JP2009192896A - Image forming apparatus and image correction method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To eliminate density unevenness in the main scanning direction and the subscanning direction. <P>SOLUTION: In the image forming apparatus, a test image including an image whose density is fixed in the main scanning direction and the subscanning direction and comprising a plurality of pages, where its density is changed stepwise for each page is formed by an image forming part; the density of the formed test image is detected by a detection part 6, and correction data for correcting image signal value is generated so that the density detected when the test image of each density is formed may be fixed in the main scanning direction and the subscanning direction, based on the result obtained by detecting the density by a correction data generating part a3; then the image signal value of each pixel is corrected, by using the correction data by a density irregularity correction part a4. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an image forming apparatus and an image correction method.

In an electrophotographic image forming apparatus such as a laser printer, an image is formed on a sheet through processes of exposure, development, transfer, and fixing.
In the image forming process, density unevenness may occur in the formed image due to device characteristics and changes with time. For example, when a conductive elastic roller that charges the photosensitive drum is installed, there is a limit to the accuracy of the installation, and therefore, the interval between the photosensitive drum and the conductive elastic roller may vary depending on the position. Further, when the frequency of use increases, the photosensitive drum may not be uniformly charged due to deterioration or the like, and density unevenness may occur.

Conventionally, in order to eliminate such density unevenness, a density distribution is detected using a test image, correction data is calculated so that the density of an image formed on a sheet is uniform, and the correction data is used. An image signal value is corrected (see, for example, Patent Document 1).
JP 2003-295529 A

  In the above-mentioned Patent Document 1, a test image having a constant density in the main scanning direction and a density gradient in the sub-scanning direction is used in one page image, and only density unevenness in the main scanning direction is used as a correction target. It was. In the vicinity of the high density value, density unevenness hardly occurs, but in the vicinity of the low density, there are many places where density unevenness occurs, and the state of density unevenness varies depending on the density level. On the other hand, even in the sub-scanning direction, density unevenness occurs due to sensitivity unevenness or eccentricity around the photosensitive member, and correction according to the density level is necessary. However, the conventional test image has one page. In particular, since a density gradient is provided in the sub-scanning direction, correction cannot be performed in the sub-scanning direction.

  An object of the present invention is to eliminate density unevenness in the main scanning direction and the sub-scanning direction.

According to the invention of claim 1,
An image forming unit;
A control unit that includes an image having a constant density in the main scanning direction and the sub-scanning direction, and causes the image forming unit to form a test image including a plurality of pages in which the density is changed step by step for each page;
A detection unit for detecting the density of the formed test image;
Based on the density detection result, correction data for correcting the image signal value is generated so that the density detected when the test image of each density is formed is constant in the main scanning direction and the sub-scanning direction. A correction data creation unit;
A density unevenness correction unit that corrects an image signal value of each pixel using the correction data;
An image forming apparatus is provided.

According to invention of Claim 2,
The detection unit detects the density at a plurality of sampling points at the same main scanning position and sub-scanning position in each density test image,
The correction data creation unit calculates a correction value obtained by correcting the image signal value so that the density detected at each sampling point matches the density of the detected test image, and performs main scanning at each sampling point as correction data. Create a three-dimensional table defining correction values for the position, sub-scanning position and image signal value,
When the density unevenness correction unit obtains a correction value corresponding to the image signal value of each pixel using the three-dimensional table, for the pixels at positions other than the sampling point, eight sampling points adjacent to the pixel An image forming apparatus according to claim 1, wherein the correction value is interpolated.

According to invention of Claim 3,
The correction data creation unit creates eight three-dimensional tables in which the correction value storage positions at the respective sampling points are shifted in accordance with the positional relationship of the eight sampling points, and stores the correction values in the respective three-dimensional tables. Add a common address to the location,
The density unevenness correction unit calculates one address indicating a main scanning position, a sub-scanning position, and an image signal value of each pixel, and obtains correction values for eight sampling points from the eight three-dimensional tables based on the addresses. The image forming apparatus according to claim 2, wherein the correction value of each pixel is obtained by interpolating the eight correction values.

According to invention of Claim 4,
Forming a test image composed of a plurality of pages including an image having a constant density in the main scanning direction and the sub-scanning direction and changing the density step by step for each page;
Detecting the density of the formed test image;
Based on the density detection result, correction data for correcting the image signal value is generated so that the density detected when the test image of each density is formed is constant in the main scanning direction and the sub-scanning direction. Process,
Correcting the image signal value of each pixel using the correction data;
An image correction method is provided.

  According to the first and fourth aspects of the invention, it is possible to correct density unevenness that occurs not only in the main scanning direction but also in the sub-scanning direction. Therefore, it is possible to eliminate density unevenness in the main scanning direction and the sub-scanning direction. Further, since the test image is composed of a plurality of pages and the density is changed stepwise for each page, the correction value at each density is obtained, so that density unevenness at each density level can be corrected.

  According to the second aspect of the present invention, since it is not necessary to hold correction values for all main scanning positions, sub-scanning positions, and image signal values, memory expansion for density unevenness correction can be avoided.

  According to the third aspect of the present invention, correction values at eight sampling points can be simultaneously read with one address. Therefore, the time for calculating the correction value can be shortened with a simple configuration.

First, the configuration will be described.
FIG. 1 shows an internal perspective view of an image forming apparatus 1 in the present embodiment. FIG. 2 is a diagram illustrating a functional configuration of the image forming apparatus 1.
As shown in FIGS. 1 and 2, the image forming apparatus 1 includes an image reading unit 2, an operation unit 3, a display unit 4, an image forming unit 5, a detection unit 6, and a control system 10. As shown in FIG. 2, the control system 10 includes a control unit 11, a storage unit 12, a memory control unit 13, an image memory 14, an image processing unit a, and the like.

  As shown in FIG. 1, the image reading unit 2 includes a document feeding unit 22 and a document scanning unit 21. The document scanning unit 21 includes an optical system and a CCD (Charge Coupled Device). The document surface conveyed by the document feeding unit 22 is optically scanned and an image signal (analog) is generated by the CCD. The generated image signal is converted into digital data by an A / D converter (not shown) and then output to the image processing unit a. Note that the image signal generated in the image reading unit 2 is an image signal separated into three colors of R (red), G (green), and B (blue).

The operation unit 3 includes a touch key and the like in addition to a start key and a numeric key for instructing start of execution, generates an operation signal corresponding to these operations, and outputs the operation signal to the control unit 11.
The display unit 4 displays various operation screens, execution results, and the like on a display unit integrated with the touch panel in accordance with display control of the control unit 11.

  As shown in FIG. 1, the image forming unit 5 includes units uY, uM, uC, and uK that perform exposure and development for each color of Y (yellow), M (magenta), C (cyan), and K (black). Further, an intermediate belt 51, a cleaning unit 52, a secondary transfer roller 53, a fixing unit 54, and a paper feeding unit 55 are provided. The units uY, uM, uC, and uK include an exposure unit u1, a development unit u2, a photosensitive drum u3, a charging unit u4, a cleaning unit u5, and a primary transfer roller u6, respectively.

  At the time of image formation, when the charging unit u4 charges the photosensitive drum u3, the exposure unit u1 irradiates laser light or the like based on the image signal to form an electrostatic latent image on the photosensitive drum u3. When toner is attached by the developing unit u2 to form a toner image on the photosensitive drum u3, the toner image is transferred to the intermediate belt 51 by the rotation of the photosensitive drum u3 and the action of the primary transfer roller u5. This exposure, development, and transfer process is performed by sequentially repeating the units uY, uM, uC, and uK for each color in accordance with the rotation of the intermediate belt 51, thereby superimposing the toner images of the respective colors on the intermediate belt 5. Form an image.

On the other hand, the paper is conveyed from the paper supply unit 55 to the secondary transfer roller 53, and the color image is transferred from the intermediate belt 51 onto the paper by the action of the secondary transfer roller 53. Thereafter, the sheet is conveyed to the fixing unit 54 and subjected to a fixing process, and then discharged onto a tray provided outside.
When the image formation is performed in this way, the toner remaining on the photosensitive drum u3 and the intermediate belt 51 is removed by the cleaning units u5 and 52.

  The detection unit 6 includes a sensor 61 for detecting the reference position of the photosensitive drum u3 and a sensor 62 for detecting the density of the toner image formed on the intermediate belt 51. The detection result is output to the image processing unit a. As the sensors 61 and 62, for example, optical sensors can be applied. By receiving the reflected light of the irradiated light, the sensor 61 detects a marker indicating the reference position provided on the photosensitive drum u3, and the sensor 62 detects a reflected light amount indicating the density of the toner image.

The installation positions of the sensors 61 and 62 are shown in FIG. 3A.
The sensor 61 is provided for each color photosensitive drum u3. The sensor 62 is provided until the toner image transferred from the photosensitive drum u3 of each color to the intermediate belt 51 reaches the position of the secondary transfer roller 53.
The arrows shown in FIG. 3A indicate the rotation direction of the photosensitive drum u3 and the rotation direction of the intermediate belt 51. The width direction of the intermediate belt 51 is the main scanning direction of the image, and the rotation direction of the intermediate belt 51, that is, the toner image conveyance direction is the sub-scanning direction of the image.

FIG. 3B is an enlarged perspective view of the installation location of the sensor 62.
As shown in FIG. 3B, two sensors 62 are coupled and arranged at positions determined as sampling points for density detection. This is because the detection result of the sensor 62 is used for density unevenness correction processing, which will be described later. However, if density detection is performed for each color, it takes time. This is because it is possible to simultaneously detect the concentrations of the two.

Next, the control system 10 will be described.
The control unit 11 includes a CPU (Central Processing Unit), a RAM (Random Access Memory), and the like, reads a program stored in the storage unit 12, and performs centralized control of operations of each unit in cooperation with the program. To do. Alternatively, various calculations are performed. For example, the control unit 11 controls the image forming operation of the image forming unit 5.

The storage unit 12 stores various programs and parameters used for executing processes.
The memory control unit 13 controls input / output of an image to / from the image memory 14.
The image memory 14 is an image storage memory, and a DRAM (Dynamic RAM) or the like is applicable.

The image processing unit a performs various image processes on the image.
FIG. 4 shows a main functional configuration of the image processing unit a.
As shown in FIG. 4, the image processing unit a includes a shading correction unit a1, a color conversion unit a2, a density unevenness correction unit a4, a γ correction unit a5, and a halftone processing unit a6. The shading correction unit a1 is provided according to three colors of R (red), G (green), and B (blue), and the density unevenness correction unit a4, the γ correction unit a5, and the halftone processing unit a6 are Y, M, and C. , K, respectively.

  The shading correction unit a1 performs a process of correcting luminance unevenness generated by the image reading unit 2 on the R, G, and B image signals input from the image reading unit 2. In addition, the I-I ′ conversion process for converting the luminance characteristic peculiar to the CCD of the image reading unit 2 into the optimum luminance characteristic according to the human visual characteristic, and the sharpening process using the MTF (Modulation Transfer Function) filter It is good also as giving etc. as needed.

  The color conversion unit a2 performs color correction on each of the R, G, and B image signals that have been subjected to the shading correction, and then performs color conversion to convert each of the R, G, and B image signals into Y, M, C, and An image signal of each color of K is generated. The image signals for each color of Y, M, C, and K are output to the correction data creation unit a3 and the density unevenness correction unit a4 corresponding to each color.

The correction data creation unit a3 creates correction data that is used by the density unevenness correction unit a4 to correct density unevenness.
Hereinafter, a method for creating correction data will be described.
FIG. 5 is a flowchart showing the flow of processing when creating correction data. As shown in FIG. 5, first, when the correction processing of the density unevenness correction unit a4 is invalidated by the control of the control unit 11 (step S1), the image forming unit 5 forms the test image (step S2). The test image is stored in advance in the storage unit 12 or the image memory 14, and is output to the halftone processing unit a6 to form an image by the image forming unit 5.

FIG. 6 shows an example of a test image.
The test image is an image used for detecting density characteristics of an image formed by the image forming unit 5. As shown in FIG. 6, a test image including a plurality of patch images having a constant density Lk (k = 0, 1, 2, 3, 4) in the main scanning direction and the sub-scanning direction, and the density Lk is stepwise for each page. Change to to create multiple pages. The image has a signal value indicating a density value for each pixel. That is, the patch image with the density Lk is an image having pixels with the image signal value Lk. Here, in the density range from the minimum density value 0 to the maximum density value 255, five pages of test images in which L0 = 0, L1 = 63, L2 = 127, L3 = 191, and L4 = 255 are set as image signal values. An example of use will be described.

  As shown in FIG. 6, the patch image is a combination of substantially rectangular images of C, M, Y, and K colors, and is arranged at a position to be a sampling point. Sampling points are provided at 20 intervals of 4 × 5 at regular intervals in the main scanning direction and the sub-scanning direction. The positions of the sampling points in the main scanning direction (hereinafter referred to as main scanning positions, indicated by X) are indicated by X = G1, G2, G3, G4, and the positions in the sub scanning direction (hereinafter referred to as sub scanning positions, Y Y = H0, H1, H2, H3, and H4. Further, the position of the image edge in the main scanning direction is indicated by X = G0, GMax as shown in FIG.

  FIG. 7 shows a state where a toner image of a test image is formed on the photosensitive drum u3. H0 to H4 shown in FIG. 7 indicate the sub-scanning position Y of the patch image formed on the photosensitive drum u3. Since the photosensitive drum u3 rotates in the direction indicated by the arrow in FIG. 7, the patch images are transferred to the intermediate belt 51 in order from the patch image whose sub-scanning position Y is H0, that is, in the order of H0 → H1 → H2 → H3 → H4. Will go. Note that after H4, the position is H0 in the next test image having the density L (k + 1), and the sub-scanning position Y is represented as H5 in order to indicate the end of the test image having the density Lk in the sub-scanning direction.

  When image formation is performed using the test image, the detection unit 6 detects density for one rotation of the photosensitive drum u3 from the reference position. For detection, the detection timing is determined in advance from the positional relationship of the sampling points indicated by the main scanning positions X of G1 to G4 and the sub-scanning positions Y of H0 to H5, and after the sensor 61 detects the reference position of the photosensitive drum u3, The detection timing of each sampling point is determined based on this reference position, and the density of the toner image transferred to the intermediate belt 51 is detected by the sensor 62 at the sampling point (step S3).

  As shown in FIG. 3B, the sensor 62 that performs density detection is provided by combining two sensors 62 according to the main scanning positions X1 to G4. One sensor 62 is provided at a position corresponding to the color C and Y patch images, and the other sensor 62 is provided at a position corresponding to the color M and K patch images. As a result, at one sampling point, the density of two colors can be detected simultaneously, such as detecting the density of the color C on one of the two sensors 62 and detecting the density of the color M on the other. Therefore, it is only necessary to first perform image formation for the colors C and Y, and then perform image formation for the colors Y and K to detect the density. In order to detect the density for four colors, the test image must be formed four times. In this embodiment, the number of times is half that.

  The detection result by the sensor 62 is input to the correction data creation unit a3. However, since the sensor 62 performs optical measurement, the correction data creation unit a3 performs an operation for converting the measured value into a color density and detects the calculated value. Treated as a density value.

Next, the correction data creation unit a3 is configured so that the density detected when the test image of each density Lk is formed is a constant density Lk in the main scanning direction and the sub-scanning direction based on the density detection result. A correction curve for correcting the image signal value is calculated (step S4).
The density detected at each sampling point can be represented by three-dimensional coordinates using three elements of a main scanning position X, a sub-scanning position Y, and an image signal I as shown in FIG. Lattice points indicated by coordinates (Gi, Hj, Lk) indicate sampling points. Note that i = 1, 2, 3, 4, j = 0, 1, 2, 3, 4, k = 0, 1, 2, 3, 4.

  Originally, in the test image having the density Lk, the same density Lk should be detected regardless of the main scanning position X and the sub-scanning position Y. Therefore, when the density detected at each sampling point is plotted on the three-dimensional coordinates, , All plot points should be on the grid points. However, when a density difference, that is, density unevenness occurs due to various causes such as sensitivity unevenness or eccentricity of the photosensitive drum u3 itself, or an accuracy error at the time of installation, the plot points are out of the lattice points. In this case, the density detected at the sampling point is not constant. For example, in the case of a test image having a certain density Lk, the density at the sub-scanning positions H0 to H5 is plotted for each of the main scanning positions G1, G2, G3, and G4. As shown in FIG. 9, the density variation occurs depending on the sub-scanning positions H0 to H5. Similarly, density unevenness may occur depending on the main scanning positions G1 to G4. Further, if the density Lk is different, the density unevenness state may be different.

  The correction data creation unit a3 creates a correction curve for correcting density unevenness using the density detection result at each sampling point (step S4). Specifically, for each main scanning position X and sub-scanning position Y indicated by coordinates (Gi, Hi), the density detected when using a test image of each density Lk as shown in FIG. 10 is plotted. Draw its approximate curve. This is a curve showing the density characteristics of the toner image detected at the sampling point. Then, a curve that is symmetrical with the drawn approximate curve with respect to the straight line indicating the target density characteristic is obtained. This obtained curve is a correction curve. The process for obtaining the correction curve is repeated for each sampling point. Since 20 approximate 4 × 5 curves are created for each position (Gi, Hj), 20 correction curves are created as a result.

  Note that the straight line indicating the target density characteristic is a straight line having an inclination of 1 so that the output image signal value is the same as the input image signal value. That is, when the image signal value of the image is Ln, the density characteristic detected from the toner image formed for the image is also Ln.

  Next, the correction data creation unit a3 creates an LUT (Look Up Table) that is correction data based on the created correction curve, and outputs it to the density unevenness correction unit a4. Specifically, an output image signal value corresponding to the image signal value Lk at each sampling point is obtained in each correction curve, and this output image signal value is obtained as a correction value (image signal value after density unevenness correction). This correction value is a value obtained by performing correction to increase or decrease the image signal value Lk of the original image in order to form an image with the target density Lk in the main scanning direction and the sub-scanning direction even when density unevenness occurs. is there. For example, if the detected density is Lk + α despite the formation of an image of the image signal value Lk due to the occurrence of density unevenness, the image signal value obtained by subtracting the increased + α from the original image signal value Lk. By forming an image with Lk-α, the density of the formed image should be Lk.

When the correction value is obtained, an LUT in which a correction value for the image signal value Lk is determined for each main scanning position X and sub-scanning position Y is created. That is, the created LUT is a three-dimensional table in which correction values are set for each of the sampling point grid points (Gi, Hj, Lk). Details of the LUT to be created will be described together with a method for correcting density unevenness.
The created LUT is output to the density unevenness correction unit a4. The density unevenness correction unit a4 sets an LUT in a correction value calculation unit described later (step S5).

The density unevenness correction unit a4 corrects the image signal value using the correction data in order to eliminate the density unevenness in the main scanning direction and the sub-scanning direction of the image.
FIG. 11 is a diagram illustrating a functional configuration of the density unevenness correction unit a4. As shown in FIG. 11, the density unevenness correction unit a4 includes a position specifying unit a41, a correction value calculation unit a42, and an interpolation calculation unit a43.

First, a method for correcting density unevenness will be described.
As described above, the correction data creation unit a3 can obtain a correction value for each sampling point indicated by each lattice point (Gi, Hj, Lk) in FIG. The density unevenness correction unit a4 uses this correction value to correct the image signal value of each pixel of the image. However, since there is no correction value for a pixel at a position other than the sampling point, the correction value at each sampling point is calculated. It is necessary to interpolate.

Interpolation is performed using correction values of eight sampling points adjacent to the pixel whose correction value is to be obtained.
12 is an enlarged view of a part of the three-dimensional coordinates of the main scanning position X, the sub-scanning position Y, and the image signal I in FIG. Eight arbitrary sampling points are indicated by points P1 to P8, correction values obtained at the points P1 to P8 are indicated by OUT1 to 8, and the position of the pixel for which the correction value is desired to be obtained is indicated by a point Q (X, Y, I). Is shown. Further, the distance in the main scanning direction between the adjacent points P1 to P8 is indicated by Xu, the distance in the sub-scanning direction is indicated by Yu, the distance in the direction of the image signal I is indicated by Iu, and the distance in the main scanning direction between the points P1 and Q is indicated. Is Xs, the distance in the sub-scanning direction is Ys, and the distance in the direction of the image signal I is Is. These Xu, Yu, Iu, Xs, Ys, and Is are values indicating the positional relationship between the points P1 to P8 and the point Q, and are used as interpolation coefficients when performing interpolation.

  In order to obtain the correction values OUT1 to P8 at the points P1 to P8, one LUT in which correction values are set for the coordinates (Gi, Hj, Lk) of all the sampling points is created, and points are calculated from the LUT. The correction values corresponding to the positions P1 to P8 (Gi, Hj, Lk) may be read out. However, in this method, eight points (Gi, Hj, Lk) are generated because eight points are designated from one LUT. Must. This requires time for the interpolation calculation. Therefore, in the present embodiment, the correction data generation unit a3 generates eight LUTs 1 to 8 corresponding to the positions of the points P1 to P8 so that the correction values OUT1 to OUT8 of the eight points P1 to P8 can be obtained with one address. .

FIG. 13 is a diagram illustrating an example of LUT1, LUT2, LUT5, and LUT6 created for points P1, P2, P5, and P6. Although not specifically shown in FIG. 13, LUTs 3, 4, 7, and 8 are similarly created for the points P3, P4, P7, and P8.
First, the configuration of the LUTs 1 to 8 will be described with reference to FIG. FIG. 14 shows an example of the LUT 1, but the other LUTs 2 to 8 have the same configuration except that the position for storing the correction value is different. In other words, each of the LUTs 1 to 8 is a 5 × 5 × 5 3 in which a table in which correction values (numbers in the figure) are stored in 5 × 5 storage positions (Gi, Hj) is created for each density Lk. It is a dimension table.

  As shown in FIG. 15, addresses Adr (numbers shown in the table of FIG. 15, Adr = 0 to 124) indicating the storage position of each correction value are assigned to the LUTs 1 to 8. This address Adr is common to the LUTs 1 to 8, and the same address Adr is used in the same storage position regardless of the LUTs 1 to 8.

  The LUTs 1 to 8 are tables created by shifting the correction value storage positions (Gi, Hj) in accordance with the main scanning position X and the sub-scanning position Y of the sampling points. For example, as shown in FIG. 13, at the points P1 and P2, which are sampling points, the main scanning position Gi is shifted by one, and the sub-scanning position Hj and the value Lk of the image signal I do not change. Therefore, the LUT 2 is created by shifting the storage position of the correction value for the main scanning position Gi of the LUT 1 by +1 in the main scanning direction.

When the storage position of the correction value of LUT1 is indicated by LUT1 [Gi, Hj, Lk], the storage position of the correction value in each of the LUTs 1 to 8 can be expressed as follows.
LUT1 [Gi, Hj, Lk]
LUT2 [Gi, Hj, Lk] = LUT1 [G (i + 1), Hj, Lk]
LUT3 [Gi, Hj, Lk] = LUT1 [Gi, Hj, L (k + 1)]
LUT4 [Gi, Hj, Lk] = LUT1 [G (i + 1), Hj, L (k + 1)]
LUT5 [Gi, Hj, Lk] = LUT1 [Gi, H (j + 1), Lk]
LUT6 [Gi, Hj, Lk] = LUT1 [G (i + 1), H (j + 1), Lk]
LUT7 [Gi, Hj, Lk] = LUT1 [Gi, H (j + 1), L (k + 1)]
LUT8 [Gi, Hj, Lk] = LUT1 [G (i + 1), H (j + 1), L (k + 1)]

  In this way, eight LUTs 1 to 8 are created according to the positional relationship of the eight points P1 to P8 (Gi, Hj, Lk), and one address Adr is adopted by adopting a common address Adr for each LUT 1 to 8. Thus, it becomes possible to obtain correction values of 8 points P1 to P8 at the same time.

  In the LUTs 1 to 8, correction values (correction values shaded in FIG. 13) for the main scanning positions G0 and GMax are added to the correction values of the sampling points of 4 × 5 × 5, and 5 × A 5 × 5 LUT is used. This is because the edge of the image cannot be set as a sampling point due to the accuracy of the apparatus of the image forming apparatus 1, and there is a region outside the density detection range, but correction is also performed outside the density detection range. This is necessary. The correction values for G0 and GMax are copies of the correction values for the adjacent main scanning positions G1 and G4, respectively. With respect to the correction values for G0 and GMax, the correction values for the main scanning positions G1 and G4 are used under the assumption that there will be no significant change in density unevenness occurring at adjacent main scanning positions G1 and G4.

Based on the main scanning position X and the sub-scanning position Y of each pixel of the image and the signal value I of each pixel, the position specifying unit a41 determines an address Adr for reading a correction value from the above-described LUTs 1 to 8 using the following formula 1. calculate. The position specifying unit a41 calculates interpolation coefficients Xs, Ys, Is, Xu, Yu, and Iu in the process of calculating the address Adr.
Adr = Xadr + Yadr × 5 + IAdr × 25 (1)

  In Equation 1, Xadr is an address indicating between which main scanning positions G0 to GMax a point Q (see FIG. 13) indicating a pixel for which a correction value is to be obtained, and Yadr is which sub-scanning position H0. This is an address indicating whether it exists in ~ H5. Iadr is an address indicating which image signal value L0 to L4 the point Q is between.

A process for calculating Xadr, Yadr, Iadr and interpolation coefficients Xs, Ys, Is, Xu, Yu, and Iu in Equation 1 will be described with reference to FIGS.
First, the position specifying unit a41 determines Xadr, Xs, and Xu depending on which main scanning position G0 to GMax is the main scanning position X of the point Q shown in FIG.

As shown in FIG. 16, when X ≦ G1 and the point Q is located between G0 and G1 (step S1; Y), the position specifying unit a41 sets Xadr = 0, Xs = X, and Xu = G1 ( Step S2).
When G1 <X ≦ G2 and the point Q is located between G1 and G2 (step S1; N, S3; Y), the position specifying unit a41 has Xadr = 1, Xs = X-G1, Xu = G2-G1. (Step S4).
When G2 <X ≦ G3 and the point Q is located between G2 and G3 (step S3; N, S5; Y), the position specifying unit a41 has Xadr = 2, Xs = X−G2, and Xu = G3−G2. (Step S6).
When G3 <X ≦ G4 and the point Q is located between G3 and G4 (step S5; N, S7; Y), the position specifying unit a41 has Xadr = 3, Xs = X-G3, Xu = G4-G3. (Step S8).
When G4 <X and the point Q is located between G4 and GMax (step S7; N), the position specifying unit a41 sets Xadr = 4, Xs = X-G4, and Xu = GMax-G4 (step S9). .

  For Yadr, as shown in FIG. 17, first, the sub-scanning position Y at the point Q is divided by the maximum value H5 of the sub-scanning position Y, and the remainder is set as the sub-scanning position Y at the point Q (step S11). Then, the sub-scanning position Y is compared with the sub-scanning positions H0 to H4 of each sampling point, and Yadr, Ys, and Yu are determined depending on which sub-scanning position H0 to H5 the point Q is between.

When Y ≦ H0 and the point Q is located between H4 and H0 (step S12; Y), the position specifying unit a41 sets Yadr = 4, YS = Y + H5-H4, and Yu = H5-H4 (step S13). .
When H0 <Y ≦ H1 and the point Q is located between H0 and H1 (step S12; N, S14; Y), the position specifying part a41 sets Yadr = 0, YS = Y, Yu = H1 (step) S15).
When H1 <Y ≦ H2 and the point Q is located between H1 and H2 (step S14; N, S16; Y), the position specifying unit a41 is Yadr = 1, YS = Y−H1, Yu = H2−H1. (Step S17).
When H2 <Y ≦ H3 and the point Q is located between H2 and H3 (step S16; N, S18; Y), the position specifying unit a41 has Yadr = 2, YS = Y−H2, and Yu = H3−H2. (Step S19).
When H3 <Y ≦ H4 and the point Q is located between H3 and H4 (step S18; N, S20; Y), the position specifying unit a41 is Yadr = 3, YS = Y−H3, Yu = H4−H3. (Step S21).
When H4 <Y and the point Q is located between H4 and H5 (step S20; N), the position specifying unit a41 sets Yadr = 4, YS = Y−H4, and Yu = H5−H4 (step S22). .

  For Iadr, the image signal value I of the pixel is compared with the image signal values L0 to L4, and Iadr, Is, and Iu are determined depending on which image signal value L0 to L4 the image signal value I is between. To do.

As shown in FIG. 18, when L0 ≦ I ≦ L1 and the point Q is located between L0 and L1 (step S31; Y), the position specifying unit a41 has Iadr = 0, Is = I, and Iu = L1. (Step S32).
When L1 <I ≦ L2 and the point Q is located between L1 and L2 (steps S31; N, S33; Y), the position specifying unit a41 has Iadr = 1, Is = I−L1, Iu = L2−L1. (Step S34).
When L2 <I ≦ L3 and the point Q is located between L2 and L3 (step S33; N, S35; Y), the position specifying unit a41 has Iadr = 2, Is = I−L2, and Iu = L3−L2. (Step S36).
When L3 <I ≦ L4 and the point Q is located between L3 and L4 (step S35; N, S37; Y), the position specifying unit a41 has Iadr = 3, Is = I−L3, Iu = L4−L3. (Step S38).
When L4 <I and the point Q is located between L4 and LMax (step S37; N), the position specifying unit a41 sets Iadr = 4, Is = I-L4, and Iu = IMax-L4 (step S39). .

Note that IMax is the maximum signal value of the image. When the minimum signal value 0 is adopted as L0 and the maximum signal value 255 is adopted as L4 as in the present embodiment, the process of step S39 is not performed.
The position specifying unit a41 outputs the calculated address Adr to each correction value calculation unit a42, and outputs interpolation coefficients Xs, Ys, Is, Xu, Yu, and Iu to the interpolation calculation unit a43.

  As shown in FIG. 11, the correction value calculation unit a <b> 42 is provided corresponding to each of the eight LUTs 1 to 8. Each correction value calculation unit a <b> 42 reads out the correction values OUT <b> 1 to 8 corresponding to the address Adr from the LUTs 1 to 8 that the correction value calculation units a <b> 42 have. Out1 to 8 are correction values of the points P1 to P8 of the eight sampling points adjacent to the point Q as described above.

For example, when Xadr = 2, Yadr = 1, and Iadr = 2 are obtained in the position specifying unit a41, Adr = 57 from Equation 1. Since Adr = 57 indicates the storage position shaded in FIG. 15, the correction value OUT1 = 127 of the coordinate position (G2, H2, L3) corresponding to Adr = 57 is read from the LUT1 shown in FIG. Similarly, correction values OUT2 to 8 corresponding to Adr = 57 are read out from the LUTs 2 to 8, respectively.
Each correction value calculation unit a42 outputs the read correction values Out1 to 8 to the interpolation calculation unit a43.

The interpolation calculation unit a43 interpolates the eight correction values Out1 to 8 using the interpolation coefficients Xs, Ys, Is, Xu, Yu, and Iu, and obtains a correction value OUT at the point Q.
The correction value OUT can be obtained by the following equation 2.
OUT = Out1 × (Iu−Is) × (Xu−Xs) × (Yu−Ys)
+ Out2 × (Iu−Is) × Xs × (Yu−Ys)
+ Out3 × Is × (Xu−Xs) × (Yu−Ys)
+ Out4 × Is × Xs × (Yu−Ys)
+ Out5 × (Iu−Is) × (Xu−Xs) × Ys
+ Out6 × (Iu−Is) × Xs × Ys
+ Out7 × Is × (Xu−Xs) × Ys
+ Out8 × Is × Xs × Ys
... (2)

  The interpolation calculation unit a43 outputs the correction value OUT obtained for each pixel to the γ correction unit a5 as an image signal after density unevenness correction.

  The γ correction unit a5 performs density gradation correction using the LUT on the image signal input from the density unevenness calculation unit a43. The density gradation correction of the γ correction unit a5 is a process for making the density gradation of the image according to the visual characteristics, and the LUT is also provided for the density gradation correction.

  The halftone processing unit a6 performs halftone processing on the image signal input from the γ correction unit a5 in order to reproduce the halftone. As the halftone processing, any method such as a screen cell method or an error diffusion method may be adopted.

  As described above, according to the present embodiment, a test image including a plurality of pages including a patch image having a constant density Lk in the main scanning direction and the sub-scanning direction and changing the density Lk step by step for each page is used. Concentration detection. Then, based on the density detection result, LUT1 to LUT8 in which correction values are set for each of the main scanning position X, the sub-operation position Y, and the image signal value Lk are created. Ask. Using a test image having a constant density in the main scanning direction and the sub-scanning direction, the density detected when the test image of each density Lk is imaged becomes a constant density Lk in the main scanning direction and the sub-scanning direction. Since the correction value obtained by correcting the image signal value is obtained, the density unevenness generated not only in the main scanning direction but also in the sub scanning direction can be corrected, and the density unevenness in the main scanning direction and the sub scanning direction can be eliminated. it can. Further, since the test image is composed of a plurality of pages and the density is changed stepwise for each page, the correction value at each density is obtained, so that density unevenness can be corrected according to the density level.

  In addition, sampling points are provided at the same main scanning position X (G1 to G4) and sub-scanning positions Y (H0 to H4) in the test image of each density Lk, and the density Lk detected at this sampling point performs the detection. A correction value is obtained so as to coincide with the density Lk of the test image, and a three-dimensional table in which the correction value is determined for the main scanning position X, the sub scanning position Y, and the image signal value Lk is created. When correcting density unevenness, a correction value corresponding to the main scanning position, sub-scanning position, and image signal value of each pixel is obtained using this three-dimensional table. For pixels at positions other than the sampling point, The eight sampling point correction values are read from the three-dimensional table and interpolated. This eliminates the need to hold correction values for all main scanning positions, sub-scanning positions, and image signal values, thereby avoiding memory expansion for density unevenness correction.

  The LUTs 1 to 8 are created by shifting the storage positions of the correction values according to the positional relationship of the eight sampling points, and the storage addresses of the LUTs 1 to 8 are assigned a common address Adr. As a result, the correction values OUT1 to OUT8 at the eight sampling points can be simultaneously read with one address Adr. Therefore, the time for calculating the correction value can be shortened with a simple configuration.

In addition, the said embodiment is a suitable example of this invention, and is not limited to this.
For example, although the density detection position by the sensor 62 is on the intermediate belt 51, the density is detected on each of the Y, M, C, and K photosensitive drums u3, and the density of the toner image formed on each photosensitive drum u3 is detected. It is good as well. Further, the configuration may be such that the density of the toner image transferred onto the paper is detected. In this case, although the cost is high, since the final density detection of the toner image is performed, correction including density unevenness caused by transfer or the like can be performed, and correction with higher accuracy is possible. .

  Furthermore, instead of always correcting density unevenness, it is possible to determine whether density unevenness has occurred based on the density detection result, and to correct density unevenness when it is determined that it has occurred. . For example, in a test image having the same density Lk, it is determined whether there is a density difference between the densities detected at two arbitrary sampling points. If there is a density difference, it is determined that density unevenness has occurred, and the correction data Create.

1 is a diagram illustrating an image forming apparatus according to an exemplary embodiment. FIG. 2 is a functional configuration diagram of the image forming apparatus in FIG. 1. It is a figure which shows the installation position of each sensor of a detection part. It is a figure which shows the installation position of the sensor which performs density | concentration detection. It is a functional block diagram of an image processing part. It is a flowchart which shows the flow of a process at the time of creating correction data. It is a figure which shows an example of the test image used when creating correction data. It is a figure which shows the relationship between a photosensitive drum and the subscanning position of a sampling point. It is a figure which shows the three-dimensional coordinate which makes a main scanning position, a subscanning position, and an image signal an element. It is a figure which shows the density | concentration characteristic in a subscanning position. It is a figure explaining the preparation method of a correction curve. It is a functional block diagram of a density nonuniformity correction part. It is a figure explaining 8-point interpolation. It is a figure which shows an example of LUT produced as correction data. It is a figure which shows an example of LUT1 among eight LUT1-8. It is a figure which shows the address of LUT. It is a figure explaining the flow of the process performed in the case of 8-point interpolation. It is a figure explaining the flow of the process performed in the case of 8-point interpolation. It is a figure explaining the flow of the process performed in the case of 8-point interpolation.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Image forming apparatus a Image processing part a3 Correction data creation part a4 Density unevenness correction part a41 Position specification part a42 Correction value calculation part a43 Interpolation calculation part a5 γ correction part 11 Control part 12 Storage part 14 Image memory 5 Image formation part u3 Photosensitive Drum 51 Intermediate belt 6 Detector 62 Sensor

Claims (4)

An image forming unit;
A control unit that includes an image having a constant density in the main scanning direction and the sub-scanning direction, and causes the image forming unit to form a test image including a plurality of pages in which the density is changed step by step for each page;
A detection unit for detecting the density of the formed test image;
Based on the density detection result, correction data for correcting the image signal value is generated so that the density detected when the test image of each density is formed is constant in the main scanning direction and the sub-scanning direction. A correction data creation unit;
A density unevenness correction unit that corrects an image signal value of each pixel using the correction data;
An image forming apparatus comprising: The detection unit detects the density at a plurality of sampling points at the same main scanning position and sub-scanning position in each density test image,
The correction data creation unit calculates a correction value obtained by correcting the image signal value so that the density detected at each sampling point matches the density of the detected test image, and performs main scanning at each sampling point as correction data. Create a three-dimensional table defining correction values for the position, sub-scanning position and image signal value,
When the density unevenness correction unit obtains a correction value corresponding to the image signal value of each pixel using the three-dimensional table, for the pixels at positions other than the sampling point, eight sampling points adjacent to the pixel The image forming apparatus according to claim 1, wherein the correction value is interpolated. The correction data creation unit creates eight three-dimensional tables in which the correction value storage positions at the respective sampling points are shifted in accordance with the positional relationship of the eight sampling points, and stores the correction values in the respective three-dimensional tables. Add a common address to the location,
The density unevenness correction unit calculates one address indicating a main scanning position, a sub-scanning position, and an image signal value of each pixel, and obtains correction values for eight sampling points from the eight three-dimensional tables based on the addresses. The image forming apparatus according to claim 2, wherein the correction value of each pixel is obtained by interpolating the eight correction values. Forming a test image composed of a plurality of pages including an image having a constant density in the main scanning direction and the sub-scanning direction and changing the density step by step for each page;
Detecting the density of the formed test image;
Based on the density detection result, correction data for correcting the image signal value is generated so that the density detected when the test image of each density is formed is constant in the main scanning direction and the sub-scanning direction. Process,
Correcting the image signal value of each pixel using the correction data;
An image correction method including:

Images