JP2020168779A - Image formation apparatus, control method of the same and program - Google Patents

Abstract

To provide a technique which prevents the occurrence of the pseudo contour in an output image after execution of the color mixture calibration.SOLUTION: A CPU 103 of a MFP 101 controls a printer 115 to form a chart used in color mixture calibration processing for generating color mixture correction data on a sheet. The chart has a plurality of patches formed on the sheet by using a plurality of types of toner. The CPU uses a scanner 119 or sensor 127 to extract a first signal value (brightness value) from a measurement value obtained by performing measurement on the chart and determine whether or not the discontinuity of the gradation occurs in the extracted first signal value. The CPU 103 controls processing for setting (registering) the target value used in the generation of the color mixture correction data on the basis of the measurement value in accordance with the determination result.SELECTED DRAWING: Figure 1

Description

The present invention relates to an image forming apparatus, a control method thereof, and a program.

In an electrophotographic image forming apparatus, an image is generally formed by using a plurality of types of coloring materials (toners) such as cyan (C), magenta (M), yellow (Y), and black (K). In such an image forming apparatus, a single color calibration technique is installed in order to generate a parameter for gradation correction that corrects the gradation characteristic of each color in the output image. In monochromatic calibration, a chart containing a patch image for calibration is printed, and the measured value obtained by scanning the chart with a scanner or a sensor is compared with the target value to generate a correction parameter (one-dimensional LUT). To do.

Further, in the electrophotographic image forming apparatus, even if the gradation characteristic of a single color is corrected by using a one-dimensional LUT, it is desired when a mixed color (for example, gray) is expressed by using toners of a plurality of colors. Color reproduction characteristics may not be obtained. Color mixing calibration is known as a technique for correcting such color reproduction characteristics of color mixing (for example, Patent Document 1). In the color mixing calibration, the color mixing chart is printed using the chart data to which the monochromatic gradation correction is applied, and the measurement value obtained by reading the color mixing chart with a scanner or a sensor is compared with the target value and the correction parameter (correction parameter ( Generates a 4-dimensional LUT). Color mixing calibration makes it possible to correct color mixing characteristics that are not sufficiently corrected by single color calibration.

Japanese Unexamined Patent Publication No. 2011-254350

The target value for the color mixing calibration can be set, for example, based on the measurement result of the color mixing chart printed at an arbitrary timing when the image forming apparatus is installed. However, the state of the image forming apparatus (printer) changes with the passage of time from the time of its installation, and the color reproduction characteristics of the printer may also change. As a result, if the target value is set (registered) in a state where the gradation of the color mixing chart output from the printer is deteriorated, the color is output when the color correction of the image data is performed after the color mixing calibration is executed. Pseudo contours may occur in the image.

The present invention has been made in view of the above-mentioned problems. An object of the present invention is to provide a technique for preventing pseudo contours from being generated in an output image after performing color mixing calibration.

The image forming apparatus according to one aspect of the present invention is an image forming apparatus having a generating means for generating color mixing correction data for bringing the color of a mixed color image formed by the image forming means closer to a target value, and the image forming apparatus. A measuring means for obtaining a measured value by measuring a chart having a plurality of patches formed on a sheet using a plurality of types of coloring materials by means, and an extracting means for extracting a first signal value from the measured value. A determination means for determining whether or not the first signal value extracted by the extraction means has a gradation discontinuity, and the target value used by the generation means according to the determination result by the determination means. It is characterized by including a control means for controlling a process set based on the measured value.

According to the present invention, it is possible to prevent the occurrence of pseudo contours in the output image after the execution of the color mixing calibration.

The block diagram which shows the configuration example of the system including the image forming apparatus. A flowchart showing the procedure of image processing by the image processing unit. A flowchart showing the procedure of the single color calibration process. A flowchart showing the procedure of the color mixing calibration process. The figure which shows the example of the chart used for the calibration process. A flowchart showing the procedure of the target value registration process (comparative example). A flowchart showing a procedure of a target value registration process. The figure which shows the example of the UI used in the registration process of a target value. The figure which shows the example of 3D-LUT used for the color mixing calibration process. The flowchart which shows the procedure of the gradation determination processing (S705). The flowchart which shows the procedure of the gradation correction processing (S707). The figure which shows the example of the lightness value extracted from the lightness data and its inclination. The figure which shows the output example of a gradation image. The flowchart which shows the procedure of the gradation determination processing (S705).

Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. The following embodiments do not limit the invention according to the claims. Although a plurality of features are described in the embodiment, not all of the plurality of features are essential to the invention, and the plurality of features may be arbitrarily combined. Further, in the attached drawings, the same or similar configurations are designated by the same reference numbers, and duplicate description is omitted.

[First Embodiment]
In the first embodiment, the first signal value (brightness value) indicating the color component having a high correlation with the high frequency component of the image is extracted from the measured value obtained by the measurement on the chart for color mixing calibration, and the extracted brightness value. It is determined whether or not there is a gradation discontinuity in. Further, according to the determination result, the process of setting the target value for the color mixing calibration based on the measured value is controlled.

<System configuration>
FIG. 1 is a diagram showing a configuration example of a system according to the present embodiment. The MFP (Multi Function Printer) 101 uses a plurality of types (4 types in this example) of coloring materials (toners) of cyan (C), magenta (M), yellow (Y), and black (K). An image forming apparatus (image processing apparatus) that forms an image on a recording material such as a sheet (paper). The MFP 101 has not only a printing function but also a scanning function of scanning an image of a document to generate a scanned image. The MFP 101 is communicably connected to an external device (other network compatible device) such as the PC 124 and the measuring instrument 128 via a network 123 such as a LAN or the Internet.

The PC 124 has a printer driver 125 for transmitting print data to the MFP 101. The measuring instrument 128 is an external measuring device of the MFP 101 located on the network 123 (or connected to the PC 124). The measuring instrument 128 has the same measuring function as the measuring unit 126 described later.

The MFP 101 includes a controller 102. The MFP 101 includes a printer (printing unit) 115, a display device (display unit) 118, a scanner (reading unit) 119, an input device 120, a storage device 121, and a network interface (I / F) 122 connected to the controller 102. Further prepare. The controller 102 includes a CPU 103. The network I / F 122 can communicate with an external device via the network 123 and receive data such as print data from the PC 124. The controller 102 includes a CPU 103, a renderer 112, and an image processing unit 114. The controller 102 further includes a RAM and a ROM (not shown). The CPU 103 controls the entire MFP 101 by reading the control program stored in the ROM in the storage device 121 or the controller 102 into the RAM and executing the control program.

The controller 102 performs the following image processing. The interpreter 104 of the CPU 103 interprets the PDL (page description language) portion of the print data received via the network 123 and the network I / F 122, and generates the intermediate language data 105. The intermediate language data 105 is input to the CMS (Color Management System) 106 or 109 according to the setting by the printer driver 125.

The CMS 106 performs color conversion of the intermediate language data 105 using the source profile 107 and the destination profile 108 to generate the intermediate language data (after CMS) 111. The source profile 107 refers to a device-dependent color space such as RGB or CMYK color space as L * a * b * (hereinafter referred to as “Lab”) or XYZ color defined by the CIE (Commission Internationale de l'Eclairage). It is a profile for converting to a device-independent color space such as space. Like the Lab color space, the XYZ color space is a device-independent color space, and colors are expressed by three types of stimulus values. The destination profile 108 is a profile for converting a device-independent color space into a device-dependent CMYK color space.

On the other hand, the CMS 109 uses the device link profile 110 to perform color conversion of the intermediate language data 105 to generate the intermediate language data (after CMS) 111. The device link profile 110 is a profile for directly converting a device-dependent color space such as RGB or a CMYK color space into a device (printer 115) -dependent CMYK color space.

In the present embodiment, the CMSs (106 and 109) are divided according to the types of profiles (107, 108 and 110), but one CMS may handle a plurality of types of profiles. Further, the type of profile is not limited to the example given in this embodiment, and any type of profile may be used as long as a device (printer 115) -dependent CMYK color space is used.

The renderer 112 generates a raster image 113 from the intermediate language data 111 generated by the CPU 103. The image processing unit 114 receives the raster image 113 or the scanned image generated by the scanner 119 as an input image, and performs image processing described later on the received input image.

The printer 115 forms an image on a sheet using C, M, Y, and K toners. The printer 115 is controlled by the controller 102. The printer 115 includes a paper feeding unit 116, a paper ejection unit 117, a measuring unit 126, and a CPU 129. The CPU 129 controls each device in the printer 115 under the control of the controller 102. The paper feed unit 116 feeds the sheet. The paper ejection unit 117 ejects the sheet on which the image is formed.

The measuring unit 126 includes a sensor 127 for acquiring the spectral reflectance and the value of the device-independent color space such as the Lab or XYZ color space as measured values, and has a measuring function using the sensor 127. The measuring unit 126 reads the image formed on the sheet by the printer 115 by the sensor 127, and transmits the information (numerical information and the like) obtained by the reading to the controller 102. The controller 102 uses the information received from the measuring unit 126 for color correction of single color or mixed color.

The scanner 119 includes an ADF (auto document feeder), optically reads an image of a document, generates a scanned image, and outputs it as image data. The scanner 119 is used for various processes such as a scan process, a copy process, and an image transmission process. The display device 118 is a UI (user interface) for displaying instructions to the user or the status of the MFP 101. The input device 120 is a UI for receiving input from the user. Some input devices are composed of a touch panel and are integrated with the display device 118. The storage device 121 is a storage device such as an HDD or SSD that stores various data such as data processed by the controller 102 or data received by the controller 102.

<Image processing for input image>
Next, with reference to FIG. 2, the image processing on the input image executed by the image processing unit 114 will be described. FIG. 2 is a flowchart showing a procedure of image processing performed on the raster image 113 or the scanned image which is an input image. The processing of each step in FIG. 2 is realized by processing by an ASIC (Application Specific Integrated Circuit) (not shown) provided in the image processing unit 114.

In S201, the image processing unit 114 determines whether the input image to be processed is a scanned image generated by the scanner 119 or a raster image 113 (bitmap-expanded) generated by the renderer 112. .. When the input image to be processed is the raster image 113, the input image is a CMYK image represented by the CMS in a CMYK color space depending on the device (printer 115). In this case, the image processing unit 114 proceeds from S201 to S207.

On the other hand, when the input image to be processed is a scanned image, the input image is an RGB image. In this case, the image processing unit 114 proceeds from S201 to S202. In S202, the image processing unit 114 performs character determination processing on the scanned image and generates character determination data. Specifically, the image processing unit 114 generates character determination data by detecting an edge or the like of a scanned image. Further, in S203, the image processing unit 114 performs color conversion processing on the scanned image (RGB image) using the color conversion parameters stored in the storage device 121 to generate a common RGB image. The common RGB image is defined in a device-independent RGB color space, and can be converted into a device-independent color space such as the Lab color space by calculation. The character determination process of S202 and the color conversion process of S203 may be executed in parallel.

After that, in S204, the image processing unit 114 performs a filter process on the scanned image (common RGB image) after the color conversion process by using the character determination data generated in S202. Here, using the character determination data, different filter processing is performed on the character portion and the other portion of the scanned image. When the filter processing is completed, in S205, the image processing unit 114 removes the ground color component from the image by performing the background skipping process on the image after the filter processing. Further, in S206, the image processing unit 114 generates a CMYK image by performing a color conversion process on the image to which the background skipping process has been performed, and proceeds to the process to S207.

In S207, the image processing unit 114 corrects the color mixture characteristic (color reproduction characteristic of the color mixture) with respect to the CMYK image by using a four-dimensional LUT (look-up table) (hereinafter referred to as "4D-LUT"). Performs processing (color correction of mixed colors). The 4D-LUT used in S207 is a four-dimensional LUT that converts a combination of C, M, Y, and K signal values (CMYK signal values) into different CMYK signal values, and is a color mixing described later with reference to FIG. Generated by the calibration process. In the present embodiment, the 4D-LUT is an example of color mixing correction data for bringing the color of the color mixing image formed by the image forming means closer to the target value (target color). By the correction process of S207, it is possible to correct the color mixing characteristic of an image formed by using toners of a plurality of colors (C, M, Y, K).

Next, in S208, the image processing unit 114 performs correction processing using a one-dimensional LUT (hereinafter, referred to as "1D-LUT") on the CMYK image after the correction processing of S207. Gradation correction (monochromatic color correction) of each color of C, M, Y, and K is performed. The 1D-LUT used in S208 is generated by a monochromatic calibration process described later with reference to FIG. In the present embodiment, the 1D-LUT is an example of monochromatic correction data for bringing the color of a monochromatic image closer to a target value.

Finally, in S209, the image processing unit 114 performs halftone processing such as screen processing or error diffusion processing on the corrected image of S208 to generate a CMYK image (binary value), and ends the processing. To do. The image processing unit 114 transmits the image data of the generated CMYK image to the printer 115 for printing based on the image data.

<Single color calibration>
FIG. 3 is a flowchart showing a procedure of a single color calibration process (single color calibration) according to the present embodiment. The processing of each step in FIG. 3 is realized by the processing by the CPU 103. The monochromatic calibration is a process for generating a 1D-LUT for monochromatic gradation correction.

In FIG. 3, the processes S301 to S304 are processes for correcting the maximum density of a single color. Further, the processes of S305 to S308 are processes for correcting the gradation of a single color.

First, in S301, the CPU 103 acquires the first chart data stored in the storage device 121. The first chart data is used to correct the maximum density of a single color, and is composed of signal values that give the maximum density of a single color for each of C, M, Y, and K. FIG. 5 is a diagram showing an example of a chart printed using the chart data (first to third chart data) stored in the storage device 121. Chart 500 is an example of a chart corresponding to the first chart data. The chart 500 is composed of images having the maximum density of each color of C, M, Y, and K.

Next, in S302, the CPU 103 causes the image processing unit 114 to perform image processing on the first chart data, and based on the first chart data after the image processing, causes the printer 115 to display a chart (chart 500 in FIG. 5). Output. In S302, the image processing unit 114 performs only halftone processing (S209), and does not perform correction processing (S207) using 4D-LUT and correction processing (S208) using 1D-LUT.

After that, in S303, the CPU 103 measures the output chart by using the scanner 119 or the sensor 127 in the measuring unit 126 to obtain the measured values for the image densities of the C, M, Y, and K colors. get. When the measurement is completed, in S304, the CPU 103 corrects the maximum density of each color by using the measured value obtained in S303 and the target value of the maximum density. In this correction, the device setting value of the printer 115 is adjusted so that the maximum density matches the target value. As a result, the printer 115 can output the density corresponding to the target value as the maximum density for each of the colors C, M, Y, and K.

Next, in S305, the CPU 103 acquires the second chart data stored in the storage device 121. The second chart data is used for monochromatic gradation correction, and is composed of signal values of monochromatic gradation data for each of C, M, Y, and K. Chart 510 in FIG. 5 is an example of a chart corresponding to the second chart data. The chart 510 is composed of patches 511 to 514 having the maximum density for each color of C, M, Y, and K, and a plurality of patches having different densities arranged in a row on the right side of patches 511 to 514.

Next, in S306, the CPU 103 causes the image processing unit 114 to perform image processing on the second chart data, and based on the second chart data after the image processing, causes the printer 115 to display a chart (chart 510 in FIG. 5). Output. At this time, as a result of the correction of S304, the printer 115 is in a state where it is possible to output the density corresponding to the target value as the maximum density for each of the colors C, M, Y, and K. In S302, the image processing unit 114 performs only halftone processing (S209), and does not perform correction processing (S207) using 4D-LUT and correction processing (S208) using 1D-LUT.

After that, in S307, the CPU 103 measures the output chart using the scanner 119 or the sensor 127 in the measuring unit 126, so that the densities of the plurality of patches are each for each of the colors C, M, Y, and K. Get the measured value of. When the measurement is completed, in S308, the CPU 103 generates a 1D-LUT for gradation correction of a single color by using the measured value obtained in S307 and the target value for gradation correction for each color.

In this way, the CPU 103 generates a 1D-LUT for gradation correction of a single color for each of the colors C, M, Y, and K. The CPU 103 stores the generated 1D-LUT in the storage device 121, and ends the monochromatic calibration.

<Mixed color calibration>
FIG. 4 is a flowchart showing a procedure of the color mixing calibration process (color mixing calibration) according to the present embodiment. The processing of each step in FIG. 4 is realized by the processing by the CPU 103. In the color mixing calibration, a color mixing chart formed by the printer 115 is measured, and a correction parameter (4D-LUT) for color mixing color correction to which the difference between the measured value obtained by the measurement and the target value is applied is applied. This is the process to generate. The color mixing calibration is performed to correct the color mixing characteristics of the device (printer 115) after the single color calibration is executed. For this reason, it is desirable to execute the color mixing calibration immediately after the above-mentioned single color calibration is completed (for example, following the single color calibration).

First, in S401, the CPU 103 acquires the third chart data stored in the storage device 121. The third chart data is used to correct the color mixing characteristics of the device, and is composed of mixed color signal values which are a combination of signal values of a plurality of colors (C, M, Y, K). Chart 520 of FIG. 5 is an example of a color mixing chart corresponding to the third chart data. Each patch (including patch 521) on the chart 520 is composed of a mixed color in which C, M, Y, and K are combined, and is formed (printed) using toners of a plurality of colors. The chart 520 includes a plurality of patches having different colors, and the plurality of patches are formed by using signal values corresponding to different Lab values.

Next, in S402, the CPU 103 causes the image processing unit 114 to perform image processing on the third chart data, and based on the third chart data after the image processing, causes the printer 115 to perform a color mixing chart (chart 520 in FIG. 5). Is output. As described above, the color mixing calibration is a process for correcting the color mixing characteristics of the device after executing the single color calibration. Therefore, the image processing unit 114 performs a correction process (S208) using the 1D-LUT generated by the single color calibration process.

After that, in S403, the CPU 103 measures the output color mixing chart using the scanner 119 or the sensor 127 in the measuring unit 126. As a result, the CPU 103 acquires a measured value indicating the color mixing characteristic of the device (printer 115) after executing the monochromatic calibration. Specifically, the CPU 103 acquires a measured value (Lab value in this embodiment) in a color space that does not depend on the device. When the measurement is performed using the scanner 119, the CPU 103 converts the RGB value obtained by the scanner 119 into a Lab value by using a color conversion parameter (not shown) such as 3D-LUT.

When the measurement of S403 is completed, the CPU 103 advances the process to S404. In S404, the CPU 103 acquires a three-dimensional LUT (hereinafter referred to as "3D-LUT") stored in the storage device 121 for conversion from the Lab color space to the CMY color space (Lab → CMY). Then, the acquired 3D-LUT is corrected. This 3D-LUT is a three-dimensional LUT that outputs a CMY value corresponding to the input Lab value.

In the correction process, the CPU 103 obtains the difference between the measured value obtained in S403 and the target value of the Lab value, and applies the obtained difference to the 3D-LUT to generate the corrected 3D-LUT. Specifically, the CPU 103 corrects by adding a difference to the Lab value on the input side of the 3D-LUT and performing a complementary operation on the obtained Lab value using the 3D-LUT before correction. Generate the later 3D-LUT.

The target value used in S404 may be a predetermined value stored in the storage device 121. Alternatively, the target value may be a value registered by outputting and measuring a color mixing chart (chart 520 of FIG. 5) for correcting the color mixing characteristic at an arbitrary timing. The registration of the target value will be described later with reference to FIG. 7.

Finally, in S405, the CPU 103 generates a 4D-LUT for correcting the color mixing characteristic. Specifically, the CPU 103 acquires a 3D-LUT for conversion from the CMY color space to the Lab color space (CMY → Lab) stored in the storage device 121. This 3D-LUT is a three-dimensional LUT that outputs a Lab value corresponding to the input CMY value. Further, the CPU 103 is for correcting the CMY value (CMY → CMY) by the calculation using the acquired 3D-LUT of CMY → Lab and the corrected 3D-LUT (3D-LUT of Lab → CMY) in S404. 3D-LUT is generated. This 3D-LUT is a three-dimensional LUT that outputs a corrected CMY value corresponding to the input CMY value. The CPU 103 generates a 4D-LUT from the 3D-LUT generated in this way so that the input value and the output value for the K color are the same.

The CPU 103 stores the 4D-LUT generated by the above process in the storage device 121, and ends the color mixing calibration.

<Comparison example>
FIG. 6 is a flowchart showing the procedure of the process of registering the target value used for the above-mentioned color mixing calibration, and shows a comparative example with respect to the registration process (FIG. 7) of the present embodiment. The processing of each step in FIG. 6 is realized by the processing by the CPU 103.

In this comparative example, the treatments of S601 to S603 are the same as the treatments of S401 to S403, respectively. In S603, when the measured value (Lab value) is obtained by the same measurement as in S403, the CPU 103 proceeds to S604. In S604, the CPU 103 sets the measured value obtained in S603 as a target value of the Lab value, saves (registers) it, and ends the process.

In the registration process of the above-mentioned comparative example, the measured value of the color mixing characteristic of the printer 115 is registered as a target value. This registration process is executed at an arbitrary timing such as when the MFP 101 is installed. By executing the registration process when the MFP 101 is installed, it is possible to execute the color mixing calibration according to the color mixing characteristics of the printer 115 when the MFP 101 is installed.

Here, FIG. 9 is a diagram showing an example of a 3D-LUT (used in S404) used for color mixing calibration. FIG. 9 illustrates the input / output relationship in the 3D-LUT, and defines the output signal value for the three-dimensional axes of L *, a *, and b * corresponding to the input signal (Lab value). The input signal values to be input are shown by solid lines at regular intervals. The points where the solid lines intersect are called grid points. Each grid point indicates a 3D-LUT input signal value (Lab value), and an output signal value (CMY value) corresponding to the input signal value represented by the grid point is defined for each grid point.

The plurality of points indicated by black circles (reference numeral 901) in FIG. 9 indicate an arrangement example of target values used for color mixing calibration (FIG. 4), respectively. As shown in FIG. 9, the target values are evenly arranged in the space in order to make corrections in the entire Lab color space. However, since the number of patches arranged on the color mixing chart (chart 520) output by S402 is limited, the number of target values is generally set to a number smaller than the number of grid points. An independent Lab value is defined for each target value. Therefore, if the Lab value is out of balance among a plurality of adjacent target values, discontinuity may occur in the gradation indicated by the plurality of target values. When an image (gradation image) having a gradation in which the gradation changes smoothly is output in a state where color mixing calibration is performed using the target value in which such gradation discontinuity occurs, the output image is displayed. Pseudo contours may occur.

FIG. 13 is a diagram showing an output example of a gradation image. Image 1301 is an example of an output image obtained when a gradation image is output in a state where color mixing calibration is performed using an appropriate target value. On the other hand, the image 1302 is an example of an output image obtained when a gradation image is output in a state where color mixing calibration is performed using a target value in which the Lab value is out of balance. In image 1302, a pseudo contour is generated in the range 1303. This pseudo contour is a phenomenon in which a sudden change in color or density appears like a contour at a position in the image where the color or density should originally change smoothly.

In the present embodiment, in order to prevent such a pseudo contour from being generated in the output image, the MFP 101 has a defect in the gradation of the target value when registering (saving) the target value for the color mixing calibration. Judge the presence or absence of. Further, the MFP 101 corrects and stores the target value according to the determination result.

<Registration process of target value>
FIG. 7 is a flowchart showing the procedure of the process of registering the target value used for the color mixing calibration (FIG. 4) according to the present embodiment. The processing of each step in FIG. 7 is realized by the processing by the CPU 103.

The processes of S701 to S703 are the same as the processes of S401 to S403, respectively. In S703, when the measured value (Lab value) is obtained by the same measurement as in S403, the CPU 103 proceeds to S704.

In S704, the CPU 103 uses each measured value (Lab value) obtained in the measurement of S703 as a signal value (brightness value) indicating lightness L among a plurality of elements (L, a, b) constituting the measured value. ) And the signal value (chromaticity value) indicating the chromaticity ab. That is, the CPU 103 generates brightness data including the brightness value and chromaticity data including the chromaticity value from each measurement value (Lab value) obtained in the measurement of S703, and stores the generated data in the storage device 121. To do.

Next, in S705, the CPU 103 performs a process of determining whether or not there is a discontinuity in the gradation indicated by the brightness value obtained in S704 (according to the procedure shown in FIG. 10 described later), and the determination process is performed. When completed, the process proceeds to S706. As described above, this gradation discontinuity causes a pseudo contour to appear in the output image.

In S706, the CPU 103 determines that there is no problem with the measured value (Lab value) obtained in S703 when there is no gradation discontinuity as a result of the determination process in S705, and proceeds to the process in S710. Proceed. In S710, the CPU 103 sets the measured value (Lab value) obtained in S703 as a target value, stores it in the storage device 121, and ends the process.

On the other hand, when the gradation discontinuity occurs as a result of the determination process of S705, the CPU 103 proceeds from S706 to S707. In S707, the CPU 103 performs a gradation correction process on the brightness value obtained in S704 (according to the procedure shown in FIG. 11 described later). This correction process is a process for suppressing the occurrence of gradation discontinuity.

When the correction process of S707 is completed, the CPU 103 then in S708, the corrected measurement value composed of the corrected brightness value (L value) and the chromaticity value (ab value) obtained in S704. (Lab value) is generated, and the process proceeds to S709.

In S709, the CPU 103 displays on the display device 118 a UI for notifying the user that the measured value has been corrected. The UI 801 shown in FIG. 8 is an example of a UI (operation screen) displayed on the display device 118 in S709. The UI801 includes a message 802 indicating that a gradation gap (pseudo contour) may occur and that the measured value is corrected and the target value is set (registered). According to the operation of the end button 803 by the user via the input device 120, the CPU 103 ends the display of the UI 801 on the display device 118.

After that, in S710, the CPU 103 sets the corrected measured value (Lab value) as a target value for color mixing calibration, stores it in the storage device 121, and ends the process.

In the correction process of S707 described above, only the brightness value (L value) is corrected, and the chromaticity value (ab value) is not corrected. The reason for this is as follows.

In general, among the components contained in a color, the luminance and lightness components have high contrast sensitivity of high frequency components, and the chromaticity component has high contrast sensitivity of low frequency components. High contrast sensitivity means that humans can visually discriminate changes in color or density. Therefore, it can be said that the high frequency component of the image has a high correlation with the brightness and brightness components of the color components of the image, and the low frequency component of the image has a high correlation with the chromaticity component of the color components of the image. .. When the brightness and brightness components of the image change, the amount of change in the high frequency component of the image becomes larger than the amount of change in the low frequency component. On the other hand, when the chromaticity component changes, the amount of change in the low-frequency component of the image becomes larger than the amount of change in the high-frequency component.

As described above, in the present embodiment, the brightness value is an example of the first signal value showing the color component having a high correlation with the high frequency component of the image. The chromaticity value is an example of a second signal value showing a color component having a high correlation with a low frequency component of an image. As the first signal value, it is also possible to use a signal value (luminance value) indicating the brightness of the image instead of the signal value (brightness value) indicating the brightness of the image.

Further, the pseudo contour illustrated in the range 1303 of FIG. 13 is a phenomenon in which an inappropriate line is seen in the output image. Since the line forming the pseudo contour is an edge component, it generally has a high frequency component. Therefore, among the color components of the measured value (Lab value) used as the target value for the color mixing calibration, the pseudo contour appearing as an inappropriate line by correcting the brightness (L) having a high contrast sensitivity of the high frequency component. It is possible to suppress the occurrence of. As a result, it is possible to make the pseudo contour invisible in the output image.

On the other hand, even if the chromaticity (ab) of the color components of the measured value (Lab value) is corrected, the influence on the generation of the pseudo contour is small. Nevertheless, when the chromaticity (ab) is corrected, the color mixing characteristic changes, and as a result, the corrected measured value may not satisfy the desired performance as the target value of the color mixing calibration. That is, the correction of the chromaticity (ab) has little effect of suppressing the generation of pseudo contours, and may inappropriately change the color mixing characteristics. Therefore, in the correction process of S707 in the present embodiment, the brightness value (L value) is corrected without correcting the chromaticity value (ab value).

As described above, in the present embodiment, only the lightness value (L value) is corrected among the color components of the measured value (Lab value) used as the target value for the color mixing calibration. That is, among the signal values constituting the measured value, the brightness value which is an example of the first signal value is corrected, and the chromaticity value which is an example of the second signal value other than the first signal value is not corrected. As a result, the generation of pseudo contours in the output image is suppressed while maintaining the color mixing characteristics required for the target value.

<Tonality determination process (S705)>
Next, with reference to FIG. 10, a specific process for determining the gradation in S705 (processing for determining whether or not gradation discontinuity occurs in the brightness value (L value) of each patch). The procedure will be described. In the following procedure, the CPU 103 determines whether there is a gradation discontinuity in the brightness value based on the slope between the plurality of brightness values (first signal value) corresponding to the different patches included in the color mixing chart. Judge whether or not.

In S1001, the CPU 103 uses the brightness data generated in S704 to process the patch to be processed (the patch of interest) and the adjacent patch among the plurality of patches included in the color mixing chart (chart 520 in FIG. 5) measured in S703. Extract the brightness value (L value) of the patch to be used. In the present embodiment, a plurality of patches corresponding to a plurality of adjacent target values in the Lab color space are arranged so as to be adjacent to each other on the color mixing chart.

FIG. 12A shows an example of the input / output relationship in the 3D-LUT (used in S404) used for the color mixing calibration as in FIG. In FIG. 12A, the plurality of points indicated by black circles (reference numeral 1201) indicate an arrangement example of target values used for color mixing calibration (FIG. 4), respectively. When the brightness values of a plurality of adjacent patches are extracted as in S1001, in the arrangement example of FIG. 12 (A), a plurality of adjacent target values in the Lab color space, such as the target values 1211 to 1213 in the range 1210, are extracted. The measured value (brightness value) corresponding to is extracted.

Next, in S1002, the CPU 103 performs a process of calculating the second-order difference value using the brightness value of the patch of interest and the brightness value of the adjacent patch, and advances the process to S1003. In S1003, the CPU 103 compares the calculated second-order difference value with the threshold value stored in advance in the storage device 121 to determine whether or not gradation discontinuity occurs in the brightness value obtained by the measurement for the color mixing chart. Is determined. Specifically, the CPU 103 determines that a gradation discontinuity has occurred when the second-order difference value is larger than the threshold value, and when the second-order difference value is not larger than the threshold value, the gradation discontinuity is determined. Judge that it has not occurred. The CPU 103 stores the determination result in the storage device 121, and proceeds to the process to S1004.

Here, FIG. 12B is a diagram showing an example of brightness values for patches corresponding to target values 1211 to 1213 within the range 1210 of FIG. 12A. The patch corresponding to the target value 1212 is set as the attention patch, and the brightness value of the attention patch is L0. Further, the brightness values of the two patches adjacent to the patch of interest (corresponding to the target values 121 and 1213, respectively) are set to L1 and L2.

In this case, the second-order difference value (dim) between the brightness value L1 of the patch of interest and the brightness values L0 and L2 of the adjacent patch is obtained by the following equation.
dif = (L2-L1)-(L1-L0) (1)
In the formula (1), (L1-L0) and (L2-L1) indicate the slope g1 and g2 of the brightness value, and the second-order difference value which is the difference between g1 and g2 indicates the amount of change in the slope.

As shown in FIG. 12B, when there is a difference between the slopes g1 and g2 (that is, the second-order difference value indicating the amount of change in the slope is large), the brightness value is large (steep) between adjacent patches. ) It can be seen that a change has occurred. This means that there is a sudden change (discontinuity) in the gradation indicated by the brightness value. As described above, since the brightness has a high contrast sensitivity of the high frequency component, there is a high possibility that a pseudo contour will occur in the output image when a large change occurs in the brightness (that is, the brightness includes the high frequency component).

Therefore, in S1003, it is determined by the determination using the calculated second-order difference value and the threshold value whether or not a large change (that is, gradation discontinuity) occurs in the brightness value between adjacent patches. Specifically, when the second-order difference value, which is the amount of change in the slope between the brightness values, is larger than the threshold value, the CPU 103 determines that the brightness value has gradation discontinuity, and the second-order difference value is the threshold value. If it is the following, it is determined that there is no gradation discontinuity in the brightness value. As shown in FIG. 12C, the second-order difference value becomes the minimum when there is no difference between the slopes g1 and g2. In this case, the contrast sensitivity of the high-frequency component is high, and the brightness does not change significantly, which means that the possibility of pseudo contours occurring in the output image is low. Therefore, in this case, it is determined that there is no gradation discontinuity in the brightness value.

When the determination of S1003 is completed, in S1004, the CPU 103 determines whether or not the processing of S1001 to S1003 is completed for all the patches included in the color mixing chart measured in S703, and if it is completed, processing is performed. To finish. On the other hand, when the patch that has not been processed (not set as the attention patch) remains, the CPU 103 sets the patch that has not been processed as the attention patch and performs the processing of S1001 to S1003. Run again.

In the present embodiment, the second-order difference value is used in the gradation determination process and the correction process described later, but any method that can detect the slope of the brightness value can be used. For example, a quadratic differential filter can be used. Other methods such as the method using the above may be used.

<Tonality correction processing (S707)>
Next, with reference to FIG. 11, a specific procedure of the gradation correction processing in S707 will be described.

In S1101, the CPU 103, similarly to S1001, from the brightness data generated in S704, among a plurality of patches included in the color mixing chart measured in S703, the patch to be processed (the patch of interest) and the adjacent patch. The brightness value (L value) of is extracted. When the brightness data is updated in S1108, which will be described later, the brightness value is extracted from the updated brightness data.

Next, in S1102, the CPU 103 performs a process of calculating the second-order difference value using the brightness value of the patch of interest and the brightness value of the adjacent patch, as in S1002. After that, in S1103, the CPU 103 determines whether or not the processing of S1101 and S1102 is completed for all the patches, and if not, returns the processing to S1101. In this way, the CPU 103 acquires the second-order difference value for all the patches and proceeds to the process to S1104.

In S1104, the CPU 103 generates correction data for correcting the brightness value in which the gradation discontinuity occurs based on the second-order difference values for all the patches, and proceeds to S1105. Here, the CPU 103 may use the second-order difference value itself as the correction data, or may use the value obtained by adjusting the second-order difference value using a predetermined coefficient as the correction data. That is, when it is determined that the brightness value has a gradation discontinuity, the CPU 103 corrects the measured value by using the second-order difference value (the amount of change in the slope between the brightness values).

In S1105, the CPU 103 generates the corrected brightness value by performing the correction processing of the brightness value for each patch using the brightness value and the correction data, and stores the corrected brightness value in the storage device 121. This correction process is performed by the following equation.
L'(x) = L (x) -α * grade (x) (2)
In the formula (2), x is the patch of interest, L'(x) is the corrected brightness value of the patch x, and L (x) is the brightness value of the patch x. Further, grad (x) indicates a second-order difference value of the brightness value, and indicates a coefficient for increasing or decreasing the second-order difference value. The correction process represented by the equation (2) is one of the optimization methods, and the grade (x) becomes smaller by repeating the correction process. That is, the second-order difference value becomes small, and it is possible to suppress the occurrence of pseudo contours in the output image.

After that, in S1106, the CPU 103 determines whether or not the correction process of S1105 has been executed a specified number of times. When the correction processing of the specified number of times is not completed, the CPU 103 proceeds to S1107, updates the brightness data stored in the storage device 121 with the corrected brightness value, and returns the processing to S1101. As a result, the processes of S1101 to S1105 are repeated until the correction process of the specified number of times is completed. When the correction process for the specified number of times is completed, the CPU 103 ends the process according to the procedure of FIG.

The CPU 103 does not end the correction process based on the number of times the correction process is performed as described above, but ends the correction process when the second-order difference value calculated in S1102 becomes equal to or less than a predetermined threshold value. You may.

As described above, in the present embodiment, the CPU 103 controls the printer 115 so as to form a chart used for the color mixing calibration process for generating the color mixing correction data on the sheet. The chart has a plurality of patches formed on a sheet using a plurality of types of toner (coloring material). The CPU 103 uses the scanner 119 or the sensor 127 in the measuring unit 126 to measure the chart and obtain the measured value. The CPU 103 extracts a first signal value (for example, a brightness value) from the obtained measured value, and determines whether or not the extracted first signal value has a gradation discontinuity. According to the determination result, the CPU 103 controls the process of setting (registering) the target value used in the generation of the color mixing correction data based on the measured value.

In the present embodiment, the CPU 103 determines whether or not gradation discontinuity occurs in the brightness value (first signal value) included in the measured value set as the target value. When the discontinuity occurs, the CPU 103 controls so that the measured value is not set as the target value as it is. Specifically, when the brightness value has a gradation discontinuity, the CPU 103 corrects the measured value by using the amount of change in the slope of the brightness value, and sets the corrected measured value as the target value. Set (register). In this way, the CPU 103 controls the process of setting (registering) the target value for the color mixing calibration based on the measured value. This makes it possible to prevent the occurrence of pseudo contours in the output image when the color correction of the color mixture is performed on the image data after the color mixing calibration is executed.

In this embodiment, the case where the target values for color mixing calibration are evenly arranged in the L * a * b * color space is shown as an example, but many target values are used for a specific color (for example, gray). The patches may be placed and the patches may be placed on the chart 520 accordingly.

[Second Embodiment]
In the first embodiment, when registering the target value for color mixing calibration, it is determined whether or not the brightness value obtained by the measurement has gradation discontinuity, and there is a defect in gradation. Is registered as a target value by correcting the measured value (that is, correcting the target value).

However, if there are other factors regarding the gradation discontinuity, such factors may be dealt with instead of correcting the target value. For example, if monochromatic calibration is not performed properly, monochromatic gradation discontinuity occurs. The occurrence of monochromatic gradation discontinuity can be a factor in the gradation discontinuity of mixed colors. In such a case, it is possible to avoid the occurrence of gradation discontinuity of color mixing by appropriately performing single color calibration.

Therefore, in the present embodiment, when it is determined that the gradation indicated by the measured value (brightness value) obtained by the measurement using the color mixing chart for correcting the color mixing characteristic has a discontinuity, another correction is made. An example of prompting the user to execute the process will be described. In the following, for the sake of simplification of the description, the description of the parts common to the first embodiment will be omitted.

FIG. 14 is a flowchart showing a procedure of a process of registering a target value used for the color mixing calibration (FIG. 4) according to the present embodiment. The processing of each step in FIG. 7 is realized by the processing by the CPU 103.

In the procedure shown in FIG. 14, the processes of S701 to S706 are the same as those of the first embodiment. In the present embodiment, in S706, the CPU 103 determines that there is no problem in the measured value (Lab value) obtained in S703 when there is no gradation discontinuity as a result of the determination process in S705. , The process proceeds to S1402. In S1402, the CPU 103 stores the measured value (Lab value) obtained in S703 as a target value in the storage device 121 as in the first embodiment, and ends the process.

On the other hand, when the gradation discontinuity occurs as a result of the determination process of S705, the CPU 103 proceeds from S706 to S1401 and cancels the registration (setting) of the target value. In S1401, the CPU 103 displays a UI on the display device 118 for prompting the user to execute another correction process that is assumed to be a factor of gradation discontinuity. In this case, the CPU 103 ends the process without saving the target value.

The UI 811 shown in FIG. 8 is an example of a UI (operation screen) displayed on the display device 118 in S1401. In this example, monochromatic calibration is assumed as a factor of gradation discontinuity. Therefore, the UI 811 includes a message 812 prompting the user to perform a monochromatic calibration for monochromatic color correction. According to the operation of the end button 813 by the user via the input device 120, the CPU 103 ends the display of the UI 811 on the display device 118. The user can operate the input device 120 according to the screen display (UI display) on the display device 118 to instruct the execution of the single color calibration. The CPU 103 can execute the monochromatic calibration according to the execution instruction from the user.

As described above, in the present embodiment, the CPU 103 measures the brightness value (first signal value) included in the measured value set as the target value when the gradation discontinuity occurs. Stop setting the target value using the value. Further, the CPU 103 displays a message prompting the execution of the monochromatic calibration on the display device 118. As a result, as in the first embodiment, it is possible to prevent a pseudo contour from being generated in the output image when the color correction of the color mixture is performed on the image data after the color mixing calibration is executed.

In this embodiment, as another correction process for solving the factor of gradation discontinuity, an example of prompting execution of single color calibration is shown. Such correction processing is not limited to single-color calibration, and may be any processing that can deal with a phenomenon in which gradation discontinuity may occur, such as processing for correcting unevenness in a page. Also, in this embodiment. When it is determined that the brightness value has gradation discontinuity, the user is urged to execute another correction process. For example, the user is asked whether to correct the target value or execute another correction process. You may display the UI to be selected.

[Other Embodiments]
The present invention supplies a program that realizes one or more functions of the above-described embodiment to a system or device via a network or storage medium, and one or more processors in the computer of the system or device reads and executes the program. It can also be realized by the processing to be performed. It can also be realized by a circuit (for example, ASIC) that realizes one or more functions.

The invention is not limited to the embodiments described above, and various modifications and modifications can be made without departing from the spirit and scope of the invention. Therefore, a claim is attached to make the scope of the invention public.

101: MFP, 103: CPU, 112: Renderer, 114: Image processing unit, 115: Printer, 119: Scanner, 121: Storage device, 122: Network I / F, 123: Network, 126: Measurement unit, 124: PC , 128: Measuring instrument

Claims (17)

An image forming apparatus having a generating means for generating color mixing correction data for bringing the color of a mixed color image formed by the image forming means closer to a target value.
A measuring means for obtaining a measured value by measuring a chart having a plurality of patches formed on a sheet using a plurality of types of coloring materials by the image forming means.
An extraction means for extracting the first signal value from the measured value, and
A determination means for determining whether or not a gradation discontinuity has occurred in the first signal value extracted by the extraction means, and
A control means for controlling a process of setting the target value used by the generation means based on the measured value according to the determination result by the determination means.
An image forming apparatus comprising the above. The image forming apparatus according to claim 1, wherein the first signal value is a signal value indicating a color component having a high correlation with a high frequency component of an image. The image forming apparatus according to claim 1 or 2, wherein the first signal value is a signal value indicating brightness or brightness of an image. The chart contains multiple patches, each with a different color mixture.
The determination means is characterized in that it determines whether or not there is a gradation discontinuity in the first signal value based on the inclination between a plurality of first signal values corresponding to different patches. The image forming apparatus according to any one of claims 1 to 3. When the amount of change in the inclination is larger than the threshold value, the determination means determines that the first signal value has a gradation discontinuity, and when the amount of change in the inclination is equal to or less than the threshold value. The image forming apparatus according to claim 4, wherein is determined that no gradation discontinuity has occurred in the first signal value. The image forming apparatus according to claim 5, wherein the determination means obtains a second-order difference value of the first signal value as the amount of change in the inclination. When the determination means determines that the first signal value has a gradation discontinuity, the determination means further includes a correction means for correcting the measured value using the amount of change in the inclination.
The control means is characterized in that, when it is determined that the first signal value has a gradation discontinuity, the measured value after correction by the correction means is set as the target value. The image forming apparatus according to any one of claims 4 to 6. The control means is characterized in that, when it is determined that the first signal value does not have gradation discontinuity, the measured value obtained by the measuring means is set as the target value. The image forming apparatus according to claim 7. According to claim 7 or 8, the correction means corrects the first signal value among the signal values constituting the measured value, and does not correct the second signal value other than the first signal value. The image forming apparatus described. The image forming apparatus according to claim 9, wherein the second signal value is a signal value indicating a color component having a high correlation with a low frequency component of an image. The image forming apparatus according to claim 9 or 10, wherein the second signal value is a signal value indicating the chromaticity of an image. The claim is characterized in that when the correction by the correction means is performed, the control means displays a message indicating that the measured value is corrected and the target value is set on the display unit of the image forming apparatus. Item 6. The image forming apparatus according to any one of Items 7 to 11. Any one of claims 1 to 6, wherein the control means cancels the setting of the target value when it is determined that the first signal value has a gradation discontinuity. The image forming apparatus according to the section. The control means performs a monochromatic calibration process for generating monochromatic correction data for bringing the color of a monochromatic image closer to a target value when it is determined that the first signal value has a gradation discontinuity. The image forming apparatus according to claim 13, wherein a message prompting execution is displayed on a display unit of the image forming apparatus. The generation means measures a chart having a plurality of patches formed on a sheet using the plurality of types of coloring materials by the image forming means by the measuring means, and the measured value obtained by the measurement and the target. The image forming apparatus according to any one of claims 1 to 14, wherein a color mixing calibration process for generating color mixing correction data to which a difference from a value is applied is performed. A control method for an image forming apparatus having a generating means for generating color mixing correction data for bringing the color of a mixed color image formed by the image forming means closer to a target value.
A measurement step of obtaining a measured value by measuring a chart having a plurality of patches formed on a sheet using a plurality of types of coloring materials by the image forming means.
An extraction step of extracting the first signal value from the measured value and
A determination step of determining whether or not a gradation discontinuity has occurred in the first signal value extracted in the extraction step, and a determination step.
A control step that controls a process of setting the target value used by the generation means based on the measured value according to the determination result in the determination step.
A method for controlling an image forming apparatus, which comprises. A program for causing a computer to execute each step of the control method of the image forming apparatus according to claim 16.