above, the creation unit 1606 obtains, for each pixel in the target area (area where inks C and M are used), the pixel value (signal value of the ink used) of the corresponding pixel in the CMYK image corresponding to that pixel, and the corresponding directional reflectance obtained in step S602 in association with each other. In this case, the signal value of ink C, the signal value of ink M, and the directional reflectance of the C wavelength (wavelength of C), M wavelength, and Y wavelength are obtained for each pixel.

[0128] Next, in step S910, the creation unit 1606 acquires the directional reflectance already determined in the processes of steps S902 to S907 for a single color ink (here, C ink) for which density characteristics have already been determined.

[0129] More specifically, since the relationship between the signal value and directional reflectance for C ink was obtained in step S904, this relationship is referenced. That is, the directional reflectance of the C wavelength, M wavelength, and Y wavelength of C ink at each pixel is obtained using the signal value of C ink at each pixel in the mixed area of ​​C ink and M ink. The directional reflectance of each wavelength obtained in this way means the directional reflectance of only C ink.

[0130] Then, in step S911, the creation unit 1601 performs a reflectance correction process to remove the influence of the directional reflectance of C ink calculated in step S910 from the directional reflectance of each pixel calculated in step S909. Specifically, in this embodiment, the directional reflectance of the C wavelength, M wavelength, and Y wavelength of the pixel is divided by the directional reflectance of the C wavelength, M wavelength, and Y wavelength of C ink only (reflectance correction process). The directional reflectance obtained in this manner is referred to here as the corrected directional reflectance.

[0131] The reflectance correction process will now be described. In this embodiment, the reflectance of an image formed on a recording medium at a certain wavelength λ is considered to be expressed as the product of the reflectances of the inks at the wavelength λ as follows.

[0132] Reflectance of formed image at wavelength λ = Π (reflectance of each ink at wavelength λ) = (reflectance of C ink at wavelength λ) x (reflectance of M ink at wavelength λ) × (reflectance of Y ink at wavelength λ) × (reflectance of K ink at wavelength λ) (Equation 11) In the example above, only C and M inks are used, Reflectance of formed image at wavelength λ = (reflectance of C ink at wavelength λ) x (reflectance of M ink at wavelength λ) (Equation 12) Therefore, the remaining directional reflectance of M ink can be found by dividing the directional reflectance of the image by the directional reflectance of C. In other words, the corrected directional reflectance in the above example estimates the directional reflectance of M ink alone.

[0133] In step S912, the creation unit 1606 creates a correction table for the new color (M ink in this example) for which the corrected directional reflectance has been calculated. That is, similar to steps S905 to S906, the signal value and corrected directional reflectance for M ink are plotted, and a correction table is created by comparing with the target density characteristics of M ink.

[0134] This completes the processing for the mixed region of C ink and M ink. Through this processing, a new correction table for M ink is obtained in addition to the correction table for C ink.

[0135] The above steps S908 to S913 are repeated to sequentially increase the number of correction tables starting from one known color. Here, an example of a mixed region of C ink and M ink has been described, but if a mixed region of C ink and Y ink exists in the column 1005, a correction table for Y ink can be created instead of M ink in the above process. Also, for example, if a "region of only C ink", a "region of C ink and M ink", and a "region of C ink, M ink, and Y ink" exist simultaneously in the target column 1005, the regions are processed in order starting from the region with the least amount of ink used, and a correction table is created for the region. That is, first, a correction table for C ink is created for the "region of only C ink" in steps S901 to S907. Then, in steps S908 to S913, a correction table for M ink is created for the "region of C ink and M ink", and then a correction table for Y ink is created for the "region of C ink, M ink, and Y ink", and so on, increasing the number of colors to be processed by one color at a time to create a correction table for a new color. Since the density fluctuation of the color for which the correction table was created is now known, the density fluctuation of the mixed color area including the new color can be obtained by performing correction to eliminate the density fluctuation of the known color. In this manner, new correction tables are created in sequence.

[0136] 6, in step S605, the correction unit 1602 performs gamma correction on the CMYK image using the correction table created in step S604 above. That is, the correction table for ink color C is used to perform gamma correction on C pixel values ​​in the CMYK image, and the correction table for ink color M is used to perform gamma correction on M pixel values ​​in the CMYK image. Also, the correction table for ink color Y is used to perform gamma correction on Y pixel values ​​in the CMYK image, and the correction table for ink color K is used to perform gamma correction on K pixel values ​​in the CMYK image.

[0137] In this way, in the second and subsequent printings, the processes of steps S406 and S407 are performed on the CMYK image that was gamma corrected in step S405, so that a formed image 400 can be obtained with its density corrected.

[0138] In this way, by correcting the CMYK image representing the ejection amount based on the correction table, the density characteristics reproduced by the image forming unit 107 can be made closer to the target density characteristics that should be reproduced, and unintended density changes can be reduced. In this embodiment, the density characteristics and correction table of a color are created by referring to an area formed with a single ink, so that a more accurate correction table can be created without being affected by mutual interference between multiple inks. Furthermore, when creating correction tables for other colors, the density characteristics obtained by referring to an area formed with a single color ink are also referenced, and correction tables are created sequentially starting from an area with a small number of colors, so that in this respect too, the correction tables can be created while suppressing the effects of interference between multiple inks. As a result, according to this embodiment, the accuracy of the density fluctuation correction process can be improved.

[0139] <Modification of the first embodiment> In the first embodiment, the target density characteristic 1608 is defined as the density characteristic to be reproduced by correction. However, instead of the defined target density characteristic, the density characteristic of any nozzle or module (head module, chip module) may be the target density characteristic 1608. For example, in the configuration shown in FIG. 2B, the density characteristic corresponding to the head module 201b located at the center or its central nozzle may be used as the target density characteristic 1608. Alternatively, it is possible to set the average value of the density characteristics corresponding to all nozzles (modules) or some nozzles (modules) as the target density characteristic 1608. In this case, although the reproducibility of the desired density characteristic for the signal value is not guaranteed, it is effective in correcting image quality defects that appear as streaky noise due to different print densities only in some nozzles or modules.

[0140] In the first embodiment, the directional reflectance with enhanced directivity at each wavelength by multiplying a matrix is ​​used as the color space for correction. However, it is not essential to use the directional reflectance. In this embodiment, the density characteristics are obtained by referring to the areas where less ink is used in sequence from the single-color ink area, thereby suppressing the influence of interference between multiple inks. Therefore, for example, in (Equation 8), the reflectance 1 may be replaced with the actually measured reflectance at each wavelength to be used as an estimated reflectance (without enhancing the directivity). In addition, if the image acquisition unit 108 does not acquire an RGB image using an RGB sensor, but has a configuration capable of acquiring the reflectance at the wavelength of each ink using a spectroscopic sensor, the measured reflectance may be used directly.

[0141] Also, in the first embodiment, as described above, the color space to be corrected is not limited to reflectance, but may be optical density, brightness, lightness, etc. From the viewpoint of saving calculation processing, it is also possible to use RGB sensor output values. The outline of the concept in this case will be described. In an area where no ink is used, it is assumed that the paper is white and the RGB sensor value is acquired as (255, 255, 255). In an area where C ink is used at 100% of the ejection amount, it is assumed that the paper is cyan and the RGB sensor value is acquired as (0, 255, 255). In this case, it is considered that C ink has the greatest effect on the R value, and the R value can be used in accordance with the reflectance of the C wavelength described above. This is the same for M ink and Y ink. From these relationships, characteristics related to the reproduced density on the recording medium are derived and corrected. When removing the influence of known other colors in the single color density characteristics from the mixed color, the decrement from the white sensor value is removed by subtraction. Compared to the first embodiment, this type of variant has the advantage of eliminating the need for conversion to reflectance, but the disadvantage is that the sensor output value is not a linear accumulation of the effects of each ink color, which may result in reduced accuracy.

[0142] In the first embodiment, for convenience of explanation, the correction table is created every time the second or subsequent prints are made, but it is not necessary to create the correction table every time, and it may be created at intervals based on the time or the number of output sheets. The intervals may be constant or not. However, when a new image is processed, if a new monochromatic area exists for the processing unit (nozzle, chip module, head module), it is preferable to create a correction table without considering the intervals and perform correction processing using the correction table. Since the accuracy of deriving density characteristics is higher from a monochromatic area than from a mixed color area, as soon as a monochromatic area is detected, the correction table that was not created from the monochromatic area is replaced with the correction table created from the monochromatic area. Note that if a new correction table is not created for the same image, the data from the input image to the halftone image does not change, so image formation may be performed based on the same halftone image without performing correction processing (application of correction table) or HT processing each time as described above.

[0143] In the first embodiment, a common target density characteristic 1608 is set for the modules and nozzles that are the units of correction. However, it is also possible to set different target density characteristics 1608 for each module and each nozzle.

[0144] In the first embodiment, the correction unit 402 is described as performing correction processing on the input CMYK image. However, the same effect can be obtained by performing a similar correction processing on the threshold matrix used in the HT processing unit 1603.

[0145] [Second embodiment] 1 may be connected to an I / F unit 109. In this case, the image forming system (image processing device) is a device having 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, and an I / F unit 109. Computer devices such as PCs (personal computers), smartphones, and tablet terminal devices can be applied to such an image forming system (image processing device).

[0146] In such an image forming system (image processing device), an image to be printed is supplied to image forming unit 107 via I / F unit 109. Also, in such an image forming system (image processing device), an image acquisition unit 108 acquires an image read from a recording medium via I / F unit 109.

[0147] The numerical values, processing timing, processing order, processing subject, data (information) structure / destination / source / storage location, etc. used in the above embodiments and variant examples are given as examples to provide a concrete explanation, and are not intended to be limiting to these examples.

[0148] In addition, any part or all of the embodiments and modifications described above may be used in appropriate combination. In addition, any part or all of the embodiments and modifications described above may be used selectively.

[0149] (Other embodiments) The present invention can also be realized by a process in which a program for implementing one or more of the 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. The present invention can also be realized by a circuit (e.g., ASIC) that implements one or more of the functions.

[0150] The invention is not limited to the above-described embodiments, and various modifications and variations are possible without departing from the spirit and scope of the invention. Accordingly, the following claims are appended to apprise the public of the scope of the invention. [Explanation of symbols]

[0151] 100: CPU 101: RAM 102: ROM 103: Operation unit 104: Display unit 105: External storage device 106: Image processing unit 107: Image forming unit 108: Image acquisition unit 109: I / F unit 110: Bus

Claims

1. a creating unit that creates correction information for reducing density variations in image formation using recording materials of multiple colors; The creating means includes: In a read image obtained by reading an image formed using recording materials of multiple colors, correction information is created corresponding to the recording materials used in the area, starting from the area with the least amount of recording material used, for which correction information has not yet been created.

13. An image processing device comprising:

2. 2. The image processing device according to claim 1, wherein the creation means creates correction information corresponding to a single recording material for an area in the read image that is formed using a single recording material, and after the creation, creates correction information corresponding to the recording materials used in the area, for which correction information has not yet been created, in order from areas formed using multiple color recording materials in the read image that use fewer recording materials.

3. If there is an area formed using a new single recording material in the read image, the creating means creates correction information corresponding to the new single recording material.

3. The image processing device according to claim 1, wherein the first and second inputs are input to the image processing device.

4. Furthermore, 4. The image processing apparatus according to claim 1, further comprising an image forming unit for forming an image on a recording medium using the recording materials of the plurality of colors.

5. the image forming means forms an image by an inkjet method, 5. The image processing apparatus according to claim 4, wherein said creating means creates a correction table for each nozzle, chip module or head module through which said image forming means ejects a recording material.

6. 6. The image processing device according to claim 5, wherein the creation means creates correction information for correcting density characteristics based on a signal value of a recording material to a target density characteristic that is a density characteristic of the nozzle, the chip module, or the head module.

7. 6. The image processing apparatus according to claim 1, wherein said creating means creates correction information for correcting density characteristics based on signal values ​​of a recording material to target density characteristics stored in advance.

8. Furthermore, 8. The image processing apparatus according to claim 1, further comprising a reading unit for reading an image formed using the recording materials of the plurality of colors.

9. Furthermore, 9. The image processing apparatus according to claim 1, further comprising a correction unit that corrects an image corresponding to a color reproduction range of the image formation using the correction information.

10. An image processing method performed by an image processing device, comprising: a creating step of creating correction information for reducing density variations in image formation using recording materials of multiple colors, In the preparation process, In a read image obtained by reading an image formed using recording materials of multiple colors, correction information is created corresponding to the recording materials used in the area, starting from the area with the least amount of recording material used, for which correction information has not yet been created.

13. An image processing method comprising:

11. A computer program for causing a computer to function as each of the means of the image processing device according to any one of claims 1 to 9.

Images