JP2017098928A - Image forming apparatus and image forming method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus that suppresses a noise generated due to a variation in period to stably perform reproduction of colors.SOLUTION: An image forming apparatus comprises: image output means 4 that outputs an image on a recording medium P by combining basic colors; gradation correction means 316 that corrects an image output by the image output means 4 by using a gradation correction value ΔCT; a measuring sensor 45 that measures reflection properties of the image; first change parameter calculation means 321 that calculates a first change parameter by using the reflection properties at a first point on the image; second change parameter calculation means 322 that calculates a second change parameter for subtracting a period variation component of the density of the image from the first change parameter; and gradation correction value generation means 319 that generates a gradation correction value ΔCT for each of the basic colors on the basis of the reflection properties, image data array R, first change parameter, and second change parameter. The second change parameter is defined by the period variation component and calculated by using the reflection properties at a second point different from the first point.SELECTED DRAWING: Figure 3

Description

  The present invention relates to an image forming apparatus and an image forming method for forming an image on a recording medium.

  In an image forming apparatus using, for example, a copying machine or a printer that forms a color image on a recording medium, the quality of an image to be formed, for example, the color to be output, can be output even when continuously outputting to a plurality of pages. It is required to reproduce or manage within a certain range.

  In such an image forming apparatus, a technique is known in which the color of an image output in a printing area is measured and corrected so as to minimize the color difference from the target color based on the colorimetric value (for example, patents). Reference 1).

  However, when such colorimetric values are obtained, if there are periodic variations close to the page interval in the colorimetric area, the periodic variations related to the colorimetric results are superimposed as noise, and the gradation characteristics are corrected. There is a risk that the accuracy may decrease.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide an image forming apparatus capable of stably reproducing a color while suppressing noise due to periodic fluctuations.

  In order to solve the above-described problems, an image forming apparatus according to the present invention outputs an image on a recording medium based on an image data array formed by combining a plurality of basic colors, based on a mixture of the basic colors. A gradation correction unit that corrects the image output by the image output unit using a gradation correction value for correcting the gradation characteristic of the basic color for each basic color, and the entire image Alternatively, a measurement sensor that measures a part of the reflection characteristics, and a first change parameter calculation unit that calculates a first change parameter using the reflection characteristics at the first point on the image measured by the measurement sensor. Second change parameter calculating means for calculating a second change parameter for subtracting a periodic fluctuation component of the density of the image from the first change parameter, the reflection characteristic, and the image Gradation correction value generating means for generating the gradation correction value of each of the basic colors based on the data array, the first change parameter, and the second change parameter, and The two change parameter is calculated by using the reflection characteristic at a second point that is determined by the periodic variation component and is different from the first point.

  According to the image forming apparatus of the present invention, it is possible to stably reproduce a color while suppressing noise due to periodic fluctuations.

1 is a diagram illustrating an example of a configuration of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a schematic diagram illustrating an example of a configuration of a control unit of the image forming apparatus illustrated in FIG. 1. FIG. 2 is a schematic diagram illustrating an example of a configuration of an image forming system of the image forming apparatus illustrated in FIG. 1. FIG. 4 is a flowchart illustrating an example of an operation of the image forming apparatus illustrated in FIG. 3. FIG. 6 is a schematic diagram illustrating an example of a periodic variation in density of an image formed on a recording medium. It is the figure which showed typically an example of the principle of calculation of the mode parameter in embodiment of this invention. It is a flowchart which shows an example of the operation | movement for calculating | requiring the gradation correction value in embodiment of this invention. It is a schematic diagram which shows an example of the periodic variation of the image formed on the recording medium in the 2nd Embodiment of this invention, and a density | concentration. It is a schematic diagram which shows an example of a structure of the image forming system of the image forming apparatus in the 2nd Embodiment of this invention. It is a figure which shows an example of the image formed on the recording medium in the 2nd Embodiment of this invention. It is a figure which shows an example of the transition of the profile according to color in the 2nd Embodiment of this invention.

FIG. 1 shows an outline of the overall configuration of an image forming apparatus as an example of the image forming apparatus in the present embodiment.
In the present embodiment, the image forming apparatus 100 includes a paper feeding unit 2 that transports a paper P that is a recording medium, and a control unit 3 that forms image information based on input document data.
The image forming apparatus 100 includes an image forming unit 4 that is an image output unit that is an electrophotographic printer engine that forms a toner image on a transfer belt 47 using image information.
The image forming apparatus 100 also includes a registration roller pair 22 that feeds the paper P supplied from the paper supply unit 2 to the transfer unit 5 at a predetermined timing.

The image forming apparatus 100 is also formed with a transfer unit 5 that is a secondary transfer unit that transfers a toner image carried on the transfer belt 47 to a sheet P at a secondary transfer position N that is a nip portion with the transfer belt 47. And a fixing unit 6 for fixing the image.
The image forming apparatus 100 also includes a paper discharge unit 7 that discharges the paper P to the outside, and a measurement sensor 45 that measures the reflection characteristics of the image. For example, the measurement sensor 45 measures the reflection characteristics of the whole or part of the image formed on the paper P, which is downstream of the secondary transfer position N in the paper P conveyance direction. Note that the position of the measurement sensor 45 may be downstream of the secondary transfer position N in the paper P transport direction, but as shown in FIG. 1, it is disposed downstream of the fixing unit 6 in the paper P transport direction. It is desirable.
By measuring the color variation of the image after fixing, the measurement sensor 45 can measure the color variation including all the color variation factors applied to the image forming apparatus 100, so that the color of the finally printed image can be stabilized with high accuracy. It can be made.
Note that the present invention is not limited to this configuration, and the measured image reflection characteristics may include the toner image reflection characteristics.

  The paper feed unit 2 includes a paper feed port 20 and a plurality of paper feed rollers 21 for conveying the paper P fed from the paper feed port 20 to the transfer unit 5.

The image forming unit 4 includes four process units 4Y, 4M, 4C, and 4K corresponding to a plurality of basic colors CMYK, that is, yellow, magenta, cyan, and black.
Since the process units 4Y, 4M, 4C, and 4K all have the same configuration and redundant description, only the process unit 4Y corresponding to yellow Y will be described here.
The process unit 4Y includes a drum-shaped photoconductor 40Y as an image carrier as a rotating body that rotates in the A direction that is counterclockwise shown in FIG. 1, and a laser unit 53Y as an optical writing device as an optical scanning device. Have. On the surface of the photoreceptor 40Y, a photosensitive layer that is a scanned surface of scanning light emitted from the laser unit 53Y is formed.
The process unit 4Y also includes a charging device 42Y provided upstream in the A direction around the photoreceptor 40Y, a developing device 43Y as a developing means, and a primary transfer means 1 provided to wind a transfer belt 47. And a next transfer roller 475Y.
The process unit 4Y also includes a cleaning device 44Y provided downstream in the A direction from the position where the primary transfer roller 475Y and the photoreceptor 40Y are in contact with each other.

The process unit 4Y also has a potential sensor that is a surface potential sensor as a surface potential detecting means for detecting the surface potential of the photoreceptor 40Y.
The process unit 4Y forms a latent image on the photoreceptor 40Y by the laser unit 53Y, thereby forming a yellow basic color toner image.
The image forming unit 4 uses the process units 4Y, 4M, 4C, and 4K to form a plurality of basic colors on the sheet P by mixing the basic colors based on image information that is an image data array. An image output means for outputting a toner image as an image. That is, the image output means outputs a mixed color image based on an image data array formed with a plurality of colors.

The transfer unit 5 includes a transfer belt 47 that rotates in the B direction, which is the clockwise direction in FIG. 1, and a drive roller 471 that is driven clockwise by a drive source.
The transfer unit 5 includes a driven roller 472 and a secondary transfer roller 473 that rotate together with the driving roller 471, and a secondary transfer counter roller 474 provided to face the secondary transfer roller 473.

  The transfer belt 47 is made by dispersing carbon powder for adjusting electric resistance in a polyimide resin with little elongation. The transfer belt 47 is wound around a driving roller 471, a driven roller 472, a secondary transfer roller 473, and primary transfer rollers 475Y, 475C, 475M, and 475K.

In the transfer unit 5, the secondary transfer counter roller 474 contacts the transfer belt 47 at the secondary transfer position N to form a nip portion. At the secondary transfer position N, the transfer unit 5 sandwiches the transfer belt 47 with the paper P between the secondary transfer counter roller 474 and the secondary transfer roller 473 and applies a secondary transfer bias to the surface of the transfer belt 47. The toner image is transferred onto the paper P.
As the secondary transfer bias of the transfer unit 5, a charge opposite to the electrostatic charge charged on the surface of the transfer belt 47 is applied.
The secondary transfer counter roller 474 conveys the paper P after the secondary transfer at the secondary transfer position N to the fixing unit 6.

The measurement sensor 45 is an in-line type that combines a plurality of monochrome line sensors, each of which has a band pass filter corresponding to each measurement target color so as to have sensitivity to three measurement target colors of red, green, and blue. It is a chromaticity measuring instrument. That is, the measurement sensor 45 has a measurement channel having three spectral characteristics corresponding to three colors of red, green, and blue (R, G, and B). The number of measurement channels that the measurement sensor 45 has is called the number of measurement channels.
The measurement sensor 45 may be any sensor having a measurement channel having at least one spectral characteristic, in other words, a measurement channel having sensitivity to one or more basic colors, and a so-called color scanner may be used. Further, as described later, a monochrome line sensor having a single spectral characteristic may be used.

The control unit 3 operates as a communication control unit for controlling bidirectional communication with a host device (for example, a personal computer) via a communication network.
The control unit 3 also operates as image data processing means for sending image data created based on document data from the host device to the image forming unit 4.
As shown in FIG. 2, the control unit 3 includes an image processing unit 30 that receives document data transmitted from a personal computer 200 or a server 201 as a host device that stores document data, and an image developed by the image processing unit 30. A gradation processing unit 31 that converts the information into image data in a format that can be received by the image forming unit 4.

The control unit 3 also includes an image inspection unit 33 that inspects the image output by the image forming unit 4 in-line to detect image information, and a tone processing unit that detects a change in color tone of the image from the detected image information. And a color tone control unit 32 for providing a gradation correction value to 31.
The control unit 3 also has an engine control unit 39 that controls the image forming unit 4.
Of the control unit 3, the engine control unit 39, the color tone control unit 32, the gradation processing unit 31, and the image inspection unit 33 are provided in the image forming apparatus 100. The image processing unit 30 is configured by software and an expansion board on a personal computer separate from the image forming apparatus 100, and is a system that can be exchanged independently of the image forming apparatus 100.

  The image processing unit 30 may be provided on a terminal separate from the image forming apparatus 100 as long as the image processing unit 30 is arranged on the image forming system that performs the image formation, or the server 201 and the personal computer 200 via the network. It may be provided on the image forming apparatus 100.

As shown in FIG. 3, the image processing unit 30 includes document color Lab conversion means 310 that converts document data Q transmitted from an external personal computer 200 or the like into Lab format, and LabCMYK conversion that converts Lab format into CMYK format. And means 311.
The image processing unit 30 includes user gradation conversion means 312 that corrects gradation based on the selected color profile.
The image processing unit 30 includes a storage device 302 as a hard disk drive serving as storage means for storing each value such as the calculated image data R temporarily or continuously during image formation.

The gradation processing unit 31 is a gradation correction value ΔCT = (Δc (c), Δm (m), Δy (y), Δk (k) for correcting the gradation characteristic of each basic color for each basic color. ), A gradation correction unit 316 as gradation correction means for correcting an image to be output by the image forming unit 4 is provided. In other words, the gradation correction unit 316 corrects the gradation characteristics of the color based on the gradation correction value.
The gradation processing unit 31 includes a gradation conversion unit 317 for converting an output image into a format that can be expressed by the image forming unit 4.
Here, Δc (c) and the like indicate that the correction amount for the input gradation value c of cyan C is Δc (c). In the following description, when it is not always necessary to clearly indicate the input gradation, it is simply written as Δc.

  The image inspection unit 33 has a scanner color Lab conversion means 318 for converting into an Lab value based on the image measured by the measurement sensor 45, that is, for obtaining a measurement value.

The color tone control unit 32 predicts the gradation of the image to be formed based on the image information input from the image processing unit 30 and outputs the gradation correction value ΔCT. A tone correction value determination unit 319 serving as a tone correction value generation unit.
The colorimetric prediction unit 34 includes a CMYKLab conversion unit 313 that converts a CMYK format into a Lab format, a scanner correction unit 325 that corrects an input value based on a reading error of the measurement sensor 45 given in advance, and a scanner color Lab conversion unit. 315. Here, the scanner color Lab conversion unit 315 and the scanner color Lab conversion unit 318 are conversion units having the same function, and thus description thereof is omitted.
The tone correction amount determination unit 319 generates a tone correction value ΔCT for each color based on the reflection characteristics, the image data array, and a first change parameter and a second change parameter described later.

An image forming method for forming an image using the image forming apparatus 100 having such a configuration will be described.
First, as shown in FIG. 3, document data Q is sent together with a print request from a personal computer 200 or server 201 on the network.
The document data Q is a complex data format including drawing commands for bitmaps, texts, and graphics that are color-designated by RGB or CMYK.
The image processing unit 30 expands the received document data Q, and arranges a pixel array composed of the basic colors of cyan, magenta, yellow, and black included in the image forming unit 4, for example, color information of each pixel in a grid pattern. The bit map data is transmitted to the gradation processing unit 31.

The gradation processing unit 31 further converts the pixels to the number of gradations that can be expressed by the image forming unit 4, and transmits the converted image data to the image forming unit 4 as image data R that is an image data array.
Based on the image data R received from the gradation processing unit 31, the image forming unit 4 outputs a toner image on the transfer belt 47 by a mixed color of yellow, magenta, cyan, and black, and performs secondary transfer of the transfer unit 5. At the position N, the toner image on the transfer belt 47 is transferred onto the paper P.
The paper P on which the toner image is formed on the surface enters the fixing unit 6 when the toner images of all colors are transferred and carried. In the fixing unit 6, when the paper P passes through the fixing nip N2 between the heating roller 161 and the fixing roller 162, the carried toner image is fixed by the action of heat and pressure, and the paper P is excellent on the paper P. A color image is formed.
The image inspection unit 33 scans an image on the paper P using the reflection characteristics of the image detected by the measurement sensor 45 as described later. The color tone control unit 32 provides a color tone to be given to the engine control unit 39 and the tone processing unit 31 so that a deviation between a target reproduction color and the color development of the output image is minimized by a tone correction operation described later. By correcting the correction amount, the color of the image output on the paper P is stabilized.

The fixed paper P that has passed through the fixing unit 6 is discharged out of the image forming apparatus 100 from the paper discharge unit 7.
Alternatively, the paper discharge unit 7 may be provided with a switching claw and a double-sided unit, and the paper P may enter the double-sided unit according to the mode of the switching claw to prepare for double-sided image formation.

  A more detailed process regarding gradation correction will be described. In the following description, the format handled by the color tone control unit 32 is the Lab (CIELab) format, and the CMYK format color data of the document data Q is converted to the Lab format. However, the format is not limited to the Lab format. Such a color expression format may be used.

The original color Lab conversion unit 310, the Lab CMYK conversion unit 311, the CMYKLab conversion unit 313, the scanner color Lab conversion units 315 and 318, and the Lab scanner color conversion 314 require a color profile as basic data for each color space conversion. To do.
Among these color profiles, the color profile necessary for conversion from the document color to the Lab value is attached to the document data Q or prepared in advance. The color profiles necessary for the scanner color Lab conversion means 315 and 318 and the Lab scanner color conversion 314 are set in advance in the color tone control unit 32 and the image inspection unit 33, respectively.

The color profiles necessary for the LabCMYK conversion unit 311 and the CMYKLab conversion unit 313 affect the color reproduction characteristics depending on the type of the paper P. Therefore, the color profile is desirably set by selecting an appropriate color profile suitable for the type of the paper P from a plurality of color profiles stored in advance in the server 201.
The change of the color profile depending on the type of the paper P may be arbitrarily performed by the user, or may be automatically performed according to the selection of the paper P that matches the input document data Q by the image processing unit 30. .
In addition, as such a color profile, for example, an ICC profile defined by an ICC (International Color Consortium) may be used.

As shown in FIG. 4, the document data Q transmitted to the image processing unit 30 is a device-independent Lab-format table of color data expressed in RGB format or CMYK format by the document color Lab conversion unit 310. The color value is converted into a docLab value (step S100).
The LabCMYK conversion unit 311 converts the docLab value into the gradation value prnCMYK, which is a set of integer gradation values of 8 bits, for each of the basic colors cyan C, magenta M, yellow Y, and black K of the image forming unit 4 (steps). S101).
The user gradation converting means 312 outputs the image data R as it is without changing the gradation value prnCMYK in the initial state (step S102).
These processes are performed simultaneously with vector data and font expansion. The resulting image data R is output as bitmap data for four colors of basic colors C, M, Y, and K obtained by quantizing the color information of the document data Q.
The output image data R is held in the storage device 302 of the image processing unit 30 for each document used for printing.

The gradation correction unit 316 corrects the color gradation characteristics based on the gradation correction value. For example, each basic color C, M, Y, K is provided with a tone correction table (TRC: Tone Response Correction), and each level of the basic color is determined using a tone correction value and a tone correction table described later. The tone is corrected (step S103).
The gradation conversion unit 317 converts the color value sent in 8 bits for each basic color using the area gradation method or the error diffusion method so as to match the number of gradations that can be expressed by the image forming unit 4. (Step S104).
In this way, the image forming unit 4 receives the image data R in a format that can be expressed by the image forming unit 4 by the gradation processing unit 31, and forms a toner image (step S105).

The toner image formed by the image forming unit 4 is transferred onto the paper P by the transfer unit 5, fixed by the fixing unit 6, and then scanned using the reflection characteristics of the image measured by the measurement sensor 45.
The image inspection unit 33 inputs the scanned image information to the scanner color Lab conversion unit 318 as an output measurement value mesCol.
The scanner color Lab conversion means 318 converts the input output measurement value mesCol into a Lab value, thereby obtaining a colorimetric value mesLab converted to a Lab value (step S106).

The color tone correction amount determination unit 319 reads a necessary portion of gradation values prnCMYK from the bitmap data stored in the storage device 302 into the page buffer in advance (step S107).
On the other hand, the color tone control unit 32 converts the CMYK format from the CMYK format to the Lab format by the CMYKLab conversion unit 313, and stores it as the output predicted value prnLab (step S108).
The predicted output value prnLab obtained by the color tone control unit 32 at this time is a value that simulates the reproduced color of the output image data R as a Lab value.

Since the predicted output value prnLab does not include a reading error specific to the measurement sensor 45, if the predicted output value prnLab is used for correction as it is, it is corrected in a state including the reading error of the measurement sensor 45. For example, when the color gamut that can be read by the measurement sensor 45 is smaller than the color gamut that can be output by the image forming unit 4, the predicted output value prnLab is larger than the output measurement value mesCol scanned by the measurement sensor 45. May be different. This is because the color gamut is compressed by the measurement sensor 45.
The scanner correction unit 325 calculates the scanner read predicted value scnCol by correcting the output predicted value prnLab based on the read error of the measurement sensor 45 given in advance (step S109).
By having such a scanner correction unit 325, even when the color gamut that can be read by the measurement sensor 45 is smaller than the color gamut that can be output by the image forming unit 4, the color reading value can be accurately predicted. Is called.

When the scanner reading predicted value scnCol is input from the scanner correction unit 325, the scanner color Lab conversion unit 315 converts the value into a Lab value and calculates a target value targetLab (step S110).
The colorimetric prediction unit 34 determines whether or not the target value targetLab has been calculated for the entire print area to be printed in advance as described above (step S111), and stores the calculated result in the storage device 302.

  The tone correction amount determination unit 319 performs tone correction value ΔCT = () for correcting the tone correction table based on the target value targetLab, the colorimetric value mesLab, and the tone value prnCMYK for these print areas. Δc, Δm, Δy, Δk) are determined.

Next, a method in which the tone correction amount determination unit 319 determines the gradation correction value ΔCT will be described.
First, prior to processing, from a print area of each page, a plurality of small color measurement areas {(xi, yi)} i having a small color change suitable for measurement of basic colors, that is, color measurement, of about several mm square. = 1,. . . , N are extracted. Here, the minute colorimetric region is represented by its center coordinates (x, y). Note that the coordinate symbols x and y described here are the coordinates y in the paper feed direction in which the paper P is conveyed at the secondary transfer position N of the image forming apparatus 100, that is, the so-called sub-scanning direction is the Y direction. Represent. Similarly, it is expressed as a coordinate x in the main scanning direction which is a direction orthogonal to the Y direction.
As a method for extracting the micro colorimetric region (xi, yi), for example, a method of selecting and extracting a region of 41 × 41 pixels at 400 dpi from an arbitrary region of about 5 mm square can be used. If the number of samples is insufficient in the minute colorimetric area (xi, yi) extracted from a single page, the previous N minute colorimetric areas (xi, yi) are extracted from several consecutive pages. A tone correction value ΔCT is determined. The process is repeated with this cycle as one cycle.
In the following description, an algorithm for calculating a correction value based on an average of measured values of these N minute colorimetric regions {(xi, yi)} i = 1,. For this reason, the sample numbers and coordinates of the individual micro-colorimetric areas are not essential here and are omitted.

The colorimetric value mesLab and the target value targetLab are extracted for the N minute colorimetric regions described above (step S201). At this time, the correction values that minimize the variation between the colorimetric value mesLab and the target value targetLab are gradation correction values ΔCT = (Δc (c), Δm (m), Δy (y), Δk (k). ).
In order to obtain the gradation correction value ΔCT, the CMYK gradation value corrected by the gradation correction unit 316 is used as a corrected CMYK gradation value, and each element is subjected to perturbation calculation according to a predetermined number of variation modes. think of. The fluctuation mode of this perturbation is defined as change mode data.
At this time, the corrected CMYK gradation value can be expressed by Equation (1).

Here, c, m, y, k are CMYK gradation values before correction, c _0, m _0, y _0, k ₀ are the above-described quasi-gradation table values, c ₁ , m ₁ , y ₁ , k _1. Is the first variation mode, c ₂ , m ₂ , y ₂ , k ₂ are the second variation modes, and θ _i , (i = {1, 2}) is the mode parameter.
However, the superscript j = c, m, y, k is not an index but a simple identification subscript. The subscript j indicates that the mode parameter θ _i ^j (i = {1,2 ,,, n}, j = {c, m, y, k}) is obtained for each basic color. i indicates that the mode parameter θ _i ^j is obtained according to the number of fluctuation modes i.
Although the number of variation modes may be three or more, here, in order to simplify the description, two variation modes of a first variation mode and a second variation mode that are first-order independent from each other will be described as an example.

  The relationship of the formula (1) can be expressed by the formula (2) with the variation of the CMYK gradation value as a vector dc and i = 1,2.

However, each element of Equation (2) is given by Equation (3) below.

The above M ₁ and M ₂ will be referred to as a first variation mode matrix and a second variation mode matrix, respectively.

At this time, if the evaluation function φ for the colorimetric value mesLab is expressed by Equation (4), it is possible to approximate the variation in printing characteristics by obtaining the mode parameters Δθ ₁ and Δθ ₂ that minimize the evaluation function φ. .
However, in the mathematical formula (4), the colorimetric value mesLab is represented by Lab _m and the target value targetLab is represented by Lab _t .
Here, the color difference evaluation function f = (f ₁ , f ₂ ,..., F _n ) is an appropriate differentiable function from the Lab space to the n-dimensional number space R ⁿ , E is an expected value (sample average), J is a Jacobian matrix of these synthesis functions f _i (Lab _m (c, m, y, k), Lab _t (c, m, y, k)).

By the way, for image formation by the image forming apparatus 100, members that move at a constant cycle, such as the photoreceptor 40Y for forming a latent image in the image forming unit 4 and the primary transfer roller 475Y for performing development, are used. Many are used. When there is some change in the member that periodically moves, the influence of these cycles may cause a concentration change including a plurality of cycle fluctuations having different cycles as shown in FIG.
When there are periodic fluctuations with a wavelength of several centimeters to several tens of centimeters, if the sampling period indicated by ○ is different from the density fluctuation period indicated by ▽, the gradation to be actually corrected indicated by the broken line May vary from the estimated value derived by sampling indicated by the alternate long and short dash line. In other words, such a periodic variation lowers the correction accuracy of the gradation characteristics.

In order to solve such a problem, in the present embodiment, the following operation is performed using the mode parameter θ _i ^j described above, so that noise due to period fluctuation is suppressed and color reproduction is stably performed. Do.

As shown in FIG. 6A, an image region divided into a plurality of portions in the Y direction by combining the minute colorimetric regions (xi, yi) with the minute colorimetric regions (xi, yi) having the same yi. Consider the sub-scanning pixel region (y0, y1, y2,... Yn).
Such sub-scanning pixel regions (y0, y1, y2,... Yn) correspond to the respective parts obtained by dividing the paper P into a plurality of parts with a constant width in the sub-scanning direction.
For convenience of explanation, the micro colorimetric area (xi, yi) and the sub-scanning pixel area (y0, y1, y2,... Yn) are greatly expressed, but when printing on A4 size paper P at 400 dpi, Since the length of the paper P in the Y direction is about 210 mm, about 3307 pixels are arranged in the Y direction. Therefore, if the minute colorimetric region (xi, yi) is 41 × 41 pixels, the sub-scanning pixel region (y0, y1, y2,... Yn) has a division number of about n = 80.

Further, the periodic fluctuation components of the image density having two different periods generated along the Y direction when the image is printed are represented as a short period fluctuation A1 and a long period fluctuation A2 in FIGS. 6B and 6C, respectively. ) Schematically. In FIGS. 6B and 6C, the periodic fluctuation component is divided into two types of short-period fluctuation A1 and long-period fluctuation A2, and the vertical axis is the density and the horizontal axis is the position in the Y direction. Actually, these periodic fluctuations of the density are superimposed and printed on the image in a synthesized state.
In addition, although the periodic variation has two different periods, it may be a periodic variation represented by two or more combined waves, or may be a periodic variation having only one period.

The measurement sensor 45 is an arbitrary one of the plurality of sub-scanning pixel areas (y0, y1, y2,... Yn), for example, each minute colorimetry area (xi, yi) included in the sub-scanning pixel area (y0). Measure the colorimetric value.
As shown in FIG. 7, the color correction amount determination unit 319 extracts a colorimetric sample of the sub-scanning pixel region (y0) from the measured colorimetric values (step S300).
The first change parameter calculation means 321 calculates the mode parameter θ ₀ ^j so as to minimize Equation 4 with the sub-scanning pixel region (y0) measured in step S300 as the first point (step S301). Save temporarily in the page buffer.

Next, the first change parameter calculation unit 321 determines whether or not the mode parameter θ _i ^j has been calculated in all printable areas of the paper P (step S302).

If the mode parameter θ _i ^j has not been calculated in all printable areas of the paper P in step S302, the subscript i is updated to i + 1, and the process returns to step S300 to measure in the sub-scanning pixel area (y1). Extract color samples.
Thereafter, this process is repeated until the mode parameter θ _i ^j is calculated for all the sub-scanning pixel regions (y0, y1, y2,... Yn).

When the mode parameter θ _i ^j is calculated in all printable areas of the paper P in step S302, the first change parameter calculation unit 321 uses the sub-scanning pixel area (yi) as a first point from the page buffer. The mode parameter θ _yi ^j is read (step S303).
The mode parameter θ _yi ^j has a function as a first change parameter.
Step S303 is a first change parameter calculation step for calculating the first change parameter.
It is assumed that the first change parameter calculation unit 321 reads the mode parameter θ ₁ ^j corresponding to the first sub-scanning pixel region (y1) as indicated by Δ in FIG. 6B.
The second change parameter calculation means 322 defines a second sub-scanning pixel region (y5) that is separated from the first sub-scanning pixel region (y1), which is the first point 321a serving as a reference, by a feature distance L in the Y direction. Select as second point 322a.
In this case, feature distance L is determined by the wavelength _{T A1} short period fluctuation A1, with respect to the wavelength _{T A1,} the sum of a half wavelength of an integral multiple wavelength _{T A1} wavelength _{T A1.} That is, it is determined to satisfy L = (N _A1 +0.5) T _A1 (N _A1 = 1, 2, 3,...).

Note that the characteristic distance L is preferably the sum of an integral multiple of the short period variation A1 and a half wavelength in order to subtract the influence of the short period variation A1, but the first point 321a and the second point 322a are It suffices if the phase is reversed in the short cycle variation A1.
In other words, the second point 322a is a characteristic distance from the first point 321a such that the first point 321a and the second point 322a are in opposite phase positions with respect to the short period variation A1. L apart.

The second change parameter calculation unit 322 calculates the color measurement result of the second sub-scanning pixel region (y5) such that the distance between the first point 321a and the second point 322a matches the feature distance L. The mode parameter θ ₅ ^j is read from the page buffer (step S304).
At this time, since the mode parameter θ ₅ ^j is averaged with the mode parameter θ ₁ ^j which is the first change parameter, the component of the short period fluctuation A1 of the image density of the mode parameter θ ₁ ^j is subtracted. It has a function as a change parameter.
Therefore, step S304 is a second change parameter calculation step.
The color tone correction amount determination unit 319 calculates a new mode parameter as the variation exclusion parameter θd by averaging the mode parameter θ ₁ ^j and the mode parameter θ ₅ ^j (step S305).

Here, only the mode parameter θ ₅ ^j is used as the second change parameter. However, as will be described later, for example, the mode parameters θ ₃ ^j and θ ₁₅ ^j may be used as long as the points are separated by the designated feature distance L. , Θ ₁₇ ^j , may be averaged.
In this case, the second sub-scanning pixel region (y3, y15, y17) is treated as a second point corresponding to a different feature distance L.

Similarly, for the first point 321b and the second point 322b shown in FIG. 6B, the mode parameter θ ₂ ^j and the mode parameter θ ₆ ^j calculated at the respective positions are averaged. It is possible to calculate the fluctuation exclusion parameter θd that cancels the influence of the short period fluctuation A1.

The fluctuation exclusion parameter θd can be calculated for all the mode parameters θ _{1,2,3,4... N} ^j for the same feature distance L, but in the sub-scanning pixel region (yi). There are portions where the density changes drastically depending on the printed image and portions that are not suitable for colorimetry.
The second change parameter calculation unit 322 calculates the second change parameter when the sub-scanning pixel area (yi) including the second point obtained according to the feature distance L is not suitable for colorimetry. It is desirable to prevent an unnecessary increase in the calculation amount.
In other words, when there is no colorimetric region where the integral multiple of the wavelength of the short period variation A1 and the sum of the half wavelengths do not exist, the mode parameter θ _yi ^j in the sub-scanning pixel region (yi) is discarded. Is desirable.
The tone correction amount determination unit 319 calculates the variation exclusion parameter θd only for the sub-scanning pixel region (yi) excluding the portion not suitable for colorimetry, and determines whether the calculation of the variation exclusion parameter θd is completed. This is performed (step S306).

The calculation of the fluctuation exclusion parameter θd may be performed across a plurality of different sheets P, or may be performed on each sheet P.
When the first point 321a and the second point 322c are selected across a plurality of sheets P, the distance between the sheets P may cause an error. Is preferably acquired as a signal from the measurement sensor 45, the registration roller pair 22, or the like.
When the fluctuation exclusion parameter θd is calculated across a plurality of sheets P, there is a possibility that the cyclic fluctuation will be excessive as shown in FIG. Is desirable.

In the above embodiment, the second point 322a is specified by determining the feature distance L when the short cycle variation A1 is known, but the short cycle variation A1 is essentially an unknown amount.
However, even if the short period variation A1 is unknown, for a certain feature distance L, the feature distance L has a certain wavelength T _AX that satisfies L = (N _AX +0.5) T _AX. There is always a periodic variation AX.
Accordingly, the periodic variation AX may be specified by specifying the feature distance L. In practice, by changing the feature distance L, a plurality of periodic components are canceled while changing the corresponding periodic variation AX. The second change mode parameter may be obtained.
The short cycle variation A1 is often caused by a rotating body such as the photoreceptor 40Y or the primary transfer roller 475Y, depending on the configuration of the image forming unit 4 and the transfer unit 5 in the image forming apparatus 100. it is conceivable that.
Therefore, such a short period fluctuation A1 may be estimated and the feature distance L may be obtained.

Now, a method for eliminating the long-period fluctuation A2 using the fluctuation exclusion parameter θd obtained based on the short-period fluctuation A1 in this way will be considered.
As shown in FIGS. 6B and 6C, the long-period fluctuation A2 sets the first points 321a and 321c and the second points 322a and 322c for each of the plurality of papers P. variation exclusion parameter [theta] d _a, calculates the [theta] d _c (step S306).
Here, the subscripts a and c are given for the purpose of clarifying the combination of the first point 321a and the second point 322a used, and a plurality of variation exclusion parameters θd that are not actually distinguished are calculated. Is done.
Such multiple variations exclusion parameter [theta] d _a, by averaging the [theta] d _c, as shown in FIG. 6 (c), change exclusion parameter [theta] d that counteracts the effects of long-period variation A2 is obtained.
Accordingly, by averaging all the fluctuation exclusion parameters θd calculated in step S306, a final mode parameter θ is obtained in which all the period fluctuations including the short period fluctuation A1 and the long period fluctuation A2 are canceled ( Step S307).

Using the mode parameter θ, the color correction amount determination unit 319 obtains a gradation correction value ΔCT that satisfies Equation 4 (step S308). That is, step S308 is a gradation correction value generation step.
As described above, the gradation correction unit 316 corrects the color gradation characteristics based on the gradation correction value ΔCT, thereby stabilizing the color development of the image output on the paper P (step S309). .

The image forming apparatus 100 according to the present embodiment includes a first change parameter calculation unit 321 that calculates the first change parameter θ ₁ ^j using the reflection characteristic at the first point 321a measured by the measurement sensor 45. ing.
The image forming apparatus 100 includes a second change parameter calculation unit 322 that calculates a second change parameter θ ₅ ^j for subtracting the periodic variation component of the image density from the first change parameter θ ₁ ^j .
The image forming apparatus 100 also generates gradation correction values ΔCT for the basic colors based on the reflection characteristics, the image data R, the first change parameter θ ₁ ^j , and the second change parameter θ ₅ ^j. A color tone correction amount determination unit 319 is provided.
Further, the second change parameter θ ₅ ^j is calculated by using the reflection characteristic at the second point 322a that is determined by the periodic fluctuation component and is different from the first point 321a.
With such a configuration, it is possible to stably reproduce a color while suppressing noise due to period fluctuation.

The characteristic distance L between the first point 321a and the second point 322a is the reverse of the first point 321a and the plurality of second points 322a with respect to the periodic fluctuation of the density of the image data R. It is set to be the phase position.
With this configuration, it is possible to stably reproduce a color while further suppressing noise due to period fluctuations.

The measurement sensor 45 measures the reflection characteristics for each sub-scanning pixel region (y0, y1, y2,... Yn) obtained by dividing the region where the image is output on the paper P into a plurality of portions.
With this configuration, noise due to periodic fluctuations in image density in the sub-scanning direction is suppressed, and color reproduction is performed stably.

The characteristic distance L between the first point 321a and the second point 322a is determined by the wavelength T _A1 of the short period variation A1, and is an integral multiple of the wavelength T _{A1 and} a half of the wavelength T _{A1 with} respect to the wavelength T _A1. It becomes the sum with the wavelength. That is, it is determined to satisfy L = (N _A1 +0.5) T _A1 (N _A1 = 1, 2, 3,...).
With this configuration, the fluctuation exclusion parameter θd works to cancel out the fluctuation component of the short-period fluctuation A1, so that noise due to the period fluctuation is further suppressed and color reproduction is performed stably.

In the present embodiment, a plurality of second points 322a and 322c are provided, and the first point 321a and the second point 322c are set separately among a plurality of sheets P to be printed continuously. Is done.
With this configuration, since the fluctuation exclusion parameter θd works to cancel out the fluctuation components of the short-period fluctuation A1 and the long-period fluctuation A2, noise is further suppressed and the color is reproduced stably.

Further, the image forming method of the present embodiment includes step S105 for outputting an image on the paper P by mixing the basic colors based on the image data R formed by combining a plurality of basic colors.
The image forming method according to the present embodiment also includes a gradation correction step for correcting an image output from the image forming unit 4 using a gradation correction value ΔCT for correcting the gradation characteristics of the basic color for each basic color. S309.
Further, there is provided a reflection characteristic measurement step S106 for measuring the reflection characteristic of the entire image or a part of the image, and a step S303 of calculating the first change parameter θ ₁ ^j using the reflection characteristic at the first point 321a on the image. doing.
The image forming method in the present embodiment also includes a step S304 of calculating a second change parameter θ ₅ ^j for subtracting the periodic variation component of the image density from the first change parameter θ ₁ ^j .
The image forming method according to the present embodiment also includes the gradation correction values of the basic colors based on the reflection characteristics, the image data R, the first change parameter θ ₁ ^j , and the second change parameter θ ₅ ^j. Step S308 for generating ΔCT.
With such a configuration, it is possible to stably reproduce a color while suppressing noise due to period fluctuation.

As a second embodiment of the present invention, a method for extracting a minute colorimetric region from a character region will be described. As already described, in order to measure the basic color using the micro colorimetric area, that is, colorimetry, there are few color changes, in other words, a plurality of several mm squares in which a certain range is composed of uniform colors. The micro colorimetric region is most preferable.
However, as shown in FIG. 8, in an image having a large contrast and a small number of homogeneous areas as in the case where the document data Q formed on the paper P includes characters, an appropriate minute colorimetric area is selected. Difficult to make a choice.
Therefore, the color tone control unit 32 in the present embodiment extracts a minute colorimetric region from the printable region of the paper P in addition to the units described in the first embodiment, as shown in FIG. As the area extracting means, an area extracting unit 323 is provided. In the present embodiment, the same components as those in the first embodiment are denoted by the same reference numerals and description thereof is omitted.

  A method in which the area extracting unit 323 extracts a minute colorimetric area when the document data Q is composed of characters as shown in FIG. 8 will be described.

FIG. 10A shows a part of document data Q composed only of character data. In FIG. 10A, a broken-line grid is shown as a grid in pixel units, and a character area F to be described later is indicated by a one-dot chain line. FIG. 10B is an enlarged view in which only the character region F is extracted and displayed.
When performing digital printing in recent years, the document data Q transmitted from an external personal computer 200 or the like usually includes character information and pixel position information of each page in which fonts are developed as described above. Yes.
Therefore, when the gradation processing unit 31 converts the document data Q to the image data R and transmits it, the font information and the pixel position information are transmitted to the color tone processing unit 32 together with the image data R.
The region extraction unit 323 corresponds to the size of one character in the region in which the character is formed in the document data Q based on the font information such as the format and size included in the image data R and the pixel position information. The rectangular area thus obtained is extracted and held as the character area F.
Although the character region F is a rectangular region here, the shape is not limited to this.

After specifying the character region F, the region extraction unit 323 extracts the pixel region of the portion where the character inside the character region F is formed as a minute colorimetric region (xi, yi).
The measurement sensor 45 measures a colorimetric value c (x, y) as a reflection characteristic in all the extracted minute colorimetric regions (xi, yi) (i = 1, 2,..., N).

Further, the region extraction unit 323 measures the colorimetric value c (x, y) for each pixel in the character region F using the measurement sensor 45 as shown in FIG. At this time, a minute colorimetric region is specified with a boundary where the difference between the colorimetric values of adjacent pixels is larger than a predetermined value.
That is, the difference between the colorimetric value c (x3, y3) and the colorimetric value c (x4, y3) between a certain minute colorimetric region (x3, y3) and the adjacent minute colorimetric region (x4, y3). Is larger than a predetermined value, a minute colorimetric region to be measured is set with the portion as a boundary. Note that in FIG. 10B, for simplification, the size per pixel is displayed to be large.
Since the area where the character is formed is an area where the contrast of the character part is large with respect to the background, the minute colorimetric area with the part where the difference between the colorimetric values of adjacent pixels is larger than the predetermined value as a boundary This is substantially equivalent to specifying the part where the character is formed.

  Using the average of the colorimetric values c (x, y) of the minute colorimetric region that is the portion where the character obtained in this way is formed, the color tone correction amount determination unit 319 uses the first change parameter generation unit 321. Then, the second correction parameter generation unit 322 is used to generate the gradation correction value ΔCT.

  Note that the method for calculating the mode parameter θ, the method for generating the gradation correction value ΔCT, and the like in this embodiment are the same as those in the first embodiment already described in FIG. . Further, as a method other than using the mode parameter θ, for example, the gradation correction value ΔCT may be calculated using only the variation in the sub-scanning direction of the micro colorimetric region (xi, yi).

In the present embodiment, the color tone control unit 32 includes a region extraction unit 323 that extracts a character region F as an image region from among regions in which an image is output.
Further, the measurement sensor 45 measures the reflection characteristics for the entire surface of the character region F extracted by the region extraction unit 323.
With such a configuration, even when an image formation area has a large contrast such as character data and a narrow uniform color tone area is output, a reduction in the accuracy of estimation of gradation characteristics is suppressed, and stable gradation is achieved. A correction value ΔCT is calculated.

In the present embodiment, the region extraction unit 323 extracts a region where a character is formed from the character region F as a minute colorimetric region.
With this configuration, even when only character data is output, a stable gradation correction value ΔCT is calculated while suppressing a decrease in estimation accuracy of gradation characteristics.

In the present embodiment, the region extraction unit 323 uses the reflection characteristics measured by the measurement sensor 45 to perform minute measurement using a portion where the difference between the colorimetric values of adjacent pixels is larger than a predetermined value as a boundary. Specify the color area.
With this configuration, even when character data is output, since the minute colorimetric region is specified after specifying the portion on which the character is printed, it is possible to suppress a decrease in the accuracy of estimation of the gradation characteristics and to stabilize the gradation. A tone correction value ΔCT is calculated.

By the way, character data is often drawn in a single color, for example, one color of black K, and the other basic colors cyan C, magenta M, and yellow Y are often not used together.
That is, when the character area F is specified and the minute colorimetric area (xi, yi) is selected, the basic colors to be measured are biased, and it is difficult to accurately correct the color tone of the unused basic colors. .

11A to 11C, the horizontal axis represents the number of print jobs, and the vertical axis represents each of the basic colors cyan C, magenta M, yellow Y, and black K when the first colorimetric result is the reference value 0. It is the figure shown as each ratio of the measurement result of each colorimetric value c (x, y) of L value, a value, and b value.
As is clear from FIGS. 11A to 11C, the profile for each color that changes in pixel density, which is a colorimetric result for each number of copies of the basic colors cyan C, magenta M, yellow Y, and black K, is 1 The following approximate expression can be approximated.
If a color-specific profile in which the pixel density change is first-order approximated is acquired in advance, even if only one of a plurality of basic colors is detected in the minute color measurement region, other than the detected basic color For the basic color, the necessary gradation correction value ΔCT can be estimated by a linear approximation formula.

Specifically, only black K is formed in the character area F, and the colorimetric value is measured by the measurement sensor 45 based on only the black K, and the fluctuation of the black K from the fluctuation exclusion parameter θd or the gradation correction value ΔCT. Predicted values ΔL _K , Δa _K , Δb _K are calculated.
The fluctuation prediction values ΔL _K , Δa _K , Δb _K are applied to the color-specific profiles in FIGS. 11A to 11C, and the color tone correction amount determination unit 319 determines the black K fluctuation prediction values ΔL _K , Δa _K , Based on Δb _K , the fluctuation prediction value of each basic color is also estimated for the other basic colors.

The tone correction amount determination unit 319 uses the color measurement results of the L value, a value, and b value of each of the basic colors cyan C, magenta M, yellow Y, and black K, which are acquired in advance, to adjust the tone correction value. ΔCT is calculated.
In other words, the tone correction amount determination unit 319 has a function as a gradation correction value estimation unit that estimates a gradation correction value ΔCT of another basic color from a profile for each basic color by a linear approximation formula. ing.
With such a configuration, even when the color that can be measured is biased, a fluctuation predicted value of another color is estimated, and the gradation correction value ΔCT is estimated by a primary approximation formula based on the estimation of the fluctuation predicted value. In addition, it suppresses a decrease in estimation accuracy of the gradation characteristic change and stably corrects the gradation characteristic.

  The color-specific profiles may be separately printed by test printing before the image forming apparatus 100 performs printing, and the color-specific profiles may be stored in the server 201 or stored in the storage device 302. Also good. Alternatively, the colorimetric value measured by the measurement sensor 45 during the printing operation may be used.

  The preferred embodiments of the present invention have been described above. However, the present invention is not limited to the specific embodiments, and the present invention described in the claims is not specifically limited by the above description. Various modifications and changes are possible within the scope of the above.

  For example, the color tone correction amount determination unit 319 corrects the gradation correction table based on the colorimetric value mesLab measured by the image inspection unit 33, the output predicted value prnLab, and the gradation value prnCMYK. A gradation correction value may be determined.

  In addition, the image forming unit of the image forming apparatus is an electrophotographic system using CMYK colors as basic colors. However, an ink jet system may be used, and the types of basic colors may be increased.

  Further, in this embodiment, the measurement sensor is installed downstream from the secondary transfer position and measures the reflection characteristics of the whole or a part of the toner image formed on the paper, but the toner formed on the transfer belt The reflection characteristics of the entire image or a part of the image may be measured.

  In addition, the image forming method in the present embodiment is not limited to the ROM provided in the image forming apparatus 100, but is also a semiconductor medium (for example, RAM, non-volatile memory), an optical medium (for example, DVD, MO, MD, CD-). R, etc.) is executed as a program stored in the program. Alternatively, the image forming method in the present embodiment may be executed by a program that can be stored in a magnetic medium (for example, a hard disk, a magnetic tape, a flexible disk, etc.) or other storage medium.

Further, in the present embodiment, only the cycle variation in the Y direction, that is, the sub-scanning direction has been described. However, the cycle in the X direction, that is, the main scanning direction, as in an image forming apparatus using an optical scanning device such as a polygon scanner, for example. It may be a configuration in which fluctuation occurs.
In that case, the fluctuation exclusion parameter that cancels the period fluctuation in the main scanning direction is calculated by replacing the sub-scanning pixel area in the present embodiment with the main scanning pixel area divided in the X direction. Can do.

  The effects described in the embodiments of the present invention are only the most preferable effects resulting from the present invention, and the effects of the present invention are limited to those described in the embodiments of the present invention. is not.

DESCRIPTION OF SYMBOLS 100 Image forming apparatus 2 Paper feed part 3 Control part 4 Tone correction means (image formation part) (image output means)
5 Transfer unit 22 Registration roller pair 6 Fixing unit 7 Paper discharge unit 20 Paper feed port 21 Paper feed roller 30 Image processing unit 31 Gradation processing unit 33 Image inspection unit 32 Color control unit 39 Engine control unit 45 Measurement sensor 47 Transfer belt 200 PC 201 Server 302 Storage device 316 Gradation correction means (gradation correction unit)
319 gradation correction value generation means (tone correction amount determination unit)
321 First change parameter generation means 321a, 321b, 321c First point 322 Second change parameter generation means 322a, 322b, 322c Second point 323 Area extraction means (area extraction unit)
325 Scanner correction unit 34 Color measurement prediction unit mesLab Measurement value (color measurement value)
f Color difference evaluation function
c ₁ , m ₁ , y ₁ , k ₁ first fluctuation mode
c ₂ , m ₂ , y ₂ , k ₂ 2nd fluctuation mode θ ₁ ^j 1st change parameter (mode parameter)
θ ₅ ^j Second change parameter (mode parameter)
θd Fluctuation exclusion parameter (mode parameter)
θ Mode parameter A1 Periodic fluctuation component (short-period fluctuation)
A2 Periodic fluctuation component (long period fluctuation)
F Character area ΔCT Tone correction value L Characteristic distance R Image data array (image data)

Japanese Patent Laying-Open No. 2015-008447

Claims (11)

An image output means for outputting an image on a recording medium by mixing the basic colors based on an image data array formed by combining a plurality of basic colors;
Gradation correction means for correcting the image output by the image output means using a gradation correction value for correcting the gradation characteristics of the basic color for each of the basic colors;
A measurement sensor for measuring the reflection characteristics of all or part of the image;
First change parameter calculation means for calculating a first change parameter using the reflection characteristic at the first point on the image measured by the measurement sensor;
Second change parameter calculation means for calculating a second change parameter for subtracting a periodic fluctuation component of the density of the image from the first change parameter;
Gradation correction value generating means for generating the gradation correction value of each of the basic colors based on the reflection characteristic, the image data array, the first change parameter, and the second change parameter; Have
The image forming apparatus, wherein the second change parameter is calculated using the reflection characteristic at a second point that is determined by the periodic variation component and is different from the first point. The image forming apparatus according to claim 1.
The distance between the first point and the second point is a position where the first point and the plurality of second points are out of phase with respect to the periodic fluctuation of the density of the image data array. An image forming apparatus that is set to be The image forming apparatus according to claim 1, wherein
The image forming apparatus, wherein the measurement sensor measures the reflection characteristic for each image region obtained by dividing a region where an image is output on the recording medium into a plurality of regions. The image forming apparatus according to any one of claims 1 to 3,
An image forming apparatus comprising a plurality of the second points. The image forming apparatus according to any one of claims 1 to 4,
The distance between the first point and the second point is a value obtained by adding a half wavelength of the wavelength to an integer multiple of the wavelength of the periodic vibration. The image forming apparatus according to any one of claims 1 to 5,
The image forming apparatus according to claim 1, wherein the first point and the second point are set separately between the plurality of recording media that are continuously printed. The image forming apparatus according to any one of claims 1 to 6,
An area extracting means for extracting an image area out of areas where the image formed by the image output means is output;
The image forming apparatus, wherein the measurement sensor measures the reflection characteristic of the entire surface of the image region extracted by the region extraction unit. The image forming apparatus according to claim 7.
The image forming apparatus, wherein the area extracting unit extracts an area where a character is formed from the areas as the image area. The image forming apparatus according to claim 8.
The region extracting means uses the reflection characteristic measured by the measurement sensor to identify the image region with a boundary where a difference between colorimetric values of adjacent pixels is larger than a predetermined value. An image forming apparatus. The image forming apparatus according to any one of claims 1 to 9,
The gradation correction value generation means includes gradation correction value estimation means for estimating the gradation correction value of another basic color from a change in the reflection characteristic of one basic color among the basic colors. An image forming apparatus. Based on an image data array formed by combining a plurality of basic colors, an image output step for outputting an image on a recording medium by mixing the basic colors;
A gradation correction step for correcting the image output by the image output means using a gradation correction value for correcting the gradation characteristics of the basic color for each of the basic colors;
A reflection characteristic measuring step for measuring a reflection characteristic of the whole or a part of the image;
A first change parameter calculating step of calculating a first change parameter using the reflection characteristic at the first point on the image;
A second change parameter calculating step of calculating a second change parameter for subtracting a periodic fluctuation component of the density of the image from the first change parameter;
A gradation correction value generation step of generating the gradation correction value of each of the basic colors based on the reflection characteristics, the image data array, the first change parameter, and the second change parameter; Have
In the image forming method, the second change parameter calculation step calculates the second change parameter using the reflection characteristic at a second point that is determined by the periodic variation component and is different from the first point.

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