Image forming apparatus and program

Abstract

To appropriately perform tone correction even if images of patches for performing tone correction are formed on a sheet in a smaller size.SOLUTION: An image forming apparatus comprises: an acquisition unit (control unit 101) that acquires a read image obtained by reading an image formation surface of a recording medium PM on which a color image is formed in which images in respective colors are overlapped with each other; a control unit (control unit 101) that forms images of tone patches P2 outside a job image formation area; and a correction unit (control unit 101) that executes tone correction in images formed by a plurality of image forming units 151 based on the read image including the tone patches P2. The tone patches P2 each include a halftone part P255 that represents a plurality of tone levels with a halftone using area modulation. The correction unit calculates a read tone value obtained by reading the tone value of the tone patches P2 from the read image, calculates a regression equation I of the read tone value relative to the positions of the tone patches P2 in a conveyance direction of the recording medium PM, and performs tone correction based on the regression equation I.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus and a program. [Background technology]

[0002] Conventionally, in image forming devices, the device characteristics, particularly the gamma characteristics, change due to environmental changes such as temperature and humidity, or changes over time, which can result in a significant deterioration in gradation. For this reason, in order to maintain high-quality image output, image forming devices are configured to form patches for gradation correction at predetermined times, detect the patches with a detection means such as a sensor, and perform gradation correction based on the detection results.

[0003] In this regard, Patent Document 1 describes an image forming device in which a patch image for correcting image forming conditions is configured to have a gradation portion in which the gradation level changes stepwise so that multiple gradation levels exist within the detection area width of the patch detection means, and a single gradation level portion in which the gradation level does not change within the detection area width. Furthermore, Patent Document 2 describes an image forming apparatus that forms a maximum gradation value compensation pattern having a length in the direction of movement of the image carrier surface to compensate for the response delay of the output of a density detection means due to a low-pass filter, immediately before a pattern portion on the surface of the image carrier where the gradation value changes continuously from the maximum gradation value to the minimum gradation value, without leaving any gap between the pattern portion and the pattern portion. Furthermore, Patent Document 3 describes an image forming device that forms a gradation patch image for determining gradation characteristics, in which the gradation level gradually changes from the maximum gradation level to the minimum gradation level, and which is either a toner image in which the gradation level changes continuously and uniformly, or a toner image in which the gradation level changes stepwise at a pitch shorter than the width of the detection area of ​​the patch sensor. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-268256 [Patent Document 2] Japanese Patent Application Laid-Open No. 2015-66779 [Patent Document 3] Patent No. 4304936 Summary of the Invention [Problem to be solved by the invention]

[0005] Incidentally, if the size of a patch for tone correction is large, the area in which an image can be formed is limited, and therefore, it is desired to form an image in a smaller size. However, in the case of an image forming device that expresses gradation through area modulation (where a screen pattern is present), if the patch for gradation correction is made small, fluctuations in the image formation density within the patch for gradation correction occur due to the phase of the screen pattern, which can result in errors in reading the gradation value.

[0006] Patent Documents 1 to 3 do not describe any technology that prevents reading errors even when the patches for tone correction are made small, and this problem remains unresolved.

[0007] The present invention has been made in consideration of such problems, and aims to provide an image forming device and program that can perform gradation correction appropriately even if patches for gradation correction are formed on paper in smaller images. [Means for solving the problem]

[0008] In order to solve the above problem, the image forming apparatus of the invention described in claim 1 comprises: a plurality of image forming units that form images using color materials based on image data; a color image forming section for forming a color image on a recording medium by superimposing the images of the respective colors formed by the image forming units; an acquisition unit that acquires a read image by reading the image forming surface of the recording medium on which the color image is formed; a control unit that forms a gradation patch image outside a job image forming area on the recording medium; a correction unit that performs gradation correction on images formed by the plurality of image forming units based on the read image including the gradation patches acquired by the acquisition unit; Equipped with The gradation patch is The recording medium conveyance method includes an ascending order section in which the gradation level changes stepwise from a minimum gradation value to a maximum gradation value in a conveyance direction of the recording medium, and a descending order section in which the gradation level changes stepwise from a maximum gradation value to a minimum gradation value in the conveyance direction, and the ascending order section and the descending order section each include: Multiple halftones using area modulation The aforementioned It has a halftone section that expresses gradation levels, the ascending section is disposed on the upstream side in the conveying direction, and the descending section is disposed on the downstream side in the conveying direction, The correction unit calculates a read gradation value by reading the gradation value of the gradation patch from the read image, and Recording A regression equation of the read gradation value with respect to the position in the feed direction is calculated, and gradation correction is performed based on the regression equation.

[0009] The invention described in claim 2 is the invention described in claim 1, The interval between patches of each tone level in the tone patch is less than 10 periods of the screen ruling in the screen pattern used to form the image of the tone patch.

[0011] Claim 3 The invention described in claim 1 or 2 In the invention described in In the tone patch, the maximum tone value portion of the ascending order section and the maximum tone value portion of the descending order section are adjacent to each other.

[0012] Claim 4 The invention described in claims 1 to 3 In the invention described in any one of the above, The correction unit calculates the regression equation in accordance with a screen ruling in a screen pattern used to form an image of the tone patch. The image forming apparatus of the invention described in claim 5 comprises: a plurality of image forming units that form images using color materials based on image data; a color image forming section for forming a color image on a recording medium by superimposing the images of the respective colors formed by the image forming units; an acquisition unit that acquires a read image by reading the image forming surface of the recording medium on which the color image is formed; a control unit that forms a gradation patch image outside a job image forming area on the recording medium; a correction unit that performs gradation correction on images formed by the plurality of image forming units based on the read image including the gradation patches acquired by the acquisition unit; Equipped with the gradation patch has a halftone portion that expresses a plurality of gradation levels by halftones using area modulation, The correction unit calculates a read gradation value by reading the gradation value of the gradation patch from the read image, calculates a regression equation for the read gradation value relative to the position of the gradation patch in the transport direction of the recording medium according to the screen line number in the screen pattern used to form the image of the gradation patch, and performs gradation correction based on the regression equation.

[0013] The invention described in claim 6 is the invention described in any one of claims 1 to 5, the gradation patches include color gradation patches for each color constituting an image using the plurality of color materials; The control unit forms tone patches in different orders on both ends in the main scanning direction of the recording medium that are outside the job image forming area.

[0014] The invention described in claim 7 is the invention described in claim 6, The control unit causes image formation of a mark for identifying the type of the gradation patch adjacent to the gradation patch having a different order among the color gradation patches. The image forming apparatus of the invention described in claim 8 comprises: a plurality of image forming units that form images using color materials based on image data; a color image forming section for forming a color image on a recording medium by superimposing the images of the respective colors formed by the image forming units; an acquisition unit that acquires a read image by reading the image forming surface of the recording medium on which the color image is formed; a control unit that forms a gradation patch image outside a job image forming area on the recording medium; a correction unit that performs gradation correction on images formed by the plurality of image forming units based on the read image including the gradation patches acquired by the acquisition unit; Equipped with the gradation patches include color gradation patches for each color constituting an image using the plurality of color materials; Each of the color gradation patches has a halftone portion that expresses a plurality of gradation levels by halftones using area modulation, the control unit forms image formation of gradation patches in which the order of the color-specific gradation patches is different on both ends in the main scanning direction of the recording medium outside the job image forming area, and forms image formation of marks that identify the types of the gradation patches adjacent to the gradation patches in which the order of the color-specific gradation patches is different; The correction unit calculates a read gradation value by reading the gradation value of the gradation patch from the read image, calculates a regression equation for the read gradation value relative to the position of the gradation patch in the transport direction of the recording medium, and performs gradation correction based on the regression equation.

[0015] Claim 9 The invention described in claim 7 or 8 In the invention described in the control unit causes a resist patch to be image-formed outside a job image forming area on the recording medium; The mark is the resist patch.

[0016] Claim 10 The invention described in claim 9 In the invention described in The correction unit corrects image forming positions in the plurality of image forming units based on the read image including the registration patch acquired by the acquisition unit.

[0017] Claim 11 The invention described in claims 1 to 10 In the invention described in any one of the above, The recording medium is a continuous medium.

[0018] Claim 12 The invention described in claims 1 to 11 In the invention described in any one of the above, The control unit causes a toner consumption patch to be image-formed on the recording medium at the same position in the main scanning direction as the gradation patch.

[0019] Claim 13 The program of the invention described in a plurality of image forming units that form images using color materials based on image data; a color image forming section for forming a color image on a recording medium by superimposing the images of the respective colors formed by the image forming units; A computer of an image forming apparatus comprising: an acquisition unit that acquires a read image by reading the image forming surface of the recording medium on which the color image is formed; a control unit for forming a gradation patch image outside a job image forming area on the recording medium; a correction unit that performs gradation correction on images formed by the plurality of image forming units based on the read image including the gradation patches acquired by the acquisition unit; It functions as The gradation patch is The recording medium conveyance method includes an ascending order section in which the gradation level changes stepwise from a minimum gradation value to a maximum gradation value in a conveyance direction of the recording medium, and a descending order section in which the gradation level changes stepwise from a maximum gradation value to a minimum gradation value in the conveyance direction, and the ascending order section and the descending order section each include: Multiple halftones using area modulation The aforementioned It has a halftone section that expresses gradation levels, the ascending section is disposed on the upstream side in the conveying direction, and the descending section is disposed on the downstream side in the conveying direction, The correction unit calculates a read gradation value by reading the gradation value of the gradation patch from the read image, and RecordingA regression equation of the read gradation value with respect to the position in the feed direction is calculated, and gradation correction is performed based on the regression equation. The program of the invention described in claim 14 is a plurality of image forming units that form images using color materials based on image data; a color image forming section for forming a color image on a recording medium by superimposing the images of the respective colors formed by the image forming units; A computer of an image forming apparatus comprising: an acquisition unit that acquires a read image by reading the image forming surface of the recording medium on which the color image is formed; a control unit for forming a gradation patch image outside a job image forming area on the recording medium; a correction unit that performs gradation correction on images formed by the plurality of image forming units based on the read image including the gradation patches acquired by the acquisition unit; It functions as the gradation patch has a halftone portion that expresses a plurality of gradation levels by halftones using area modulation, The correction unit calculates a read gradation value by reading the gradation value of the gradation patch from the read image, calculates a regression equation for the read gradation value relative to the position of the gradation patch in the transport direction of the recording medium according to the screen line number in the screen pattern used to form the image of the gradation patch, and performs gradation correction based on the regression equation. The program of the invention described in claim 15 is a plurality of image forming units that form images using color materials based on image data; a color image forming section for forming a color image on a recording medium by superimposing the images of the respective colors formed by the image forming units; A computer of an image forming apparatus comprising: an acquisition unit that acquires a read image by reading the image forming surface of the recording medium on which the color image is formed; a control unit for forming a gradation patch image outside a job image forming area on the recording medium; a correction unit that performs gradation correction on images formed by the plurality of image forming units based on the read image including the gradation patches acquired by the acquisition unit; It functions as the gradation patches include color gradation patches for each color constituting an image using the plurality of color materials; Each of the color gradation patches has a halftone portion that expresses a plurality of gradation levels by halftones using area modulation, the control unit forms image formation of gradation patches in which the order of the color-specific gradation patches is different on both ends in the main scanning direction of the recording medium outside the job image forming area, and forms image formation of marks that identify the types of the gradation patches adjacent to the gradation patches in which the order of the color-specific gradation patches is different; The correction unit calculates a read gradation value by reading the gradation value of the gradation patch from the read image, calculates a regression equation for the read gradation value relative to the position of the gradation patch in the transport direction of the recording medium, and performs gradation correction based on the regression equation. [Effects of the Invention]

[0020] According to the present invention, it is possible to provide an image forming apparatus and a program that can perform suitable gradation correction even when a patch for gradation correction is formed as a smaller image on a sheet. [Brief explanation of the drawings]

[0021] [Figure 1] 1 is a block diagram showing a configuration of an image forming apparatus according to an embodiment; [Figure 2A] 10A and 10B are diagrams illustrating an example of a recording medium on which a job image and a patch image are formed. [Figure 2B] FIG. 10 is a diagram illustrating an example of a gradation patch. [Figure 2C] FIG. 10 is a diagram illustrating an example of a gradation patch. [Figure 2D] FIG. 10 is a diagram illustrating an example of a resist patch. [Figure 2E] FIG. 10 is a diagram illustrating an example of a resist patch. [Figure 3] 10 is a flowchart showing the flow of an image forming process. [Figure 4] 10 is a flowchart showing the flow of tone correction processing. [Figure 5] FIG. 10 is a diagram showing an example of read gradation values ​​of a gradation patch. [Figure 6] FIG. 10 is a diagram showing an example of an average value of coordinate-converted read grayscale values. [Figure 7] 10A and 10B are diagrams showing an example of a recording medium on which an image of a job, a patch, and an eye mark are formed according to Modification 1. [Figure 8] FIG. 10 is a diagram showing an example of read gradation values ​​of a high-line screen according to Modification 2. DETAILED DESCRIPTION OF THE INVENTION

[0022] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings, but the scope of the invention is not limited to the illustrated examples.

[0023] [Configuration of Image Forming Apparatus] First, the configuration of an image forming apparatus 100 according to an embodiment of the present invention will be described. Fig. 1 is a diagram showing the main configuration of an image forming apparatus 100. As shown in Fig. 1, the image forming apparatus 100 is an apparatus that continuously forms images on a roll-shaped recording medium PM.

[0024] As shown in FIG. 1, the image forming apparatus 100 is configured to include a control unit 101, a communication unit 102, an operation display unit 103, a memory unit 104, a paper feed unit 105, a conveying unit 106, a winding unit 107, an image forming unit 150, a fixing unit 170, an image scanner 180, etc.

[0025] The control unit 101 includes a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), etc. The CPU of the control unit 101 reads a program corresponding to the processing content from the ROM, loads it into the RAM, and centrally controls the operations of each unit of the image forming apparatus 100 in cooperation with the loaded program.

[0026] The communication unit 102 is configured by a communication control card such as a LAN (Local Area Network) card, and transmits and receives various data to and from external devices connected to a communication network such as a LAN or WAN (Wide Area Network).

[0027] The operation display unit 103 includes a display unit 103a such as a liquid crystal display or an organic EL display, and an input unit 103b configured with various operation keys, a touch panel overlaid on the screen of the display unit 103a, a numeric keypad, etc. The operation display unit 103 displays various information on the display unit 103a, and converts user input operations to the input unit 103b into operation signals and outputs the signals to the control unit 101.

[0028] The storage unit 104 is configured by, for example, a non-volatile semiconductor memory (so-called flash memory), a hard disk drive, etc. The storage unit 104 stores various data including various setting information related to the image forming apparatus 100, job information (job setting information and job image data), etc. The memory unit 104 also stores image data of a toner consumption patch P1, which will be described later, image data of a gradation patch P2 for correcting the density gradation of the image to be formed, and image data of a registration patch P3 for correcting the image formation position in multiple image forming units 151.

[0029] The paper feed unit 105 includes a paper feed roll 105a and a paper feed roller 105b. A continuous recording medium PM, such as a roll of film, roll paper, or fabric, is wound around the paper feed roll 105a. The paper feed roll 105a rotates at a speed corresponding to the conveyance speed of the recording medium PM in accordance with instructions from the control unit 101, and sends the recording medium PM to the conveyance path. The paper feed roller 105b stretches the recording medium PM with an appropriate tension while sending the recording medium PM toward the image forming unit 150. Note that the recording medium PM does not have to be a continuous medium that can be rolled up; in that case, the individual recording media PM may be fed and discharged in order. The conveying unit 106 has a conveying path and a plurality of conveying roller pairs such as registration roller pairs, and conveys the recording medium PM fed from the paper feeding unit 105 within the image forming apparatus 100. The winding unit 107 has a winding roll 107a and a winding roller 107b. The winding roller 107b sends out the recording medium PM on which an image has been formed toward the winding roll 107a. The winding roll 107a rotates at a speed corresponding to the conveying speed of the recording medium PM in accordance with instructions from the control unit 101, and winds up the recording medium PM on which the image has been formed.

[0030] The image forming unit 150 prints an image on a recording medium PM based on the job setting information and image data, and generates a printed matter. The image forming section 150 includes an image forming unit 151, a roller 152, an intermediate transfer belt 153, a secondary transfer roller 154, and the like. In this embodiment, the image forming section 150 has image forming units 151 for each color: Y (yellow), M (magenta), C (cyan), K (black), and S (spot color; white in this embodiment), and is capable of printing using white toner in addition to images using the usual Y, M, C, and K toners. In the image forming section 150, toner images of each color are formed by the image forming units 151 and sequentially transferred onto the intermediate transfer belt 153, where the five color toner images are superimposed. The intermediate transfer belt 153 is an endless belt wound around multiple rollers, and rotates in the direction of the arrow shown in Figure 1 in accordance with the rotation of each roller. Secondary transfer roller 154 transfers the toner image on intermediate transfer belt 153 onto recording medium PM fed from paper feed unit 105. More specifically, the recording medium PM and intermediate transfer belt 153 are sandwiched in a transfer nip N formed by secondary transfer roller 154 being pressed against roller 152, and when a predetermined transfer voltage is applied to secondary transfer roller 154, the toner forming the toner image on secondary transfer roller 154 is attracted to the recording medium PM and transferred to the recording medium PM, thereby forming (printing) an image. Roller 152, intermediate transfer belt 153, and secondary transfer roller 154 form a color image forming unit.

[0031] The fixing unit 170 fixes the toner image printed on the recording medium PM to the recording medium PM using heat and pressure. In FIG. 1, the image forming unit 150 is illustrated as an image forming unit of a so-called electrophotographic type, but the printing type is not limited to this, and the image forming unit may be of another printing type, such as an inkjet type.

[0032] The image scanner 180 reads the image-formed surface of the recording medium PM that has been printed by the image forming unit 150 and fixed by the fixing unit 170, and outputs the read image data (read image) to the control unit 101. The image scanner 180 is configured, for example, by a color scanner. The image scanner 180 is disposed downstream of the fixing unit 170, and is configured to read the image while the recording medium PM is being transported.

[0033] The control unit 101 also acquires a read image obtained by reading the image forming surface of the recording medium PM on which a color image is formed. Here, the control unit 101 functions as an acquisition unit. Furthermore, the control unit 101 controls the image forming unit 150 to form an image of a gradation patch P2 outside a job image forming area (to be described later) on the recording medium PM. Here, the control unit 101 functions as a control unit. Furthermore, based on the read image including the gradation patch P2 acquired by the acquisition unit, the control unit 101 performs gradation correction on the images formed by the multiple image forming units 151. Here, the control unit 101 functions as a correction unit.

[0034] Here, the toner consumption patch P1, gradation patch P2, and registration patch P3 that are formed on the recording medium PM by the control unit 101 controlling the image forming unit 150 will be described. Figure 2A is a diagram showing an example of a recording medium PM on which a job image, a toner consumption patch P1, gradation patches P2a and P2b (when gradation patches P2a and P2b are not distinguished, they will be referred to as gradation patch P2 below), and resist patches P3a to P3d (when resist patches P3a to P3d are not distinguished, they will be referred to as resist patch P3 below) are image-formed. 2A, area A is the job image forming area where the job image is formed. Area B is outside the job image forming area on both sides of the job image forming area in the main scanning direction, and is the area where toner consumption patch P1, gradation patch P2, and registration patch P3 are image-formed. By forming the toner consumption patch P1, gradation patch P2, and registration patch P3 in area B, it is possible to ensure a larger area A, which is the job image forming area. The toner consumption patch P1 is a patch for consuming old toner when printing a job that consumes a small amount of toner.

[0035] 2B shows gradation patch P2a. Gradation patch P2a has a boundary line P21 that indicates the leading end of gradation patch P2 in the transport direction of recording medium PM and is a detection mark for detecting gradation patch P2a, a boundary line P22 that indicates the boundary between the first gradation patch portion P25 and the second gradation patch portion P24, and a boundary line P23 that indicates the trailing end of gradation patch P2 in the transport direction of recording medium PM and is a detection mark for detecting gradation patch P2a, the first gradation patch portion P25, and the second gradation patch portion P24. Here, boundary line P22 may be omitted from gradation patch P2. Since gradation patch P2 has at least two boundary lines P21 and P23, the leading and trailing ends of gradation patch P2 can be detected when detecting gradation patch P2, and based on this, the position where gradation patch P2 is image-formed and the magnification of gradation patch P2 in the transport direction of recording medium PM can be obtained.

[0036] The first gradation patch portion P25 is made up of a plurality of color gradation patches with gradually different gradation values ​​for Y, M, C, and K. That is, the first gradation patch portion P25 has a Y color gradation patch P25y, an M color gradation patch P25m, a C color gradation patch P25c, and a K color gradation patch P25k. When there is no need to distinguish between the Y color gradation patch P25y, the M color gradation patch P25m, the C color gradation patch P25c, and the K color gradation patch P25k, they will be referred to as color gradation patches P25x. Fig. 2B shows the detailed configuration of the M color gradation patch P25m. In the example shown in Fig. 2B, black boundaries are added to make the boundaries between the patches of each gradation level easier to see, but in reality, there are no boundaries between the patches of each gradation level. The M color gradation patch P25m has an ascending section P252 that changes in 11 gradation levels in a stepped manner from the minimum gradation value to the maximum gradation value from the upstream side to the downstream side in the transport direction of the recording medium PM, and a descending section P251 that changes in 11 gradation levels in a stepped manner from the maximum gradation value to the minimum gradation value. Additionally, the ascending order section P252 and the descending order section P251 each have a minimum gradation value section P253, a maximum gradation value section P254, and a halftone section P255. As shown in FIG. 2B, the maximum gradation value section P254 is adjacent to a portion of the halftone section P255 whose gradation value is close to that of the maximum gradation value section P254. Therefore, when the gradation patch P2 is read by the image scanner 180, the influence of light leaking from the portion with low gradation values ​​in the maximum gradation value section P254 can be minimized.

[0037] In addition, the width of the patches of each gradation level in the halftone section P255 in the conveying direction of the recording medium PM is equally spaced, and the width of the patches of the minimum gradation value section P253 and the maximum gradation value section P254 in the conveying direction of the recording medium PM is wider than the width of each gradation level in the halftone section P255. The gradation patch P2 is smaller than conventional patches, with the intervals between patches of each gradation level in the ascending portion P252 and the descending portion P251 being less than 10 periods of the screen ruling in the screen pattern used to form the image of the gradation patch P2. This configuration is also the same for the Y color gradation patch P25y, the C color gradation patch P25c, and the K color gradation patch P25k. 2B, the color gradation patches P25x are arranged adjacent to each other in the minimum gradation value portion P253. This makes it possible to suppress the influence of light leakage between the color gradation patches P25x when the gradation patches P2 are read by the image scanner 180. Furthermore, since there is no need to leave a gap between the color gradation patches P25x to avoid the influence of light leakage, the length of the gradation patches P2 in the transport direction of the recording medium PM can be minimized.

[0038] The second gradation patch portion P24 is made up of a plurality of color gradation patches with gradually varying gradation values ​​of secondary or tertiary colors composed of toners of multiple colors from among Y, M, C, and K. That is, the second gradation patch portion P24 has an R (Red) color gradation patch P24r composed of Y and M, a G (Green) color gradation patch P24g composed of Y and C, a B (Blue) color gradation patch P24b composed of M and C, and a 3C color gradation patch P243c composed of Y, M, and C. When the R color gradation patch P24r, the G color gradation patch P24g, the B color gradation patch P24b, and the 3C color gradation patch P243c are not to be distinguished from one another, they are referred to as color gradation patches P24x. The configuration of each color gradation patch P24x is similar to that of the M color gradation patch P25m, and they are arranged adjacent to each other. 2C shows gradation patch P2b. Like gradation patch P2a, gradation patch P2b has boundaries P21 to P23, a first gradation patch portion P25, and a second gradation patch portion P24, but the positions of the first gradation patch portion P25 and the second gradation patch portion P24 are opposite to those of gradation patch P2a, with boundary line P22 sandwiched between them.

[0039] Figure 2D shows identical resist patches P3a and P3c, and Figure 2E shows identical resist patches P3b and P3d. The resist patch P3 has reference color patches P31 and P32 and a measurement target color patch portion P33. The measurement target color patch portion P33 is arranged so as to be sandwiched between the reference color patches P31 and P32. Here, the reference color is, for example, K, and the measurement target colors are Y, M, and C. The resist patches P3a and P3c shown in FIG. 2D and the resist patches P3b and P3d shown in FIG. 2E have different sizes and shapes of the measurement target color patches included in the measurement target color patch section 33.

[0040] Returning to FIG. 2A, enlarged sections 1 to 4 will be described. A portion of the resist patch P3a and the gradation patch P2a is shown in enlarged portion 1. The resist patch P3a is arranged so that the upward direction of the Y axis shown in FIG. 2D coincides with the conveyance direction of the recording medium PM. The resist patch P3a is also arranged so that the reference color patch P32 and the boundary line P21 overlap. A portion of the resist patch P3b and the gradation patch P2a is shown in enlarged portion 2. The resist patch P3b is arranged so that the upward direction of the Y axis shown in FIG. 2E coincides with the conveyance direction of the recording medium PM. The resist patch P3b is also arranged so that the reference color patch P31 and the boundary line P23 overlap. The enlarged portion 3 shows a portion of the resist patch P3c and the gradation patch P2b. The resist patch P3c is arranged so that the upward direction of the Y axis shown in FIG. 2D coincides with the conveyance direction of the recording medium PM. The resist patch P3c is also arranged so that the reference color patch P32 and the boundary line P21 overlap. The enlarged portion 4 shows a portion of the resist patch P3d and the gradation patch P2b. The resist patch P3d is arranged so that the upward direction of the Y axis shown in FIG. 2E coincides with the conveyance direction of the recording medium PM. The resist patch P3d is also arranged so that the reference color patch P31 and the boundary line P23 overlap. By arranging the reference color patch P31 so that it overlaps with the boundary line P23, and the reference color patch P32 so that it overlaps with the boundary line P21, the area in which the gradation patch P2 and the resist patch P3 are image-formed can be made smaller in the transport direction of the recording medium PM. Furthermore, by arranging identical resist patches P3a and P3c at the leading edge of the gradation patch P2 in the transport direction of the recording medium PM, and identical resist patches P3b and P3d at the trailing edge of the gradation patch P2 in the transport direction of the recording medium PM, the control unit 101 can identify the leading and trailing edges of the gradation patch P2 in the transport direction of the recording medium PM based on the read image including the resist patch P3.

[0041] Here, the same resist patch P3 may be placed on the boundaries P21 and P23 of the gradation patch P2a, and a resist patch P3 of a different form from the resist patch P3 placed on the gradation patch P2a may be placed on the boundaries P21 and P23 of the gradation patch P2b. In this way, depending on the form of the resist patch P3, the control unit 101 identifies the type of the gradation patch P2 based on the read image including the resist patch P3.

[0042] 2A indicates the reading range of the image scanner 180. As shown in Fig. 2A, the gradation patch P2 and the resist patch P3 are formed as images on the recording medium PM within a range that is surely within the range IS.

[0043] [Operation of Image Forming Apparatus] Next, the operation of image forming apparatus 100 will be described. 3 shows a flowchart of the image forming process executed by the image forming apparatus 100. The image forming process is executed by a program stored in the control unit 101 of the image forming apparatus 100.

[0044] In the image forming process, first, the control unit 101 acquires the image data of the job, the image data of the toner consumption patch P1, the image data of the gradation patch P2, and the image data of the resist patch P3 from the memory unit 104, and controls the image forming unit 150 to form the image of the job and each patch on the recording medium PM (step S1). Next, the control unit 101 acquires the read image on the recording medium PM read by the image scanner 180, and determines whether or not the gradation patch P2 has been detected based on the read image (step S2). If the gradation patch P2 has not been detected (step S2; NO), the control unit 101 moves the process to step S2. Furthermore, if the gradation patch P2 is detected (step S2; YES), the control unit 101 executes a gradation correction process (step S3). FIG. 4 shows a flowchart of the tone correction process.

[0045] In the tone correction process, first, the control unit 101 calculates the read tone value of the tone patch P2 based on the read image data read by the image scanner 180 (step S31). An example of read-out gradation values ​​of gradation patch P2b is shown in the upper diagram of Figure 5. The horizontal axis of the upper diagram of Figure 5 represents the position in the transport direction of the recording medium PM, and the vertical axis represents the read-out gradation value. Next, the control unit 101 detects the positions of the rear edge D1 of the boundary line P21, the front edge D2 and rear edge D3 of the boundary line P22, and the front edge D4 of the boundary line P23 based on the read gradation value calculated in step S31 (step S32). The upper diagram in FIG. 5 shows the positions of edges D1 to D4 of the boundary lines P21 to P23 detected in step S32. Next, the control unit 101 calculates the center coordinates E1 to E3 of the boundary lines P21 to P23 based on the positions of the edges D1 to D4 detected in step S32 (step S33). The upper diagram in FIG. 5 shows the positions of the center coordinates E1 to E3 of the boundary lines P21 to P23 calculated in step S33. Here, instead of calculating the center coordinates E1 to E3 of the boundaries P21 to P23 in step S33, the positions of the centers of gravity of the boundaries P21 to P23 may be calculated.

[0046] Next, the control unit 101 identifies the positions F of the leading and trailing ends of the color gradation patches P25x and P24x of Y, M, C, K, R, G, B, and 3C based on the center coordinates E1 to E3 of the boundary lines P21 to P23 calculated in step S33, and extracts the areas of the color gradation patches P25x and P24x (step S34). The upper diagram in FIG. 5 shows the positions F of the leading and trailing ends of the color gradation patches P25x and P24x identified in step S34. Next, the control unit 101 turns back the area from the leading edge to the trailing edge of the color gradation patches P25x and P24x at the center coordinate G, and calculates the average value of the read gradation values ​​in the turned back area (step S35). The middle diagram in FIG. 5 shows a diagram of the Y color gradation patch P25y folded back at the center coordinate G. In this way, the color gradation patches P25x and P24x have an ascending section P252 and a descending section P251, and by calculating the average value of the read gradation values ​​in the portion folded back at the center coordinate G, it is possible to suppress the influence of the edge effect that occurs in the adjacent area of ​​the portion where the gradation has changed, such as so-called sweeping, in the dry electrophotographic method, for example. 2A, in step S35, if gradation patches P2a and P2b have been acquired in the scanned image, the average value of the folded read gradation values ​​of the Y-color gradation patch P25y in gradation patch P2a and the average value of the folded read gradation values ​​of the Y-color gradation patch P25y in gradation patch P2b may be further averaged. This also applies to the color gradation patches of other colors. Because the gradation characteristics may differ at each end of the recording medium PM in the main scanning direction, the above process allows for more accurate gradation correction.

[0047] Next, the control unit 101 specifies a data range H to be used in the regression equation calculation so that the average values ​​of the read gradation values ​​folded back in step S35 are spaced equally apart for each gradation level (step S36). The position of data range H is shown in the lower diagram of FIG. Next, the control unit 101 calculates the regression equation I in the data range H identified in step S36 (step S37). The regression formula I is a predetermined function and may be a polynomial. If the regression formula I is a polynomial of a high degree (for example, fourth degree or higher), the control unit 101 performs coordinate transformation on the average value of the read gradation values ​​in the data range H identified in step S36 so that the position in the transport direction of the recording medium PM and the average value of the read gradation values ​​have a substantially linear relationship. FIG. 6 shows an example of the average value of the coordinate-transformed read gradation values. Then, the control unit 101 may calculate a linear regression formula I or a regression formula I of a polynomial of a lower degree for the average value of the coordinate-transformed read gradation values. If the regression formula I is a polynomial of a high degree, there is a possibility that the difference between the regression formula I and the average value of the read gradation values ​​will be large outside the data range H. This can be prevented by lowering the degree of the regression formula I as described above.

[0048] Next, the control unit 101 calculates the gradation value of each gradation level based on the regression formula I calculated in step S37 (step S38). Next, the control unit 101 corrects the image data of the job based on the gradation values ​​of each gradation level calculated in step S38, forms the corrected image of the job (step S39), and ends this process. Note that the method of correcting the image data performed in step S39 is not limited. Correction may be performed by changing the gradation values ​​of image data that has been processed by a RIP (Raster Image Processor), or by adjusting a conversion lookup table during halftone (area modulation) processing. Alternatively, correction may be performed by adjusting the control parameters of each image forming unit 151, etc.

[0049] Returning to the description of the image forming process shown in FIG. Next, the control unit 101 determines whether or not the image formation of the job is completed (step S5). If the image formation of the job is not completed (step S5; NO), the control unit 101 shifts the process to step S3. Moreover, if the image formation of the job is completed (step S5; YES), the control unit 101 ends this process.

[0050] (Variation 1) Next, a first modification of the above embodiment will be described. The following description will focus on the differences from the above embodiment. The configuration of the image forming apparatus 100 of this modification is the same as that of the image forming apparatus 100 of the above embodiment.

[0051] In the image forming process of this modified example, the control unit 101 controls the image forming unit 150 to form an image of the job, a toner consumption patch P1, a gradation patch P2, a registration patch P3, and an eye mark MK on the recording medium PM. The eye mark MK is a mark that indicates the image forming position of the image of the job. FIG. 7 is a diagram showing an example of a recording medium PM on which a job image, a toner consumption patch P1, gradation patches P2a and P2b, registration patches P3a to P3d, and an eye mark MK are formed. 7, area A is the job image forming area where the job image is formed. Area B is outside the job image forming area on one side of the job image forming area in the main scanning direction, and is the area where the toner consumption patch P1, gradation patch P2, and registration patch P3 are image-formed. Area C is outside the job image forming area on the opposite side of area B in the main scanning direction, and is the area where the eye mark MK is image-formed. In the example shown in FIG. 7, tone correction can be performed by performing the same processing as the image forming processing in the above embodiment.

[0052] (Variation 2) Next, a second modification of the above embodiment will be described. The following description will focus on the differences from the above embodiment. The configuration of the image forming apparatus 100 of this modification is the same as that of the image forming apparatus 100 of the above embodiment.

[0053] An example of the read gradation values ​​of the gradation patch P2 of this modified example is shown in Figure 8. The horizontal axis of Figure 8 represents the number of samples and indicates the position in the conveyance direction of the recording medium PM. The vertical axis represents the read gradation values, and the reflectance for each R, G, and B filter is displayed. The read gradation value of the gradation patch P2 shown in FIG. 8 is obtained by reading an image of the gradation patch P2 formed with a screen pattern having a high screen ruling (for example, 175 rulings or more, more preferably 230 rulings or more). In the example shown in Fig. 8, the fluctuation of the read grayscale value at each grayscale level is small enough to allow the read grayscale value at each grayscale level to be read. However, the read grayscale value of the high-line screen shown in Fig. 8 has an unstable curve of rising and falling read grayscale values. In particular, the curve of rising and falling read grayscale values ​​of R shown in Fig. 8 loses its bowl-like shape. In this case, a regression equation with a higher order and closer to the fluctuations in the curve of the rising and falling read gradation values ​​may be calculated, thereby enabling gradation correction to be performed with higher accuracy.

[0054] (Variation 3) Next, a third modification of the above embodiment will be described. The following description will focus on the differences from the above embodiment. The configuration of the image forming apparatus 100 of this modification is the same as that of the image forming apparatus 100 of the above embodiment.

[0055] In the gradation correction process of this modified example, the control unit 101 performs registration correction processing before calculating the read gradation value of the gradation patch P2 in step S31. Specifically, the control unit 101 calculates the registration correction value based on the registration patch P3 image-formed in step S1 of the image formation process in the read image data. The control unit 101 then calculates the read gradation value of the gradation patch P2 corrected based on the registration correction value. This allows for more accurate gradation correction to be performed even when color misregistration occurs. Here, the image forming apparatus 100 may be configured to include a detection unit that detects the toner image formed on the intermediate transfer belt 153, detect the toner image formed on the intermediate transfer belt 153 using the detection unit, and perform resist correction based on the detection result.

[0056] As described above, the image forming apparatus 100 includes a plurality of image forming units 151 that form images using color materials based on image data, a color image forming section (roller 152, intermediate transfer belt 153, and secondary transfer roller 154) that forms a color image on the recording medium PM by superimposing images of each color formed by the image forming units 151, an acquisition section (controller 101) that acquires a read image by reading the image forming surface of the recording medium PM on which the color image has been formed, and a control section (controller The image forming apparatus includes a control unit (control unit 101) that performs gradation correction on an image formed by a plurality of image forming units 151 based on a read image including a gradation patch P2 acquired by the acquisition unit, and a correction unit (control unit 101) that performs gradation correction on an image formed by a plurality of image forming units 151 based on a read image including a gradation patch P2 acquired by the acquisition unit, wherein the gradation patch P2 has a halftone unit P255 that expresses a plurality of gradation levels by halftone using area modulation, and the correction unit calculates read gradation values ​​by reading the gradation values ​​of the gradation patch P2 from the read image, calculates a regression equation I of the read gradation values ​​relative to the position of the gradation patch P2 in the conveying direction of the recording medium PM, and performs gradation correction based on the regression equation I. Therefore, even if the patch for tone correction is formed as a smaller image on the paper, tone correction can be performed suitably.

[0057] Furthermore, in the image forming apparatus 100, the interval between patches of each gradation level in the gradation patch P2 is less than 10 periods of the screen ruling in the screen pattern used to form the image of the gradation patch P2. Therefore, the gradation patch P2 can be formed as a smaller image, and the gradation patch P2 can be formed as an image even in a limited area of ​​the recording medium PM.

[0058] In addition, in the image forming device 100, the gradation patch P2 has an ascending section P252 in which the gradation level changes stepwise in the conveying direction from the minimum gradation value to the maximum gradation value, and a descending section P251 in which the gradation level changes stepwise in the conveying direction from the maximum gradation value to the minimum gradation value, with the ascending section P252 being located upstream in the conveying direction and the descending section P251 being located downstream in the conveying direction. Therefore, the edge effect can be suppressed.

[0059] Furthermore, in the gradation patch P2 of the image forming apparatus 100, the maximum gradation value portion P254 of the ascending portion P252 and the maximum gradation value portion P254 of the descending portion P251 are adjacent to each other. Therefore, the influence of light coming around from the small gradation value portion in the maximum gradation value portion P254 can be minimized.

[0060] Furthermore, in the image forming apparatus 100, the correction unit calculates the regression formula I in accordance with the screen ruling in the screen pattern used to form the image of the gradation patch P2. Therefore, it is possible to perform tone correction with higher accuracy.

[0061] In addition, in the image forming device 100, the gradation patch P2 has color-specific gradation patches P24x, P25x for each color that makes up an image made of multiple color materials, and the control unit forms gradation patches P2 with different orders of color-specific gradation patches P24x, P25x at each end in the main scanning direction of the recording medium PM, which is outside the job image forming area. Therefore, when the gradation differs at each end of the recording medium PM in the main scanning direction, it is possible to perform gradation correction with higher accuracy.

[0062] Furthermore, in the image forming apparatus 100, the control unit causes an image formation of a mark that identifies the type of gradation patch P2 adjacent to the gradation patch P2 in which the order of the color gradation patches P24x and P25x is different. Therefore, the type of the tone patch P2 can be easily identified.

[0063] Furthermore, in the image forming apparatus 100, the control unit forms an image of a registration patch P3 outside the job image forming area of ​​the recording medium PM, and the mark is the registration patch P3. Therefore, an area for separately forming an image of a mark becomes unnecessary, and the area for forming an image of the gradation patch P2 can be minimized.

[0064] In the image forming apparatus 100, the correction section corrects the image forming positions of the plurality of image forming units 151 based on the read image including the registration patch P3 acquired by the acquisition section. Therefore, even when color misregistration occurs, more accurate tone correction can be achieved by performing registration correction.

[0065] In the image forming apparatus 100, the recording medium is a continuous medium. Therefore, tone correction can be performed even during image formation on a continuous medium.

[0066] Furthermore, in the image forming apparatus 100, the control unit causes the toner consumption patch P1 to be image-formed on the recording medium PM at the same position in the main scanning direction as the gradation patch P2. Therefore, a wider job image forming area can be secured on the recording medium PM.

[0067] The description of the above embodiment is a preferred example of the image forming apparatus according to the present invention, and the present invention is not limited to this.

[0068] For example, in the above embodiment and modified example, the image forming apparatus 100 forms an image on the recording medium PM, which is a rollable continuous medium, but this is not limited to this. The recording medium PM may also be a sheet of paper or other medium that is not continuous. By applying the present invention to an image forming apparatus that forms an image on a sheet of paper or other medium, gradation correction in response to environmental changes such as temperature during continuous printing can be performed without temporarily interrupting printing, which can contribute to cost reduction by improving the efficiency of printing work and reducing downtime.

[0069] In the above embodiment and modified examples, the reference color is K (black), but other colors may be used as the reference color. However, because the K patch is displayed at a high density in all of the R, G, and B separated image data, it is preferable to use K as the reference color.

[0070] Furthermore, in the above embodiment and modified examples, the read image is read by the image scanner 180, but the reading unit that reads the read image may be provided outside the image forming apparatus.

[0071] In the above description, examples have been disclosed in which a nonvolatile semiconductor memory or a hard disk is used as a computer-readable medium storing a program for executing each process, but this is not limiting. Other computer-readable media may also be portable recording media such as CD-ROMs. Furthermore, a carrier wave may also be used as a medium for providing program data via a communication line.

[0072] In addition, the detailed configuration and detailed operation of each part of the image inspection device can be modified as appropriate without departing from the spirit of the present invention. [Explanation of symbols]

[0073] 100 Image forming device 101 control unit (acquisition unit, control unit, correction unit) 102 Communications Department 103 Operation display section 104 Storage section 105 Paper feed section 105a Paper feed roll 105b Paper feed roller 106 Conveyor 107 Winding section 107a Winding roll 107b Winding roller 150 Image forming unit 151 Image forming unit 152 Roller (color image forming section) 153 Secondary transfer belt (color image forming section) 154 Secondary transfer roller (color image forming section) 170 Fixing unit 180 Image Scanner N Transfer nip

Claims

1. a plurality of image forming units that form images using color materials based on image data; a color image forming section for forming a color image on a recording medium by superimposing the images of the respective colors formed by the image forming units; an acquisition unit that acquires a read image by reading the image forming surface of the recording medium on which the color image is formed; a control unit that forms a gradation patch image outside a job image forming area on the recording medium; a correction unit that performs gradation correction on images formed by the plurality of image forming units based on the read image including the gradation patches acquired by the acquisition unit; Equipped with the gradation patch comprises an ascending section in which the gradation level changes stepwise in the conveying direction of the recording medium from a minimum gradation value to a maximum gradation value, and a descending section in which the gradation level changes stepwise in the conveying direction from a maximum gradation value to a minimum gradation value, the ascending section and the descending section each comprising a halftone section that expresses the plurality of gradation levels by halftones using area modulation, the ascending section being arranged on the upstream side in the conveying direction, and the descending section being arranged on the downstream side in the conveying direction; The correction unit calculates a read gradation value by reading the gradation value of the gradation patch from the read image, calculates a regression equation for the read gradation value relative to the position of the gradation patch in the conveying direction, and performs gradation correction based on the regression equation.

2. 2. The image forming apparatus according to claim 1, wherein the interval between patches of each tone level in the tone patches is less than 10 periods of the screen ruling in a screen pattern used to form the image of the tone patches.

3. 3. The image forming apparatus according to claim 1, wherein in the tone patch, the maximum tone value portion of the ascending order portion and the maximum tone value portion of the descending order portion are adjacent to each other.

4. 4. The image forming apparatus according to claim 1, wherein the correction section calculates the regression equation in accordance with a screen ruling in a screen pattern used to form an image of the tone patch.

5. A plurality of image forming units that form images using color materials based on image data; a color image forming section for forming a color image on a recording medium by superimposing the images of the respective colors formed by the image forming units; an acquisition unit that acquires a read image by reading the image forming surface of the recording medium on which the color image is formed; a control unit that forms a gradation patch image outside a job image forming area on the recording medium; a correction unit that performs gradation correction on images formed by the plurality of image forming units based on the read image including the gradation patches acquired by the acquisition unit; Equipped with the gradation patch has a halftone portion that expresses a plurality of gradation levels by halftones using area modulation, The correction unit calculates a read gradation value by reading the gradation value of the gradation patch from the read image, calculates a regression equation for the read gradation value relative to the position of the gradation patch in the transport direction of the recording medium according to the screen line number in the screen pattern used to form the image of the gradation patch, and performs gradation correction based on the regression equation.

6. the gradation patches include color gradation patches for each color constituting an image using the plurality of color materials; 6. An image forming apparatus according to claim 1, wherein the control unit forms images of gradation patches in which the order of the color gradation patches is different at each of both ends in the main scanning direction of the recording medium that are outside the job image forming area.

7. The image forming apparatus according to claim 6 , wherein the control unit forms an image of a mark that identifies the type of the gradation patch adjacent to a gradation patch that is in a different order from the color gradation patches.

8. A plurality of image forming units that form images using color materials based on image data; a color image forming section for forming a color image on a recording medium by superimposing the images of the respective colors formed by the image forming units; an acquisition unit that acquires a read image by reading the image forming surface of the recording medium on which the color image is formed; a control unit that forms a gradation patch image outside a job image forming area on the recording medium; a correction unit that performs gradation correction on images formed by the plurality of image forming units based on the read image including the gradation patches acquired by the acquisition unit; Equipped with the gradation patches include color gradation patches for each color constituting an image using the plurality of color materials; Each of the color gradation patches has a halftone portion that expresses a plurality of gradation levels by halftones using area modulation, the control unit forms image formation of gradation patches in which the order of the color-specific gradation patches is different on both ends in the main scanning direction of the recording medium outside the job image forming area, and forms image formation of marks that identify the types of the gradation patches adjacent to the gradation patches in which the order of the color-specific gradation patches is different; The correction unit calculates a read gradation value by reading the gradation value of the gradation patch from the read image, calculates a regression equation for the read gradation value relative to the position of the gradation patch in the transport direction of the recording medium, and performs gradation correction based on the regression equation.

9. the control unit causes a resist patch to be image-formed outside a job image forming area on the recording medium; 9. The image forming apparatus according to claim 7, wherein the mark is the resist patch.

10. The image forming apparatus according to claim 9 , wherein the correction section corrects image forming positions in the plurality of image forming units based on the read image including the registration patch acquired by the acquisition section.

11. The image forming apparatus according to claim 1 , wherein the recording medium is a continuous medium.

12. The image forming apparatus according to claim 1 , wherein the control unit forms an image of a toner consumption patch on the recording medium at the same position in the main scanning direction as the gradation patch.

13. a plurality of image forming units that form images using color materials based on image data; a color image forming section for forming a color image on a recording medium by superimposing the images of the respective colors formed by the image forming units; A computer of an image forming apparatus comprising: an acquisition unit that acquires a read image by reading the image forming surface of the recording medium on which the color image is formed; a control unit for forming a gradation patch image outside a job image forming area on the recording medium; a correction unit that performs gradation correction on images formed by the plurality of image forming units based on the read image including the gradation patches acquired by the acquisition unit; It functions as the gradation patch comprises an ascending section in which the gradation level changes stepwise in the conveying direction of the recording medium from a minimum gradation value to a maximum gradation value, and a descending section in which the gradation level changes stepwise in the conveying direction from a maximum gradation value to a minimum gradation value, the ascending section and the descending section each comprising a halftone section that expresses the plurality of gradation levels by halftones using area modulation, the ascending section being arranged on the upstream side in the conveying direction, and the descending section being arranged on the downstream side in the conveying direction; The correction unit is a program that calculates a read gradation value by reading the gradation value of the gradation patch from the read image, calculates a regression equation for the read gradation value relative to the position of the gradation patch in the transport direction, and performs gradation correction based on the regression equation.

14. A plurality of image forming units that form images using color materials based on image data; a color image forming section for forming a color image on a recording medium by superimposing the images of the respective colors formed by the image forming units; A computer of an image forming apparatus comprising: an acquisition unit that acquires a read image by reading the image forming surface of the recording medium on which the color image is formed; a control unit for forming a gradation patch image outside a job image forming area on the recording medium; a correction unit that performs gradation correction on images formed by the plurality of image forming units based on the read image including the gradation patches acquired by the acquisition unit; It functions as the gradation patch has a halftone portion that expresses a plurality of gradation levels by halftones using area modulation, The correction unit is a program that calculates a read gradation value by reading the gradation value of the gradation patch from the read image, calculates a regression equation for the read gradation value relative to the position of the gradation patch in the transport direction of the recording medium according to the screen line number in the screen pattern used to form the image of the gradation patch, and performs gradation correction based on the regression equation.

15. A plurality of image forming units that form images using color materials based on image data; a color image forming section for forming a color image on a recording medium by superimposing the images of the respective colors formed by the image forming units; A computer of an image forming apparatus comprising: an acquisition unit that acquires a read image by reading the image forming surface of the recording medium on which the color image is formed; a control unit for forming a gradation patch image outside a job image forming area on the recording medium; a correction unit that performs gradation correction on images formed by the plurality of image forming units based on the read image including the gradation patches acquired by the acquisition unit; It functions as the gradation patches include color gradation patches for each color constituting an image using the plurality of color materials; Each of the color gradation patches has a halftone portion that expresses a plurality of gradation levels by halftones using area modulation, the control unit forms image formation of gradation patches in which the order of the color-specific gradation patches is different on both ends in the main scanning direction of the recording medium outside the job image forming area, and forms image formation of marks that identify the types of the gradation patches adjacent to the gradation patches in which the order of the color-specific gradation patches is different; The correction unit is a program that calculates a read gradation value by reading the gradation value of the gradation patch from the read image, calculates a regression equation for the read gradation value relative to the position of the gradation patch in the transport direction of the recording medium, and performs gradation correction based on the regression equation.

Images