JP2007184730A - Density correction table generating apparatus, density correction table generating method, program, and storage medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technology of generating a correction table whereby an interpolation error can be limited within a permissible range and the memory capacity required for which can be suppressed. <P>SOLUTION: The density correction table generating apparatus disclosed herein includes: an input output characteristic acquisition means for acquiring an input output characteristic of an image forming apparatus; an interpolation characteristic calculation means for calculating an interpolation characteristic between two points in the input output characteristic; an interpolation error calculation means for calculating a representative value of the interpolation errors; and a density correction table update means for adding a data point corresponding to the representative value to a density correction table stored in a storage means when the representative value satisfies a predetermined condition. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a technique for correcting density unevenness of an image formed by an image forming apparatus.

  An image forming apparatus having a droplet discharge mechanism, such as an ink jet printer, has a plurality of nozzles that discharge ink in order to increase printing speed. Here, ideally, it is desirable that all the nozzles are arranged in a line at a uniform interval. However, in reality, there is a certain variation in the nozzle interval due to a problem in manufacturing technology. Also, there may be variations in the amount of ink ejected from the nozzles. When printing is performed using such a print head, the ink ejected from the head varies in landing positions and sizes due to variations in nozzle spacing. That is, the image (dot) formed on the paper reflects the nozzle variation. In particular, in a one-pass type image forming apparatus only in the paper feeding direction such as a line head type ink jet printer, such nozzle variation causes a so-called banding phenomenon.

As a technique for suppressing the banding phenomenon caused by nozzle variation, there is a technique for compensating nozzle variation by image processing (see, for example, Patent Document 1). Patent Document 1 discloses a technique for compensating for uneven printing density due to nozzle variation using a correction table. That is, in Patent Document 1, a correction coefficient obtained based on a printing density when a so-called solid pattern is printed is stored in an image forming apparatus in advance, and the gradation value of a pixel is multiplied by the correction coefficient at the time of printing. A technique for correcting density unevenness is disclosed.
Japanese Patent Laid-Open No. 1-129667

  There are various methods for increasing the accuracy of density correction. Among these, increasing the number of data stored in the correction table is the most direct method. From the viewpoint of improving accuracy, it is desirable to store data for all gradation values that can be output by the printer. However, as the amount of data stored in the correction table increases, there is a problem that a large amount of memory must be secured. On the other hand, from the viewpoint of saving memory, it is desirable that the amount of data stored in the correction table is small. When the data amount of the correction table is reduced, it is necessary to calculate correction values by linear interpolation between data stored in the correction table. However, when linear interpolation is performed on a region where the input / output characteristics are not linear, there is a problem that an error due to the interpolation becomes large.

  The present invention has been made in view of the above circumstances, and provides a technique for creating a correction table capable of keeping an interpolation error within an allowable range and suppressing a memory capacity.

  In order to solve the above-described problems, the present invention provides a storage unit that stores a density correction table used for density correction of an image forming apparatus, and input / output characteristics of the image forming apparatus, the input / output having a plurality of data points. A data point included in the density correction table includes a predetermined number of data points among the input / output characteristics acquisition means for acquiring characteristics and the data points of the input / output characteristics acquired by the input / output characteristics acquisition means. Data that is adjacent to the reference data point in a specific direction, with one of the data points specified by the initialization means as a reference data point serving as a reference for processing. Interpolation characteristics between these two points by data point specifying means for specifying a point as an adjacent data point and interpolation processing between the reference data point and the adjacent data point. Interpolation characteristic calculation means for calculating the interpolation characteristic, and the interpolation characteristic calculated by the interpolation characteristic calculation means and the input / output characteristics are compared to calculate an interpolation error between the reference data point and the adjacent data point Interpolation error calculating means, interpolation error determining means for determining whether the interpolation error calculated by the interpolation error calculating means is less than or less than a predetermined allowable error, and the interpolation error determining means When it is determined that the error is not less than the tolerance or less than the tolerance, the input / output characteristic satisfies a predetermined condition among the data points existing between the reference data point and the adjacent data point Density correction table updating means for adding data points to the density correction table stored in the storage means; and the density correction table. When the data point added by the data update unit is specified as a new adjacent data point, the adjacent data point update unit, and the determination unit determines that the interpolation error is less than or less than the allowable error Among the data points specified by the initialization means, the reference data point update means for specifying the adjacent data point as a new reference data point, and the reference data point update means specified as a new reference data point End determination means for determining whether there is a data point adjacent to the data point in the specific direction, and when the end determination means determines that there is no data point adjacent to the reference data point in the specific direction, A density correction table generating device having density correction table determining means for determining data points included in the density correction table; Provide a position.

  In a preferred aspect, in the density correction table generating device, the image forming apparatus has a plurality of nozzles, and the input / output characteristics are a plurality of maximum output values and minimum output values, and each of the plurality of nozzles A plurality of maximum output values and minimum output values corresponding to the data point, and the initialization means includes a data point corresponding to one output value of the plurality of maximum output values and one of the plurality of minimum output values. The data point corresponding to the output value may be specified as the initial value. In this case, one output value among the plurality of maximum output values is a minimum output value among the plurality of maximum output values, and one output value among the plurality of minimum output values is the plurality of minimum output values. It may be the maximum output value among the output values.

  In still another preferred aspect, the density correction table generating device may be the maximum value of the interpolation.

  In yet another preferred aspect, in the density correction table generation device, the input / output characteristic acquisition unit may include data points for all gradation values that can be output by the image forming apparatus.

  The present invention also provides a program for causing a computer device to realize the above functions and a storage medium storing the program.

  Furthermore, the present invention provides a density correction table generation method for generating a density correction table for use in density correction of an image forming apparatus on a computer device, wherein the input / output characteristics of the image forming apparatus are a plurality of data points. An input / output characteristic acquisition step for acquiring the input / output characteristics, and a predetermined number of data points among the data points of the input / output characteristics are specified as initial values of the data points included in the density correction table. Of the data points specified in the initialization step and the initialization step, one data point is used as a reference data point serving as a reference for processing, and a data point adjacent to the reference data point in a specific direction is used as an adjacent data point. A data point identification step to be identified and an interpolation process between the reference data point and the adjacent data point can be performed between these two points. An interpolation error calculation step for calculating an interpolation characteristic, and an interpolation error for calculating a representative value of an interpolation error between the reference data point and the adjacent data point by comparing the interpolation characteristic and the input / output characteristic. A calculation step; an interpolation error determination step for determining whether a representative value of the interpolation error is less than or less than a predetermined allowable error; and the representative value of the interpolation error in the interpolation error determination step is the allowable error In a density correction table update step for adding a data point corresponding to the representative value to a density correction table stored in the storage means, and in the density correction table update step An adjacent data point update step for identifying the added data point as a new adjacent data point; and When it is determined that the representative value of the inter-error is equal to or less than the allowable error or less than the allowable error, the reference data point update step for specifying the adjacent data point as a new reference data point and the initialization step are specified. An end determination step for determining whether there is a data point adjacent to the data point specified as a new reference data point by the reference data point update means among the data points, and the end determination step; A correction table generation method including a density correction table determination step for determining a data point included in the density correction table when it is determined that there is no data point adjacent to the reference data point in the specific direction. A table generation method is provided.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
<1. Image forming system>
FIG. 1 is a block diagram showing a functional configuration of an image forming system 1 according to the first embodiment of the present invention. The image forming system 1 includes an image forming apparatus 200 and a PC (Personal Computer) 100. The image forming apparatus 200 is an apparatus that forms an image on a sheet (recording material) by ejecting ink according to control data. The PC 100 is a computer device that controls the image forming apparatus 200. The PC 100 includes an application 108 such as a word processor and image processing software, and a device driver 109 for controlling the image forming apparatus 200. The application 108 delivers the image data to the device driver 109 in response to a user instruction input or the like. The device driver 109 controls the image data to be processed (RGB (red, green, blue) color multi-value) for injecting ink from the nozzles for each color of CMYK (cyan, magenta, yellow, black). And having the function of outputting the control data to the image forming apparatus 200.

  The device driver 109 has the following functions in detail. The resolution conversion unit 101 converts the input color multivalued image data into a resolution that can be processed by the image forming apparatus 200. The color space conversion unit 102 converts image data in RGB format into image data in CMYK format. The density correction unit 103 performs density correction processing on the image data using the density correction table TB1. Here, the density correction table TB1 is obtained by reading the density correction table TB1 stored in the image forming apparatus 200 and storing it in the PC 100. Details of the density correction table TB1 will be described later. The quantization unit 104 performs a binarization process for converting the multivalued CMYK data into binary CMYK data. The rasterizing unit 105 generates control data from the binary CMYK data. The image forming unit 250 of the image forming apparatus 200 ejects CMYK inks according to the control data. Thus, the image forming system 1 forms an image on a sheet (recording material).

  FIG. 2 is a block diagram illustrating a hardware configuration of the image forming apparatus 200. In the present embodiment, the image forming apparatus 200 is a line head type ink jet printer. A CPU (Central Processing Unit) 210 reads and executes a print processing program stored in a ROM (Read Only Memory) 220. The ROM 220 stores a density correction table TB1 unique to the image forming apparatus 200 (details of the density correction table TB1 will be described later). The ROM 220 is preferably a rewritable EEPROM (Electronically Erasable and Programmable Read Only Memory). A RAM (Random Access Memory) 230 functions as a work area when the CPU 210 executes a program. The I / F 240 is an interface for transmitting and receiving data and control signals to and from other devices such as the PC 100. The RAM 230 also stores data received via the I / F 240. The image forming unit 250 forms an image on a sheet according to the nozzle control data under the control of the CPU 210. The above components are connected to each other via a bus 290. When the CPU 210 executes the application 108 and the device driver 109, the image forming apparatus 200 has functions corresponding to the functional components shown in FIG.

  As illustrated in FIG. 2, the image forming unit 250 further has a configuration described below. The line head 251 is a print head having a plurality (z) of nozzles (not shown) that eject ink. The nozzle may be of any structure, such as a piezo type that discharges droplets with a piezoelectric body, or a heating type that discharges by heating. The line head 251 has a size that is equal to or larger than the maximum width of paper that can be printed by the image forming apparatus 200. The ink tank 252 supplies ink to the nozzles, and is provided for each color of CMYK. In this embodiment, the image forming apparatus 200 forms an image using four colors of ink. However, the image forming apparatus 200 may be configured to use six colors, seven colors, or more colors. The page buffer 257 is a memory that stores nozzle control data for one page of an image. The head drive circuit 253 outputs, to the line head 251, a control signal for causing ink droplets to be ejected from a specified nozzle among a plurality of nozzles mounted on the line head 251 under the control of the control unit 254. . In this way, ink droplets are ejected from the designated nozzle onto the paper. The ink droplets ejected from the nozzles form dots on the paper. Hereinafter, for convenience of description, discharging ink droplets from the nozzles is expressed as “dots on” and not discharging ink droplets from the nozzles as “dots off”. For example, “data for designating ON / OFF of dots” means data for designating whether or not to eject ink droplets for a designated nozzle. “Dot” means an ink droplet ejected from a nozzle or an image formed on the paper by the ink droplet.

  Since the line head 251 has a size equal to or larger than the paper width, dots for one line can be formed. The motor 255 is a motor that moves the paper in a predetermined direction (paper feed). The motor drive circuit 256 outputs a drive signal to the motor 255 under the control of the control unit 254. When the motor 255 moves the paper by one line, the next line is drawn. In this way, the image forming apparatus 200 can form an image on one sheet only by scanning in one direction (paper feeding).

  FIG. 3 is a block diagram illustrating a hardware configuration of the PC 100. The CPU 110 is a control unit that controls each component of the PC 100. The CPU 110 reads and executes a control data generation program (device driver) stored in an HDD (Hard Disk Drive) 150. The RAM 130 functions as a work area when the CPU 110 executes a program. The ROM 120 stores a program and the like necessary for starting up the PC 100. The I / F 140 is an interface for transmitting and receiving data and control signals to and from other devices such as the image forming apparatus 200. The HDD 150 is a storage device that stores various data and programs. The HDD 150 stores a density correction table TB1 read from the image forming apparatus 200. The keyboard 160 and the display 170 are user interfaces for the user to perform operation input to the PC 100. The above components are connected to each other by a bus 190. When the CPU 210 executes the print processing program, the PC 100 has functions corresponding to the functional components shown in FIG. Although not shown, the PC 100 and the image forming apparatus 200 are connected to each other via a wired or wireless connection via the I / F 140 and the I / F 240.

  FIG. 4 is a flowchart showing the operation of the image forming system 1. When a power supply (not shown) of the image forming apparatus 200 is turned on, the CPU 210 reads a print processing program from the ROM 220 and executes it. When the print processing program is executed, the CPU 210 waits for input of control data. In the PC 100, when a print instruction is input from the application 108, the CPU 110 reads the device driver 109 of the image forming apparatus 200 from the HDD 150 and executes it. First, the CPU 110 reads out image data to be processed from the HDD 150 and stores it in the RAM 130 (step S100). In the present embodiment, the input image data is RGB color multivalued image data. The image forming apparatus 200 is an ink jet printer that forms an image with CMYK four color inks. Therefore, the image forming apparatus 200 needs to convert the color space of the image data from RGB to CMYK. The input image data has a gradation value for each pixel, but the ink ejected from the nozzles of the image forming apparatus 200 is dot on / off (dot hitting / shotting) for a dot of a certain size. The intermediate gradation cannot be expressed with only two gradations. Also, there are three types of dot sizes S, M, and L that can be formed by the image forming apparatus 200. Therefore, in the image forming apparatus 200, one pixel of input image data is associated with a dot matrix of a dot × a dot, and gradation expression is performed by the number of dots drawn in the dot matrix. The dot matrix is not limited to a square and may be a rectangle. Therefore, it is necessary to convert the input image data into data that designates dot on / off. For this purpose, as described below, the processing for converting the resolution of the image data into a resolution corresponding to the number of nozzles, and the two-gradation designating the on / off of the dots in the multi-gradation image data It is necessary to convert it to data. The dot sizes that can be formed by the image forming apparatus 200 are not limited to three types, and may be larger or smaller.

  Subsequently, the CPU 110 determines the resolution of the input image data. If the resolution of the input image data is different from the resolution that can be processed by the image forming apparatus 200, the CPU 110 performs a resolution conversion process that converts the input image data to a resolution that can be processed by the image forming apparatus 200 (step S110). The CPU 110 stores the image data after resolution conversion in the RAM 130. Subsequently, the CPU 110 converts the RGB format image data into the CMYK format image data in order to adapt the resolution-converted image data to the color space of the image forming apparatus 200 (step S120). The CPU 110 stores the color-converted image data in the RAM 130. Subsequently, the CPU 110 performs density correction processing on the image data after color conversion using the density correction table TB1 (step S130). Details of the density correction processing will be described later.

  Subsequently, the CPU 110 performs binarization (quantization) processing on the image data after density correction by a dither matrix method, an error diffusion method, or the like (step S140). The CPU 110 stores the image data after density correction in the RAM 130. The CPU 110 performs rasterization processing for generating control data from the image data after density correction (step S150). CPU 110 outputs the generated control data to image forming apparatus 200. The image forming unit 250 of the image forming apparatus 200 forms an image on a sheet according to the control data. The image forming apparatus 200 forms an image whose density has been corrected in this way on a sheet.

<2. Generation of density correction table>
Next, a method for generating the density correction table TB1 will be described. Here, black ink (K) will be described as an example, but a density correction table is created for each color.
FIG. 5 is a block diagram showing a functional configuration of the density correction table generation system 2 according to the present embodiment. The density correction table generation system 2 includes an image forming apparatus 200, a PC 300, and a scanner 400. Although not shown, the PC 300 and the image forming apparatus 200, and the PC 300 and the scanner 400 are connected by wire or wirelessly. The PC 300 stores a test pattern 301 for generating a density correction table. The image forming unit 250 of the image forming apparatus 200 outputs the image D according to the test pattern 301. The scanner 400 reads the image D and generates a scanned image. The scanner 400 outputs the generated scan image to the PC 300. The PC 300 stores the received scan image as the scan image 304. The density measurement unit 303 of the PC 300 calculates the density of the scan image 304 output from the scanner 400. The density correction table generation unit 302 generates a density correction table based on the density measurement result by the density measurement unit 303. The image forming apparatus 200 stores the density correction table generated by the PC 300 as the density correction table TB1.

  FIG. 6 is a block diagram illustrating details of the density correction table generation unit 302. The input / output characteristic acquisition unit 3021 generates the input / output characteristics of the image forming apparatus 200 from the density measurement result by the density measurement unit 303. The input / output characteristic includes a plurality of data points. The interpolation characteristic calculation unit 3022 first specifies at least two data points to be recorded in the density correction table TB1 from a plurality of data points. The interpolation characteristic calculation unit 3022 specifies a reference data point that is a reference for processing among at least two data points and an adjacent data point that is adjacent to the reference data point in a specific direction. The interpolation characteristic calculation unit 3022 calculates the interpolation characteristic between these two points from the reference data point and the adjacent data point. The interpolation error calculation unit 3023 calculates the maximum value of the interpolation error between these two points by comparing the interpolation characteristics with the input / output characteristics. The interpolation error determination unit 3024 determines whether the maximum value of the interpolation error is equal to or less than a predetermined allowable error (or less than the allowable error). When the interpolation error determination unit 3024 determines that the maximum value of the interpolation error is not less than a predetermined allowable error, the density correction table update unit 3025 sets the data point corresponding to the maximum value according to a predetermined condition. , Added to the density correction table TB1. When the density correction table TB1 is updated, the interpolation characteristic calculation unit 3022 calculates the interpolation characteristic between the reference data point and the adjacent data point by using the added data point as a new adjacent data point. When the interpolation error determination unit 3024 determines that the maximum value of the interpolation error is not less than a predetermined allowable error, the adjacent data point is set as a new reference data point. Details of these processes will be described later. As described above, in the present embodiment, the density correction table generation system 2 uses the maximum value of the interpolation error as the representative value of the interpolation error. However, the density correction table generation system 2 may use a representative value other than the maximum value.

  FIG. 7 is a diagram illustrating a hardware configuration of the PC 300. Since the hardware configuration of the PC 300 is basically the same as that of the PC 100, a detailed description thereof will be omitted, and only differences from the PC 100 will be described. The HDD 350 stores a density correction table generation program for generating a density correction table. The HDD 350 stores a test pattern 301 for generating a density correction table.

  FIG. 8 is a flowchart showing the density correction table generation process. When the user instructs generation of a density correction table by a method such as operating the keyboard 360, the CPU 310 of the PC 300 reads out and executes a density correction table generation program from the HDD 350. When the CPU 310 executes the density correction table generation program, the PC 300 has a function as the density correction table generation apparatus shown in FIG. First, in step S <b> 20, the CPU 310 acquires input / output characteristics of the image forming apparatus 200. Input / output characteristics are information indicating the relationship between input luminance and output luminance. The details of the input / output characteristic acquisition processing are as follows.

  FIG. 9 is a flowchart showing details of the input / output characteristic acquisition processing. In step S201, the CPU 310 generates a test pattern image. CPU 310 reads test pattern 301 stored in HDD 350. The CPU 310 outputs the read test pattern 301 to the image forming apparatus 200 via the I / F 340. The test pattern 301 is image data for outputting all gradation values that can be output by the nozzles of the image forming apparatus 200. When the image forming apparatus 200 can express, for example, 8-bit gradation, the test pattern 301 is an image including a 256 gradation pattern from 0 to 255. When the data of the test pattern 301 is received, in step S202, the image forming apparatus 200 prints the test pattern on the sheet according to the received data. Since this test pattern is for generating a density correction table, the density correction processing is not performed when the test pattern is printed. Therefore, the test pattern is printed in a state including printing unevenness due to the physical characteristics of the nozzles.

  Next, in step S203, the scanner 400 reads the image D of the printed test pattern. In order to measure the luminance value for each nozzle in the subsequent processing, reading is performed at a resolution higher than the printing resolution when reading the image. For example, when printing is performed at a resolution of 720 dpi (dot per inch), reading is performed at a resolution of 2880 dpi. In this case, four points of density data can be acquired for one printing dot. The scanner 400 outputs the read image D data to the PC 300. The CPU 310 of the PC 300 stores the input image data as a scan image 304 in the HDD 350.

  Subsequently, the CPU 310 associates density data (output luminance) in the scanned image 304 with each nozzle. The CPU 310 identifies the position corresponding to the data whose density is below a predetermined threshold as the end of the test pattern. In this way, the CPU 310 specifies data corresponding to, for example, the left corner of the test pattern. The CPU 310 specifies the density data of 4 vertical points × 4 horizontal points = 16 points from the specified left corner as the density data corresponding to the pixel (nozzle) at the left corner of the test pattern, that is, the density data of the unit area. The same applies to the other nozzles.

  Next, in step S204, the CPU 310 generates input / output characteristics. The CPU 310 can obtain the input luminance from the image data of the test pattern 301. Further, the CPU 310 can obtain output luminance from the scanned image 304. The input / output characteristic includes a plurality of data points. In the case of 8-bit image data, since the input luminance is 256 gradations from 0 to 255, the input / output characteristics of a nozzle include 256 data points. Each data point includes input luminance and output luminance corresponding to the input luminance. In the following description, a data point corresponding to input luminance x and output luminance y is represented as (x, y).

FIG. 10 is a diagram illustrating input / output characteristics. The CPU 310 acquires an input / output characteristic T _best for each of the z nozzles included in the image forming apparatus 200. In FIG. 10, input / output characteristics T _best ^{0 to} T _best ^{2 of} three nozzles are shown. Ideally, all nozzles should have the same input / output characteristics. However, in reality, the input / output characteristics of each nozzle are not completely the same due to reasons such as manufacturing errors. The CPU 310 stores the generated z sets of input / output characteristics T _best ^{0 to} T _best ^z−1 in the RAM 130.

  A description will be given with reference to FIG. 8 again. When the input / output characteristics are acquired, in step S21, the CPU 310 initializes the density correction table. That is, the CPU 310 specifies initial values of data points included in the density correction table. Details are as follows.

FIG. 11 is a flowchart showing details of the initialization process. First, the CPU 310 secures a storage area for the density correction table TB1 in the RAM 330. At this point, there are no data points included in the density correction table. Next, in step S111, the CPU 310 calculates the output luminance of each nozzle at the maximum input luminance from the z sets of input / output characteristics. Here, in the case of an 8-bit image, the maximum input luminance = 255. For example, the CPU 310 calculates from the input / output characteristics T _best ^{0 to} T _best ² that the output luminances corresponding to the input luminance = 255 are 252, 251, and 245, respectively. Next, in step S112, the CPU 310 specifies the minimum value _OH ^min among the output values for the maximum input luminance. In the previous example, O _H ^min = 245.

Next, in step S113, the CPU 310 calculates the output luminance of each nozzle at the minimum input luminance from the input / output characteristics. Here, in the case of an 8-bit image, the minimum input luminance = 0. For example, the CPU 310 calculates from the input / output characteristics T _best ^{0 to} T _best ² that the output luminances corresponding to the input luminance = 0 are 40, 43, and 38, respectively. Next, in step S114, the CPU 310 specifies the maximum value O _L ^max among the output values for the minimum input luminance. In the previous example, O _L ^max = 38.

Next, in step S115, the CPU 310 adds data points corresponding to O _H ^min and O _L ^max to the density correction table as initial values. That is, in the above example, two data points represented by (255, O _H ^min ) and (0, O _L ^max ) are added to the density correction table. That is, the density correction table stored in the RAM 330 is updated so that the density correction table includes these two data points. When these data points are added to the density correction table, the CPU 310 ends the initialization process. Note that the number of data points specified as the initial value is not limited to two. The CPU 310 may specify three or more data points as initial values. For example, the CPU 310 may add three data points of O _H ^min , O _L ^max , and data points corresponding to these intermediate points as initial values to the density correction table. Further, the order of the processes for calculating O _H ^min and O _L ^max is not limited to that shown in FIG. CPU310 _is the _O ^{L max} may be calculated before the ^{O H min.}

A description will be given with reference to FIG. 8 again. The following processing is performed on the processing target nozzles by specifying the processing target nozzles one by one in order. The processing target nozzle is specified by setting a parameter indicating the processing target nozzle, for example. When the density correction table is initialized, in step S22, the CPU 310 specifies a reference data point serving as a reference for processing and an adjacent data point adjacent to the reference data point in a specific direction among data points included in the density correction table. To do. In the present embodiment, the CPU 310 identifies the data point having the smallest input luminance value as the reference data point, and the CPU 310 determines the data point adjacent to the reference data point in the direction in which the input luminance value increases as the adjacent data point. As specified. That is, the data point (0, O _L ^max ) is specified as the reference data point, and the data point (255, O _H ^min ) is specified as the adjacent data point. CPU 310 stores an identifier for specifying the reference data point and the adjacent data point in RAM 330. Note that the CPU 310 may specify the data point with the maximum input luminance value as the reference data point. CPU 310 may specify a data point adjacent to the reference data point in the direction in which the input luminance value decreases as an adjacent data point.

Next, in step S23, the CPU 310 calculates a required output characteristic. The required output characteristic is defined by a straight line connecting a point (minimum input luminance, O _L ^max ) and a point (maximum input luminance, O _H ^min ) in the input luminance-output luminance coordinate system. That is, the CPU 310 calculates a parameter for specifying a straight line connecting the point (0, O _L ^max ) and the point (255, O _H ^min ). The CPU 310 stores parameters that specify the required output characteristics in the RAM 330. Next, in step S24, the CPU 310 calculates an interpolation error. Details are as follows.

FIG. 12 is a flowchart showing details of the interpolation error calculation process. FIG. 13 is a diagram for explaining the interpolation error calculation process. First, the CPU 310 secures a storage area for the parameter i in the RAM 330. The parameter i is a parameter indicating the input luminance. In the present embodiment, the parameter i is an integer that satisfies 0 ≦ i ≦ 255. CPU 310 sets the initial value of parameter i to the input luminance of the reference data point. In this embodiment, the initial value is determined as i = 0. In step S141, the CPU 310 calculates an interpolation characteristic between the reference data point and the adjacent data point.
In this embodiment, linear interpolation is performed. That is, the CPU 310 calculates a parameter for specifying a straight line connecting the point (0, O _L ^max ) and the point (255, O _H ^min ). The CPU 310 stores parameters for specifying the interpolation characteristics in the RAM 330. In the first process of this embodiment, the required output characteristic and the interpolation characteristic are the same straight line.

Next, in step S142, the CPU 310 calculates the output luminance Oi when the input luminance Ii = i from the required output characteristics. The CPU 310 stores the calculated value of the output luminance Oi in the RAM 330. Next, in step S143, the CPU 310 calculates a corrected input luminance Si when the output luminance = Oi from the required output characteristics. The CPU 310 stores the calculated input luminance Si value in the RAM 330. When the required output characteristic and the interpolation characteristic are the same straight line, Ii = Si. Next, in step S144, the CPU 310 calculates the input luminance Pi when the output luminance = Oi from the input / output characteristic T _best ⁱ . As described above, the input / output characteristics include a plurality of data points. The CPU 310 determines whether the input / output characteristics include a data point with output luminance = Oi. When the input / output characteristics include a data point with output luminance = Oi, the CPU 310 determines Pi so as to be equal to the input luminance value of the data point. When the input / output characteristic does not include a data point with output luminance = Oi, the CPU 310 calculates Pi by interpolation. That is, the CPU 310 calculates the input luminance Pi corresponding to the output luminance = Oi using two data points before and after the output luminance = Oi. The CPU 310 can use any one of linear interpolation, n-order polynomial interpolation, exponential interpolation, logarithmic interpolation, and the like. The CPU 310 stores the calculated value of the input luminance Pi in the RAM 330. Note that the CPU 310 may set Pi as the input luminance of the data point where the difference between the output luminance and Oi is the minimum, regardless of interpolation.

  Next, in step S145, the CPU 310 calculates an interpolation error Di. The interpolation error Di is defined by the absolute value of the difference between the input luminance Si due to the interpolation characteristic and the input luminance Pi due to the input / output characteristic. That is, Di = | Si-Pi |. The CPU 310 stores the calculated interpolation error Di, the input luminance Pi corresponding to the interpolation error Di, and the output luminance Di corresponding to the input luminance Pi in the RAM 330. Next, in step S146, the CPU 310 determines whether calculation of necessary complementary errors has been completed. That is, it is determined whether the parameter i is equal to the input luminance of the adjacent data point. When the parameter i is smaller than the input luminance of the adjacent data point (step S146: NO), the CPU 310 updates i as i = i + 1. When the parameter i is updated, the CPU 310 repeatedly executes the processes of steps S142 to S145. When parameter i is equal to the input luminance of the adjacent data point (step S146: YES), CPU 310 ends the interpolation error calculation process. When the interpolation error processing is completed, the RAM 330 stores an interpolation error characteristic indicating a change in interpolation error between the reference data point and the adjacent data point.

  A description will be given with reference to FIG. 8 again. When the interpolation error is calculated, in step S25, the CPU 310 determines whether the interpolation error is less than or equal to the allowable error (or less than the allowable error). Specifically, the CPU 310 specifies the maximum value Dimax among the interpolation errors Di between the reference data point and the adjacent data point. CPU 310 compares maximum value Dimax with an allowable error. If the maximum value Dimax exceeds the allowable error (step S25: NO), the CPU 310 shifts the process to step S26. When the maximum value Dimax is equal to or smaller than the allowable error (step S25: YES), the CPU 310 shifts the process to step S28. The PC 300 stores an allowable error in the HDD 350 or the ROM 320 in advance. The allowable error is determined based on the accuracy of the binarization process. In the present embodiment, the tolerance is the same value for all input luminances. Note that the tolerance may vary depending on the input luminance. For example, the tolerance may be such that the tolerance is maximized at a point where the tolerance is the largest and the smallest, and the tolerance is minimized at a point where the input luminance is an intermediate gradation.

  In step S26, the CPU 310 updates the density correction table by adding a data point corresponding to Dimax. The CPU 310 extracts from the RAM 330 the input luminance Pi and the output luminance Oi that give Dimax. Hereinafter, the input luminance Pi and the output luminance Oi that give Dimax are expressed as IDimax and ODimax, respectively. The CPU 310 adds the data point (IDimax, ODimax) to the density correction table. Instead of adding the data points (IDimax, ODimax) to the density correction table, the CPU 310 selects the data point or the output luminance that has the smallest difference between the input luminance and IDimax among the data points included in the input / output characteristics. A data point having a minimum difference from ODimax may be added to the density correction table.

  Next, in step S27, the CPU 310 sets the data point newly added to the density correction table as a new adjacent data point. When the adjacent data point is updated, the CPU 310 moves the process to step S23.

  FIG. 14 is a diagram illustrating the density correction table update process. In FIG. 14, points A and B are data points stored as initial values in the density correction table. Data point A is a reference data point and data point B is an adjacent data point. Through the processing described above, the data point C corresponding to the maximum interpolation error is newly added to the density correction table. Then, the data point C is specified as a new adjacent data point. That is, data point A is a reference data point and data point C is an adjacent data point. When the adjacent data point is updated in this way, the interpolation error calculation process in step S24 is performed again.

  FIG. 15 is a diagram for explaining the interpolation error calculation process. Here, the interpolation error Di is calculated in the region between the reference data point (data point A) and the adjacent data point (data point C). In the same manner as described above, in the region between the data point A and the data point C, it is determined whether the maximum value Dimax of the interpolation error Di is equal to or less than the allowable error. If Dimax exceeds the allowable error, a data point corresponding to Dimax is added to the density correction table.

  FIG. 16 is a diagram illustrating the density correction table update process. In the example of FIG. 16, a data point D is newly added to the density correction table. The CPU 310 updates the adjacent data point with the data point D as a new adjacent data point.

  A description will be given with reference to FIG. 8 again. When the maximum value Dimax of the interpolation error Di is equal to or smaller than the allowable error (step S25: YES), in step S28, the CPU 310 updates the reference data point with the previous adjacent data point as a new reference data point. In the example of FIG. 16, when the maximum value Dimax of the interpolation error Di in the region between the reference data point A and the adjacent data point D is equal to or less than the allowable error, the data point D is specified as a new reference data point. Next, in step S29, the CPU 310 determines whether or not processing has been completed for all data points included in the density correction table. Specifically, it is determined whether there is a data point adjacent to a new reference data point in a specific direction (in which the input luminance increases). In the example of FIG. 16, an adjacent data point (data point C) exists at the data point D that is the reference data point. Therefore, it is determined that processing has not been completed for all data points included in the density correction table. When it is determined that the processing has not been completed for all data points included in the density correction table (step S29: NO), the CPU 310 uses a data point adjacent to the new reference data point as a new adjacent data point. Update data points. When the new reference data point and the adjacent data point are specified in this way, the CPU 310 moves the process to step S24.

  In the example of FIG. 16, consider a case where data point C is a reference data point and data point B is an adjacent data point. When the maximum value Dimax of the interpolation error Di in the region between the reference data point and the adjacent data point is equal to or smaller than the allowable error, the CPU 310 has completed the processing for all the data points included in the density correction table in step S28. Judge. Here, the data point B is specified as a new reference data point, but the data point B does not have an adjacent data point in the specific direction. Therefore, CPU 310 determines that the processing has been completed for all data points included in the density correction table. In this case (step S29: YES), the CPU 310 determines in step S30 whether an unprocessed density correction table (that is, an unprocessed nozzle) exists. When it is determined that there is an unprocessed density correction table (step S30: YES), the CPU 310 updates the density correction table (processing target nozzle) to be processed. When the processing target nozzle is updated, the CPU 310 shifts the processing to step S21. When it is determined that there is no unprocessed density correction table (step S30: NO), the CPU 310 determines the density correction table. For example, in the state shown in FIG. 16, the density correction table is determined as a table including four data points A to D. CPU 310 stores the determined density correction table TB1 in HDD 350. The CPU 310 ends the density correction table generation process.

  Next, the CPU 310 transmits a table update request for requesting the update of the density correction table and the density correction table TB1 to the image forming apparatus 200. When receiving the table update request, the CPU 210 of the image forming apparatus 200 stores the received density correction table TB1 in the ROM 220. In this way, the image forming apparatus 200 stores the density correction table TB1 for correcting density unevenness peculiar to its own nozzle.

  FIG. 17 is a diagram illustrating the generated density correction table TB1. The density correction table TB1 has a plurality of density correction tables. Each of the plurality of density correction tables corresponds to one nozzle among the plurality of nozzles of the image forming apparatus 200. In the input / output characteristics obtained by interpolation using the density correction table generated in this way, the error from the input / output characteristics obtained from the test pattern is within a predetermined allowable error. Therefore, according to the present embodiment, it is possible to obtain a density correction table in which the interpolation error is within an allowable range while suppressing the memory capacity. In order to further reduce the necessary memory capacity, the CPU 310 may compress the density correction table TB1. For example, the CPU 310 may reduce the amount of data by assigning a common index to the data of the portion where the input luminance and the output luminance are common in FIG.

<3. Density correction processing>
Next, density correction processing using the density correction table TB1 generated as described above will be described. The density correction process described here is details of the density correction process in step S130 of FIG. The outline of the density correction process is as follows. The processing is performed by sequentially specifying “own pixels” to be processed among the pixels constituting the image data one by one. The correction value of the own pixel is calculated using the density correction table TB1. In the following, only the process relating to the black ink (K) will be described, but the density correction process is similarly performed for the other color components (cyan, yellow, magenta).

  FIG. 18 is a flowchart showing details of the density correction processing according to the present embodiment. First, the CPU 110 of the PC 100 calculates an output luminance range that can be output from all nozzles (step S301). That is, MaxMin and MinMax are calculated from the density correction table TB1. In the following, for the sake of simplicity, description will be made in consideration of only the three nozzles # 00 to # 02. The density correction table for nozzles # 00 to # 02 is shown in FIG. Therefore, MaxMin = 245.0 and MinMax = 38.0.

  Next, the CPU 110 initializes parameters x and y indicating positions in the image (step S302). The parameters x and y are position parameters for specifying the own pixel. x is a parameter indicating the position in the image width direction (paper feed direction), and y is a parameter indicating the position in the image height direction (direction perpendicular to the paper feed direction) (ie, nozzle number). In the present embodiment, the CPU 110 initializes x and y to 0, respectively. In the following description, a nozzle that outputs an ink dot that forms its own pixel is referred to as “own nozzle”.

Subsequently, the CPU 110 calculates a required output luminance (step S303). The required output luminance is like a target value of output luminance. In the present embodiment, the input image is expressed with a luminance of 256 gradations (0 to 255). However, as described above, the luminance that can be output by the image forming apparatus 200 is limited to the range of MinMax to MaxMin (38 to 245 in this embodiment). Therefore, it is necessary to convert the luminance of the input image to the required output luminance so that the luminance of the input image falls within the output luminance range. The CPU 110 converts the luminance I of the input image into the required output luminance C _req according to the following equation (2).
C _req = (MaxMin−MinMax) / C _max × I + MinMax (2)
Here, C _max indicates the maximum luminance (C _max = 255 in the present embodiment).

Assuming that a solid image having a luminance of 128 is input as an input image, C _req = 142 is obtained by substituting MaxMin = 245, MinMax = 39, C _max = 255, and I = 128 into the equation (2) (decimal point). Rounded down below). Since the input image is a solid image, the required output luminance is 142 for all pixels.

  A description will be given with reference to FIG. 18 again. Subsequently, the CPU 110 acquires a density correction table for the own nozzle, that is, the nozzle #y (step S304). This process is performed as follows, for example. The CPU 110 transmits a table transmission request for requesting transmission of the density correction table to the image forming apparatus 200. When receiving the table transmission request, the CPU 210 of the image forming apparatus 200 reads the density correction table TB1 from the RAM 230. The CPU 210 transmits a table transmission response including the read density correction table TB1 to the PC 100 that is the transmission source of the table transmission request. When receiving the table transmission response, the CPU 110 of the PC 100 extracts the density correction table TB1 from the received table transmission response. The CPU 110 stores the extracted density correction table TB1 in the HDD 150. The CPU 110 extracts the density correction table for the nozzle #y from the density correction table TB1 stored in the HDD 150. The CPU 110 stores the extracted density correction table in the RAM 130.

  Note that reading of the density correction table TB1 from the image forming apparatus 200 to the PC 100 may be performed prior to the flow illustrated in FIG. 18, for example, when a printer driver is installed in the PC 100. Further, the image forming apparatus 200 may read all of the density correction table TB1, or may read only a part if necessary.

  Next, the CPU 110 calculates the correction value of the own pixel based on the density correction table of the own nozzle (step S308). Specifically, CPU 110 calculates a correction value for input luminance such that the output luminance matches the required output luminance. When the required output luminance value is not in the density correction table, a correction value is calculated by interpolation. Now, since the required output luminance of the own pixel is 142, linear interpolation is performed using data when the required output luminance is 150.3 and 130.0. In order to set the required output luminance to 142, the input luminance correction value is 162. The CPU 110 stores the correction value calculated in this way in the RAM 130.

  Next, in step S309, the CPU 110 updates the nozzle number y. Specifically, the nozzle number y is updated with y = y + 1. The CPU 110 determines whether the processing has been completed for all nozzles (step S310). When the processing has not been completed for all the nozzles (step S310: NO), the CPU 110 repeatedly executes the processing of steps S304 to S309 described above. In the present embodiment, when the process of steps S304 to S309 is completed for nozzle # 00 (pixel [x, 0]), the process of steps S304 to S309 is subsequently performed for nozzle # 01 (pixel [x, 1]). . The processes in steps S304 to S309 for the pixel [x, 1] are basically performed in the same manner as the processes in steps S304 to S309 for the pixel [x, 0].

  On the other hand, when the processing is completed for all the nozzles for one line (step S310: YES), the CPU 110 updates the value of x as x = x + 1 and returns the nozzle number to 0 (step S311). The CPU 110 determines whether the processing has been completed for all pixels (step S312). When the processing has not been completed for all the pixels (step S310: NO), the CPU 110 repeatedly executes the processing of steps S304 to S311. When the processing has been completed for all pixels (step S312: YES), the CPU 110 completes the density correction processing.

  The CPU 110 outputs control data including the correction value calculated as described above to the image forming apparatus 200. The image forming apparatus forms an image according to the correction value.

  As described above, according to the present embodiment, image correction is performed in consideration of the output characteristics of the own nozzle corresponding to the subject pixel to be corrected. The correction process is performed based on a density correction table that stores information indicating the output characteristics of the nozzles. Further, if the discharge characteristics (output luminance) of the continuous nozzles are less than an allowable value, the density correction table data is integrated. As a result, high image quality can be achieved by correction using the density correction table, and the memory capacity for storing the density correction table can be reduced.

<4. Other embodiments>
The present invention is not limited to the above-described embodiment, and various modifications can be made.
In the above-described embodiment, the image forming apparatus 200 is a line head type ink jet printer. However, the image forming apparatus 200 may be another type of printer such as a multi-pass type ink jet printer.

  In the above-described embodiment, the PC 100 that is the image processing apparatus and the PC 300 that is the density correction table generating apparatus are separate apparatuses, but a single apparatus is used as the image processing apparatus and the density correction table generating apparatus. It may have a function. Further, the function as the image processing device or the density correction table generation device may be distributed to two or more computer devices.

In the above-described embodiment, the example in which the input / output characteristic T _best is a downwardly convex curve has been described. However, the shape of the input / output characteristic is not limited to that described in the embodiment. The present invention can be applied to input / output characteristics of any shape.

2 is a block diagram illustrating a functional configuration of the image forming system 1. FIG. 2 is a block diagram illustrating a hardware configuration of an image forming apparatus 200. FIG. 2 is a block diagram illustrating a hardware configuration of a PC 100. FIG. 3 is a flowchart showing the operation of the image forming system 1. 2 is a block diagram showing a functional configuration of a density correction table generation system 2. FIG. 4 is a block diagram showing details of a density correction table generation unit 302. FIG. It is a figure which shows the hardware constitutions of PC300. It is a flowchart which shows a density | concentration correction table production | generation process. It is a flowchart which shows the detail of an input / output characteristic acquisition process. It is a figure which illustrates an input / output characteristic. It is a flowchart which shows the detail of an initialization process. It is a flowchart which shows the detail of an interpolation error calculation process. It is a figure explaining an interpolation error calculation process. It is a figure explaining a density correction table update process. It is a figure explaining an interpolation error calculation process. It is a figure explaining a density correction table update process. It is a figure which illustrates the produced | generated density correction table. It is a flowchart which shows the detail of the density correction process which concerns on this embodiment.

Explanation of symbols

TB1 ... density correction table, 1 ... image forming system, 2 ... density correction table generation system, 100 ... PC, 100 ... density correction table generation device, 101 ... resolution conversion unit, 102 ... color space conversion unit, 103 ... density correction unit , 104 ... Quantization unit, 105 ... Rasterization unit, 108 ... Application, 109 ... Device driver, 110 ... CPU, 120 ... ROM, 130 ... RAM, 140 ... I / F, 150 ... HDD, 160 ... Keyboard, 170 ... Display , 200 ... Image forming apparatus, 210 ... CPU, 220 ... ROM, 230 ... RAM, 240 ... I / F, 250 ... Image forming unit, 251 ... Line head, 252 ... Ink tank, 253 ... Head drive circuit, 254 ... Control , 255, motor, 256, motor drive circuit, 257, page buffer, 2 DESCRIPTION OF SYMBOLS 0 ... Bus, 300 ... PC, 301 ... Test pattern, 302 ... Density correction table generation unit, 303 ... Density measurement unit, 304 ... Scanned image, 310 ... CPU, 320 ... ROM, 330 ... RAM, 340 ... I / F, 350 ... HDD, 360 ... Keyboard, 400 ... Scanner, 3021 ... Input / output characteristic acquisition unit, 3022 ... Interpolation characteristic calculation unit, 3023 ... Interpolation error calculation unit, 3024 ... Interpolation error determination unit, 3025 ... Density correction table update unit

Claims (8)

Storage means for storing a density correction table used for density correction of the image forming apparatus;
Input / output characteristic acquisition means for acquiring input / output characteristics of the image forming apparatus, the input / output characteristics having a plurality of data points;
Among the data points of the input / output characteristics acquired by the input / output characteristics acquisition means, initialization means for specifying a predetermined number of data points as initial values of the data points included in the density correction table;
Data point specification that specifies one data point among the data points specified by the initialization means as a reference data point as a reference for processing and a data point adjacent to the reference data point in a specific direction as an adjacent data point Means,
An interpolation characteristic calculating means for calculating an interpolation characteristic between these two points by an interpolation process between the reference data point and the adjacent data point;
An interpolation error calculating means for calculating a representative value of an interpolation error between the reference data point and the adjacent data point by comparing the interpolation characteristic calculated by the interpolation characteristic calculating means with the input / output characteristic; ,
Interpolation error determination means for determining whether a representative value of the interpolation error calculated by the interpolation error calculation means is equal to or less than a predetermined allowable error or less than the allowable error;
When the interpolation error determining means determines that the representative value of the interpolation error is not less than or less than the allowable error or less than the allowable error, the data point corresponding to the representative value is stored in the density correction table stored in the storage means. Density correction table update means to be added;
An adjacent data point update means for specifying the data point added by the density correction table update means as a new adjacent data point;
Reference data point update means for specifying the adjacent data point as a new reference data point when the interpolation error determination means determines that the representative value of the interpolation error is equal to or less than the allowable error or less than the allowable error;
An end determination unit that determines whether there is a data point adjacent to the data point specified as a new reference data point by the reference data point update unit among the data points specified by the initialization unit in the specific direction; ,
Density correction table determining means for determining data points included in the density correction table when the end determining means determines that there is no data point adjacent to the reference data point in the specific direction. Table generator. The image forming apparatus has a plurality of nozzles;
The input / output characteristics are a plurality of maximum output values and minimum output values, each including a plurality of maximum output values and minimum output values corresponding to each of the plurality of nozzles;
The initialization means specifies, as an initial value, a data point corresponding to one output value of the plurality of maximum output values and a data point corresponding to one output value of the plurality of minimum output values; The density correction table generating device according to claim 1. One output value of the plurality of maximum output values is a minimum output value of the plurality of maximum output values,
The density correction table generation device according to claim 2, wherein one output value among the plurality of minimum output values is a maximum output value among the plurality of minimum output values.   The density correction table generation device according to claim 1, wherein a representative value of the interpolation error is a maximum value of the interpolation error.   2. The density correction table generation device according to claim 1, wherein the input / output characteristic acquisition unit includes data points for all gradation values that can be output by the image forming apparatus. A density correction table generation method for generating a density correction table used for density correction of an image forming apparatus,
Input / output characteristics of the image forming apparatus, an input / output characteristics acquisition step for acquiring input / output characteristics having a plurality of data points;
An initialization step of specifying a predetermined number of data points among the data points of the input / output characteristics as initial values of the data points included in the density correction table;
Data point specification that specifies one data point among the data points specified in the initialization step as a reference data point serving as a reference for processing and a data point adjacent to the reference data point in a specific direction as an adjacent data point Steps,
An interpolation characteristic calculating step of calculating an interpolation characteristic between these two points by an interpolation process between the reference data point and the adjacent data point;
An interpolation error calculating step of calculating a representative value of an interpolation error between the reference data point and the adjacent data point by comparing the interpolation characteristic and the input / output characteristic;
An interpolation error determining step of determining whether a representative value of the interpolation error is equal to or less than a predetermined allowable error or less than the allowable error;
When it is determined in the interpolation error determination step that the representative value of the interpolation error is not less than the allowable error or less than the allowable error, the data point corresponding to the representative value is stored in the density correction table stored in the storage means. A density correction table update step to be added;
An adjacent data point update step for specifying the data point added in the density correction table update step as a new adjacent data point;
A reference data point update step for specifying the adjacent data point as a new reference data point when it is determined that a representative value of the interpolation error is equal to or less than the allowable error or less than the allowable error;
An end determination step of determining whether a data point specified as a new reference data point by the reference data point update unit among the data points specified in the initialization step is adjacent to the data point in the specific direction; ,
A correction table having a density correction table determination step for determining a data point included in the density correction table when it is determined in the end determination step that there is no data point adjacent to the reference data point in the specific direction; Generation method. Computer equipment,
Storage means for storing a density correction table used for density correction of the image forming apparatus;
Input / output characteristic acquisition means for acquiring input / output characteristics of the image forming apparatus, the input / output characteristics having a plurality of data points;
Among the data points of the input / output characteristics acquired by the input / output characteristics acquisition means, initialization means for specifying a predetermined number of data points as initial values of the data points included in the density correction table;
Data point specification that specifies one data point among the data points specified by the initialization means as a reference data point as a reference for processing and a data point adjacent to the reference data point in a specific direction as an adjacent data point Means,
An interpolation characteristic calculating means for calculating an interpolation characteristic between these two points by an interpolation process between the reference data point and the adjacent data point;
An interpolation error calculating means for calculating a representative value of an interpolation error between the reference data point and the adjacent data point by comparing the interpolation characteristic calculated by the interpolation characteristic calculating means with the input / output characteristic; ,
Interpolation error determination means for determining whether a representative value of the interpolation error calculated by the interpolation error calculation means is equal to or less than a predetermined allowable error or less than the allowable error;
When the interpolation error determining means determines that the representative value of the interpolation error is not less than or less than the allowable error or less than the allowable error, the data point corresponding to the representative value is stored in the density correction table stored in the storage means. Density correction table update means to be added;
An adjacent data point update means for specifying the data point added by the density correction table update means as a new adjacent data point;
Reference data point update means for specifying the adjacent data point as a new reference data point when the interpolation error determination means determines that the representative value of the interpolation error is equal to or less than the allowable error or less than the allowable error;
An end determination unit that determines whether there is a data point adjacent to the data point specified as a new reference data point by the reference data point update unit among the data points specified by the initialization unit in the specific direction; ,
When the end determination means determines that there is no data point adjacent to the reference data point in the specific direction, the end determination means functions as density correction table determination means for determining a data point included in the density correction table. program. Computer equipment,
Storage means for storing a density correction table used for density correction of the image forming apparatus;
Input / output characteristic acquisition means for acquiring input / output characteristics of the image forming apparatus, the input / output characteristics having a plurality of data points;
Among the data points of the input / output characteristics acquired by the input / output characteristics acquisition means, initialization means for specifying a predetermined number of data points as initial values of the data points included in the density correction table;
Data point specification that specifies one data point among the data points specified by the initialization means as a reference data point as a reference for processing and a data point adjacent to the reference data point in a specific direction as an adjacent data point Means,
An interpolation characteristic calculating means for calculating an interpolation characteristic between these two points by an interpolation process between the reference data point and the adjacent data point;
An interpolation error calculating means for calculating a representative value of an interpolation error between the reference data point and the adjacent data point by comparing the interpolation characteristic calculated by the interpolation characteristic calculating means with the input / output characteristic; ,
Interpolation error determination means for determining whether a representative value of the interpolation error calculated by the interpolation error calculation means is equal to or less than a predetermined allowable error or less than the allowable error;
When the interpolation error determining means determines that the representative value of the interpolation error is not less than or less than the allowable error or less than the allowable error, the data point corresponding to the representative value is stored in the density correction table stored in the storage means. Density correction table update means to be added;
An adjacent data point update means for specifying the data point added by the density correction table update means as a new adjacent data point;
Reference data point update means for specifying the adjacent data point as a new reference data point when the interpolation error determination means determines that the representative value of the interpolation error is equal to or less than the allowable error or less than the allowable error;
An end determination unit that determines whether there is a data point adjacent to the data point specified as a new reference data point by the reference data point update unit among the data points specified by the initialization unit in the specific direction; ,
When the end determination means determines that there is no data point adjacent to the reference data point in the specific direction, the end determination means functions as density correction table determination means for determining a data point included in the density correction table. A storage medium that stores programs.

Images