Patent Publication Number: US-2023164284-A1

Title: Image forming apparatus capable of forming image on sheet with uneven surface, image forming condition adjustment method

Description:
INCORPORATION BY REFERENCE 
     This application is based upon and claims the benefit of priority from the corresponding Japanese Patent Application No. 2021-189948 filed on Nov. 24, 2021 and Japanese Patent Application No. 2021-189949 filed on Nov. 24, 2021, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     The present disclosure relates to an image forming apparatus and an image forming condition adjustment method. 
     In an image forming apparatus of an electrophotographic method, an image may be formed on a sheet with an uneven surface such as a sheet of embossed paper. Toner is hardly adhered to a lower part of the uneven surface of the sheet. As a result, in the image forming apparatus, when an image is formed on a sheet with an uneven surface, an image forming condition, such as a transfer current that is supplied to a transfer portion that transfers a toner image to the sheet, is adjusted for the purpose of restricting the degradation of the formed image. 
     In addition, there is known, as a related technology, an image forming apparatus that adjusts the image forming condition based on the detection result of the sheet surface shape. In this image forming apparatus, the image forming condition is adjusted based on a variation width of a voltage that is applied to a transfer portion connected to a constant current power supply, when a sheet passes through a position where a toner image is transferred to the sheet by the transfer portion. 
     SUMMARY 
     An image forming apparatus according to an aspect of the present disclosure includes an image forming portion, a first acquisition processing portion, a second acquisition processing portion, and an adjustment processing portion. The image forming portion forms an image on a sheet. The first acquisition processing portion acquires a captured image of the sheet. The second acquisition processing portion acquires a specific value that indicates a difference between a gradation value of a pixel that, among pixels included in the captured image acquired by the first acquisition processing portion, corresponds to a higher part of an outer surface of the sheet, and a gradation value of a pixel that, among the pixels included in the captured image, corresponds to a lower part that is lower than the higher part. The adjustment processing portion adjusts an image forming condition of the image forming portion based on the specific value acquired by the second acquisition processing portion. 
     An image forming condition adjustment method according to another aspect of the present disclosure is executed in an image forming apparatus including an image forming portion that forms an image on a sheet, and includes a first acquisition step, a second acquisition step, and an adjustment step. In the first acquisition step, a captured image of the sheet is acquired. In the second acquisition step, a specific value is acquired, wherein the specific value indicates a difference between a gradation value of a pixel that, among pixels included in the captured image acquired in the first acquisition step, corresponds to a higher part of an outer surface of the sheet, and a gradation value of a pixel that, among the pixels included in the captured image, corresponds to a lower part that is lower than the higher part. In the adjustment step, an image forming condition of the image forming portion is adjusted based on the specific value acquired in the second acquisition step. 
     An image forming apparatus according to a further aspect of the present disclosure includes an image forming portion, a transfer processing portion, a first acquisition processing portion, a detection processing portion, a second acquisition processing portion, and an adjustment processing portion. The image forming portion forms an image on a sheet using toners of colors C, M, and Y. The transfer processing portion transfers, to the sheet, a specific toner image that includes a first toner layer of a first color formed on an image-carrying member and a second toner layer of a second color formed on the first toner layer, the first color being any one of the colors C, M, and Y, the second color being any one of the colors C, M, and Y and being different from the first color. The first acquisition processing portion acquires a captured image of the sheet. The detection processing portion detects a specific image that corresponds to the specific toner image included in the captured image acquired by the first acquisition processing portion. The second acquisition processing portion acquires a skewness of a histogram of gradation values of a color mixture of the second color and a third color in the pixels included in the specific image detected by the detection processing portion, the third color being a color different from the first color and the second color among the colors C, M, and Y. The adjustment processing portion adjusts the image forming condition of the image forming portion based on the skewness acquired by the second acquisition processing portion. 
     An image forming condition adjustment method according to a still further aspect of the present disclosure is executed in an image forming apparatus including an image forming portion that forms an image on a sheet using toners of colors C, M, and Y, and includes a transfer step, a first acquisition step, a detection step, a second acquisition step, and an adjustment step. In the transfer step, a specific toner image is transferred to the sheet, wherein the specific toner image includes a first toner layer of a first color formed on an image-carrying member and a second toner layer of a second color formed on the first toner layer, the first color being any one of the colors C, M, and Y, the second color being any one of the colors C, M, and Y and being different from the first color. In the first acquisition step, a captured image of the sheet is acquired. In the detection step, a specific image is detected, wherein the specific image corresponds to the specific toner image included in the captured image acquired in the first acquisition step. In the second acquisition step, a skewness of a histogram of gradation values of a color mixture of the second color and a third color in the pixels included in the specific image detected in the detection step, is acquired, the third color being a color different from the first color and the second color among the colors C, M, and Y In the adjustment step, the image forming condition of the image forming portion is adjusted based on the skewness acquired in the second acquisition step. 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description with reference where appropriate to the accompanying drawings. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Furthermore, the claimed subject matter is not limited to implementations that solve any or all disadvantages noted in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a cross-section diagram showing a configuration of an image forming apparatus according to a first embodiment of the present disclosure. 
         FIG.  2    is a block diagram showing a system configuration of the image forming apparatus according to the first embodiment of the present disclosure. 
         FIG.  3    is a cross-section diagram showing a configuration of an image forming portion of the image forming apparatus according to the first embodiment of the present disclosure. 
         FIG.  4    is a diagram showing an example of a histogram acquired by the image forming apparatus according to the first embodiment of the present disclosure. 
         FIG.  5    is a diagram showing an example of the histogram acquired by the image forming apparatus according to the first embodiment of the present disclosure. 
         FIG.  6    is a diagram showing an example of first table data used to adjust an image forming condition in the image forming apparatus according to the first embodiment of the present disclosure. 
         FIG.  7    is a diagram showing an example of second table data used to adjust an image forming condition in the image forming apparatus according to the first embodiment of the present disclosure. 
         FIG.  8    is a diagram showing an example of third table data used to adjust an image forming condition in the image forming apparatus according to the first embodiment of the present disclosure. 
         FIG.  9    is a diagram showing an example of fourth table data used to adjust an image forming condition in the image forming apparatus according to the first embodiment of the present disclosure. 
         FIG.  10    is a diagram showing an example of fifth table data used to adjust an image forming condition in the image forming apparatus according to the first embodiment of the present disclosure. 
         FIG.  11    is a flowchart showing an example of a first image forming condition adjustment process executed by the image forming apparatus according to the first embodiment of the present disclosure. 
         FIG.  12    is a block diagram showing a system configuration of an image forming apparatus according to a second embodiment of the present disclosure. 
         FIG.  13    is a diagram showing an example of a histogram acquired by the image forming apparatus according to the second embodiment of the present disclosure. 
         FIG.  14    is a diagram showing an example of the histogram acquired by the image forming apparatus according to the second embodiment of the present disclosure. 
         FIG.  15    is a flowchart showing an example of a second image forming condition adjustment process executed by the image forming apparatus according to the second embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following describes embodiments of the present disclosure with reference to the accompanying drawings. It should be noted that the following embodiments are examples of specific embodiments of the present disclosure and should not limit the technical scope of the present disclosure. 
     First Embodiment 
     First, a description is given of a configuration of an image forming apparatus  100  according to a first embodiment of the present disclosure with reference to  FIG.  1    and  FIG.  2   . 
     It is noted that, for the sake of explanation, a vertical direction in a state where the image forming apparatus  100  is usably installed (the state shown in  FIG.  1   ), is defined as an up-down direction D 1 . In addition, a front-rear direction D 2  is defined on the supposition that the left side of the image forming apparatus  100  in  FIG.  1    is a front side (front). Furthermore, a left-right direction D 3  is defined based on the image forming apparatus  100  in the installation state viewed from the front side. 
     The image forming apparatus  100  is a multifunction peripheral having a plurality of functions such as a scan function for reading image data from a document sheet, a print function for forming an image based on image data, a facsimile function, and a copy function. It is noted that the present disclosure is applicable to an image forming apparatus such as a printer, a facsimile apparatus, and a copier. 
     As shown in  FIG.  1    and  FIG.  2   , the image forming apparatus  100  includes an ADF (Auto Document Feeder)  1 , an image reading portion  2 , an image forming portion  3 , a sheet feed portion  4 , an operation/display portion  5 , a storage portion  6 , and a control portion  7 . 
     The ADF  1  conveys a document sheet that is a reading target of the scan function. The ADF  1  includes a document sheet setting portion, a plurality of conveyance rollers, a document sheet pressing member, and a sheet discharge portion. 
     The image reading portion  2  realizes the scan function. The image reading portion  2  includes a document sheet table, a light source, a plurality of mirrors, an optical lens, and a CCD (Charge Coupled Device). 
     The image forming portion  3  realizes the print function. Specifically, the image forming portion  3  forms, by an electrophotographic method, a color or monochrome image on a sheet supplied from the sheet feed portion  4 . 
     The sheet feed portion  4  supplies a sheet to the image forming portion  3 . The sheet feed portion  4  includes a sheet feed cassette, a manual feed tray, and a plurality of conveyance rollers. 
     The operation/display portion  5  is a user interface of the image forming apparatus  100 . The operation/display portion  5  includes a display portion and an operation portion. The display portion is, for example, a liquid crystal display and displays various types of information in response to control instructions from the control portion  7 . The operation portion is composed of, for example, operation keys or a touch panel through which various types of information are input to the control portion  7  in response to user operations. 
     The storage portion  6  is a nonvolatile storage device. For example, the storage portion  6  is a storage device such as: a nonvolatile memory such as a flash memory or an EEPROM; an SSD (Solid State Drive); or an HDD (Hard Disk Drive). 
     The control portion  7  comprehensively controls the image forming apparatus  100 . As shown in  FIG.  2   , the control portion  7  includes a CPU  11 , a ROM  12 , and a RAM  13 . The CPU  11  is a processor that executes various calculation processes. The ROM  12  is a nonvolatile storage device in which various information such as control programs for causing the CPU  11  to execute various processes are preliminarily stored. The RAM  13  is a volatile or nonvolatile storage device that is used as a temporary storage memory (working area) for the various processes executed by the CPU  11 . The CPU  11  comprehensively controls the image forming apparatus  100  by executing the various control programs preliminarily stored in the ROM  12 . 
     It is noted that the control portion  7  may be a control portion provided independently of a main control portion that comprehensively controls the image forming apparatus  100 . In addition, the control portion  7  may be formed as an electronic circuit such as an integrated circuit (ASIC). 
     Configuration of Image Forming Portion  3   
     Next, a configuration of the image forming portion  3  is described with reference to  FIG.  1    to  FIG.  3   . Here,  FIG.  3    is a cross-section diagram showing a configuration of a plurality of image forming units  20 , an intermediate transfer belt  26 , and a secondary transfer roller  27 . 
     As shown in  FIG.  1   , the image forming portion  3  includes four image forming units  20 , a laser scanning unit  25 , the intermediate transfer belt  26 , the secondary transfer roller  27 , a fixing device  28 , and a sheet discharge tray  29 . In addition, as shown in  FIG.  2   , the image forming portion  3  includes a power supply  41  and an image capturing portion  42 . 
     Of the four image forming units  20 , an image forming unit  21  (see  FIG.  3   ) forms a Y (yellow) toner image. Of the four image forming units  20 , an image forming unit  22  (see  FIG.  3   ) forms a C (cyan) toner image. Of the four image forming units  20 , an image forming unit  23  (see  FIG.  3   ) forms an M (magenta) toner image. Of the four image forming units  20 , an image forming unit  24  (see  FIG.  3   ) forms a K (black) toner image. That is, the image forming portion  3  forms an image on a sheet using toners of colors C, M, Y, and K. As shown in  FIG.  1    and  FIG.  3   , the four image forming units  20  are arranged in order of yellow, cyan, magenta, and black from the front side of the image forming apparatus  100  along the front-rear direction D 2 . 
     As shown in  FIG.  3   , each of the image forming units  20  includes a photoconductor drum  31 , a charging roller  32 , a developing device  33 , a primary transfer roller  34 , and a drum cleaning portion  35 . In addition, each of the image forming units  20  includes a toner container  36  shown in  FIG.  1   . 
     On a surface of the photoconductor drum  31 , an electrostatic latent image is formed. For example, the photoconductor drum  31  has a photosensitive layer formed from amorphous silicon. Upon receiving a rotational driving force supplied from a motor (not shown), the photoconductor drum  31  rotates in a rotation direction D 4  shown in  FIG.  3   . This allows the photoconductor drum  31  to convey the electrostatic latent image formed on its surface. 
     Upon receiving a supply of a predetermined charging voltage, the charging roller  32  electrically charges the surface of the photoconductor drum  31 . For example, the charging roller  32  charges the surface of the photoconductor drum  31  to a positive polarity. The surface of the photoconductor drum  31  charged by the charging roller  32  is irradiated with light that is emitted from the laser scanning unit  25  based on image data. This forms an electrostatic latent image on the surface of the photoconductor drum  31 . 
     The developing device  33  develops the electrostatic latent image formed on the surface of the photoconductor drum  31 . The developing device  33  includes a pair of stirring members, a magnet roller, and a developing roller. The pair of stirring members stir developer stored inside the developing device  33 , wherein the developer includes toner and carrier. As the developer is stirred, the toner included in the developer makes friction with the carrier included in the developer, and the toner is charged to the positive polarity. The magnet roller draws up the developer stirred by the pair of stirring members and supplies the toner included in the developer to the developing roller. The developing roller conveys the toner supplied from the magnet roller to a position facing the photoconductor drum  31 . In addition, upon receiving an application of a predetermined developing bias voltage, the developing roller supplies the toner conveyed to the position facing the photoconductor drum  31 , to the photoconductor drum  31 . This allows the electrostatic latent image formed on the surface of the photoconductor drum  31  to be visualized (developed). It is noted that the toner is supplied from the toner container  36  to the developing device  33 . 
     The primary transfer roller  34 , upon receiving a supply of a predetermined primary transfer current, transfers a toner image formed on the surface of the photoconductor drum  31  to an outer peripheral surface of the intermediate transfer belt  26 . As shown in  FIG.  3   , the primary transfer roller  34  is disposed to face the photoconductor drum  31  across the intermediate transfer belt  26 . 
     The drum cleaning portion  35  removes the toner that has remained on the surface of the photoconductor drum  31  after the transfer of the toner image by the primary transfer roller  34 . 
     The laser scanning unit  25  emits light based on the image data, to the surfaces of the photoconductor drums  31  of the image forming units  20 . 
     The intermediate transfer belt  26  is a belt member of an endless shape to which toner images formed on the surfaces of the photoconductor drums  31  of the image forming units  20  are transferred. The intermediate transfer belt  26  is stretched by a drive roller  26 A (see  FIG.  3   ) and a stretch roller  26 B (see  FIG.  3   ) with a predetermined tension. The intermediate transfer belt  26  rotates in a rotation direction D 5  shown in  FIG.  3    when the drive roller  26 A rotates upon receiving a rotational driving force supplied from a motor (not shown). This allows the intermediate transfer belt  26  to convey the toner image formed on the outer peripheral surface thereof to a secondary transfer position P 1  (see  FIG.  3   ) where the toner image is transferred to a sheet by the secondary transfer roller  27 . It is noted that the outer peripheral surface of the intermediate transfer belt  26  from which the toner image has been transferred by the secondary transfer roller  27  is cleaned by a belt cleaning portion  26 C shown in  FIG.  3   . 
     The secondary transfer roller  27 , upon receiving a supply of a predetermined secondary transfer current, transfers the toner image that has been transferred to the outer peripheral surface of the intermediate transfer belt  26 , to a sheet supplied from the sheet feed portion  4 . As shown in  FIG.  3   , the secondary transfer roller  27  is disposed to face the drive roller  26 A across the intermediate transfer belt  26 . The secondary transfer roller  27  is biased by a biasing member (not shown) towards the drive roller  26 A so that the secondary transfer roller  27  comes in contact with the intermediate transfer belt  26  with a predetermined nip pressure. The secondary transfer roller  27 , at the secondary transfer position P 1  (see  FIG.  3   ) where it comes in contact with the intermediate transfer belt  26 , transfers the toner image formed on the intermediate transfer belt  26  to the sheet. The secondary transfer roller  27  is an example of a transfer portion of the present disclosure. 
     The fixing device  28  fixes the toner image transferred to the sheet by the secondary transfer roller  27 , to the sheet. As shown in  FIG.  1   , the fixing device  28  includes a fixing roller  28 A and a pressure roller  28 B. The fixing roller  28 A is heated to a predetermined fixing temperature by a heater (not shown). The fixing roller  28 A is rotated at a predetermined speed. The pressure roller  28 B is biased by a biasing member (not shown) towards the fixing roller  28 A so that the pressure roller  28 B comes in contact with the fixing roller  28 A with a predetermined nip pressure. Between the fixing roller  28 A and the pressure roller  28 B is formed a fixing nip portion P 2  (see  FIG.  1   ) that heats and pressurizes a sheet. The toner image transferred to the sheet is heated and pressurized so as to be fixed to the sheet when the sheet passes through the fixing nip portion P 2 . The fixing device  28  is an example of a fixing portion of the present disclosure. 
     The sheet to which the toner image has been fixed by the fixing device  28  is discharged to the sheet discharge tray  29 . 
     The power supply  41  is a constant current power supply that supplies the secondary transfer current to the secondary transfer roller  27 . The power supply  41  supplies the secondary transfer current set by the control portion  7  to the secondary transfer roller  27 . For example, the secondary transfer current is a current of a negative polarity. 
     The image capturing portion  42  captures an image of a sheet that has been conveyed via the secondary transfer position P 1  (see  FIG.  1   ) where the toner image is transferred to a sheet by the secondary transfer roller  27 . In other words, the image capturing portion  42  reads an image of a sheet that has been conveyed via the secondary transfer position P 1 . Specifically, the image capturing portion  42  captures the image of the sheet at a downstream of the secondary transfer position P 1  in a sheet conveyance path R 1  (see the two-dot chain line with an arrow shown in  FIG.  1   ) that extends from the sheet feed cassette to the sheet discharge tray  29  via the secondary transfer position P 1  (see  FIG.  1   ) and the fixing nip portion P 2  (see  FIG.  1   ). For example, as shown in  FIG.  1   , the image capturing portion  42  is disposed at a downstream of the fixing nip portion P 2  in the conveyance path R 1 . The secondary transfer position P 1  is an example of an image forming position of the present disclosure. It is noted that the image capturing portion  42  may be disposed at an upstream of the fixing nip portion P 2  in the conveyance path R 1 . 
     For example, the image capturing portion  42  is a CIS (Contact Image Sensor) that includes a light emitting portion and a light receiving portion. The light emitting portion emits light toward a surface of a sheet that is conveyed along the conveyance path R 1 . The light receiving portion receives the light that has been emitted from the light emitting portion and reflected on the surface of the sheet, and outputs an electric signal that corresponds to an amount of received light. 
     The electric signal output from the light receiving portion of the image capturing portion  42  is converted into a digital signal (image data) by an analog front-end circuit (not shown). For example, the analog front-end circuit converts the electric signal output from the light receiving portion of the image capturing portion  42  into image data that represents colors of the pixels by R, G, and B of 256 gradations. The image data output from the analog front-end circuit is input to the control portion  7 . 
     Meanwhile, in the image forming apparatus  100 , an image may be formed on a sheet with an uneven surface such as a sheet of embossed paper. The toner is hardly adhered to a lower part of the uneven surface of the sheet. As a result, in the image forming apparatus  100 , when an image is formed on a sheet with an uneven surface, an image forming condition such as the secondary transfer current is adjusted for the purpose of restricting the degradation of the formed image. 
     In addition, there is known, as a related technology, an image forming apparatus that adjusts the image forming condition based on the detection result of the sheet surface shape. In this image forming apparatus, the image forming condition is adjusted based on a variation width of a voltage that is applied to a transfer portion connected to a constant current power supply, when a sheet passes through a position where a toner image is transferred to the sheet by the transfer portion. 
     Here, on the sheet with the uneven surface, the larger the difference in the surface height is, the more hardly the toner is adhered to the lower part. However, the variation width of the voltage detected in the image forming apparatus of the related technology reflects both the difference in the sheet surface height and a ratio of the lower part to a higher part that is higher than the lower part, in the width direction of the sheet. Thus, the image forming apparatus of the related technology cannot adjust the image forming condition accurately. 
     On the other hand, as described in the following, the image forming apparatus  100  according to the first embodiment of the present disclosure can adjust the image forming condition accurately when an image is formed on a sheet with an uneven surface. 
     Configuration of Control Portion  7   
     Next, a configuration of the control portion  7  is described with reference to  FIG.  2   . 
     As shown in  FIG.  2   , the control portion  7  includes a first acquisition processing portion  51 , a second acquisition processing portion  52 , and an adjustment processing portion  53 . 
     Specifically, a first image forming condition adjustment program for causing the CPU  11  to function as the above-described portions is preliminarily stored in the ROM  12  of the control portion  7 . The CPU  11  functions as the above-described portions by executing the first image forming condition adjustment program stored in the ROM  12 . 
     It is noted that the first image forming condition adjustment program may be recorded on a non-transitory computer-readable recording medium such as a CD, a DVD, or a flash memory, and may be read from the recording medium and installed in a storage device such as the storage portion  6 . In addition, a part or all of the first acquisition processing portion  51 , the second acquisition processing portion  52 , and the adjustment processing portion  53  may be composed of an electronic circuit such as an integrated circuit (ASIC). 
     The first acquisition processing portion  51  acquires a captured image of a sheet. 
     Specifically, the first acquisition processing portion  51  acquires the captured image of the sheet by using the image capturing portion  42 . 
     For example, when a predetermined conveyance timing arrives, the first acquisition processing portion  51  causes a sheet stored in the sheet feed cassette to be conveyed along the conveyance path R 1 . Subsequently, the first acquisition processing portion  51  uses the image capturing portion  42  to capture an image of the sheet that is conveyed upon the arrival of the conveyance timing. 
     For example, the conveyance timing is a timing when an instruction to execute a print process for forming an image on a sheet has been input. It is noted that the conveyance timing may be a timing when a predetermined user operation has been performed on the operation/display portion  5 . 
     It is noted that the first acquisition processing portion  51  may acquire the captured image of the sheet by using the image reading portion  2 . For example, when the predetermined user operation has been performed on the operation/display portion  5 , the first acquisition processing portion  51  may use the image reading portion  2  to capture an image of a sheet that is placed on the document sheet table or a sheet that is conveyed by the ADF  1 . 
     The second acquisition processing portion  52  acquires a specific value that indicates a difference between: a gradation value of a pixel, among pixels included in the captured image of the sheet acquired by the first acquisition processing portion  51 , that corresponds to the higher part of an outer surface of the sheet; and a gradation value of a pixel, among the pixels included in the captured image, that corresponds to the lower part that is lower than the higher part. 
     Specifically, the second acquisition processing portion  52  acquires, as the specific value, a difference between: a gradation value that, in a histogram of gradation values of the pixels included in the captured image of the sheet, corresponds to a local maximum value of the highest frequency; and a gradation value that corresponds to a local maximum value of the second highest frequency. 
     For example, the second acquisition processing portion  52  acquires a histogram of gradation values of G (green) that shows an appearance frequency for each gradation value of G in the captured image of the sheet acquired by the first acquisition processing portion  51 , based on gradation values of G of the pixels included in the captured image. Specifically, the second acquisition processing portion  52  acquires the histogram of gradation values of G by totaling, for each gradation value of G, the number of appearances of a pixel having a gradation value of G in the captured image of the sheet. 
     Here,  FIG.  4    and  FIG.  5    show examples of the histogram of gradation values of G acquired by the second acquisition processing portion  52 . 
       FIG.  4    shows an example of the histogram of gradation values of G acquired by the second acquisition processing portion  52  when an image of a first sheet having an uneven surface is captured. The first sheet is a sheet whose surface includes a flat part and a plurality of recesses. 
       FIG.  5    shows an example of the histogram of gradation values of G acquired by the second acquisition processing portion  52  when an image of a second sheet having an even surface is captured. 
     As shown in  FIG.  5   , when the first acquisition processing portion  51  acquires a captured image of the second sheet, only one peak (local maximum value) of frequency appears. A gradation value of G that corresponds to the peak is a gradation value of G of a pixel corresponding to the surface of the second sheet. 
     On the other hand, as shown in  FIG.  4   , when the first acquisition processing portion  51  acquires a captured image of the first sheet, two peaks (local maximum values) of frequency appears. A gradation value of G that corresponds to, of the two peaks, the peak of a higher frequency is a gradation value of G of a pixel corresponding to the flat part of the first sheet. In addition, a gradation value of G that corresponds to, of the two peaks, the peak of a lower frequency is a gradation value of G of a pixel corresponding to a recess of the first sheet. 
     For example, when the acquired histogram of gradation values of G includes a plurality of peaks, the second acquisition processing portion  52  acquires, as the specific value, a difference between a gradation value corresponding to a peak (local maximum value) of the highest frequency and a gradation value corresponding to a peak (local maximum value) of the second highest frequency. On the other hand, when the acquired histogram of gradation values of G does not include a plurality of peaks, the second acquisition processing portion  52  does not acquire the specific value. 
     It is noted that when the acquired histogram of gradation values of G includes three or more peaks, the second acquisition processing portion  52  may acquire, as the specific value, a difference between: a gradation value corresponding to a peak that appears in an area including the highest frequency; and a gradation value corresponding to a peak that appears in an area including the lowest frequency. 
     In addition, when the first acquisition processing portion  51  has acquired a captured image of a sheet, the second acquisition processing portion  52  may acquire a histogram of gradation values of R or B among gradation values of R, G, and B of pixels included in the captured image. In addition, the second acquisition processing portion  52  may acquire a histogram of average values of gradation values of R, G, and B. 
     In addition, the second acquisition processing portion  52  may detect a plurality of areas of different heights included a sheet whose image has been captured, based on an image indicating stepped parts included in the captured image of the sheet. In this case, the second acquisition processing portion  52  may acquire the specific value by determining, among the plurality of areas included in the sheet whose image has been captured, an area having the highest gradation value of pixel as the higher part and determining an area having the lowest gradation value of pixel as the lower part. 
     The adjustment processing portion  53  adjusts the image forming condition of the image forming portion  3  based on the specific value acquired by the second acquisition processing portion  52 . 
     Specifically, the adjustment processing portion  53  adjusts the nip pressure at the secondary transfer position P 1  based on the specific value acquired by the second acquisition processing portion  52 . The nip pressure at the secondary transfer position P 1  is an example of the image forming condition of the present disclosure, and an example of a transfer condition under which a toner image is transferred by a transfer portion of the present disclosure. 
     In addition, the adjustment processing portion  53  adjusts the secondary transfer current supplied to the secondary transfer roller  27 , based on the specific value acquired by the second acquisition processing portion  52 . The secondary transfer current is an example of the image forming condition of the present disclosure, and an example of the transfer condition under which the toner image is transferred by the transfer portion of the present disclosure. 
     In addition, the adjustment processing portion  53  adjusts the fixing temperature of the fixing roller  28 A based on the specific value acquired by the second acquisition processing portion  52 . The fixing temperature is an example of the image forming condition of the present disclosure, and an example of a fixing condition under which the toner image is fixed by the fixing portion of the present disclosure. 
     In addition, the adjustment processing portion  53  adjusts a nip pressure at the fixing nip portion P 2  based on the specific value acquired by the second acquisition processing portion  52 . The nip pressure at the fixing nip portion P 2  is an example of the image forming condition of the present disclosure, and an example of the fixing condition under which the toner image is fixed by the fixing portion of the present disclosure. 
     In addition, the adjustment processing portion  53  adjusts a rotation speed of the fixing roller  28 A based on the specific value acquired by the second acquisition processing portion  52 . The rotation speed of the fixing roller  28 A is an example of the image forming condition of the present disclosure, and an example of the fixing condition under which the toner image is fixed by the fixing portion of the present disclosure. 
     For example, when the second acquisition processing portion  52  has acquired the specific value, the adjustment processing portion  53  determines an unevenness depth level of the sheet based on the specific value. For example, in the image forming apparatus  100 , the unevenness depth level is determined as one of six levels from level 1 (most shallow) to level 6 (most deep) depending on the height of the specific value. 
     In addition, in the image forming apparatus  100 , a first table data TD 11  (see  FIG.  6   ) is preliminarily stored in the storage portion  6 , wherein the first table data TD 11  shows a correspondence relationship between the unevenness depth level and an adjustment amount of the nip pressure at the secondary transfer position P 1 . 
     The adjustment processing portion  53  consults the first table data TD 11  to identify an adjustment amount of the nip pressure at the secondary transfer position P 1  corresponding to the determined unevenness depth level. Subsequently, the adjustment processing portion  53  adjusts the nip pressure at the secondary transfer position P 1  based on the identified adjustment amount. 
     In addition, in the image forming apparatus  100 , a second table data TD 12  (see  FIG.  7   ) is preliminarily stored in the storage portion  6 , wherein the second table data TD 12  shows a correspondence relationship between the unevenness depth level and an adjustment amount of the secondary transfer current. 
     The adjustment processing portion  53  consults the second table data TD 12  to identify an adjustment amount of the secondary transfer current corresponding to the determined unevenness depth level. Subsequently, the adjustment processing portion  53  adjusts the secondary transfer current based on the identified adjustment amount. 
     In addition, in the image forming apparatus  100 , a third table data TD 13  (see  FIG.  8   ) is preliminarily stored in the storage portion  6 , wherein the third table data TD 13  shows a correspondence relationship between the unevenness depth level and an adjustment amount of the fixing temperature. 
     The adjustment processing portion  53  consults the third table data TD 13  to identify an adjustment amount of the fixing temperature corresponding to the determined unevenness depth level. Subsequently, the adjustment processing portion  53  adjusts the fixing temperature based on the identified adjustment amount. 
     In addition, in the image forming apparatus  100 , a fourth table data TD 14  (see  FIG.  9   ) is preliminarily stored in the storage portion  6 , wherein the fourth table data TD 14  shows a correspondence relationship between the unevenness depth level and an adjustment amount of the nip pressure at the fixing nip portion P 2 . 
     The adjustment processing portion  53  consults the fourth table data TD 14  to identify an adjustment amount of the nip pressure at the fixing nip portion P 2  corresponding to the determined unevenness depth level. Subsequently, the adjustment processing portion  53  adjusts the nip pressure at the fixing nip portion P 2  based on the identified adjustment amount. 
     In addition, in the image forming apparatus  100 , a fifth table data TD 15  (see  FIG.  10   ) is preliminarily stored in the storage portion  6 , wherein the fifth table data TD 15  shows a correspondence relationship between the unevenness depth level and an adjustment amount of the rotation speed of the fixing roller  28 A. 
     The adjustment processing portion  53  consults the fifth table data TD 15  to identify an adjustment amount of the rotation speed of the fixing roller  28 A corresponding to the determined unevenness depth level. Subsequently, the adjustment processing portion  53  adjusts the rotation speed of the fixing roller  28 A based on the identified adjustment amount. 
     It is noted that the adjustment processing portion  53  may acquire an adjustment amount of an image forming condition corresponding to a specific value acquired by the second acquisition processing portion  52 , by using a relational expression that represents a relationship between the specific value and the adjustment amount of the image forming condition. 
     First Image Forming Condition Adjustment Process 
     In the following, with reference to  FIG.  11   , a description is given of an example of the procedure of a first image forming condition adjustment process executed by the control portion  7  in the image forming apparatus  100 , as well as a first image forming condition adjustment method of the present disclosure. Here, steps S 11 , S 12 , ... represent numbers assigned to the processing procedures (steps) executed by the control portion  7 . It is noted that the control portion  7  executes the first image forming condition adjustment process when the conveyance timing has arrived. 
     Step S 11   
     First, in step S 11 , the control portion  7  causes a sheet stored in the sheet feed cassette to be conveyed along the conveyance path R 1 . 
     Step S 12   
     In step S 12 , the control portion  7  uses the image capturing portion  42  to acquire a captured image of the sheet that is conveyed by the process of step S 11 . Here, the process of step S 12  is an example of a first acquisition step of the present disclosure, and is executed by the first acquisition processing portion  51  of the control portion  7 . 
     Step S 13   
     In step S 13 , the control portion  7  acquires, based on the captured image acquired in step S 12 , a histogram of gradation values of G of pixels included in the captured image. 
     Step S 14   
     In step S 14 , the control portion  7  determines whether or not the histogram of gradation values of G acquired in step S 13  includes a plurality of peaks. 
     Here, upon determining that the histogram of gradation values of G acquired in step S 13  includes a plurality of peaks (Yes side at S 14 ), the control portion  7  moves the process to step S 15 . In addition, upon determining that the histogram does not include a plurality of peaks (No side at S 14 ), the control portion  7  ends the first image forming condition adjustment process. 
     Step S 15   
     In step S 15 , the control portion  7  acquires the specific value. Here, the process of step S 15  is an example of a second acquisition step of the present disclosure, and is executed by the second acquisition processing portion  52  of the control portion  7 . 
     Specifically, the control portion  7  acquires, as the specific value, a difference between: a gradation value corresponding to a peak (local maximum value) of the highest frequency; and a gradation value corresponding to a peak (local maximum value) of the second highest frequency in the histogram of gradation values of G acquired in step S 13 . 
     Step S 16   
     In step S 16 , the control portion  7  adjusts the image forming condition based on the specific value acquired in step S 15 . Here, the process of step S 16  is an example of an adjustment step of the present disclosure, and is executed by the adjustment processing portion  53  of the control portion  7 . 
     Specifically, the control portion  7  determines the unevenness depth level based on the specific value acquired in step S 15 . 
     In addition, the control portion  7  consults the first table data TD 11  (see  FIG.  6   ) to identify an adjustment amount of the nip pressure at the secondary transfer position P 1  corresponding to the determined unevenness depth level. Subsequently, the control portion  7  adjusts the nip pressure at the secondary transfer position P 1  based on the identified adjustment amount. 
     In addition, the control portion  7  consults the second table data TD 12  (see  FIG.  7   ) to identify an adjustment amount of the secondary transfer current corresponding to the determined unevenness depth level. Subsequently, the control portion  7  adjusts the secondary transfer current based on the identified adjustment amount. 
     In addition, the control portion  7  consults the third table data TD 13  (see  FIG.  8   ) to identify an adjustment amount of the fixing temperature corresponding to the determined unevenness depth level. Subsequently, the control portion  7  adjusts the fixing temperature based on the identified adjustment amount. 
     In addition, the control portion  7  consults the fourth table data TD 14  (see  FIG.  9   ) to identify an adjustment amount of the nip pressure at the fixing nip portion P 2  corresponding to the determined unevenness depth level. Subsequently, the control portion  7  adjusts the nip pressure at the fixing nip portion P 2  based on the identified adjustment amount. 
     In addition, the control portion  7  consults the fifth table data TD 15  (see  FIG.  10   ) to identify an adjustment amount of the rotation speed of the fixing roller  28 A corresponding to the determined unevenness depth level. Subsequently, the control portion  7  adjusts the rotation speed of the fixing roller  28 A based on the identified adjustment amount. 
     As described above, the image forming apparatus  100  acquires the specific value that indicates a difference between: a gradation value of a pixel, among pixels included in a captured image of a sheet, that corresponds to the higher part of an outer surface of the sheet; and a gradation value of a pixel, among the pixels included in the captured image, that corresponds to the lower part. In addition, the image forming condition is adjusted based on the acquired specific value. With this configuration, when an image is formed on a sheet with an uneven surface, compared with the image forming apparatus of the related technology, it is possible to adjust the image forming condition accurately. 
     In addition, the image forming apparatus  100  acquires, as the specific value, a difference between: a gradation value that corresponds to a local maximum value of the highest frequency in a histogram of gradation values of the pixels included in the captured image of the sheet; and a gradation value that corresponds to a local maximum value of the second highest frequency. With this configuration, compared with a configuration where a plurality of areas of different heights that are included a sheet whose image has been captured, are detected from the captured image of the sheet, and the specific value is acquired based on the detection result, it is possible to acquire the specific value by a simple process. 
     Second Embodiment 
     Next, a description is given of a configuration of an image forming apparatus  200  according to a second embodiment of the present disclosure with reference to  FIG.  12   . 
     The image forming apparatus  200  according to the second embodiment includes the control portion  7  that has a different configuration from that of the image forming apparatus  100  according to the first embodiment. It is noted that the image forming apparatus  100  and the image forming apparatus  200  have the same components other than the control portion  7 . 
     As shown in  FIG.  12   , the control portion  7  of the image forming apparatus  200  includes a transfer processing portion  61 , a first acquisition processing portion  62 , a detection processing portion  63 , a second acquisition processing portion  64 , and an adjustment processing portion  65 . 
     Specifically, a second image forming condition adjustment program for causing the CPU  11  to function as the above-described portions is preliminarily stored in the ROM  12  of the control portion  7 . The CPU  11  functions as the above-described portions by executing the second image forming condition adjustment program stored in the ROM  12 . 
     It is noted that the second image forming condition adjustment program may be recorded on a non-transitory computer-readable recording medium such as a CD, a DVD, or a flash memory, and may be read from the recording medium and installed in a storage device such as the storage portion  6 . In addition, a part or all of the transfer processing portion  61 , the first acquisition processing portion  62 , the detection processing portion  63 , the second acquisition processing portion  64 , and the adjustment processing portion  65  may be composed of an electronic circuit such as an integrated circuit (ASIC). 
     The transfer processing portion  61  transfers, to a sheet, a specific toner image that includes: a first toner layer of a first color formed on the intermediate transfer belt  26 ; and a second toner layer of a second color formed on the first toner layer, wherein the first color is any one of colors C, M, and Y, and the second color is any one of the colors C, M, and Y and is different from the first color. The intermediate transfer belt  26  is an example of an image-carrying member of the present disclosure. 
     For example, the first toner layer is a toner image of C (cyan). In addition, the second toner layer is a toner image of M (magenta). In this case, the first color is C (cyan), and the second color is M (magenta). In addition, the specific toner image is a toner image of B (blue) that is a color mixture of the first color and the second color. 
     For example, in the image forming apparatus  200 , first image data that is used by the image forming unit  22  to form the first toner layer is preliminarily stored in the storage portion  6 . The first image data includes a first image that corresponds to the first toner layer formed on the intermediate transfer belt  26  by the image forming unit  22 . For example, the first image is a rectangular image of a predetermined size. In addition, the first image is a single-color image of C (cyan) having a predetermined specific density. 
     In addition, in the image forming apparatus  200 , second image data that is used by the image forming unit  23  to form the second toner layer is preliminarily stored in the storage portion  6 . The second image data includes a second image that corresponds to the second toner layer formed by the image forming unit  23  on the first toner layer. For example, the second image has the same shape as the first image. In addition, the second image is a single-color image of M (magenta) having the specific density. 
     The transfer processing portion  61  transfers the specific toner image to a sheet by using the image forming unit  22 , the image forming unit  23 , the laser scanning unit  25 , the intermediate transfer belt  26 , the secondary transfer roller  27 , the sheet feed portion  4 , the first image data, and the second image data. Specifically, the transfer processing portion  61  forms the first toner layer on the photoconductor drum  31  of the image forming unit  22 , and transfers the first toner layer onto the intermediate transfer belt  26 . In addition, the transfer processing portion  61  forms the second toner layer on the photoconductor drum  31  of the image forming unit  23 , and transfers the second toner layer onto the first toner layer formed on the intermediate transfer belt  26 . This forms the specific toner image on the intermediate transfer belt  26 . Subsequently, the transfer processing portion  61  transfers the specific toner image from the intermediate transfer belt  26  to a sheet conveyed by the sheet feed portion  4 . This forms, on the sheet, a toner image in which the layers of the specific toner image are arranged upside down. 
     For example, when the conveyance timing arrives, the transfer processing portion  61  causes a sheet stored in the sheet feed cassette to be conveyed along the conveyance path R 1 . Subsequently, the transfer processing portion  61  transfers the specific toner image to the sheet that is conveyed upon the arrival of the conveyance timing. 
     The first acquisition processing portion  62  acquires the captured image of the sheet. 
     Specifically, the first acquisition processing portion  62  uses the image capturing portion  42  to acquires the captured image of the sheet to which the specific toner image has been transferred by the transfer processing portion  61 . 
     It is noted that the first acquisition processing portion  62  may acquire the captured image of the sheet by using the image reading portion  2 . For example, when a predetermined user operation is received after the sheet to which the specific toner image has been transferred by the transfer processing portion  61  is discharged to the sheet discharge tray  29 , the first acquisition processing portion  62  may use the image reading portion  2  to capture an image of a sheet that is placed on the document sheet table or a sheet that is conveyed by the ADF  1 . 
     The detection processing portion  63  detects a specific image that corresponds to the specific toner image included in the captured image of the sheet acquired by the first acquisition processing portion  62 . 
     For example, the detection processing portion  63  detects, as the specific image, a colored area (an area of a color that is different from a base color of the sheet) having the same shape as the specific toner image included in the captured image of the sheet. 
     The second acquisition processing portion  64  acquires a skewness of a histogram of gradation values of a color mixture of the second color and a third color in the pixels included in the specific image detected by the detection processing portion  63 , the third color being a color different from the first color and the second color among C, M, and Y 
     For example, when the first color is C (cyan) and the second color is M (magenta), the third color is Y (yellow). In this case, the color mixture of the second color and the third color is R (red). 
     For example, the second acquisition processing portion  64  acquires a histogram of gradation values of R (red) that indicates an appearance frequency for each gradation value of R in the specific image, based on gradation values of R of the pixels included in the specific image detected by the detection processing portion  63 . Specifically, the second acquisition processing portion  64  acquires the histogram of gradation values of R by totaling, for each gradation value of R, the number of appearances of a pixel having a gradation value of R in the specific image. Subsequently, the second acquisition processing portion  64  calculates the skewness of the histogram based on the acquired histogram. 
     Here,  FIG.  13    and  FIG.  14    show examples of the histogram of gradation values of R acquired by the second acquisition processing portion  64 . 
       FIG.  13    shows an example of the histogram of gradation values of R acquired by the second acquisition processing portion  64  when the specific image indicates the specific toner image transferred to the first sheet. 
     In addition,  FIG.  14    shows an example of the histogram of gradation values of R acquired by the second acquisition processing portion  64  when the specific image indicates the specific toner image transferred to the second sheet. 
     As shown in  FIG.  14   , when the specific image indicates the specific toner image transferred to the second sheet, the skewness of the histogram of gradation values of R is substantially 0 (zero). 
     On the other hand, as shown in  FIG.  13   , when the specific image indicates the specific toner image transferred to the first sheet, the skewness of the histogram of gradation values of R is a value of the positive side. The value becomes higher as the difference in height between the flat part and the recesses of the first sheet becomes larger. This is because the transfer of the first layer to the recesses tends to be insufficient since the distance to the recesses from the first layer is larger than that from the second layer, and the second toner layer transferred to the sheet tends to be exposed correspondingly. 
     The adjustment processing portion  65  adjusts the image forming condition of the image forming portion  3  based on the skewness acquired by the second acquisition processing portion  64 . 
     Specifically, the adjustment processing portion  65  adjusts the nip pressure at the secondary transfer position P 1  based on the skewness acquired by the second acquisition processing portion  64 . 
     In addition, the adjustment processing portion  65  adjusts the secondary transfer current supplied to the secondary transfer roller  27 , based on the skewness acquired by the second acquisition processing portion  64 . 
     For example, when the second acquisition processing portion  64  has acquired the skewness, the adjustment processing portion  65  determines the unevenness depth level based on the skewness. For example, in the image forming apparatus  200 , the unevenness depth level is determined as one of six levels from level 1 (most shallow) to level 6 (most deep) depending on the height of the skewness. It is noted that when the difference in sheet surface height is large, the skewness acquired by the second acquisition processing portion  64  may be a value of the negative side. In this case, the adjustment processing portion  65  may determine the unevenness depth level as the maximum (level 6). 
     In addition, the adjustment processing portion  65  consults the first table data TD 11  (see  FIG.  6   ) to identify an adjustment amount of the nip pressure at the secondary transfer position P 1  corresponding to the determined unevenness depth level. Subsequently, the adjustment processing portion  65  adjusts the nip pressure at the secondary transfer position P 1  based on the identified adjustment amount. 
     In addition, the adjustment processing portion  65  consults the second table data TD 12  (see  FIG.  7   ) to identify an adjustment amount of the secondary transfer current corresponding to the determined unevenness depth level. Subsequently, the adjustment processing portion  65  adjusts the secondary transfer current based on the identified adjustment amount. 
     It is noted that the adjustment processing portion  65  may adjust one or more of the fixing temperature, the nip pressure at the fixing nip portion P 2 , and the rotation speed of the fixing roller  28 A, based on the skewness acquired by the second acquisition processing portion  64 . 
     It is noted that the adjustment processing portion  65  may acquire an adjustment amount of an image forming condition corresponding to a skewness acquired by the second acquisition processing portion  64 , by using a relational expression that represents a relationship between the skewness and the adjustment amount of the image forming condition. 
     Second Image Forming Condition Adjustment Process 
     In the following, with reference to  FIG.  15   , a description is given of an example of the procedure of a second image forming condition adjustment process executed by the control portion  7  in the image forming apparatus  200 , as well as a second image forming condition adjustment method of the present disclosure. It is noted that the control portion  7  executes the second image forming condition adjustment process when the conveyance timing has arrived. 
     Step S 21   
     First, in step S 21 , the control portion  7  causes a sheet stored in the sheet feed cassette to be conveyed along the conveyance path R 1 . 
     Step S 22   
     In step S 22 , the control portion  7  transfers the specific toner image to a sheet conveyed by the process of step S 21 . Here, the process of step S 22  is an example of a transfer step of the present disclosure, and is executed by the transfer processing portion  61  of the control portion  7 . 
     Specifically, the control portion  7  forms the first toner layer on the photoconductor drum  31  of the image forming unit  22 , and transfers the first toner layer onto the intermediate transfer belt  26 . In addition, the control portion  7  forms the second toner layer on the photoconductor drum  31  of the image forming unit  23 , and transfers the second toner layer onto the first toner layer formed on the intermediate transfer belt  26 . This forms the specific toner image on the intermediate transfer belt  26 . Subsequently, the control portion  7  transfers the specific toner image from the intermediate transfer belt  26  to the sheet conveyed by the process of step S 21 . 
     Step S 23   
     In step S 23 , the control portion  7  uses the image capturing portion  42  to acquire a captured image of the sheet to which the specific toner image has been transferred. Here, the process of step S 23  is an example of the first acquisition step of the present disclosure, and is executed by the first acquisition processing portion  62  of the control portion  7 . 
     Step S 24   
     In step S 24 , the control portion  7  detects the specific image from the captured image of the sheet acquired in step S 23 . Here, the process of step S 24  is an example of a detection step of the present disclosure, and is executed by the detection processing portion  63  of the control portion  7 . 
     Specifically, the control portion  7  detects, as the specific image, a colored area having the same shape as the specific toner image included in the captured image of the sheet. 
     Step S 25   
     In step S 25 , the control portion  7  acquires a histogram of gradation values of a color mixture of the second color and the third color in the pixels included in the specific image detected in step S 24 . 
     Specifically, the control portion  7  acquires the histogram of gradation values of R based on the gradation values of R of the pixels included in the specific image detected in step S 24 . 
     Step S 26   
     In step S 26 , the control portion  7  acquires the skewness of the histogram based on the histogram acquired in step S 25 . Here, the processes of steps S 25  and S 26  are an example of the second acquisition step of the present disclosure, and are executed by the second acquisition processing portion  64  of the control portion  7 . 
     Step S 27   
     In step S 27 , the control portion  7  determines whether or not the skewness acquired in step S 26  is different from 0 (zero). 
     Here, upon determining that the skewness acquired in step S 26  is different from 0 (zero) (Yes side at S 27 ), the control portion  7  moves the process to step S 28 . In addition, upon determining that the skewness acquired in step S 26  is 0 (zero) (No side at S 27 ), the control portion  7  ends the second image forming condition adjustment process. It is noted that the control portion  7  may determine that the skewness acquired in step S 26  is 0 (zero) when the acquired skewness is very small. 
     Step S 28   
     In step S 28 , the control portion  7  adjusts the image forming condition based on the skewness acquired in step S 26 . Here, the process of step S 28  is an example of the adjustment step of the present disclosure, and is executed by the adjustment processing portion  65  of the control portion  7 . 
     Specifically, the control portion  7  determines the unevenness depth level based on the skewness acquired in step S 26 . 
     In addition, the control portion  7  consults the first table data TD 11  (see  FIG.  6   ) to identify an adjustment amount of the nip pressure at the secondary transfer position P 1  corresponding to the determined unevenness depth level. Subsequently, the control portion  7  adjusts the nip pressure at the secondary transfer position P 1  based on the identified adjustment amount. 
     In addition, the control portion  7  consults the second table data TD 12  (see  FIG.  7   ) to identify an adjustment amount of the secondary transfer current corresponding to the determined unevenness depth level. Subsequently, the control portion  7  adjusts the secondary transfer current based on the identified adjustment amount. 
     As described above, in the image forming apparatus  200 , the specific toner image that includes: the first toner layer of the first color formed on the intermediate transfer belt  26 ; and the second toner layer of the second color formed on the first toner layer, is transferred to a sheet. In addition, a skewness of a histogram of gradation values of a color mixture of the second color and the third color is acquired based on the specific image corresponding to the specific toner image included in the captured image of the sheet. Furthermore, the image forming condition is adjusted based on the acquired skewness. With this configuration, when an image is formed on a sheet with an uneven surface, compared with the image forming apparatus of the related technology, it is possible to adjust the image forming condition accurately. 
     It is noted that the image forming apparatus  200  may acquire a skewness corresponding to the specific image that is included in the captured image of the sheet to which the specific toner image has been transferred, based on the specific image, and acquire the specific value based on pixels of a sheet area included in the captured image that is different from the specific image. In this case, the image forming apparatus  200  may adjust the image forming condition based on both the acquired skewness and the acquired specific value. For example, the image forming apparatus  200  may adjust the image forming condition based on a simple average value or a weighted average value of: an unevenness depth level corresponding to the acquired skewness; and an unevenness depth level corresponding to the acquired specific value. In addition, the image forming apparatus  200  may adjust the image forming condition based on any one of the acquired skewness and the acquired specific value that corresponds to a higher unevenness depth level. 
     In addition, the image forming apparatus  200  may execute the processes of steps S 11  to S 15  of the first image forming condition adjustment process, and determine whether or not to execute the second image forming condition adjustment process, based on the specific value acquired by the process of step S 15 . For example, the image forming apparatus  200  may execute the process of step S 16  when the specific value acquired by the process of step S 15  exceeds a predetermined threshold, and execute the second image forming condition adjustment process instead of the process of step S 16  when the specific value is equal to or smaller than the threshold. In addition, the image forming apparatus  200  may execute the second image forming condition adjustment process instead of the process of step S 16  when the specific value acquired by the process of step S 15  exceeds the threshold, and execute the process of step S 16  when the specific value is equal to or smaller than the threshold. In addition, the image forming apparatus  200  may execute the second image forming condition adjustment process instead of the process of step S 16  when the specific value acquired by the process of step S 15  exceeds the threshold, and end the first image forming condition adjustment process without executing the process of step S 16  when the specific value is equal to or smaller than the threshold. 
     It is to be understood that the embodiments herein are illustrative and not restrictive, since the scope of the disclosure is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds thereof are therefore intended to be embraced by the claims.