JP5870548B2 - Image forming apparatus and correction value calculation method - Google Patents

Description

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

  In an electrophotographic image forming apparatus, density unevenness may occur in an image printed on paper. Irregular density unevenness occurs on the entire surface of the sheet due to various factors such as uneven charging of the photosensitive member, fluctuations in rotational speed, and uneven distance between the photosensitive member and the exposed portion or between the photosensitive member and the developing portion. For such density unevenness, the image forming apparatus performs gradation correction processing on the image data to reduce the density unevenness.

  For example, many methods for printing a pattern having a plurality of patch images with different gradation values and calculating a correction value for gradation correction processing are disclosed (for example, see Patent Documents 1 and 2). According to Patent Documents 1 and 2, the density value of the printed pattern is measured, and a correction γ table is created from the density characteristic of the measured density value and the target gradation characteristic. With the correction γ table, it is possible to obtain the pixel value after the gradation correction process corresponding to the pixel value (gradation value) of each pixel of the image data.

JP 2007-264364 A JP 2010-134366 A

  According to Patent Documents 1 and 2, the same pattern is formed twice in one period and measured from the pattern of the same gradation value in accordance with the period of the photoconductor in order to cope with the density unevenness generated on the entire surface. The density value is averaged to obtain a correction value. With this method, a correction value corresponding to density unevenness at least every half cycle can be obtained, but it cannot be said that the correction value reduces all density unevenness occurring on the entire surface of the paper.

  From the viewpoint of improving the correction accuracy, a pattern covering the entire surface of the paper, not a partial patch image, can be used for calculating the correction value. By changing the gradation value of the pattern in units of pages, measuring the density at a plurality of positions from one gradation value pattern, and obtaining the correction value from the average value, not only in the main scanning direction of the paper A more optimal correction value can be obtained for the entire density unevenness occurring in the sub-scanning direction. However, since it is necessary to print the pattern on the entire surface, consumption of paper, toner, and the like increases.

  An object of the present invention is to calculate an optimal correction value in order to reduce the overall density unevenness that occurs in the main scanning direction and the sub-scanning direction of a sheet regardless of the pattern.

According to the invention of claim 1,
A printing unit that prints on the paper a plurality of patterns having different gradation values and having a width in the main scanning direction of the paper;
A control unit that obtains density values measured at a plurality of positions in the main scanning direction of each gradation value pattern printed on the paper, and calculates correction values of the respective gradation values according to the positions in the main scanning direction; ,
An image processing unit that performs gradation correction processing on image data using the correction value calculated by the control unit,
The controller is
A density characteristic indicating a correlation between a position in the main scanning direction where the density value is measured and the density value is created for each gradation value,
The density characteristic of each gradation value is divided by the range and normalized, a representative characteristic of the normalized density characteristic of each gradation value is created, and a reference position where the correction value is 0 is determined by the representative characteristic,
In the density characteristic of each gradation value obtained by multiplying the representative characteristic by the density characteristic range of each gradation value, each gradation according to the position in the main scanning direction based on the determined density value of the reference position An image forming apparatus for calculating a correction value is provided.

According to invention of Claim 2,
A printing unit that prints on the paper a plurality of patterns having different gradation values and having a width in the main scanning direction of the paper;
A control unit that acquires density values measured at a plurality of positions in the main scanning direction of each gradation value pattern printed on the paper, and calculates correction values of the respective gradation values according to the positions in the main scanning direction;
An image processing unit that performs gradation correction processing on image data using the correction value calculated by the control unit,
The controller is
A density characteristic indicating a correlation between a position in the main scanning direction where the density value is measured and the density value is created for each gradation value,
A reference position with a correction value of 0 is temporarily determined in the density characteristics of each gradation value, and a correction value at each position is calculated from the difference between the density value at the reference position and the density value at each position in the main scanning direction. Create a correction value characteristic indicating the correlation between the position in the scanning direction and the correction value,
The correction value characteristic of each gradation value is normalized by dividing by the range, a representative characteristic of the normalized correction value characteristic of each gradation value is created, and the reference position where the correction value is 0 by the representative characteristic is recorded. Decide
Based on the correction value characteristic of each gradation value obtained by multiplying the representative characteristic by the range of the correction value characteristic of each gradation value, the correction value of the reference position determined in this way is used as a reference according to the position in the main scanning direction. An image forming apparatus that recalculates the correction value of each gradation value is provided.

According to invention of Claim 3,
The printing unit prints a pattern of each gradation value on a plurality of pages of paper,
The said control part acquires the density value measured from the pattern printed on the paper of each page, creates the said density characteristic for all the pages from the said density value, and produces the representative characteristic. Is provided.

According to invention of Claim 4,
A step of printing on the paper a plurality of patterns having different gradation values and having a width in the main scanning direction of the paper;
Obtain density values measured at a plurality of positions in the main scanning direction of each gradation value pattern printed on the paper, and correlate the density values with the positions in the main scanning direction at which the density values are measured. Creating a density characteristic to be shown for each gradation value;
A step of normalizing by dividing the density characteristic of each gradation value by the range, creating a representative characteristic of the normalized density characteristic of each gradation value, and determining a reference position where the correction value is 0 based on the representative characteristic When,
In the density characteristic of each gradation value obtained by multiplying the representative characteristic by the density characteristic range of each gradation value, each gradation according to the position in the main scanning direction based on the determined density value of the reference position Calculating a correction value of the value;
A correction value calculation method is provided.

According to the invention of claim 5,
A step of printing on the paper a plurality of patterns having different gradation values and having a width in the main scanning direction of the paper;
Density values measured at a plurality of positions in the main scanning direction of each gradation value pattern printed on the paper are acquired, and the correlation between the positions in the main scanning direction where the density values are measured and the density values is shown. Creating density characteristics for each gradation value;
A reference position with a correction value of 0 is temporarily determined in the density characteristics of each gradation value, and a correction value at each position is calculated from the difference between the density value at the reference position and the density value at each position in the main scanning direction. Creating a correction value characteristic indicating the correlation between the position in the scanning direction and the correction value;
The correction value characteristic of each gradation value is normalized by dividing by the range, a representative characteristic of the normalized correction value characteristic of each gradation value is created, and the reference position where the correction value is 0 by the representative characteristic is recorded. A step of determining;
Based on the correction value characteristic of each gradation value obtained by multiplying the representative characteristic by the range of the correction value characteristic of each gradation value, the correction value of the reference position determined in this way is used as a reference according to the position in the main scanning direction. Recalculating the correction value of each gradation value;
A correction value calculation method is provided.

According to the invention of claim 6,
In the step of printing the pattern, a pattern of each gradation value is printed on a plurality of pages of paper,
In the step of creating the density characteristic, a density value measured from a pattern printed on the paper of each page is acquired, and the density characteristic for all pages is created from the density value,
6. The correction value calculation method according to claim 4, wherein in the step of determining the reference position based on the representative characteristic, the reference position is determined based on the representative characteristic created based on the density characteristic for all pages.

  According to the present invention, since a unique reference position is determined for each gradation value from the representative characteristics, the main scanning direction and the sub-scanning direction of the paper are used regardless of where the gradation value pattern is arranged on the paper. The optimum correction value can be calculated in order to minimize the density unevenness that occurs in the image. The optimum correction value can be calculated for any pattern having a width in the main scanning direction, and the correction accuracy can be improved.

1 is a functional block diagram of an image forming apparatus according to the present embodiment. 4 is a flowchart of correction value calculation processing executed by the image forming apparatus according to the first embodiment. (A), (b), (c) An example of the correction pattern printed for calculating the correction value is shown. Another example of the correction pattern is shown. Another example of the correction pattern is shown. Another example of the correction pattern is shown. An example of the position in the main scanning direction where the density is measured in the correction pattern is shown. The characteristic diagram of the density characteristic of each gradation value which changes by normalization and creation of a representative characteristic is shown. A specific example of the correction value is shown. 10 is a flowchart of correction value calculation processing executed by the image forming apparatus according to the second embodiment. The characteristic diagram of the correction value characteristic of each gradation value which changes by normalization and creation of a representative characteristic is shown. A specific example of the correction value is shown.

  Embodiments of the present invention will be described below with reference to the drawings.

<First Embodiment>
FIG. 1 shows an image forming apparatus 1 according to the present embodiment.
The image forming apparatus 1 can print document data created by an application of a user terminal. The image forming apparatus 1 can also copy a document.
As shown in FIG. 1, the image forming apparatus 1 includes a control unit 11, a storage unit 12, a display unit 13, an operation unit 14, a communication unit 15, an I / F 16, a scanner 21, an image processing unit 22, a memory 23, and a printing unit 24. It is configured with.

The control unit 11 includes a CPU (Central Processing Unit), a RAM (Random Access Memory), and the like. The control unit 11 reads out a program stored in the storage unit 12 and cooperates with the program in each unit of the image forming apparatus 1. To control the operation.
For example, the control unit 11 outputs the print setting input via the operation unit 14 to the image processing unit 22 as a print command.
Further, the control unit 11 executes a correction value calculation process, and calculates a correction value used for the gradation correction process of the image processing unit 22.

  The storage unit 12 stores programs executed by the control unit 11, files necessary for execution of the programs, data, and the like. The storage unit 12 stores correction pattern data to be printed for calculation of correction values for gradation correction processing.

The display unit 13 displays an operation screen or the like on a touch panel or a display according to the control of the control unit 11.
The operation unit 14 generates an operation signal corresponding to the operation of the touch panel or the operation key and outputs the operation signal to the control unit 11. In addition to basic print settings such as the number of copies and the paper size, optional print settings can be made via the operation unit 14. The optional print settings include addition of a page number, watermark, copy-forgery-inhibited pattern image, image rotation, reduction or enlargement, page allocation, density designation, and the like.

  The communication unit 15 includes a communication interface and communicates with a controller and the like. The controller rasterizes PDL (Page Description Language) data transmitted from the user terminal, and generates bitmap format image data. The PDL data includes, as a print command, optional print settings in addition to basic print settings such as the number of copies and paper size. Since the controller extracts the print command from the PDL data and transmits it to the image forming apparatus 1 together with the image data, the communication unit 15 receives the image data and the print command from the controller and outputs them to the image processing unit 22.

The I / F 16 is an interface such as a USB (Universal Serial Bus).
The scanner 21 scans a document and generates image data. The image data is output to the image processing unit 22.

  The image processing unit 22 performs various image processes on the image data input from the communication unit 15 or the scanner 21. For example, since the color system of the scanner 21 is R (red), G (green), and B (blue), the image processing unit 22 performs color conversion processing on the image data generated by the scanner 21 and prints it. Image data of Y (yellow), M (magenta), C (cyan), and K (black), which are color materials used in the unit 24, is generated. The image processing unit 22 compresses the image data and stores it in the memory 23, reads the compressed image data from the memory 23, and decompresses the image data. When a print command is input, the image processing unit 22 enlarges or reduces the image data in accordance with the print command, and performs image processing such as page number addition and page allocation.

The image processing unit 22 performs gradation correction processing on the image data using the correction value calculated by the control unit 11. Since the LUT (Look Up Table) in which the correction value of each gradation value is set for each position in the main scanning direction is created by the control unit 11, the image processing unit 22 uses the pixel value (gradation value) of each pixel of the image data. ) And the correction value corresponding to the position in the main scanning direction are acquired from the LUT, and the correction value is added to the pixel value of each pixel to obtain the pixel value after the gradation correction processing.
The image processing unit 22 screen-processes the image data that has been subjected to the gradation correction processing, performs PWM (Pulse Width Modulation) conversion, and outputs the result to the printing unit 24.

  The printing unit 24 includes an exposure unit, a developing unit, a photosensitive drum, a fixing device, and the like, and performs an electrophotographic printing process. In the printing process, the exposure unit irradiates a charged photosensitive drum with laser light based on the PWM-converted image data. An electrostatic latent image is formed on the photosensitive drum. When the developing unit develops the photosensitive drum, a toner image is formed on the photosensitive drum, and the toner image is primarily transferred to an intermediate belt or the like. The same image formation is repeated for the four colors of YMCK toner, and the toner images for the four colors are superimposed on the intermediate belt to form a color image. The color image is secondarily transferred onto the paper, and the paper is conveyed to a fixing device and fixed.

FIG. 2 is a flowchart of correction value calculation processing executed by the control unit 11 of the image forming apparatus 1. The control unit 11 periodically executes this correction value calculation process, and updates the correction value used by the image processing unit 22 for the gradation correction process.
As shown in FIG. 2, the control unit 11 reads a correction pattern from the storage unit 12, and causes the printing unit 24 to print the pattern on a sheet (step S1). As the correction pattern, a plurality of patterns having different gradation values are arranged on one page or a plurality of pages of paper. Each gradation value pattern has a width in the main scanning direction of the paper, and specifies density unevenness in the main scanning direction by measuring density values of patterns printed at a plurality of positions in the main scanning direction. Can do.

FIG. 3A, FIG. 3B, and FIG. 3C show examples of correction patterns.
The pattern T11 shown in FIG. 3A is a belt-like pattern having a width in the main scanning direction of the paper in units of C, M, Y, and K as color materials of the printing unit 24. A plurality are arranged in the direction, that is, in the sub-scanning direction. There are three patterns of gradation values 255, 127, and 63 for each color. The color of each gradation value pattern is (C): cyan, (M): magenta, (Y): yellow, (K): black. Show. The length of each strip-shaped pattern in the sub-scanning direction is preferably as close as possible to the size of characters that can be printed by the printing unit 24. For example, if the paper size is A3, it is about 293 mm.

By printing the pattern T11 for a plurality of pages and using it for calculation of correction values, correction values corresponding to density unevenness for a plurality of pages can be calculated, and correction accuracy can be improved.
In addition to the pattern T11, a pattern T12 shown in FIG. 3B may be printed and used for calculating the correction value. The pattern T12 has C, M, Y, and K color units, and gradation values 223, 159, and 92 are arranged in the sub-scanning direction. By adding the pattern T12 to the pattern T11, the gradation value can be increased by three for each color, and a correction value with high correction accuracy can be obtained.

  Instead of arranging the band-like patterns of each gradation value in units of colors as in patterns T11 and T12, they may be arranged in units of gradation values as in pattern T21 shown in FIG. Good.

FIG. 4 shows another example of the correction pattern.
In the patterns T31 and T32 shown in FIG. 4, strip-like patterns having three gradation values 255, 127, and 63 for each color are repeatedly arranged twice. Since the pattern of all colors does not fit in one page by repeating the arrangement twice, a pattern T3 1 of C and M colors is arranged on the first page, and a pattern T32 of Y and K colors is arranged on the second page. ing. The length of the pattern for one color in the sub-scanning direction is the same as the length of one turn of the photosensitive drum of the printing unit 24. By arranging the pattern of the same gradation value twice for one cycle of the photosensitive drum, the correction value can be calculated so that density unevenness generated not only in the main scanning direction but also in the sub-scanning direction is reduced.

  As shown in FIG. 5, the above-described pattern T21 in which the band-like pattern is arranged in the gradation value unit and the pattern T22 in which the arrangement position of the band-like pattern is replaced in the gradation value unit may be used in combination. According to the patterns T21 and T22, the belt-like pattern having the same gradation value is printed twice at different positions in the sub-scanning direction. Therefore, like the correction patterns T31 and T32 shown in FIG. In addition, the correction value can be calculated so that density unevenness in the sub-scanning direction is reduced.

  From the viewpoint of optimizing the correction value so as to reduce the density unevenness generated in the main scanning direction and the sub-scanning direction, that is, the entire surface of the sheet, as the correction pattern, as shown in FIG. It is also possible to use patterns T41 to T43 in which these patterns are arranged on the entire page. As shown in FIG. 6, a pattern T41 having a gradation value 255 having a width in the main scanning direction and the sub-scanning direction is arranged on the entire first page. Similarly, a pattern T42 having a gradation value 127 is arranged on the second page, and a pattern T43 having a gradation value 63 is arranged on the third page.

  In any of the above correction patterns, the greater the number of patterns to be printed, the more accurately the density unevenness can be specified, so that a correction value with high correction accuracy can be calculated. On the other hand, if the number of patterns increases, the consumption of paper and toner increases due to pattern printing. What pattern is printed and how much is printed may be appropriately determined from the relationship between the accuracy of correction and the cost.

  The user measures the density value of the printed pattern with a colorimeter or the like, and inputs the measurement result to the image forming apparatus 1. Since the printed pattern has a width in the main scanning direction, the user measures density values at a plurality of positions in the main scanning direction of each gradation value pattern. The number of measurement positions is arbitrary, but the correction value is optimized as the number of measurement positions increases. For example, when the patterns T21 and T22 shown in FIG. 5 are printed, the user may measure the density value of each gradation value pattern at six positions x1 to x6 in the main scanning direction as shown in FIG.

  The user may connect the colorimeter directly to the I / F 16 and input the measurement result directly from the colorimeter to the image forming apparatus 1. Alternatively, the user may store the measurement result in the USB memory and the USB memory in the I / F 16. You may connect and input a measurement result. Alternatively, the user himself / herself may input the measurement result via the operation unit 14.

The control unit 11 acquires a density value measurement result from a colorimeter or USB memory connected to the I / F 16. The control unit 11 divides the acquired density values according to the colors Y, M, C, and K, and calculates the following correction values in color units using the density values of each color.
First, the control unit 11 plots the measured density value against the position in the main scanning direction where the density value is measured, and creates a density characteristic of each gradation value (step S2). Since the measured density value is a discrete value, the control unit 11 interpolates the density value at each measurement position to create a density characteristic. Preferably, the control unit 11 performs a process such as smoothing on the created density characteristic to remove noise such as streaks in advance. It is possible to calculate an appropriate correction value by removing noise.

  A characteristic diagram A1 in FIG. 8 shows an example of density characteristics created for the three gradation values 255, 127, and 63, respectively. The horizontal axis of the density characteristic is the position in the main scanning direction where the density value is measured, and the vertical axis is the measured density value. The density value between each measurement position is interpolated, and a smooth characteristic curve is drawn.

According to a normal correction value calculation method, a position in the main scanning direction that takes an intermediate value of the density value in the density characteristics of each tone value is determined as a reference position, and the correction value of each tone value is determined based on this reference position. Is determined. The correction value at the reference position is 0, and the correction values at the other positions are determined by the following equation according to the difference between the density value at the reference position and the density value at the other position.
Correction value = (density value at reference position−density value at other position) × coefficient α
The coefficient α is a coefficient for converting density values into gradation values.

  In the characteristic diagram A1 shown in FIG. 8, when the measurement position that takes the intermediate value of the density value is determined as the reference position, the reference position indicated by the arrow varies with the gradation values 255, 127, and 63. This is because different density unevenness occurs in the main scanning direction for each gradation value, and the range between the minimum value and the maximum value of the density value varies. The reason why the density unevenness in the main scanning direction differs depending on the gradation value is that the density unevenness is generated not only in the main scanning direction but also in the sub-scanning direction, depending on the gradation value. is not. If there is no density unevenness in the sub-scanning direction, the density unevenness in the main scanning direction is essentially the same for each gradation value, and the reference position should be substantially the same.

  Nevertheless, when a different reference position is determined for each gradation value and a correction value for each gradation value is obtained as in the prior art, the correction value is the optimal correction for all density unevenness occurring on the paper. Not necessarily a value. This is because the same density unevenness does not always occur in the main scanning direction between the position in the sub-scanning direction and the position in the other sub-scanning direction where the gradation pattern of the correction value is printed. For example, if a correction value obtained at a position in the sub-scanning direction in which the density unevenness in the main scanning direction is relatively small is applied to a position in another sub-scanning direction in which the density unevenness in the main scanning direction is large, the correction is insufficient. In this case, the correction becomes excessive.

  Thus, according to the conventional correction value calculation method, the correction value is biased depending on the arrangement position of the belt-like pattern in the sub-scanning direction. As shown in FIG. 6, it is sufficient to use a pattern arranged on the entire page, but the cost increases. Therefore, in order to minimize all density unevenness that occurs in the main scanning direction and sub-scanning direction regardless of the pattern, the correction value calculation method according to the present embodiment creates representative characteristics from the density characteristics of each gradation value. Then, a reference position common to each gradation value is determined based on the representative characteristic.

First, the control unit 11 normalizes by dividing the density characteristic of each gradation value by its range (maximum value-minimum value) (step S3). Specifically, the control unit 11 uses the maximum value and the minimum value of the density characteristics of each gradation value to obtain a density value normalized by the following equation. The control unit 11 plots the normalized density value to obtain a normalized density characteristic.
Normalized concentration value = measured concentration value / (maximum value−minimum value)
A characteristic diagram A2 in FIG. 8 shows the density characteristics of each normalized gradation value. As a result of normalization, the range of density values that have varied among the tone values in the density characteristics is unified to 1.

The control unit 11 creates a representative characteristic of the density characteristic of each normalized gradation value (step S4). Specifically, the control unit 11 adds up the density values having the same position in the main scanning direction in the density characteristics of each gradation value, divides the added density value by 3, and calculates the averaged density value in the main scanning direction. A representative characteristic is created by plotting each position.
A characteristic diagram A3 in FIG. 8 shows a representative characteristic created from the density characteristics of each normalized gradation value in the characteristic diagram A2 .

  The patterns T21 and T22 are printed on two pages of paper, and each gradation value pattern is printed on a separate paper. In this way, when a correction pattern is printed on a plurality of pages of paper, the control unit 11 acquires the density value measured from the pattern of gradation values printed on the paper of each page, and the density value It is preferable to create density characteristics from each, normalize and average to create representative characteristics for all pages. Accordingly, it is possible to calculate an optimum correction value for correcting not only the density unevenness that occurs in one page of paper but also the density unevenness that occurs in each page of paper. When the density unevenness varies from page to page, it is possible to obtain an optimum correction value for reducing not only the density unevenness of the paper of a certain page but also the density unevenness of all pages as much as possible.

The control unit 11 determines a reference position in the created representative characteristic (step S5) . For example, other positions taking the average value of the density values of the representative characteristic may be determined maximum value of the range of density values, the minimum value, the position to take an intermediate value such as a reference position. A characteristic diagram A3 in FIG. 8 is an example in which a position in the main scanning direction (position indicated by an arrow in the characteristic diagram A3) that takes an intermediate value of the density value range of the representative characteristic is determined as the reference position.

Next, the range of the control unit 11 density characteristics of the tone values to the representative characteristics, i.e. used during normalization - by multiplying the (maximum value-minimum value) to obtain the density characteristics of each tone value (step S6 ).
A characteristic diagram A4 in FIG. 8 shows density characteristics of each gradation value obtained by multiplying the representative characteristic in the characteristic diagram A3 by the original range. As shown in the characteristic diagram A4, the density characteristic of each gradation value has the same positions as the representative characteristic shown in the characteristic diagram A3, such as the maximum value, the minimum value, their intermediate value, and the average value.

In the density characteristic of each gradation value obtained by multiplying the representative characteristic by the original range, the control unit 11 uses each density level corresponding to the position in the main scanning direction based on the density value at the reference position determined by the representative characteristic. A correction value for the tone value is calculated (step S7). The control unit 11 determines the correction value of the reference position as 0, and sets the correction value of the position in the other main scanning direction according to the difference obtained by subtracting the density value of the position in the other main scanning direction from the density value of the reference position. It is determined by the following formula.
Correction value = (Density value at reference position−Density at position in other main scanning direction) × Coefficient α
The coefficient α is a coefficient for converting density values into gradation values.

FIG. 9 shows a specific example of the correction value calculated from the density characteristic of each gradation value.
Table a1 in FIG. 9 shows correction values calculated using the position x4 as a reference position.
The control unit 11 linearly interpolates the correction values calculated for the gradation values 255, 127, and 63 to calculate correction values between the gradation values 255, 127, and 63. Similarly, the control unit 11 calculates a correction value between the positions x1 to x6 by linear interpolation, and creates an LUT in which the calculated correction value of each gradation value is determined for each position in the main scanning direction. Table a2 in FIG. 9 is an example of an LUT. The control unit 11 sets the LUT as the LUT that the image processing unit 22 uses for the gradation correction process (step S8).

  As described above, according to the first embodiment, the image forming apparatus 1 has a plurality of patterns with different gradation values, and has a pattern having a width in the main scanning direction on the sheet. The density value measured at a plurality of positions in the main scanning direction of the printing unit 24 to be printed and the pattern of each gradation value printed on the paper is acquired, and the correction value of each gradation value according to the position in the main scanning direction A control unit 11 that calculates the image data, and an image processing unit 22 that performs gradation correction processing on the image data using the correction value calculated by the control unit 11. The control unit 11 creates, for each gradation value, a density characteristic indicating the correlation between the position in the main scanning direction where the density value is measured and the density value, and normalizes the density characteristic of each gradation value by dividing it by the range. . The control unit 11 creates a representative characteristic of the density characteristic of each normalized gradation value, and determines a reference position where the correction value is 0 based on the representative characteristic. In the density characteristic of each gradation value obtained by multiplying the representative gradation by the density characteristic range of each gradation value, the control unit 11 sets the position in the main scanning direction based on the density value of the determined reference position. A correction value for each gradation value is calculated.

  Since the unique reference position is determined from the representative characteristics of the density characteristics of each gradation value, the entire surface of the sheet, that is, the main scanning direction and the sub-scanning direction, regardless of where the gradation value pattern is arranged on the sheet. An optimum correction value can be obtained in order to minimize the density unevenness that occurs. As long as the pattern has a width in the main scanning direction of the sheet, an optimum correction value can be calculated and the correction accuracy can be improved.

<Second Embodiment>
Compared to the first embodiment in which the reference position is determined by the representative characteristic of the density characteristic and the correction value is calculated, the correction value is calculated first and the representative characteristic is created from the correction value characteristic as the second embodiment. An example in which the reference position is determined and the correction value is recalculated will be described.

  The image processing apparatus according to the second embodiment has the same configuration as the image processing apparatus 1 according to the first embodiment, and only the processing content of the correction value calculation process by the control unit 11 is different. Therefore, the illustration of the image processing apparatus according to the second embodiment is omitted, and the same reference numerals are used for the same components as those of the image processing apparatus 1, and only the correction value calculation process according to the second embodiment is described. To do.

FIG. 10 is a flowchart of the correction value calculation process according to the second embodiment.
Steps S21 and S22 shown in FIG. 10 are the same processing contents as steps S1 and S2 of the correction calculation processing (FIG. 2) according to the first embodiment. That is, the control unit 11 reads a correction pattern from the storage unit 12 and causes the printing unit 24 to print the pattern on a sheet (step S21). The user measures the density value of the printed pattern with a colorimeter or the like, and inputs the measurement result to the image forming apparatus 1. The control unit 11 acquires a density value measurement result from a colorimeter or USB memory connected to the I / F 16. The point that the following correction values are calculated in color units is the same as in the first embodiment. The control unit 11 plots the measured density value against the position in the main scanning direction where the density value is measured, and creates a density characteristic of each gradation value (step S22).

  The control unit 11 provisionally determines a reference position where the correction value is 0 in the density characteristic of each gradation value (step S23). For example, as shown in the characteristic diagram A1 in FIG. 8, the control unit 11 provisionally determines a position that takes an intermediate value between the minimum value and the maximum value in the density characteristic of each gradation value as the reference position. Since the density characteristic of each gradation value has a different density value range, the temporarily determined reference position also varies.

The control unit 11 calculates a correction value from the difference between the density value of the temporarily determined reference position and the density value of each position in the main scanning direction by the following formula, and plots it for each gradation value to create a correction value characteristic ( Step S24).
Correction value = (density value at reference position−density value at other position) × coefficient α
The coefficient α is a coefficient for converting density values into gradation values.
A characteristic diagram B1 in FIG. 11 shows a correction value characteristic created from the density characteristic of each gradation shown in the characteristic diagram A1 in FIG. The correction value of the reference position temporarily determined in each correction value characteristic is zero.

The control unit 11 normalizes the correction value characteristic of each gradation value by dividing by the range (maximum value−minimum value) (step S25). Specifically, the control unit 11 uses the maximum value and the minimum value of the correction value characteristics of each gradation value to obtain a correction value normalized by the following equation. The control unit 11 plots the normalized correction value to obtain a normalized correction value characteristic.
Normalized correction value = correction value / (maximum value-minimum value)
A characteristic diagram B2 in FIG. 11 shows the normalized correction value characteristic of each gradation value. As a result of normalization, the range of correction values that have varied among the gradation values in the correction value characteristics is unified to 1.

The control unit 11 creates a representative characteristic of the correction value characteristic of each normalized gradation value. Specifically, the control unit 11 adds the correction values having the same position in the main scanning direction in the normalized correction value characteristic of each gradation value, and divides the added correction value by 3 and averages the correction values. Values are plotted for each position in the main scanning direction to create a representative characteristic.
A characteristic diagram B3 in FIG. 11 shows a representative characteristic created from the correction value characteristic of each normalized gradation value in the characteristic diagram B2 .

  When a correction pattern is printed on a plurality of pages of paper, the control unit 11 acquires density values measured from patterns of gradation values printed on each page, and obtains correction value characteristics from the density values. It is preferable to create representative characteristics for all pages by creating, normalizing, and averaging. Accordingly, it is possible to calculate an optimum correction value for correcting not only the density unevenness that occurs in one page of paper but also the density unevenness that occurs in each page of paper. When the density unevenness varies from page to page, it is possible to obtain an optimum correction value for reducing not only the density unevenness of the paper of a certain page but also the density unevenness of all pages as much as possible.

The control unit 11 finally determines the reference position in the created representative characteristic (step S26) . For example, other positions taking the average value of the correction value of the representative characteristic, the maximum value of the correction-value range, the minimum value, the position or the like to take an intermediate value, optionally a reference position can be determined. A characteristic diagram B3 in FIG. 11 is an example in which the position in the main scanning direction (position indicated by the arrow in the characteristic diagram B3) taking the intermediate value of the correction value range of the representative characteristic is determined as the reference position.

Next, the control unit 11 range of the correction value characteristic of each tone value representative characteristics, i.e. used during normalization - by multiplying the (maximum value-minimum value) to obtain the corrected value characteristics of each tone value ( Step S27).
A characteristic diagram B4 in FIG. 8 shows the correction value characteristic of each gradation value obtained by multiplying the representative characteristic in the characteristic diagram B3 by the original range. As shown in the characteristic diagram B4, the correction value characteristic of each gradation value coincides with the representative characteristic shown in the characteristic diagram B3 in the positions of the maximum value, the minimum value, and the like.

  The control unit 11 recalculates a correction value from the correction value characteristic of each gradation value obtained by multiplying the representative characteristic by the original range with reference to the correction value at the reference position determined by the representative characteristic (see FIG. Step S28). The control unit 11 adds the amount of change from the temporarily determined reference position correction value to the determined reference position correction value to the denormalized correction value, and uses the determined reference position correction value. A new correction value to be 0 is recalculated.

FIG. 12 shows a specific example of the correction value calculated from the density characteristic of each gradation value.
A table b1 in FIG. 12 shows the correction value characteristics created based on the reference positions temporarily determined by the gradation values 255, 127, and 63 from the density characteristics. The reference position of the gradation value 255 varies as position x4, the reference position of the gradation value 127 varies as x3, and the reference position of the gradation value 63 varies as x5. Based on the representative characteristic of the correction value characteristic, the position x4 is determined as a reference position common to the gradation values 255, 127, and 63, and the correction value characteristic of the recalculated correction value is shown in Table b2. If the measurement result is the same density value, the correction finally obtained is the correction value calculation process according to the first embodiment or the correction value calculation process according to the second embodiment. The value is the same.

  The control unit 11 linearly interpolates the recalculated correction value to calculate a correction value between the gradation values 255, 127, and 63. Similarly, the control unit 11 calculates a correction value between the positions x1 to x6 by linear interpolation, and creates an LUT in which the calculated correction value of each gradation value is determined for each position in the main scanning direction. Table b3 in FIG. 12 is an example of an LUT. The control unit 11 sets the LUT as an LUT that the image processing unit 22 uses for gradation correction processing (step S29).

  As described above, according to the second embodiment, the image forming apparatus 1 prints a pattern having a plurality of patterns having different gradation values and having a width in the main scanning direction on the sheet. Density values measured at a plurality of positions in the main scanning direction of the gradation value pattern printed on the paper and the gradation values printed on the paper, and the correction values of the gradation values corresponding to the positions in the main scanning direction are obtained. A control unit 11 for calculating, and an image processing unit 22 for performing gradation correction processing on the image data using the correction value calculated by the control unit 11 are provided. The control unit 11 creates a density characteristic indicating a correlation between the position in the main scanning direction where the density value is measured and the density value for each gradation value. The control unit 11 provisionally determines a reference position where the correction value is 0 in the density characteristics of each gradation value, and the correction value at each position is determined from the difference between the density value at the reference position and the density value at each position in the main scanning direction. And a correction value characteristic indicating the correlation between the position in the main scanning direction and the correction value is created. The control unit 11 divides and normalizes the correction value characteristic of each gradation value by its range, creates a representative characteristic of the normalized correction value characteristic of each gradation value, and sets the correction value to 0 by the representative characteristic. Determine the reference position to be used. Based on the correction value characteristic of each gradation value obtained by multiplying the representative characteristic by the range of the correction value characteristic of each gradation value, the control unit 11 determines the position in the main scanning direction based on the correction value of the determined reference position. The correction value of each gradation value corresponding to is recalculated.

  Since the only reference position is determined from the representative characteristic of the correction value characteristic of each gradation value, the entire surface of the sheet, that is, the main scanning direction and the sub-scanning direction, regardless of where the gradation value pattern is arranged on the sheet. Thus, it is possible to obtain an optimum correction value for minimizing the density unevenness occurring in the image. An optimum correction value can be calculated for any pattern having a width in the main scanning direction, and correction accuracy can be improved.

The first and second embodiments described above are preferred examples of the present invention, and the present invention is not limited to this.
For example, the correction pattern is not limited to the examples shown in FIGS. 3 to 6 as long as a pattern having a width in the main scanning direction of the paper is arranged for each gradation value. Regardless of which pattern is used, according to the correction value calculation method shown in the first or second embodiment, an optimal correction value is calculated for all density unevenness occurring in the main scanning direction and the sub-scanning direction of the paper. This can improve the accuracy of correction.

  Further, the correction value may be calculated after adjusting the printing unit 24 so that the density value at the determined reference position matches the target density value. Thereby, a more accurate correction value can be obtained.

  Further, as a computer-readable medium for the correction value calculation processing program, a non-volatile memory such as a ROM or a flash memory, or a portable recording medium such as a CD-ROM can be applied. A carrier wave is also used as a medium for providing program data via a communication line.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 11 Control part 12 Memory | storage part 13 Display part 14 Operation part 15 Communication part 16 I / F
21 Scanner 22 Image processing unit 23 Memory 24 Printing unit

Claims (6)

A printing unit that prints on the paper a plurality of patterns having different gradation values and having a width in the main scanning direction of the paper;
A control unit that obtains density values measured at a plurality of positions in the main scanning direction of each gradation value pattern printed on the paper, and calculates correction values of the respective gradation values according to the positions in the main scanning direction; ,
An image processing unit that performs gradation correction processing on image data using the correction value calculated by the control unit,
The controller is
A density characteristic indicating a correlation between a position in the main scanning direction where the density value is measured and the density value is created for each gradation value,
The density characteristic of each gradation value is divided by the range and normalized, a representative characteristic of the normalized density characteristic of each gradation value is created, and a reference position where the correction value is 0 is determined by the representative characteristic,
In the density characteristic of each gradation value obtained by multiplying the representative characteristic by the density characteristic range of each gradation value, each gradation according to the position in the main scanning direction based on the determined density value of the reference position An image forming apparatus that calculates a correction value. A printing unit that prints on the paper a plurality of patterns having different gradation values and having a width in the main scanning direction of the paper;
A control unit that acquires density values measured at a plurality of positions in the main scanning direction of each gradation value pattern printed on the paper, and calculates correction values of the respective gradation values according to the positions in the main scanning direction;
An image processing unit that performs gradation correction processing on image data using the correction value calculated by the control unit,
The controller is
A density characteristic indicating a correlation between a position in the main scanning direction where the density value is measured and the density value is created for each gradation value,
A reference position with a correction value of 0 is temporarily determined in the density characteristics of each gradation value, and a correction value at each position is calculated from the difference between the density value at the reference position and the density value at each position in the main scanning direction. Create a correction value characteristic indicating the correlation between the position in the scanning direction and the correction value,
The correction value characteristic of each gradation value is normalized by dividing by the range, a representative characteristic of the normalized correction value characteristic of each gradation value is created, and the reference position where the correction value is 0 by the representative characteristic is recorded. Decide
Based on the correction value characteristic of each gradation value obtained by multiplying the representative characteristic by the range of the correction value characteristic of each gradation value, the correction value of the reference position determined in this way is used as a reference according to the position in the main scanning direction. An image forming apparatus that recalculates the correction value of each gradation value. The printing unit prints a pattern of each gradation value on a plurality of pages of paper,
The said control part acquires the density value measured from the pattern printed on the paper of each page, creates the said density characteristic for all the pages from the said density value, and produces the representative characteristic. The image forming apparatus described in 1. A step of printing on the paper a plurality of patterns having different gradation values and having a width in the main scanning direction of the paper;
Obtain density values measured at a plurality of positions in the main scanning direction of each gradation value pattern printed on the paper, and correlate the density values with the positions in the main scanning direction at which the density values are measured. Creating a density characteristic to be shown for each gradation value;
A step of normalizing by dividing the density characteristic of each gradation value by the range, creating a representative characteristic of the normalized density characteristic of each gradation value, and determining a reference position where the correction value is 0 based on the representative characteristic When,
In the density characteristic of each gradation value obtained by multiplying the representative characteristic by the density characteristic range of each gradation value, each gradation according to the position in the main scanning direction based on the determined density value of the reference position Calculating a correction value of the value;
Correction value calculation method including A step of printing on the paper a plurality of patterns having different gradation values and having a width in the main scanning direction of the paper;
Density values measured at a plurality of positions in the main scanning direction of each gradation value pattern printed on the paper are acquired, and the correlation between the positions in the main scanning direction where the density values are measured and the density values is shown. Creating density characteristics for each gradation value;
A reference position with a correction value of 0 is temporarily determined in the density characteristics of each gradation value, and a correction value at each position is calculated from the difference between the density value at the reference position and the density value at each position in the main scanning direction. Creating a correction value characteristic indicating the correlation between the position in the scanning direction and the correction value;
The correction value characteristic of each gradation value is normalized by dividing by the range, a representative characteristic of the normalized correction value characteristic of each gradation value is created, and the reference position where the correction value is 0 by the representative characteristic is recorded. A step of determining;
Based on the correction value characteristic of each gradation value obtained by multiplying the representative characteristic by the range of the correction value characteristic of each gradation value, the correction value of the reference position determined in this way is used as a reference according to the position in the main scanning direction. Recalculating the correction value of each gradation value;
Correction value calculation method including In the step of printing the pattern, a pattern of each gradation value is printed on a plurality of pages of paper,
In the step of creating the density characteristic, a density value measured from a pattern printed on the paper of each page is acquired, and the density characteristic for all pages is created from the density value,
6. The correction value calculation method according to claim 4, wherein in the step of determining the reference position based on the representative characteristic, the reference position is determined based on the representative characteristic created based on the density characteristic for all pages.

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