JP2017204786A - Image forming apparatus, image forming unit, and program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming system capable of improving stability of reproduction color in a print job, regardless of imaging conditions and printing conditions.SOLUTION: When a second measurement sensor (second measurement means) obtains measured values of the number exceeding a predetermined value from a color image formed on a sheet (print medium) based on document data (input image), a gradation correction unit (gradation correction means) corrects gradation characteristic variation by using gradation correction parameters estimated by a color tone correction amount determination unit (second gradation correction parameter estimation means). Meanwhile, when the second measurement sensor cannot obtain measured values of the number exceeding a predetermined value from a color image, the gradation correction unit corrects gradation characteristic variation by using gradation correction parameters estimated by a color tone correction amount determination unit (first gradation correction parameter estimation means), based on the reflection characteristics of a color measurement patch. A color tone correction amount determination unit (first gradation correction parameter estimation means) corrects gradation characteristic variation by using gradation correction parameters estimated based on the reflection characteristics of the color measurement patch measured by a first measurement sensor (first measurement means).SELECTED DRAWING: Figure 3

Description

  The present invention relates to an image forming apparatus, an image forming unit, and a program.

  In digital printing apparatuses such as an electrophotographic system and an ink jet system that presuppose a large amount of printing, stability of output colors is required when continuous output such as hundreds or thousands of sheets is performed. However, unlike full-scale commercial printing, the operating environment of a digital printing apparatus is often not always strictly controlled for output color stability. For this reason, when it is necessary to manage the output color strictly to some extent, it is necessary to frequently stop the machine and calibrate the output color.

  According to such a conventional digital printing apparatus, since it is necessary to repeatedly perform calibration, there have been problems such as generation of lost paper, stoppage of jobs, and increase in the number of work steps. In order to solve this problem, Japanese Patent Application Laid-Open No. 2004-228561 measures the color of an output image of an electrophotographic printing apparatus, and adjusts the tone correction curve (TRC) so that the difference between the colorimetric value and a previously stored reference color is minimized. ) Is corrected. In Patent Document 2, the drift of the entire color space of the reproduction color is corrected based on the in-line color measurement result using a model that approximates the variation principal component. Furthermore, in Patent Document 3, the change mode of the gradation reproduction curve is approximated by its main component.

  These methods can characterize changes in reproduced colors that have inherent infinite degrees of freedom with fewer parameters. However, if there is not enough colorimetric information on the output image, a separate colorimetric patch is available. Must be provided outside the area of the user image. For example, in Patent Document 4, a colorimetric patch called a control strip is formed on a margin on the premise that margins are premised on cutting in the subsequent process, and image density fluctuations are monitored. However, in the case of a digital printer that frequently uses cut sheet printing that is not predicated on cutting, there may be no region where control strips can be placed. In such a case, for example, in Patent Document 5, a colorimetric patch is formed on the intermediate transfer member, and the density of the colorimetric patch is observed.

  However, according to such a conventional digital printing apparatus, the correspondence relationship between the patch density on the intermediate transfer member and the density on the printing medium as a final result differs depending on the image forming condition and the printing condition. The image forming conditions are, for example, parameters such as developing bias, laser exposure intensity, and toner charge amount. The printing conditions are parameters such as the type of paper that is the printing medium and the number of dotted lines. Therefore, it is difficult to maintain the stability of the reproduced color regardless of the image forming conditions and the printing conditions.

  The present invention has been made in view of the above, and an object of the present invention is to provide an image forming system capable of improving the stability of reproduced colors in a print job regardless of image forming conditions and printing conditions. It is what.

  In order to solve the above-described problems and achieve the object, the present invention develops an input image composed of a plurality of basic colors into a printable format based on a color profile registered in a color profile registration unit. In the image forming apparatus that forms a color image on a print medium by mixing the basic colors, the intermediate image is temporarily formed before the input image is transferred onto the print medium. Colorimetric patch forming means for forming a plurality of colorimetric patches combining the gradation values of different basic colors, first measuring means for measuring reflection characteristics of the colorimetric patches, and reflection characteristics of the colorimetric patches And a first gradation correction parameter estimating means for associating the reflection characteristic of the colorimetric patch with a gradation correction parameter for correcting a gradation characteristic variation of the image forming apparatus, and the printing medium Based on a second measuring means for measuring at least a part of the reflection characteristics of the color image formed by transferring the input image to the input image and at least a part of the reflection characteristics of the color image. The second gradation correction parameter estimation means for estimating the gradation correction parameter and the second measurement means have obtained a number of measurement values exceeding a predetermined value from the color image. As described above, the gradation characteristic variation is corrected using the gradation correction parameter estimated by the second gradation correction parameter estimation means, and the second measurement means performs the predetermined measurement from the color image. The gradation characteristic change using the gradation correction parameter estimated by the first gradation correction parameter estimation means on condition that the measured value exceeding the value cannot be obtained. Wherein the of having, a gradation correction means for correcting.

  According to the present invention, it is possible to improve the stability of reproduced colors in a print job regardless of image forming conditions and printing conditions.

FIG. 1 is a hardware block diagram showing a hardware configuration of the image forming apparatus according to the present embodiment. FIG. 2 is a functional block diagram showing a functional configuration of the image forming apparatus according to the present embodiment. FIG. 3 is a diagram for explaining the flow of image data in the image data processing performed by the image forming apparatus. FIG. 4 is a configuration diagram showing an example of a schematic configuration of a laser printer to which the image forming apparatus according to the present embodiment is applied. FIG. 5 is a flowchart showing the flow of colorimetric region extraction processing. FIG. 6 is a diagram showing an example of the arrangement of the colorimetric patches formed on the intermediate transfer belt. FIG. 7 is a block diagram illustrating an example of the internal configuration of the preprocessing unit. FIG. 8 is a diagram illustrating an example of a halftone dot configuration of a colorimetric patch, and FIG. 8A is a diagram illustrating an example of a low density gradation pattern. FIG. 8B is a diagram showing an example of the intermediate density gradation pattern. FIG. 9 is a diagram illustrating an example of the configuration of the mode curve. FIG. 10 is a diagram illustrating an example of mode curve synthesis. FIG. 11 is a flowchart illustrating a flow of a series of processes performed by the image forming system. FIG. 12 is a flowchart showing the flow of the parameter conversion matrix search process.

  The best mode for carrying out the invention will be described below with reference to the drawings.

(Description of hardware configuration of image forming apparatus)
FIG. 1 is a hardware block diagram illustrating a hardware configuration of the image forming apparatus 8. As shown in FIG. 1, the image forming apparatus 8 includes a CPU (Central Processing Unit) 100 and a memory 102. The CPU 100 and the memory 102 are connected by an internal bus B1. The memory 102 includes a ROM (Read Only Memory) and a RAM (Random Access Memory). The memory 102 stores a program P1 executed by the CPU 100, document data 30 received from a user PC 1 described later, a color profile received from a server 7 described later, and the like. The CPU 100 controls the operation of the image forming apparatus 8 by reading the program P1 from the memory 102 and executing it.

  The program P1 is provided by being incorporated in the memory 102 in advance. The program P1 is a file in an installable or executable format and is recorded on a computer-readable recording medium such as a CD-ROM, a flexible disk (FD), a CD-R, or a DVD (Digital Versatile Disc). And may be configured to be provided. Furthermore, the program P1 may be stored on a computer connected to a network such as the Internet and provided by being downloaded via the network. The program P1 may be provided or distributed via a network such as the Internet.

  Further, the image forming apparatus 8 includes a printer engine 4. The printer engine 4 is an image forming unit that forms an image by electrophotography and executes printing of the document data 30. The printer engine 4 is connected to the internal bus B1 via the printer engine I / F 106. The printer engine 4 includes a first measurement sensor 58 that is a first measurement unit. The function of the first measurement sensor 58 will be described later.

  The image forming apparatus 8 further includes a scanner 27. The scanner 27 is connected to the internal bus B1to via the scanner I / F 108. The scanner 27 includes a second measurement sensor 59 which is a second measurement unit. The functions of the scanner 27 and the second measurement sensor 59 will be described later.

  Further, the image forming apparatus 8 includes a network I / F 104 connected to the internal bus B1. The user PC 1 and the server 7 are connected to the network I / F. The user PC 1 stores document data 30 that is an example of an input image. The user PC 1 transmits the document data 30 to the image forming apparatus 8 as necessary. The server 7 is an example of a color profile registration unit, and stores information such as a color profile used for color conversion (to be described later) required by the image forming apparatus 8.

(Description of functional configuration of image forming apparatus)
FIG. 2 is a functional block diagram showing a functional configuration of the image forming apparatus 8 according to the present embodiment. The image forming apparatus 8 according to the present embodiment includes an image processing unit 3, an engine control unit 9, a gradation processing unit 31, an image inspection unit 5, a color tone control unit 28, and the printer engine 4 described above. The image forming apparatus 8 is connected to the user PC 1 and the server 7 via the network 2.

  The image processing unit 3 develops document data 30 as an input image input from a user PC (Personal Computer) 1 via the network 2 into a printable format. Then, the image processing unit 3 converts the developed document data 30 into a pixel array (bitmap data or a compression format equivalent thereto) configured with the basic colors of the printer engine 4 and sends it to the gradation processing unit 31. Send it out. The document data 30 is expressed in a data format including bitmap data color-designated by RGB or CMYK, text and graphics drawing commands, and the like.

  The engine control unit 9 controls the printer engine 4. The engine control unit 9 is mounted in the same housing as the printer engine 4 and constitutes an image forming unit 32 together with the color tone control unit 28, the gradation processing unit 31, and the image inspection unit 5.

  The gradation processing unit 31 converts the pixel array developed by the image processing unit 3 into the number of gradations that can be output by the printer engine 4.

  The image inspection unit 5 includes the second measurement sensor 59 and the scanner 27 described above. The second measurement sensor 59 includes an RGB line sensor or a monochrome line sensor. The image inspection unit 5 performs colorimetry on the surface by the second measurement sensor 59 while feeding the paper by the paper feeding mechanism provided in the scanner 27 before the image formed by the printer engine 4 is printed out. By doing inline inspection.

  Specifically, the second measurement sensor 59 irradiates the image formed by the printer engine 4 with light and receives the reflected light, thereby measuring the reflectance of the image two-dimensionally on the surface. To do. Then, as a result of the in-line inspection, the image inspection unit 5 detects a color tone variation (density variation, hue variation, etc.) of the output image 6 which is an example of a color image, and a deviation from the target color (colorimetric predicted value) is detected. The reproduced color of the output image 6 is stabilized by correcting the parameters of the engine control unit 9 and the gradation processing unit 31 so as to be minimized.

  The tone control unit 28 calculates tone correction parameters for correcting the tone of the output image 6 based on the tone variation of the output image 6 detected by the image inspection unit 5. Then, the color tone control unit 28 gives the calculated gradation correction parameter to the gradation processing unit 31.

  Each function unit of the engine control unit 9, the gradation processing unit 31, the image inspection unit 5, and the color tone control unit 28 can be executed by executing the program P1 read from the memory 102 by the CPU 100 (FIG. 1). In this manner, it is expanded on the memory 102. Each functional unit operates according to the procedure described in the program P1.

  The image processing unit 3 is configured separately from the image forming unit 32, for example, by software on a PC and an expansion board, and has a system configuration that can be exchanged for the image forming unit 32. Note that the image processing unit 3 may be integrated with the image forming unit 32.

(Color profile setting)
Next, the flow of image data in the image data processing performed by the image forming apparatus 8 will be described with reference to FIG. In FIG. 3, image processing related to drawing commands and text development is omitted in order to avoid complicated description. Further, in the following example including FIG. 3, the data to be compared by the color tone correction amount determination unit 19 is CIELab (hereinafter simply referred to as Lab), but the color expression for comparison does not necessarily have to be Lab. Although it does not need to be three-dimensional, in order to avoid the description being abstract, it will be described below as Lab. The document data 30 will be described as image data assuming specific printing device characteristics of RGB or CMYK (for example, JapanColor2001Coated). In FIG. 3, for convenience of description, the color tone control unit 28 is arranged on the lower side, unlike FIG.

  Various conversion units illustrated in FIG. 3, that is, the document color-Lab conversion unit 10, the Lab-CMYK conversion unit 11, the CMYK-Lab conversion unit 13, the scanner color-Lab conversion units 15, 18, and the Lab-scanner color conversion unit 14. Requires basic data called color profiles for each color space conversion (ICC profiles defined by ICC (International Color Consortium) are widely known as examples of such color profiles).

  Among these color profiles, the color profile necessary for the document color-Lab conversion unit 10 is attached to the document data or prepared in advance. The color profiles necessary for the scanner color-Lab conversion units 15 and 18 and the Lab-scanner color conversion unit 14 are fixedly set in advance in the color tone control unit 28 and the image inspection unit 5. On the other hand, the color profiles required for the Lab-CMYK conversion unit 11 and the CMYK-Lab conversion unit 13 have different color reproduction characteristics depending on the paper that is an example of the print medium set in the printer engine 4. An appropriate color profile (hereinafter referred to as a paper profile) selected prior to the start is downloaded from the server 7 and set in both the image processing unit 3 and the color tone control unit 28. The user selects the paper profile via the user PC 1 or the user interface of the image processing unit 3.

(Description of image forming color path)
Under such a color profile setting, the document data 30 described in RGB, CMYK, etc. is first converted into Lab values (in FIG. 3), which are device-independent color values by the document color-Lab conversion unit 10. docLab). The Lab-CMYK conversion unit 11 converts docLab into an 8-bit integer gradation value of each color of the basic colors CMYK of the printer engine 4. The user gradation conversion unit 12 outputs the CMYK value (prnCMYK in FIG. 3) as it is by default without changing it. These color conversions are processed at the same time as vector data and font expansion, and the resulting CMYK data is output as bitmap data. The document data 30 developed as a bitmap of CMYK values in this way is temporarily held in the storage device 22 for each print document. The subsequent image forming unit 23 forms the output image 6 on the designated paper based on the CMYK bitmap image data (prnCMYK in FIG. 3).

  The gradation correction unit 16 is an example of a gradation correction unit. The gradation correction unit 16 includes a gradation correction curve (TRC: Tone Reproduction Curve) for each of C, M, Y, and K colors, and C, M, Y, K using these gradation correction curves. Correct each value. This gradation correction curve is corrected so as to keep the reproduced color of the final output image stable in accordance with the gradation correction parameter which is a table value given from the tone correction amount determination unit 19.

  The gradation conversion unit 17 converts the color value sent in 8 bits for each color of C, M, Y, and K into the number of gradations that can be expressed by one dot by the printer engine 4 using the area gradation method and error diffusion. Decrease using the law. The printer engine 4 forms the output image 6 on the paper based on the CMYK signal reduced by the gradation converting unit 17.

  On the other hand, the CMYK value (prnCMYK) input also to the color tone control unit 28 is returned again to the Lab value (prnLab in FIG. 3) by the CMYK-Lab conversion unit 13. The CMYK-Lab conversion unit 13 is a model that simulates the reproduction color of the output image 6 output from the image forming unit 23 as a Lab value. The simulated output predicted value (prnLab) and the measured value (mesCol) of the output image 6 scanned by the scanner 27 of the image inspection unit 5 are converted into Lab values by the scanner color-Lab conversion unit 18 to be colorimetric values. Using (mesLab), the tone correction amount determination unit 19 corrects the TRC set in the gradation correction unit 16. In the present embodiment, the scanner correction unit 25 corrects a reading error that is specific to the scanner 27 and occurs depending on the input image, thereby improving the accuracy of scanner reading color prediction.

  Specifically, the scanner correction unit 25 uses the Lab value (prnLab) output from the CMYK-Lab conversion unit 13 as a colorimetric value in the scanner color space via the Lab-scanner color conversion unit 14 (scnCol). The converted value is converted into a predicted reading value (scnCol ′) of the scanner 27.

  Then, the scanner color-Lab conversion unit 15 serving as a colorimetric value conversion unit converts the read predicted value (scnCol ′) output from the scanner correction unit 25 again into a colorimetric predicted value (targetLab) that is a device-independent Lab value. Convert to By this series of processing from the Lab-scanner color conversion unit 14 to the scanner color-Lab conversion unit 15, the read color generated when the color gamut that the scanner 27 can read is smaller than the color gamut that the printer engine 4 can output. Saturation (color gamut compression) is also simulated.

  In FIG. 3, the CMYK-Lab conversion unit 13, the Lab-scanner color conversion unit 14, the scanner correction unit 25, and the scanner color-Lab conversion unit 15 constitute a colorimetric value prediction model unit 21. Generation of a colorimetric prediction value (targetLab) by the colorimetric value prediction model unit 21 requires a large calculation load (memory amount used and processing time). For this reason, actually, a colorimetric prediction value is generated in advance from data for the required number of pages and stored in the storage device 26, and the subsequent color tone correction amount determination unit 19 performs the measurement that is stored in the storage device 26. Use color prediction values. The storage device 26 is an example of a storage unit. As described above, the colorimetric prediction value (targetLab) as the scanner reading result including the error characteristics on the scanner 27 side is stored in advance, so that the color tone correction amount determination unit 19 in the subsequent stage performs the TRC correction amount and the color mixture correction. The table value can be calculated efficiently in real time. As a result, the print color prnLab simulated by the printer color gamut compression unit 20 is converted into a target color targetLab to be measured by the image inspection unit 5. The tone correction amount determination unit 19 includes a target Lab that is a target color, a measurement value mesLab that is actually obtained from the image inspection unit 5, a prnCMYK that is an input value to the gradation correction unit 16, and an engine from the engine control unit 9. Based on the information, the setting value of TRC is determined.

(Description of printer engine configuration)
FIG. 4 is a cross-sectional view illustrating a schematic configuration of a laser printer 90 as an example of the printer engine 4. First, the configuration and operation of the developing unit 60k will be described. The photosensitive drum 50k rotates in the direction of arrow A in FIG. The rotational position of the photosensitive drum 50k is detected by a rotation detector 57 provided at the end of the photosensitive drum 50k. First, the charger 52 applies a uniform charge to the surface of the photosensitive drum 50k cleaned by the cleaning roller 51, with respect to the photosensitive drum 50k. Next, an electrostatic latent image is formed on the photosensitive drum 50k by scanning the surface of the photosensitive drum 50k while the laser beam 55 emitted from the laser unit 53 blinks in accordance with a signal output from the exposure control device 65. Form.

  The scanning direction of the laser beam 55 at this time is the main scanning direction, and the rotation direction of the photosensitive drum 50k (arrow A) is the sub-scanning direction. The formed electrostatic latent image is developed with black (K) toner supplied from the developing roller 54 and charged to a potential opposite to that of the photosensitive drum 50k, and becomes a toner image. The developed toner image is transferred to the intermediate transfer belt 61. The developing units 60c, 60m, and 60y have the same configuration, and form toner images of cyan (C), magenta (M), and yellow (Y) on the surfaces of the photosensitive drums 50c, 50m, and 50y, respectively. .

  The formed toner images are sequentially transferred onto the intermediate transfer belt 61 in a superimposed manner. A first measurement sensor 58 serving as a first measurement unit detects a density change of a colorimetric patch, which will be described later, formed on the intermediate transfer belt 61 outside the area of the user image formed in the document data 30. To do. The belt cleaning mechanism 63 following the first measurement sensor 58 contacts the intermediate transfer belt 61 in synchronization with the position of the color measurement patch, thereby removing the color measurement patch on the intermediate transfer belt 61. In addition, a preprocessing unit 64 is provided at the subsequent stage of the first measurement sensor 58. The function of the preprocessing unit 64 will be described later. The transfer roller 62 collects the C, M, Y, and K toner images superimposed on the intermediate transfer belt 61 on the sheet conveyed on the sheet conveyance path 66 from the right side to the left side in FIG. Transcript. The fixing device 56 fixes the toner image transferred on the paper onto the paper surface by heat-pressing.

(Description of processing for extracting a colorimetric region from a user image)
Next, a process for extracting a colorimetric area from a user image area will be described with reference to FIG. FIG. 5 is a flowchart showing the flow of colorimetric region extraction processing.

  First, the color tone correction amount determination unit 19 reads, from the storage device 22, image data converted into CMYK bitmap for one page of the document data 30 to be printed into the storage device 26 (step S100).

  Next, the color tone correction amount determination unit 19 extracts a colorimetric region that is a colorimetric target from the read image data (step S101).

  Specifically, for example, as disclosed in Patent Document 1, a method of extracting a plurality of small regions of a fixed size (for example, 41 × 41 pixels at 400 dpi) with a gradually changing flatness is used. At this time, the extracted colorimetric region is described by center coordinates (xi, yi) in pixel units (subscript i is a sample number). xi is a position in the main scanning direction, and yi is a position in the sub-scanning direction (paper feeding direction). For each colorimetric area, the average value of the CMYK input tone value and scanner colorimetric value is treated as a value in the colorimetric area. Note that, in the case of reproduction color stabilization management in printing in which the same type of document is repeated, the sample of the colorimetric area to be extracted extends over a single page or a plurality of pages as a repetition unit. In such a case, the sub-scanning coordinate yi is the number of repeated pages, or a group of those divided into an appropriate number of pages (about several pages), and the first page writing position is the origin. It can be reused by describing and storing in continuous coordinates including between sheets.

  Next, the color tone correction amount determination unit 19 confirms whether or not extraction of colorimetric areas for the number of print cycle pages has been completed (step S102). When the extraction of the colorimetric region is completed (step S102; Yes), the process of FIG. 5 is terminated. Otherwise (step S102; No), the process returns to step 100.

(Explanation of colorimetric patch placement)
Next, the arrangement of the colorimetric patches formed on the intermediate transfer belt 61 will be described with reference to FIG. FIG. 6 shows the arrangement of colorimetric patches 80c, 81c, 82c, 80m, 81m, 82m, 80y, 81y, 82y, 80k, 81k, 82k (hereinafter referred to as 80c to 82k) formed on the intermediate transfer belt 61. It is a figure which shows an example. 6 is a view of the intermediate transfer belt 61 of the laser printer 90 shown in FIG.

  The intermediate transfer belt 61 moves at a constant speed in the direction indicated by the arrow 72. Printing is performed continuously, but there is a predetermined interval or more between the sheets sent to the sheet conveyance path 66 in FIG. In FIG. 6, the corresponding paper positions are indicated by broken lines 70a and 70b. The portion beyond the right end of the intermediate transfer belt 61 is wound downward toward the laser printer 90, and is not shown in FIG. Hereinafter, the broken line 70a is referred to as a preceding sheet position, and the broken line 70b is referred to as a subsequent sheet position. In FIG. 6, a user image formed on the intermediate transfer belt 61 at a position corresponding to the preceding paper position 70a is referred to as a preceding image 71a. A user image formed at a position corresponding to the subsequent sheet position 70b is referred to as a subsequent image 71b. As shown in FIG. 6, the colorimetric patches 80c to 82k are formed in a gap between the preceding paper position 70a and the subsequent paper position 70b. Here, the toner image formed on the intermediate transfer belt 61 and before being transferred to a sheet as a printing medium is referred to as an intermediate image.

  The colorimetric patches 80c, 81c, and 82c are halftone dots of a single cyan color. The colorimetric patch 80c is solid 100%, the colorimetric patch 81c is low density (for example, 22.2%), and the colorimetric patch 82c is medium density (for example, 50%). The colorimetric patches 80m, 81m, 82m, 80y, 81y, 82y, 80k, 81k, 82k are also arranged in the same halftone dot percentage, with 80m, 81m, 82m being magenta, 80y, 81y, 82y being yellow, 80k, 81k and 82k correspond to black.

  Above the belt surface of the intermediate transfer belt 61, a first measurement sensor 58 is disposed along the belt surface of the intermediate transfer belt 61 in a direction orthogonal to the arrow 72 that is the traveling direction of the intermediate transfer belt 61. ing.

  The first measurement sensor 58 is an optical sensor that measures the reflectance of the colorimetric patches 80c to 82k by irradiating the colorimetric patches 80c to 82k with light and receiving the reflected light. As shown in FIG. 6, the first measurement sensor 58 includes measurement sensors 58c1 to 58k3 at positions corresponding to the colorimetric patches 80c to 82k, respectively.

  In the present embodiment, the first measurement sensor 58 measures the reflection characteristics of the color measurement patches 80c to 82k formed on the intermediate transfer belt 61, but the reflection characteristics of the color measurement patches 80c to 82k. The position for measuring is not limited to the intermediate transfer belt 61. In other words, it may be before the toner image is transferred to the paper as the printing medium. For example, immediately after the electrostatic latent images formed on the photosensitive drums 50y, 50m, 50c, and 50k are developed, the reflection characteristics of the developed toner images are expressed as the photosensitive drums 50y, 50m, 50c, and 50k. You may measure above. In this case, the toner images formed on the photosensitive drums 50y, 50m, 50c, and 50k are intermediate images. In this case, the first measurement sensor 58 is installed toward the surface of each of the photosensitive drums 50y, 50m, 50c, and 50k.

(Description of pre-processing of colorimetric patch measurement values)
The tone correction amount determination unit 19 (FIG. 3) estimates the tone correction parameter for the signal output by the preprocessing unit 64 (FIG. 4) provided after the first measurement sensor 58. Perform software processing. At this time, the tone correction amount determination unit 19 functions as a first tone correction parameter estimation unit. Here, for the sake of convenience, the output signals of the measurement sensors 58c1 to 58k3 are positive values, and it is assumed that a higher value is output as the density of the colorimetric patches 80c to 82k is higher.

  The preprocessing unit 64 performs signal processing for improving the S / N of the measurement values that are the measurement results of the measurement sensors 58c1 to 58k3. Normally, any halftone dot colorimetric patch has fine fluctuation noise in the sub-scanning direction (the direction opposite to the arrow 72 in FIG. 6). Many of these fluctuation noise components in the sub-scanning direction are caused by rotating parts such as the developing roller 54 and the photosensitive drums 50c to 50k in FIG. Therefore, the preprocessing unit 64 detects a contrast variation with a high S / N by extracting a difference signal between the colorimetric patches prior to signal smoothing in the sub-scanning direction. Of course, since the average fluctuation component can improve the S / N by simply averaging, a signal that is less susceptible to sub-scanning period fluctuations by converting it into a sum signal and a difference signal at the input stage. Is detected.

  Next, the contents of signal processing performed by the preprocessing unit 64 will be described with reference to FIG. FIG. 7 is a block diagram illustrating an example of processing the input signals from the cyan measurement sensors 58c1, 58c2, and 58c3 with respect to the internal configuration of the preprocessing unit 64.

  First, the preprocessing unit 64 includes an adder 40 and a subtractor 41 from a measurement sensor 58c3 for a medium density halftone dot (colorimetric patch 82c) and a measurement sensor 58c2 for a low density halftone dot (colorimetric patch 81c), respectively. To calculate the added value and the difference value, respectively. Next, the preprocessing unit 64 accumulates samples for a certain period corresponding to the lengths of the colorimetric patches 80c to 82k in the direction of the arrow 72 by the integrators 43b and 43c. On the other hand, the preprocessing unit 64 similarly accumulates the output of the measurement sensor 58c1 for the solid patch (colorimetric patch 80c) by the integrator 43a to obtain the accumulated value s. Furthermore, the dividers 42a and 42b normalize the previous addition value (a + b) and the difference value (a−b) with the accumulated value s to obtain output values p and q.

  Thereafter, the accumulated value s is sent to the engine control unit 9 (FIG. 3). The engine control unit 9 uses printing parameters (image forming conditions) such as a developing bias, laser exposure intensity, and toner charge amount as control signals for maintaining a measured value for the solid patch at a constant value. The output values p and q are used by the color tone correction amount determination unit 19. The preprocessing unit 64 described above may be realized using either an analog circuit or a digital circuit. The function of the preprocessing unit 64 may be implemented as software processing that operates on the color correction amount determination unit 19.

(Description of halftone dot configuration of colorimetric patch)
Next, the halftone dot configuration of the colorimetric patch will be described with reference to FIG. FIG. 8A is a diagram showing an example of a low density gradation pattern 45 (halftone dot = 22.2%) representing intermediate gradation values used for the colorimetric patches 81c, 81m, 81y, 81k. FIG. 8B is a diagram showing an example of an intermediate density gradation pattern 46 (halftone dot = 50%) representing intermediate gradation values used for the colorimetric patches 82c, 82m, 82y, and 82k.

となっている。ただし、これは理論上の画素を表す図であって、実際のトナー像はこれよりも若干広がりを持つぼやけた像となる。これらの中間階調値を表す低濃度階調パターン４５と中間濃度階調パターン４６は、ユーザ画像を印刷して出力画像６（カラー画像）を生成する際に使用する網点パターンとは異なる別の網点パターン（面積階調パターン）として使用される。 Each grid in FIGS. 8A and 8B corresponds to one theoretical pixel of 600 dpi, and white represents no toner and black represents toner. Therefore, the number of lines of these halftone dots is

It has become. However, this is a diagram showing a theoretical pixel, and an actual toner image is a blurred image having a slightly wider area. The low density gradation pattern 45 and the intermediate density gradation pattern 46 representing these intermediate gradation values are different from the halftone dot pattern used when the user image is printed and the output image 6 (color image) is generated. Used as a halftone dot pattern (area gradation pattern).

  In the present embodiment, the colorimetric values of the colorimetric patches 80c to 82k on the intermediate transfer belt 61 are not directly used as the intermediate tone density change amount, but indirectly to the density change of the user image being used. The method of associating is taken. Therefore, as the low density gradation pattern 45 and the intermediate density gradation pattern 46, a halftone pattern different from the halftone pattern used by the user image can be used.

  In general, the higher the number of halftone dots, the more the intermediate density tends to change more sensitively to changes in the environment and printing conditions. The number of lines exceeding 200 lpi in this embodiment is a high density among the dot densities used in the electrophotographic system. Even when the user image is expressed by a relatively low number of halftone dots, the density change detection by the first measurement sensor 58 can be performed by using such a high number of lines gradation pattern for density change detection. S / N is secured. Of course, the low density gradation pattern 45 and the intermediate density gradation pattern 46 are merely examples, and may be a combination of patterns different from these as long as sufficient S / N for density change detection is ensured. .

  The low density gradation pattern 45 and the intermediate density gradation pattern 46 usually need to be formed outside the printing area (the area between the preceding paper position 70a and the subsequent paper position 70b in FIG. 6). Therefore, in the present embodiment, the exposure control device 65 (FIG. 4) generates signal generation means (colorimetric patch formation) that generates the low density gradation pattern 45 and the intermediate density gradation pattern 46 separately from the user image. Means).

(Description of TRC correction amount determination method)
Next, the TRC correction amount determination method based on the colorimetric values of the colorimetric region on the user image (for example, the subsequent image 71b (FIG. 6)) performed by the color tone correction amount determination unit 19 will be described. At this time, the tone correction amount determination unit 19 functions as second tone correction parameter estimation means. That is, as described above, the color correction amount determination unit 19 functions as a first gradation correction parameter estimation unit that estimates a gradation correction parameter based on the reflection characteristics of the colorimetric patches 80c to 82k. As will be described, it also acts as second gradation correction parameter estimating means for estimating the gradation correction parameter based on the reflection characteristic of the user image.

ここで、ｃ，ｍ，ｙ，ｋは補正前のＣＭＹＫ階調値、ｃ_０，ｍ_０，ｙ_０，ｋ_０は基準階調特性、ｃ_１，ｍ_１，ｙ_１，ｋ_１は第１変動モード、ｃ_２，ｍ_２，ｙ_２，ｋ_２は第２変動モードである。また、θ^ｊ _ｉ（ｉ＝｛０，１，２｝，ｊ＝｛ｃ，ｍ，ｙ，ｋ｝）はモードパラメータである（上付きの添字は指数ではなく、単なる識別用の添字）。特に、モードパラメータθ^ｊ _ｉは実スカラーであり、基準階調特性および各変動モードは、各ｃ，ｍ，ｙ，ｋ階調値の入力レンジＤ上で定義された同一次元の独立な実ベクトルである。なお、モードパラメータは、数学的には入力レンジＤ上の実数値関数のなすベクトル空間の低次元部分空間の基底だが、実装上は、配列や、配列と補間の組み合わせ等で実装された曲線と考えてよい。なお、入力レンジＤは、通常、［０，１］区間の実数や、［０，１００］区間や［０，２５５］区間の整数等で実装される。 In the following description, the variable for each sample is considered as a random variable, and the subscript of the sample number is omitted. First, CMYK gradation value after the correction by the gradation correction unit 16 of FIG. 3 corresponding to the colorimetric region ^{(~ c, ~ m, ~} y, ~ k) and. Then, considering the case where each element of the corrected CMYK gradation value is corrected by combining two fluctuation modes defined in advance, it is placed as shown in Equation (1) (the number of fluctuation modes is set to 2). This is for simplification of explanation, and the expansion of the number of modes in the following explanation is easily inferred).

Here, c, m, y, and k are CMYK gradation values before correction, c ₀ , m ₀ , y ₀ , and k ₀ are reference gradation characteristics, and c ₁ , m ₁ , y ₁ , and k ₁ are _first values. The fluctuation mode, c ₂ , m ₂ , y ₂ , k ₂ is the second fluctuation mode. In addition, θ ^j _i (i = {0, 1, 2}, j = {c, m, y, k}) is a mode parameter (the superscript is not an exponent but a simple identification subscript). In particular, the mode parameter θ ^j _i is a real scalar, and the reference tone characteristic and each variation mode are independent real vectors of the same dimension defined on the input range D of each c, m, y, k tone value. It is. Note that the mode parameter is mathematically the basis of the low-dimensional subspace of the vector space formed by the real valued function on the input range D. However, on implementation, the mode parameter is a curve implemented by an array or a combination of an array and an interpolation. You can think about it. The input range D is usually implemented by a real number in the [0, 1] section, an integer in the [0, 100] section, or the [0, 255] section.

At this time, the relationship of the primary term of Formula (1) is expressed by Formula (2) by matrix notation.

そして、式（３）の評価関数Φを最小化するΔθ_１，Δθ_２によって、印刷特性変動を近似する。 Here, the evaluation function Φ for the colorimetric value is defined by Expression (3).

Then, the printing characteristic variation is approximated by Δθ ₁ and Δθ ₂ that minimize the evaluation function Φ of Expression (3).

また、式（３）において、Ｅは期待値（本実施の形態の場合はサンプル平均）である。ここで、微小正定数λは変化量Δθ_ｉの大きさを抑えるためのペナルティであり、式（３）を最小化する解を安定化させる正則化加重として作用する。この微小正定数λの値としては、後述する拡張行列^〜Ｍの固有値との関係で適切な値を、予め実験に基づいて定めておく。このとき、拡張行列^〜Ｍを式（５）とすると、式（３）を最小化する変化量Δθ_１，Δθ_２は、式（６）により求められる。

However, in Equation (3), Lab _m is a Lab value corresponding to mesLab in FIG. Also, Labt: (c, m, y, k) → (L, a, b) is an estimated model of colorimetric values corresponding to the colorimetric value prediction model unit 21 of FIG. It is. Further, f = (f ₁ , f ₂ ,..., F _n ): Lab → R ⁿ is an appropriate differentiable function from the Lab space to the n-dimensional number space R ⁿ . J is a Jacobi matrix of these synthesis functions f _i (Lab _m (c, m, y, k), Lab _t (c, m, y, k)) expressed by the equation (4). .

In Equation (3), E is an expected value (sample average in this embodiment). Here, the small positive constant λ is a penalty for suppressing the magnitude of the change amount Δθ _i , and acts as a regularization weight that stabilizes the solution that minimizes the expression (3). As the value of the small positive constant λ, an appropriate value is determined in advance based on experiments in relation to the eigenvalues of an extended matrix ^to M described later. At this time, when the extended matrix ^~ M in the formula (5), the change amount [Delta] [theta] ₁ that minimizes Equation (3), [Delta] [theta] ₂ is obtained by equation (6).

In particular, in Equation (6), when sufficiently independent data is secured even when n is smaller than the number of input channels (four channels c, m, y, and k in this embodiment). Is an extended matrix ^to M (because it is a positive definite symmetric matrix), the total eigenvalue is larger than the small positive constant λ, and a sufficiently meaningful solution can be obtained from equation (6).

等を利用することができる。明度差のみを評価する２番目の例でイエローの明度差の分解能が不足する場合には、３番目の例の様に、イエローの識別性の高い色度ｂの差分値を評価に加えることによって精度が確保される。 As an example of the differentiable function f described above,

Etc. can be used. If the resolution of the lightness difference of yellow is insufficient in the second example in which only the lightness difference is evaluated, as in the third example, the difference value of chromaticity b with high yellow discrimination is added to the evaluation. Accuracy is ensured.

  As can be inferred from the above example, the observation value directly required for the estimation is the output of the differentiable function f, and Lab that is an intermediate input is not necessarily required. Therefore, even if the signal to be input to the color tone correction amount determination unit 19 in FIG. 3 is a part of the Lab value or RGB value, it is possible to sufficiently distinguish the C, M, Y, and K color monochromatic gradations Can be substituted if it is a characteristic signal (in the case of a monochrome sensor, it can be adjusted by the spectral characteristics of a filter provided in front of the sensor). In particular, when a monochrome line sensor is used as the scanner 27 in FIG. 3, the scanner correction unit 25 and the scanner color-Lab conversion units 15 and 18 in the above description are simplified accordingly.

特に、式（７）において、予めキャリブレーションによって基準階調特性ｃ_０，ｍ_０，ｙ_０，ｋ_０が十分に線形化されている仮定の下では、右辺の偏微分係数は省略可能である。フィードバックプロセスでは、変化量Δθ_１，Δθ_２を色調補正量決定部１９に保持しておき、累積した変化量Δθ_１，Δθ_２に基づいて、式（７）から各色のＴＲＣ補正量が決定される。 From the variations Δθ ₁ and Δθ _{2 of} the mode parameter θ obtained by Expression (6), gradation correction amounts Δc, Δm, Δy, and Δk that are gradation correction parameters are approximately obtained by Expression (7), respectively. .

In particular, in Equation (7), the partial differential coefficient on the right side can be omitted under the assumption that the reference gradation characteristics c ₀ , m ₀ , y ₀ , k ₀ are sufficiently linearized by calibration in advance. . In the feedback process, the change amounts Δθ ₁ and Δθ ₂ are held in the color tone correction amount determination unit 19, and the TRC correction amounts of the respective colors are determined from Expression (7) based on the accumulated change amounts Δθ ₁ and Δθ _2. The

(Description of configuration example of mode curve (basis function))
Next, a configuration example of the mode curve and a synthesis example thereof will be described with reference to FIGS. FIG. 9 is a diagram illustrating an example of the configuration of the mode curve. FIG. 10 is a diagram illustrating an example of mode curve synthesis. In the above-described equation (1), the case of two variation modes has been described as an example in order to avoid complication, but FIG. 9 illustrates selection of a mode curve (basis function) in the case of three variation modes. An example is shown. The horizontal axis is the input gradation value (halftone dot%), and the vertical axis is the gain normalized to 1 at the maximum value. Curve 47 corresponds to the first mode curve (M1), curve 48 corresponds to the second mode curve (M2), and curve 49 corresponds to the third mode curve (M3). As these mode curves, it is desirable to prepare a combination that can effectively approximate the gradation characteristic variation with as few modes as possible. However, it is not always necessary to stick to the main component.

  FIG. 10 shows an example of synthesizing the gradation correction curve using these mode curves M1 to M3. The horizontal axis in FIG. 10 represents the input gradation value x, and the vertical axis represents the output gradation value y. A gradation correction curve 44 shown in FIG. 10 represents a correction curve generated by combining the three mode curves M1 to M3 in FIG. 9 by the coefficients shown in FIG. In this way, a sufficiently smooth gradation correction curve 44 is generated by approximation by a linear sum of the three mode curves M1 to M3.

  In the case of the mode curves M1 to M3 of the present embodiment, the assumed gradation characteristic variation is approximately -10 to 10 with respect to the uniform reference gradation characteristic (y = x). The product sum is approximated as a sufficiently smooth curve without failure. For this reason, in the case of this embodiment, a common mode curve can be used for all the colors C, M, Y, and K, but different modes more specific to the gradation characteristic variation tendency for each color. By using a combination of curves, it is possible to approximate the gradation characteristic variation with a smaller number of modes. Approximating gradation characteristic fluctuations with a small number of modes makes it possible to estimate gradation characteristic fluctuations from a more biased small number of samples, but against subtle differences in gradation characteristic fluctuation trends due to differences in printing conditions, etc. The versatility will be impaired.

Next, a method for associating the output value p = ^t (p, q) of the preprocessing unit 64 with the reflectance of the colorimetric patch formed on the intermediate transfer belt 61 described above and the mode parameter θ will be described.

For a specific basic color, a case will be described in which the variation mode is assumed to be three modes and the mode parameters are set to θ = ^t (θ ₁ , θ ₂ , θ ₃ ) (similar arguments are C, M, Y, Applicable for each color of K). In actual gradation characteristic fluctuations, these three fluctuation modes do not necessarily change independently but change with a certain degree of dependency. In particular, in the range of minute fluctuations under the condition that the solid density is maintained at a constant value by process control, the gradation characteristic fluctuations focusing on the intermediate density characteristic are “dark dim”, “high contrast ⇔ low contrast” It is generally characterized by the following two characteristics. Therefore, assuming that the linear relationship Δθ = M _p · Δp holds between the output value p = ^t (p, q) and the change amount Δθ of the mode parameter θ, the parameter conversion matrix M is obtained by the least square method. _p is determined. That is, the equation (9) is obtained by obtaining the parameter transformation matrix M _p that minimizes the equation (8). In the equation (9), I is a unit matrix.

However, the minute positive constant λ _p is a regularization weight similar to the minute positive constant λ described above, and may be an appropriate constant that stabilizes the solution. The expected value E is estimated from the average of samples when there is sufficient color information in the input image. Each element of the parameter conversion matrix M _p determined in this way is stored together with image forming conditions such as a laser exposure intensity control value, a development bias setting value, and a toner charge amount, together with a set sheet identification number which is a printing condition. It is stored in the device 26. Each element of the parameter conversion matrix M _p stored in the storage device 26 is referred to as a default setting value of the parameter conversion matrix M _p in similar image forming conditions and printing conditions. The information stored in the storage device 26 is shared with the server 7 via the network 2 in FIG. Note that the above-described example is an example in which the gradation variation amount is not so large and can be approximated by linear approximation, but of course, it may be replaced by a machine learning method using a neural network or the like.

(Description of the flow of a series of processing performed by the image forming system)
Next, a flow of a series of processes performed by the image forming apparatus 8 will be described with reference to FIG. FIG. 11 is a flowchart showing a flow of a series of processes performed by the image forming apparatus 8. Note that the processing described in FIG. 11 is all performed by the color tone correction amount determination unit 19 (FIG. 3).

Here, a description will be given of a case where the default values of the parameter conversion matrix M _p in Expression (9) that associate the output values p of the colorimetric patches 80c to 82k on the intermediate transfer belt 61 with the change amount Δθ of the mode parameter θ are given. To do.

First, the reference gradation characteristics c ₀ , m ₀ , y ₀ , k ₀ of each color are set in the gradation correction unit 16 (step S200). Next, each element of the change amount Δθ = ^t (Δθ ₁ , Δθ ₂ ,...) Of the mode parameter θ and the average value Δθavg of the change amount Δθ are initialized to 0 (step S201). Furthermore, a data queue for accumulating a series of data described later is initialized (step S202).

Next, the color tone correction amount determination unit 19 acts as a first tone correction parameter estimation unit, and calculates a change amount Δp from the reference value of the output value p obtained from the colorimetric values of the colorimetric patches 80c to 82k. The change amount Δθ of the mode parameter θ of the target color (C, M, Y, K) is calculated based on the equation (9), and the calculated change amount is set to Δθ _p (step S203).

Further, the color tone correction amount determination unit 19 acts as second tone correction parameter estimation means, and uses the CMYK data (prnCMYK) of the input image (user image) and the colorimetric values of the user image output by the scanner 27. The change amount Δθ of the mode parameter θ is calculated from a certain output image read value (mesLab) and the predicted value (targetLab) by the colorimetric value prediction model unit 21 based on the equation (6). Then, the partial vector corresponding to the target color in the calculated change amount Δθ is set to Δθ _u (step S204).

  The color tone correction amount determination unit 19 evaluates whether the number of effective samples of measurement values extracted for calculation from within the page is sufficient depending on whether a predetermined threshold value (predetermined value) is exceeded (step S205). When the number of valid samples is sufficient (step S205; Yes), the process proceeds to step S206, and otherwise (step S205; No), the process proceeds to step S207.

The color correction amount determination unit 19 sets the flag flg to “1” indicating that the number of valid samples is sufficient, and sets the partial vector Δθ _u as the change amount Δθ of the mode parameter θ (step S206).

Tone correction amount determination unit 19, a flag flg, with number of effective samples is set to "0" indicating that it is not sufficient, the amount of change [Delta] [theta] _p of mode parameter theta _p and the variation [Delta] [theta] mode parameter theta (step S207).

The flag flg, the output value p, the partial vector Δθ _u and the change amount Δθ are added to the data queue (step S208).

  It is determined whether the process has been repeated for the predetermined number of pages (N) for each color of C, M, Y, and K (step S209). For example, N = 10 is set as the predetermined number of pages. When the process is repeated for the predetermined number of pages (step S209; Yes), the process proceeds to step S210, and otherwise (step S209; No), the process returns to step S203.

  When the processing for the predetermined number of pages is executed, the change amount Δθ of the mode parameter θ relating to the latest N CMYK colors on the data queue is accumulated in the average value Δθavg (step S210). By this averaging process, the estimation result of the change amount Δθ based on the measurement of the reflection characteristic of the color measurement patch and the estimation result of the change amount Δθ based on the color measurement result of the user image are smoothly matched.

  Using the accumulated change amount Δθ of the mode parameter θ, the gradation correction amounts Δc, Δm, Δy, Δk are calculated by the equation (7) (step S211). When the gradation correction amounts Δc, Δm, Δy, and Δk are provided as an 8-bit table, each gradation correction amount Δc, Δm, Δy, Δk has an input gradation n / 2.55% (n = 0, 1,... , 255).

  Next, the calculated gradation correction amounts Δc, Δm, Δy, and Δk are added to the respective reference gradation characteristics that are also given as an array of 256 elements, and set in the gradation correction unit 16 of FIG. (Step S212).

  It is determined whether the sum of the latest N flags flg on the data queue exceeds a threshold value (predetermined value) (step S213). When the threshold value is exceeded (step S213; Yes), the process proceeds to step S214, and otherwise (step S213; No), the process proceeds to step S215.

In step S213, stored on the data queue {p, [Delta] [theta] _u} from the N sets of data related to, the parameter conversion matrix M _p obtained by the formula (9), and updates the parameter transformation matrix M _p so far ( Step S214).

  The color tone correction amount determination unit 19 determines whether the print job is completed (step S215). When the print job is completed (step S215; Yes), the processing in FIG. 11 is terminated. In other cases (step S215; No), the process returns to step S200.

(Description of the flow of search processing of parameter conversion matrix M _p)
Next, the flow of the search process for the parameter conversion matrix M _p will be described with reference to FIG. Figure 12 is a flowchart showing the flow of search processing of parameter conversion matrix M _p. This search process is performed by the tone correction amount determination unit 19 (FIG. 3).

First, it is confirmed whether a default value of an effective parameter conversion matrix M _p is defined under the current printing conditions (step S220). When the default value of the parameter conversion matrix M _p is defined (step S220; Yes), the process proceeds to step S226, and otherwise (step S220; No), the process proceeds to step S221.

Tone correction amount determination unit 19 searches the storage device 26 to check whether the default value of the parameter transformation matrix M _p are defined (step S221). When the default value of the parameter conversion matrix M _p is defined (step S221; Yes), the process proceeds to step S227, and otherwise (step S221; No), the process proceeds to step S222.

The color tone correction amount determination unit 19 searches the server 7 to check whether a default value of the parameter conversion matrix M _p is defined (step S222). When the default value of the parameter conversion matrix M _p is defined (step S222; Yes), the process proceeds to step S227, and otherwise (step S222; No), the process proceeds to step S223.

  The color tone correction amount determination unit 19 causes the printer engine 4 to output a predetermined number (N) of color test charts (for example, IT8.7 / 3) instead of the user image (step S223).

The tone correction amount determination unit 19 executes the flowchart of FIG. 11 on the output result of the color test chart of the printer engine 4 to generate the parameter conversion matrix M _p (step S224).

The color tone correction amount determination unit 19 stores the generated parameter conversion matrix M _p in the storage device 26 (step S225).

  The image forming apparatus 8 executes a normal print job by executing the processing shown in the flowchart of FIG. 11 (step S226).

When the default value of the parameter conversion matrix M _p is defined in step S221 or step S222, the color tone correction amount determination unit 19 sets the parameter conversion matrix M _p (step S227). Thereafter, the process proceeds to step S226.

  As described above, according to the image forming apparatus 8 according to the present embodiment, on the paper (print medium) based on the original data 30 (input image) and the color profile registered in the server 7 (color profile registration unit). When the second measurement sensor 59 (second measurement unit) has obtained a number of measurement values exceeding a predetermined value from the output image 6 (color image) formed on the tone correction unit 16 The (gradation correction means) corrects the gradation characteristic variation using the gradation correction parameters estimated by the tone correction amount determination unit 19 (second gradation correction parameter estimation means). On the other hand, when the second measurement sensor 59 cannot obtain a number of measurement values exceeding the predetermined value from the output image 6, the gradation correction unit 16 uses the first measurement sensor 58 (first measurement sensor 58). Based on the reflection characteristics of the colorimetric patches 80c to 82k, which are toner images (intermediate images) temporarily formed on the intermediate transfer belt 61 by the exposure controller 65 (colorimetric patch forming means) measured by The tone characteristic variation is corrected using the tone correction amounts Δc, Δm, Δy, Δk (tone correction parameters) estimated by the color tone correction amount determining unit 19 (first tone correction parameter estimating means). Therefore, when there is not enough color information in the document data 30, the gradation correction amounts Δc, Δm, Δy, associated with the reflection characteristics of the colorimetric patches 80c to 82k by the first gradation correction parameter estimation means. The gradation characteristic variation can be corrected accurately using Δk. That is, it is possible to improve the stability of reproduced colors in a print job regardless of image forming conditions and printing conditions. Further, for example, even when only the intermediate gradation information related to a specific primary color is insufficient, the gradation characteristics using the gradation correction parameter estimated by the first gradation correction parameter estimation means for only that primary color. Variation correction is performed, and for the remaining primary colors, gradation characteristic variation correction can be performed using the estimation result obtained by the second gradation correction parameter estimation means.

  Further, according to the image forming apparatus 8 according to the present embodiment, when the second measurement sensor 59 has obtained a number of measurement values that exceeds a predetermined value, the color tone correction amount determination unit 19 (the first correction value determination unit 19) The gradation correction parameter estimation means) is the reflection characteristics of the colorimetric patches 80c to 82k measured by the first measurement sensor 58 and the level estimated by the color tone correction amount determination unit 19 (second gradation correction parameter estimation means). The correspondence relationship with the tone correction parameter is learned. When the second measurement sensor 59 cannot obtain a number of measurement values exceeding a predetermined value from the output image 6 (color image), the gradation correction unit 16 (gradation correction means) The gradation characteristic variation is corrected using the correspondence relationship. Accordingly, since the relationship between the colorimetric values of the colorimetric patches 80c to 82k on the intermediate transfer belt 61 and the actual gradation variation can be learned from the original data 30 (input image) including sufficient color information, it is sufficient in advance. By learning with a color test chart output having accurate color information, the relationship between the variation in the component of the image forming apparatus 8 and the intermediate patch detection signal caused by the compatibility with the paper as the printing medium and the actual gradation variation Can cope with the difference.

  Then, according to the image forming apparatus 8 according to the present embodiment, the reflection characteristics of the colorimetric patch, the tone correction parameter estimated by the color tone correction amount determination unit 19 (second tone correction parameter estimation means), Since the learning result of the correspondence relationship is stored in the storage device 26 (storage means) and reused, the second and subsequent learning can be omitted. In addition, when the image forming apparatus 8 includes a communication unit, the learning result is stored in the server 7 connected to the image forming apparatus 8 via the communication unit, so that information can be shared between systems at different locations. Therefore, it is possible to share the initial settings and to detect an abnormality in the system state.

  Furthermore, according to the image forming apparatus 8 according to the present embodiment, the colorimetric patches 80c to 82k formed on the intermediate transfer belt 61 have a high number of lines whose density changes sensitively to changes in image forming conditions. By using the halftone patch, the S / N of the density change detection of the colorimetric patch is improved. In particular, the intermediate density change and the contrast change can be associated with the change mode of the gradation characteristic by using two steps of the high-sensitivity intermediate gradation patch.

  Further, according to the image forming apparatus 8 according to the present embodiment, the number of control parameters can be reduced by approximating the change in the gradation characteristics with a small number of basis functions, and the gradation characteristics due to overfit can be reduced. Rippling can be prevented. In addition, since the number of colorimetric patches 80c to 82k formed on the intermediate transfer belt 61 can be reduced, wasteful toner consumption can be reduced. In particular, if the solid density is handled separately, the gradation characteristic shape can be roughly characterized by three degrees of freedom of intermediate density, highlight skip, and contrast.

  Further, according to the image forming apparatus 8 according to the present embodiment, since the number of gradation correction parameters is three or less per basic color of the image forming apparatus 8, typical gradation characteristic changes are accurately approximated. be able to. Further, if the correction accuracy requirement is not so high, or if the correlation between the three gradation correction parameters is high, the gradation correction parameters can be reduced.

  In addition, according to the image forming apparatus 8 according to the present embodiment, at least two or more intermediate gradation patches are generated, and the tone correction amount determination unit 19 (first gradation correction parameter estimation unit) The reflection characteristics of the colorimetric patches 80c to 82k are obtained by using the difference value of the reflection characteristics of the two different intermediate gradation colorimetric patches 80c to 82k obtained by the first measurement sensor 58 (first measurement means). Corresponds to gradation correction parameters. Therefore, after printing a color image without intermediate gradation information for a long period of time and then shifting to printing of a color image rich in intermediate gradation information, the printer engine 4 at the start of printing output after the transition. It is possible to avoid fluctuations in the estimated value of the gradation correction parameter due to the irregular output image abnormal value caused by the instability of the state.

  Further, according to the image forming apparatus 8 according to the present embodiment, the color tone correction amount determination unit 19 includes the tone correction parameter estimated based on the first tone correction parameter estimation unit, and the second tone correction. By interpolating the gradation correction parameter estimated based on the parameter estimation means, the gradation correction parameter is switched stepwise. Therefore, the estimation results of the change amount Δθ of the mode parameter θ obtained by the first gradation correction parameter estimation unit and the second gradation correction parameter estimation unit can be smoothly matched.

  In addition, according to the image forming unit 32 according to the present embodiment, the output image 6 (formed on the paper (printing medium) based on the document data 30 (input image) having a printable format read from the outside. When the second measurement sensor 59 (second measurement unit) has obtained a number of measurement values exceeding a predetermined value from the color image), the gradation correction unit 16 (gradation correction unit) The tone characteristic variation is corrected using the tone correction parameter estimated by the color tone correction amount determination unit 19 (second tone correction parameter estimation means). On the other hand, when the second measurement sensor 59 cannot obtain a number of measurement values exceeding the predetermined value from the output image 6, the gradation correction unit 16 uses the first measurement sensor 58 (first measurement sensor 58). Based on the reflection characteristics of the colorimetric patches 80c to 82k, which are toner images (intermediate images) temporarily formed on the intermediate transfer belt 61 by the exposure controller 65 (colorimetric patch forming means) measured by The tone characteristic variation is corrected using the tone correction amounts Δc, Δm, Δy, Δk (tone correction parameters) estimated by the color tone correction amount determining unit 19 (first tone correction parameter estimating means). Therefore, when there is not enough color information in the document data 30, the gradation correction amounts Δc, Δm, Δy, associated with the reflection characteristics of the colorimetric patches 80c to 82k by the first gradation correction parameter estimation means. The gradation characteristic variation can be corrected accurately using Δk. That is, it is possible to improve the stability of reproduced colors in a print job regardless of image forming conditions and printing conditions.

  Although the present embodiment has been described above, the above-described embodiment is an example of a preferred embodiment of the present invention, but the specific configuration, processing content, and the like are the same as those described in the embodiment. The present invention is not limited, and various modifications can be made without departing from the scope of the present invention.

  For example, instead of causing the CPU 100 to operate according to the program P1, the image forming apparatus 8 implements a hardware by installing a dedicated ASIC (Application Specific Integrated Circuit) having the same calculation function and control function as those executed by the program P1. May be operated automatically.

  6 ... Output image (color image), 7 ... Server (color profile registration unit), 8 ... Image forming apparatus, 16 ... Tone correction unit (tone correction unit), 19 ... Tone correction amount determination unit (first floor) Tone correction parameter estimation means, second gradation correction parameter estimation means, 26 ... storage device (storage means), 30 ... original data (input image), 32 ... image forming unit, 58 ... first measurement sensor (first) 1, 59 ... second measurement sensor (second measurement means), 65 ... exposure control device (colorimetric patch forming means), Δc, Δm, Δy, Δk ... gradation correction amount (tone correction) Parameter).

JP 2012-70360 A Japanese Patent No. 5150096 US Pat. No. 5,749,020 Japanese Patent No. 5341940 JP 2003-189103 A

Claims (10)

An input image composed of a plurality of basic colors is developed into a printable format based on a color profile registered in a color profile registration unit, and a color image is formed on a print medium by mixing the basic colors. In the image forming apparatus to
Colorimetric patch forming means for forming a plurality of colorimetric patches combining gradation values of different basic colors when temporarily forming an intermediate image before transferring the input image onto the print medium When,
First measuring means for measuring reflection characteristics of the colorimetric patch;
First gradation correction parameter estimating means for associating the reflection characteristic of the colorimetric patch with the gradation correction parameter for correcting the gradation characteristic variation of the image forming apparatus based on the reflection characteristic of the colorimetric patch;
Second measuring means for measuring at least a part of reflection characteristics of the color image formed by transferring the input image onto the print medium;
Second gradation correction parameter estimation means for estimating the gradation correction parameter based on the input image and the reflection characteristics of at least a part of the color image;
The gradation correction parameter estimated by the second gradation correction parameter estimation means on the condition that the second measurement means has obtained a number of measurement values exceeding a predetermined value from the color image. The first characteristic is corrected on the condition that the second measurement means cannot obtain the number of measurement values exceeding the predetermined value from the color image. Gradation correction means for correcting the gradation characteristic variation using the gradation correction parameter estimated by the gradation correction parameter estimation means;
An image forming apparatus comprising: The first gradation correction parameter estimating means is configured to obtain the colorimetric patch from the color image on condition that the second measuring means has obtained a number of the measured values exceeding the predetermined value. And learning the correspondence between the reflection characteristics of the image and the gradation correction parameter estimated by the second gradation correction parameter estimation means,
On the condition that the number of measurement values exceeding the predetermined value cannot be obtained from the color image by the second measurement unit, the gradation correction unit uses the correspondence relationship to generate the gradation. The image forming apparatus according to claim 1, wherein the characteristic variation is corrected. The correspondence relationship, the print medium to which the color image is transferred, the image forming condition and the printing condition when the color image is formed, and the stored information for reusing the stored information by the gradation correcting unit Having means,
The image forming apparatus according to claim 2. The colorimetric patch forming unit generates the colorimetric patch that combines at least two or more intermediate gradation values with an area gradation pattern different from that of the color image. 4. The image forming apparatus according to any one of items 3. The first gradation correction parameter estimation means and the second gradation correction parameter estimation means are configured to change the gradation characteristic variation according to a change mode serving as a basis for approximating at least one gradation characteristic variation prepared in advance. 5. The image forming apparatus according to claim 1, wherein a coefficient when the correction amount is approximated by a linear sum of the change modes is used as the gradation correction parameter. 6. The image forming apparatus according to claim 1, wherein the number of gradation correction parameters is three or less per basic color of the image forming apparatus. The colorimetric patch forming unit generates the colorimetric patch by combining at least two intermediate gradation values, and the first tone correction parameter estimating unit is obtained by the first measuring unit. The difference value of the reflection characteristic of the colorimetric patch of two different intermediate gradations is used when associating the reflection characteristic of the colorimetric patch with the gradation correction parameter. The image forming apparatus according to any one of the above. The first gradation correction parameter estimation means and the second gradation correction parameter estimation means include the gradation correction parameter estimated based on the first gradation correction parameter estimation means, and the second floor. The image forming apparatus according to claim 1, wherein the gradation correction parameter estimated based on the tone correction parameter estimation unit is switched in a stepwise manner by interpolating. In an image forming unit that reads an input image having a printable format composed of a plurality of basic colors and forms a color image on a print medium based on the input image,
Colorimetric patch forming means for forming a plurality of colorimetric patches combining gradation values of different basic colors when temporarily forming an intermediate image before transferring the input image onto the print medium When,
First measuring means for measuring reflection characteristics of the colorimetric patch;
First gradation correction parameter estimating means for associating the reflection characteristic of the colorimetric patch with the gradation correction parameter for correcting the gradation characteristic variation of the image forming unit based on the reflection characteristic of the colorimetric patch;
Second measuring means for measuring at least a part of reflection characteristics of the color image formed by transferring the input image onto the print medium;
Second gradation correction parameter estimation means for estimating the gradation correction parameter based on the input image and the reflection characteristics of at least a part of the color image;
The gradation correction parameter estimated by the second gradation correction parameter estimation means on the condition that the second measurement means has obtained a number of measurement values exceeding a predetermined value from the color image. The first characteristic is corrected on the condition that the second measurement means cannot obtain the number of measurement values exceeding the predetermined value from the color image. Gradation correction means for correcting the gradation characteristic variation using the gradation correction parameter estimated by the gradation correction parameter estimation means;
An image forming unit comprising: Against the computer,
An input image composed of a plurality of basic colors is developed into a printable format based on a color profile registered in a color profile registration unit, and a color image is formed on a print medium by mixing the basic colors. In the image forming apparatus to
A colorimetric patch forming step of forming a plurality of colorimetric patches combining tone values of the different basic colors when temporarily forming an intermediate image before transferring the input image onto the print medium; When,
A first measurement step of measuring the reflection characteristic of the colorimetric patch;
A first gradation correction parameter estimating step for associating the reflection characteristic of the colorimetric patch with the gradation correction parameter for correcting the gradation characteristic variation of the image forming apparatus based on the reflection characteristic of the colorimetric patch;
A second measuring step of measuring at least a part of the reflection characteristics of the color image formed by transferring the input image onto the print medium;
A second gradation correction parameter estimation step for estimating the gradation correction parameter based on the input image and the reflection characteristics of at least a part of the color image;
The gradation correction parameter estimated in the second gradation correction parameter estimation step on the condition that the second measurement step has obtained a number of measurement values exceeding a predetermined value from the color image. The first characteristic is corrected on the condition that the measurement value exceeding the predetermined value cannot be obtained from the color image by the second measurement step from the color image. A gradation correction step of correcting the gradation characteristic variation using the gradation correction parameter estimated in the gradation correction parameter estimation step of
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